Neurophysiology of Silence Part B: Theory and Review (Volume 280) (Progress in Brain Research, Volume 280) [1 ed.] 0443236135, 9780443236136

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Progress in Brain Research Volume 280

Neurophysiology of Silence Part B: Theory and Review

Serial Editor

Vincent Walsh Institute of Cognitive Neuroscience University College London 17 Queen Square London WC1N 3AR UK

Editorial Board Tanya Calvey, Cape Town, South Africa University of Cape Town Hamed Ekhtiari, USA University of Minnesota Chi Ieong Lau, Taipei, Taiwan Shin Kong Wu Ho-Su Memorial Hospital Shane O’Mara, Dublin, Ireland Trinity College Flavia H. Santos, Ireland University College Dublin

Progress in Brain Research Volume 280

Neurophysiology of Silence Part B: Theory and Review Edited by

Tal Dotan Ben-Soussan Research Institute for Neuroscience, Education, and Didactics, Patrizio Paoletti Foundation for Development and Communication, Assisi, Italy

Joseph Glicksohn Department of Criminology, Bar-Ilan University; The Leslie and Susan Gonda (Goldschmied) Multidisciplinary Brain Research Center, Bar-Ilan University, Ramat Gan, Israel

Narayanan Srinivasan Department of Cognitive Science, Indian Institute of Technology Kanpur, Kanpur, India

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford, OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States First edition 2023 Copyright © 2023 Elsevier B.V. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/ permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-443-23613-6 ISSN: 0079-6123 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Zoe Kruze Acquisitions Editor: Mariana Kuhl Developmental Editor: Federico Paulo Mendoza Production Project Manager: Abdulla Sait Cover Designer: Mark Rogers Typeset by STRAIVE, India

Contributors Michele Antonelli Department of Public Health, Reggio Emilia, Italy Vered Aviv The Jerusalem Academy of Music and Dance, Jerusalem, Israel Henk Barendregt Faculty of Science, Radboud University, Nijmegen, The Netherlands Giuseppe Augusto Chiarenza Centro Internazionale Disturbi di Apprendimento, Attenzione, Iperattivita` (CIDAAI), Milano, Italy Avijit Chowdhury Meditation Research Program, Department of Psychiatry, Massachusetts General Hospital, Harvard Medical School, Boston, MA, United States Kathryn J. Devaney UC Berkeley Center for the Science of Psychedelics, Berkeley, CA, United States Davide Donelli Division of Cardiology, Azienda Ospedaliero-Universitaria di Parma, Parma, Italy Mark A. Elliott School of Psychology, National University of Ireland, Galway, Galway, Republic of Ireland Prakash Chandra Kavi Center for Brain and Cognition, Universitat Pompeu Fabra, Barcelona, Spain Ruben E. Laukkonen Faculty of Health, Southern Cross University, Gold Coast, QLD, Australia Davide Lazzeroni Prevention and Rehabilitation Unit, IRCCS Fondazione Don Gnocchi, Parma, Italy Teodora Milojevi c School of Psychology, National University of Ireland, Galway, Galway, Republic of Ireland Matteo Rizzato “Humandive”, Pordenone, Italy Matthew D. Sacchet Meditation Research Program, Department of Psychiatry, Massachusetts General Hospital, Harvard Medical School, Boston, MA, United States

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Heleen A. Slagter Department of Experimental and Applied Psychology, Vrije Universiteit Amsterdam, the Netherlands & Institute for Brain and Behavior, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands

Contents Contributors .............................................................................................................. v Preface ..................................................................................................................... xi

CHAPTER 1 The causal influence of conscious engagement on photonic behavior: A review of the mind-matter interaction.......................................................................1

Teodora Milojevi c and Mark A. Elliott 1. Introduction ....................................................................................... 1 1.1 Theoretical background to mind-matter interaction ................. 1 1.2 Quantum mechanics and the double-slit experiment ................ 3 1.3 Interpretations of the quantum measurement problem ............. 4 1.4 Focused attention and the double-slit experiment .................... 4 1.5 The effect of attention at large distances .................................. 7 1.6 Double-slit interference, one photon at a time ......................... 8 1.7 Psychological predictors of performance in the experiments.............................................................................. 10 1.8 Critique: the advanced meta-experimental protocol ............... 11 2. Discussion ....................................................................................... 13 References ............................................................................................ 15

CHAPTER 2 The psychophysiology of “covert” goal-directed behavior........................................................................17 Giuseppe Augusto Chiarenza Introduction ..................................................................................... 18 The method ..................................................................................... 19 The movement-related potentials (MRPs) ..................................... 20 Covert goal-directed behavior and the development of movement-related potentials ........................................................... 23 5. MRPs and developmental dyslexia ................................................ 25 6. Application of MRPs in other clinical situations ........................... 31 7. Conclusions..................................................................................... 34 Appendix .............................................................................................. 35 The skilled performance task....................................................... 35 EEG recording .............................................................................. 36 Data analysis................................................................................. 36 References ............................................................................................ 37 1. 2. 3. 4.

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CHAPTER 3 Conscious entry into sleep: Yoga Nidra and accessing subtler states of consciousness.................43 Prakash Chandra Kavi 1. Introduction..................................................................................... 44 2. MPE and states of consciousness ................................................... 45 2.1 Heuristic mapping of lucid dreaming and lucid dreamless sleep to a typical sleep architecture ........................................ 45 2.2 MPE ......................................................................................... 46 2.3 Four main states of consciousness, according to the Upanishads ............................................................................... 47 3. Conscious entry into sleep and “Yoga Nidra”............................... 48 3.1 “Yoga Nidra”........................................................................... 48 4. Experimental methodology ............................................................. 51 4.1 Plausible experiments .............................................................. 51 5. Witnessing deep sleep and dreams ................................................. 52 5.1 “Clear light of sleep” in bon-Buddhist traditions................... 53 5.2 Bodies of consciousness .......................................................... 53 5.3 Present-day meditation schools that teach awareness during dreams and sleep .......................................................... 54 6. Discussions ..................................................................................... 55 6.1 Future possibilities in conducting advanced meditation research .................................................................................... 55 6.2 Conscious entry into NREM 2 sleep during a “Yoga Nidra” meditation as a starting point for investigating the phenomena of “conscious entry into sleep” ............................ 55 Acknowledgments ................................................................................ 56 References ............................................................................................ 56

CHAPTER 4 Cessations of consciousness in meditation: Advancing a scientific understanding of nirodha sam apatti......................................................................61 Ruben E. Laukkonen, Matthew D. Sacchet, Henk Barendregt, Kathryn J. Devaney, Avijit Chowdhury, and Heleen A. Slagter 1. Introduction..................................................................................... 62 2. Nirodha and nirodha samapatti ...................................................... 63 3. Buddhist meditation context ........................................................... 65 3.1 The four rupa (form/material) jh anas..................................... 66 3.2 The four arupa (formless/immaterial) jh anas ......................... 66 3.3 Nirodha sama˜patti: Preparation, procedure, and prerequisites ............................................................................. 67

Contents

4. Contemplative science context ....................................................... 68 5. Preliminary findings ....................................................................... 70 6. Theoretical frameworks: Predictive processing............................. 72 7. Theoretical frameworks: Neural dis-integration ............................ 75 8. Discussion ....................................................................................... 77 9. Challenges and future directions .................................................... 79 10. Conclusion ...................................................................................... 81 Acknowledgments .................................................................................. 82 References.............................................................................................. 82

CHAPTER 5 Moving through silence in dance: A neural perspective...................................................................89 Vered Aviv Introduction ..................................................................................... 89 What is silence? What could silence in dance represent/be? ........ 90 The neural basis of the perception of human movement .............. 92 Conditions for silence in dance ...................................................... 95 Further elaboration of silence in dance with regards to communication............................................................................... 97 6. Why pursue silence in dance? ........................................................ 98 7. Conclusions..................................................................................... 98 References ............................................................................................ 99 1. 2. 3. 4. 5.

CHAPTER 6 Silence and its effects on the autonomic nervous system: A systematic review......................................103 Davide Donelli, Davide Lazzeroni, Matteo Rizzato, and Michele Antonelli 1. Introduction ................................................................................... 104 1.1 Background............................................................................ 104 1.2 Silence in history and philosophy ......................................... 104 1.3 Types of silence..................................................................... 105 1.4 The human autonomic nervous system, a peak evolutionary achievement: An introduction to the Polyvagal Theory ................................................................... 106 2. Methods ........................................................................................ 115 2.1 Literature search methods ..................................................... 115 3. Results ........................................................................................... 116 3.1 Description of the literature findings .................................... 116 4. Discussion ..................................................................................... 126 4.1 Effects of silence on the autonomic nervous system ............ 126 5. Conclusions................................................................................... 134 References .......................................................................................... 135

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Preface This Neurophysiology of Silence volume of Progress in Brain Research (PBR) includes several contributions by authors from different perspectives on silence, from its psychophysiological properties to its more philosophical and contemplative implications. The first part (Volume 277) focused on experimental studies. The second part focuses on theoretical contributions to the Neurophysiology of Silence. The first chapter authored by Mark A. Elliott and Teodora Milojevic titled “The causal influence of conscious engagement on photonic behavior: A review of the mind-matter interaction” provides an extended review of empirical studies on the influence of attention on photonic behavior. Giuseppe Chiarenza contributed a work titled “The psychophysiology of ‘covert’ goal-directed behavior,” which examines silence in the context of covert behavior and goal-oriented movement. This is then followed by two contributions, which explore silence in the context of different states of consciousness. More specifically, Prakash Chandra Kavi’s chapter titled “Conscious entry into sleep: Yoga Nidra and accessing subtler states of consciousness” examines the state of Yoga Nidra. This is followed by Ruben Laukkonen and colleagues’ “Cessations of consciousness in meditation: Advancing a scientific understanding of Nirodha Samapatti,” which highlights the current knowledge on Nirodha Samapatti. The chapter authored by Vered Aviv titled “Moving through silence in dance: A neural perspective” focuses on the interplay between silence and movement, in particular silence and dancing. Finally, Davide Donelli and colleagues present a review titled “Silence and its effects on the autonomic nervous system: A systematic review” in which they highlight how silence affects the functions of our autonomic nervous system. Together, these great contributions demonstrate the richness of the experience of silence and the different ways it can be addressed, examined, and applied. We would like to express our gratitude to all authors for their involvement and shared aim of expanding our current knowledge on the multidisciplinary and multifaceted phenomenon of silence. This volume was born from the International Conference on the Neurophysiology of Silence (ICONS) held in Assisi, Italy, in the summer of 2021, which was led by the Research Institute for Neuroscience, Education, and Didactics (RINED) of the Fondazione Patrizio Paoletti. In this event, we explored the Neurophysiology of Silence in relation to the current times and the challenges as well as the opportunities they offer. We hope that this work will help to spread the importance, validity, and possibility of voluntary silence, well-being, and inner peace. Tal Dotan Ben-Soussan, Joseph Glicksohn, and Narayanan Srinivasan

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CHAPTER

The causal influence of conscious engagement on photonic behavior: A review of the mind-matter interaction

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Teodora Milojevic and Mark A. Elliott* School of Psychology, National University of Ireland, Galway, Galway, Republic of Ireland *Correspondence author: e-mail address: [email protected]

Abstract The well-known, quantum physics “double-slit” experiment was the first demonstration of wave-particle duality of light—photons naturally behave like waves, but once they are registered by a conscious observer they switch to behaving like particles. In recent years, a new avenue of research has reported a psychophysical interaction occurring when focused attention was employed in the double-slit experiment. In this context, the act of focusing attention to photons passing through the double-slit appears to collapse their wave function thus causing a shift toward particle-like behavior reflected in a decreased intensity of wave interference. Contrary to the common belief that physical events have a unidirectional, first-order causal effect on cognition, these studies suggest that mental activities are capable of influencing physical systems. The present paper provides an extended review of findings on this psychophysical phenomenon, as well as recommendations for future research.

Keywords Mind-matter interaction, Photonic behavior, Consciousness, Focused attention, Wave-particle duality, Double-slit experiment

1 Introduction 1.1 Theoretical background to mind-matter interaction Mind-matter interaction refers to an ability of mental activity to directly influence different types of physical system. It is one of the crucial phenomena of human experience as it aims to explain the causal relationship between two fundamental Progress in Brain Research, Volume 280, ISSN 0079-6123, https://doi.org/10.1016/bs.pbr.2023.03.005 Copyright © 2023 Elsevier B.V. All rights reserved.

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aspects—the mental and the physical. The causal properties of mind and its role in our surroundings have been a topic of debate for many centuries and, to date, there remains a lack of empirical clarity due to the complexity of the relationship and the lack of scientific attention dedicated to it. This paper will provide an overview of how perspectives on mind-matter interaction have evolved; outline the contributions of quantum mechanics to our understanding of this phenomenon; review studies employing focused attention in the double-slit experiment, which gives way to studying the effect of mind onto matter; and finally, advise on future directions in this line of research. For the largest part of human history, dating back to ancient philosophers, substance dualism was a prevalent view of mind and matter. The main proponent of this view in Europe in the modern era was Rene Descartes whose ideas are referred to as Cartesian dualism. According to Decartes, mind and matter represent two radically distinct entities. With the progression of science, this view was abandoned and replaced by physicalism to account for the obvious interaction that exists between physical and mental aspects of our experience—events elicit impressions and thoughts manifest into actions. This doctrine posits that the world is primarily physical in its nature and physical substances have the only causal effect, whereas mental phenomena are merely a byproduct of their interaction (Bernstein, 1968). Modern science has favored physicalism and is largely focused on studying material events. This has created a skew in the empirical investigation whereby mind was not explored as a viable independent variable. In psychology, and particularly in the behaviorist approach, the study of mind was reduced to the study of behavior because the latter is directly observable and quantifiable, and thus more convenient for an exploration through the scientific method (Strawson, 2019). Subjective, conscious experiences were reduced to purely neural events, and such reduction was simultaneously eliminativist of mental states as having place in science. As regards mind, physicalism isn’t without its challenges, with one of the main ones being the “knowledge argument.” Namely, there are truths about consciousness which cannot be deduced from the physical paradigm limited to material events (Alter, 2017). In opposition to physicalism are dual-aspect theories which provide an alternative and more holistic explanation of the subject at hand. Mind and matter are two expressions of an unseparated, underlying domain through which their interactions are moderated (Atmanspacher, 2014). Some of the notable proponents of dual-aspect theories are the physicists, David Bohm, Wolfgang Pauli, and the psychologist, Carl Gustav Jung. Bohm (1973) distinguished between the implicate, “enfolded” order, and the explicate, “unfolded” order. The former is the primary, allencompassing, psychophysical aspect of reality which is empirically inaccessible. The latter emerges through the unfoldment of the implicate order and represents the domain of reality experienced through our senses. The distinction between the mind and matter only exists at the surface level, explicate order, and the two are correlated by their shared origin in the implicate order. In line with this is the concept of “unus mundus” popularized by Pauli and Jung (Atmanspacher and Fach, 2013). This postulates existence of a unified reality which, through the process of decomposition, gives rise to the mental and physical aspects of our experience. Just like the implicate order, unus mundus is empirically inaccessible and it can only be

1 Introduction

approximated “from the mental side through the (collective) unconscious and from the physical side through the world’s quantum nature” (Dechamps, 2019, p. 5).

1.2 Quantum mechanics and the double-slit experiment A significant contributor to our understanding of the role of mind in our surroundings is the field of quantum mechanics, which heavily influenced the formation of dualaspect theories, and particularly the Pauli-Jung conjecture (Atmanspacher, 2020). Quantum mechanics emerged at the beginning of the 20th century due to a number of discoveries that could not be explained within the paradigms of classical physics. One such discovery was the theory of wave-particle duality. Namely, there are two possible forms of matter—waves and particles. Waves represent transfer of energy through space, and they are quite peculiar in nature. They exist in multiple places at the same time so their physical properties cannot be exactly determined, only approximated by the probabilistic wave function. On the other hand, particles are concrete objects with precisely describable physical properties. All macroscopic systems can be reduced to molecules, molecules to atoms, and atoms to quantum entities (i.e., electrons, protons, photons). The latter represent the fundamental building blocks of all matter and something curious occurs at this level: all quantum entities naturally behave like waves, but once they become a subject of consciousness through measurement or observation (terms used interchangeably in this regard), they switch to behaving like particles. This occurrence gives rise to the quantum measurement problem, whereby measurement refers to the conscious processing of a quantum state. It sets forth that conscious agents performing measurement largely affect what is to be measured (Jammer, 1974). It is considered a problem because it contradicts the common-sense doctrine of realism, according to which the world and measurement are independent of one another. The first demonstration of wave-particle duality was Thomas Young’s double-slit experiment performed in 1801 (Greenberger et al., 2009). The apparatus used in the experiment is an elongated, sealed, metal tube consisting of three integral parts: a bulb at one end, a plate pierced by two parallel slits in the middle, and a detector screen at the other end. When the bulb illuminates the double-slit, waves of light pass through it, they break into two new waves which then interact with each other. In places where a peak of one wave meets a peak of another wave their intensity is reinforced; and, in places where a peak of one wave meets a trough of another wave, they cancel each other out. This occurrence is known as the interference pattern, and it is projected onto the detector screen as intermittent dark and bright bands of light. Things became curious when the experiment was performed with a filter placed on the bulb which allowed only a single photon to be in flight at each given time. A single photon passing through one of the slits had nothing to interfere with, yet the interference pattern was still observed. Detectors were then placed at the two slits with the purpose of registering the path that single photons took. However, this only raised more questions than answers. Once the measurement was introduced, photons changed their behavior from waves to particles, thus projecting two parallel strips of light onto the detector screen instead of the expected interference pattern.

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1.3 Interpretations of the quantum measurement problem Since the development of quantum mechanics, several interpretations of the quantum measurement problem have been proposed; however, to date, a consensus has not been reached due to a lack of empirical evidence and disagreement regarding what exactly constitutes a measurement. Some physicists favor a physical interpretation such as Zehs quantum decoherence, according to which a system loses its coherence due to interaction with environment (Zeh, 1970). More specifically, when the system is coupled with its environment, a transfer of energy occurs—elements of the quantum wave function are released into the environment and acquire phases from the environment. Through continuous decoherence, quantum entities start to exhibit classical states. On the other hand, proponents of a mental interpretation of the quantum measurement problem, such as Niels Bohr and Werner Heisenberg, proposed that measurement causes a collapse of the wave function—once observed, quantum wave-like potentials are collapsed into classical particle-like realities (Heisenberg and Bohr, 1958). John von Neumann developed this view further by defining the measurement process (Neumann, 1955). All quantum entities, and by extension everything in the universe, naturally exist in a superposition state—the ability to be in multiple states at the same time. Once the entity, or the measurement reading, is registered by a conscious agent, the probabilistic state described by the wave function is collapsed into a tangible reality. This means that the world explained by quantum physics and the world delivered to us through our senses are two very different things, whereby consciousness plays the crucial role in the transition from probability to tangibility. Henry Stapp is one of the more contemporary proponents of this view who suggested that consciousness intervenes with a quantum system and selects one outcome to be realized among the varying quantum possibilities (Stapp, 2011). Von Neumanns interpretation is also referred to as the “consciousness collapse theory” and is at the central interest of the present paper.

1.4 Focused attention and the double-slit experiment The von Neumann interpretation motivated a new line of research which set out to explore whether other aspects of consciousness besides observation, such as attention and intention, collapse the wave function of photons in the double-slit experiment. Research was conducted by having participants focus their attention toward the double-slit and then withdraw their attention, in an alternating and repeated fashion, while the double-slit apparatus was running. Essentially, participants took on the role of detectors used in the original experiment by visualizing the two slits and photons passing through one or the other. It was hypothesized that gaining information about the path that photons take by extra-sensory means would stir photons toward exhibiting more particle-like behavior which would subsequently decrease wave interference. Following the attention-focusing task, participants withdrew their attention and relaxed. Each epoch lasted for 20–30 s and was repeated

1 Introduction

some 20–60 times. Control sessions were run without anyone present. To increase the association between participants’ attention and the apparatus, visual feedback was provided in a form of an indicator bar reflecting the real-time change in the interference pattern. A detector inside of the apparatus registered the interference pattern and by forming a differential measure between focused and withdrawn attention conditions it was possible to determine the effect of attention on the wave-particle duality of photons. Over the course of these studies, various components of wave interference were examined and computed into z-scores for the purpose of analyses. The pioneering studies were conducted by two research teams at York and Princeton universities, jointly reported in Ibison and Jeffers (1998). The measured variable was the interference contrast between the two attention conditions. Participants at York University were instructed to imagine that they could identify the which-path information about the light beam in the apparatus. Participants at Princeton University received a more goal-oriented instruction to mentally intend the indicator bar presented on a screen, reflecting real-time interference contrast, to remain as low as possible. The study at York University yielded non-significant results (z ¼ 0.48). However, it is worth noting that secondary analysis of the data revealed that 14 out of 74 test sessions produced results significant at 1% level, meaning that the overall non-significant result could be ascribed to an excess of negative results. The Princeton team reported a marginally significant effect of human intention on the interference pattern contrast at the 5% level (z ¼ 1.65). These studies were particularly beneficial for inspiring further research characterized by more refined designs and an exploration of various physical and psychological factors pertinent to the experiment (Radin et al., 2012). The state-of-the-art research findings on the effect of focused attention on the wave-particle duality of photons are credited to Dean Radin and his colleagues from the Institute of Noetic Sciences who conducted a total of 16 experiments. Several double-slit optical systems were used throughout, consisting of a 5–10 mW linearly charged laser, a neutral density filter that reduced the divergence of light, a double-slit, and a charge-coupled device line camera which recorded the interference pattern. The majority of the experiments were carried out in an electromagnetically and acoustically shielded chamber for the purpose of reducing the potential interfering effects of ambient vibrations. Samples in these studies were comprised of both meditator and nonmeditator groups and examination of the differences between the two groups allowed the researchers to ascribe the observed effect more accurately to attentional focus. Besides visual, audio feedback on performance was provided so that the experiment could be performed with eyes closed. A pitch of a continuous sound indicated the real-time intensity profile of the recorded interference pattern. Participants held the image of the double-slit in their minds eye and interacted with it by: mentally blocking one of the slits; visualizing photons passing through a distinct slit; or “becoming one” with the apparatus in a contemplative way. Alternatively, they could mentally intend the readings provided through feedback to go in the direction of the hypothesis. With increased internal validity, these studies provide better insight into the psychophysical interactions within the double-slit apparatus.

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The summary of the statistical results derived from these experiments is presented in Table 1. That focused attention perturbs the wave function of photons in the double-slit experiment was demonstrated in 250 test sessions conducted across six experiments (Radin et al., 2012). The variable of interest was the ratio between single-slit and double-slit spectral power, R. The value R decreased significantly in the attention-toward condition relative to the attention-away condition. This shift equated to a 4.36 sigma deviation from a null effect (P ¼ 6  106), whereas the analysis of control sessions conducted without participants present resulted in a null deviation of 0.43 sigma (P ¼ 0.67). Each experiment explored a distinct variable to provide a deeper insight into the nature of the psychophysical interaction and to ensure that the observed effect was indeed due to focused attention. Using EEG to record alpha band event-related desynchronization (ERD, 8–12 Hz), it was revealed that this electrocortical marker of shifts in attention was weakly but positively correlated with the decrease in R (r ¼ 0.027, P ¼ 0.004). The effect was also observed in a retrocausal version of the experiment, whereby data were generated and recorded without being observed and 3 months prior to being used in the experiment. Participants were presented with a strip-chart displaying the prerecorded data which they mentally intended to go as low as possible. The attention-condition assignments were generated and assigned during the observation phase. Experienced meditators significantly decreased the value R (es ¼ 80, z ¼ 2.53, P ¼ 0.006). This result particularly supports the consciousness collapse interpretation of the quantum measurement problem, which suggests that wave function is collapsed when observation takes place, and not when the event is generated. Factors such as temperature, sound vibrations, and signal drifts were examined and showed no significant impact on the observed psychophysical effect. Guerrer (2018) carried out five exploratory and four formative experiments as a conceptual replication of Radin DS-experiment, using a similar protocol and a modified setup and analysis. Data collected in the exploratory mode revealed highly significant differences between the two attention conditions (z ¼ 6.37, P ¼ 1.89  1010, es ¼ 0.50  0.08). Following this, the analysis method and variables of interest were pre-registered for the subsequent four experiments. The formative experiments failed to reach global significance when analyzed based on a directional hypothesis. Table 1 Summary of statistical results from studies employing focused attention in the double-slit optical system.a Publication

No. of experiments

N

σ

P

Radin et al. (2012) Radin et al. (2013) Radin et al. (2015) Radin, Michael, & Delorme (2016) Guerrer (2018)

6 3 6 1 5 4

250 2159 122 2985 160 80

4.36 6.81 3.95 5.72 6.37 2.75

6  106 4.8  1012 3.77  105a 1.05  108 1.89  1010 6.02  103a

a

Results obtained in post-hoc analyses.

1 Introduction

However, a post hoc bidirectional analysis resulted in a significant outcome (z ¼ 2.75, P ¼ 6.02  103, es ¼ 0.31  0.22). Examination of control sessions consistently yielded non-significant results.

1.5 The effect of attention at large distances The first set of experiments was conducted with participants seated 2–3 m away from the double-slit apparatus. The researchers were concerned that participants might have naturally leaned toward the apparatus while focusing their attention and then leaned back during the relaxation periods. This change in proximity might have introduced artifacts due to human body heat and vibrations. To isolate the apparatus from participants, two experiments were conducted over the Internet. The apparatus was located in the Institute of Noetic Sciences in California and linked to a web server accessible to anyone. Conducting an experiment outside of the controlled laboratory environment poses numerous issues to the validity of the experiment; therefore, certain measures were devised to guard against them. Completion of a multi-step registration procedure was required to ascertain that the number of potentially frivolous participants was kept at a minimum. Only one person at a time was allowed to partake in the experiment. The server waited for a handshake after each concentration epoch in order to ensure that participants haven’t left the test session before its completion. Control sessions were conducted with a computer programmed to simulate human participants, indistinguishable to the server and the double-slit apparatus from the actual participants. Sessions completed less than half-way were excluded from the analyses. The first online experiment examined the spectral power and phase associated with the double-slit component of the interference pattern. Analysis of 2089 test sessions revealed a significant decrease in the measured variable (z ¼ 4.31, es ¼ 0.09  0.02, P ¼ 2.6  106), whereas control sessions showed no effect (z ¼ 0.51, P ¼ 0.61) (Radin et al., 2013). The second experiment looked at fringe visibility which provides a relative measure of interference as it accounts for fluctuations in its overall intensity (Radin, Michael, & Delorme, 2016). Over the course of 2 years, 2985 test sessions were contributed and resulted in a 5.72 sigma deviation in fringe visibility at the center of the interference pattern in line with the direction of participants’ intention (P ¼ 1.05  108), whereby 17 out of 20 experimental fringes deviated significantly. By contrast, the control sessions were associated with a 0.17 sigma deviation (P ¼ 0.26), and none of the 20 control fringes reached statistical significance. Maximum effect was observed when data were lagged 9 s, which was expected due to Internet transmission and human task-switching delays. In both studies, participants located across six continents took part, ranging in distance from 1 km to 18,000 km. There was no correlation between the distance of the participants and the observed effect. The data gathered in 2013 and 2014, jointly analyzed and reported in Radin et al. (2016), underwent two independent reanalyzes. Baer (2015) analyzed the 2013 data in a way to avoid the complex preprocessing and statistical procedures that were previously used. The measure of interest were photon counts recorded at the interference

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pattern minima which are expected to increase due to a psychophysical interaction. In both the experimental (i.e., attention-toward and attention-away) and control conditions, photon count means, and standard deviations were calculated and compared as normalized percentage differences. A consistent trend in data was observed with a small average increase in photon counts when participants focused their attention vs. when they were relaxed, while the same analysis in the control condition, where a computer simulated human participants, yielded substantially lower and negative values. When the normalized percent differences were averaged across all interference pattern minima, the cumulative probability for a psychophysical effect in the experimental and control conditions was 51.3% and 50% respectively, reflecting a small but non-negligible difference. The strongest effect was observed at the 8th minimum (68% vs. 52%), and the weakest effect at the 2nd minimum (50.8% vs. 50.4%). The results of these analyses are indicative of a psychophysical interaction; however, standard deviations were generally larger than the average percent values which indicates substantive noise in the measurement. Baer suggested that apparati with a stable power supply need to be used in order to obtain more decisive results. Trembley (2019) conducted a thorough analysis of both data sets by examining different components of the data and performing multiple statistical tests. Radin et al. (2016) measured a change in fringe visibility at a specific point of the interference pattern (fringe 9) and analyzed data in 9 fringe lags. This was because an optimal effect was observed at this specific delay in their first online study. In this reanalysis, all fringes were taken into consideration, and data were analyzed at each lag between 0 and 25 s. Certain results were consistently observed despite the test the data were subjected to: There was a significant decrease in fringe visibility in the 2013 experimental data, in line with the direction of participants’ intention. Surprisingly, a significant change in the opposite direction was observed in the control data. Generally, the change in fringe visibility was non-significant in the 2014 human sessions; and in cases when it was significant, the change was in the opposite direction. The control data were largely non-significant. Throughout the tests “average effect sizes were systematically slightly higher in the human sessions than in controls” (p. 16). A trimming procedure for removal of outliers revealed that, as the percentage of eliminated outliers increased, so did the significance of anomalies, indicating that the core of the data is truly anomalous. There was no strong evidence of a systematic bias explaining the observed anomalies. Nevertheless, the results cannot be considered conclusive due to a large number of statistical tests that were performed. Trembley reported that the data are anomalous; however, there might be some other factors besides a psychophysical interaction causing these anomalies.

1.6 Double-slit interference, one photon at a time The continuous laser beams used in the above-discussed experiments illuminate hundreds of trillions of photons per second. To explore the effect of focused attention on single quanta, Radin et al. (2015) carried out an identical experimental protocol using

1 Introduction

a single-photon double-slit apparatus. Instead of a laser, the apparatus has a small incandescent bulb with a filter on which allows only a single photon to be in flight at each given time. A photomultipler tube (PMT) detects light and a digital circuit counts the number of photons that arrive at a distinct point of the interference pattern. In front of the PMT is a single slit with adjustable position to allow for measuring events at a particular peak or trough. In accordance with the consciousness collapse hypothesis, any means of gaining which-path information about photons collapses the interference pattern, reflected in decreased photon counts at a fringe maximum (i.e., peak) and increased photon counts at a fringe minimum (i.e. trough). The latter was the measured point in this study. Another hypothesis was the “consciousness influence hypothesis” which predicts that the interference pattern will shift in the direction of the goal of interaction, as operationally defined by the feedback provided in the experiments. A total of 132 test sessions were conducted across six experiments. Two of the six resulted in a significant effect of focused attention on the perturbation in the interference pattern, measured as an increase of photon counts at the interference minimum. One experiment explored the role of motivation and its effect on attention. This was achieved by introducing a more engaging form of visual feedback—an aesthetically pleasing LED Buddha statue whereby the level of brightness corresponded to the real-time number of registered photons. The experiment was carried out in complete darkness and the task was to increase the illumination level of the Buddha statue by means of attentional focus. Such experimental setup seemed to have had a fostering effect on the participants’ performance and subsequent increase of photon counts (z ¼ 2.55, P ¼ 0.01, es ¼ 0.57). Another experiment yielded particularly strong and significant results, however, in the opposite direction (z ¼ 4.50, P ¼ 6.8  106, es ¼ 0.90). Some of the participants remarked on the tone of the audio feedback provided in the experimental condition being more distracting than helpful which might have caused them to lower the tone rather than to increase it as instructed. Two experiments shed light on the effect of cognitive training and repeated testing of selected individuals. Groups of two participants underwent trainings prior to each test session for several weeks. An audio program was recorded by a hypnotherapist with the goal of instilling confidence that the psychophysical task could be accomplished with ease. An examination of results in chronological order revealed a significant upwards trend in the correlation between test session and photon counts (r ¼ 0.53, P ¼ 0.004, two-tail), meaning that with the progression of test sessions the number of photon counts recorded at the fringe minimum increased. Additionally, EEG neurofeedback was designed to train attention switching between the experimental and control conditions. Since increased alpha power at central occipital and central frontal sites is associated with relaxation and decreased alpha power at the same two sites with concentration, the neurofeedback training trained this electrocortical activity, one after the other. There was a significant downwards trend in the correlation between test session and recorded photon counts (r ¼ 0.51, P ¼ 0.002, two-tail). Hypnosis had a relaxing and motivating effect on participants, while, according to their reports neurofeedback training put participants to sleep and

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made completing the test sessions increasingly difficult. This might explain the observed effect in the opposite direction as they might have intended the tone of the audio feedback to decrease, opposed to the nominal goal of the experiment. The overall result for the six experiments was z ¼ 0.58, P ¼ 0.56. However, when the Chi-square statistic was calculated to assess the shift in variance, the outcome was highly significant, z ¼ 3.95, P ¼ 3.77  105, suggesting that a psychophysical effect was observed in these studies, but the direction of that effect was not constant. Results from these experiments indicate that it is not the mere action of observation that collapses the wave function, but rather the goal of the psychophysical interaction, which is consistent with the consciousness influence hypothesis. An analysis of environmental factors such as temperature, humidity, and vibration revealed no artifactual influence on the reported results.

1.7 Psychological predictors of performance in the experiments A number of psychological factors, such as motivation, personality, belief in extra-sensory perception, and competence in attention-focusing tasks have affected performance in the double-slit experiment. People who were highly motivated, operationalized as those who contributed 10 or more sessions, performed better than those who contributed no more than two sessions (Radin et al., 2013). Personality traits of openness and absorption correlate with perturbations observed in the interference pattern. Absorption, which reflects one’s disposition to become deeply involved with a task while focusing, is particularly deviant from the population norm among people who exhibit superior performance in the experiment. Belief in the existence of extra-sensory perception often predisposes successful performance in extra-sensory perception tasks (Schmeidler & Murphy, 1946). Such was the case in the present body of research, with an exception of one study which failed to find a significant correlation between this factor and the performance in the experiment (Radin et al., 2013). A hypothesized explanation for this unexpected occurrence was a strong skew toward high belief present in the data. The most powerful predictor of the performance in the experiments was competence in attention-focusing tasks. Maintaining focused attention for periods between 20 and 30 s may not sound overly challenging. However, due to the nature of the human mind which tends to wander and get caught up in different fantasies, holding a stable and intense focus is a demanding task, particularly for untrained individuals. In order to ensure compliance with the task at hand, a requisite for a large portion of participants who contributed to these studies was experience in attention-focusing techniques. On average, experienced meditators produced 2.5 times larger effects than non-meditators (Radin et al., 2012). Superior performance was also observed among people who were engaged in mental disciplines that require focused attention, such as music, intentional healing, sport, and art. Optimal results were usually observed when data were lagged for 2–3 s, which makes sense considering that it takes some time to reorient attention after the relaxation period. These findings are particularly important because they indicate that the observed decrease in the

1 Introduction

intensity of the interference pattern was indeed due to focused attention and not some physical or analytical artifacts. One study failed to demonstrate a correlation between meditation experience and performance in the experiment (Radin et al., 2013). However, the study was conducted outside of the controlled laboratory environment which might have been enough of a reason to explain the lack of the anticipated correlation.

