Abstract
Consciousness is private. Although conscious beings directly access their own conscious experiences, the consciousness of others must be inferred through overt report: observable behaviours — such as overt facial expressions, vocalizations and body gestures — that suggest the level, state and content of consciousness. However, overt report is limited because it can be erroneous (for example, resulting from wilful deception or being subject to recall error), absent (for example, during sleep and paralysis) or conflict with research goals (for example, in no-report paradigms and resting-state studies). These limitations encourage the search for covert measures of consciousness: physiological signals that disclose consciousness without relying on overt behaviour. This Review highlights emerging covert measures of consciousness in humans, including eye, skin, respiratory and heart signals. We also address the challenge of distinguishing physiological signals linked to conscious versus unconscious neural processing. Finally, we consider the ethical implications of infringing on the innate privacy of consciousness.
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References
Koch, C., Massimini, M., Boly, M. & Tononi, G. Neural correlates of consciousness: progress and problems. Nat. Rev. Neurosci. 17, 307–321 (2016).
Nagel, T. What is it like to be a bat? Philos. Rev. 83, 435–450 (1974).
Kronemer, S. I. et al. Human visual consciousness involves large scale cortical and subcortical networks independent of task report and eye movement activity. Nat. Commun. 13, 7342 (2022).
Snider, S. B. & Edlow, B. L. Magnetic resonance imaging in disorders of consciousness. Curr. Opin. Neurol. 33, 676–683 (2022).
Young, M. J., Edlow, B. L. & Bodien, Y. G. Covert consciousness. NeuroRehabilitation 54, 23–42 (2024).
Sandin, R. H., Enlund, G., Samuelsson, P. & Lennmarken, C. Awareness during anaesthesia: a prospective case study. Lancet 355, 707–711 (2000).
Birch, J., Schnell, A. K. & Clayton, N. S. Dimensions of animal consciousness. Trends Cogn. Sci. 24, 789–801 (2020).
Schooler, J. W. Re-representing consciousness: dissociations between experience and meta-consciousness. Trends Cogn. Sci. 6, 339–344 (2002).
Hurlburt, R. T. & Heavey, C. L. Investigating pristine inner experience: implications for experience sampling and questionnaires. Conscious. Cogn. 31, 148–159 (2015).
Garric, C. et al. Dissociation between objective and subjective perceptual experiences in a population of hemianopic patients: a new form of blindsight? Cortex 117, 299–310 (2019).
Levine, F. M. & Lee De Simone, L. The effects of experimenter gender on pain report in male and female subjects. Pain 44, 69–72 (1991).
Ekman, P. & Friesen, W. V. Nonverbal leakage and clues to deception. Psychiatry 32, 88–106 (1969).
Block, N. On a confusion about a function of consciousness. Behav. Brain Sci. 18, 227–247 (1995).
Roediger, H. L. & McDermott, K. B. Creating false memories: remembering words not presented in lists. J. Exp. Psychol. Learn. 21, 803–814 (1995).
Loftus, E. F. & Palmer, J. C. Reconstruction of automobile destruction: example of the interaction between language and memory. J. Verb. Learn. Verb. Behav. 13, 585–589 (1974).
Tateno, A., Jorge, R. E. & Robinson, R. G. Pathological laughing and crying following traumatic brain injury. J. Neuropsychiatry Clin. Neurosci. 16, 426–434 (2004).
Cummings, J. L. et al. Defining and diagnosing involuntary emotional expression disorder. CNS Spectr. 11, 1–7 (2006).
Childs, N. L., Mercer, W. N. & Childs, H. W. Accuracy of diagnosis of persistent vegetative state. Neurology 43, 1465–1467 (1993).
Vallotton, C. D. Signs of emotion: what can preverbal children “Say” about internal states? Infant. Ment. Health J. 29, 234–258 (2008).
Terrace, H. S., Petitto, L. A., Sanders, R. J. & Bever, T. G. Can an ape create a sentence? Science 206, 891–902 (1979).
Aru, J., Bachmann, T., Singer, W. & Melloni, L. Distilling the neural correlates of consciousness. Neurosci. Biobehav. Rev. 36, 737–746 (2012).
Tsuchiya, N., Wilke, M., Frassle, S. & Lamme, V. A. F. No-report paradigms: extracting the true neural correlates of consciousness. Trends Cogn. Sci. 19, 757–770 (2015).
Frässle, S., Sommer, J., Jansen, A., Naber, M. & Einhäuser, W. Binocular rivalry: frontal activity relates to introspection and action but not to perception. J. Neurosci. 34, 1738–1747 (2014).
Block, N. What is wrong with the no-report paradigm and how to fix it. Trends Cogn. Sci. 23, 1003–1013 (2019).
Michel, M. & Morales, J. Minority reports: consciousness and the prefrontal cortex. Mind Lang. 35, 493–513 (2020).
Gonzalez-Castillo, J., Kam, J. W. Y., Hoy, C. W. & Bandettini, P. A. How to interpret resting-state fMRI: ask your participants. J. Neurosci. 41, 1130–1141 (2021).
Stoll, J. et al. Pupil responses allow communication in locked-in syndrome patients. Curr. Biol. 23, R647–R648 (2013).
Monti, M. M. et al. Willful modulation of brain activity in disorders of consciousness. N. Engl. J. Med. 362, 579–589 (2010).
Martinez-Conde, S., Macknik, S. L., Troncoso, X. G. & Dyar, T. A. Microsaccades counteract visual fading during fixation. Neuron 49, 297–305 (2006).
Otero-Millan, J., Macknik, S. L. & Martinez-Conde, S. Microsaccades and blinks trigger illusory rotation in the “rotating snakes” illusion. J. Neurosci. 32, 6043–6051 (2012).
Sulutvedt, U. et al. Brightness perception changes related to pupil size. Vis. Res. 178, 41–47 (2021).
Posner, J. B., Saper, C. B., Schiff, N. D. & Claassen, J. Plum and Posner’s Diagnosis and Treatment of Stupor and Coma 5th edn (Oxford Univ. Press, 2019).
Aru, J., Suzuki, M., Rutiku, R., Larkum, M. E. & Bachmann, T. Coupling the state and contents of consciousness. Front. Syst. Neurosci. 13, 43 (2019).
Bayne, T., Hohwy, J. & Owen, A. M. Are there levels of consciousness? Trends Cogn. Sci. 20, 405–413 (2016).
Vaitl, D. et al. Psychobiology of altered states of consciousness. Psychol. Bull. 131, 98–127 (2005).
Blanke, O. & Metzinger, T. Full-body illusions and minimal phenomenal selfhood. Trends Cogn. Sci. 13, 7–13 (2009).
Lewis, C. I. Mind and the World-Order: Outline of a Theory of Knowledge (Charles Scribner’s Sons, 1929).
Redinbaugh, M. J. et al. Thalamus modulates consciousness via layer-specific control of cortex. Neuron 106, 66–75 e12 (2020).
Di Perri, C. et al. Neural correlates of consciousness in patients who have emerged from a minimally conscious state: a cross-sectional multimodal imaging study. Lancet Neurol. 15, 830–842 (2016).
Varley, T. F. et al. Fractal dimension of cortical functional connectivity networks and severity of disorders of consciousness. PLoS ONE 15, e0223812 (2020).
Casali, A. G. et al. A theoretically based index of consciousness independent of sensory processing and behavior. Sci. Transl. Med. 5, 198ra105 (2013).
Stender, J. et al. The minimal energetic requirement of sustained awareness after brain injury. Curr. Biol. 26, 1494–1499 (2016).
Aubinet, C. et al. Brain metabolism but not gray matter volume underlies the presence of language function in the minimally conscious state (MCS): MCS+ versus MCS− neuroimaging differences. Neurorehabil Neural Repair 34, 172–184 (2020).
Mélotte, E. et al. Is oral feeding compatible with an unresponsive wakefulness syndrome? J. Neurol. 265, 954–961 (2018).
Horikawa, T., Tamaki, M., Miyawaki, Y. & Kamitani, Y. Neural decoding of visual imagery during sleep. Science 340, 639–642 (2013).
Vetter, P. et al. Decoding natural sounds in early “visual” cortex of congenitally blind individuals. Curr. Biol. 30, 3039–3044 (2020).
Bannert, M. M. & Bartels, A. Human V4 activity patterns predict behavioral performance in imagery of object color. J. Neurosci. 38, 3657–3668 (2018).
Emmerling, T. C., Zimmermann, J., Sorger, B., Frost, M. A. & Goebel, R. Decoding the direction of imagined visual motion using 7T ultra-high field fMRI. Neuroimage 125, 61–73 (2016).
Willett, F. R., Avansino, D. T., Hochberg, L. R., Henderson, J. M. & Shenoy, K. V. High-performance brain-to-text communication via handwriting. Nature 593, 249–254 (2021).
Hatamimajoumerd, E., Ratan Murty, N. A., Pitts, M. & Cohen, M. A. Decoding perceptual awareness across the brain with a no-report fMRI masking paradigm. Curr. Biol. 32, 4139–4149 (2022).
Hajonides, J. E., Nobre, A. C., van Ede, F. & Stokes, M. G. Decoding visual colour from scalp electroencephalography measurements. Neuroimage 237, 118030 (2021).
Koide-Majima, N., Nishimoto, S. & Majima, K. Mental image reconstruction from human brain activity: neural decoding of mental imagery via deep neural network-based Bayesian estimation. Neural Netw. 170, 349–363 (2024).
Gaziv, G. et al. Self-supervised natural image reconstruction and large-scale semantic classification from brain activity. Neuroimage 254, 119121 (2022).
Ronconi, L., Balestrieri, E., Baldauf, D. & Melcher, D. Distinct cortical networks subserve spatio-temporal sampling in vision through different oscillatory rhythms. J. Cogn. Neurosci. 36, 572–589 (2023).
Fovet, T. et al. Decoding activity in Broca’s area predicts the occurrence of auditory hallucinations across subjects. Biol. Psychiatry 91, 194–201 (2022).
Franke, K. et al. State-dependent pupil dilation rapidly shifts visual feature selectivity. Nature 610, 128–134 (2022).
Murphy, P. R., O’Connell, R. G., O’Sullivan, M., Robertson, I. H. & Balsters, J. H. Pupil diameter covaries with BOLD activity in human locus coeruleus. Hum. Brain Mapp. 35, 4140–4154 (2014).
Reimer, J. et al. Pupil fluctuations track fast switching of cortical states during quiet wakefulness. Neuron 84, 355–362 (2014).
Pfeffer, T. et al. Coupling of pupil- and neuronal population dynamics reveals diverse influences of arousal on cortical processing. eLife 11, e71890 (2022).
Einhäuser, W., Koch, C. & Carter, O. L. Pupil dilation betrays the timing of decisions. Front. Hum. Neurosci. 4, 18 (2010).
McGinley, M. J. et al. Waking state: rapid variations modulate neural and behavioral responses. Neuron 87, 1143–1161 (2015).
Farraj, Y. et al. Measuring pupil size and light response through closed eyelids. Biomed. Opt. Express 12, 6485–6495 (2021).
Goldfine, A. M. & Schiff, N. D. Consciousness: its neurobiology and the major classes of impairment. Neurol. Clin. 29, 723–737 (2011).
Fiebelkorn, I. C. & Kastner, S. A rhythmic theory of attention. Trends Cogn. Sci. 23, 87–101 (2019).
Blumenfeld, H. Brain mechanisms of conscious awareness: detect, pulse, switch, and wave. Neuroscientist 29, 9–18 (2023).
Joshi, S., Li, Y., Kalwani, R. & Gold, J. I. Relationships between pupil diameter and neuronal activity in the locus coeruleus, colliculi, and cingulate cortex. Neuron 89, 221–234 (2016).
Reimer, J. et al. Pupil fluctuations track rapid changes in adrenergic and cholinergic activity in cortex. Nat. Commun. 7, 13289 (2016).
Schneider, M. et al. Spontaneous pupil dilations during the resting state are associated with activation of the salience network. Neuroimage 139, 189–201 (2016).
Van Egroo, M., Gaggioni, G., Cespedes-Ortiz, C., Ly, J. Q. M. & Vandewalle, G. Steady-state pupil size varies with circadian phase and sleep homeostasis in healthy young men. Clocks Sleep. 1, 240–258 (2019).
Wilhelm, B., Wilhem, H., Lüdtke, H., Streicher, P. & Adler, M. Pupillographic assessment of sleepiness in sleep-deprived healthy subjects. Sleep 21, 258–265 (1998).
Lieberman, J. D. et al. Use of admission Glasgow coma score, pupil size, and pupil reactivity to determine outcome for trauma patients. J. Trauma 55, 437–442 (2003).
Emami, P. et al. Impact of Glasgow coma scale score and pupil parameters on mortality rate and outcome in pediatric and adult severe traumatic brain injury: a retrospective, multicenter cohort study. J. Neurosurg. 126, 760–767 (2017).
Isbell, H. Comparison of the reactions induced by psilocybin and LSD-25 in man. Psychopharmacologia 1, 29–38 (1959).
Franklin, M. S., Broadway, J. M., Mrazek, M. D., Smallwood, J. & Schooler, J. W. Window to the wandering mind: pupillometry of spontaneous thought while reading. Q. J. Exp. Psychol. 66, 2289–2294 (2013).
Schwartz, Z. P., Buran, B. N. & David, S. V. Pupil-associated states modulate excitability but not stimulus selectivity in primary auditory cortex. J. Neurophysiol. 123, 191–208 (2020).
van Kempen, J. et al. Behavioural and neural signatures of perceptual decision-making are modulated by pupil-linked arousal. eLife 8, e42541 (2019).
Sangare, A. et al. Pupil dilation response elicited by violations of auditory regularities is a promising but challenging approach to probe consciousness at the bedside. Sci. Rep. 13, 20331 (2023).
Naber, M., Frässle, S. & Einhäuser, W. Perceptual rivalry: reflexes reveal the gradual nature of visual awareness. PLoS ONE 6, e20910 (2011).
Bárány, E. H. & Halldén, U. Phasic inhibition of the light reflex of the pupil during retinal rivalry. J. Neurophysiol. 11, 25–30 (1948).
Binda, P., Pereverzeva, M. & Murray, S. O. Pupil size reflects the focus of feature-based attention. J. Neurophysiol. 112, 3046–3052 (2014).
Unsworth, N. & Robison, M. K. Pupillary correlates of covert shifts of attention during working memory maintenance. Atten. Percept. Psychophys. 79, 782–795 (2017).
Mathot, S., van der Linden, L., Grainger, J. & Vitu, F. The pupillary light response reflects eye-movement preparation. J. Exp. Psychol. Hum. Percept. Perform. 41, 28–35 (2015).
Gusso, M. M. et al. More than a feeling: scalp EEG and eye signals in conscious tactile perception. Conscious. Cogn. 105, 103411 (2022).
Naber, M., Alvarez, G. A. & Nakayama, K. Tracking the allocation of attention using human pupillary oscillations. Front. Psychol. 4, 919 (2013).
Zavagno, D., Tommasi, L. & Laeng, B. The eye pupil’s response to static and dynamic illusions of luminosity and darkness. Iperception 8, 2041669517717754 (2017).
Laeng, B. & Endestad, T. Bright illusions reduce the eye’s pupil. Proc. Natl Acad. Sci. USA 109, 2162–2167 (2012).
Sperandio, I., Bond, N. & Binda, P. Pupil size as a gateway into conscious interpretation of brightness. Front. Neurol. 9, 1070 (2018).
Laeng, B. & Sulutvedt, U. The eye pupil adjusts to imaginary light. Psychol. Sci. 25, 188–197 (2014).