1.8 Critique: the advanced meta-experimental protocol To explore whether a factor other than human conscious intention could account for the observed effects, Radin et al., (2020a, see also Walleczek and von Stillfried, 2019) carried out a conceptual replication study. The specific goal was to identify whether an artifact in the form of a systematic methodological error could be responsible for the previously reported decrease in the double-slit light-interference intensity. This was the first mind-matter interaction study that employed the advanced meta-experimental protocol (AMP), which is particularly suited for detecting and quantifying Type 1 and Type 2 errors in experiments yielding weak effects. Unlike previous experiments which adopted exploratory study design, the replication implemented strictly predictive, confirmatory design with pre-registered data collection, processing, and statistical analysis. Data were collected using a single blind procedure and independently analyzed by Walleczek and Stillfried (2019). The AMP involves a differential analysis between four possible combinations of epoch-pairs X (attention-toward) and O (attention-away): 1) standard experiment (X/O); 2) systematic negative control (O/O); 3) systematic positive control (X/X); and 4) systematic time-reversed control (O/X). Each test session generated a randomized sequence of the four epoch-pairs, repeated five times. In a true-experimental condition, participants alternately directed their attention toward and away from the apparatus, whereas a sham-experimental condition employed an identical protocol, only without participants present. Sham experiments were performed before each test session using the same epoch-pair sequence. With four epoch-pairs systematically applied in two conditions, the overall dataset was partitioned into eight, non-overlapping subsets. In an artifact-free scenario, sham experiments are expected to produce identical and non-significant results, whereas in true experiments only the comparison of X/O and O/X epoch-pairs are expected to result in significant differences. The results of the analysis did not provide support for the psychophysical hypothesis as there were no significant differences in the double-slit light-interference intensity in the true-experimental X/O and O/X conditions. The Xs/Os comparison in the sham experiment revealed a significant decrease in the measured variable without participants present, thus identifying a significant Type 1 error. In the commissioned replication study, the false-positive detection rate was 50%, and the absolute mean difference between XS and OS conditions expressed in percentage was dm ¼ 0.0159%—ten times larger than the percent effect size reported in the 2016 online study (0.001%). Walleczek and Stillfried (2019) concluded that

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the method employed in Radin DS-experiment does not exclusively measure an anomalous consciousness effect, but it is rather sensitive to some other, currently unknown, confounding variables. It is worth noting several differentiating factors between the commissioned replication study and the earlier Radin DS-experiments. Provision of real-time audio feedback was omitted as participants in early testing phase remarked on finding it distracting. The omission of feedback poses a challenge to comparing the present results with the previous ones. Due to the randomized sequencing of epoch-pairs, there were instances (i.e., …OX XX XO… or …XX XX XX…) when participants had to focus for 2–3 min at a time, thus presenting them with a significant, additional cognitive task. The sample consisted of 25 participants who contributed ten sessions each. The PANAS mood-reporting questionnaire administered before and after each test sessions showed a significant decrease in the “positive mood” (r ¼ 0.28, df ¼ 248, P ¼ 6  106) and “attentiveness” subscales (r ¼ 0.22, df ¼ 248, P ¼ 0.0005) over sessions. Therefore, it is possible that the randomized sequencing and increased number of test sessions introduced conditions which were too taxing on participants’ ability to maintain attention to the task. Following the critique, Radin et al. (2020a) published their account in which they outlined the necessity to perform a multiple comparisons adjustment, and presented the results of a variance analysis that was not pre-registered, but included before the data were examined. They argued that conducting eight mean comparisons causes a statistical inflation due to which there is a 34% chance for detecting a false-positive result and, after applying the false discovery rate (FDR) method to adjust for multiple comparisons, none of the eight P-values were significant at P < 0.05. On the other hand, Walleczek and Stillfried (2020) argued that adjustment for multiple comparisons would be in error considering that the mean differences were calculated for non-overlapping datasets. A secondary analysis of a shift in variance was conducted as previous mindmatter interaction studies indicated that intentional efforts can sometimes produce results in the opposite direction due to a number of unconscious factors as well as demand characteristics within the experimental procedure. Application of the FDR method to eight variance P-values yielded a significant outcome in the experimental O/X condition, which was one of the two conditions aimed at identifying true-positive effects. A correlational analysis between the z-scores obtained in the experimental O/X and X/O conditions was significantly negative, rE ¼ 0.14, P ¼ 0.03 (two-tail), whereas the correlation between the same conditions in sham experiments was non-significant, rC ¼ 0.02, P ¼ 0.71. These results indicated that the mean-shift in variance within sessions was a true effect of consciousness, which was not direction-wise consistent across sessions. When different metrics were considered, such as spectral analysis and fringe visibility, regarded as more direct measures of the intensity of an interference pattern, both provided significant support for the psychophysical hypothesis. In the experimental data, the former resulted in a 3.4 sigma effect (P ¼ 0.0003), and the latter in 2.3 sigma, for 7 of 22 fringes after adjusting for Type 1 error inflation. The sham data were non-significant, and examination of temperature and vibration fluctuations did not show any confounding influence (Radin et al., 2021).

2 Discussion

2 Discussion The studies reviewed in this paper demonstrate an attention-modulated mind-matter interaction in the double-slit experiment. The act of focusing attention toward photons passing through the double-slit appears to attenuate their wave interference and cause a slight shift toward particle-like behavior. The observed effect is in line with von Neumann’s interpretation of the quantum measurement problem according to which quantum wave-like potentials are collapsed into classical particle-like realities when registered by a conscious observer. The psychophysical effect was reported by three research teams and was: (1) independent of the distance between the participant and the apparatus; (2) larger among those with experience in attention-focusing tasks; (3) correlated with an electrocortical marker of shifts in attention; (4) mediated by one’s motivation, ability to become absorbed in a task, and belief in extra-sensory perception; (5) observed even retrocausally; and, (6) not due to environmental artifacts such as temperature, humidity, and ambient vibrations. Twenty-nine experiments have been conducted to date with eleven yielding significant results (P < 0.05, two-tailed), not including those obtained in post-hoc analyses. Only one result would be expected to have occurred by chance, with the cumulative binomial probability P < 107 (Radin et al., 2020b). Independent analyses of data and the commissioned replication study provide further insight into the reported effect when different measures, protocols, and statistical methods are employed. A simpler analysis conducted by Baer (2015) provided results for both the experimental and control sessions which were consistently in line with the predictions of the psychophysical hypothesis. A large portion of Trembley’s (2019) thorough analysis supported the proposition of the anomalous consciousness effect in data. However, identification of a significant effect in one of the control datasets, also reported by Walleczek and Stillfried (2019), was indicative of the presence of a yet unknown confounding variable. The advanced meta-experimental protocol found no true-positive effects and reported a 50% detection-rate of a false-positive effect. The commissioned replication study opened up debates regarding the necessity for a multiple comparisons adjustment as well as the study design that future experiments should implement. Walleczek and Stillfried advised discontinuing further exploratory research as even small modifications bear the potential of misidentifying false-positives as true-positive results, and instead recommend to follow a strictly predictive, confirmatory design. However, this line of research is in its early stages, and it explores the effects of attention and intention which, just as any other cognitive functions, depend on a myriad of internal and external factors, and are therefore not easily replicable. For these reasons, implementing an exploratory study design with pre-registered replications would be most suitable for future investigation (Radin et al., 2020a). Consciousness seems to have a goal-directed rather than a passive observational effect. The wave function wasn’t merely collapsed by participants’ intention to extract which-path information about photons. Instead, the shift in photon behavior conformed to the specific goal of the interaction, as demonstrated in Radin et al. (2015). The mechanism underlying the reported psychophysical interaction is

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unknown because there is no known physical link between one’s mental efforts and subsequent changes observed in a distant physical system. However, dual-aspect theories might offer an explanation. Bohm, Pauli, and Jung (Atmanspacher, 2020) proposed that mind and matter, two distinct aspects of human experience, are structurally correlated within the shared, underlying psychophysical domain. Their interaction is moderated at this level and then reflected onto the surface, empirically accessible level of reality. The transition between the two is characterized by an epistemic split, or in physical terms, by measurement (Dechamps, 2019). Consciousness persists as the most complex and mysterious matter in science and, as such, it has given rise to numerous theories and models attempting to explain the nature of it. Physicalist theories propose consciousness as a localized and emergent property of brain activity. This approach has been the starting point in contemporary neuroscience research which has focused on searching for neural correlates of consciousness. The research informs on brain regions in which neurons act in a collective and coordinated fashion to generate different conscious percepts. However, physicalist theories fail to explain why and how phenomenological experiences arise, which has been coined “the hard problem” of consciousness (Chalmers, 1995). The problem might be due to the basic assumption of physicalist theories which ignores the possibility of a non-physical aspect of consciousness. In contrast, many non-physicalist theories propose that consciousness is fundamental in nature, is precedent of space-time and matter, and, as such, has causal properties on the physical world. There have been over a thousand studies to date reporting an effect of mental efforts onto outcomes observed in a wide range of physical systems, from the quantum to the macroscopic scale. Research motivated by non-physicalist theories also provides support for remote viewing phenomena such as perceiving information from distant locations, another person, and non-inferable future events (see Wahbeh et al., 2022). Meta-analyses of these studies demonstrate that the results are real in a statistical context, and that they cannot be assigned to design limitations or selective reporting (Carden˜a, 2018; Radin and Nelson, 2000). Non-physicalist theories imply a non-local property whereby there exists one universal consciousness, not restricted to specific points in space and time. Despite the two approaches discussed here being contradictory in nature, they do not necessarily exclude one another, but rather explain individual aspects of a larger whole. While non-physicalist theories target the origin and the scope of capacities of consciousness, physicalist theories explain how consciousness is integrated and processed in the brain. The research reviewed in this paper represents a novel approach to studying mind-matter interaction phenomena by combining the fields of psychology and physics. As such, it has the potential to provide a more complete account of the role of the mind in our surroundings. The results imply a non-local property of consciousness with its causal effects reaching much farther than one’s own body; and, at the same time, they contradict the mainstream views about the nature of reality. The information presented here can guide future studies in devising optimal experimental conditions and protocols which would allow for obtaining stronger and more decisive effects. At present, this line of research would benefit most from an investigation

References

of the mechanism underlying the psychophysical interaction, such as by employing brain imaging, or recording brain activity (an EEG for example) concurrently with participation in the experiment. This, in turn, has the potential to contribute to other areas of research, particularly those concerning techniques for improving mental health and well-being (Cannard, 2022). Selecting and running sessions with participants who demonstrate special propensity toward psychophysical interaction, identifying potential confounding variables, providing more specific instructions on how to mentally interact with the apparatus, as well as using an apparatus with lower noise levels should all contribute to gaining a deeper insight into the effects of focused attention on the behavior of photons. Consciousness is an inextricable part of all human experience and quantum entities are the fundamental building blocks of all matter. By understanding how the two interact at the smallest scale possible, we can better understand and deal with the events at the observable, macroscopic scale.

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Ibison, M., Jeffers, S., 1998. A double-slit diffraction experiment to investigate claims of consciousness-related anomalies. J. Sci. Explor. 12 (4), 543–550. Jammer, M., 1974. The philosophy of quantum mechanics: The interpretations of quantum mechanics in historical perspective. Wiley. Neumann, J.V., 1955. Mathematical Foundations of Quantum Mechanics. Princeton University Press. Radin, D., Michel, L., Delorme, A., 2016. Psychophysical modulation of fringe visibility in a distant double-slit optical system. Phys. Essays 29 (1), 14–22. https://doi.org/ 10.4006/0836-1398-29.1.014. Radin, D., Michel, L., Galdamez, K., Wendland, P., Rickenbach, R., Delorme, A., 2012. Consciousness and the double-slit interference pattern: six experiments. Phys. Essays 25 (2), 157–171. https://doi.org/10.4006/0836-1398-25.2.157. Radin, D., Michel, L., Johnston, J., Delorme, A., 2013. Psychophysical interactions with a double-slit interference pattern. Phys. Essays 26 (4), 553–566. https://doi.org/10.4006/ 0836-1398-26.4.553. Radin, D., Michel, L., Pierce, A., Delorme, A., 2015. Psychophysical interactions with a single-photon double-slit optical system. Quantum Biosystems 6 (1), 82–98. Radin, D.I., Nelson, R.D., 2000. Meta-Analysis of Mind-Matter Interaction Experiments: 1959 to 2000. Radin, D., Wahbeh, H., Michel, L., Delorme, A., 2020a. Psychophysical effects in double-slit interference patterns: Response to a critique., https://doi.org/10.31234/osf.io/9csgu. Radin, D., Wahbeh, H., Michel, L., Delorme, A., 2020b. Commentary: false-positive effect in the radin double-slit experiment on observer consciousness as determined with the advanced meta-experimental protocol. Front. Psychol. 11. https://doi.org/10.3389/ fpsyg.2020.00726. Radin, D., Wahbeh, H., Michel, L., Delorme, A., 2021. Psychophysical interactions with a double-slit interference pattern: exploratory evidence of a causal influence. Phys. Essays 34 (1), 79–88. https://doi.org/10.4006/0836-1398-34.1.79. Schmeidler, G.R., Murphy, G., 1946. The influence of belief and disbelief in ESP upon individual scoring levels. J. Exp. Psychol. 36 (3), 271–276. https://doi.org/10.1037/h0054099. Stapp, H., 2011. Mindful Universe: Quantum Mechanics and the Participating Observer. Springer. Strawson, G., 2019. A hundred years of consciousness: “a long training in absurdity”. Estudios De Filosofı´a 59, 9–43. https://doi.org/10.17533/udea.ef.n59a02. Tremblay, N., 2019. Independent re-analysis of alleged mind-matter interaction in double-slit experimental data. PloS One 14 (2). https://doi.org/10.1371/journal.pone.0211511. Wahbeh, H., Radin, D., Cannard, C., Delorme, A., 2022. What if consciousness is not an emergent property of the brain? Observational and empirical challenges to materialistic models. Front. Psychol. 13. https://doi.org/10.3389/fpsyg.2022.955594. Walleczek, J., von Stillfried, N., 2019. False-positive effect in the Radin double-slit experiment on observer consciousness as determined with the advanced meta-experimental protocol. Front. Psychol. 10. https://doi.org/10.3389/fpsyg.2019.01891. Walleczek, J., von Stillfried, N., 2020. Response: commentary: false-positive effect in the Radin double-slit experiment on observer consciousness as determined with the advanced meta-experimental protocol. Front. Psychol. 11. https://doi.org/10.3389/fpsyg.2020.596125. Zeh, H.D., 1970. On the interpretation of measurement in quantum theory. Found. Phys. 1 (1), 69–76. https://doi.org/10.1007/bf00708656.

CHAPTER

The psychophysiology of “covert” goal-directed behavior

2

Giuseppe Augusto Chiarenza* Centro Internazionale Disturbi di Apprendimento, Attenzione, Iperattivita` (CIDAAI), Milano, Italy *Corresponding author: e-mail address: [email protected]

Abstract Covert behavior is defined as behavior that is not directly visible and is thus comparable to a type of behavioral silence that requires modern psychophysiological techniques to reveal. Goal-directed behavior is teleologically purposive. Fundamentally, there are two approaches to accounting for purposeful behavior. One is the cybernetic approach, which views behavior as homeostatic and largely reflexive. The other one views behavior as a cognitive process that involves an interaction between neural events representing the previous experience, the present state of the individual, and the occurrence of particular features in the environment. This review, based on published data, presents a non-invasive psychophysiological method for investigating the electrical brain activity associated with those “silent” behaviors such as intention, evaluation of results, and memorization. Movement-related potentials (MRPs) are ideal for studying these processes. The MRPs are recorded during the execution of the skilled performance task (SPT). This task requires the execution of fast ballistic movements with the thumbs of both hands, learning a precise and short time interval between the two thumb presses, and scoring the highest number of target performances. The subject receives real-time feedback about the results of his performance. The MRPs associated with this task and present during covert behavior are the Bereitschaftspotential (BP) present before the onset of movement and the Skilled Performance Positivity (SPP) after movement, which coincides with the subject’s awareness of the success or failure of his performance. These potentials show a maturational trend, reaching the adult form around the age of 10 when formal and abstract thinking progress. SPT and MRPs are particularly suitable to study neurodevelopmental disorders. Children with developmental dyslexia show abnormal MRPs, both in latency and amplitude, in different brain areas.

Keywords Covert behavior, Movement related potentials, Bereitschaftspotential, Skilled performance positivity, Development, Dyslexia, Education

Progress in Brain Research, Volume 280, ISSN 0079-6123, https://doi.org/10.1016/bs.pbr.2023.01.006 Copyright © 2023 Elsevier B.V. All rights reserved.

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1 Introduction Covert behavior, a form of behavioral silence that is not directly observable, requires modern psychophysiological techniques to be revealed. Most of the human behavior we observe seems intentional and teleological (from Greek τέλος, telos “end, purpose” and -λογία, logia, directed towards a goal). It often seems to be a search for a goal previously defined by a model or idea in the brain. Exactly how this goal is achieved, however, can vary. The formulation of strategies, their realization through actions, the evaluation of results, and comparisons with experiences can be explained in different ways. Fundamentally, there are two approaches to accounting for purposeful behavior. One is the cybernetic approach, which views behavior as homeostatic and largely reflexive, and the other is the cognitive approach, which assumes that higher animals possess consciousness, have ideas, and can think about the significance of the information from the environment. According to the former model, at birth, the behavior of the newborn is largely reflex; an organism is endowed with innate patterns of behavior explained as reflexes triggered by the stimulus or as the reduction of drives. As the individual develops, other mechanisms may serve to distribute information to additional regions of the brain. In this manner, multisensory and multivariate transactions begin to modulate genetically specific processes that were initially more simply determined, and a cognitive model of the environment is gradually built, incorporating features of individual experience as well as species-specific features. Numerous observations have established the great power of the cybernetic approach to account for many complex as well as simple behaviors in humans and other mammals, as well as in insects, fish, and birds. As we ascend the phylogenetic tree, this approach becomes unsatisfactory. Behaviors emerge that cannot be explained plausibly as innate or conditioned reflexes. For example, a response learned to a specified stimulus can be elicited by generalization to a novel stimulus that activates very different afferent pathways; learned responses can be executed by using muscles that achieve the desired purpose but that were never before used for that behavior; animals and humans can learn new skills by watching the behavior of another individual. The Italian neurophysiologist Giacomo Rizzolatti and his colleagues have found that neurons in the rostral-ventral part of the premotor cortex of the monkey (area f5, corresponding to Brodmann 44 Broca’s area in humans) not only respond when they perform certain gestures like taking some peanuts but even when they see other monkeys perform the same gesture. These cells are called mirror neurons (Gallese et al., 1996). This system of neurons allows a person to recognize the gestures of others and their meaning, thus contributing to social contact and interaction. Researchers who have studied mirror neurons have identified two new features of the motor system. First, the motor system is not active only during the actual execution of the movement; some parts of this system are active even when the action is imagined or is prepared for action. Secondly, some neurons are active not only when performing a well-circumscribed movement but when we watch another perform the same action or when we hear sounds that belong to that action. Mirror neurons, therefore,

2 The method

represent the multimodal nature of the actions, play a role in the concept of what a monkey or a man is doing, and can distinguish various types of activities to help their planning. The observation of the movements and actions of others also encourages imitation. Mirror neurons foster an understanding of the motor behavior of others, the imitation of gestures, and learning actions (Rizzolatti and Craighero, 2004). The authors show that the mind is relational and deals with some of the practical consequences of this; for example, a child will not learn to speak by watching television. Therefore, one can view behavior as resulting from a cognitive process that involves an interaction between neural events representing the previous experience, the present state of the individual, and the occurrence of particular features in the environment. Such behaviors consist of an attempt to match new experiences against an idea reflecting past experiences. It is cognitive rather than reflexive, involving thinking rather than activation of specific neural pathways constituting stimulusresponse circuits (John, 1980). Therefore, for this type of interaction to occur, there must be (a) information on the presence of a goal in the environment, (b) information on the action necessary to attain that goal, and (c) an idea that can be tested against the goal and can be updated according to the results achieved. We have all the necessary and sufficient conditions to observe when the subject begins a voluntary action to interact with the environment. I present a non-invasive physiological method with which we can describe the interaction of human beings with the environment during covert and overt goaldirected behavior. This review, based on previously published data, illustrates the development of MRPs associated with goal-directed behavior in children, explains the rationale for applying it in children with developmental dyslexia and adds new research on MRPs in clinical situations. Furthermore, this chapter has a historical perspective of the last 40 years on this topic, offering the opportunity to discuss those publications in light of the research of recent years that supports the current materials.

2 The method When a subject is engaged in a self-paced, voluntary, and skilled task, called a “skilled performance task” (SPT), and receives visual feedback in real-time about his motor performance, as it happens in our days with videogames, a characteristic sequence of brain potentials is recorded from the scalp in both children and adults. The brain electrical activity associated with this task is called Movement Related Potentials (MRPs). The SPT is self-paced, voluntary, goal-directed, and interactive. To perform adequately, it requires the following skills: bimanual coordination, bimanual ballistic movements, adaptive programming, learning proper timing, and performance improvement. The task provides online knowledge of results and feedback. See the appendix for a detailed description of the skilled performance task and the recording conditions.

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3 The movement-related potentials (MRPs) Based on the spatial-temporal characteristic of these potentials and their relationship to electromyographic activity and performance, four successive periods can be identified during a skilled task: the premotor period, the sensory-motor period, the motor completion period, and the post-motor period (Fig. 1). (Papakostopoulos, 1978a). The Bereitschaftspotential (BP) is characteristic of the premotor period, and its onset precedes the EMG activities of 800–1200 ms (Kornhuber and Deecke, 1965). Its amplitude is higher during skilled and goal-oriented tasks than during unskilled and non-purposeful ones (Chiarenza et al., 1980). Its scalp distribution is prevalent in the central and precentral areas. Recently, Paradiso et al. (2004) have demonstrated the involvement of the thalamus in the preparation of self-paced movements. Neither the scalp nor the thalamus showed pre-movement potentials with passive wrist extensions. The thalamic MRP preceded both contralateral and ipsilateral wrist movements. There was no significant difference between the onset time of thalamic MRP and cortical MRP the maximum amplitude occurred at contacts located in the ventral lateral nucleus. During unimanual action, the BP is predominantly in the contra-lateral hemisphere, while during bimanual skilled action, the BP is symmetric in the right-hand subjects but not in the left-hand subjects (Papakostopoulos, 1980a, 1980b). This potential seems to reflect the strategic organization of the ideokinetic elements necessary to achieve the goal.

FIG. 1 Schematic diagram of Movement-related potentials recorded in Cz and left forearm electromyogram, during the execution of the skilled performance task.

3 The movement-related potentials (MRPs)

The sensory-motor period starts from the onset of phasic electromyographic activity and lasts for about 200 ms. It is during this period that behavior becomes manifest. It coincides with the appearance on the scalp of the Motor Cortex Potential (MCP) and N100. This potential following the movement onset is of special interest. MCP is a negative potential, which follows the BP and reaches its peak after 40–80 ms from the EMG peak. It is present during unskilled actions and increases in amplitude during a self-paced, ballistic and sustained motor performance (Grunewald-Zuberbier et al., 1979; Papakostopoulos, 1978b). It is present in both children and adults, its amplitude decreases with senescence (Papakostopoulos and Banerji, 1980). Scalp, cortical and magnetic recordings of MRPs with the same skilled task have clearly shown that the source of the MCP is located anteriorly to the central fissure (Chiarenza et al., 1991; Papakostopoulos and Crow, 1984; Rektor et al., 1998) (Fig. 2). This potential seems to be an index of reafferent peripheral input coming from the skin, tendons, and muscle receptors. (Papakostopoulos and Crow, 1984). MCP is followed by the cortical evoked potential, N100, the expression of the early stages of visual perceptual processing of the brain response to the visual stimulus, i.e., the oscilloscope’s sweep. This potential, due to the inhibition phenomenon, is reduced in amplitude in the precentral areas. (Papakostopoulos et al., 1975). The motor completion period is characterized by the decline of the EMG activities and, on the scalp, by a positive peak with a latency of 200 ms (P200). This potential seems to reflect the reafferent activity of the deeper structure of the CNS (Vaughan et al., 1968). The post-motor period is characterized by the return of the electromyographic activities to the preceding rest conditions and by the presence of a large positive potential with a latency of 460 ms, denominated as Skilled Performance Positivity (SPP) (Papakostopoulos, 1980c). This potential has a central and parietal scalp distribution. Magnetic signals, recorded over the hand somatomotor area during the SPT, show that SPP gets contributions from the somatomotor cortex (Chiarenza et al., 1991). Therefore, one might even inquire about the appropriateness of the classical differentiation between sensory, motor, and cognitive activities. SPP is observed in healthy subjects when they are asked to perform a motor-perceptual task that requires precision, timing, and improvement of performance level by providing adequate real-time feedback information on the outcome. The SPP is absent during unskilled tasks and when adequate feedback is not provided to the subject (Papakostopoulos et al., 1986). From a chronological standpoint, SPP coincides with the subject’s awareness of the success or failure of his performance. This positivity appears when the subject looks for information about his outcome, that is to say, information relevant to the efficiency of his psychomotor pre-programmed organization (Chiarenza et al., 1990b). The knowledge of the outcome is likely to be used to influence future actions. The MRPs of the premotor period, BP, and of the post-motor period, SPP, are indices of covert, “silent” behavior, while the MRPs of the sensory-motor period, MCP and N100, and the MRPs of the motor completion period, P200, are signs of overt behavior.

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FIG. 2 24-Channel recordings of magnetic fields evoked by median nerve stimulation (above) and of movement-related cerebral magnetic fields during the skilled performance task (below) in subject JK. The peak of the MEF clearly follows the peak of the EMG, its source is located anteriorly to the central sulcus, about 2 cm more medially, and slightly anteriorly from the source of N20m; this difference was statistically significant at the 95% confidence levels for all subjects. The schematic head shows the measurement locations. Signals were recorded over the hand somatomotor area of the left hemisphere in all subjects (N ¼ 4). The upper trace of each pair shows the field gradient in the vertical (y) direction and the lower in the horizontal (x) direction. The superimposed traces in the lower part of the figure illustrates the replicability of the recordings. The vertical lines show the EMG onset. The passband is 0.05–500 Hz for somatosensory evoked fields (SEFs) and 0.05–100 Hz for the movement-related fields. The insert shows the ǝBr/ǝy signal from location 9 and the simultaneously recorded rectified EMG. RF ¼ Readiness Fields; MEF ¼ Movement-Evoked Fields; SPF¼Skilled Performance Fields. Chiarenza, G.A., Hari, R.K., Karhu, J.J., Tessore S. 1991. Brain activity associated with skilled finger movements: multichannel magnetic recordings. Brain Topogr., 3, 433–439.

4 Covert goal-directed behavior and the development of MRPs

4 Covert goal-directed behavior and the development of movement-related potentials The age-related changes in the Movement Related Potentials from childhood to adult life provide further guidance on the various stages of organization of “covert” goaldirected behavior (Chiarenza et al., 1984) (Fig. 3). In children aged six to seven, only MCP, N100, and P200 are present; they represent the sensory information coming from the subject and environment. Therefore, despite the awareness of the movement carried out by the children, the BP characteristic of the preparatory period and covert behavior, is absent in these young subjects (Chiarenza et al., 1995). Around 9–10 years of age, BP starts to emerge in the frontal and central areas. During this period of age, the children acquire the adult capacity of abstract thinking (Bruner, 1970). They conceive many possible ways in which they could operate and many alternative ways in which they could perform better. We suppose that these different possibilities, these “strategic qualities,” i.e., the intentionality of action to achieve a goal, are reflected in the BP amplitude.

FIG. 3 The averaged movement-related brain potentials during the execution of the skilled motor task, in normal subjects of 6 and 12 years old, along with (bottom) left arm (LEMG) and right arm (REMG) electromyographic activity. In this and the following figures, the vertical bar indicates the trigger point and a  2 μV calibration signal; BP ¼ Bereitschaftspotential; MCP ¼ Motor Cortex Potential, SPP ¼ Skilled Performance Positivity; PAN¼Post Action Negativity.

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FIG. 4 The Movement Related Potentials (MRPs) of a subject 7 years old. The MRPs are averaged according to performance time. Wrong interval 2 ¼ time performance between 20 and 40 ms; target interval 3 ¼ time performance between 40 and 60 ms; wrong interval 4 ¼ time performance between 60 and 80 ms. Note the presence of Post Action Negativity (PAN) in Fz and Cz in the wrong interval 2 and 4 and the presence of Skilled Performance Positivity (SPP) in Fz, Cz, and Pz, in the target interval 3.

In children, aged 6–7, during the post-motor period, another potential with a negative polarity denominated Post-Action Negativity, (PAN), appears in the frontal and central regions. This negative potential is linked to poor performance (Fig. 4). Negative potentials, like PAN, after task completion have been associated with rare, novel, unexpected events or error detection (Courchesne et al., 1987; Dehaene, 2018; Hillyard and Kutas, 1983). This negativity could represent the preconceptual stage of representational intelligence, in which reality is dependent on and extinguishes in the moment of immediate perception (Inhelder and Piaget, 1958). The surprise or novelty of the outcome of the performance predominates in these children. PAN decreases with age and disappears by the age of 8–9. Until 9 years of age, SPP is present only in the posterior areas, and its amplitude increases with age. Conversely, SPP latency decreases significantly with age. Around 10 years of age, SPP also appears in the prefrontal and frontal regions. The presence of SPP in these areas is probably related to the maturation of longdistance connections between the parietal and frontal areas. The parietal areas have

5 MRPs and developmental dyslexia

mainly associative somatosensory and visual perceptual functions, while intention, motivation, attention, programming, and evaluation of voluntary and goal-directed behavior are functions of the frontal lobes (Fuster, 1985). These areas have a different maturational trend: parietal areas reach adult levels of synaptic formation around 10–12 months of age, while frontal areas do so at around 10 years of age (Thompson and Nelson, 2001). Therefore, in the parietal areas, SPP could be the expression of more perceptual functions, while in the frontal regions, SPP may represent functions that are more abstract. This viewpoint holds that the emergence of SPP in frontal areas corresponds to a new way of brain functioning: the outcome and knowledge of results are used to match them with projects and past experiences to improve goal-directed behavior (Chiarenza, 2022; Chiarenza et al., 1990b). After 11 years of age, thinking is propositional. In the period of formal thinking, the children take the outcome of their performance, put it in sentence form, and begin to find relationships among sentences. These potentials are affected by age. In particular, the MCP slowly decreases in amplitude and disappears; the SPP also decreases in amplitude and slightly increases in latency. The BP amplitude appears to be unaffected by age (Papakostopoulos et al., 1990; Papakostopoulos and Banerji, 1980).

5 MRPs and developmental dyslexia Lack of fluency and prosody are characteristic symptoms of developmental dyslexia (DSM-52013). Numerous clinical observations have reported that subjects with developmental dyslexia have difficulty in the motor organization when they execute neuromotor tasks (Abercrombie et al., 1964; Connoly and Stratton, 1968) both simple (Bruininks and Bruininks, 1977; Denhoff et al., 1968; Lewis et al., 1970; Pyfer and Carlson, 1972) and complex (Klicpera et al., 1981; Owen et al., 1971). These difficulties in the execution of motor tasks are associated with minor motor neurological signs such as dysrhythmia, kinetic movements, or mirror movements (Adams et al., 1974; Kennard, 1960; Stine et al., 1975; Wolff and Hurwitz, 1973). These motor impairments have been related to a disturbance in time organization in the performance of motor skills (Denckla, 1973; Klicpera et al., 1981). More recently, Nicolson and Fawcett (2005) reported that dyslexic children have difficulties when required to undertake fast, fluent, over-learned skills or novel skills that involve the blending of two actions, and their performance after extensive practice is slower and more error-prone. They argue that dyslexic children have difficulties both with the initial proceduralization of skills and with the “quality” of those skills post-training. The time organization of skilled tasks in developmental dyslexia at the neurophysiological level has not been widely investigated. In my opinion, the reason for the neglect of the motor component of dyslexia lies in the fact that all experimental designs, both neurophysiological and behavioral, were built on the stimulus-response model. In this way, only events related to

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stimulus presentation—auditory or visual—and stimulus processing are observed. This model can describe only phenomena that occur in the interval between the stimulus and the response of the subject. Phenomena before the onset of the stimulus and after the onset of the response, the covert behavior, are missing. Therefore, the Movement Related Potentials represent a fundamental and unique contribution to observing the “silent” behavior before and after the stimulus. Furthermore, the skilled performance task appears to be particularly well suited to studying the temporal organization of movement because it requires the subjects to learn and reproduce a precise and very short time interval, i.e., between 40 and 60 ms, by performing two self-paced ballistic movements. Temporal organization is a fundamental function for learning, language, reading, writing and thinking. This task, to be executed properly, requires precise coordination of two ballistic movements that can only be realized with advanced programming of the two movements. Learning the proper timing leads to an improvement in performance, thanks to feedback on the outcome. This information is provided in real-time and is used to correct performances that do not fall within the target area, i.e., 40–60 ms. Many studies have clarified the contribution of the cerebellum to the organization and timing of fast voluntary movements (Baresˇ et al., 2018). It has been widely accepted that the motor cortices and the basal ganglia are involved in the generation of self-initiated movements. The cerebellum is involved in the timing of volitional movements (Lee and Assad, 2003; Okano and Tanji, 1987; Romo and Schultz, 1992; Schultz and Romo, 1992). Other studies have also suggested that the basal ganglia process rhythmic timing while the cerebellum measures a single time interval. Recent studies using the self-timing task have shown that the time course of preparatory activity differs between neurons recorded from the cerebellar dentate nucleus and the striatal caudate nucleus (Tanaka et al., 2021). Neurons in the cerebellum start their activity about half a second before self-initiated movements (Kunimatsu et al., 2018). At the microscopic level, the cerebellar granular layer appears especially well suited for the timing operations required to confer millisecond precision for cerebellar computations. This precision clock takes place in the cerebellum at the granular layer, where Purkinje cells and granule cells both synapse through parallel and climbing fibers. It is more likely that these neural networks work in conjunction across both suband supra-second time scales as a unified entity (Ohmae et al., 2017). In this network, the cerebellar system is principally involved in initiation and adjustment during the acquisition of tasks, while the striato-thalamo-cortical network is more involved in the termination of timing (Pettera et al., 2016). These neural networks appear to be dysregulated in dyslexic children (Heim and Keil, 2004). Llina´s (1993) calls neural dyschronia the inefficient integration of information from various brain areas. We can observe this dysregulation at the scalp level using the Movement Related Potentials during a skilled performance. In addition, since the assumptions in dyslexia predict poor reading skills, a perceptual-motor task like SPT, which lies outside the domain of reading, is particularly suitable to test this hypothesis.

5 MRPs and developmental dyslexia

Using this task, we have shown that children with developmental dyslexia show significant differences during covert and overt behavior, at the behavioral and neurophysiological levels in comparison with normal children (Fig. 5). Dyslexic subjects have a lower percentage of performance in the target area and are much slower than normal subjects. Their performance time exceeded 100 ms, and they were also less accurate, with a percentage of near-target area performance lower than the percentage of normal subjects (Chiarenza et al., 1982a, 1982b). At the neurophysiological level, in the premotor period, with no evidence of behavioral manifestation, BP is reduced in the frontal, central, precentral, and parietal areas. Its onset was significantly delayed and started only 100 ms before the movement. It has been recently suggested that BP could consist of at least two components. The first begins 1.2 s before the movement and lasts for about 450 to 600 ms and the second is characterized by a steeper negative ramp lasting 300–400 ms. The first component is associated with processes related to the representation of the action and motor programming, while the second is considered the most effective part of the process and therefore the most automatic (Shibasaki, 2012). Intracerebral recordings of the BP have confirmed this observation (Kukleta et al., 2012). The first component is absent in children with developmental dyslexia, while the second one is considerably reduced in amplitude. From a behavioral point of view, these children are slow and not very accurate. The short duration of the BP and its reduced amplitude, resulting a deficit that leads to poor performance (Chiarenza et al., 1990a). During the sensory-motor period, the latency of the visual evoked potential to the oscilloscope’s sweep is significantly increased in prefrontal, frontal, central, and precentral areas compared to the control group. Latency expresses the speed with which information is carried out through the nervous system. The visual response in dyslexic children reached different cortical areas at different times. The latency of P200 in the prefrontal areas is also significantly increased during the motor completion period. Its amplitude is also reduced in all the cerebral areas recorded. The observation that P200 is present in the absence of external stimuli (Vaughan et al., 1968) and increases in amplitude with force (Wilke and Lansing, 1973) supports the hypothesis that P200 is a reafferent somatosensory potential related to movement. The reduced amplitude in dyslexic children in the various brain areas, also during the same performance interval (target performance), could indicate a defect in the integration of reafferent kinaesthetic information. Very interesting information also came from the observation of the MRPs during the other fragment of covert or “silent” behavior, the post-motor period. The grand average of MRPs of all the trials shows that SPP is absent in the frontocentral and precentral regions and present in the parietal regions but with reduced amplitude (Fig. 5). When separate averages for target performance and near-target performance are made, it is possible to note that SPP, related to target performance, is present in the fronto-central regions even though the amplitude is considerably reduced (Fig. 6). The averages of MRPs for wrong performances show that SPP is absent in the fronto-central regions and is substituted by the Post-action Negativity, which is the typical potential of children aged 6–7 years (Fig. 7). Similar negative

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FIG. 5 The average movement-related brain potentials during the execution of the skilled motor task, in normal subjects (thin line) and dyslexic subjects (thick line), recorded in Fpz (Fpz ¼ middle prefrontal), Fz (Fz ¼ middle frontal), and Pz (Pz ¼ middle parietal), along with (bottom) arm electromyographic activity (EMG). Dyslexic subjects showed a reduced BP amplitude of a very short duration, indicating non-adequate preparation. Reduced MCP amplitude indicates a lack of kinesthetic processing. N100 and reduced P200 amplitude indicate a deficit of visual perception and reafferent activity respectively. Reduced SPP amplitude in the parietal regions and the presence of PAN in the central and frontal regions suggest a reduced ability to evaluate target performance and non-target performance respectively (for more details see Chiarenza Journal of learning disabilities, 1990).