Hustá, C., Dalmaijer, E., Belopolsky, A. & Mathôt, S. The pupillary light response reflects visual working memory content. J. Exp. Psychol. Hum. Percept. Perform. 45, 1522–1528 (2019).
Mathôt, S., Grainger, J. & Strijkers, K. Pupillary responses to words that convey a sense of brightness or darkness. Psychol. Sci. 28, 1116–1124 (2017).
Kay, L., Keogh, R., Andrillon, T. & Pearson, J. The pupillary light response as a physiological index of aphantasia, sensory and phenomenological imagery strength. eLife 11, e72484 (2022).
Gao, J., Ko, A., Yabe, Y., Goodale, M. A. & Chen, J. Pupil size is modulated by the size of equal-luminance gratings. J. Vis. 20, 4 (2020).
Tortelli, C., Turi, M., Burr, D. C. & Binda, P. Pupillary responses obey Emmert’s law and co-vary with autistic traits. J. Autism Dev. Disord. 51, 2908–2919 (2021).
Vemuri, K., Srivastava, A., Agrawal, S. & Anand, M. Age, pupil size differences, and color choices for the “dress” and the “jacket”. J. Opt. Soc. Am. A Opt. Image Sci. Vis. 35, B347–B355 (2018).
Beukema, S., Olson, J. A., Jennings, B. J. & Kingdom, F. A. A. Pupil dilation to illusory motion in peripheral drift images: perception versus reality. J. Vis. 17, 1–13 (2017).
Sahraie, A. & Barbur, J. L. Pupil response triggered by the onset of coherent motion. Graefes Arch. Clin. Exp. Ophthalmol. 235, 494–500 (1997).
Castellotti, S., Francisci, C. & Del Viva, M. M. Pupillary response to real, illusory, and implied motion. PLoS ONE 16, e0254105 (2021).
Sapir, A., Hershman, R. & Henik, A. Top-down effect on pupillary response: evidence from shape from shading. Cognition 212, 104664 (2021).
Sulutvedt, U., Mannix, T. K. & Laeng, B. Gaze and the eye pupil adjust to imagined size and distance. Cogn. Sci. 42, 3159–3176 (2018).
Enright, J. T. Perspective vergence: oculomotor responses to line drawings. Vis. Res. 27, 1513–1526 (1987).
Mathot, S. Pupillometry: psychology, physiology, and function. J. Cogn. 1, 16 (2018).
Castaldi, E., Pomè, A., Cicchini, G. M., Burr, D. & Binda, P. The pupil responds spontaneously to perceived numerosity. Nat. Commun. 12, 5944 (2021).
Hupé, J. M., Lamirel, C. & Lorenceau, J. Pupil dynamics during bistable motion perception. J. Vis. 9, 10 (2009).
Brascamp, J. W., de Hollander, G., Wertheimer, M. D., DePew, A. N. & Knapen, T. Separable pupillary signatures of perception and action during perceptual multistability. eLlife 10, e66161 (2021).
Turi, M., Burr, D. C. & Binda, P. Pupillometry reveals perceptual differences that are tightly linked to autistic traits in typical adults. eLife 7, e32399 (2018).
Einhäuser, W., Stout, J., Koch, C. & Carter, O. Pupil dilation reflects perceptual selection and predicts subsequent stability in perceptual rivalry. Proc. Natl Acad. Sci. USA 105, 1704–1709 (2008).
Acquafredda, M., Binda, P. & Lunghi, C. Attention cueing in rivalry: insights from pupillometry. eNeuro 9, ENEURO.0497-21.2022 (2022).
Lowe, S. W. & Ogle, K. N. Dynamics of the pupil during binocular rivalry. Arch. Ophthalmol. 75, 395–403 (1966).
Kloosterman, N. A. et al. Pupil size tracks perceptual content and surprise. Eur. J. Neurosci. 41, 1068–1078 (2015).
Strauch, C., Greiter, L. & Huckauf, A. Pupil dilation but not microsaccade rate robustly reveals decision formation. Sci. Rep. 8, 13165 (2018).
de Gee, J. W., Knapen, T. & Donner, T. H. Decision-related pupil dilation reflects upcoming choice and individual bias. Proc. Natl Acad. Sci. USA 111, E618–E625 (2014).
Rozado, D., Lochner, M., Engelke, U. & Dünser, A. Detecting intention through motor-imagery-triggered pupil dilations. Hum. Comput. Interact. 34, 83–113 (2019).
Richer, F. & Beatty, J. Pupillary dilations in movement preparation and execution. Psychophysiology 22, 204–207 (1985).
Mischkowski, D., Palacios-Barrios, E. E., Banker, L., Dildine, T. C. & Atlas, L. Y. Pain or nociception? Subjective experience mediates the effects of acute noxious heat on autonomic responses — corrected and republished. Pain 160, 1469–1481 (2019).
van Hooijdonk, R. et al. Touch-induced pupil size reflects stimulus intensity, not subjective pleasantness. Exp. Brain Res. 237, 201–210 (2019).
Ellermeier, W. & Westphal, W. Gender differences in pain ratings and pupil reactions to painful pressure stimuli. Pain 61, 435–439 (1995).
Lewandowska, K., Gagol, A., Sikora-Wachowicz, B., Marek, T. & Fąfrowicz, M. Saying “yes” when you want to say “no” — pupil dilation reflects evidence accumulation in a visual working memory recognition task. Int. J. Psychophysiol. 139, 18–32 (2019).
Jessen, S., Altvater-Mackensen, N. & Grossmann, T. Pupillary responses reveal infants’ discrimination of facial emotions independent of conscious perception. Cognition 150, 163–169 (2016).
Kinner, V. L. et al. What our eyes tell us about feelings: tracking pupillary responses during emotion regulation processes. Psychophysiology 54, 508–518 (2017).
Bradley, M. M., Miccoli, L., Escrig, M. A. & Lang, P. J. The pupil as a measure of emotional arousal and autonomic activation. Psychophysiology 45, 602–607 (2008).
Partala, T. & Surakka, V. Pupil size variation as an indication of affective processing. Int. J. Hum. Comput Stud. 59, 185–198 (2003).
Kashihara, K., Okanoya, K. & Kawai, N. Emotional attention modulates microsaccadic rate and direction. Psychol. Res. 78, 166–179 (2014).
de’Sperati, C., Roatta, S., Zovetti, N. & Baroni, T. Decoding overt shifts of attention in depth through pupillary and cortical frequency tagging. J. Neural Eng. 18, 036008 (2021).
Kang, O. & Wheatley, T. Pupil dilation patterns reflect the contents of consciousness. Conscious. Cogn. 35, 128–135 (2015).
Kang, O. & Banaji, M. R. Pupillometric decoding of high-level musical imagery. Conscious. Cogn. 77, 102862 (2020).
Monster, A. W., Chan, H. C. & O’Connor, D. Long-term trends in human eye blink rate. Biotelem. Patient Monit. 5, 206–222 (1978).
Doane, M. G. Interactions of eyelids and tears in corneal wetting and the dynamics of the normal human eyeblink. Am. J. Ophthalmol. 89, 507–516 (1980).
Guipponi, O., Odouard, S., Pinède, S., Wardak, C. & Ben Hamed, S. fMRI cortical correlates of spontaneous eye blinks in the nonhuman primate. Cereb. Cortex 25, 2333–2345 (2015).
Golan, T. et al. Human intracranial recordings link suppressed transients rather than ‘filling-in’ to perceptual continuity across blinks. eLife 5, e17243 (2016).
Kern, M., Schulze-Bonhage, A. & Ball, T. Blink- and saccade-related suppression effects in early visual areas of the human brain: intracranial EEG investigations during natural viewing conditions. Neuroimage 230, 117788 (2021).
Blin, O., Masson, G., Azulay, J. P., Fondarai, J. & Serratrice, G. Apomorphine-induced blinking and yawning in healthy volunteers. Br. J. Clin. Pharmacol. 30, 769–773 (1990).
Nakano, T. Blink-related dynamic switching between internal and external orienting networks while viewing videos. Neurosci. Res. 96, 54–58 (2015).
Holland, M. K. & Tarlow, G. Blinking and thinking. Percept. Mot. Skills 41, 503–506 (1975).
Slagter, H. A., Georgopoulou, K. & Frank, M. J. Spontaneous eye blink rate predicts learning from negative, but not positive, outcomes. Neuropsychologia 71, 126–132 (2015).
Jongkees, B. J. & Colzato, L. S. Spontaneous eye blink rate as predictor of dopamine-related cognitive function — a review. Neurosci. Biobehav. Rev. 71, 58–82 (2016).
Nakano, T., Kato, M., Morito, Y., Itoi, S. & Kitazawa, S. Blink-related momentary activation of the default mode network while viewing videos. Proc. Natl Acad. Sci. USA 110, 702–706 (2013).
Volkmann, F. C., Riggs, L. A. & Moore, R. K. Eyeblinks and visual suppression. Science 207, 900–902 (1980).
Hari, R., Salmellin, R., Tissari, S. O., Kajola, M. & Virsu, V. Visual stability during eyeblinks. Nature 367, 121–122 (1994).
Bristow, D., Frith, C. & Rees, G. Two distinct neural effects of blinking on human visual processing. Neuroimage 27, 136–145 (2005).
Ranti, C., Jones, W., Klin, A. & Shultz, S. Blink rate patterns provide a reliable measure of individual engagement with scene content. Sci. Rep. 10, 8267 (2020).
Nakano, T., Yamamoto, Y., Kitajo, K., Takahashi, T. & Kitazawa, S. Synchronization of spontaneous eyeblinks while viewing video stories. Proc. Biol. Sci. 276, 3635–3644 (2009).
Hoppe, D., Helfmann, S. & Rothkopf, C. A. Humans quickly learn to blink strategically in response to environmental task demands. Proc. Natl Acad. Sci. USA 115, 2246–2251 (2018).
Shultz, S., Klin, A. & Jones, W. Inhibition of eye blinking reveals subjective perceptions of stimulus salience. Proc. Natl Acad. Sci. USA 108, 21270–21275 (2011).
Abeles, D., Amit, R., Tal-Perry, N., Carrasco, M. & Yuval-Greenberg, S. Oculomotor inhibition precedes temporally expected auditory targets. Nat. Commun. 11, 3524 (2020).
Bonfiglio, L. et al. Cortical source of blink-related delta oscillations and their correlation with levels of consciousness. Hum. Brain Mapp. 34, 2178–2189 (2013).
Bonfiglio, L. et al. Spectral parameters modulation and source localization of blink-related alpha and low-beta oscillations differentiate minimally conscious state from vegetative state/unresponsive wakefulness syndrome. PLoS ONE 9, e93252 (2014).
Magliacano, A. et al. Spontaneous eye blinking as a diagnostic marker in prolonged disorders of consciousness. Sci. Rep. 11, 22393 (2021).
Bonfiglio, L. et al. Spontaneous blinking behaviour in persistent vegetative and minimally conscious states: relationships with evolution and outcome. Brain Res. Bull. 68, 163–170 (2005).
Maffei, A. & Angrilli, A. Spontaneous eye blink rate: an index of dopaminergic component of sustained attention and fatigue. Int. J. Psychophysiol. 123, 58–63 (2018).
Jacobs, L., Feldman, M. & Bender, M. B. Eye movements during sleep. I. The pattern in the normal human. Arch. Neurol. 25, 151–159 (1971).
Zargari Marandi, R., Madeleine, P., Omland, O., Vuillerme, N. & Samani, A. Eye movement characteristics reflected fatigue development in both young and elderly individuals. Sci. Rep. 8, 13148 (2018).
Stern, J. A., Boyer, D. & Schroeder, D. Blink rate: a possible measure of fatigue. Hum. Factors 36, 285–297 (1994).
Barbato, G. et al. Effects of sleep deprivation on spontaneous eye blink rate and alpha EEG power. Biol. Psychiatry 38, 340–341 (1995).
Crevits, L., Simons, B. & Wildenbeest, J. Effect of sleep deprivation on saccades and eyelid blinking. Eur. Neurol. 50, 176–180 (2003).
Ponder, E. & Kennedy, W. P. On the act of blinking. Q. J. Exp. Physiol. 18, 89–198 (1927).
Nakano, T. & Miyazaki, Y. Blink synchronization is an indicator of interest while viewing videos. Int. J. Psychophysiol. 135, 1–11 (2019).
Smilek, D., Carriere, J. S. & Cheyne, J. A. Out of mind, out of sight: eye blinking as indicator and embodiment of mind wandering. Psychol. Sci. 21, 786–789 (2010).
Mason, M. F. et al. Wandering minds: the default network and stimulus-independent thought. Science 315, 393–395 (2007).
Zhang, H., Anderson, N. C. & Miller, K. F. Refixation patterns of mind-wandering during real-world scene perception. J. Exp. Psychol. Hum. Percept. Perform. 47, 36–52 (2021).
Tebecis, A. K. & Provins, K. A. Hypnosis and eye movements. Biol. Psychol. 3, 31–47 (1975).
Brych, M., Murali, S. & Handel, B. The role of blinks, microsaccades and their retinal consequences in bistable motion perception. Front. Psychol. 12, 647256 (2021).
Ito, J. et al. Perceptual switching, eye movements, and the bus paradox. Perception 32, 681–698 (2003).
Kalisvaart, J. P. & Goossens, J. Influence of retinal image shifts and extra-retinal eye movement signals on binocular rivalry alternations. PLoS ONE 8, e61702 (2013).
Murali, S. & Händel, B. Spontaneous eye blinks map the probability of perceptual reinterpretation during visual and auditory ambiguity. Cogn. Sci. 48, e13414 (2024).
Bonneh, Y. S., Adini, Y. & Polat, U. Contrast sensitivity revealed by spontaneous eyeblinks: evidence for a common mechanism of oculomotor inhibition. J. Vis. 16, 1–15 (2016).
Paparella, G. et al. Painful stimulation increases spontaneous blink rate in healthy subjects. Sci. Rep. 10, 20014 (2020).
Bradley, M. M., Codispoti, M., Cuthbert, B. N. & Lang, P. J. Emotion and motivation I: defensive and appetitive reactions in picture processing. Emotion 1, 276–298 (2001).
Hare, R., Wood, K., Britain, S. & Shadman, J. Autonomic responses to affective visual stimulation. Psychophysiology 7, 408–417 (1970).
Terhune, D. B., Sullivan, J. G. & Simola, J. M. Time dilates after spontaneous blinking. Curr. Biol. 26, R459–R460 (2016).
Rosenzweig, G. & Bonneh, Y. S. Familiarity revealed by involuntary eye movements on the fringe of awareness. Sci. Rep. 9, 3029 (2019).
Walcher, S., Korner, C. & Benedek, M. Looking for ideas: eye behavior during goal-directed internally focused cognition. Conscious. Cogn. 53, 165–175 (2017).
Annerer-Walcher, S., Korner, C., Beaty, R. E. & Benedek, M. Eye behavior predicts susceptibility to visual distraction during internally directed cognition. Atten. Percept. Psychophys. 82, 3432–3444 (2020).
Callara, A. L., Greco, A., Scilingo, E. P. & Bonfiglio, L. Neuronal correlates of eyeblinks are an expression of primary consciousness phenomena. Sci. Rep. 13, 12617 (2023).
Van Opstal, F., De Loof, E., Verguts, T. & Cleeremans, A. Spontaneous eyeblinks during breaking continuous flash suppression are associated with increased detection times. J. Vis. 16, 21 (2016).
Mahanama, B. et al. Eye movement and pupil measures: a review. Front. Comp. Sci. 3, 1–22 (2022).