5 MRPs and developmental dyslexia

FIG. 6 The average of the movement-related brain potentials associated with target performance in normal subjects (dashed line) and dyslexic subjects (continuous line). The potential associated with knowledge of results (SPP) is present in all areas of the brain but of reduced amplitude.

potentials in frontal areas are recorded when novel, rare, or unexpected stimuli are presented (Courchesne et al., 1987). It is highly likely that children with developmental dyslexia perceive their unsuccessful performance as an unexpected, although likely, event. The presence of PAN in these children could be related to a different strategy activated during processing when the target was missed. These children seemed to give

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FIG. 7 The average of the movement-related brain potentials associated with non-target performance in normal subjects (dashed line) and dyslexic subjects (continuous line). The SPP potential associated with the evaluation and knowledge of results is only present in the parietal areas (perceptual activity), while in the central and frontal areas the post-action negative potential (PAN) is recorded, an expression of the failure to process the error (Chiarenza, 1990).

6 Application of MRPs in other clinical situations

significance to the target performance only as the unique possible result. They do not appear to recognize the failed performance, as evidenced by the presence of SPP during a poor performance in normal children. The dyslexic children do not seem to treasure the experience enough to plan strategies that are more suitable for the task required. Finally, these children with developmental dyslexia have deficits in programming and temporal organization of movements, integration of visual and kinaesthetic sensory processes, performance evaluation, and correction. These studies demonstrate that dyslexia is not only a phonological or gestalt deficit but also a praxic disorder in which praxic abilities, such as motor programming, temporal organization, sequential and sensory-motor integration, and evaluation processes, are required and somehow defective in dyslexia. Dyslexia can be defined psychophysiologically as a disorder of programming and integrating ideokinetic elements, associated with a deficit in the fast processing and integration of sensory information and a reduced efficiency of error system analysis. All these phenomena occur at different levels and at different times in the central nervous system during reading (Chiarenza, 2017). When these structures are unable to perform their functions due to a variety of factors, both covert and overt behaviors are somehow impaired.

6 Application of MRPs in other clinical situations Despite the enormous potential of this task for the electrophysiological study of executive functions, it has not been used on a large scale both to validate these studies in normal subjects and to demonstrate its usefulness in the clinical field as a measure of therapeutic and rehabilitative treatments. Fattapposta et al. (1996) observed that Olympic gun-shooting champions, during long-term practice with SPT, achieved a higher number of target performances than the control group. These high levels of performance were associated with a decrease in BP amplitude and an increase in SPP amplitude. Olympic champions with more practice recruited fewer resources to achieve optimal performance, and this was reflected in the magnitude of the BP. In the psychopharmacological field, MRPs have been used to evaluate, in normal subjects, the effects of acute and chronic administration of Piracetam, a drug used to improve learning and memory abilities (Chiarenza et al., 1987, 1990b). There was no statistically significant difference between placebo or Piracetam treatment with regard to “performance,” which was already optimal at baseline. On the contrary, the MRBMs were significantly modified by treatment. In particular, the Bereitschaftspotential was present as a positive shift during acute treatment with Piracetam and increased after chronic treatment. Skilled performance positivity (SPP) amplitudes were significantly increased, and SPP latency was reduced, by chronic treatment with Piracetam. In the clinical field, two studies have employed SPT to evaluate MRPs in subjects with Parkinson’s disease (PD) (Fattapposta et al., 2000; Papakostopoulos and Banerji, 1980). One of these studies also investigated the

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changes in MRPs after treatment with L-dopa (Fattapposta et al., 2002). Subjects with PD had a significantly lower number of target performances associated with an increased BP amplitude and decreased SPP amplitude. The authors interpreted these amplitude changes as a need for more attentional resources to complete the task and a reduced capacity for evaluating results. The treatment with L-dopa improved the number of target performances; the BP amplitude decreased significantly, and the SPP amplitude increased. These studies show that MRPs are indices sensitive to the severity of a clinical situation and accurate for evaluating the effects of drug treatment. Bockova´ and Rektor (2019) indirectly confirmed these observations in a review. The authors reviewed 85 articles on oscillatory dynamics, connectivity studies, and deep brain stimulation in individuals with PD. The authors discovered disrupted movement-related gamma oscillations in the basal ganglia, as well as cortico-subcortical and cortico-cortical motor loops, which were suppressible by dopaminergic medication and high-frequency deep brain stimulation. Event-related desynchronization (ERD) and synchronization (ERS) MRPs were employed in subjects with amyotrophic lateral sclerosis (ALS) to quantify cortical sensorimotor processes during volitional movements (Bizovicar et al., 2014). When compared to controls, patients had significantly lower resting alpha spectral power density (SPD) and beta spectral response density (ERD). Additionally, patients exhibited merely unilateral post-movement ERS (beta rebound), whereas this phenomenon was bilateral in controls. ERD/ERS amplitudes did not correlate with corresponding MRPs for either patients or controls. MRPs have been used to monitor the effect of motor rehabilitation in hemiparetic patients following a stroke (Fattapposta et al., 2008), before therapy and after 3, 9, and 12 months. The results highlighted a different trend for BP and SPP. At baseline, hemiparetic patients scored a lower number of target performances and had a lower BP amplitude than controls; SPP was absent. The number of correct performances did not improve significantly during the subsequent recordings. BP amplitude showed a mild increase in the second, third, and fourth recordings, while SPP amplitude revealed a slight improvement in the second and a marked improvement in the third and fourth recordings, with no statistically significant difference from controls. In these hemiparetic subjects, the ability to plan a movement seems to be much more limited than the ability to evaluate their own performance. Other interesting information on the utility of MRPs in the neurological and psychiatric clinical fields comes from studies in subjects with mental retardation, where the organic damage is known as Down syndrome (Chiarenza, 1993), and in subjects affected by schizophrenia (Chiarenza et al., 1985; Chiarenza and Cazzullo, 1989). The MRPs of subjects with Down syndrome and those of two control groups, one of the same mental age and another of the same chronological age, were compared. In this way, any differences in the MRPs found in subjects with Down syndrome were to be attributed to the characteristics of Down syndrome and not to differences related to chronological or mental age. The performance of the SPT presupposes that the subjects understand the necessary requirements to accomplish the task and are able to maintain their commitment to the task. The subjects with Down syndrome

6 Application of MRPs in other clinical situations

performed the SPT task according to the instructions, even if they needed a much longer time to practice. Successful performance depends mostly on the correct sequence of the two ballistic movements. Preparation of the two movements requires a fine-tuning of a central clock to control information coming from afferent and efferent systems, i.e., the information coming from the two finger presses (Hirsch and Sherrick, 1961; Rosenbaum and Patashnik, 1980). The interval between these two consecutive movements is too short (40–60 ms for target performance) for proprioceptive and exteroceptive feedback from the first movement to act as a trigger for the second one (Lashley, 1951). Therefore, the two thumbs’ movements have to be programmed as a single unit (Chiarenza et al., 1985; Kelso et al., 1979). Throughout the task, the performance of subjects with Down syndrome was significantly below that of subjects in the two control groups in both accuracy and speed and did not change with practice, as did that of the subjects in the two groups. Most of the performances of the subjects with Down syndrome were placed in the first time interval, 0–20 ms, or in the last interval, the one greater than 100 ms, and they scored a very low number of target performances (Chiarenza et al., 1985). It seems that subjects with Down syndrome have difficulty tuning this central clock to control information coming from the two movements. Moreover, this study confirms that Down syndrome subjects have particular difficulties when a sequence of movements has to be programmed to coincide with an external event (Henderson et al., 1981a). The specific problem of motor programming in subjects with Down syndrome seems to be solely in the temporal component and not in the spatial one (Henderson et al., 1981b). Another explanation for these low performances comes from the observation of MRPs. Subjects with Down syndrome had an absent or greatly reduced BP. Furthermore, the motor cortex potential (MCP), the proprioceptive afferent information processing index, was absent in the precentral cortex. Suitable proprioceptive information is of fundamental importance for the preparation and execution of movements. The MCP’s lack of elaboration of this sensory feedback may be responsible for the poor capacity of a temporal organization to carry out complex motor acts. Animal experiments and observations of patients with damage to the posterior spinal cord columns show that total or partial deafferentation prevents the temporal control of a motor sequence (Dubrovsky and Garcia-Hill, 1973). The absence or reduced amplitude of the MCP is also explained by the particular anatomopathological condition of the cerebral cortex of these subjects. In fact, it is known that the cerebral cortex in people with Down syndrome, in addition to having a reduced number of synapses, is characterized by an early Alzheimer’s disease phenomenon (Becker et al., 1986). The MCP is the first to decrease in amplitude in adults older than 60 years (Papakostopoulos and Banerji, 1980). Finally, SPP in subjects with Down syndrome was present in all cerebral areas recorded with reduced amplitude and normal latency. The fact that the latency was normal compared to the control groups suggests that these subjects had a good speed in recognizing and evaluating the results of their performances but were unable to experience the results obtained as a consequence of their programming and processing deficits of afferent kinaesthetic and proprioceptive information.

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The application of MRPs with SPT in the psychiatric field also offers us some interesting observations, in particular the comparison of MRPs in chronic schizophrenic subjects (Chiarenza et al., 1985) with those recorded in subjects after the first schizophrenic episode (Chiarenza and Cazzullo, 1989). Subjects with chronic schizophrenia have a high level of variability in their motor performance. This variability has been attributed to a failure to attend to a task properly, to maintain a set, or to change the settings quickly when necessary (Buss and Lang, 1965; Kristofferson, 1967; Lang and Buss, 1965; Shakow, 1963), or to some interference that interrupts the organization of a preparatory set during which the organization of correct programs occurs (Callaway, 1970). There is some neurophysiological evidence to support these assumptions, but, in most studies, there was no simultaneous assessment of performance and brain activities (Dongier, 1973; Shagass et al., 1978; Timsit-Berthier, 1973; Timsit-Berthier et al., 1973; Westphal et al., 1983). Our results bridge this gap. The BP, MCP, and SPP of chronic schizophrenics are reduced in amplitude and sometimes absent across all brain regions. This reduction can be attributed to various phenomena. To what extent the reduced BP amplitude indexes deficiencies in maintaining a set (Buss and Lang, 1965), appropriately changing a set (Lang and Buss, 1965), or interference by irrelevant activities, it is difficult to decide. However, lack of evaluation of results (reduced or absent SPP) and lack of sufficient proprioceptive information (absent or reduced MCP) do not help in developing an appropriate preparatory set. The presented neurophysiological data show that both of these conditions do occur in schizophrenic patients. The brain potentials associated with knowledge of results, SPP, and the potentials reflecting reafferent sensory feedback, MCP, either were both reduced or absent in schizophrenic patients. In this context, it could be argued that impairments in developing an appropriate change of set are the most likely explanation for impairments in using performance outcome information to achieve a greater number of “target performances.” In terms of MRPs, the only difference between acute and chronic schizophrenia is a lower BP amplitude over the frontal regions (Chiarenza and Cazzullo, 1989). It seems that the first difficulty that patients encounter at the onset of this pathology is their poor programming ability to conceive a sequence of movements aimed at reaching a goal.

7 Conclusions Purposeful behavior is one of the most sophisticated manifestations of the human mind. It includes numerous processes that can be described in various ways according to the domain in question: psychological, neuropsychological, and neurophysiological, just to mention the most common ones. On a psychological level, goal-directed behavior can be described in terms of processes of abstraction, programming, analysis of the events around us, evaluation of our behaviors, learning, and adaptation. On a neuropsychological level, it can be viewed in terms of programming, attention, perception, inhibition, motivation, and engagement, as well as

Appendix

cognition and memorization. At the neurophysiological level, all the processes described above find their functional equivalent in the four periods when the movement-related potentials occur during a goal-directed behavior, such as the skilled performance task. Furthermore, this task has a unique feature that distinguishes it from all the other tasks commonly used to record MRPs or Even-related potentials (ERPs): it offers the possibility of studying sensory, motor and cognitive processes all together and at the same time. The uniqueness and strength of the neurophysiological approach with goal-directed tasks consist of visualizing the cerebral manifestations during behavioral silence. The possibility of observing its most intimate mechanisms during these periods of behavioral silence with new imaging techniques is one of the most exciting research projects of this century. Finally, if by intelligence we mean the ability to imagine the future, correctly evaluate our actions and behaviors, and predict the consequences of our acts, then MRPs are a sensitive and accurate tool to measure it (Chiarenza, 2022). Finally, yet importantly, this neurophysiological research could have an impact on the education field. First, teaching should foster self-regulation and self-directedness. It has been observed that the brain is much more active and involved when it has to spontaneously undertake a task rather than respond to a simple input (Chiarenza et al., 2013). We need to encourage the integration and elaboration of different sensory experiences to enrich our internal world and social relationships. Equally important is teaching students how to consistently evaluate their work through immediate and constant feedback. In this way, students are helped to activate processes for correcting any errors and developing more suitable and effective behaviors.

Appendix The skilled performance task The subject sat in an armchair placed 70 cm from an oscilloscope in a lighted and electrically shielded room, holding a push-button in each hand. The excursion of the button was 5 mm. The task consisted of starting a sweep of the oscilloscope trace by pressing the left-hand button with the left thumb and stopping it by pressing the right-hand button with the right thumb, within a predetermined area of the oscilloscope screen, 40–60 ms from the onset of the sweep; the sweep velocity was 1 mm/ms. Performance within this time interval was defined as the “target performance.” After a verbal explanation and practical demonstration of the task, subjects were given enough time to master the task to achieve a sufficient number of target performances. None of the subjects had previous experience with this task or any other type of motor test. The practice also enabled the subjects to learn how to control eye movements and blinking while executing the task and to pace their trials at intervals of between 7 and 20 s. Subjects were asked to remain relaxed during the task and avoid muscular preparatory movements before pressing.

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EEG recording Ag/AgCl electrodes were fixed to the scalp with collodion over the prefrontal (Fpz), frontal (Fz), central (Cz), right precentral (RPC), left precentral (LPC), parietal (Pz), right parietal (P4), and left parietal (P3) regions, according to the International 10/20 system. Each electrode was referred to the linked mastoid. A bipolar EOG was also recorded. The EMG was recorded from the left and right bodies of the long flexor muscle of the thumb. Electrode impedance was less than 3 kΩ. The EEG was amplified with Physio-Amp Marazza preamplifiers: high-pass and low-pass filters (6 dB/octave) were respectively 0.019 Hz–70Hz for the EEG and 5.3 Hz–700 Hz for the EMG. The analysis started with sampling for each channel, a square wave of 25 μV for calibration, and equalization procedures. A trigger pulse generated by the left-hand button press was used to initiate the data samples for each channel. The sampling rate was 250 Hz for 2.2 s preceding the trigger pulse and 1 s immediately following it. An average of the first 1 s of the sample was used to establish a baseline from which all amplitudes were measured. Amplitude values were equalized across channels based on a stored calibration pulse.

Data analysis Performance was measured as the time interval between the two presses and defined as “performance time” (PT). Target performance (TP) was defined as the number of tries in which the sweep was terminated within the correct 40–60 ms target interval. Sweep terminations less than 40 ms or greater than 60 ms were designated as errors. Accuracy, defined as “performance shift” (PS), was calculated as the distance of the end of the sweep from the target area. 100 consecutive trials without muscular artifacts, blinking, or eye movements were averaged and measured for each subject. The amplitude and onset of BP were measured as follows: the moving average method was employed to determine BP onset. It was established that BP onset, if present, should appear within a time frame of 1200 ms before EMG onset. Two averages were calculated for each point in this range, the first being the mean of the potentials at all points preceding the fixed one, excluding the last nine, and the second being the mean of the potentials at the 20 points around the fixed point (nine preceding the fixed point itself and ten following). Consequently, the point at which the second mean constantly remained greater as an absolute value than the first was chosen as that corresponding to the onset of BP. The twenty-point value corresponding to an interval of 80 ms was chosen based on experimental data because the result was ideal for correctly identifying the onset of BP. To isolate the left EMG onset, the same moving average method was used. In each step, the slope of a simple linear regression was calculated. A significant change in the slope indicated EMG onset. The mean BP amplitude was computed for 200-ms periods immediately preceding the left EMG onset. SPP amplitude was taken as the average value over 200 ms centered on the main positive peak value (SPP) in the latency band between 350 and 650 ms from the left-hand trigger. This value was measured from the baseline. SPP latencies were measured from the left-hand trigger pulse.

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CHAPTER

Conscious entry into sleep: Yoga Nidra and accessing subtler states of consciousness

3

Prakash Chandra Kavi* Center for Brain and Cognition, Universitat Pompeu Fabra, Barcelona, Spain *Corresponding author: e-mail address: [email protected]

Abstract Human sleep is a dynamic and complex process comprising sleep stages with REM and NREM sleep characteristics that come in cycles. During sleep, there is a loss of responsiveness or a perceptual loss of conscious awareness with increasing thresholds for wakefulness as sleep progresses. There are brief bursts of wakefulness or Wake After Sleep Onset (WASO) throughout a nocturnal sleep. Conscious experience during nocturnal sleep is known to occur during lucid dreaming when one is aware during dreams when the dream is occurring. Most cultures have known lucid dreaming since antiquity. However, conscious experience during dreamless sleep is relatively lesser known. Nevertheless, selected Indo-Tibetan meditation literature has documented it since antiquity. Minimal Phenomenal Experience (MPE) research describes lucid dreamless sleep as its target phenomenology. “Conscious entry into sleep” posits tonic alertness is maintained post sleep onset through the sleep stages for sustained durations of time until an eventual loss of conscious awareness. Entering sleep consciously and being aware during dreamless sleep, including Slow Wave Activity, is plausibly to be in the state of “Yoga Nidra” or Yogic sleep. An attentive sleepful state provides access to subtler states of consciousness and significantly deepens the levels of silence. It is phenomenologically distinct from hypnagogic hallucinations and lucid dreaming. Unfortunately, sleep studies validating this phenomenology are yet to be done. Therefore, an experimental methodology akin to those used in lucid dreaming experiments is described.

Keywords Lucid dreaming, Neurophenomenology, Lucid dreamless sleep, Yoga Nidra, Sleep onset, MPE, Meditation, Advaita, Clear light, Hypnagogic state

Progress in Brain Research, Volume 280, ISSN 0079-6123, https://doi.org/10.1016/bs.pbr.2022.12.012 Copyright © 2023 Elsevier B.V. All rights reserved.

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CHAPTER 3 Conscious entry into sleep

1 Introduction A generally accepted behavioral description of sleep is: (1) the sleeper goes into a particular body position(s) with muscle relaxation, quiescence, and no movements though there can be multiple changes in body positions (De Koninck et al., 1983); (2) loss of awareness as indicated by progressively increased arousal thresholds in different sleep stages (Comsa et al., 2019; Laureys et al., 2007); (3) it is a reversible process daily, unlike coma and other related disorders of consciousness, which is not a quickly reversible process (Brown et al., 2010); and (4) regulation by a homeostatic and circadian process, like the need for more sleep when sleep is deprived (Borb and Achermann, 1999). The modern classification of sleep using the American Association of Sleep Medicine (American Academy of Sleep Medicine, 2014) describes five sleep stages: rapid eye movement (REM) sleep, three levels of non-REM sleep (N1, N2, N3), and wake (W). Sleep staging is typically done by visual scoring of neural activity of the brain using electroencephalography (EEG), eye movement using electrooculography (EOG), and chin muscles movement using electromyography (EMG). And with sensors measuring cardiac and respiratory activity, it is collectively referred to as polysomnography (PSG) (Rundo and Downey, 2019). PSG is the gold standard for diagnosing sleep disorders, especially sleep apnea, and conducting sleep studies. Major sleep disorders are listed in ICSD-3 (Sateia, 2014). A typical nocturnal sleep of a healthy adult is 7–9 h and can have 4–6 REM cycles. Sleep is entered through NREM sleep, and the first REM cycle typically lasts for 70–100 min, whereas later REM cycles last for 90–120 min. Generally, multiple bursts of Wake After Sleep Onset (WASO) are observed during nightly sleep. In the first 2 REM cycles, N3 predominates. REM sleep is predominant in the later REM cycles. Furthermore, in the first NREM episodes of the night, there is greater Slow Wave Sleep (SWS) in the frontal than in parietal and occipital regions (Werth et al., 1996). In the later NREM stages, SWS moves progressively to posterior and subcortical regions (Gennaro et al., 2004). These above-described patterns are typical of a healthy adult’s sleep architecture (Carskadon and Dement, 2011; Tan et al., 2019; Yetton et al., 2018). In the first half of sleep, predominantly NREM, a set of brain networks are progressively taken off-line or deactivated. Then, put back together or reactivated during each subsequent episode of REM until the brain finally returns to waking (Dang-Vu et al., 2005; Maquet et al., 2000; Muzur et al., 2002). During the process of falling asleep in nocturnal sleep, a person is usually unaware of this transition from wake to sleep, starting with early drowsiness and then in further deep sleep stages (Ogilvie, 2001). This transition from wake to sleep is called the hypnagogic state and can contain mental imagery and thought forms. However, a person is retrospectively aware of these sleep and wake states (Hori et al., 1994). Lucid dreaming (Baird et al., 2019; Van Eeden, 1913) is a phenomenon in which a person is aware during the dream when the dream is occurring in real-time. It has been known since antiquity in both Western (Aristotle, 1941) and eastern cultures

2 MPE and states of consciousness

(Padmasambhava, 1998). In addition, especially in Indo-Tibetan cultures, some meditation traditions have aimed to develop an awareness of the dream and sleep states (LaBerge, 2003; Norbu and Katz, 1992; Wallace and Hodel, 2012). Lucid dreams occur more frequently during REM sleep stages in the later REM cycles (LaBerge, 1990). REM dream reports are typically longer, vivid, with more imagery compared to NREM dream reports which have more thought-like mentation (Hobson et al., 2000). The view “Lucid dream ¼ REM sleep” is still persistent as many lucid dream induction methods target REM sleep stages (Stumbrys et al., 2012). In addition, Sleep Onset REM is observed in cases of narcolepsy (Dodet et al., 2014). In some cases, lucid dreams have been recorded in a lab setting in NREM sleep stages, mostly N1 and rare instances of N2. Hence, it may be helpful to use the terms “REM lucidity” and “NREM lucidity” (Stumbrys and Erlacher, 2012). However, lucidity in N3 is yet to be observed. Recently, “interactive dreaming” experiments (Konkoly et al., 2021) were conducted during Lucid Dreaming episodes to establish 2-way communication between the experimenter and the sleeper. These used an SVLD (Signal Verified Lucid Dreaming) approach where the participant moved the eyes horizontally using LRLR or Left Right Left Right scans to indicate lucidity and signal responses. LRLR eye movement is the gold standard for signaling lucidity (Hearne, 1978; La Berge et al., 1981). Lucid dreaming researchers have mentioned the need to update the standard view that sleeping individuals are oblivious to the world around them. They have demonstrated that meaningful comprehension and dialogue are possible (Konkoly et al., 2021).

2 MPE and states of consciousness 2.1 Heuristic mapping of lucid dreaming and lucid dreamless sleep to a typical sleep architecture A sleep hypnogram describes a healthy adult’s typical sleep architecture, as shown in Fig. 1. Heuristically thinking, the Red lines show potential lucid dreaming stages, whereas the Purple lines show lucid dreamless sleep. The diagram shows that lucid dreamless sleep is more plausible in the first half of the night. However, the sleep architecture for someone being lucid in dreamless sleep could be different from a typical sleep architecture. Furthermore, the sleep stages do not occur as steep lines as shown in a hypnogram but are a continuous process. Therefore, an EEG spectrogram may provide a better representation than a hypnogram. Again, the sleep stage transitions detected in PSG need not be the same across the whole brain. In an fMRI-based spatio-temporal neuroimaging analysis in studying the transitions across sleep stages, several brain regions corresponded with sleep stage transitions differentlyeven if the overall PSG sleep stage transitions from the whole brain perspective remained consistent (Stevner et al., 2019).

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Dashed lines represent WASO. There can be more, only indicative

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FIG. 1 Heuristic lucid dreaming and lucid dreamless sleep stages.

2.2 MPE Minimal Phenomenal Experience (MPE) (Metzinger, 2014; Windt, 2015) models aim to describe the minimal model of consciousness as “only a minimal model can give us a deep scientific understanding of the essence of phenomenal experience.” And hence it attempts to study the phenomenal nature of “pure consciousness” during meditation as its entry point. At an abstract level, this form of consciousness is an unpartitioned epistemic space characterized by the presence of tonic alertness (Metzinger, 2020). The target phenomenology of MPE is lucid dreamless sleep (Metzinger, 2019). It is the phenomenon where a person is aware during dreamless deep sleep. A celebrated teacher of meditation, J. Krishnamurthi has spoken on various occasions about being conscious during sleep and has described it as an attentive sleepful state (Krishnamurthi, 1974). Furthermore, many other meditation teachers and gurus across the ages have discussed the phenomenology of lucid dreamless sleep to a great extent. However, there is currently no available scientific evidence in the form of a sleep data recording that shows someone being aware during dreamless deep sleep.

2.2.1 Consciousness in dreamless sleep A classical debate between two schools of Indian philosophy, the Advaita Vedanta vs. the Nyaya school, is whether consciousness is present or absent in dreamless sleep. Advaita schools describe consciousness as a continuum being present in dreamless sleep. It considers “jagrat sushupti” (or “awake in deep sleep” in Sanskrit) as an advanced stage of an accomplished Yogi. Also, Buddhist concepts of “bhavanga” and “alaya-vigyan” uphold the view that consciousness is present in dreamless sleep. However, the Nyaya school says consciousness is absent in dreamless sleep (Thompson, 2015).

2 MPE and states of consciousness

Modern neuroscience seems to uphold the view that consciousness is something that goes away or disappears in dreamless sleep and comes back after one wakes up or goes back to a dream (Searle, 2000; Tononi, 2008). It is phenomenologically closer to the Nyaya school’s viewpoint. Modern neuroscience’s approach to sleep and consciousness research has epistemological differences from classical Advaita Vedanta, Yogic and Buddhist philosophy. Therefore, a new sleep taxonomy from a phenomenological perspective could help describe lucid dreamless sleep (Thompson, 2014; Windt et al., 2016). Some modern hypotheses view consciousness as a sleep-wake continuum (Glicksohn, 1989; Paoletti et al., 2022). Furthermore, an integrated approach to understanding the hypnagogic and hypnopompic (the transition state between sleeping and waking up) states from an intra-person perspective could provide new directions to understanding the sleep-wake continuum (Glicksohn, 2019).

2.3 Four main states of consciousness, according to the Upanishads Classical ancient Indian meditation literature, including Mandukya Upanishad (Baars, 2013; Nikhilananda, 1990; Srinivasan, 2020), describes four states of consciousness. A simplified description of the four states of consciousness is in Table 1. A healthy sleeper has a phenomenological understanding of the first three states of consciousness or waking, dreaming, and deep sleep to varying degrees in daily life. Using standard AASM guidelines, waking (“jagrat”) is equivalent to the waking state. Dreaming (“swapna”) is plausibly congruent to REM with vivid imagery but can also include parts of NREM sleep stages that have thoughts and mentation, especially 1 and 2. Though NREM 3 sleep stage may also have minimal mentation, it is known to have ongoing memory consolidation (McNamara et al., 2010; Wei et al., 2018). Finally, deep sleep mapped to “sushupti” phenomenologically lacks mentation but is also refreshing and peaceful and may correspond primarily to NREM 3 (Thompson, 2015). The Sanskrit word “turiya” literally means “the Fourth,” referring to the fourth state of consciousness. It is a state of “para” or the transcendental state. And also described as “Advaita,” or non-duality that is beyond the subject-object duality. This transcendental state is attained due to the “pratyaksh” or direct knowledge of the Table 1 Four main states of consciousness, according to Mandukya Upanishad. State of consciousness

Direction of attention

Content of consciousness

Waking (“Jagrat”) Dreaming (“Swapna”) Deep Sleep (“Sushupti”) Fourth (“Turiya”)

Outward Inward Inward Inward/outward

Present Present Absent Absent

Awareness levels High Generally low Extremely low Very high/“here and now”

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Witness or the Primordial Consciousness, also variously referred to as “drshta”/”sakshin”/”atman.” Subject-object duality remains due to “avidya” (or absence of the direct knowledge of the Witness) in the first three states of consciousness.

2.3.1 Between the normal state of consciousness and “turiya,” there are three intermediate stages “There are four main states of consciousness: waking, dreaming, deep sleep, and turiya. There are also three intermediate states: unmani, ahladini, and samadhi, totaling seven states. Between waking and dreaming is the intermediate state called unmani. Between dreaming and sleeping is the intermediate state called ahladini. Between sleep and turiya is the intermediate state called samadhi.” (Rama, 2007). “Turiya” is not just the fourth one but what underlies the three states (Lakshmanjoo, 2015; Rama, 2007; Ramana and Osborne, 1970). Notably, three phenomenological stages can plausibly lead a person from a state of normal healthy sleep to a transcendental sleep. Thereby attaining the transcendental state of “turiya.” The second and third intermediate stages are incredibly advanced, and the first intermediate stage can be considered a gateway stage for going into the next two. This chapter focuses on this gateway stage.

2.3.2 Transcendental state descriptions in multiple cultures and traditions There are numerous interpretations of “samadhi” and non-dual awareness in various meditation literature across traditions. It can be considered an extremely advanced meditation state where consciousness is without content or very little content (Bharati, 1986; Josipovic and Miskovic, 2020; Ramana and Osborne, 1970; Srinivasan, 2020). However, only the “samadhi” that leads to the direct knowledge of the Witness is relevant in the context of attaining “turiya.” Therefore, the subject-object duality persists even if one is conscious during sleep until reaching the final “samadhi.” Subject-object duality is transcended only after “samadhi,” which leads to the “para” or the transcendental state of “turiya.” It makes one a “Buddha” or “The Awakened One.” Notably, the word “Buddha” is a synonym for any person who is “Awakened” and does not only refer to the historical Shakyamuni Buddha. Another expression referring to the same state of consciousness is “Jivanmukta” (or “The Liberated One”) while being alive (Bhattacharyya, 1951). In Tantrik tradition, it is called attaining “Mahamudra siddhi,” and such a being is called a “Mahasiddha” (Namgyal, 2006).

3 Conscious entry into sleep and “Yoga Nidra” 3.1 “Yoga Nidra” “Yoga Nidra” or Yogic Sleep is described variously by different meditation teachers. A necessary clarification is that the “Yoga Nidra” meditation is not the actual “Yoga Nidra” but is an ancient meditation technique of perhaps Tantrik origin from India.

3 Conscious entry into sleep and “Yoga Nidra”

This meditation is done in the waking state, like any other meditation but is unique as it is a practice aimed at consciously entering sleep during the night. It may involve guided meditation done in a group or even can be done individually without any guidance. It is usually done in a supine position keeping the body completely still and may involve visualization or centering of attention while scanning the bodily parts. It usually culminates with the meditator focusing attention on the Heart Center and plausibly going into a state of conscious sleep. An accomplished “Yoga Nidra” meditator will be able to consciously enter the NREM sleep state every night, including the stage of dreamless sleep. Being aware during dreamless deep sleep is to be in the state of “Yoga Nidra” (Bharati, 2014; Parker, 2019; Rama, 2007; Saraswati, 1998). Although this is an extremely rare phenomenon, Swami Rama demonstrated the phenomenology of Yoga Nidra, or lucid dreamless sleep in a controlled environment (Bharati, 2014; Green and Green, 1977; Parker, 2019). “Yoga Nidra” is phenomenologically congruent to being lucid in the first 2 REM cycles. Hence, a qualified “subject” is also a “lucid sleeper” who can enter sleep consciously and be tonically alert throughout the sleep stages, including dreamless deep sleep at the N3 sleep stage.

3.1.1 Plausible stages of “Yoga Nidra” There are two plausible stages, as highlighted in Table 2. These stages are only indicative as it is tricky to do the phenomenological mapping to a standard sleep architecture based on AASM guidelines that are from a physiological standpoint. There can be bursts of WASO and changes to different sleep stages, and the described order of sleep stages is only to aid comprehension. It is posited that there will plausibly be higher beta and gamma activity than healthy controls and lucid dreamers. Secondly, it will be a very restful and relaxed period of sleep. The “Yoga Nidra” basic stage essentially witnesses the hypnagogic state, N1 sleep, and N2 sleep. However, it is plausible that tonic alertness gets lost on entering N2. Hence, it is helpful to subdivide this stage into the Preparatory Stage and the “Yoga Nidra” basic stage. In the Preparatory stage, tonic alertness continues throughout the hypnagogic state, and sleep is entered consciously via N1 but subsequently gets lost just before entering the N2 sleep stage. On the other hand, in the “Yoga Nidra” basic stage, tonic alertness is maintained in the early stages of N2 but may eventually get lost as shown in Fig. 2A. Or “Yoga Nidra” Basic ¼ “Lucid NREM 2”. Tonic alertness may again appear during lucid dreaming or later REM cycles for extended periods that continue through the hypnopompic stage leading to a Table 2 Plausible stages of “Yoga Nidra.”

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A Lights Off

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Time (in hrs) Yellow Line shows plausible sleep stages for "Yoga Nidra" Advanced Light Green and Dark Green lines shows plausible hypnagogic and hypnopompic stage Dashed Red Line shows plausible lucid REM and lucid NREM All timelines of the sleep stages, hypnagogic stage and WASO are indicative Conscious Transition to Wake Loss of Tonic Alertness Tonic Alertness begins

FIG. 2 (A) Plausible sleep stages for Yoga Nidra Basic. (B) Plausible sleep stages for “Yoga Nidra” advanced.

conscious transition to the waking state. When the “Yoga Nidra” basic stage is stable, the “lucid sleeper” should be able to sleep consciously every night, unless there are some unusual circumstances. And plausibly, there is a greater frequency of dreams that are less vivid and lack perceivable content, often referred to as “white dreams” (Lewin, 1946; Siclari et al., 2013). It may lead to experiencing bursts of luminosity during deep sleep. Interestingly, the “lucid sleeper” may sleep for longer durations, have excessive sleepiness, and appear exhausted, even though there is lucidity during sleep. This sleep behavior is described as “Sushuptipaad” in Akshi Upanishad (Warrier, 1967).