Intoy, J. & Rucci, M. Finely tuned eye movements enhance visual acuity. Nat. Commun. 11, 795 (2020).
Rucci, M., Iovin, R., Poletti, M. & Santini, F. Miniature eye movements enhance fine spatial detail. Nature 447, 851–854 (2007).
Pritchard, R. M. Stabilized images on the retina. Sci. Am. 204, 72–78 (1961).
Coiner, B. et al. Functional neuroanatomy of the human eye movement network: a review and atlas. Brain Struct. Funct. 224, 2603–2617 (2019).
Hafed, Z. M., Goffart, L. & Krauzlis, R. J. A neural mechanism for microsaccade generation in the primate superior colliculus. Science 323, 940–943 (2009).
Bremmer, F., Kubischik, M., Hoffmann, K. P. & Krekelberg, B. Neural dynamics of saccadic suppression. J. Neurosci. 29, 12374–12383 (2009).
Golan, T. et al. Increasing suppression of saccade-related transients along the human visual hierarchy. eLife 6, e27819 (2017).
Sdoia, S., Conversi, D., Pecchinenda, A. & Ferlazzo, F. Access to consciousness of briefly presented visual events is modulated by transcranial direct current stimulation of left dorsolateral prefrontal cortex. Sci. Rep. 9, 10950 (2019).
Schall, J. D. The neural selection and control of saccades by the frontal eye field. Philos. Trans. R. Soc. Lond. B 357, 1073–1082 (2002).
Rivaud, S., Müri, R. M., Gaymard, B., Vermersch, A. I. & Pierrot-Deseilligny, C. Eye movement disorders after frontal eye field lesions in humans. Exp. Brain Res. 102, 110–120 (1994).
Ting, W. K., Perez Velazquez, J. L. & Cusimano, M. D. Eye movement measurement in diagnostic assessment of disorders of consciousness. Front. Neurol. 5, 137 (2014).
Overbeek, B., Eilander, H., Lavrijsen, J. & Koopmans, R. Are visual functions diagnostic signs of the minimally conscious state? An integrative review. Brain Inj. 33, 234–234 (2019).
Yagi, T. & Baba, S. Evaluation of the brain-stem function by the auditory brain-stem response and the caloric vestibular reaction in comatose patient. Arch. Otorhinolaryngol. 238, 33–43 (1983).
Schlosser, H. G., Unterberg, A. & Clarke, A. Using video-oculography for galvanic evoked vestibulo-ocular monitoring in comatose patients. J. Neurosci. Methods 145, 127–131 (2005).
Trojano, L. et al. Quantitative assessment of visual behavior in disorders of consciousness. J. Neurol. 259, 1888–1895 (2012).
Giacino, J. T. & Kalmar, K. The vegetative and minimally conscious states: a comparison of clinical features and functional outcome. J. Head Trauma Rehab. 12, 36–51 (1997).
Dolce, G. et al. Visual pursuit in the severe disorder of consciousness. J. Neurotrauma 28, 1149–1154 (2011).
Kallio, S., Hyönä, J., Revonsuo, A., Sikka, P. & Nummenmaa, L. The existence of a hypnotic state revealed by eye movements. PLoS ONE 6, e26374 (2011).
Weitzenhoffer, A. M. Hypnosis and eye movements. I. Preliminary report on a possible slow eye movement correlate of hypnosis. Am. J. Clin. Hypn. 11, 221–227 (1969).
Lange, E. B., Zweck, F. & Sinn, P. Microsaccade-rate indicates absorption by music listening. Conscious. Cogn. 55, 59–78 (2017).
Hebbard, F. W. & Fischer, R. Effect of psilocybin, LSD, and mescaline on small, involuntary eye movements. Psychopharmacologia 9, 146–156 (1966).
Aserinsky, E. & Kleitman, N. Regularly occurring periods of eye motility, and concomitant phenomena, during sleep. Science 118, 273–274 (1953).
Zils, E., Sprenger, A., Heide, W., Born, J. & Gais, S. Differential effects of sleep deprivation on saccadic eye movements. Sleep 28, 1109–1115 (2005).
De Gennaro, L., Ferrara, M., Curcio, G. & Bertini, M. Visual search performance across 40 h of continuous wakefulness: measures of speed and accuracy and relation with oculomotor performance. Physiol. Behav. 74, 197–204 (2001).
Di Stasi, L. L. et al. Microsaccade and drift dynamics reflect mental fatigue. Eur. J. Neurosci. 38, 2389–2398 (2013).
Di Stasi, L. L. et al. Towards a driver fatigue test based on the saccadic main sequence: a partial validation by subjective report data. Transp. Res. C. Emerg. Technol. 21, 122–133 (2012).
Grace, P. M., Stanford, T., Gentgall, M. & Rolan, P. E. Utility of saccadic eye movement analysis as an objective biomarker to detect the sedative interaction between opioids and sleep deprivation in opioid-naive and opioid-tolerant populations. J. Psychopharmacol. 24, 1631–1640 (2010).
Scholes, C., McGraw, P. V., Nyström, M. & Roach, N. W. Fixational eye movements predict visual sensitivity. Proc. Biol. Sci. 282, 20151568 (2015).
Engbert, R. & Kliegl, R. Microsaccades uncover the orientation of covert attention. Vis. Res. 43, 1035–1045 (2003).
Pastukhov, A., Vonau, V., Stonkute, S. & Braun, J. Spatial and temporal attention revealed by microsaccades. Vis. Res. 85, 45–57 (2013).
Kaufmann, B. C. et al. Eyetracking during free visual exploration detects neglect more reliably than paper-pencil tests. Cortex 129, 223–235 (2020).
Ro, T., Rorden, C., Driver, J. & Rafal, R. Ipsilesional biases in saccades but not perception after lesions of the human inferior parietal lobule. J. Cogn. Neurosci. 13, 920–929 (2001).
Laubrock, J., Engbert, R. & Kliegl, R. Fixational eye movements predict the perceived direction of ambiguous apparent motion. J. Vis. 8, 13.1–13.17 (2008).
Madelain, L. & Krauzlis, R. J. Pursuit of the ineffable: perceptual and motor reversals during the tracking of apparent motion. J. Vis. 3, 642–653 (2003).
Troncoso, X. G., Macknik, S. L., Otero-Millan, J. & Martinez-Conde, S. Microsaccades drive illusory motion in the enigma illusion. Proc. Natl Acad. Sci. USA 105, 16033–16038 (2008).
Piras, A., Raffi, M., Persiani, M., Perazzolo, M. & Squatrito, S. Effect of heading perception on microsaccade dynamics. Behav. Brain Res. 312, 246–252 (2016).
Yarbus, A. L. Eye Movements and Vision (Plenum, 1967).
Senzai, Y. & Scanziani, M. A cognitive process occurring during sleep is revealed by rapid eye movements. Science 377, 999–1004 (2022).
Herman, J. H. et al. Evidence for a directional correspondence between eye movements and dream imagery in REM sleep. Sleep 7, 52–63 (1984).
Dement, W. & Wolpert, E. A. The relation of eye movements, body motility, and external stimuli to dream content. J. Exp. Psychol. 55, 543–553 (1958).
Hobson, J. A., Goldfrank, F. & Snyder, F. Respiration and mental activity in sleep. J. Psychiatr. Res. 3, 79–90 (1965).
Gurtner, L. M., Hartmann, M. & Mast, F. W. Eye movements during visual imagery and perception show spatial correspondence but have unique temporal signatures. Cognition 210, 104597 (2021).
Brandt, S. A. & Stark, L. W. Spontaneous eye movements during visual imagery reflect the content of the visual scene. J. Cogn. Neurosci. 9, 27–38 (1997).
Totten, E. Eye movement during visual imagery. Comp. Psychol. Monogr. 11, 46 (1935).
Johansson, R., Holsanova, J. & Holmqvist, K. Pictures and spoken descriptions elicit similar eye movements during mental imagery, both in light and in complete darkness. Cogn. Sci. 30, 1053–1079 (2006).
Bonneh, Y. S., Adini, Y. & Polat, U. Contrast sensitivity revealed by microsaccades. J. Vis. 15, 11 (2015).
Yu, G., Yang, M., Yu, P. & Dorris, M. C. Time compression of visual perception around microsaccades. J. Neurophysiol. 118, 416–424 (2017).
Morrone, M. C., Ross, J. & Burr, D. Saccadic eye movements cause compression of time as well as space. Nat. Neurosci. 8, 950–954 (2005).
Ross, J., Morrone, M. C. & Burr, D. C. Compression of visual space before saccades. Nature 386, 598–601 (1997).
Enoksson, P. Binocular rivalry and monocular dominance studied with optokinetic nystagmus. Acta Ophthalmol. 41, 544–563 (1963).
White, A. L., Moreland, J. C. & Rolfs, M. Oculomotor freezing indicates conscious detection free of decision bias. J. Neurophysiol. 127, 571–585 (2022).
Kronemer, S. I. et al. Eye metrics are a marker of visual conscious awareness and neural processing in cerebral blindness. Preprint at bioRxiv https://doi.org/10.1101/2025.01.06.631506 (2025)
White, A. L. & Rolfs, M. Oculomotor inhibition covaries with conscious detection. J. Neurophysiol. 116, 1507–1521 (2016).
Badde, S., Myers, C. F., Yuval-Greenberg, S. & Carrasco, M. Oculomotor freezing reflects tactile temporal expectation and aids tactile perception. Nat. Commun. 11, 3341 (2020).
Schiff, N. D. Cognitive motor dissociation following severe brain injuries. JAMA Neurol. 72, 1413–1415 (2015).
Ekman, P. & Friesen, W. V. Detecting deception from the body or face. J. Personal. Soc. Psychol. 29, 288–298 (1974).
Kapoustina, O., Echegaray-Benites, C. & Gélinas, C. Fluctuations in vital signs and behavioural responses of brain surgery patients in the intensive care unit: are they valid indicators of pain? J. Adv. Nurs. 70, 2562–2576 (2014).
Chatelle, C. et al. Assessment of nociception and pain in participants in an unresponsive or minimally conscious state after acquired brain injury: the relation between the coma recovery scale-revised and the nociception coma scale-revised. Arch. Phys. Med. Rehabil. 99, 1755–1762 (2018).
Gelinas, C. et al. Behaviors indicative of pain in brain-injured adult patients with different levels of consciousness in the Intensive care unit. J. Pain. Symptom Manag. 57, 761–773 (2019).
Arbour, C. et al. Detecting pain in traumatic brain-injured patients with different levels of consciousness during common procedures in the ICU: typical or atypical behaviors? Clin. J. Pain 30, 960–969 (2014).
Roulin, M. J. & Ramelet, A. S. Behavioral changes in brain-injured critical care adults with different levels of consciousness during nociceptive stimulation: an observational study. Intensive Care Med. 40, 1115–1123 (2014).
Pincherle, A. et al. Motor behavior unmasks residual cognition in disorders of consciousness. Ann. Neurol. 85, 443–447 (2019).
Remi, J. et al. The crossed leg sign indicates a favorable outcome after severe stroke. Neurology 77, 1453–1456 (2011).
Mélotte, E. et al. Swallowing in individuals with disorders of consciousness: a cohort study. Ann. Phys. Rehabil. Med. 64, 101403 (2021).
Mat, B. et al. New behavioral signs of consciousness in patients with severe brain injuries. Semin. Neurol. 42, 259–272 (2022).
Ouellette, S. M. & Simpson, C. Monitoring for intraoperative-awareness. AORN J. 68, 950–961 (1998).
Darwin, C. The Expression of Emotions in Man and Animals (D. Appleton and Company, 1872).
Takalkar, M., Xu, M., Wu, Q. & Chaczko, Z. A survey: facial micro-expression recognition. Multimed. Tools Appl. 77, 19301–19325 (2017).
Lang, P. J., Greenwald, M. K., Bradley, M. M. & Hamm, A. O. Looking at pictures: affective, facial, visceral, and behavioral reactions. Psychophysiology 30, 261–273 (1993).
Wolf, K. et al. The facial pattern of disgust, appetence, excited joy and relaxed joy: an improved facial EMG study. Scand. J. Psychol. 46, 403–409 (2005).
Rudokaite, J., Ertugrul, I. O., Ong, S., Janssen, M. P. & Huis In ‘t Veld, E. Predicting vasovagal reactions to needles from facial action units. J. Clin. Med. 12, 1644 (2023).
Schwartz, G. E., Brown, S. L. & Ahern, G. L. Facial muscle patterning and subjective experience during affective imagery: sex differences. Psychophysiology 17, 75–82 (1980).
Fridlund, A. J., Schwartz, G. E. & Fowler, S. C. Pattern recognition of self-reported emotional state from multiple-site facial EMG activity during affective imagery. Psychophysiology 21, 622–637 (1984).
Witvliet, C. V. & Vrana, S. R. Psychophysiological responses as indices of affective dimensions. Psychophysiology 32, 436–443 (1995).
Schwartz, G. E., Fair, P. L., Salt, P., Mandel, M. R. & Klerman, G. L. Facial muscle patterning to affective imagery in depressed and nondepressed subjects. Science 192, 489–491 (1976).
Raduga, M. ‘I love you’: the first phrase detected from dreams. Sleep Sci. 15, 149–157 (2022).
Raduga, M. Detecting lucid dreams only by submentalis electromyography. Sleep Med. 88, 221–230 (2021).
McGuigan, F. J. & Tanner, R. G. Covert oral behavior during conversational and visual dreams. Psychon. Sci. 23, 263–264 (1971).
Shimizu, A. & Inoue, T. Dreamed speech and speech muscle activity. Psychophysiology 23, 210–214 (1986).
Fenwick, P. et al. Lucid dreaming: correspondence between dreamed and actual events in one subject during REM sleep. Biol. Psychol. 18, 243–252 (1984).
Gardner, R. Jr., Grossman, W. I., Roffwarg, H. P. & Weiner, H. The relationship of small limb movements during REM sleep to dreamed limb action. Psychosom. Med. 37, 147–159 (1975).
Wolpert, E. A. Studies in psychophysiology of dreams. II. An electromyographic study of dreaming. AMA Arch. Gen. Psychiatry 2, 231–241 (1960).
Shepard, R. N. & Metzler, J. Mental rotation of three-dimensional objects. Science 171, 701–703 (1971).
Libet, B. The neural time factor in conscious and unconscious events. Ciba Found. Symp. 174, 123–137 (1993).
Cederblad, A. M. H., Visokomogilski, A., Andersen, S. K., MacLeod, M. J. & Sahraie, A. Conscious awareness modulates processing speed in the redundant signal effect. Exp. Brain Res. 239, 1877–1893 (2021).
Ben-Haim, M. S. et al. Disentangling perceptual awareness from nonconscious processing in rhesus monkeys (Macaca mulatta). Proc. Natl Acad. Sci. USA 118, e2017543118 (2021).
Wilson, S. A note on an associated movement of the eyes and ears in man. Rev. Neurol. Psychiatry 6, 331–336 (1908).
Strauss, D. J. et al. Vestigial auriculomotor activity indicates the direction of auditory attention in humans. eLife 9, e54536 (2020).
Salameh, C., Perchet, C., Hagiwara, K. & Garcia-Larrea, L. Sympathetic skin response as an objective tool to estimate stimulus-associated arousal in a human model of hyperalgesia. Neurophysiol. Clin. 52, 436–445 (2022).
Mangina, C. A. & Beuzeron-Mangina, J. H. Direct electrical stimulation of specific human brain structures and bilateral electrodermal activity. Int. J. Psychophysiol. 22, 1–8 (1996).