4 Experimental methodology

Over time, witnessing different sleep stages will lead to being tonically alert, even in N3. And when there is stability in having “Yoga Nidra” on a daily basis, it is a highly advanced state, and there will be perceivable luminosity during deep sleep as shown in Fig. 2B. Or “Yoga Nidra” Advanced ¼ “Lucid NREM 3.”

4 Experimental methodology It is posited that to “consciously enter sleep,” the “lucid sleeper” will need significantly greater interoceptive abilities than a healthy sleeper and even a lucid dreamer. These include heightened awareness of thoughts, the thinking process, breath and breathing, bodily parts, and movements. Therefore, the following output mechanisms may plausibly verify the signals as events in a PSG-like system: (1) LRLR eye movements; (2) Volitional breathing. The loss of tonic alertness in a particular sleep stage can determine the loss of lucidity. Table 2 provides a phenomenological perspective from the point of view of “Yoga Nidra” that captures the eventual loss of tonic alertness. A critical issue while conducting these experiments is the length of the sleep segment of a particular sleep stage and occurrences of WASO episodes that can add a lot of complexity. Furthermore, these sleep stages are much more complex and may not occur in the simplistic manner shown in Fig. 1.

4.1 Plausible experiments The studies may be conducted in 2 phases to validate “conscious entry to sleep.” First, it shall be limited to the “Yoga Nidra” Basic stage or conscious entry into sleep till N2. Then, after getting a pool of subjects or “lucid sleeper(s)” who are successful in the Basic Stage, it could be extended to test “Yoga Nidra.” Each “lucid sleeper” could do these experiments for 15 consecutive nights to provide reliable and sufficient data. Then, again, these experiments can be repeated after 6 months to a year to gather longitudinal data.

4.1.1 Real-time 2-way communication between an experimenter and the “lucid sleeper” Taking cues from the “interactive dreaming” (Konkoly et al., 2021) experiments, a simple arithmetic task like 7–4 can be presented to the “lucid sleeper” as an auditory stimulus when the PSG indicates NREM sleep stage N2. A correct response signal is 3 demonstrated using 3 left-right horizontal eye movements (LRLRLR) or three volitional breaths. Lucidity Status is updated to “Yoga Nidra Basic,” given a correct response. Similarly, during N3, another simple arithmetic task, say 5–3, can be presented as an auditory stimulus. Again, suppose the “lucid sleeper” provides a correct response signal 2, demonstrated by 2 left-right horizontal eye movements (LRLR) or two volitional breaths. In that case, the Lucidity Status is “Yoga Nidra Advanced.”

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Similarly, different audio cues can be presented in different sleep stages, corresponding to Table 2. Hence, this will lead to a pathway to lucidity for each phenomenological stage. And they are objectively validated, further demonstrating tonic alertness. For example, suppose the “lucid sleeper” provided at least one validated response to an external stimulus but not so in the next stage. In that case, the “lucid sleeper” can be woken up and asked the response akin to a Sleep Report (Foulkes, 1962). An expected response demonstrates that auditory cognition was still functioning, and the sleeper was tonically alert, but muscle atonia impeded the sleeper from answering.

4.1.2 Self-identification of sleep stages by the “lucid sleeper” The “lucid sleeper” can indicate lucidity by choosing either LRLR eye movements, volitional breathing, or a combination. For example, on entering N1 sleep, the “lucid sleeper” can provide an output signal in the form of an LRLR eye movement— another output signal or LRLR eye movement after entering N2 and another in REM. In addition, two or more output signals at random time intervals can indicate tonic alertness if the sleep stage duration is longer than 10 min. It is also plausible that the “lucid sleeper” can provide an output signal on entering a phenomenologically distinct sleep stage within 2–3 s. It is an advanced state showing higher interoceptive awareness. Furthermore, the “lucid sleeper” can respond to each change in sleep stage corresponding to the phenomenological stages described in Table 2, including identifying the moment of entry into sleep. Thus, a “lucid sleeper” can objectively indicate being epistemically aware of the states of waking, dreaming (REM) and light sleep (NREM 1–2), and deep sleep (later stages of NREM3).

4.1.3 Conducting experiments during the completion stage of a “Yoga Nidra” meditation The critical stage in a “Yoga Nidra” meditation is towards its very end, where one prepares to “consciously enter sleep.” (Rama, 2007; Saraswati, 1998) Therefore, conducting the experiments described in Sections 4.1.1 and 4.1.2 while completing a “Yoga Nidra” meditation is plausible. Another alternative is to provide an audio stimulus with distinct inputs corresponding to each sleep stage and get responses from the meditator after waking up as part of a Sleep Report. Valid responses demonstrate that auditory cognition was active, indicating tonic alertness during sleep.

5 Witnessing deep sleep and dreams In due course, the “lucid sleeper” will become lucid for longer durations and in later REM cycles. As awareness of dreams grows, there is a witnessing of sleep stages, including dreamless sleep and dreams. The vivid imagery in dreams may start getting abated. Witnessing dreams may ultimately lead to a transcendence of dreams

5 Witnessing deep sleep and dreams

altogether. At this stage, there is plausibly a stable perception of the light of the Witness or the primordial consciousness. However, it is a highly advanced stage described as the stage of “Bhavashoonya” in Akshi Upanishad (Warrier, 1967).

5.1 “Clear light of sleep” in bon-Buddhist traditions In Bon-Buddhist traditions, there is an excellent emphasis on Dream Yoga, which teaches meditation methods and techniques to induce and enhance lucid dreaming. Modern lucid dreaming induction methods (Aspy, 2020; Stumbrys et al., 2012) have close similarities to these techniques, without considering the ritualistic and metaphysical ideas of Dream Yoga teachings. The teachings of Dream Yoga usually culminate with Sleep Yoga practices that lead to the Clear Light of Sleep (Lop€ on and Dixey, 2002; Wangyal, 1998). The approach seems different from Yogic/Advaitic methods, which start with consciously entering sleep first and emphasize lucid dreamless sleep rather than lucid dreaming. Nevertheless, the goal in both Yogic and Bon-Buddhist traditions is to eventually reach the stage to have the ability to be lucid in both dreams and sleep, to get the direct perception of the Clear Light of Sleep during nocturnal sleep. Thereby, the Yogi or “lucid sleeper” acquires the required stability in entering subtler bodies of consciousness. In the Completion Stage of the Highest Yoga Tantra, Dalai Lama 14th describes, “at a certain level of this practice, the clear light will manifest. If you’ve arrived at that point in your experience and practice, then it’s very easy for you to recognize the clear light of sleep when that naturally occurs. And if you have arrived at the point where you can recognize dreamless sleep as dreamless sleep, then it’s very easy for you to recognize the dream as the dream.” (His Holiness the Dalai Lama and Varela, 2002; Lodoe, 1995).

5.2 Bodies of consciousness Meditation traditions from Indo-Tibetan literature usually describe progress in meditation with access to sutler bodies of consciousness starting from the gross level or the physical body. Table 3 shows a simplistic description of “bodies of consciousness” related to the Yogic/Advaitic (Wayman and Aiyar, 1982) and Bon-Buddhist (Wayman, 1977) traditions. Of course, there are differences in approaches between the two traditions, Table 3 A simplistic description of bodies of consciousness. Yogic/Advaitic traditions

Bon-Buddhist traditions

“Sthoola Sharir” (Gross Body) “Sookshma Sharir” (Subtle Body) “Karana Sharir” (Causal Body)

Gross Body—Gross Mind Subtle Body—Subtle Mind Very Subtle Body—Very Subtle Mind

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and each tradition has multiple sub-traditions. However, there are certain essential similarities, and this table points out the similarities rather than the differences. Recently, there has been an attempt to describe these subtle bodies using an interoceptive map of the central nervous system (Loizzo, 2016). An intuitive description of bodies of consciousness is an epistemic capacity to access phenomenology that is not limited to the waking state. And these bodies of consciousness can be considered “energy bodies” that manifest when a meditator can access the dreaming and deep sleep state corresponding to the “Subtle Body” and the “Very Subtle Body,” respectively. These bodies are generally present in a latent form or an unmanifested state. The physical body is the “Gross Body,” and the part of the brain that does sensory processing is the “Gross Mind.” The experience of “Subtle Body” suitably manifests only when one becomes aware of the dreaming state and can consciously enter sleep. “Kundalini,” often described in various meditation literature, is an experience of the energy phenomenon of the “Subtle Body.” The “chakras” of “kundalini” can be described as “energy centers” at the intersection of the “Subtle Body” with the “Gross Body,” where the attention naturally rests. It is incredibly challenging to find these subtle bodies and “chakras” as part of human anatomy. Instead, these “chakras” can be directly experienced by an advanced meditator with high interoceptive awareness during advanced stages of meditation. In Bon-Buddhist traditions, the “dream body,” which is a manifestation of the “Subtle Body,” is attained during the completion stage of advanced Tantric meditations like Guhya Samaja tantra. It is an Emanation body or is separate from the physical body and is an excellent example of OBE (Out of Body Experience). Certain Buddhist traditions describe it as the “impure illusory body” (Chang, 1986; Wayman, 1977). Some studies have demonstrated a close affinity between lucid dreaming and OBE experience (Hunt, 1989). Therefore, conscious entry into sleep and being lucid in sleep and dreams is essentially getting familiarity with Subtle Body-Subtle Mind.

5.3 Present-day meditation schools that teach awareness during dreams and sleep Many meditation traditions have died out due to a lack of continuity and changing social norms. However, even to this day, selected international meditation organizations teach meditations to be done during sleep. For example, multiple schools teach Yoga Nidra meditations, including the Himalayan Yogic tradition of Swami Rama from Rishikesh, India. Another school, Ligmincha Foundation, is of Bon-Buddhist Dzogchen tradition of Tibetan origin and teaches Dream and Sleep Yoga. However, it is not mandatory to undergo these meditations to have a conscious experience during sleep. There have been multiple meditation teachers who have spoken extensively on this topic. Among them was J. Krishnamurthi, who was against following any methodical practice as part of a meditation.

6 Discussions

6 Discussions This chapter supports the point of view held by Advaita and multiple other schools of philosophy that posit that consciousness is present in dreamless sleep. As opposed to the Nyaya school and some modern theories of consciousness, posit consciousness is absent in dreamless sleep, as discussed in Section 2.2.1.

6.1 Future possibilities in conducting advanced meditation research Current neuroscience literature on most meditation phenomenology focuses primarily on the waking state, where the activity levels in the brain are considerably different and higher than during the sleep stages. Therefore, advanced states of consciousness described in meditation literature like “samadhi,” “turiya,” and “consciousness as such” cannot be suitably interpreted only from the waking state perspective. Additionally, there is a lack of availability of “subjects” who can and are willing to demonstrate advanced phenomenologies such as conscious entry into sleep and lucid dreamless sleep in a laboratory setting. Thus, using traditional meditation research methods for finding “subjects” who are “lucid sleepers” could be extremely challenging due to the rarity of such phenomenology. And this is perhaps the single most significant reason why there is no available sleep data on lucid dreamless sleep yet. In the last decade, there have been a lot of advances in wearables technology, including sleep wearables for sleep monitoring. The quality of sleep studies conducted using such research-grade wearables is comparable to standard PSG tests as a tool for doing preliminary investigations. Hence, using a two-phased approach to conduct empirical studies is suggested. Firstly, starting with wearable technologies can help conduct pilot studies on potential “subjects” even in their own homes or a familiar setting. After that, advanced tests in a controlled laboratory environment could be conducted systematically with an advanced PSG-like setup. This approach can substantially reduce the cost, time, and effort of conducting this research. Perhaps this trend has already started in a few research labs.

6.2 Conscious entry into NREM 2 sleep during a “Yoga Nidra” meditation as a starting point for investigating the phenomena of “conscious entry into sleep” Scientific investigation of “conscious entry to sleep” could start with a microphenomenological interview of “Yoga Nidra” meditators as potential “subjects” since this meditation is quite popular in many countries throughout the world and across cultures. Then, the selected “subjects” could receive sleep monitoring wearables to be worn during a “Yoga Nidra” meditation session. Studies described in Section 4.1.3 will typically last 1–2 h, mainly during the daytime, compared to nightly sleep. And can be done in the comfort of their homes or a familiar setting.

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Furthermore, for each “subject,” at least 15 sessions should be recorded. A successful study should demonstrate the phenomenology equivalent to conscious entry into N2 sleep or the “Yoga Nidra” Basic stage. After that, sleep studies during nocturnal sleep could further validate the phenomenology of “conscious entry into sleep” and “Yoga Nidra” basic stages. These studies could also be conducted in a controlled environment like a sleep laboratory setting.

Acknowledgments I want to thank Joseph Glicksohn for the review comments, Thomas Metzinger for the MPE Workshop, and Stephen Parker for answering questions on Yoga Nidra meditation. Also, Antonino Raffone and Gorka Zamora Lopez for discussions that helped me crystallize my thoughts.

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CHAPTER

Cessations of consciousness in meditation: Advancing a scientific understanding of nirodha sam
apatti

4

Ruben E. Laukkonena,*, Matthew D. Sacchetb, Henk Barendregtc, Kathryn J. Devaneyd, Avijit Chowdhuryb, and Heleen A. Slagtere a

Faculty of Health, Southern Cross University, Gold Coast, QLD, Australia Meditation Research Program, Department of Psychiatry, Massachusetts General Hospital, Harvard Medical School, Boston, MA, United States c Faculty of Science, Radboud University, Nijmegen, The Netherlands d UC Berkeley Center for the Science of Psychedelics, Berkeley, CA, United States e Department of Experimental and Applied Psychology, Vrije Universiteit Amsterdam, the Netherlands & Institute for Brain and Behavior, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands *Corresponding author: e-mail address: [email protected]

b

Abstract Absence of consciousness can occur due to a concussion, anesthetization, intoxication, epileptic seizure, or other fainting/syncope episode caused by lack of blood flow to the brain. However, some meditation practitioners also report that it is possible to undergo a total absence of consciousness during meditation, lasting up to 7 days, and that these “cessations” can be consistently induced. One form of extended cessation (i.e., nirodha sam
apatti) is thought to be different from sleep because practitioners are said to be completely impervious to external stimulation. That is, they cannot be ’woken up’ from the cessation state as one might be from a dream. Cessations are also associated with the absence of any time experience or tiredness, and are said to involve a stiff rather than a relaxed body. Emergence from meditation-induced cessations is said to have profound effects on subsequent cognition and experience (e.g., resulting in a sudden sense of clarity, openness, and possibly insights). In this paper, we briefly outline the historical context for cessation events, present preliminary data from two labs, set a research agenda for their study, and provide an initial framework for understanding what meditation induced cessation may reveal about the mind and brain. We conclude by integrating these so-called nirodha and nirodha sam
apatti experiences—as they are known in classical Buddhism—into current cognitive-neurocomputational and active inference frameworks of meditation. Progress in Brain Research, Volume 280, ISSN 0079-6123, https://doi.org/10.1016/bs.pbr.2022.12.007 Copyright © 2023 Elsevier B.V. All rights reserved.

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Keywords Consciousness, Meditation, Cessation, Awareness, Active inference, Predictive processing, Nirodha sam
apatti, Jh
ana, Fruition

1 Introduction Many unique states of mind have been described by meditators and contemplatives. These can range from ecstatic and mystical absorptions to out-of-body experiences, and even states of so-called pure consciousness (Metzinger, 2020). However, as yet, no scientific papers that we are aware of have explored a meditation-induced event known in P
ali (the liturgical language of Theravada Buddhism) as nirodha sam
apatti (NS), which literally means “cessation attainment,” but often is rendered as “cessation of feeling and perception” (Nanamoli and Bodhi, 1995). Compared to other non-ordinary experiences that scientists might be tempted to dismiss due to their inherently subjective and variable nature, the NS experience is concrete: an internally induced absence of consciousness. The event is outwardly comparable to general anesthesia and differentiated from deep sleep in that after a NS event there is no sensation of time having passed, there are no dreams, and one cannot be ’woken up’ by physical stimulation or pain (Nanamoli and Bodhi, 1995).a Clearly, in terms of understanding the mind and brain, the capacity to voluntarily turn off consciousness, analogously to general anesthesia, is immensely interesting, also given how rare the capacity is and its implications for our understanding of top-down processing in the brain.b There are also notable after-effects of NS (and other cessation) experiences involving a profound sense of clarity, which some meditators describe as a kind of inner “reset,” which further differentiates this experience from (coming out of ) sleep or general anesthesia. The aim of the present paper is to describe the cessation event to a scientific audience, to contextualize it within contemporary cognitive neuroscience, and to generate a research agenda. All authors on the present paper—including PIs of three independent research programs and a Theravada Buddhist meditation teacher—have been directly involved in collecting neuroimaging and physiological data on cessation events. Hence, this paper developed out of a desire to generate a clearer characterization of what is so far known about the state(s) of cessation, how it might be integrated in the mind sciences, and what might be learned by studying it further. Below we begin with a relatively colloquial description of nirodha sam
apatti and a more general phenomenon nirodha—and how these are said to occur in the course meditation according to canonical Buddhist texts, known collectively as a

With some exceptions, for example when a meditator “intends” to be woken up by a certain stimulation. b Consider also the question “what the evolutionary imperative of such a capacity might be?.” As we discuss in later sections, one speculative possibility is that the capacity for NS is evolutionary spandrel related to a latent capacity for hibernation. Curiously, and similar to other mammals, meditators have inclined for thousands of years to practice intensively in caves.

2 Nirodha and nirodha sam
apatti

the P
ali Canon.c In later sections we focus our attention on what nirodha might reveal about the plasticity of mind and brain, describe some of the initial studies we have conducted, explore how these phenomena might be disruptive to certain metaphysical viewpoints about the nature of self, and finally what they mean for the practice and theory of meditation. Note that we are not approaching the topic as historians, but as scientists: We draw on a combination of relevant textual evidence, on phenomenological reports from contemporary practitioners, conversations with meditation teachers, and on our own understanding from studying cessation (both nirodha and NS) under laboratory conditions based on preliminary data that we have collected. Moreover, we present an attempt to place NS within the influential framework of active inference or predictive processing (Friston, 2009), that has rapidly gained influence in the cognitive and neurosciences in recent years as an all-encompassing theory of brain and mental functioning.

2 Nirodha and nirodha sam
apatti There is a distinction to be made between nirodha (cessation) and nirodha sam
apatti (translates to “cessation attainment” but also called “cessation of feeling and perception”). Nirodha events can happen spontaneously in the course of deep meditation and are experienced as a short “gap” or “cut” in the stream of consciousness in the realm of milliseconds or seconds, often followed by a sense of clarity and openness. The subsequent changes to cognition can be short- (e.g., minutes to hours) or long-lasting (e.g., days, weeks, months, years or permanent). Nirodha cessations can represent an important stage of progress in meditation and is sometimes equated with other phases of meditation known as path (magga) or realization and fruition (phala) (e.g., Berkovich-Ohana, 2017; Sayadaw, 2016).d On the other hand, nirodha sam
apatti (NS) involves a much more intentional process where one needs to be able to pass through various stages of samadhi (i.e., concentration, serenity, or tranquility) and then willfully create the conditions for NS to occur for a pre-specified amount of time (ranging from very short to up to as long as 7 days, Buddhaghosa, 2020).e Moreover, it is said that NS is only available to the most advanced meditators who have already experienced multiple brief cessations. Note that we refer to both nirodha and NS as different forms of cessation. To illustrate the nature of NS, the Maha Vedalla sutta (Nanamoli and Bodhi, 1995) distinguishes the state from death as follows: c The P
ali Canon consists of three collections of books, 1. The Vinaya (rules that the monks have to obey), 2. The Suttas, discourses ascribed to the Buddha and his close students (Bodhi, 2000; Nanamoli and Bodhi, 1995; Walshe, 1995; and two more series), and 3. The Abhidhamma (summarized in Bodhi, 2012), also called Buddhist psychology. In the later Visuddhimagga (Buddhaghosa, 2020), the teachings of the Suttas have been systematized. d There may be disagreements about the value of cessations for fruition depending on the Buddhist tradition. e While this is considered the common way to enter into nirodha sam
apatti, some meditators report alternative methods that may also work.

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“In the case of the one who is dead, who has completed his time, his bodily, verbal and mental fabrications have ceased and subsided, his vitality is exhausted, his heat subsided, and his faculties are scattered. But in the case of a monk who has attained the cessation of perception and feeling, his bodily, verbal and mental fabrications have ceased and subsided, his vitality is not exhausted, his heat has not subsided, and his faculties are exceptionally clear.”

The above quote indicates that NS is different from death because only the person who has died has lost their physiological capacity to maintain a normal body temperature, their life energy (or vitality), and their capacity to regulate their body (faculties). On the other hand, NS is similar to death because one is also no longer experiencing any perceptions and feelings and they have no bodily, verbal, or mental fabrications (i.e., experiences, which are characterized as fabrications in Buddhism, Burbea, 2014). Thus, according to Buddhist literature, NS is a kind of suspended animation where key physiological processes remain intact (although possibly slowed down), but all conscious experience has ceased (Buswell and Lopez, 2014). Cessation (both nirodha and NS) is therefore also distinct from experiences of pure consciousness and non-dual awareness (Josipovic, 2019; Laukkonen and Slagter, 2021; Metzinger, 2020; Millie`re et al., 2018; Milliere and Metzinger, 2020), selfless experiences (Deane, 2020; Dor-Ziderman et al., 2013; Milliere and Metzinger, 2020), or other spiritual or altered states of consciousness that might be elicited through meditation or other means (e.g., psychoactive substances, psychological disorders, or brain traumas). Experiences of selflessness or events that might resemble pure consciousness or non-dual awareness may happen either before or after cessation, but not during cessation. This distinction between pure consciousness and cessation is clarified further in the theoretical frameworks sections. Although our goal in this paper is to focus on the peculiar prospect that meditation can induce an absence of consciousness, why might it be of value to a meditator to undergo cessation? Anecdotal evidence from participants in our research, which is consistent with the ancient texts (Buddhaghosa, 2020; Thera, 1961), proposes that cessation causes an important transformation for how the mind works and triggers a profound clarity and openness, although this is yet to be tested. Moreover, in some cases, what is discovered upon emergence is believed to be important because one is exposed experientially to the progressive reconstitution of the mind. In Buddhism, the processes through which the sams
aricf mind assembles itself are known as paticcasamupp
ada. (dependent origination or arising), sometimes dissected into 12˙ stages or nid
anas (i.e., links). The experience and understanding of dependent origination, which some meditators associate with the stages of how the mind reassembles, is an important hallmark of contemplative realization or understanding in all Buddhist traditions (Boisvert, 1995).

f

Samsara represents a mind or life that is characterized by dukkha or unsatisfactoriness.

3 Buddhist meditation context

We also acknowledge that there are many uses of the term nirodha even within Buddhism and across Buddhist traditions. For instance, and as noted above, in some cases nirodha is associated with other terms, such as path(/magga)/fruition(/phala) (Berkovich-Ohana, 2017; Sayadaw, 2016). However, for the purposes of our current presentation, we equate nirodha specifically with the experienced absence of consciousness that can occur, which may or may not be associated with the other stages of meditation progress. On the other hand, NS refers to the willful entry into such an absence for a determined period. Thus, for scientific purposes and based on reported phenomenology of subjects we have worked with in the laboratory, here we define cessation as: “The absence of all experience and consciousness—with no retrospective awareness of anything having taken place during the absence—accompanied by a subsequent profound sense of clarity, openness, and vitality.”

Other important experiences may occur after cessation, but these may not be consistent for all meditators (e.g., insights or pure awareness events), hence we have adopted the characteristics that seem to be the most consistent or “necessary and sufficient” conditions.

3 Buddhist meditation context In order to contextualize cessation, below we begin with the soteriological description of Theravada Buddhist jh
ana meditation, the prerequisites for being able to spontaneously enter nirodha, and the procedure to actually enter into and emerge from this state on demandg (i.e., NS). Practicing meditation in order to attain certain levels of jh
ana has received relatively little attention within science (with a few exceptions, Dennison, 2019; Hagerty et al., 2013). Jh
ana practice encourages maintaining one “object” of attention largely at the exclusion of others (although in some traditions this can involve an accompanying openness; Vimalaramsi, 2015). This kind of samatha (tranquility) practice—sometimes dubbed focused attention (Lutz et al., 2008)—when ongoing, is thought to lead to various reliable changes in one’s experience as the meditation deepens. It can be used as a support for mindfulness or vipassan
a (insight) practices (Catherine, 2010; Nanamoli and Bodhi, 1995), or mindfulness can be brought into the samatha meditation and be present throughout the jh
anas. The jh
anas can also be one-pointed (highly focused) or more open and aware (Catherine, 2010; Vimalaramsi, 2015). There are eight jh
anas and after the eighth

g

Note that although some meditators can voluntarily enter cessation, they cannot choose to do so in the same way that one might turn off a light switch. Instead, the meditator creates certain conditions and intentions before meditation, then engages in the practice that leads to a voluntary but spontaneous cessation (Buddhaghosa, 2020).

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jh
ana (or subsequent signless state, tbd), if the conditions are right (Buddhaghosa, 2020; Thera, 1961), NS occurs. Notably, NS requires a combination of mastery in both insight (vipassan
a) and samatha (i.e., jh
ana) practice (Buddhaghosa, 2020; Thera, 1961). One way to think of jh
anas are as levels of deepening samatha (concentration) meditation. Another way is to think of them as levels of serenity or ’letting go’ (Armstrong, 2021; Johnson, 2017), which uncover aspects of the mind that usually go unnoticed.


3.1 The four rupa (form/material) jhanas The first four jh
anas have different characteristics described in terms of thinking/ attention, feeling, and the quality of awareness. Each jh
ana, somewhat counterintuitively, is a reduction in some mental quality or habit. Hence why they have sometimes been referred to as levels of cessation (Armstrong, 2021). The first four jh
anas are known as rupa or “form” jh
anas because they involve some experience of the senses and the object of focus is “material” in nature. The four form/material jh
anas are presented in Table 1 (Bodhi, 2012; Nanamoli and Bodhi, 1995).

3.2 The four arupa (formless/immaterial) jha
nas The four subsequent jh
anas are known as formless or arupa jh
anas because they no longer include a sensory experience. Instead, the key characteristics of these jh
anas are mental and their qualities predominate when sensory experience has largely (or totally) abated. Moreover, the basis of the arupa jh
anas is equanimity, i.e., jh
ana four. These jh
anas and their translations are presented in Table 2 (Bodhi, 2012; Nanamoli and Bodhi, 1995). As an illustrative example of the transition from the last form jh
ana to the first formless jh
ana, the fifth jh
ana known as a
k
as
anan˜c
ayatana or “infinite space” is described as follows:

Table 1 Description and translation of the four “form/material” jh
anas.
jhana ONE: TWO: THREE: FOUR:

Qualities or objects of mind in Pali and in English Vitakka Vicara

Applied att. Sustained att.

Piti Sukkha Piti Sukkha

Joy bliss Joy bliss

Karuna Mudita Karuna Mudita Karuna Mudita Upekka

Compassion Empathy Compassion Empathy Compassion Empathy Equanimity

Note: Att. ¼ Attention/thought. There are variations in translations, however for our purposes it is
nas involves a reduction in different qualities of mind sufficient to note that the progression of the jha
nas already feel “positive” (or mind activities) that are usually considered positive. Hence, the initial jha
nas are more stable and calm. Note that some traditions use karuna and mudita as but the later jha objects of meditation in the form of metta.

3 Buddhist meditation context

Table 2 Description and translation of the four “formless/immaterial” jh
anas.
jhana

Qualities of mind in Pali and in English

FIVE: SIX: SEVEN: EIGHT:


a
sa
nan˜ca
yatana Ak Vin˜n˜a
n·an˜ca
yatana
˜can˜n˜a
yatana Akin Nevasan˜n˜a
na
san˜n˜a
yatana

Infinite/boundless Space Infinite/boundless Consciousness Nothingness Neither Perception nor Non-Perception

Note: The use of the word consciousness in classical Buddhism tends to be different than the way consciousness is construed in contemporary science (i.e., as the presence of phenomenality as such).
nas seven and eight still have some, albeit subtle, phenomenal experience present. There is Clearly, jha
na known as the signless state. also a transitional state after the eighth jha

“By passing entirely beyond bodily sensation, by the disappearance of all sense of resistance, and by non-attraction to the perception of diversity, seeing that space is infinite, he reaches and remains in the sphere of infinite space (Walshe, 1995, 9:14).”

It is beyond the scope of this paper to detail the experience of each of the arupa jh
anas. It is sufficient to say that in the current context, like the first four jh
anas, each progression of the formless jh
anas is thought to represent the discovery of more subtle and deeper qualities of the mind, that is, with less and less contents of consciousness. Put differently, as one’s relaxed concentration (samatha) deepens and greater letting go occurs (Armstrong, 2021; Johnson, 2017) then the qualities of the present jh
ana disappear and give rise to the qualities of the next jh
ana, and so on. Contemporary practitioners of jh
ana meditation may also use particular instructions at each stage in order to progress the mind from one jh
ana to the next (e.g., Burbea, 2014; Catherine, 2010). Note that in jh
anas 1–4 there is a gradual refinement of the mental factors present. In jh
anas 5–7 the mind-state remains as in jh
ana four (equanimity), but the qualities of the mind are increasingly refined. After jh
ana eight is the ’signless state’, where the difference between subject and object seems to melt away, and hence is comparable to a non-dual state.

3.3 Nirodha sama˜patti: Preparation, procedure, and prerequisites As mentioned in the introduction, although rare, a nirodha event can occur in the course of meditation and does not necessarily require one to pass through all of the jh
anas (Berkovich-Ohana, 2017). However, in order to master NS and be able to turn off consciousness at will and for a predetermined amount of time, it is suggested that one needs to be capable of passing easily through all eight jh
anas (Buddhaghosa, 2020; Thera, 1961). According to these texts, it is necessary that the meditator is able to: 1. 2. 3. 4. 5.

Direct attention to a chosen jh
ana Enter the chosen jh
ana Predetermine how long one will be in the chosen jh
ana Emerge from the jh
ana Reflect on the jh
ana

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But even more is said to be needed in order to be able to willfully induce nirodha (Buddhaghosa, 2020). The meditator must also be at an advanced stage of “liberation,”h known as an An
ag
amī or “non-returner” (i.e., partially enlightened, having eradicated aversion and sensual desire, but still with “conceit” and other hindrances remaining) or an Arahant (i.e., being fully enlightened, Nanamoli and Bodhi, 1995). And finally, the meditator must be able to repeat easily the insights leading to this stage. There are also certain directions for preparation and intentions that the meditator should engage in before entering NS. For example, the meditator is directed to set an intention for the number of hours or days to remain in NS and to know that they will not die during the state. It is also advised that the meditator does not enter NS for longer than 6 days (or 7 days at the most) because this could be dangerous for the body (Buddhaghosa, 2020). Some practitioners also engage in training determinations (cf. capacity 3 above) in order to refine their ability to automatically or physiologically track the passage of time. For example, the practitioner may set an intention to sit for a specific number of minutes and regularly test themselves in order to improve their ability to predict when the intended amount of time has passed (Buddhaghosa, 2020). Once the practitioner has mastered meditating for the chosen amount of time, then they are said to be able to set an intention to enter NS for a prespecified time, and the body is said to naturally “wake up” the meditator after that time has passed (Buddhaghosa, 2020). It is also highly likely that many contemplatives from traditions outside of Buddhism have undergone similar experiences, either intentionally or spontaneously in the course of meditation. There are certainly recognizable anecdotes to be found from renowned teachers in other traditions, including Ramakrishna (Saradananda, 2015) and Ramana Maharshi (Godman, 2017). In both of these cases, the contemplatives report experiencing states during which the body’s normal functioning changed dramatically, and the body remained stiff and unresponsive to external stimulation.

4 Contemplative science context Next, we briefly consider how cessations fit into the broader literature on meditation and styles of practice. Currently, contemplative science literature is following several branches of inquiry. These include smaller branches conducting basic mechanistic investigations into the neural systems involved in meditation as well as psychobiological assessments of meditation correlates in long term practitioners.

h

Enlightenment in classical Buddhism is defined by the absence of specific mental qualities known as fetters (or obstacles). Examples of fetters (samyojana), of which there are 10, include: Ignorance, ill will, desire, conceit, and restlessness (Bodhi, 2000). From a cognitive perspective, this could map onto a decrease in harmful habits (see also footnote i).

4 Contemplative science context

The smaller branch is complementing a main branch investigating mindfulness training interventions for alleviation of clinical symptoms or increased wellbeing. Cessation has received little attention in the contemplative science literature, no doubt partially due to the technical challenges of studying the phenomenon (nirodha cessations are very brief, e.g., milliseconds), and longer periods of NS require significant practice expertise to attain, thereby rendering practitioners exceedingly rare and difficult to recruit. However, Davis and Vago (2013) report preliminary data that in meditators trained be Shinzen Young, the BOLD signal dramatically increased in front-polar cortex during “Gone” events (which share some, but not all, features with nirodha). They also emphasize the importance of quantitative measures to attenuate bias in self-reports of meditative states, measures which will be particularly important in verifying practitioner reports of cessation in future studies (Davis and Vago, 2013). While NS requires specific cultivation to achieve, nirodha can occur in several different practice contexts. Previously, meditation techniques have been categorized according to four types of practices: focused attention (FA), open monitoring (OM), loving kindness (LK) and nondual (ND) meditation (Josipovic, 2010; Laukkonen and Slagter, 2021; Lutz et al., 2007, 2008). Briefly, FA meditation involves exclusive focus on one “object” of meditation (e.g., the breath) at the exclusion of others. OM involves a more open and relaxed attentional scope, and emphasizes a nonjudgmental or mindful observation of experience (vipassan
a meditation falls in this category). Relative to OM practice, ND may involve no “attention” as it is usually understood. ND releases the separation between subject (self ) and object (other) in favor of resting in and as awareness (Laukkonen and Slagter, 2021; i.e., the deepest form of inner “silence” Paoletti et al., 2022; Paoletti and Ben-Soussan, 2020). However, it has also been noted (Malinowski, 2013) that several meditation practices draw on all of these techniques, and that they exist on a continuum more than as separate categories. Moreover, for advanced meditators, ND awareness may be reflexively present at all levels of practice (Josipovic, 2019; Laukkonen and Slagter, 2021; Paoletti et al., 2022). More recently, this system has been refined from a set of bins (FA, OM, LK) to a series of dimensions, along which any practice can vary (Dahl et al., 2015; Lutz et al., 2015). It has been proposed that the relevant dimensions for a typology of meditation are: dereification, object orientation, and meta-awareness, with meditation techniques sorted into “families” of practice: attentional (e.g., jh
ana, samatha with support, mantra), constructive (e.g., bhramavihara/loving kindness practices), and deconstructive (e.g., Vipassan
a, Koan practice) (Dahl et al., 2015; Lutz et al., 2015). Attentional practices draw upon cognitive processes of attention regulation and meta-awareness, constructive practices recruit perspective taking and reappraisal, and deconstructive practices rely upon the cognitive process of self-inquiry. The attentional family of practices, including mindfulness meditation, has received the most scientific attention, with a relative paucity of studies on constructive and deconstructive practices, including insight and “do nothing” practices (Dahl et al., 2015; Lutz et al., 2015).