Boucsein, W. Electrodermal Activity 2nd edn (Springer, 2012).
Altintop, C. G., Latifoğlu, F., Akin, A. K., İleri, R. & Yazar, M. A. Analysis of consciousness level using galvanic skin response during therapeutic effect. J. Med. Syst. 45, 1–12 (2020).
Daltrozzo, J. et al. Emotional electrodermal response in coma and other low-responsive patients. Neurosci. Lett. 475, 44–47 (2010).
Salvato, G. et al. Autonomic responses to emotional linguistic stimuli and amplitude of low-frequency fluctuations predict outcome after severe brain injury. Neuroimage Clin. 28, 102356 (2020).
Reale, G. et al. The immediate effects of immersive virtual reality on autonomic nervous system function in patients with disorders of consciousness after severe acquired brain injury: a pilot study. J. Clin. Med. 12, 7639 (2023).
Luaute, J. et al. Electrodermal reactivity to emotional stimuli in healthy subjects and patients with disorders of consciousness. Ann. Phys. Rehabil. Med. 61, 401–406 (2018).
Storm, H. et al. Skin conductance correlates with perioperative stress. Acta Anaesthesiol. Scand. 46, 887–895 (2002).
Kurzová, A., Hess, L., Slíva, J. & Málek, J. Can changes in skin impedance be used to monitor sedation after midazolam and during recovery from anesthesia? Physiol. Res. 70, 265–272 (2021).
Storm, H., Shafiei, M., Myre, K. & Raeder, J. Palmar skin conductance compared to a developed stress score and to noxious and awakening stimuli on patients in anaesthesia. Acta Anaesthesiol. Scand. 49, 798–803 (2005).
Herlan, A., Ottenbacher, J., Schneider, J., Riemann, D. & Feige, B. Electrodermal activity patterns in sleep stages and their utility for sleep versus wake classification. J. Sleep Res. 28, 1–8 (2019).
Lester, B. K., Burch, N. R. & Dossett, R. C. Nocturnal EEG-GSR profiles: the influence of presleep states. Psychophysiology 3, 238–248 (1967).
Johnson, L. C. & Lubin, A. Spontaneous electrodermal activity during waking and sleeping. Psychophysiology 3, 8–17 (1966).
Richter, C. P. The significance of changes in the electrical resistance of the body during sleep. Proc. Natl Acad. Sci. USA 12, 214–222 (1926).
Walrath, L. C. & Hamilton, D. W. Autonomic correlates of meditation and hypnosis. Am. J. Clin. Hypn. 17, 190–197 (1975).
Block, R. I., Ghoneim, M. M., Fowles, D. C., Kumar, V. & Pathak, D. Effects of a subanesthetic concentration of nitrous oxide on establishment, elicitation, and semantic and phonemic generalization of classically conditioned skin conductance responses. Pharmacol. Biochem. Behav. 28, 7–14 (1987).
Block, R. I., Ghoneim, M. M., Kumar, V. & Pathak, D. Psychedelic effects of a subanesthetic concentration of nitrous oxide. Anesth. Prog. 37, 271–276 (1990).
Loggia, M. L., Juneau, M. & Bushnell, C. M. Autonomic responses to heat pain: heart rate, skin conductance, and their relation to verbal ratings and stimulus intensity. Pain 152, 592–598 (2011).
Ledowski, T. et al. Monitoring of skin conductance to assess postoperative pain intensity. Br. J. Anaesth. 97, 862–865 (2006).
Feinstein, B., Langton, J. N., Jameson, R. M. & Schiller, F. Experiments on pain referred from deep somatic tissues. J. Bone Joint Surg. Am. 36-A, 981–997 (1954).
Folkins, C. H., Lawson, K. D., Opton, E. M. Jr. & Lazarus, R. S. Desensitization and the experimental reduction of threat. J. Abnorm. Psychol. 73, 100–113 (1968).
Hoppe, J. M., Holmes, E. A. & Agren, T. Exploring the neural basis of fear produced by mental imagery: imaginal exposure in individuals fearful of spiders. Philos. Trans. R. Soc. Lond. B Biol. Sci. 376, 20190690 (2021).
Hauri, P. & Van de Castle, R. L. Psychophysiological parallels in dreams. Psychosom. Med. 35, 297–308 (1973).
He, W., Boesveldt, S., Delplanque, S., de Graaf, C. & de Wijk, R. A. Sensory-specific satiety: added insights from autonomic nervous system responses and facial expressions. Physiol. Behav. 170, 12–18 (2017).
Zaman, J., Van de Pavert, I., Van Oudenhove, L. & Van Diest, I. The use of stimulus perception to account for variability in skin conductance responses to interoceptive stimuli. Psychophysiology 57, e13494 (2020).
Vallar, G. & Ronchi, R. Somatoparaphrenia: a body delusion. A review of the neuropsychological literature. Exp. Brain Res. 192, 533–551 (2009).
Ramachandran, V. S., Rogers-Ramachandran, D. & Cobb, S. Touching the phantom limb. Nature 377, 489–490 (1995).
Gülbetekin, E. et al. Effects of right or left face stimulation on self and other perception in enfacement illusion. Soc. Neurosci. 16, 189–205 (2021).
Armel, K. C. & Ramachandran, V. S. Projecting sensations to external objects: evidence from skin conductance response. Proc. Biol. Sci. 270, 1499–1506 (2003).
Ocklenburg, S., Ruther, N., Peterburs, J., Pinnow, M. & Gunturkun, O. Laterality in the rubber hand illusion. Laterality 16, 174–187 (2011).
Palomo, P. et al. Subjective, behavioral, and physiological responses to the rubber hand illusion do not vary with age in the adult phase. Conscious. Cogn. 58, 90–96 (2018).
Moseley, G. L. et al. Psychologically induced cooling of a specific body part caused by the illusory ownership of an artificial counterpart. Proc. Natl Acad. Sci. USA 105, 13169–13173 (2008).
Kammers, M. P. M., Rose, K. & Haggard, P. Feeling numb: temperature, but not thermal pain, modulates feeling of body ownership. Neuropsychologia 49, 1316–1321 (2011).
Ehrsson, H. H., Wiech, K., Weiskopf, N., Dolan, R. J. & Passingham, R. E. Threatening a rubber hand that you feel is yours elicits a cortical anxiety response. Proc. Natl Acad. Sci. USA 104, 9828–9833 (2007).
Williams, L. M. et al. Mapping the time course of nonconscious and conscious perception of fear: an integration of central and peripheral measures. Hum. Brain Mapp. 21, 64–74 (2004).
Scott, R. B., Minati, L., Dienes, Z., Critchley, H. D. & Seth, A. K. Detecting conscious awareness from involuntary autonomic responses. Conscious. Cogn. 20, 936–942 (2011).
Chuen, L., Sears, D. & McAdams, S. Psychophysiological responses to auditory change. Psychophysiology 53, 891–904 (2016).
Lapate, R. C., Rokers, B., Li, T. & Davidson, R. J. Nonconscious emotional activation colors first impressions: a regulatory role for conscious awareness. Psychol. Sci. 25, 349–357 (2014).
Wilms, L. & Oberfeld, D. Color and emotion: effects of hue, saturation, and brightness. Psychol. Res. 82, 896–914 (2018).
Zieliński, P. An arousal effect of colors saturation a study of self-reported ratings and electrodermal responses. J. Psychophysiol. 30, 9–16 (2016).
Grossberg, J. M. & Wilson, H. K. Physiological changes accompanying the visualization of fearful and neutral situations. J. Personal. Soc. Psychol. 10, 124–133 (1968).
Cortese, B. M. et al. Olfactory cue reactivity in nicotine-dependent adult smokers. Psychol. Addict. Behav. 29, 91–96 (2015).
Wicken, M., Keogh, R. & Pearson, J. The critical role of mental imagery in human emotion: insights from fear-based imagery and aphantasia. Proc. Biol. Sci. 288, 20210267 (2021).