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While the key contributing practice factors for cessation have not yet been described directly, Desbordes et al. (2015) advanced equanimity as a viable outcome measure of meditative experience, and all of the practice categories in the deconstructive family (e.g., vipassana) advanced by Dahl et al. (2015) simultaneously depend on equanimity for efficacy and cultivate equanimity as an outcome of practice. Taken together, these works suggest that practices of cultivating and sustaining equanimity, some degree of focused attention, and deconstruction may be necessary for nirodha (King, 1977). However, the deep absorptive concentration associated with the jh
anas is said to only be necessary for NS (Buddhaghosa, 2020). Although often touted as a risk-free way to move the practitioner toward a greater sense of well-being and flourishing, meditation practice is not without risks (Lindahl et al., 2017; Schlosser et al., 2019). Lasting adverse effects of even a short (8-day) mindfulness intervention include hyperarousal and dissociation (Britton et al., 2021; Lindahl et al., 2020). Given the level of expertise required for both nirodha and NS, it is relevant to note that in long term practitioners, documented adverse effects are predicted based on having been on an overnight meditation retreat (Schlosser et al., 2019). Thus, understanding how to resource and support practitioners pursuing these states in long—multiple day, week, or month—retreats is vital (e.g., adequate preparation and beneficial integrative practices). The full extent of preparation and integration that is sufficient to mitigate adverse effects is not known, but prior work suggests that factors that can ameliorate difficult experiences include mentors and friends working directly with the practitioner in their own social context (Lindahl et al., 2017), and having practitioners balance deconstructive practices with other types of practices, such as samatha or loving kindness (Schlosser et al., 2019). It may also be supportive to have a conceptual framework that manages expectations such that difficult experiences are not as surprising to the practitioner, and to have trained psychologists available when particularly troubling experiences occur.

5 Preliminary findings Below we outline preliminary findings from two experiments (in preparation for publication). As research into cessation is in its infancy, we describe these findings as they may support others who might be starting out on a similar research project and they may be useful in the development of (initial) neurocognitive models for generating hypotheses. These findings cannot yet be used to draw strong conclusions. In a recent pilot study conducted by authors MDS and AC, we used EEG to examine the neural correlate of nirodha in an adept meditator with over 6000 hours spent in meditation retreats. In this case, what the participant considers nirodha was observed during vipassan
a/open-monitoring/insight meditation practice. In some traditions, nirodha is believed to be a culmination of the Stages of Insight meditation (Sayadaw, 2016), whereas NS is the culmination of both insight and samatha practice. As described earlier, this meditator reported nirodha as a momentary extinction of

5 Preliminary findings

experience that was followed by profound alterations of consciousness,i a sort of “reset” that is characterized by mental clarity. In the study, we employed a neurophenomenological approach where our phenomenologically trained subject systematically evaluated the mental and physiological processes relevant to nirodha as he experienced them, and these evaluations were used to classify and select 37 high-grade nirodha events for subsequent EEG-based analysis. Preliminary results have shown that 20 s before nirodha (not NS), there begins a linear decrease in large-scale functional neural interactions in the alpha-band, as reflected by an EEG-based measure of functional connectivity. Markedly, this interaction was lowest immediately following a cessation, and there was a gradual increase from 3 to 40s post cessation wherein these interactions returned to prior levels. The modulation of network integration was unique to the alpha frequency band—there were no significant differences in functional neural interactions before or after nirodha in the delta, theta, or beta frequency bands. This decrease in global functional connectivity may indicate a gradual reduction in information exchange between different brain areas and ultimately to the experience of a “cut” from consciousness during nirodha. It is interesting to note that a study by van Lutterveld et al. (2017) previously reported increased alpha-band network integration during meditation in experienced meditators. In comparison, our results suggest that although progressive meditative stages may be characterized by increased alpha-synchronization, nirodha events appear to be experienced following a gradual decrease in overall brain connectivity. Another study by Berkovich-Ohana (2017), also using EEG, analyzed data from two adept meditators as they experienced three nirodha events (termed fruitions in the paper) each (i.e., a total of six nirodhas), and reported increased global long-range gamma (25–45 Hz) synchronization during states of nirodha as compared to non-nirodha states. The authors interpret the increased gamma signature as possibly offering an underlying mechanism for the un-learning of habitual conditioning and mental patterns. In another EEG study of an adept meditator from authors RL, HB, MS, AC, and HS (in preparation for publication), we compared conditions of resting wakefulness

i

A typical description of alterations of consciousness following nirodha may include: a sense of increased clarity, less grasping of experience (i.e., de-reification), increased energy and vibrancy, “openness” of mind and emotions, greater mindfulness, increased cognitive flexibility, less selfcenteredness, less concern for the past or future, just to name a few. Future research may benefit from interviewing experienced practitioners to confirm (e.g., via factor analysis) the consistency of these anecdotal characteristics, and their relative importance or strength. According to Buddhist literature (Buddhaghosa, 2020) the first nirodha/fruition associated with awakening leads to the absence of a subset of unwholesome mental factors (cetasikas), such as wrong view about self, greed, hatred, conceit (subsequent nirodhas/fruitions lead to the reduction or absence of yet other factors). One speculation is that as a consequence of putting an end to these unwholesome mental states (or bad habits) one may experience some of the side-effects described above, e.g., increased clarity and energy may follow because unwholesome habits cost energy. How nirodha/fruition could prevent particular mental states or habits from arising in the future is not known, and requires empirical investigation. We provide one potential computational mechanism in the discussion.

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and a short duration of sleep (i.e., nap) to nirodha sam
apatti. As described above, this particular type of cessation is the result of a hybrid vipassan
a and jh
ana or concentration meditation practice, and can be induced for a prespecified period of time. Interestingly, our result, albeit in n ¼ 1, corroborated the previous finding concerning nirodha in that the nirodha sam
apatti was also characterized by attenuated alpha-synchronization in comparison to the awake and nap conditions. Similar desynchronization of alpha has been found with ketamine (Blain-Moraes et al., 2014) and propofol (Kallionp€a€a et al., 2020; Lee et al., 2013) induced unconsciousness, discussed further in the theoretical frameworks section.

6 Theoretical frameworks: Predictive processing Some of us (Laukkonen and Slagter, 2021) have previously proposed that the effects of meditation on (conscious) experience can be understood from the perspective of the predictive brain or active inference (Feldman and Friston, 2010; Friston, 2009). This perspective starts from the notion that our brains have no direct access to the outside world, only to their own sensory activation patterns, induced by electrical signals conveyed by the senses. Hence, in order to see and guide actions, brains must instantiate predictive models about the likely hidden causes of their sensory input, and continuously minimize prediction errors to ensure model reliability. In fact, in this perspective, the main imperative of the brain and mind is to minimize prediction errors. Prediction error minimization can occur either by changing the predictions, through model updating (perceptual inference), or by generating the predicted sensory input through action (active inference). Importantly, not just perception and action, but all brain functions, including high-level cognitive processes, are understood as minimizing prediction errors, albeit at different time scales. That is, the deep temporal and hierarchical structure of generative models in the human brain allows for the entertainment of “what if” beliefs (or counterfactual hypotheses) about the world, and hypothesis testing that does not entail overt action (Friston et al., 2018). The ability to have beliefs about what it is like to act, to revisit how past events unfolded, and to internally simulate and weigh the potential consequences of one’s future actions underlies our ability to purposefully evaluate, imagine, remember, plan and make judgments (Buzsa´ki et al., 2014). Such covert or mental actions (Metzinger, 2017) allow the brain to reduce ambiguity over the outcomes expected under various policies in service of future action. Crucially, brains not only predict upcoming sensory input, but also estimate the reliability of the sensory input (a second-order prediction). Obviously, it would be detrimental if the brain were simply at the whim of external influences, since sensory input can be noisy and/or not representative of the world at large. Therefore, brains also need to predict the reliability or precision of sensory input, which requires integrating information over time or experiences (Friston, 2009). It has been proposed that attention maps onto precision weighting, in line with empirical findings showing that attention can modulate the gain of sensory signals (Feldman and Friston, 2010; Hohwy, 2012). In the predictive processing framework, the mind is hence constructed through past experience, which shows notable parallels to Buddhist ideas about the mind

6 Theoretical frameworks: Predictive processing

as constructed in nature and conditioned by the past (Laukkonen and Slagter, 2021; see also Lutz et al., 2019 for an overview specific to FA meditation, as well as Pagnoni, 2019 for Zen practice). We previously put forward that predictive processing may hence provide a unifying framework for understanding the wide range of effects of meditation on mental experience (Laukkonen and Slagter, 2021). Specifically, we proposed that three main styles of meditation described earlier— FA meditation, OM meditation, and ND awareness meditation—can radically change ordinary mental experience by bringing the practitioner more and more into the present moment through physical and mental inaction and underlying changes in predictive processing. More specifically, we proposed that these styles of meditation can be placed on a continuum, and gradually reduce the temporal depth of predictive processing in the brain. First, in FA meditation, high precision is assigned to one source of presentmoment sensory input, typically breath sensations, which automatically reduces the precision assigned to other events that may normally habitually arise in experience (i.e., mind wandering thoughts at temporally deeper levels in the processing hierarchy). The way that reducing precision of thoughts reduces their arising is similar to the way that, while engaged with reading, we are not aware of the feeling of our shirt resting on our backs. Then, in OM meditation, any content of experience is assigned equal precision, and hence, consequently relatively low precision (bare attention), logically inducing a non-reactive mode of experiencing or a shift to pure sensing without evaluation. Finally, in ND meditation, a state of complete presentmoment awareness is induced by releasing any (precision) expectations about even the very next possible moment. In this state, also the most temporally shallow mental processes should disappear. Note that even seemingly direct experiences, like that of a teacup, demand a complex process of construction from past experience and include anticipation of possible changes in sensory input (e.g., proprioceptive and sensory changes related to drinking from the cup), which intrinsically relies on active inference and inferring oneself as a hidden cause of sensory input in the future. Thus, if awareness rests only in the now, even very basic structures of ordinary cognition should logically fall away, including conceptualization, self-awareness and time perception. One proposal is that the state of non-dual awareness reflects a representation of tonic or intrinsic alertness (Metzinger, 2020), a proposal that aligns with the fact that this state is unaffected by transient events, devoid of conceptual content, and accompanied by the feeling of wakefulness (Gamma and Metzinger, 2021). This predictive model of tonic alertness is temporally shallow, given that it does not entertain complex state transitions or sequences over time. The question arises, how can the unusual state of cessation—assuming it exists as described above—be explained within this predictive processing framework? While non-dual awareness is characterized by wakeful awareness in the present moment (Metzinger, 2020), cessation is characterized by an absence of awareness and wakefulness. Speculatively, cessation could thus reflect a final release of the expectation to be aware or alert. Clearly, if the state of cessation can be sustained across multiple days, the normal wake-sleep cycle must be disrupted, supporting the idea that the

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very basic expectation to periodically be awake is not entertained in the brain. Yet, it is conceivable that cessation also shares characteristics with deep sleep or other states that are characterized by a lack of awareness at the (neuro)physiological level, as from an evolutionary point of view, it seems unlikely that one can induce an entirely novel state that the body did not evolve to occupy in the past. These may include physiological changes that would support an ability to sustain the state for longer time periods, without, for example, being disrupted by signals of hunger or thirst, in particular a reduction in arousal, body temperature and metabolic rate, as also seen during sleep (Tononi and Cirelli, 2006). Indeed, anecdotal evidence suggests that the body of practitioners in NS is cooled down, their heart rate is lowered, and their breathing barely perceptible.j The brain does not only minimize exteroceptive prediction errors, but also interoceptive prediction errors, that signal deviations from expected bodily states. Notably, it cannot directly minimize interoceptive prediction errors that signal deviance from expected physiological states: it can only do so indirectly, via active inference (Pezzulo et al., 2015). Take the example of hunger. Hunger arises when certain predicted sensory signals from stretch receptors gastrointestinal (GI) tract and from certain hormones that track food intake do not arrive in the brain. The brain can only resolve these interoceptive prediction errors through action, i.e., by eating. Thus, the brain can only indirectly, in the future, minimize interoceptive prediction errors through minimization of proprioceptive and exteroceptive prediction errors. Indeed, many allostatic processes are anticipatory building on the fact that many homeostatic processes are periodic and hence predictable (Barrett and Simmons, 2015). For example, we typically eat well before we get hungry (Drazen et al., 2006). The fact that, for example, hunger signals are not anticipated, experienced or acted upon during cessation thus suggests a disconnect at higher levels in the hierarchy where generative models encode associations between sensorimotor and interoceptive events (Pezzulo et al., 2015), in line with our proposal that meditation progressively reduces hierarchical, temporally deep predictive processing in the brain (Laukkonen and Slagter, 2021). It is also possible that a very low metabolic rate during NS prevents the need to eat or drink, similar to what is observed in daily torpor and longer periods of torpor (e.g., hibernation) in mammals (Ruf and Geiser, 2015). Strikingly, the state of NS is allegedly accompanied by immobility, as is also the case during torpor, and coming out of the state of NS is said to be accompanied by a feeling of renewed energy, and torpor serves energy conservation. Yet, although the human body can much better deal with body temperatures below its lower bound than once thought possible, as shown in medical studies in which hypothermia is induced in injured patients to prevent organ damage, there is no evidence to suggest that humans, as homeotherms, can willfully induce hypothermia (or torpor) (Ruf and Geiser, 2015). j

Although, in our preliminary data involving a period of 90 min NS, these physiological changes were not observed. Hence, either dramatic changes to physiology take time (i.e., possibly days or weeks, as they usually do in hibernating animals, Blanco et al., 2016; Tøien et al., 2011), or these characteristics are not necessary conditions of NS.

7 Theoretical frameworks: Neural dis-integration

On the other hand, there is evidence from studies in experts in g-tummo meditation (a form of advanced Tibetan Buddhist meditation training) that suggests that they are able to enhance their body temperature, albeit likely only to slightly above the normal range (Kozhevnikov et al., 2013). Moreover, some work has associated g-tummo meditation with a hypometabolic physiological state, as shown by reductions in oxygen consumption and metabolic rate (Benson et al., 1990). For example, one study in three meditation experts reported that they could up- and down regulate their metabolic rate by 61% and 64%, respectively, which is much lower than the reduction in metabolic rate that occurs in regular sleep (Benson et al., 1990), although this finding warrants replication given that it is one of the only studies showing such radical results. Nevertheless, these findings do provide some promise that meditation can have powerful effects on physiological processes usually considered automatic. Yet, these changes are induced through processes with temporal depth (that rely on active inference): imagery and forceful breathing. Of further note, both sleep and torpor are associated with changes in the activity of the ascending reticular activation system (ARAS) which mediates tonic alertness and arousal and is responsible for human consciousness level (Laureys et al., 2009). Meditation, including the state of non-dual or pure awareness, has also been related to changes in arousal and the ARAS system (Britton et al., 2014; Metzinger, 2020). Thus, some forms of advanced meditation may modulate predictive processes related to controlling physiological systems that are critical to maintaining (the potential for) wakefulness, resulting in NS. Future studies should systematically investigate (neuro)physiological changes over time during both short and longer periods of NS to better understand the mechanisms underlying the state and their similarity or differences to other states characterized by a loss of awareness, such as deep sleep and torpor. Future studies are also necessary to clarify to what extent spontaneous and willfully induced NS rely on similar mechanisms. To summarize, NS may reflect a final release of the expectation to be awake or aware. This could be brought about through (neuro)physiological changes that support a low-arousal, hypometabolic state. Coming out of NS may follow a reverse path in which the mind is progressively reconstructed going from simple wakefulness (e.g., pure awareness, Metzinger, 2020) to temporally shallow (e.g., sensory experience) to temporally deep predictive processes (e.g., thinking) (Laukkonen and Slagter, 2021).

7 Theoretical frameworks: Neural dis-integration In addition to the above proposed mechanism of cessation related to a gradual deconstruction of hierarchical predictive processing, what might be the neural correlate of such a transition to unconsciousness? As described earlier, one replicated finding from our two unpublished studies is a reduction in alpha functional connectivity as measured by EEG (a preliminary finding). Yet, interestingly, a breakdown in alpha connectivity has been robustly connected to transitions from consciousness to

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unconsciousness when induced by ketamine (Blain-Moraes et al., 2014) and propofol (Kallionp€a€a et al., 2020; Lee et al., 2013) using the same measures as in our cessation research, i.e., the phase lag index.k This breakdown in normal synchronization may indicate an impairment in corticocortical communication (Supp et al., 2011) and thus a failure to bind together elements of a coherent conscious experience. Hence, it may be that some meditation styles, through their deconstructive elements (Dahl et al., 2015), actively dis-integrate or unbind aspects of phenomenology that results in a breakdown of functional connectivity, and thus unconsciousness. It is reasonable to expect that this final breakdown of consciousness can only occur once other, higher (in the hierarchy), levels of integration have been sufficiently relaxed, as described above (Laukkonen and Slagter, 2021). Notably, the strength of network connectivity in subjects before anesthesia predicted their susceptibility to propofol (Chennu et al., 2016) suggesting that individuals may vary in their state or trait connectivity and that this can potentially determine the likelihood of unconsciousness. Meditation may progressively decrease the connectivity of aspects of conscious experience and thus make an episode of “unconsciousness” more likely to occur. This way of looking at cessation also shows consistencies with theories that propose consciousness rests on the capacity to integrate information (e.g., Tononi, 2004, 2008). According to information integration theory (IIT, Tononi, 2005, 2008), one of the central features of consciousness is that it is a unitary phenomenon. That is, we always only experience a singular conscious experience instead of two or more separate consciousnesses. As noted by Tononi (2008, p. 219): “…underlying the unity of experience must be causal interactions among certain elements within the brain. This means that these elements work together as an integrated system, which is why their performance, unlike that of the camera, breaks down if they are disconnected.”

That is why dis-integrating (i.e., dis-unifying) consciousness may unravel an essential and necessary feature of consciousness without which it does not occur. But how does consciousness remerge? This is a key question for future research. For now, we speculate that the processes are likely to be similar to re-emerging from deep sleep or other non-conscious state, e.g., via the release of particular hormones and neurotransmitters (Fuller et al., 2006). Albeit speculative, there is an aspect of meditation that relates very closely to this kind of dis-integration of phenomenology and consciousness. A key concept in Buddhism is that of the five aggregates (khandhas) that are said to represent the totality of human existence from a phenomenological standpoint. These five aggregates are form, sensations, perceptions, mental activity, and consciousness. Without entering

k The breakdown of alpha functional connectivity is one of the few common characteristics of propofol and ketamine induced unconsciousness, which otherwise seem to involve very different mechanisms at the neurochemical level (Kushikata et al., 2016) and at the systems level (Lu et al., 2008).

8 Discussion

into a detailed discussion of the aggregates, it is taught that the meditator should discover that none of the aspects of experience (including consciousness) has any independent existence, and that each of the aggregates are characterized by suffering (dukkha), impermanence (anicc
a), and non-self (anatta) (Buddhaghosa, 2020; Nanamoli and Bodhi, 1995; Sayadaw, 2016). As the meditation practice thus deconstructs the qualities of experience that make it into a whole, it is possible that the unbinding or dis-integrating of these aggregates play a role in the gradual breakdown of functional connectivity, information integration, and associated consciousness. Somewhat similarly, the practice of advaita vedanta also involves a deconstruction of one’s experience through processes of self-enquiry (i.e., questioning the nature and continued existence of elements of experience such as the self, Nisargadatta, 2012), and this can anecdotally lead to similar events totally void of sensorial experience (Godman, 2017).l In sum, inducing a state of unconscious from within may involve a combination of an initial reduction of hierarchically deep predictive processes (e.g., through jh
ana and vipassan
a meditation or FA, OM, and ND) followed by a final deconstruction/ disintegration of phenomenology as such (i.e., awareness), that may be explained in a complimentary way through a reduction in mechanisms underlying expectations of wakefulness (cf. Metzinger) and a reduction in neural integration (i.e., an unbinding of the elements of experience that bind together a coherent consciousness, Tononi, 2004). Since all conscious experience is unified (Tononi, 2005), to dis-unify experience from within is to eventually give rise to unconsciousness—or cessation.

8 Discussion “Our very existence is in the atmosphere of non-existence.”

Bhagavad Gita, approximately 300 BCE (Bhaktivedanta and Prabhupada, 1972). In the history of cognitive neuroscience and neuropsychology, many unique case studies have informed our understanding of how the mind and brain work and the malleability and vulnerability of our conscious experience. We add meditationinduced cessations to this list, which is the intriguing capacity of some advanced meditators to “turn off” consciousness from within. Even though such events are rare, what might they reveal about the way human psychology and biology work? And what are the implications for our understanding of the top-down malleability of consciousness and its contents? In this paper, we sought to provide an introductory overview of the state of cessation in its two documented forms, nirodha and nirodha sam
apatti. We have reviewed historical descriptions and accounts from classical Buddhism, discussed preliminary data from several studies of different practitioners, l

As in the famous case of Ramana Maharshi, wherein he was so absorbed in his meditation that insects were eating away at his body and food had to be placed in his mouth so that he would eat (Godman, 2017).

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contextualized cessation within the field of contemplative neuroscience, and provided an initial theoretical explanation for its occurrence. Here we briefly reiterate the process by which nirodha is thought to arise. First, short “nirodha” cessations is thought to occur as a result of insight/vipassan
a practice which is characterized by non-judgmental and equanimous observation of experience through the lenses of anicc
a (impermanence), anatta (non-self ) and dukkha (dissatisfaction). In this context, cessations occur in deep stages of practice as a culmination of the Stages of Insight, and are experienced as a momentary cut, or absence, in consciousness (Sayadaw, 1994, 2016). A more extended, and more intentional form of cessation is nirodha sam
apatti, wherein the advanced meditator can induce a period of absence for a predetermined amount of time, and up to 7 days (Buddhaghosa, 2020; Thera, 1961). NS is thought to be a consequence of mastering both insight and jh
ana practices. Next, we turn our attention to outstanding questions and a research agenda. We have proposed that the absence of consciousness associated with cessation may be the logical result of progressively reducing abstract predictive processing in the brain (Laukkonen and Slagter, 2021; see also Paoletti et al., 2022). We have also proposed that this flattening of hierarchical processing can be even further “broken down” via a reduction in functional connectivity that might give rise to a disintegration in the binding elements of consciousness, as evidenced by a reduction in alpha synchronization during cessation. However, we have not yet proposed a mechanism for the possible consequences of nirodha to the mind and brain. Although speculative, one possibility that captures the strong phenomenological sense of clarity and openness associated with the post-cessation state is a reset of the precision-weighting landscape at levels of the brain’s functional hierarchy that maintain temporally deep (abstract) beliefs about the world and self. One role of precision-weighting is to register one’s confidence in the reliability of priors vs prediction errors (Carhart-Harris and Friston, 2019; Feldman and Friston, 2010; Haarsma et al., 2021). From a Bayesian perspective, the more precise one’s priors, the less driven by bottom-up input the system is, as the new data is rigidly ignored if it does not match up with existing expectations. Alternatively, if one’s priors have low precision and the prediction errors (i.e., the input) have high expected precision, then one is willing to revise their generative model in light of new evidence. If priors undergo a precision-reset—the extent of which may be determined by the depth of the cessation—this would manifest as greater confidence in, and attention to, present moment sensory experience, or phenomenologically a sense of clarity and freshness, as if everything is new or as if “seen for the first time.” Or put simply, cessation would result in an experience of the present moment that is less conditioned by past beliefs. Hence, one possibility is that cessation leads to a kind of inner reset of the precision-weighting landscape at higher-levels in the processing hierarchy, reducing one’s computational trust in priors that encode deep beliefs about (oneself in) the world—which have just been revealed to be highly vulnerable and volatile in light

9 Challenges and future directions

of cessation—and hence increasing the vividness, attention, and confidence, associated with sensorial data. In essence, this represents a shift in the system such that the generative model is driven by bottom-up input more than top-down expectations. This hypothesis could be tested using any paradigm that tracks the degree of topdown effects on perception or cognition. Notably, the presence of visual illusions or other cognitively driven distortions might be weakened or more flexible following nirodha. Logically, higher-level priors with relaxed precision, which are also implicated in psychedelic states (Carhart-Harris and Friston, 2019), should also result in greater cognitive/perceptual flexibility that could be evidenced by smaller decrements in performance as a consequence of task-switching, or improved problemsolving in tasks that require creative restructuring of representations, such as insight problems (Laukkonen and Tangen, 2017; Laukkonen et al., 2021; Ohlsson, 1984). A complimentary possibility is that deep states of meditation such as cessation and ND awareness have an effect on hyperpriors on precision (Sandved-Smith et al., 2021). Hyperpriors on precision are “…prior beliefs about the precision of beliefs about the state of the world” (Friston et al., 2013, p. 1). Or put simply, beliefs about uncertainty in general. The profound surprise initiated by cessation or ND awareness may lead the organism to also relax hyperpriors on precision due to the experience of what was once taken as “permanent/certain” to be in fact “impermanent/uncertain” (e.g., the self, world, and one’s beliefs about them). This could foreseeably explain some trait effects often touted by meditators following such experiences. For instance, a relaxing of the hyperprior on precision would result in less “grasping”: the organism now expects that things are always changing so does not resist when things do in fact change.

9 Challenges and future directions One additional challenge to conducting research on cessation is the discrepancy between phenomenological absence and the absence of processing at a physiological level. As in comatose states or deep sleep, the brain may continue to respond to stimuli even though they are not registered consciously (Cossy et al., 2014; Morlet and Fischer, 2014; Strauss et al., 2015). Hence, the degree to which cessation actually disrupts automatic brain (sensory/perceptual) processes is unknown, although some level of breakdown in higher-order functioning is reasonable to expect (e.g., markers of language comprehension or emotional reactivity). Relatedly, it difficult to know to what extent the feedforward sweep of processing is affected during cessation. Since the meditator is able to set an “intention” to re-emerge from NS given certain external conditions (e.g., a fire), some feedforward processing is certainly necessary as well as some minimal higher-order inferential processing that does not yet give rise to consciousness. Nevertheless, ordinary responses to pain or surprising/startling stimuli should considerably diminish during cessation (Antonova et al., 2015; Grant and Rainville, 2009; Levenson et al., 2012). Moreover, changes to physiological

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processes are likely to take time, as in cases of torpor or hibernation (Ruf and Geiser, 2015). Hence, the most radical changes to processes like breathing, heart rate, or neural responses may only occur after the meditator has remained in NS for an extended period, e.g., 1 or 2 days, or longer. As to a research agenda, as noted above, very few scientific studies have so far examined the mental and (neuro)physiological changes that accompany nirodha and NS, and what we know about the state is largely based on self-reports (long) after the NS experience occurred, and introspective reports can be inaccurate and biased (Nisbett and Wilson, 1977). Innovative methods are necessary to determine mental changes because an individual in a NS state is not able to respond or complete tasks using traditional cognitive-behavioral paradigms. We particularly propose two approaches: No-report paradigms and microphenomenology. No-report paradigms combined with measurements of brain activity allow researchers to make inferences about the nature of cognitive processing and consciousness (Tsuchiya et al., 2015), and have already proved useful in determining the extent to which mental operations are preserved in the comatose or asleep brain. Studies in comatose patients, for example, have used no-report paradigms and event-related potentials and shown normal auditory deviancy detection, but abnormalities in how unconscious brains track and integrate information over longer periods of time, or presented patients with their own vs close relatives’ names to determine the presence of self-related processing as an index of preserved awareness (Morlet and Fischer, 2014; Tzovara et al., 2015). A similar approach can be taken to study the state of cessation, as these methods allow us to probe changes in sensory responding (e.g., to a startling sound) and conscious dynamics without disrupting the meditation. Microphenomenology (Petitmengin et al., 2017, 2019), on the other hand, permits researchers to rigorously unravel the dynamics of experience through carefully structured interviews, and may be specifically useful in the study of meditation and consciousness, as expert meditators may be able to more reliably induce and maintain particular states and more accurately describe them (Lutz et al., 2007). Microphenomenology may be particularly valuable for uncovering changes to experience in the brief moments before and after cessation, which in turn can give rise to empirical hypotheses. For instance, it could be that the moments before cessation involve an unbinding of the unity of experience, consistent with the breakdown in functional connectivity and/or states of pure consciousness (Metzinger, 2020). Such combined neurophenomenological (Varela, 1996) modeling is a particularly promising approach. Microphenomenology may also help to better delineate to what extent nirodha and NS are subjectively different or similar, and whether it experientially matters whether one was in cessation for, e.g., 90 min vs 6 days. In addition to no-report paradigms and neuro-phenomenology, as mentioned above, it is important that future studies include physiological measurements to track changes in bodily state, such as heart rate, breathing, skin conductance, blood oxygenation, temperature, and bodily movements. Such measurements, combined with measurements of “resting”-state brain activity, are crucial for determining how nirodha sam
apatti may qualitatively differ from deep sleep or comatose states,

10 Conclusion

and presents a fundamentally different altered state of awareness, which would have large implications for current scientific theories of consciousness. Moreover, (neuro) physiological measures can also reveal to what extent practitioners can precisely sustain the state for the intended duration,m the precise beginning and end time of the state, and if this state can be characterized as an unusual hypometabolic state, that possibly shares characteristics with torpor in other mammals, and how long it may take for such a state to arise. Finally, this approach may also shed light on the extent to which nirodha sam
apatti is stable over time at the (neuro)physiological level, or, for example, still contains remnants of the regular sleep cycle.

10 Conclusion We find the study of cessations—rare states of internally induced transitions from consciousness to unconsciousness—promising for the future of cognitive neuroscience, much in the same way as externally induced events have provided insights into the nature of the mind and brain (e.g., brain traumas, unusual psychiatric conditions, comatose, and drug-induced states, and so on). To this end, there are advanced meditators who are uniquely able to consistently and safely induce these states under laboratory conditions. Our initial results reveal that meditative cessations show similar results to propofol and ketamine induced states of unconsciousness, and we have presented an initial theoretical explanation for how meditation can gradually abate hierarchical predictive processing until cognition and experience ceases. We also believe that the science of cessations will provide new insights for our understanding of the nature of consciousness—and possibly even the hard problem (Chalmers, 2017). Unlike anesthesia or trauma induced states of unconsciousness, the specific events surrounding a cessation are introspectively available to meditators retrospectively and can be induced successively many times, thereby affording repeated measurements with minimal side effects. Taken together, this may increase the quality of our “microscope” on the neurophenomenological constituents of (altered states of ) experience and consciousness itself, without the confounding element of a drug or other invasive procedure.

m

Interestingly, in our study (unpublished; RL, HB, HS), the participant intended to enter NS for 90 min. During NS, the researchers noted no perceptible eye and body movements. Moreover, the EEG, respiration, and heart rate appeared to remain unchanged for the entire 90 min. After roughly 90 min, the researchers observed a notable increase in heart rate (i.e., an increase of 10 beats per minute) as well as changes in the EEG and EOG (e.g., first indication of eye movements appeared at around 90 min). The participant opened their eyes after 104 min and indicated that they were fully awake. Clearly, such an observation requires careful replication and a more rigorous design, nevertheless, this first observation highlights a curious potential capacity associated with NS—to wake up out of cessation automatically, after a prespecified time, via simply the intention to do so.

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Acknowledgments We would like to acknowledge archeologist Igor Djakovic for first raising the possibility of hibernation in the context of cessation to RL. We also thank Jakob Hohwy for the insight to consider the role of hyperpriors in cessation, and Koen vd Biggelaar from the Suttav
ada Foundation for his helpful comments on the manuscript. Finally, we thank both Yair Pinto and Marco Dekker for their support throughout this research. Prof. Slagter and Dr. Laukkonen are supported by the European Research Council Starting Grant (679399). Dr. Sacchet and the Meditation Research Program are supported by the National Institute of Mental Health (Project Number R01MH125850), Dimension Giving Fund, Ad Astra Chandaria Foundation, Brain and Behavior Research Foundation (Grant Number 28972), BIAL Foundation (Grant Number 099/2020), Emergence Benefactors, The Ride for Mental Health, Gatto Foundation, and individual donors.

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Moving through silence in dance: A neural perspective

5 Vered Aviv*

The Jerusalem Academy of Music and Dance, Jerusalem, Israel *Corresponding author: e-mail address: [email protected]

Abstract The word “silence” typically refers to the auditory modality, signifying an absence of sound or noise, being quiet. One may then ask: could we attribute the notion of silence to the domain of dance, e.g., when a movement is absent and the dancer stops moving? Is it at all useful to think in terms of silence when referring to dance? In this chapter, my exploration of these questions is based on recent studies in brain research, which demonstrate the remarkable facility of specific regions in the human brain to perceive visually referred biological and, in particular, human motion, leading to prediction of future movements of the human body. I will argue that merely ceasing motion is an insufficient condition for creating a perception of silence in the mind of a spectator of dance. Rather, the experience of silence in dance is a special situation where the static position of the dancer does not imply motion, and is unlikely to evoke interpretation of the intentions or the emotional expression of the dancer. For this to happen, the position of the dancer, while being still, should be held effortlessly, aimlessly, and with a minimal expression of emotion and intention. Furthermore, I suggest that dynamics, repetitive movement (such as that of Sufi whirling dervishes), can also be perceived as silence in dance because of the high level of predictability and evenness of the movement. These moments of silence in dance, which are so rare in our daily lives, invite us to experience the human body from a new, “out of the box” perspective that is the essence of all the arts.

Keywords Human body, Human movement perception, Neuro-aesthetics, Action observation, Stillness, Implied motion

1 Introduction The term silence conveys a complexity of ideas—varying from the absence of an entity or process (lack of sound, lack of thoughts) to a state of extremely active concentration and presence (such as in meditation) (Schwartz, 1999). Therefore, when using the term silence in the context of dance, we need to clarify to which of the Progress in Brain Research, Volume 280, ISSN 0079-6123, https://doi.org/10.1016/bs.pbr.2022.12.009 Copyright © 2023 Elsevier B.V. All rights reserved.

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qualities of silence we are referring to. In other words, we need to characterize which aspects of silence are relevant to the experience of dance. Active concentration means the process of attempting to block out incoming thoughts and concentrate only on one aspect, either physiological—such as one’s own breathing (inhale and exhale) or the mental repetition of some words. The term presence means the concentration of the mind on present sensations (pain, alignment of the body, etc.). This is to distinguish from the state of remembering something from the past or simulating a future situation. Presence also implies awareness, self-awareness, consciousness and alertness (Giannachi and Kaye, 2011; Pini, 2019). Dance, the artistic expression via movement of the human body, can be related to the notion of silence. Dance, as an implicit non-verbal endeavor, operates at times in the realm of silence in the acoustic sense, where no sound is involved as part of the performance (no music, no spoken words, no vocal expression and minimal body sounds). Yet, even under such circumstances, dance is often experienced by spectators as a non-silent event (Orgs et al., 2016; Schwartz, 1999). When dance conveys a certain message, communicates a narrative of an episode, when it presents a series of actions taken by the dancers (such as lifting, jumping, walking) or when transmitting a clear emotional expression by the performers—then dance is unlikely to be perceived as silent (Jaworski, 1992; Schwartz, 1999; Sontag, 1969). Although performed in an acoustic silence, the situations described above correlate with verbal, explicit experiences and are therefore considered non-silent in their nature (Schwartz, 1999). This invokes the questions: In which situations or under what circumstances does the notion of silence relate to dance? In addition, if such a notion does exist in dance, what might it signify to the spectator and to the dancer, and how similar or different might their experiences be? This chapter begins by adapting the term “silence” to the domain of dance, describing the various aspects related to the notion of silence that are relevant to dance, and continues by examining their manifestation in dance. In particular, how silence in dance is related to particular cases where motion has ceased. Next, the brain response to movement and non-moving situations of the human body is briefly discussed. Specifically, the brain’s response to distinct situations such as: stillness; implied motion within a frozen position; retaining a position in the middle of a movement; and to a repetitive movement. I will then discuss what these specific situations of silence in dance might offer to the mind of a spectator, and speculate on the benefits from such moments of silence in dance.

2 What is silence? What could silence in dance represent/be? Many scholars who investigate the phenomenon of silence, avoid defining it, because of its complexity and multi-faceted expressions (see for example, Bindeman, 2017; Catterall, 2005; Jaworski, 1992; Schwartz, 1999; Voegelin, 2010). Yet, they do

2 What is silence? What could silence in dance represent/be?

characterize some properties of silence, explore its nature, and categorize it. Silence can be categorized as absolute or pure—a genuine emptiness. But such pure silence or “emptiness” is an abstract idea as it is not realistic in the physiological world (Schwartz, 1999; Sontag, 1969). What we actually sense and perceive in our daily lives is an impure silence that, in terms of acoustics, is a lack of excessive noise (Schwartz, 1999). In terms of our own experience, it is the silence as we perceive and interpret the sensory (silent) input—such as an experience of the arrest of time, stillness of motion or complete darkness (Alvarez, 2020; Sontag, 1969). Silence can also be categorized as either a passive or an active experience. Passive silence is grasped as empty, quiet or peaceful, and contains no meaning. It might lead to self-reflection. Whereas active silence is experienced as “full” because it contains and conveys a meaning (an extra-linguistic meaning) or a purpose (as in meditation). Active silence directs the participants towards interpretation of the situation (Sontag, 1969). Silence also contributes to the dramatic aspect of the scene, by emphasizing a critical moment (Lissa, 1964; Schwartz, 1999), for instance, the 40 3300 silent piece of music by John Cage (Pritchett, 2009). Silence may also serve as a rest or a preparation for a future stimulus, for example, before the performance begins (Lissa, 1964). Researchers do not necessarily avoid defining a silent art (Lissa, 1964; Withers, 1997). Withers (1997) defines art as silent when it is experienced rather than comprehended (addressed to the spirit vs the mind, as she refers to it). The mind is referred to our thought life and our reasoning powers whereas the spirit is referred to our experiences. According to Withers another prerequisite of silent art is that the time element is removed or when “time is frozen” (see also Sontag, 1969). This is in contrast to non-silent art, which expresses a sequence of thoughts that evolve within time, therefore, according to Withers (1997), time is a prerequisite component for non-silent art. Non-silent art is addressed to our cognition, to our mind (Withers, 1997). Withers adds that the essence of silence in art is centered around the intensity of its concentration on the now (the present), the absence of narrative, the economy of the symbol and harmonious use of the medium materials (such as colors and lines in the case of painting). Other elements, such as ambiguity of the artistic message or scene, might be also related to silent art (Withers, 1997). Art works might be silent in a subtle way: they might make noise, but the meaning of the noise may not be clear or obvious. Or their meaning may not even be present, such as in abstract visual art. Therefore, it is possible for sound in an art work to be silent, in the sense that it has no meaning or that its meaning is obscure, removed from any conventional interpretation; e.g., white noise (Aviv, 2014; Schwartz, 1999; Withers, 1997). Jaworski (1997) introduced the idea of silence as a metaphor for communication, so a frozen gesture of an artist on the stage can be considered as an instance of silence. In light of the above, it seems clear that silence in general, and silent art in particular, can be expressed in many ways. Although the experienced silence can never be pure, we can experience silence in different varieties, ranging from an impure, passive (empty) silence to an impure active communicative and dramatic silence. Forms of silence have different expressions in different media (acoustics, plastic arts, etc.) and they might serve diverse purposes.

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With respect to dance, silence is mentioned in several essays (Hamera, 1990; Main, 2010; Schwartz, 1999; Withers, 1997). Silence in dance is mostly referred to in relation to the acoustic aspect—either the absence of music during the dance piece or the minimal presences of noise made by the dancers, such as during breathing or foot tapping (Schwartz, 1999). But even if no acoustic sound is present during dance, still there has been a claim that the repertoire of steps in ballet are referred to as a vocabulary partly because they contain symbols of meaning. And if they do have a meaning they cannot be considered as being silent. Gestures are considered as a lower level of silence in dance, because they induce the audience to initiate the process of constructing interpretations (Hamera, 1990; Jaworski, 1997; Schwartz, 1999). In summary, the more abstract the dance piece, i.e., the more remote from direct and clear interpretation, then the more silent it can be considered (Aviv, 2017; Schwartz, 1999). However, none of these studies explored the notion of silence in dance in the context of movement. Movement is the medium by which dance is expressed, analogous to sound which is the medium by which music is expressed. Absence of sound is silence in the music domain while absence of movement can be considered as silence in dance domain. One should note that there is always some movement within the body’s muscular and skeletal systems. In order to keep a position muscles contracts in such a way that the limbs/body sway around a base. These postural movements are usually small and executed by reflexes (Adrian and Cooper, 1995). Therefore, I do not use the term cessation of motion but rather I refer to absence of visible motion, which could also be thought as absence of intentional motion. This is similar to music in the sense that silence in music do not necessarily means absolute acoustic silence, but rather a cessation of intentional acoustic signals in music. In this chapter, I examine which situations, in terms of movement, could be considered as silence in dance. Three possible forms of silence in dance will be discussed: stillness—the ceasing of movement at the end of a motion or between two movement phrases (such as standing still after completing a jump); holding a position—the stopping of motion in the midst of a movement, (for instance, the arrest of movement in the middle of stepping, freezing a movement); and the special case of a repetitive ongoing motion (such as the Sufi whirling dervishes; see Cakmak et al., 2017), which might also be considered as a version of silence in dance. These different cases are discussed below with respect to movement perception.

3 The neural basis of the perception of human movement In order to further discuss silence in dance, let’s first look at the perception of human motion. The section below presents the major tools by which we analyze human motion and interpret it. First, I will indicate which brain areas respond to biological motion and which areas respond to human motion specifically. Then, I will present the phenomenon of implied motion perception and the brain areas involved processing this phenomenon. Because time is a major component of movement

3 The neural basis of the perception of human movement

perception, I will highlight difference in the perception of time when looking at real motion versus implied motion. In the final part of this section, I will discuss how people infer emotional states when observing the moving body. There is a consensus among many researchers that a key goal of the brain, and specifically that of the human brain, is to understand motion (see Johansson, 1973; Llina´s, 2002; Sommer and Wurtz, 2008; Zeki, 1993). Indeed, many brain researchers agree that the nervous system evolved primarily to enable animals (including humans) to move through the environment safely. Animals must be able to predict, on the basis of incoming sensory information (visual, auditory, etc.), the outcome of each movement, both their own as well as that of other animals or objects (Blakemore and Decety, 2001; Llina´s, 2002). A few brain areas have been found to be involved in responding to visually referred biological motion (Fig. 1). Based on brain imaging studies, brain connectivity and neural activity during movement, these areas have been identified to include: the STS (ventral and posterior parts of the superior temporal sulcus and the fusiform gyrus); the middle temporal cortex; parietal regions; inferior frontal gyrus; bilateral insula; left lateral cerebellum; and left intraparietal cortex (Blakemore and Decety, 2001; Grezes et al., 2001; Grosbras et al., 2012; Grossman and Blake, 2001; Peelen and Downing, 2007; Saxe et al., 2004; Sokolov et al., 2018). All these brain regions are activated in response to biological motion, and some (e.g., the occipitotemporal brain system) respond specifically to movement of the human body, as was shown in several studies (for reviews, see Blakemore and Decety, 2001 and Peelen and Downing, 2007). These areas do not respond to non-biological motion or to biologically impossible motions (Blakemore and Decety, 2001).

FIG. 1 Brain activity during perception of biological motion. (A–F) Regions showing higher responses for walker-present compared with walker-absent displays are located in the bilateral MTC (A), right STS (B), right FFG (C), left lateral cerebellar lobule Crus I (LCB) (D), right anterior insula (INS) (E), and right IFG (F). (G) Location of the seven network nodes of the cerebrocerebellar network active during biological motion processing. Adapted from Sokolov, A.A., Zeidman, P., Erb, M., Ryvlin, P., Friston, K. J., Pavlova, M.A., 2018. Structural and effective brain connectivity underlying biological motion detection. Proc. Natl. Acad. Sci. U S A 115 (51), E12034–E12042, with permission.

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The human body (in both moving and stationary conditions), as well as its specific parts, induce a selective response in the EBA (extrastriate body area), in the right lateral occipitotemporal cortex and in the fusiform body area (FBE) (Downing et al., 2001; Grosbras et al., 2012; Peelen and Downing, 2005, 2007; Pitcher et al., 2019). Human drawings, in the form of stick figures and silhouettes of humans, also triggered a significant response in the EBA; this led to the conclusion that this area responds not to the surface properties of the body but rather to its representational outline (Peelen and Downing, 2007). Some regions of the human brain are wired to respond to biological motion to such an extent that even a stationary stimulus that implies motion, such as when watching a photograph of a man throwing a ball, induces a response in the visual motion detection areas (the medial temporal visual area, MT/V5 and the medial superior temporal area, MST) and, consequently, leads to a perception of motion (Blakemore and Decety, 2001; Kourtzi and Kanwisher, 2000; Peelen and Downing, 2007). Not every stationary photograph of a person induces a perception of motion; for instance, a man sitting on an armchair is unlikely to elicit a perception of motion. It is important to note that biological motion that is biomechanically possible to implement, is processed in a different path in the brain than a non-biological motion (either of an object or biologically impossible motion) (Blakemore and Decety, 2001). Some lateral brain areas (EBA) respond more strongly to moving vs static bodies, while other ventral brain areas (FBA) do not demonstrate differences in response to moving vs static figure (Pitcher et al., 2019). Although sharing a similar brain path, the time perception of a real motion seems different (overestimated) than time perception of implied motion of human (not overestimated). This was demonstrated by studying dance movements when viewed as real motion as compared with the same dance movements shown as implied motion (Sgouramani et al., 2019). This could be due to the fact that real motion conveys a greater amount of contextual change as compared to implied motion and therefore, is judged as lasting longer. This argues for distinct processing mechanisms or differential involvement of the same brain areas for real versus implied motion (Sgouramani et al., 2019; see also Nather et al., 2013). It is worth noting that implied motion perception is based on action prediction and the understanding of the consequence and the meaning of the upcoming action. In addition, it has been shown that observation of human motion may elicit an automatic attribution of intention to that person’s action, a process which involves the medial prefrontal cortex and the STS (Blake and Shiffrar, 2007; Cole and Millett, 2019; Gallagher et al., 2000; Wellman, 2018). The attribution of intention may also lead to evaluation of that person’s inner state, based on the interpretation of their movement (Cole and Millett, 2019; Schaafsma et al., 2015 and see Fig. 2) In this process, looking at a person’s action may evoke the sensory motor representation of the action, which will lead (automatically and unconsciously) to the estimation of the intention of the person’s action, based on the understanding of self-intention while having similar sensory motor input (Brass and Heyes, 2005; Heyes, 2011). It is worth noting that specific movement characteristics convey certain emotions and influence dance expressions and their interpretations. For example, it was shown

4 Conditions for silence in dance

FIG. 2 Three examples of neural responses to actions at three conceptual levels. Responses to biological motion and goal-directed action in the superior temporal sulcus (STS, green), thinking about people’s beliefs and desires in the temporo-parietal junction (TPJ, blue), and to people’s stable personality traits and preferences in the medial prefrontal cortex (MPFC, red). Adapted from Koster-Hale, J., Saxe, R., 2013. Theory of mind: a neural prediction problem. Neuron, 79 (5), 836–848, with permission.

that patterns of body movement that portrayed a set of 12 emotions (e.g., joy, amusement, interest, ager, panic, anxiety) could be differentiated by the spectators (Dael et al., 2012). In research focusing on dancers’ expression of emotions (Sawada et al., 2003), the dancers were asked to express certain emotions using characteristics of only a particular body part and specific action, such as adduction of the right extended arm. Their expressed emotions were accurately perceived by naı¨ve spectators. Other studies show that spectators accurately perceived the emotional meanings expressed by the dancers, based on the type and various dynamical parameters of the movements such as velocity and frequency (Shikanai et al., 2013; Van Dyck et al., 2017). The neural underpinnings of the perception of emotional states were explored by Bachmann et al. (2018). To summarize. The section above was aimed to emphasize that a significant part of the human brain is specialized (and becomes expert) in analyzing human motion (both actual and implied). Being expert, the spectator automatically tends to interpret the intentions and emotions conveyed by the moving body of the other. The question than arises: are there conditions whereby, albeit such an automatic (and strong) reaction to movement of the human body, the spectator might interpret the movement or stillness as being “silent”?

4 Conditions for silence in dance Based on the above, I maintain that stillness in dance (ceasing movement between two movement phrases), or holding a position (in the midst of a movement) are insufficient conditions for claiming that it corresponds to silence in dance. This is because ceasing movement may invoke an implied movement perception in the

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spectator’s brain. Under this condition, although static, the spectator’s brain may (perhaps very actively) attempt to predict the next movement and/or feel a set of emotions corresponding to the predicted movement. It is therefore hardly justified to relate the term “silence” to such instances of stillness during dance that evoke emotional/predictive responses (see Fig. 3). This means that, in order to create a perception of silence in dance, the choreographer needs to design instances of a unique type of motion/dance that do not invoke a prediction in the spectator’s mind of the next steps that the dancer might take. This means a state of stillness that is not part of a clear action (such as lifting a partner). This requires an abstracted type of movement, which is unlikely to invoke an implied-motion response or to produce referred emotions or intentions to the dancer (for discussion on abstracted art, see Aviv, 2017). This, I claim, are very uniquely moments indeed, that should be carefully designed. An interesting, perhaps somewhat unexpected, possibility of silence in dance are cases of repetitive long-lasting ongoing motion (like the Sufi whirling dervishes). From the spectator’s point of view, watching whirling for a long time offers a situation of a very predictable movement, with little or no accelerations, which might be perceived as an effortless condition, not leading to a specific action other than itself. This is a meditative situation for the Sufi dervishes (Cakmak et al., 2017). Such whirling might also induce a silence-like perception in the spectator’s brain because the

A

B

FIG. 3 A non-implied motion photograph of a dancer (A) vs an implied-motion photograph of a dancer (B). The dancer in panel (A) is situated in a stable position which she could hold for a long time—a situation that does not necessarily lead to prediction of motion (stillness). The dancer could also start moving out of this position in several ways and it is hard to predict the upcoming movement away from this position. The dancer in panel (B) is holding a position in the midst of a movement and her next step is rather easily predicted by the spectator. She also has more than one option to continue moving but the spectator can predict which path is more likely to be taken. Photographed by Vered Aviv, 2013 #.

5 Further elaboration of silence in dance with regards to communication

brain is free to respond to the movement as such, it is a movement not leading to a specific related meaning or intention. A movement that induces a minimal need for interpretation by the spectator’s mind might therefore be associated with silence in dance. In addition to the above considerations, we should also consider another component that relates to the notion of silence in dance, namely, the bodily effort invested in keeping a position (during stillness or in repetitive motion). Humans are very good in visually evaluating the degree of effort applied by others when we watch them move. The quality of movement (or dance) is detected by the spectator who distinguishes whether the movement is performed effortlessly or by using a high degree of muscle power/effort (for details on the brain network involved in force perception, force estimation and the role of the cerebellum in these tasks, see Casiraghi et al., 2019). Note that goals and intentions can only be deduced from the visually kinematic data available to the spectator (Orgs et al., 2016). Indeed, when we watch a person executing a movement or dance, our own motor system is activated in a similar way (via the “mirror system” including the premotor, parietal and cerebellar cortices) and we therefore make a good estimation regarding the amount of force applied by the other when performing such movement (Calvo-Merino et al., 2006; Casiraghi et al., 2019; Orgs et al., 2016; Rizzolatti and Craighero, 2004). Therefore, the circumstances when the dancer applies high significant bodily-effort, either in stillness or in motion, is unlikely to invoke a perception of silence in dance. Muscle power/effort is involved in force perception and force estimation. Force estimation is necessary for understanding the actions of others, predicting appropriate reactions and subsequent interactions. In this context, perceiving the force applied to objects by others is crucial for understanding their intentions, for predicting the success of self-generated actions, and for dynamic movement control in interactions (Casiraghi et al., 2019).

5 Further elaboration of silence in dance with regards to communication The complexity of attributing the notion of silence to dance emerges from the fact that dance is mediated by a human body—a body that, intrinsically, transmits intentions, emotions and ideas which are perceived as such by the spectator (Brass and Heyes, 2005; Calvo-Merino et al., 2006; Casiraghi et al., 2019; Heyes, 2011; Leach, 2013; Orgs et al., 2016; Rizzolatti and Craighero, 2004). Some researchers emphasize that dance is a human socio-cultural activity that involves both the moving dancer and a spectator. They point out that movement in dance serves communicative purpose, that it is expressive by its nature and it acts toward exchanging emotions, intentions and ideas between people, namely, the choreographer, dancer, and audience (Leach, 2013; Orgs et al., 2016). It has been shown that emotional expression is referred to static as well as dynamic human figures, activating the STS, FBA and premotor cortex areas of the spectator (De Gelder et al., 2010). Hence, as such, silence in dance should be bare of movements which (for the spectator) convey

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intentions and emotions. Silence in dance is aiming to minimalize the communication of messages that might lead to association of narrative or high emotional subject matter which, by definition, are typical to many traditional (non-silent) dances.

6 Why pursue silence in dance? Although hard to achieve, going against the “natural” way by which the brain interprets movement and the respective emotions it conveys, I suggest that it is worthwhile to pursue silence in dance. First, it provides the spectator’s brain with a special condition (rarely, if at all, attained in our daily lives), enabling the brain to explore new (unfamiliar) experiences. Furthermore, it shifts the spectator’s mind from their highly habitual way of interpreting the moving human body (with its survival function) to a realm whereby the human body is uninterpretable as a moving entity. There on the stage, although well identified and familiar, the dancer does not provide specific interpretations, predictions or cues for interpretation of expressed emotions. There is silence. Such episode will be based on the highly developed skills of the dancer to optimally transmit the message of the dance as well as precise expression of emotion relevant to silence (Orgs et al., 2016). This unique moment will invite the spectator to wonder and perhaps to generate internal, maybe new, brain states. It may lead to a different perception of time at that moment. Under such uncommon situation(s), the perception of time itself can change, as was shown to be the case by Sgouramani et al. (2019). It is also possible that such situations might lead to meditative-like brain states (e.g., when viewing Sufi whirling). Clearly, non-silent dance may evoke feelings/experiences/patterns of thinking that go beyond the habitual way of interpreting a moving human body. However, it reasonable to assume that such new pattern of thinking/feeling is more likely to be evoked, because the viewers are exposed to the unique situation of silence in dance, whereby no predictions, no emotions and no effort are expressed by the dancer. Such situation is rare indeed (and probably new) for the spectator’s experience and therefore is likely to induce new experiences and perceptions.

7 Conclusions Silence in dance, as in other time-dependent art forms, is a powerful tool for emphasizing and creating critical and dramatic moments during the art performance (Lissa, 1964). In addition, by applying (the rare condition of ) silence in dance one can achieve a spectacular and/or expressive performance (Main, 2010). In this paper, I have shown that silence in dance is expressed by minimal or no movement—such as in stillness, or holding of motion or otherwise via an invariable repetitive movement. A ceasing of motion is not a sufficient condition to create a perception of silence in dance. The absence of implied motion in a static position,

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an effortless movement (or effortlessly kept position), and a minimal expression of emotions and intentions are all necessary conditions for the experience of silence in dance. Such a situation is far from our daily experience of looking at human bodies, understanding their actions, and interpreting their intentions. This situation can be created in the artificial, artistic, controlled realm of dance and might invite us to experience the silence of the human body (and brain) as a sublime emptiness or as a dramatic moment in the artistic domain of dance. This situation, which induces the perception and experience of silence in the spectator brain, is indeed very demanding for the dancer motion-wise and expressively. Yet, in some rare situations, such as the Sufi whirling, the dancer and spectator might share a similar synchronized transcendental experience of silence.

References Adrian, M.J., Cooper, J.M., 1995. Biomechanics of Human Movement. Indiana University, Brown & Benchmark Pub. Alvarez, L.P., 2020. Sound, silence, resonance, and embodiment: choreographic synaesthesia. Idea J. 17 (02), 215–229. Aviv, V., 2014. What does the brain tell us about abstract art? Front. Hum. Neurosci. 8, 85. Aviv, V., 2017. Abstracting dance: detaching ourselves from the habitual perception of the moving body. Front. Psychol. 8, 776. Bachmann, J., Munzert, J., Kr€uger, B., 2018. Neural underpinnings of the perception of emotional states derived from biological human motion: a review of neuroimaging research. Front. Psychol. 9, 1763. Bindeman, S., 2017. Silence in Philosophy, Literature, and Art. Rodopi Publisher, Brill. Blake, R., Shiffrar, M., 2007. Perception of human motion. Annu. Rev. Psychol. 58, 47–73. Blakemore, S.J., Decety, J., 2001. From the perception of action to the understanding of intention. Nat. Rev. Neurosci. 2 (8), 561–567. Brass, M., Heyes, C., 2005. Imitation: is cognitive neuroscience solving the correspondence problem? Trends Cogn. Sci. 9 (10), 489–495. Cakmak, Y.O., Ekinci, G., Heinecke, A., C¸avdar, S., 2017. A possible role of prolonged whirling episodes on structural plasticity of the cortical networks and altered vertigo perception: the cortex of sufi whirling dervishes. Front. Hum. Neurosci. 11, 3. Calvo-Merino, B., Gre`zes, J., Glaser, D.E., Passingham, R.E., Haggard, P., 2006. Seeing or doing? Influence of visual and motor familiarity in action observation. Curr. Biol. 16 (19), 1905–1910. Casiraghi, L., Alahmadi, A.A., Monteverdi, A., Palesi, F., Castellazzi, G., Savini, G., D’Angelo, E., 2019. I see your effort: force-related bold effects in an extended action execution–observation network involving the cerebellum. Cereb. Cortex 29 (3), 1351–1368. Catterall, J.S., 2005. Conversation and silence: transfer of learning through the arts. J. Learn. Through Arts 1 (1), 1–12. Cole, G.G., Millett, A.C., 2019. The closing of the theory of mind: a critique of perspectivetaking. Psychon. Bull. Rev. 26 (6), 1787–1802.

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Dael, N., Mortillaro, M., Scherer, K.R., 2012. Emotion expression in body action and posture. Emotion 12 (5), 1085. De Gelder, B., van den Stock, J., Meeren, H.K.M., Sinke, C.B.A., Kret, M.E., Tamietto, M., 2010. Standing up for the body. Recent progress in uncovering the networks involved in the perception of bodies and bodily expressions. Neurosci. Biobehav. Rev. 34, 513–527. Downing, P.E., Jiang, Y., Shuman, M., Kanwisher, N., 2001. A cortical area selective for visual processing of the human body. Science 293 (5539), 2470–2473. Gallagher, H.L., Happe, F., Brunswick, N., Fletcher, P.C., Frith, U., Frith, C.D., 2000. Reading the mind in cartoons and stories: an fMRI study of ‘theory of mind’ in verbal and nonverbal tasks. Neuropsychologia 38 (1), 11–21. Giannachi, G., Kaye, N., 2011. Performing Presence: Between the Live and the Simulated. Manchester University Press, Manchester, UK. Grezes, J., Fonlupt, P., Bertenthal, B., Delon-Martin, C., Segebarth, C., Decety, J., 2001. Does perception of biological motion rely on specific brain regions? Neuroimage 13 (5), 775–785. Grosbras, M.H., Beaton, S., Eickhoff, S.B., 2012. Brain regions involved in human movement perception: a quantitative voxel-based meta-analysis. Hum. Brain Mapp. 33 (2), 431–454. Grossman, E.D., Blake, R., 2001. Brain activity evoked by inverted and imagined biological motion. Vision Res. 41 (1011), 1475–1482. Hamera, J., 1990. Silence that reflects: Butoh, Ma, and a crosscultural gaze. Text Perform. Q. 10 (53–60). Heyes, C., 2011. Automatic imitation. Psychol. Bull. 137, 463–483. Jaworski, A., 1992. The Power of Silence: Social and Pragmatic Perspectives. Sage Publications. Jaworski, A., 1997. Metacommunicative and metaphorical silences. In: Jaworski, A. (Ed.), Silence: Interdisciplinary Perspectives. Mouton de Gruyter, Berlin, pp. 381–401. Johansson, G., 1973. Visual perception of biological motion and a model for its analysis. Percept. Psychophys. 14 (2), 201–211. Kourtzi, Z., Kanwisher, N., 2000. Activation in human MT/MST by static images with implied motion. J. Cogn. Neurosci. 12 (1), 48–55. Leach, J., 2013. Choreographic objects. J. Cult. Econ. https://doi.org/10.1080/17530350. 2013.858058. Lissa, Z., 1964. Aesthetic functions of silence and rests in music. J. Aesthet. Art Critic. 22 (4), 443–454. Llina´s, R.R., 2002. I of the vortex: From Neurons to Self. MIT press. Main, L., 2010. The spectacle of silence and stillness. In: Pierce, K. (Ed.), Dance & Spectacle, The 2010 SDHS Conference July 9–11, 2010, University of Surrey, U.K. Dance and Spectacle. Society of Dance History Scholars, pp. 169–176. Nather, F.C., Bueno, J.L.O., Bigand, E., 2013. Body movement implied by static images modulates eye movements and subjective time estimation. Psychol. Neurosci. 6 (3), 261–270. Orgs, G., Caspersen, D., Haggard, P., 2016. You move, I watch, it matters: aesthetic communication in dance. In: Obhi, S.S., Cross, E.S. (Eds.), Shared Representations: Sensorimotor Foundations of Social Life. Cambridge University Press, Cambridge, UK, pp. 627–653. Peelen, M.V., Downing, P.E., 2005. Selectivity for the human body in the fusiform gyrus. J. Neurophysiol. 93 (1), 603–608. Peelen, M.V., Downing, P.E., 2007. The neural basis of visual body perception. Nat. Rev. Neurosci. 8 (8), 636–648.

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Pini, S., 2019. Stage Presence in Dance: A Cognitive Ecological Ethnographic Approach. Doctoral dissertation. Macquarie University, Sydney, Australia. Pitcher, D., Ianni, G., Ungerleider, L.G., 2019. A functional dissociation of face-, body-and scene-selective brain areas based on their response to moving and static stimuli. Sci. Rep. 9 (1), 1–9. Pritchett, J., 2009. What Silence Taught John Cage: The Story of 40 3300 . In: The anarchy of silence: John Cage and experimental art, pp. 166–177. Rizzolatti, G., Craighero, L., 2004. The mirror-neuron system. Annu. Rev. Neurosci. 27, 169–192. Sawada, M., Suda, K., Ishii, M., 2003. Expression of emotions in dance: relation between arm movement characteristics and emotion. Percept. Mot. Skills 97 (3), 697–708. Saxe, R., Xiao, D.K., Kovacs, G., Perrett, D.I., Kanwisher, N., 2004. A region of right posterior superior temporal sulcus responds to observed intentional actions. Neuropsychologia 42 (11), 1435–1446. Schaafsma, S.M., Pfaff, D.W., Spunt, R.P., Adolphs, R., 2015. Deconstructing and reconstructing theory of mind. Trends Cogn. Sci. 19 (2), 65–72. Schwartz, L., 1999. Understanding silence: meaning and interpretation. Perform. Res. 4 (3), 8–11. Sgouramani, H., Moutoussis, K., Vatakis, A., 2019. Move still: the effects of implied and real motion on the duration estimates of dance steps. Perception 48 (7), 616–628. Shikanai, N., Sawada, M., Ishii, M., 2013. Development of the movements impressions emotions model: evaluation of movements and impressions related to the perception of emotions in dance. J. Nonverbal Behav. 37 (2), 107–112. Sokolov, A.A., Zeidman, P., Erb, M., Ryvlin, P., Friston, K.J., Pavlova, M.A., 2018. Structural and effective brain connectivity underlying biological motion detection. Proc. Natl. Acad. Sci. 115 (51), E12034–E12042. Sommer, M.A., Wurtz, R.H., 2008. Brain circuits for the internal monitoring of movements. Annu. Rev. Neurosci. 31, 317. Sontag, S., 1969. The aesthetics of silence. In: Styles of Radical Will, 334. Penguin books LTD, London. Van Dyck, E., Burger, B., Orlandatou, K., 2017. The communication of emotions in dance. In: The Routledge Companion to Embodied Music Interaction. Routledge, pp. 122–130. Voegelin, S., 2010. Listening to Noise and Silence: Towards a Philosophy of Sound Art. Bloomsbury Publishing, USA. Wellman, H.M., 2018. Theory of mind: the state of the art. Eur. J. Dev. Psychol. 15 (6), 728–755. Withers, S., 1997. Silence and Communication in Art. In: Jaworski, A. (Ed.), Silence: Interdisciplinary Perspectives. Mouton de Gruyter, Berlin, pp. 351–366. Zeki, S., 1993. A Vision of the Brain. Blackwell scientific publications, Oxford.

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Silence and its effects on the autonomic nervous system: A systematic review

6

Davide Donellia,⁎, Davide Lazzeronib, Matteo Rizzatoc, and Michele Antonellid a

Division of Cardiology, Azienda Ospedaliero-Universitaria di Parma, Parma, Italy Prevention and Rehabilitation Unit, IRCCS Fondazione Don Gnocchi, Parma, Italy c “Humandive”, Pordenone, Italy d Department of Public Health, Reggio Emilia, Italy ⁎ Corresponding author: e-mail address: [email protected]

b

Abstract This systematic review explores the influence of silence on the autonomic nervous system. The Polyvagal Theory has been used as a reference model to describe the autonomic nervous system by explaining its role in emotional regulation, social engagement, and adaptive physiological responses. PubMed, Scopus, PsycInfo, EMBASE, and Google Scholar were systematically searched up until July 2023 for relevant studies. The literature search yielded 511 results, and 37 studies were eventually included in this review. Silence affects the autonomic nervous system differently based on whether it is inner or outer silence. Inner silence enhances activity of the ventral vagus, favoring social engagement, and reducing sympathetic nervous system activity and physiological stress. Outer silence, conversely, can induce a heightened state of alertness, potentially triggering vagal brake removal and sympathetic nervous system activation, though with training, it can foster inner silence, preventing such activation. The autonomic nervous system response to silence can also be influenced by other factors such as context, familiarity with silence, presence and quality of outer noise, and empathy.

Keywords Silence, Autonomic nervous system, Polyvagal theory, Experience sharing, Well-being, Systematic review

Progress in Brain Research, Volume 280, ISSN 0079-6123, https://doi.org/10.1016/bs.pbr.2023.08.001 Copyright © 2023 Elsevier B.V. All rights reserved.

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1 Introduction 1.1 Background Silence, as an absence of sound, is generally perceived to hold a distinct quality that allows for moments of stillness and tranquility in our lives. The effects of silence on our body and mind have gained increasing attention in recent years, as our modern world becomes increasingly filled with noise and constant sensory stimulation (Jariwala et al., 2017). Despite being often undervalued, silence represents an interesting aspect of our environment that has the potential to significantly impact our well-being (Ben-Soussan et al., 2021). In this systematic review, we aim to provide a deeper understanding of the mechanisms through which silence influences our autonomic nervous system activity and, by examining the available scientific literature, we seek to shed light on the potential benefits of silence for promoting well-being. Through an exploration of various physiological markers such as heart rate, blood pressure, hormone levels, and brain activity, we can gain insight into how silence influences the body’s stress response, relaxation processes, and overall physiological balance. In this regard, the Polyvagal Theory provides a robust framework for understanding the response of the autonomic nervous system to external stimuli and how it influences our physiological and emotional states in different situations (Porges, 2011), including in the absence of sounds. In particular, the Polyvagal Theory underscores the importance of safety and cues of danger in shaping our autonomic responses, and it can help to better understand the effects of silence on our body and behavior. After reviewing the available evidence on the topic, we will explore the practical applications of silence in different settings, considering how intentional incorporation of silence may contribute to stress reduction, improved cognitive function, and overall well-being.

1.2 Silence in history and philosophy Silence has played a profound role in both history and philosophy, influencing various cultural, intellectual, and spiritual realms (Sardello, 2011; Ben-Soussan et al., 2021). Throughout the ages, silence has been recognized as a powerful means of contemplation and inner reflection. Esteemed thinkers like Socrates, Plato, and Buddha emphasized its significance in fostering self-awareness and gaining deeper insights into the human experience. In the realm of spirituality and religion, silence holds a deep association. Many faiths and spiritual traditions incorporate silence as a pathway to connect with the divine, attain inner peace, and transcend the limitations of the ego. Practices such as silent meditation, prayer, and contemplation are prevalent across diverse belief systems, including Buddhism, Hinduism, Christianity, and Taoism. Philosophy, too, embraces silence as a vital element in the pursuit of wisdom. Philosophers engage in moments of thoughtful silence to explore profound questions, grapple with paradoxes, and challenge established assumptions. By allowing

1 Introduction

for the suspension of judgment, silence enables new insights to emerge and encourages intellectual growth. Silence also serves as a response to the limits of language and the ineffable aspects of human experience. It acknowledges that certain emotions, experiences, and truths elude adequate expression through words alone. In this way, silence becomes a form of communication, conveying profound meaning beyond linguistic boundaries (Khan and Masud, 2017; Nakane, 2007). Culturally and ritually, silence holds significant value. It is woven into ceremonies, rituals, and communal practices as a symbol of respect, reverence, or commemoration (Flanagan, 1985). Moments of silence during memorial services, national remembrance days, or sacred ceremonies create a space for collective reflection and connection (Ehrenhaus, 1988). Moreover, silence can be a potent form of social and political expression. It can be employed as a means of protest, resistance, or solidarity. Silent vigils, sit-ins, and protests have been used throughout history to raise awareness, challenge the status quo, and give voice to marginalized communities (Lavender, 2022). In essence, silence encompasses a rich tapestry of meanings and applications in history and philosophy. It facilitates introspection, fosters spiritual connection, fuels philosophical inquiry, conveys unspoken truths, carries cultural significance, and serves as a tool for social and political expression.

1.3 Types of silence Silence can be categorized into different types based on various factors (Kurzon, 2007; Paoletti and Ben-Soussan, 2020). Here are some definitions of types of silence which have been proposed: •

•

•

•

Outer Silence: this type of silence refers to the absence of environmental sounds. It occurs in situations where there is a lack of auditory stimuli, such as being in a quiet room, a secluded natural environment, or during the nighttime when the surroundings are typically quieter. Inner Silence: inner or internal silence relates to the absence of mental chatter or inner dialogue. It is a state of quietude within one’s mind, free from racing thoughts, distractions, and mental noise. inner silence is often associated with practices like meditation, mindfulness, and deep relaxation techniques. Social Silence: social silence pertains to the absence of verbal communication or the deliberate choice to refrain from speaking. It can occur during moments of mutual understanding, respect, or when individuals choose not to engage in conversation. Social silence can be observed in situations such as meditation groups, silent retreats, or certain cultural practices. Emotional Silence: emotional silence refers to the absence of emotional expression or the withholding of one’s emotions. It can occur when individuals suppress or hide their feelings, often due to social norms, fear of judgment, or the desire to maintain a composed outward appearance. Emotional silence can manifest as a lack of emotional response or a guarded demeanor.

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•

•

Ritualistic Silence: ritualistic silence is observed in specific cultural, religious, or ceremonial practices where participants voluntarily abstain from speaking or making noise as a form of reverence, contemplation, or spiritual connection. Examples include silent prayer, silent vigils, or specific moments of silence during rituals or ceremonies. Reflective Silence: reflective silence refers to intentional periods of quiet reflection and introspection. It is a time for self-examination, deep thinking, and gaining insight. Reflective silence can be self-imposed or facilitated through activities like journaling, solo nature walks, or dedicated quiet contemplation.

These types of silence can often overlap or intersect, and their experience may vary depending on individual preferences, cultural backgrounds, and situational contexts. Exploring and embracing different types of silence can offer individuals opportunities for self-insight, relaxation, and enhanced well-being. The physiological and psychological effects of silence can exhibit variations in their manifestation and intensity among individuals (Pfeifer and Wittmann, 2020). Each person’s response to silence can differ due to factors such as personal predispositions, sensory sensitivities, psychological states, and contextual factors, as well as the status of the autonomic nervous system, as described by the Polyvagal Theory.

1.4 The human autonomic nervous system, a peak evolutionary achievement: An introduction to the Polyvagal Theory The Polyvagal Theory is arguably the best model to date for describing the overall functioning of the human autonomic nervous system. It emerged as a consequence of Prof. Stephen Porges’s efforts to comprehend the interplay between physiological states and behavior. In the late 1960s, the integration between physiological states with behavior was described with limited models, the main one being the theory of arousal, which was vaguely defined and predominantly associated with the sympathetic nervous system (Porges, 2011). However, this model seemed unsatisfactory and Prof. Porges’s vision was to employ physiological measures as a conduit to understand psychological states, particularly in clinical settings. The first clue in this direction was the observation that attention stabilized heart rate. This observation sparked interest in linking vagal tone to heart rate variability, leading to the development of quantification techniques to characterize rhythms in the beat-to-beat heart rate pattern and validation studies to demonstrate that the amplitude of the respiratory rhythm in heart rate was an indicator of vagal influences on the heart (Porges, 2009). A crucial observation for the development of the Polyvagal Theory was the so-called “vagal paradox,” which refers to the fact that high vagal tone is not always beneficial. In newborns, for example, it can sometimes lead to dangerous bradycardia (Porges, 2011). The Polyvagal Theory, formulated in the early 1980s, introduced the concept of vagal tone and highlighted the importance of the vagus nerve in regulating

1 Introduction

physiological responses. The theory posits that the autonomic nervous system comprises three neural circuits that dynamically interact to influence behavior and physiological states, emphasizing the role of the vagus nerve and the parasympathetic nervous system against the dominant view which considered the sympathetic nervous system as the primary mediator of arousal and stress (Porges, 2007, 2009). In particular, the theory implies that there are two vagal systems, one potentially lethal (the Vegetative Vagus, originating in the Dorsal Motor Nucleus of the Vagus) and the other protective (the Smart or Ventral Vagus, originating in the Nucleus Ambiguus), and introduced the concept of neuroception, which refers to how neural circuits distinguish between safe and dangerous environments without conscious awareness. It also emphasizes how the autonomic nervous system regulates social engagement through the action of the myelinated vagus nerve originating from the nucleus ambiguus (ventral vagus), critical for social communication, including facial expression and vocalization (Porges, 2001, 2003).

1.4.1 The central idea of safety: Neuroception To understand the Polyvagal Theory, it is helpful to start with the central concept of “safety,” which can be explained through the idea of neuroception (Porges, 2009, 2011). Neuroception refers to the process by which our nervous system continuously evaluates cues from the environment to determine whether it is safe, dangerous, or potentially life-threatening. This evaluation occurs at a subconscious level, without conscious awareness (Porges, 2004). Based on these assessments, our nervous system regulates our physiological and behavioral responses. For example, when a person is detected as safe, neurobiologically determined prosocial behaviors are triggered. Conversely, the detection of danger activates neural processes that facilitate adaptive defense behaviors such as fight, flight, or freeze. The concepts of social engagement and defense behaviors are contingent on the level of risk perceived in the environment: these behaviors can be adaptive or maladaptive, depending on the context. An individual’s inability to appropriately regulate these systems, either failing to inhibit defense mechanisms in safe environments or failing to activate them in risky ones, may characterize pathology (Porges, 2011; Porges and Carter, 2017). Accurate neuroception aligns cognitive awareness of risk with physiological responses, while faulty neuroception can lead to maladaptive physiological reactivity and inappropriate defensive behaviors, which are often associated with specific psychiatric disorders. When the nervous system detects safety, metabolic demands adjust accordingly. Stress responses, such as increases in heart rate and cortisol levels, are dampened. Conversely, a neuroception of danger can trigger physiological states that support “freezing” or “shutdown” behaviors, characterized by significant drops in blood pressure and heart rate, fainting, and apnea. If neuroception identifies a person as safe, a neural circuit actively inhibits areas of the brain that organize defensive strategies. However, slight changes in perceived biological movements (vitality forms) can shift neuroception from “safe” to “dangerous,” disrupting prosocial neural systems

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and triggering defensive strategies (Di Cesare et al., 2021; Donelli et al., 2018; Donelli and Rizzato, 2018; Porges, 2011; Stern, 2010). In the presence of a safe individual, the active inhibition of brain areas controlling defense strategies allows for spontaneous social behavior. Conversely, perceived risky situations activate the brain circuits regulating defense strategies, leading to aggressive behavior or withdrawal. The nervous system’s ability to discern environmental safety or danger relies on specific neural mechanisms, with one of its primary actors being the amygdala (Porges, 2004). The amygdalae, small nut-shaped structures found within the medial temporal lobe adjacent to the ventral hippocampus, are complex formations, featuring many nuclei and established connections with numerous other brain regions, thus rendering them some of the most interconnected structures. Functionally, the amygdala is divided into two primary segments: the basolateral amygdala, which facilitates Pavlovian learning, and the central amygdala, integral to attentional processes (Carrere and Alexandre, 2015; Domı´nguez-Borra`s and Vuilleumier, 2022; Fernando et al., 2013). The basolateral amygdala is fed with an array of sensory information from the cortex, due to its associations with the insular, prefrontal, cingulate, and parietal cortices. In contrast, the central amygdala exerts significant influence over the brainstem by projecting to the hypothalamus and brainstem nuclei, including the mesencephalic reticular formation, thus coordinating autonomic, behavioral, and neuroendocrine responses (Pessoa, 2010). The amygdala is primarily activated in response to fear (Bocchio et al., 2016). Its overarching role can be best described as a “sentinel,” identifying the nature of a stimulus and determining responses to threats such as stress hormone release (adrenaline, noradrenaline, cortisol), modulation of the activity of different brain areas (hypothalamus, thalamus, nuclei of the trigeminal and the facial nerves, ventral tegmental area, locus coeruleus, laterodorsal tegmental nucleus, and nucleus accumbens), and initiation of emotional learning processes (these responses can vary from person to person on the basis of subjective experiences and neurobiological factors) (Cunningham and Payne, 2017; West et al., 2021). In situations where the brain struggles to predict the meaning or value of sensations, or the appropriate response to them, especially under conditions of ambiguity, the amygdala steps in, stimulating attention and other brain regions to gather further data until clarity is achieved. Its principal role likely pertains to the emotional processes linked to potential threats. Essentially, when an ambiguous stimulus, whether internal or external, is analyzed by the amygdala, recognized as potentially hazardous, and contextualized, the central amygdala initiates an emotional and autonomic response (Pessoa, 2010; Lindquist et al., 2012). It is crucial to note that the perception of danger or threat can stem from both external (e.g., a dangerous person or situation) and internal/visceral (e.g. fever, pain, or physical illness through interoception) environments (Harricharan et al., 2021; Price et al., 2003; Weiss et al., 2014). This process describes quite precisely the activity referred to as neuroception in the Polyvagal Theory, as the subsequent autonomic responses are evoked in accordance with it.

1 Introduction

The function of “sentinel” is facilitated by the amygdala’s ability to receive sensory information via two pathways: the “low road,” a direct route from the thalamus that bypasses the cortex, delivering swift, albeit generalized, information; and the “high road”, originating from the cortex, which receives information from the thalamus. The latter, while slower, conveys more detailed and comprehensive data (LeDoux, 2012). This dual circuitry enables the amygdala to rapidly process information via the “low road,” priming it to promptly trigger an autonomic response should the “high road” affirm that the sensory stimulus is harmful. This illustrates the amygdala’s correlation with fear, given its responsibility to prepare us for potentially threatening situations in a timely manner. As a result, individuals with a selective amygdala deficiency are unable to experience fear and often find themselves in precarious situations without any awareness (Feinstein et al., 2011). Concerning risk evaluation at a higher cortical level, functional magnetic resonance imaging (fMRI) studies suggest that the temporal cortex plays a significant role in this process (Gosselin et al., 2005; Fiddick, 2011). The fusiform gyrus and the superior temporal sulcus appear to be involved in assessing biological movement and intentionality, via the detection of changes in factors such as movement, vocalizations, and facial expressions—all of which are pivotal in gauging a person’s trustworthiness. Depending on variations in these factors, these two areas can transmit signals of safety or danger, which are relayed to the amygdala through their temporal connections. Therefore, it is plausible that these cortical areas, which regulate the amygdala’s control over the autonomic response via the brainstem (primarily prosocial, fight-flight, or freezing mechanisms), exert a certain degree of control over the amygdala itself (Feinstein et al., 2011; Porges, 2011).

1.4.2 The adaptive responses of the autonomic nervous system Depending on how dangerous the situation is evaluated, there may be three main autonomic responses, which reflect the recruitment of phylogenetically increasingly ancient neuroanatomical structures (Porges, 2011):
Safe situation: high Smart Vagus tone. The activity of the Smart Vagus is also known as “vagal brake,” which refers to the regulatory function of the vagus nerve over heart rate. Under this perspective, a higher “vagal tone” correlates with a slower heart rate, effectively acting as a “brake.” This reflects the body’s adaptive response to environmental stimuli; greater vagal tone promotes a state of calm and social engagement by reducing heart rate, whereas a decrease in vagal tone facilitates an increased cardiac output, readying the organism for fight or flight responses. When the Smart Vagus tone is high, this reflects in a favorable autonomic condition to prosociality (the Social Engagement System): slow heartbeat, high control on phonatory muscles, control on mimic muscles, and connection between orbicular ocular muscles and hot motor system. This occurs in normal safety and tranquility conditions, so that the vagus exerts its “Rest-andDigest Response” function and predisposition to sociality, inhibiting the influence of the sympathetic nervous system on the heart.

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Dangerous situation: Smart Vagus tone reduction (removal of “vagal brake”), and sympathetic system tone increase, that is the condition of preparation to mobilization also called “Fight-or-Flight Response.” In this case, the vagal tone exerting control on the motor aspects necessary for pro-socialization is lost, whereas the sympathetic tone exerting control on the physiology necessary for mobilization is favored (e.g., catecholamine, vasodilation, cardiac output increase) increasing metabolic activity and cardiac output (Asarian et al., 2012; Cardinali, 2017).
Potentially fatal situation: high Vegetative Vagus tone, which leads to immobilization (“Freeze Response”), in other words to the most phylogenetically antique defense mechanism, that is death feigning. In this case, the parasympathetic tone of the vegetative Vagus prevails on the sympathetic tone, leads to immobilization and predisposes to a reduction of metabolic demands (physiological changes such as slowed heart rate and breathing, and decreased blood pressure) necessary to preserve brain oxygenation during death feigning. In this case, parasympathetic tone increase acts on sphincters giving rise to the popular saying “wet his shorts” (Tindle and Tadi, 2022).

1.4.3 The functional anatomy of the vagal system This division into three phylogenetically ordered responses of the autonomic nervous system can resolve the “vagal paradox” because the same neural structure (the vagus nerve) has evolved to serve a dual purpose: both survival and sociality. The vagus nerve has several branches, with up to 80% of its fibers being afferent (Yuan and Silberstein, 2016). It is also lateralized and asymmetrical, with the right vagus being more potent in regulating the heart rate (Sitdikov et al., 2000). The vagus nerve originates from two separate nuclei in the medulla: the dorsal motor nucleus of the vagus (DMNX) and the nucleus ambiguus (NA), the former being the vegetative vagus which manages reflexive regulation of visceral functions, and the latter the smart vagus which involves active processes like attention, motion, emotion, and communication. Each system has separate neuroanatomical characteristics, different origins in terms of ontogeny and phylogeny, and employs diverse adaptive strategies (Baker and Lui, 2022). The distinction between the viscerotropic organization of the two vagal nuclei, DMNX and NA, is critical. The DMNX primarily projects to subdiaphragmatic structures such as the stomach and intestines (Huang et al., 1993). In contrast, the NA projects to supradiaphragmatic structures including the larynx, pharynx, soft palate, esophagus, bronchi, and heart. The NA contains both special visceral efferents (voluntary motor fibers) and general visceral efferents (involuntary motor fibers). The special visceral efferents from the dorsal portion of the NA are involved in motor projections to organs like the larynx, pharynx, and esophagus. The general visceral efferents from the ventral portion of the NA are involved in motor projections to the heart and bronchi (Wang et al., 2001).

1 Introduction

Additionally, the nucleus tractus solitarius (NTS) is located near the DMNX and serves as the terminus for many afferent pathways traveling through the vagus from peripheral organs. These three structures (DMNX, NA, NTS) form the primary central regulatory component of the vagal system (Porges, 2001, 2011). The Polyvagal Theory proposes that the different branches of the vagus nerve have distinct roles in regulating visceral functions. For instance, during orienting reflexes, there could be an increase in vagal outflow from one branch (dorsal vagus) to produce bradycardia and a withdrawal of vagal outflow from another branch to suppress respiratory sinus arrhythmia (smart vagus). Respiratory Sinus Arrhythmia (RSA) is a naturally occurring variation in heart rate that occurs during a breathing cycle. During inhalation, the heart rate typically increases, and during exhalation, the heart rate decreases. RSA is a measure of vagal tone, often utilized as a non-invasive method to assess cardiac and autonomic nervous system health. The NA forms part of a shared neuronal network that generates a cardiorespiratory rhythm, implying that the output from the NA branch of the vagus that ends on the sinoatrial node of the heart shares a frequency common to both respiratory and cardiac systems. In contrast, the output from the dorsal motor nucleus lacks a respiratory rhythm. From this observation stems the idea that the functional output of the NA vagus on the heart can be tracked by respiratory sinus arrhythmia (RSA). This implies that the concept of vagal tone may not be generalized to all vagal efferent pathways, but may need to be limited to a specific branch or subsystem of the vagus being evaluated (Porges and Lewis, 2010; Porges, 2011). Therefore, the vagal system is not monolithic, but rather comprises different components, including general visceral efferent fibers controlling smooth and cardiac muscle, and special visceral efferent fibers that regulate the somatic muscles of the larynx, pharynx, and esophagus. These muscles have crucial roles in vocalization, sucking, and swallowing, and they synchronize these processes with respiration. Additionally, the vagal system has neuroanatomical connections to source nuclei responsible for controlling facial expressions, mastication, and head turning.

1.4.4 The repurposing of ancient survival systems for social engagement It is necessary to spend a few more words regarding the third autonomic response related to immobilization. In fact, immobilization as a “freezing” strategy is highly beneficial for survival even if it can trigger potentially lethal physiological changes, such as a dramatic slowing of heart rate, cessation of breathing, and a drop in blood pressure. However, immobilization itself is also crucial in prosocial activities essential to mammalian life, including conception, childbirth, nursing, and social bonding. These activities necessitate a degree of immobilization, such as a mother restraining movement while nursing or the functional immobilization of a child during an embrace. Evolution has gradually repurposed neural circuits initially involved in freezing behaviors to serve intimate social needs. Over time, these brain structures developed receptors for oxytocin, a neuropeptide released during childbirth, nursing, and activities that establish social bonds (Kendrick, 2000; Neumann, 2008). The release of

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oxytocin in a perceived safe environment allows us to enjoy the comfort of an embrace without fear. However, if our nervous system identifies someone as dangerous, even with the release of oxytocin, we resist the attempted embrace. This understanding of the freeze response and its role in both defense and prosocial behaviors provides an important perspective on human behavior and physiological responses (Porges, 2011, 2021). Mammals, particularly primates, have developed brain structures that regulate both social and defensive behaviors. This evolution has been driven by the need to distinguish friend from foe, evaluate environmental safety, and communicate within social units. As the vertebrate nervous system became more complex, needing to respond not only to threats to survival or reproduction but also to sociality, its affective and behavioral repertoire expanded, leading to a nervous system that enables humans to express emotions, communicate, and regulate bodily and behavioral states. According to the Polyvagal Theory, the formation of positive attachments and social bonds need appropriate conditions, as follows: the presence of three distinct neural circuits supporting social engagement behaviors, mobilization, and immobilization are essential; the nervous system must evaluate environmental risk and regulate adaptive behavior to match the perceived safety or danger of an environment, independent of conscious awareness; perception of safety is necessary before social engagement behaviors can occur, accompanied by the physiological benefits of the activation of the social engagement system; immobilization without fear is required to allow social behaviors associated with nursing, reproduction, and the formation of strong pair bonds; oxytocin must be released to enable immobilization without fear by blocking defensive freezing behaviors (Porges, 2011, 2021).

1.4.5 Face, eyes, hearing, and touch: The doors to social engagement and coregulation The face, hearing, and touch are of particular importance in modulating the state of autonomic activation, through neuroception. The observation of others’ faces, which conveys the majority of information about visceral and emotional states, the act of hearing others’ voices and their tone and inflection, and the observation of others’ vitality forms that provide clues about intentions and internal states are crucial for accurate neuroception. “Vitality forms” is a term coined by Daniel Stern to describe the dynamic nature of social exchanges (Stern, 2010). These forms, evident in both verbal and physical interactions, reflect the underlying emotional state and are characterized by how we move and speak. They are determined by five aspects: Movement, Time, Force, Space, and Intention, which together represent our holistic experience of an action. Recent neuroscientific studies have found a correlation between vitality forms and specific brain structures, further validating this concept (Di Cesare et al., 2021). In fact, only when a human interaction is recognized as safe the process of co-regulation can take place.

1 Introduction

Co-regulation is understood as a reciprocal process where individuals are interconnected in their emotional and physiological states. This is particularly relevant in the context of social engagement, where the physiological state of one individual can influence and be influenced by the state of another (Rizzato and Donelli, 2014; Seth and Friston, 2016; H€ ubner et al., 2021). Co-regulation is achieved through a two-way exchange of social and biological signals, primarily mediated by the autonomic nervous system as the effector, and by the mirror neurons system as the perceptor (Donelli and Rizzato, 2018). For instance, the presence, voice, and actions of a calming person can soothe someone who is anxious or upset, slowing down their heart rate and helping them feel safer. Other contexts can induce affective misalignment: during parent-infant interactions, when the overall arousal level of the dyad was high, parents responded to elevated arousal in the child by decreasing their own arousal, thereby helping to regulate the infant’s affective state (Wass et al., 2019). On the one hand, the human face, with its intricate musculature and innervation, plays a crucial role in conveying emotional states and facilitating social interaction, thus being fundamental to assess if other people may represent a menace or not. The lower part of the face is primarily controlled by voluntary motor pathways, while the upper part is regulated by involuntary motor pathways, which are more closely linked to the autonomic nervous system and internal states (Porges, 2011, 2021). This dichotomy implies that the upper part of the face, being less susceptible to voluntary control, provides a more reliable indicator of an individual’s visceral state, thus allowing co-regulation or defense. Two distinct types of facial motor control can be identified: a “hot motor control system” for the involuntary part, and a “cold motor control system” for the voluntary part (Keysers, 2011). The cold motor control system, which comprises the primary motor cortex and premotor cortex, is involved in instinctive actions such as chewing and articulating sounds. Conversely, the hot motor control system, which encompasses regions surrounding the medial cingulate sulcus, is responsible for emotional involuntary motor behaviors like laughter or expressions of disgust. Both these systems converge on the brainstem, specifically on the nuclei of cranial nerves that control facial, masticatory and phonatory muscles, and regulate the parasympathetic control on the heart and bronchi (Porges, 2004, 2011, 2021). Despite converging on the same nuclei, these systems carry different motor programs, as they are coded by different areas. This distinction is evident in certain patients who, due to lesions in either the cold or hot motor control system (e.g., in Parkinson’s disease and in ischemic stroke), are unable to produce voluntary or involuntary expressions, respectively (Keysers, 2011). Touch also plays a role in neuroception, allowing us to identify the interaction as safe or dangerous based on the vitality forms perceived through tactile stimuli. However, this modality is more typical in infants, where touch is a key element in facilitating the proper maturation of the autonomic nervous system (which is still not completely myelinated) through co-regulation with the mother (Rattaz et al., 2022). With the maturation of the autonomic nervous system, neuroception relies more on the senses of vision, hearing, and interoception, while tactile interaction

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takes on a predominantly role in social bonding and stress coping (Crucianelli and Filippetti, 2020; Hauser et al., 2019). Hearing plays a crucial role in the assessment of the safety of the environment as well: from a phylogenetic perspective, it is the most important sense for defending against potential dangers and predator aggression. For mammals, including humans, sound waves from the environment impact the eardrum and are then transferred to the inner ear via these ossicles (Ugarteburu et al., 2022). The ossicular chain in land mammals primarily serves to reduce the impact of low-frequency sounds through bone conduction. The contraction of two muscles, the stapedius and the tensor tympani, which are innervated by branches of the facial and trigeminal nerves respectively, can further attenuate sounds. This contraction reduces the compliance of the ossicular chain, damping the amplitude of low-frequency sounds reaching the inner ear from the environment: a tightened ossicular chain reduces eardrum movement, and only higher frequencies impacting the eardrum are conveyed to the inner ear and brain’s auditory processing areas (Fournier et al., 2022). This results in a significant reduction in the perception of low-frequency sounds, which facilitates the extraction of higher frequency sounds associated with human voice and other mammalian vocalizations (Porges, 2004, 2011; Porges and Lewis, 2010). The Polyvagal Theory lays special emphasis on the phylogenetic transformations in the neural governance of the autonomic nervous system, noting the evolutionary convergence of this regulatory shift with the control of the middle ear muscles to facilitate mammalian vocal communication. During defensive states where the middle ear muscles remain relaxed, the categorization of acoustic stimuli is predicated upon intensity. Conversely, during safe social engagement states, acoustic stimuli prioritization is based on pitch. In these safe states, the amplification of frequencies linked to conspecific vocalizations is selective, leading to the attenuation of other frequencies (Porges, 2004, 2011; Porges and Lewis, 2010). On the other hand, during defensive states, the intense, low-frequency sounds that signal a danger (e.g., a predator) can be more readily detected, whereas the subtle, high frequencies of conspecific vocalizations tend to get lost within the background noise. With regard to social engagement behaviors, the social engagement system acts a shift in the autonomic state to suppress sympathetic activity and enhance parasympathetic tone (smart vagus), whilst concurrently augmenting the neural tone to the striated muscles of the face and head. This includes facial expressions, increased “emotional” cueing of the eyes associated with more extensive eye contact, amplified prosody, and the enhancement of auditory discernment through the contraction of the middle ear muscles. During social interactions, the stiffening of the ossicular chain actively modifies the transfer function of the middle ear, dampening low-frequency sounds and thereby boosting the ability to discern conspecific vocalizations. Nonetheless, from a phylogenetic perspective, this selective attention towards conspecific vocalizations carries a cost, making it more challenging to detect the lower acoustic frequencies generated by predators. Therefore, the recognition and creation of safe environments (for example, nests or homes) assume a pivotal role in enabling the social engagement

2 Methods

system to foster prosocial behavior (Porges, 2004, 2011; Porges and Lewis, 2010). This mechanism of selective hearing depending on the neuroception of the environment is crucial for understanding how silence can interact with the autonomic nervous system.

2 Methods 2.1 Literature search methods The study protocol of this systematic review was registered in the Open Science Framework (OSF) under the following DOI:10.17605/OSF.IO/Q2XRY. PubMed, Scopus, PsycInfo, and EMBASE were systematically screened to find studies about the effects of silence on autonomic nervous system activity. Google Scholar was also searched to scan the so-called “gray literature” (doctoral theses, conference proceedings, technical reports, etc.). The search was conducted on July 16th, 2023. No restrictions were imposed in terms of language and publication date. The following search strategy was applied for PubMed: ("silence"[Title/ Abstract] OR "silent environment"[Title/Abstract] OR "silent space"[Title/Abstract] OR "silent room"[Title/Abstract] OR "silent meditation"[Title/Abstract] OR "no sound"[Title/Abstract] OR "soundless*"[Title/Abstract] OR "absence of sound" [Title/Abstract] OR "absence of noise"[Title/Abstract] OR "noiseless*"[Title/Abstract]) AND ("autonomic"[Title/Abstract] OR "Heart Rate Variability"[Title/Abstract] OR "stress hormone*"[Title/Abstract] OR "skin conductance"[Title/Abstract] OR "vagus" [Title/Abstract] OR "vagal"[Title/Abstract] OR "polyvagal"[Title/Abstract] OR "parasympathetic"[Title/Abstract] OR "sympathetic"[Title/Abstract]). Analogue search strategies were used to screen the other scientific databases. The PICOS criteria applied for study inclusion were the following: •

•

• •

Population (P): healthy subjects or patients with a neuropsychiatric condition whose pathomechanisms can help explain the effects of silence on the autonomic nervous system (i.e., neurodegenerative diseases, stroke recovery, misophonia, auditory impairment, etc.). Laboratory studies were also considered, but only secondarily, to collect any relevant preclinical evidence on the topic. Intervention (I): exposure to outer silence, which refers to the absence of external sounds, or experience of a state of inner silence through meditation, namely a peace of mind when thoughts subside (Paoletti and Ben-Soussan, 2020). Studies about the physiological effects of activities performed in silence were also briefly described to get the whole picture. Control (C): any type of control, including no comparison (pre-post studies or case reports). Outcomes (O): all valid measures to estimate autonomic nervous system activity, including heart rate (and its variability), blood pressure, respiratory frequency, stress hormone levels, and changes in skin conductance. Studies about the other effects of silence on the body and mind were briefly mentioned too for the sake of completeness.

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•

Study type (S): any type of clinical and physiological studies, preferably controlled and with a full description of their methods. The bibliography of literature reviews was searched for relevant studies through snowballing.

All research items retrieved through the database search were screened with the help of a dedicated software (EndNote Program). The following data were extracted from each study eligible for inclusion: characteristics of the participants, type of silence (outer or inner), comparison, physiological outcomes, and study design. In order to account for their methodological quality, it was checked whether the included clinical studies had a full description of their study protocol following the recommendations of the EQUATOR network (https://www.equator-network.org/). Since the evidence from relevant studies was expected to be quite heterogeneous and scant, no quantitative synthesis was planned. Therefore, the most important findings of the included studies were qualitatively summarized and critically discussed, with a keen focus on the effects of silence on the autonomic nervous system activity as explained by the Polyvagal Theory.

3 Results 3.1 Description of the literature findings Numerous scientific studies have investigated the effects of silence on the body and mind, shedding light on its impact on both physiological and psychological realms. In the following passage, we provide an overview of these effects, encompassing various aspects of human well-being influenced by the presence of silence. We will then focus on the nervous system activity. The literature search yielded 511 results, and 37 studies eventually met the criteria for inclusion in this review. The details of the article screening process are described in a flow diagram (Fig. 1). A summary of the most important findings of the included studies is reported in Tables 1–3. In particular, Table 1 describes the results of physiological studies about both outer and inner silence (Arslan et al., 2022; Bernardi et al., 2006, 2009; Blase and van Waning, 2019; Cysarz and B€ussing, 2005; Dillman Carpentier and Potter, 2007; Dousty et al., 2011; Harada et al., 2017; Hilz et al., 2014; Hoshi et al., 2022; Kirste et al., 2015; Perez-Lloret et al., 2014; Philips et al., 2019; Selvaraj et al., 2008; Sharma et al., 2015; Siragusa et al., 2020; Tayade and Tucker, 2022; Trivedi et al., 2020; Ukaegbe and Tucker, 2022; Vishnubhotla et al., 2021), Table 2 shows the findings of studies about the effects of different activities performed in a silent environment (Bellamy et al., 2021; Desai et al., 2015; Hasegawa et al., 2004; Khalfa et al., 2003; Labbe et al., 2007; Proverbio et al., 2015; Radun et al., 2021; Song et al., 2021; Voisin et al., 2006), and Table 3 reports the effect of exposure to outer silence in patients with different health conditions (Akdemir et al., 2010; Cotoia et al., 2018; De Haro and Bleda, 2019; FerrerTorres and Gimenez-Llort, 2021; O’Kelly et al., 2013; Leonard et al., 2021; Pin˜eros et al., 2023; Suedfeld and Hare, 1977).

3 Results

Records identified through database searching (n=511) PubMed (n=121) PsycINFO (n=27) EMBASE (n=167) Scopus (n=191) Google Scholar (n=5, after a pre-screening of 100 records)

Duplicates removed (n=224)

Records screened by title and abstract (n=287)

Studies excluded (n=240)

Full-text articles assessed for eligibility (n=47)

Studies excluded after a full-text assessment (n=10)

Studies included in the review (n=37)

FIG. 1 Details of the article screening process. The reasons for exclusion of studies after the full-text assessment were the following: irrelevant topic (n ¼ 2), no outcomes of interest (n ¼ 4), and absence of a group exposed to silence (n ¼ 4).

Among the physiological studies about the effects of outer silence, the number of participants ranged from 10 to 62, exposure to silence lasted on average for a few minutes (minimum: 1 min—maximum: 10 min) and it was mostly compared with the effects of listening to different types of music (Table 1). Particularly, in all but two studies (Tayade and Tucker, 2022; Ukaegbe and Tucker, 2022), silence was the control condition, whereas the intervention implied listening to specific sounds or music (Table 1). In two studies, it was underscored that resting in complete silence for some minutes can trigger tinnitus in around half of the people, suggesting that short-term auditory sensory deprivation can sometimes lead to the perception of inner-generated sounds (Tayade and Tucker, 2022; Ukaegbe and Tucker, 2022). Moreover, physiological studies have shown that the absence of sounds actually triggers a heightened state of alertness within the auditory cortex (Voisin et al., 2006). In the experiment conducted by Voisin and colleagues, two frontal activations (right dorsolateral prefrontal and inferior frontal) were first found (regardless of the side where sound was searched for), and this is compatible with the well established role of these regions in

117

Table 1 Summary of physiological studies included in this systematic review (part 1): scientific evidence about the effects of outer or inner silence on nervous system activity in healthy subjects or laboratory models. Population

Intervention

Comparison

Outcomes

Refs.

Outer silence 62 Healthy subjects (10 M, 52 F; 18–35 years old)

Exposure to silence for 10 min

None

Ukaegbe and Tucker (2022)

58 Healthy subjects (58 M, 0 F; 18–35 years old) 32 Healthy subjects (M/F ?; 19–24 years old)

Exposure to silence for 10 min Exposure to silence for two periods of 30 sec

None

30 Healthy subjects (18 M, 12 F; 18–34 years old)

Exposure to silence for 3 min

Listening to classical or another type of music

25 Healthy subjects (?)

Exposure to silence for 3.5 min

Listening to classical, new age or romantic melodies

25 Healthy subjects (?)

Exposure to silence for around 1 min

Listening to different types of music

63% Of the participants perceived tinnitus while sitting in silence, and, of these, 95% reported it within 5 minutes. 41% Of the participants perceived some type of tinnitus during or after 10 minutes of silence. The bandwidth of the polarization and depolarization of heart rate and R-wave amplitude increased in response to music in comparison with silence. Only classical music can reduce sympathetic nervous activity, while silence or other types of music do not have this effect. There are types of music capable of modulating the autonomic nervous system activity in a more pronounced way than silence, especially in terms of relaxation and vagal stimulation. Sympathetic activity was greater during music processing than during silence.

25 Healthy subjects (10 M, 15 F; 18–45 years old)

Exposure to silence for 5 min

Listening to classical music composed by Rossini, Stravinsky, Satie, and Minkus

12 Musicians + 12 healthy controls (?)

Exposure to silence for 5 min

Listening to different types of music

Listening to arousal or relaxing music

The brain tissue pulsatility significantly decreased with relaxing music compared to silence. The heart rate and skin conductance, but not HRV, were also reduced with relaxing music. Music, especially in trained subjects, may first concentrate attention during faster rhythms, then induce relaxation during slower rhythms or silence.

Tayade and Tucker (2022) Dousty et al. (2011)

Harada et al. (2017) Perez-Lloret et al. (2014)

Dillman Carpentier and Potter (2007) Siragusa et al. (2020)

Bernardi et al. (2006)

24 Healthy subjects, including 12 musicians (8 M, 16 F; 23–27 years old)

Exposure to silence for 5 and 2 min before and after listening to the music

Listening to classical pieces composed by Beethoven, Puccini, Bach, and Verdi

Unlike some classical music, silence showed progressive reduction in heart rate and other physiological variables (respiration, blood pressures, middle cerebral artery flow velocity, and skin vasomotion). During silence, autonomic modulation was lower - but showed sympathetic predominance - in older than younger subjects.

Bernardi et al. (2009)

10 Young (22.8  1.7 years old) + 10 older (61.7  7.7 years) subjects (10 M, 10 F) 17 Healthy subjects (6 M, 11 F; 21.9  0.4 years old) 10 Healthy subjects (4 M, 6 F; 27.7  2.9 years old)

Exposure to silence for 5 min

Listening to relaxing or aggressive music

Exposure to silence for several 10-min intervals

Listening to classical music composed by Bach or Mozart Listening to music, audiobook, or white noise

Unlike silence, some classical music can increase vagal tone and suppress sympathetic nervous system activity. The HRV analysis revealed no significant difference among the explored conditions in both multiple sleep latency and maintenance of wakefulness tests.

Hoshi et al. (2022)

Silent meditation

Didgeridoo sound meditation

Philips et al. (2019)

36 Healthy subjects (0 M, 36 F; 20–60 years old)

Silent meditation

Non-silent active meditation

20 Mental health therapists (M/F ?; 52 years old on average) 13 Trained meditators + 4 healthy subjects (10 M, 7 F; 18+ years old)

Shamatha quiescence meditation

No meditation

The participants who practiced sound meditation experienced significantly more relaxation than those who meditated in silence. Non-silent active meditation can improve the participants’ mood and parasympathetic nervous system activity, while these effects are not observed after silent meditation. Regular practice of Shamatha meditation was associated with reduced cortisol levels, sympathetic activity, and increased vagal tone.

Samyama silent meditation

No meditation

The silent meditation program favorably increased the resting-state functional connectivity between the salience and default mode networks.

Vishnubhotla et al. (2021)

Inner silence 74 Healthy subjects (16 M, 48 F; 18+ years old)

Silent resting in different parts of the day

Hilz et al. (2014)

Arslan et al. (2022)

Trivedi et al. (2020)

Blase and van Waning (2019)

Continued

Table 1 Summary of physiological studies included in this systematic review (part 1): scientific evidence about the effects of outer or inner silence on nervous system activity in healthy subjects or laboratory models.—cont’d Population

Intervention

Comparison

Outcomes

Refs.

9 Healthy subjects, including a Zen master (5 M, 4 F; 43  7 years old) 8 Trained yoga practitioners (5 M, 3 F; 26  2 years old) A Himalayan spiritual master aged 81 years old

Zen meditation

None

Cysarz and €ssing Bu (2005)

Silent resting

Isha yoga, including “AUM” chanting

Silent meditation

None

Zen meditation was associated with significantly lower heart rate and respiratory frequency, as it happens with vagal activation and sympathetic inhibition. During sound meditation (“AUM” chanting), there was an increase in sympathetic activity, which ceased during silent resting. Silent meditation can increase the activation of the ventral vagus over the sympathetic nervous system.

Silence (isolation from all sounds)

Exposure to white noise, previously recorded pup calls or classical music

If compared with different acoustic stimuli, silence can trigger neuronal regeneration in mice.

Kirste et al. (2015)

Other studies Laboratory mice

F ¼ Females; HRV ¼ Heart Rate Variability; M ¼ Males; Ref. ¼ Bibliographic reference. The studies are ordered on the basis of their sample size.

Selvaraj et al. (2008) Sharma et al. (2015)

Table 2 Summary of studies included in this systematic review (part 2): scientific evidence about the effects on the nervous system of activities performed in a silent environment.

Population 38 Autistic + 37 neurotypical adults 59 Healthy adults (workers) 56 Healthy adults (college students)

Activity performed in a silent environment

Control condition

Outcomes

Refs.

Making a shopping decision

Social and non-social sound conditions

Bellamy et al. (2021)

Performing different tasks requiring concentration Coping with stress

Noise or speech

Silence eased decision-making in both autistic and neurotypical adults. However, no differences were found in terms of heart rate variability. Compared to silence and noise, working during speech was more annoying and led to increased sympathetic activity. Listening to self-select or classical music, after exposure to a stressor, significantly reduced physiological arousal compared to listening to heavy metal music or sitting in silence. More efficient and faster recall of faces occurred under conditions of silence or when participants were listening to emotionally touching music. Post-exercise recovery was enhanced by slow music, while this effect was not observed with fast music or silence.

 Labbe et al. (2007)

In the presence of music, salivary cortisol levels ceased to increase after the stressor, whereas it continued to increase for 30 min in the silent condition. Viewing a forest scene in silence can help increase parasympathetic activity, but doing it with natural environmental sounds has a more pronounced effect on relaxation. When trying to detect a sound emerging from silence, different cortical (frontal cortex, temporal and parietal lobe) and subcortical (left thalamus and caudate nuclei bilaterally) areas are activated. The mental task was associated with increased sympathetic activity. Listening to the music helped decrease stress levels, while silence did not have this effect.

Khalfa et al. (2003)

54 Healthy adults (university students)

Doing a face memory test

30 Healthy subjects, both adolescents and adults 24 Healthy subjects (students)

Post-exercise recovery following moderate exercise

Listening to different types of music Listening to emotionally touching music or the sound of rain Listening to slow or fast music

Recovery from a psychologically stressful task Viewing the image of a forest

Listening to relaxing music

11 Healthy adults

Detecting a sound emerging from silence

None

8 Healthy adults

Performing a mental calculation task

Listening to orchestral music (Bolero)

20 Healthy adults (university students)

Listening to forest sounds

Ref. ¼ Bibliographic reference. The studies are ordered on the basis of their sample size.

Radun et al. (2021)

Proverbio et al. (2015) Desai et al. (2015)

Song et al. (2021) Voisin et al. (2006)

Hasegawa et al. (2004)

Table 3 Summary of clinical studies included in this systematic review (part 3): scientific evidence about the effects of outer silence in patients affected by health conditions. Population

Intervention

Comparison

Outcomes

Design

Refs.

60 Patients with peripheral arterial disease + 30 healthy subjects

Exposure to silence for 10 min

De Haro and Bleda (2019)

Waiting in a silent environment for 30 min

RCT

Cotoia et al., 2018

33 Concussed + 51 nonconcussed athletes

Exposure to silence for 30 min

CS

onard Le et al., 2021

23 Patients with schizophrenia + 23 healthy subjects

Exposure to silence

In all participants, flow-mediated arterial dilation significantly increased 10 min after listening to the music, but this effect was not observed after silence. Tibetan music helped decrease stress-related sympathetic activation, while silence was not associated with this benefit. The skin conductance results showed greater and faster post-stress recovery after listening to the music compared with silence for concussed athletes. Music-to-silence changes in skin conductance were more pronounced in healthy subjects than in patients with schizophrenia.

RCT

60 Patients waiting for urologic surgery

Listening to classical music composed by Go´recki Listening to Tibetan music

CS

Akdemir et al. (2010)

21 Patients in vegetative or minimally conscious states + 20 healthy subjects

Exposure to silence for 5 min

Brain activity, measured with EEG, was enhanced during listening to preferred music. The same was not observed with silence.

CS

O’Kelly et al., 2013

24 Patients with misophonia assessed before and after the COVID-19 confinement period 27 Patients with COVID-19 (Sars-Cov-2 infection)

Exposure to silence while invited to imagine triggering sounds

Pre-post study

FerrerTorres and nezGime Llort (2021)

Exposure to silence followed by music

Listening to the music followed by silence

Crossover RCT

Pin˜eros et al., 2023

16 Patients with snake phobia

A 5-h experience in a silent dark room followed by viewing slides of snakes

Viewing slides of snakes

An increase in physiological arousal after the confinement period was identified in the study participants, even when exposed to silence and simply invited to imagine misophonia-triggering sounds. Intervention activated the parasympathetic nervous system, while comparison was associated with a greater activation of the sympathetic nervous system. The experimental group showed significant reductions in heart-rate acceleration and in heightening of skin conductance when viewing slides of snakes the day after the session were significantly correlated.

CS

Suedfeld and Hare (1977)

Listening to classical music composed by Mozart Listening to classical Turkish music or white noise Exposure to preferred music, disliked music, or white noise Exposure to triggering and non-triggering sounds

CS ¼ Controlled Study; RCT ¼ Randomized Controlled Trial; Ref. ¼ Bibliographic reference. The studies are ordered on the basis of their sample size.

3 Results

attentional control. Thereafter, an increased activity of the superior temporal cortex was detected, contralateral to the side where sound was expected to be present. The area extended from the vicinity of Heschl’s gyrus to the surrounding areas (planum temporale/anterior lateral areas). This heightened state can be understood as the brain’s natural inclination to search for auditory input, as if it anticipates and seeks out sounds even when none are present. This finding is consistent with the Polyvagal Theory, as previously discussed. In line with this evidence, studies about the effects of silence compared with some types of music on the autonomic nervous system indicate that only classical, relaxing or new-age music can reduce sympathetic nervous activity and improve the ventral vagal tone, while silence is not associated with these effects (Harada et al., 2017; Hoshi et al., 2022; Perez-Lloret et al., 2014; Siragusa et al., 2020). Additionally, it was observed that the sympathetic predominance during silence is more pronounced in elderly subjects than in younger individuals (Hilz et al., 2014). On the other hand, music with a faster rhythm can actually increase the sympathetic tone more than silence (Bernardi et al., 2006, 2009; Dillman Carpentier and Potter, 2007; Dousty et al., 2011), as it probably induces an increased state of alertness. These effects are observed both in experienced musicians and in other people (Bernardi et al., 2009). A possible explanation for this phenomenon from a polyvagal perspective may lie in the fact that some types of relaxing or classical music have a greater affinity with a specific type of vocal prosody typical of soothing social interactions, while silence generally favors a state of alertness, which is even more pronounced with non-relaxing music genres. In fact, any external stressor is perceived (neuroception) as a potential threat, and silence creates the condition in which the middle ear focuses on low frequencies that could potentially originate from threats. Conversely, relaxing music, by simulating human vocal prosody (such as a mother’s voice to a child), partly adjusts the middle ear’s perception towards higher frequencies, reducing sensitivity to low frequencies that could be perceived as threatening and thus stressful. Additionally, relaxing music partly increases ventral vagal tone, enhancing control over heart rate and respiration, indirectly reducing stress levels. Among the physiological studies about the effects of inner silence, the number of participants ranged from 1 to 74, and silent meditation was either evaluated by itself or compared with other forms of meditation involving mantra chanting or listening to specific sounds (Table 1). In general, silent meditation was not found to be associated with enhanced ventral vagal tone (Philips et al., 2019; Trivedi et al., 2020), apart from cases of well-trained people who can more voluntarily control these effects regardless of the specific meditation technique (Blase and van Waning, 2019; Cysarz and B€ ussing, 2005; Sharma et al., 2015; Vishnubhotla et al., 2021). In fact, in non-experienced individuals, sound or non-silent meditation can increase parasympathetic activity, while these changes are not observed when meditation is performed in silence (Philips et al., 2019; Trivedi et al., 2020). Additionally, in trained meditators, mantra chanting can lead to some degree of sympathetic activation, whereas meditating in silence can revert these effects (Selvaraj et al., 2008) and even induce a reduction in cortisol levels (Blase and van Waning, 2019). In line with this

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evidence, surveys reported in the scientific literature indicate that those who are more experienced with meditation techniques often prefer practicing in silence (Liu and Rice, 2019). Usually, silence can favor a shift from a predominant parasympathetic activity of the ventral vagus to the activation of the sympathetic nervous system. This may be explained by the fact that, as mentioned above, external silence can increase a state of arousal, since the individual’s attention tends to be more focused on detecting potential sources of danger emerging from the environment. However, with sufficient training, one may voluntarily redirect attention to foster a condition of inner silence and prevent the outer silence-induced activation of the sympathetic nervous system. In fact, several studies have demonstrated that, to some degree and under certain conditions, autonomic functions may be controlled voluntarily (Yates, 1980). From a molecular perspective, genes associated with glucocorticoid signaling, serotonergic signaling, and neurotrophins are among the psychophysical stress-related targets susceptible to epigenetic dysregulation. Surprisingly, silent meditation practices appear to impact the same gene targets, such as FKBP5, SLC6A4, and BDNF, consequently influencing endocrine, neuronal, and behavioral functions (Venditti et al., 2020). This suggests that achieving a state of inner silence through meditation may counteract the detrimental effects of a stressful environment. However, the interplay between stress and meditation on shared epigenetic mechanisms remains unclear, and further molecular and epigenetic evidence is needed to establish a cause-and-effect relationship. It is plausible that various types of meditation, by improving immune function, metabolism, stress response pathways, and promoting neuroplasticity, can influence energy conservation mechanisms, enhance homeostasis, and reinforce the reciprocal relationship between mind and body, leading to enhanced relaxation abilities and positive psychological outcomes. If we consider the studies about the effects on the nervous system of activities performed in a silent environment, the number of participants ranged from 8 to 75 and the control condition often implied accomplishing the same task while listening to some music, environmental noises or natural sounds (Table 2). It was demonstrated that silence does not reduce psychophysical stress levels associated with specific mental or physical exercises (Desai et al., 2015; Hasegawa et al., 2004; Khalfa et al., 2003; Labbe et al., 2007), but it can help in performing different tasks requiring high levels of concentration (Bellamy et al., 2021; Proverbio et al., 2015; Radun et al., 2021). Additionally, pictures of pleasing wild environments, such as forests, can better increase the ventral vagal tone when accompanied by natural sounds than when contemplated in full silence (Song et al., 2021). Taken together, these results are in line with the evidence from the other studies included in this review, and suggest that silence can ease the accomplishment of mental tasks by inducing a heightened state of self-awareness and attention, but it does not improve stress levels for the same reasons. In a laboratory study with mice (Table 1), researchers discovered that silence, unlike white noise or other auditory stimuli, led to an increase in the production of precursor cells involved in neurogenesis: in particular, after 7 days, the group exposed to silence exhibited a significantly greater number of new neurons compared

3 Results

to the control group (Kirste et al., 2015). These findings emphasize the distinctive and beneficial effect of silence on promoting the generation of new neurons, distinguishing it from ambient background noise. In fact, it has been hypothesized that, in neurodegenerative conditions such as Alzheimer’s disease, resting states can be associated with reduced sensory input and cognitive activity, providing opportunities for the brain to engage in restorative processes (Sheline and Raichle, 2013). Gaining a deeper understanding of these phenomena holds the potential to mitigate the progression of neurodegenerative disorders, expedite the recovery process following brain injuries, and pave the way for novel treatments targeting these health conditions. In the clinical studies included in this review, the number of participants varied from a minimum of 16 to a maximum of 90, and the conditions of interest were neuropsychiatric problems, cardiovascular illnesses, COVID-19- and surgery-related stress (Table 3). In none of the studies included, the authors reported a reference to internationally-recognized methodological standards, as reported in the EQUATOR network (https://www.equator-network.org/). The main findings of these studies suggest that, as opposed to silence, classical, Tibetan or preferred music can temporarily improve flow-mediated arterial dilation in subjects with peripheral arterial disease, decrease stress levels in patients waiting for surgical procedures, favor a faster post-stress recovery in concussed athletes, and increase brain activity in vegetative patients (Cotoia et al., 2018; De Haro and Bleda, 2019; Leonard et al., 2021; O’Kelly et al., 2013). Additionally, it was demonstrated that music-to-silence autonomic changes are more pronounced in healthy subjects than in patients with schizophrenia, indicating that silence-induced autonomic changes may be hampered in individuals with some neuropsychiatric disorders (Akdemir et al., 2010): from a polyvagal perspective, this piece of evidence can be explained by the fact that individuals with psychosis often experience difficulties in regulating their autonomic nervous system normally, leading to a chronically reduced ventral vagal tone. In patients isolated for COVID-19, silence, preceded by music, led to an increased sympathetic tone (Pin˜eros et al., 2023). A physiological arousal after the confinement period was also identified in COVID-19 patients suffering from misophonia, even when exposed to silence and simply invited to imagine misophonia-triggering sounds (Ferrer-Torres and Gimenez-Llort, 2021). Finally, in a study with patients trying to overcome snake phobia, sensory deprivation helped decrease the subjects’ fear, but only when the deprivation involved both visual and auditory stimuli, as it happens while resting in a dark room (Suedfeld and Hare, 1977). Overall, when interpreted in light of the Polyvagal Theory, these studies indicate that silence does not help improve the ventral vagal tone, but rather favors some degree of sympathetic activation. Other studies reported in the scientific literature and not focused on the effects of silence show that noise reduction can be associated with a decrease in blood pressure and heart rate (Bernardi et al., 2006; Dousty et al., 2011; Fleming, 2019; van Kempen et al., 2002), whereas overexposure to urban noises can cause negative changes for cardiovascular health, leading to increased risk of developing hypertension and heart diseases (Stansfeld et al., 2000). Scientific studies also show that, during sleep, noise

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pollution can increase heart rate, blood pressure, breathing and alter the neuroendocrine system functions, compromising the individual’s health in the long run (Kawada, 2011). In other words, even though silence does not improve the ventral vagal tone (as it happens with relaxing music), it still acts as a benefit when compared to noise pollution, namely unpleasant sounds, granting the auditory system an opportunity to regain its sensitivity to sounds and improving stress-related conditions worsened by noise exposure.

4 Discussion 4.1 Effects of silence on the autonomic nervous system To discuss the impact of silence on the autonomic nervous system, it is helpful to use the distinction between inner and outer silence (Paoletti and Ben-Soussan, 2020). Inner Silence is related to the concept of internal environment. It is associated with the individual’s psychological, neurological, and physiological mechanisms. Inner silence can be fostered through practices like meditation, which are considered internal intentional acts. Outer Silence, on the other hand, is connected with the concept of external environment, referring to extrinsic factors like perceptual deprivation (Ben-Soussan et al., 2019; Paoletti and Ben-Soussan, 2020). This involves creating conditions in one’s external environment that minimize sensory input. As a result, we have two types of noise: inner and outer, which by definition disrupt the state of silence. The inner noise can primarily refer to the internal dialogue, the flow of mental associations that constantly represents the background of our consciousness, but it can also encompass the baseline perception of stimuli originating from within the organism and the viscera (interoception). Outer noise is easily understood, especially when referring to sounds and the sense of hearing. However, external noise can also be considered as the neuroceptive stimuli originating from other individuals, such as their visceral and emotional states, communicated through their movements, face, and voice, as we discussed above. This latter type of outer noise ultimately translates into inner noise, as these stimuli are internalized through the mirror neuron system and converted into sensations within the observer’s organism through interoceptive mechanisms. To better understand this last concept, a digression on interoception and the experience sharing system and the underlying mechanisms is necessary:
With regard to the concept of the experience sharing system, our comprehension of others is founded on the fact that we internally simulate the experiences of others as we observe them. This internal simulation is made possible by the mirror system within our brain, a network of mirror neurons that can activate in response to both performing an action ourselves or observing someone else perform that same action (Bonini et al., 2022). These mirror neurons have the unique attribute of being responsive to both motor commands and sensory stimuli (Bonini et al., 2022). Different populations of mirror neurons exist, each responsible for a series

4 Discussion

of specific movements, which, when coordinated properly in terms of kinematic characteristics, result in a goal-directed action. A notable feature of these motor neurons is their ability to activate not only during the execution of an intentiondirected action but also when observing someone else carry out the same action. Consequently, the interaction of mirror neurons gives rise to the mirror system, whose key function is to allow the observer’s brain to internally simulate the observed actions, although in a latent state (Gibson, 1977; Rizzolatti and Sinigaglia, 2008). The mirror system in our brains creates an internal model, anchored in our motor knowledge of the actions we observe others performing. This internal modeling develops in the premotor areas of our brain (Rizzolatti and Sinigaglia, 2008). This implies that even though the populations of mirror neurons that fire when we witness another person perform an action are the same ones that fire when we personally carry out the same action, the representation remains latent during observation, as opposed to when performing the action, where the representation triggers a command to the primary motor areas to initiate the actual movement. Without this distinction, we would be constantly compelled to mimic all actions within our vicinity. This is due to the intervention of the prefrontal cortex and hence the exercise of will, required to transition from the latent state to execution. Notably, the bi-modality of mirror neurons extends beyond the visuo-motor domain to include other populations of “other-selective” neurons, as they have been recently labeled (Bonini et al., 2022), and in fact have been identified across various species and brain areas. In primates, the sensorimotor and emotional mirror neuron networks have been found in several regions. The sensorimotor network includes the ventral premotor cortex (PMv), primary motor cortex (M1), inferior parietal lobule (IPL), anterior intraparietal area (AIP), dorsal premotor (PMd), mesial premotor (PMm) cortex, prefrontal cortex (PFC), and secondary somatosensory cortex (SII). The emotional network, on the other hand, includes the anterior cingulate cortex (ACC), amygdala, and insula. Additional evidence in humans suggests that the basal ganglia and the cerebellum might also play a role in these networks (Albertini et al., 2020, 2021; Bonini et al., 2010; Bruni et al., 2018; Caruana et al., 2011; Errante and Fogassi, 2020; Falcone et al., 2017; Ferroni et al., 2021; Hihara et al., 2015; Keysers et al., 2004; Lanzilotto et al., 2019, 2020; Livi et al., 2019; Livneh et al., 2012; Papadourakis and Raos, 2019; Simone et al., 2017; Vigneswaran et al., 2013; Wicker et al., 2003; Yoshida et al., 2011). In fact, human brain regions involved in the control and regulation of emotions become active when witnessing emotional displays of others. A network including the amygdala, the insula, and the cingulate cortex has a role in the expression, experience, and perception of facial and bodily emotional displays (de Gelder, de Borst and Watson, 2015; Woolley et al., 2015; Wang et al., 2017; Caruana et al., 2020). Similarly to the motor system, the mirror system offers insights into understanding how we perceive others’ feelings. This mirror system prepares us to understand the sensations that the observed person experiences, allowing us to develop an internal portrayal of their emotional state within our own bodies.

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This process induces specific visceral and somatosensory responses, generated by signals from our insula to the hypothalamus, and hence to the autonomic nervous system, as well as by the activation of the somatosensory areas. These induced feelings afford us an insight, so to speak, into the other person’s emotional state at that moment, simulating the observed person’s internal state within ourselves. Our ability to comprehend and recognize what the other person is feeling arises from our past direct experiences of those emotions, providing a frame of reference. In other words, the mirror mechanism is thought to reflect a sensorimotor simulation of others’ emotions rather than a mere muscle-specific resonance, supporting the idea that affective empathy depends on the capacity of social stimuli to trigger visceromotor, not simply somatomotor, effects in the observer. Others’ emotional displays afford visceromotor reactions in the observer’s brain, ranging from aligned visceromotor simulation to misaligned complementary responses that promote social regulation or adaptive mutual behaviors. From this perspective, the autonomic nervous system plays a central role as it represents the true effector of visceromotor responses that can be elicited by mechanisms of affective empathy. This process is also referred to as “embodied simulation” (Gallese and Sinigaglia, 2011), and its discovery has led to a new understanding of human relationships: no longer is there a discrete “you and I”, but rather an ongoing “us.” This novel perspective on the study of human relationships has been termed “intersubjectivity” (Ammaniti and Gallese, 2014). It is important to note that the modulation of empathic responses is much more complex compared to the modulation of motor responses. For example, empathic responses are affected by the context in which others’ emotional displays are observed and can afford very different visceromotor and neurobehavioral reactions (e.g., seeing a person injured on the ground may induce empathic alignment or even rage and hostility, depending on whether the person is a passerby injured by a criminal or a criminal injured by a police officer). However, this discussion exceeds too much the scope of this review (for further details on this topic see (Donelli and Rizzato, 2018)). The central role of the mirror system in empathy and emotion recognition, as well as the theory of embodied simulation, has been vigorously debated over the years (Lamm and Majdandzˇic, 2015). However, the authors support this perspective because, even when reducing the role of the mirror system to embodied simulation of motor acts, it would still remain essential for emotion recognition (and consequently their embodied simulation in the observer) through sensorimotor signals conveyed by facial expressions and vitality forms of gestures and vocalizations.
On the other hand, interoception refers to the perception of internal bodily states, and it involves the awareness of one’s own physical state, which is fundamental also for the experience sharing system described above (Craig, 2003, 2009; Critchley et al. 2004; Donelli and Rizzato, 2018; Chen et al., 2021). Examples of interoceptive stimuli include thirst, pain, sensual touch, itch, heartbeat, and bladder distension.

4 Discussion

There are two major ascending peripheral neural pathways that transmit interoceptive signals to the Central Nervous System (CNS): ganglia residing in the cranial/vagal pathways, such as nodose or jugular ganglia, often project to the nucleus tractus solitarii (NTS) of the brainstem, whereas dorsal root ganglia, located along the spinal nerve pathway, project information to the brain through the spinal cord. Interoceptive information is often first processed in subcortical structures of the brain such as the medial NTS, the parabrachial nucleus (PB), and the ventromedial nucleus of the thalamus. These neurons may project to higher brain regions including the hypothalamus, insula, anterior cingulate cortex, and somatosensory cortex for further integration and interpretation (Craig, 2002; Saper, 2002; Critchley and Harrison, 2013; Berntson and Khalsa, 2021; Chen et al., 2021). The insula emerged as a crucial cortical node in the interoceptive system. Primary interoceptive information is relayed from the ventromedial nucleus of the thalamus to the posterior insula, and integration with exteroceptive sensorimotor and proprioceptive information most likely takes place within the posterior and central regions. The anterior insular cortex (AIC) is most strongly connected to paralimbic cortical regions and may be involved in connections between interoceptive and emotional states. The insula therefore may serve as a key interoceptive hub for integrating and regulating signals from the internal and external environments (Penfield and Faulk, 1955; Mesulam and Mufson, 1982; Craig, 2009; Kurth et al., 2010; Uddin et al., 2017). The anterior insula receives visceral inputs, such as signals from the internal organs, as well as somatic inputs from the body, and integrates them to provide a representation of the physical state of the body. This creates a sort of somatotopy, or a mapping of the body’s internal sensations and physiology, within the anterior insula. The anterior insula is involved in processing various emotional and physical feelings such as pleasure, pain, disgust, love, fear, sadness, happiness, sexual arousal, and more. The activation of the insula can be triggered by different areas of the brain, depending on the context, such as through the premotor cortex by observing facial expressions or through language processing regions like Broca’s area and the temporal lobe when reading an emotional story. Moreover, there is a correlation between the intensity of emotional experiences and interoceptive capacity; individuals who are more attuned to their internal bodily states, such as their heartbeat, are also more aware of their emotions (Critchley et al., 2004). The anterior insula in particular plays a key role in interoception and in recognizing others’ emotions: it has the ability to perceive the body’s internal conditions and interpret this visceral sensitivity, and, by integrating visual and auditory information, is capable of connecting data obtained from observing or hearing an individual experiencing an emotion to those neuronal circuits within us that can viscerally replicate the same emotion. In fact, the insula also has outputs, or efferences, that can influence physiological homeostasis by acting in first place on the autonomic nervous system, part of the ‘central autonomic

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network’ (CAN) (Benarroch, 1993), enabling it to reproduce emotional states at the visceral level (Critchley et al. 2004; Chen et al., 2021). The CAN is composed of key components such as the anterior cingulate cortex, insular cortex, thalamus, hypothalamus, amygdala, periaqueductal gray (PAG), parabrachial nucleus, NTS, locus coeruleus, and ventrolateral medulla: these components have a broad influence on the sympathetic and parasympathetic autonomic control of internal states. Neurons that are active in the parasympathetic system extend from the rostral insula and medial prefrontal cortex to the viscerae through a chain of three synapses. On the other hand, neurons active in the sympathetic system extend from the primary/secondary motor and primary somatosensory cortex through four synapses. This implies that distinct cortical networks regulate viscerae activity, and it backs the idea that the processing of gut signals for motivation and affect involves a network of sensorimotor feedback loops between the brain and the periphery. Other insular outputs can impact energy metabolism, thermoregulation, feeding, digestion, fluid and electrolyte balance, sleep-wake cycling, reproductive processes, and stress endocrine responses via the hypothalamus. Furthermore, the insula has an impact on the immune system through a reflexive control mechanism, influencing immune responses and the progression of inflammatory diseases (Berntson and Khalsa, 2021). These notions are fundamental to understanding the concept of inner noise, indirectly generated by the presence of other individuals and social interactions. As discussed earlier, if an individual is in the presence of one or more individuals who possess an "inner noisy" state (characterized by reduced ventral vagal tone and increased sympathetic or dorsal vagal tone), such as anger, pain, disgust, hatred, depression, anxiety, etc., then through the experience sharing system underlying affective empathy and emotion recognition, that individual will, to a greater or lesser extent, replicate the same visceromotor activation pattern observed in the others. At the same time, an individual’s level of interoception also influences this mechanism from two perspectives: from the viewpoint of the observed individual, as greater interoceptive capacity leads to a heightened perceived intensity of emotions, potentially resulting in increased external expression through gestures, vocalizations, and facial expressions; and from the viewpoint of the observing individual, as greater interoceptive capacity leads to a heightened intensity of observed emotional states that are embodied-simulated within themselves. Given the previous definitions of inner and outer silence, it is possible to analyze now how these two types of silence can interact with the autonomic nervous system, although scientific evidence in this area is limited. Regarding inner silence, it is plausible that the experience of this state may impact the autonomic nervous system regulation through the ability to voluntarily foster a state of inner silence. This state, often cultivated through practices such as meditation, has been shown to enhance the activity of the ventral vagus, thus favoring the recruiting of the Social Engagement System, and reduce the activity of the sympathetic nervous system, thereby reducing physiological stress markers like cortisol (Mohan et al., 2011; Vandana et al., 2011; Koncz et al., 2021). Regular

4 Discussion

practice of meditation can improve heart rate variability (HRV), a key indicator of autonomic nervous system flexibility and an individual’s ability to adapt to stress (Lehrer and Gevirtz, 2014). In a polyvagal perspective, these changes are understood as a manifestation of the ventral vagal complex’s regulatory effects. However, it is important to note that studies indicate the enhancement of ventral vagal tone may not be associated with silent meditation for individuals unless they have undergone sufficient training. As the retrieved studies show, as the practice progresses, individuals might develop the ability to shift their attention away from potentially stressful thoughts or internal dialogues—the “inner noise”—reducing the likelihood of sympathetic nervous system activation. This notion aligns with the Polyvagal Theory, which posits that the ventral vagus, associated with parasympathetic activity, promotes a calming effect and inhibits defensive responses triggered by the sympathetic nervous system. Therefore, through inner silence, it may be possible to achieve a balance between parasympathetic and sympathetic activity, thus promoting autonomic regulation. Conversely, outer silence might exert different influences on the autonomic nervous system. It has been found that outer silence can induce a heightened state of alertness within the auditory cortex (Voisin et al., 2006). This alertness, derived from the brain’s anticipation and active search for sound, can induce a state of arousal that potentially triggers the activation of the sympathetic nervous system. From a polyvagal perspective, the response to outer silence could be intimately tied to the system’s evolutionary function of detecting threats. As discussed above, an absence of sound triggers a heightened state of alertness within the auditory cortex, likely reflecting the brain’s survival instinct to anticipate and search for potential danger cues in the environment. This state of alertness is associated with the activation of the sympathetic nervous system, also known as the “fight or flight” response, and leads to a decreased activation of the ventral vagal complex or “vagal brake” removal. Hence, outer silence might not typically be associated with an enhancement of ventral vagal tone, reflecting the primary role of the auditory system in threat detection. However, with adequate training and adaptation, an individual might be able to voluntarily redirect attention during periods of outer silence to foster a condition of inner silence. This could potentially prevent the outer silence-induced activation of the sympathetic nervous system, allowing for a better autonomic balance between the sympathetic and parasympathetic branches. Therefore, outer silence might initially cause a sympathetic arousal but, given appropriate conditioning, can become a trigger for inner silence and its associated ventral vagus dominance. Thus, the effects of outer silence on autonomic regulation might be primarily determined by the individual’s ability to foster inner silence in the context of an externally quiet environment. The precise mechanisms by which such training could increase inner silence remain unknown and could be a topic of interest for future research. One could speculate that such a result could be achieved through two mechanisms: on one hand, voluntary relaxation aimed at minimizing sympathetic activity and simultaneous enhancement of ventral vagal activity, achieved through a consistently

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renewed intentional effort (prefrontal activity); on the other hand, intentional effort directed at suppressing the neuroceptive alert state caused by outer silence. This condition would thus enable the attainment of inner silence even in an environment of outer silence, benefiting from the positive effects of outer silence on focus and concentration without experiencing sympathetic activation effects. Different scenarios of autonomic nervous system activation can be configured based on various elements:
The presence or absence of outer acoustic silence and the individual’s familiarity and habituation with it: The presence of outer acoustic silence can activate the more ancient and instinctive parts of the autonomic nervous system, predisposing individuals to listen to the surrounding environment with a focus on lower frequencies associated with potential threats. This can result in a reduced ability to discern nuances in communication between individuals, leading to decreased attention and comprehension. From an autonomic perspective, this predisposition caused by outer acoustic silence can result in both the removal of vagal brake and sympathetic system stimulation, leading to a fight-or-flight state. In individuals unaccustomed to outer silence, it can also trigger activation of the dorsal vagus system, leading to the freeze response and, for example, panic attacks. It is not difficult to imagine different psychopathological conditions that favor this type of scenario, such as autism. For individuals who are accustomed to a noisy external environment since childhood, silence may represent a threatening stimulus, independent of the detection of low frequencies within it that could be associated with objective threats, unless specific training has been provided. On the other hand, the phenomenon of using white noise to facilitate sleep in certain individuals is well-known. Conversely, individuals accustomed to quiet external environments may have developed the ability to modulate neuroception of the surrounding environment, allowing for activation of higher-level autonomic circuits, increased ventral vagal tone, and thereby promoting a rest-and-digest state and predisposition to social interaction. Other individuals may have intentionally developed the ability to dwell in outer acoustic silence through prefrontal effort, as we have learned from the studies collected in the systematic review.
The context in which outer acoustic silence manifests: This element is crucial as it is linked to the recognition of specific environmental characteristics when outer acoustic silence is experienced. If the environment is recognized as familiar (e.g., home, workplace), outer acoustic silence may not trigger a neuroceptive alertness but instead facilitate concurrent activation of ventral vagal and dorsal vagal/sympathetic systems, i.e., safe immobilization and mobilization, which are prerequisites for bonding between human beings through physical contact, rest, and sexual activity. On the other hand, if outer silence occurs in a completely new environment, where the orienting reflex is easily recruited, neuroceptive alertness and a reduction in ventral vagal tone in favor of the sympathetic system are more plausible. However, there are special situations, such as certain religious places like churches and monasteries, where particular architectural features or

4 Discussion













specific visual sensory stimuli may modulate individuals’ neuroception, creating a sense of familiarity and perceiving a “safe” silence despite the novelty of the location. The quality and type of outer acoustic noise one is exposed to: outer acoustic noise can also consist of music, which, if it possesses characteristics similar to human vocal prosody, can have a calming effect by increasing the neuroception of safety and thus reducing sympathetic tone while increasing vagal inhibition. Music which is similar to human vocal prosody refers to a type of music that resembles the frequencies typically used in human communication under normal and non-threatening conditions. As discussed above, classical music is an example of this type, other examples may be new-age or relaxing music, or even the listening to some ASMR artists. Examples of music that do not represent human vocal prosody and may lead to sympathetic activation include heavy metal or certain electronic music. The level of training and ability of individuals to create inner silence: This element is crucial as it implies intentional intervention by individuals and therefore prefrontal effort. Voluntary training in creating or dwelling in inner silence means skill in overcoming the natural and instinctive response of the human organism to react with autonomic defense mechanisms (sympathetic activation) that may arise from thoughts and associations producing inner noise. Similarly, such voluntary training means skill in overcoming sympathetic activation that may arise from interoception of one’s viscera, a condition often facilitated by meditative practices, whether generated by a state of illness or health at the basal level. The presence or absence of outer emotional silence and the individual’s familiarity and habituation with it: The visceral and emotional states of the individuals around us represent a form of outer “emotional” noise, as we discussed above. Thanks to the experience sharing system we are able to understand what happens to others, their intentions, and their feelings (Rizzato and Donelli, 2014; Donelli and Rizzato, 2018). When neuroception is of safety, the activation of the Social Engagement System with the cardiac/respiratory/ phonatory control provided by the ventral vagus allows for the co-regulation of physiological states. As a result, the quality of the internal states of the people around us influences our internal state to a large extent, often unmodifiable. The presence of sick, angry, or emotionally negative individuals, for example, represents a form of outer emotional noise. On the other hand, the presence of a sick person who has learned to achieve inner silence (voluntary high vagal brake) does not represent a source of outer emotional noise because their control over their own autonomic nervous system allows for a state that ensures effective and beneficial co-regulation for those around them. The empathic sensitivity of an individual: This can expose them to an overstimulation of outer-inner noise, perceiving the emotional states of others in an excessively intense manner (see Donelli and Rizzato, 2018; Donelli et al., 2018), known as empathic distress, making it more difficult or impossible to achieve inner silence.

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All these elements can act individually or interact with each other, indicating the significant role of silence in regulating the human autonomic nervous system. Such a prominent role in influencing the autonomic nervous system means that silence indirectly has a profound impact on many aspects of human life, ranging from physical health and well-being to emotions and volition. Consequently, silence should become a central element of our lives. New studies are needed in this direction to further clarify, with experimental evidence, the effects of silence on the autonomic nervous system. Moreover, silence should be integrated into human lives to enhance quality of life from various perspectives. From a practical point of view, strategies incorporating silence into daily routines can be beneficial for well-being, despite the current scarcity of evidence. These include mindful pauses, which entail taking deliberate breaks throughout the day to embrace moments of silence. This practice facilitates restoration and stress reduction by finding a quiet environment, focusing attention on the breath or present moment, and minimizing distractions, thus contributing to improved well-being (Tolson, 2022). Silent commuting is another practical strategy. By refraining from auditory stimulation, such as turning off radios or audio devices, individuals can embrace and adapt to the absence of sound. This approach promotes introspection, relaxation, and mental rejuvenation during transit. Taking technology breaks, which involves designating specific periods for silence by disconnecting from digital screens, muting notifications, and setting electronic devices aside, which facilitates well-being (Radtke et al., 2022). During these breaks, engaging in activities that encourage relaxation, like reading, meditating, or spending time in natural environments, can further enhance the beneficial effects. Individuals can also incorporate quiet outdoor moments into their routines. Spending time in natural settings that offer tranquility and minimizing auditory stimuli allows for immersion in peaceful surroundings. Activities such as leisurely walks, contemplation in serene environments, or seeking solace in gardens can promote mental restoration and a sense of well-being (Antonelli et al., 2019; Hansen, Jones and Tocchini, 2017). Lastly, adopting mindfulness practices like meditation, yoga, or tai chi provides structured approaches to incorporate silence and present-moment awareness. These practices have been shown to enhance well-being by cultivating attentional regulation, self-awareness, and stress reduction (Grossman et al., 2004; Zou et al., 2018). Therefore, by intentionally integrating moments of silence into daily routines, individuals can experience notable benefits due to a better control over their autonomic nervous system such as stress reduction, increased self-awareness, and overall well-being.

5 Conclusions In conclusion, silence plays a pivotal role in emotional regulation, social engagement, and adaptive physiological responses mediated by the autonomic nervous system, as outlined in the Polyvagal Theory. The impact of silence, whether inner

References

or outer, significantly influences the autonomic nervous system. Inner silence promotes social engagement and reduces physiological stress by enhancing the activity of the ventral vagus. Outer silence, while initially inducing alertness and potential sympathetic nervous system activation, can foster inner silence with training, thereby preventing such activation. The response to silence is multifaceted, influenced by context, familiarity with silence, outer noise, and empathic sensitivity. The interplay of these factors underscores the profound role of silence in regulating the autonomic nervous system, thereby affecting physical health, well-being, emotions, and volition. Therefore, integrating silence into our daily lives is essential, as it can allow for a deeper understanding of oneself, others, and the world, opening doors to profound insights and transformative experiences. Future research should aim to provide experimental evidence elucidating the effects of silence on the autonomic nervous system, as improving our understanding of its potential could promote health, well-being, and awareness.

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