Palma, J. A. & Benarroch, E. E. Neural control of the heart: recent concepts and clinical correlations. Neurology 83, 261–271 (2014).
Skora, L. I., Livermore, J. J. A. & Roelofs, K. The functional role of cardiac activity in perception and action. Neurosci. Biobehav. Rev. 137, 104655 (2022).
Baguley, I. J. et al. Dysautonomia after traumatic brain injury: a forgotten syndrome? J. Neurol. Neurosurg. Psychiatry 67, 39–43 (1999).
Riganello, F. et al. A heartbeat away from consciousness: heart rate variability entropy can discriminate disorders of consciousness and is correlated with resting-state fMRI brain connectivity of the central autonomic network. Front. Neurol. 9, 769 (2018).
Raimondo, F. et al. Brain–heart interactions reveal consciousness in noncommunicating patients. Ann. Neurol. 82, 578–591 (2017).
Sattin, D. et al. Analyzing the loss and the recovery of consciousness: functional connectivity patterns and changes in heart rate variability during propofol-induced anesthesia. Front. Syst. Neurosci. 15, 652080 (2021).
Scheffer, G. J., Tenvoorde, B. J., Karemaker, J. M., Ros, H. H. & Delange, J. J. Effects of thiopentone, etomidate and propofol on beat-to-beat cardiovascular signals in man. Anaesthesia 48, 849–855 (1993).
Moerman, N., Bonke, B. & Oosting, J. Awareness and recall during general anesthesia. facts and feelings. Anesthesiology 79, 454–464 (1993).
DeBenedittis, G., Cigada, M., Bianchi, A., Signorini, M. G. & Cerutti, S. Autonomic changes during hypnosis: a heart rate variability power spectrum analysis as a marker of sympatho-vagal balance. Int. J. Clin. Exp. Hypn. 42, 140–152 (1994).
Oswald, V. et al. Autonomic nervous system modulation during self-induced non-ordinary states of consciousness. Sci. Rep. 13, 151811 (2023).
Cauthen, N. R. & Prymak, C. A. Meditation versus relaxation: an examination of the physiological effects of relaxation training and of different levels of experience with transcendental meditation. J. Consult. Clin. Psychol. 45, 496–497 (1977).
Yüksel, R., Ozcan, O. & Dane, S. The effects of hypnosis on heart rate variability. Int. J. Clin. Exp. Hypn. 61, 162–171 (2013).
Uslu, T. et al. Cerebral blood flow evaluation during the hypnotic state with transcranial doppler sonography. Int. J. Clin. Exp. Hypn. 60, 81–87 (2012).
Somers, V. K., Dyken, M. E., Mark, A. L. & Abboud, F. M. Sympathetic-nerve activity during sleep in normal subjects. N. Engl. J. Med. 328, 303–307 (1993).
Trinder, J. et al. Autonomic activity during human sleep as a function of time and sleep stage. J. Sleep Res. 10, 253–264 (2001).
Gronfier, C., Simon, C., Piquard, F., Ehrhart, J. & Brandenberger, G. Neuroendocrine processes underlying ultradian sleep regulation in man. J. Clin. Endocrinol. Metab. 84, 2686–2690 (1999).
Chouchou, F. & Desseilles, M. Heart rate variability: a tool to explore the sleeping brain? Front. Neurosci. 8, 402 (2014).
Brylowski, A., Levitan, L. & Laberge, S. H-Reflex suppression and autonomic activation during lucid rem-sleep: a case-study. Sleep 12, 374–378 (1989).
Nardelli, M. et al. Activation of brain–heart axis during REM sleep: a trigger for dreaming. Am. J. Physiol. Regul. Integr. Comp. Physiol. 321, R951–R959 (2021).
LaBerge, S., Levitan, L. & Dement, W. C. Lucid dreaming: physiological correlates of consciousness during REM sleep. J. Mind Behav. 7, 251–258 (1986).
Al, E. et al. Heart–brain interactions shape somatosensory perception and evoked potentials. Proc. Natl Acad. Sci. USA 117, 10575–10584 (2020).
Walker, B. B. & Sandman, C. A. Visual evoked-potentials change as heart-rate and carotid pressure change. Psychophysiology 19, 520–527 (1982).
Sandman, C. A. Augmentation of the auditory event related potentials of the brain during diastole. Int. J. Psychophysiol. 2, 111–119 (1984).
Leupin, V. & Britz, J. Interoceptive signals shape the earliest markers and neural pathway to awareness at the visual threshold. Proc. Natl Acad. Sci. USA 121, e2311953121 (2024).
Motyka, P. et al. Interactions between cardiac activity and conscious somatosensory perception. Psychophysiology 56, e13424 (2019).
Grund, M. et al. Respiration, heartbeat, and conscious tactile perception. J. Neurosci. 42, 643–656 (2022).
Sandman, C. A., McCanne, T. R., Kaiser, D. N. & Diamond, B. Heart rate and cardiac phase influences on visual perception. J. Comp. Physiol. Psychol. 91, 189–202 (1977).
Cobos, M. I., Guerra, P. M., Vila, J. & Chica, A. B. Heart-rate modulations reveal attention and consciousness interactions. Psychophysiology 56, e13295 (2019).
Salomon, R. et al. The insula mediates access to awareness of visual stimuli presented synchronously to the heartbeat. J. Neurosci. 36, 5115–5127 (2016).
Dworkin, B. R. et al. Central effects of baroreceptor activation in humans: attenuation of skeletal reflexes and pain perception. Proc. Natl Acad. Sci. USA 91, 6329–6333 (1994).
Pramme, L., Larra, M. F., Schächinger, H. & Frings, C. Cardiac cycle time effects on mask inhibition. Biol. Psychol. 100, 115–121 (2014).
Burton, A. R., Birznieks, I., Bolton, P. S., Henderson, L. A. & Macefield, V. G. Effects of deep and superficial experimentally induced acute pain on muscle sympathetic nerve activity in human subjects. J. Physiol. 587, 183–193 (2009).
Kyle, B. N. & McNeil, D. W. Autonomic arousal and experimentally induced pain: a critical review of the literature. Pain Res. Manag. 19, 159–167 (2014).
Burton, A. R., Brown, R. & Macefield, V. G. Selective activation of muscle and skin nociceptors does not trigger exaggerated sympathetic responses in spinal-injured subjects. Spinal Cord 46, 660–665 (2008).
Lang, P. J., Kozak, M. J., Miller, G. A., Levin, D. N. & McLean, A. Jr. Emotional imagery: conceptual structure and pattern of somato-visceral response. Psychophysiology 17, 179–192 (1980).
Wang, Y. & Morgan, W. P. The effect of imagery perspectives on the psychophysiological responses to imagined exercise. Behav. Brain Res. 52, 167–174 (1992).
Erlacher, D. & Schredl, M. Cardiovascular responses to dreamed physical exercise during REM lucid dreaming. Dreaming 18, 112–121 (2008).
Decety, J., Jeannerod, M., Germain, M. & Pastene, J. Vegetative response during imagined movement is proportional to mental effort. Behav. Brain Res. 42, 1–5 (1991).
Park, H. D., Correia, S., Ducorps, A. & Tallon-Baudry, C. Spontaneous fluctuations in neural responses to heartbeats predict visual detection. Nat. Neurosci. 17, 612–618 (2014).
Sadeghi, S., Wittmann, M., De Rosa, E. & Anderson, A. K. Wrinkles in subsecond time perception are synchronized to the heart. Psychophysiology 60, e14270 (2023).
Ogden, R. S., Dobbins, C., Slade, K., McIntyre, J. & Fairclough, S. The psychophysiological mechanisms of real-world time experience. Sci. Rep. 12, 12890 (2022).
Schwarz, M. A., Winkler, I. & Sedlmeier, P. The heart beat does not make us tick: the impacts of heart rate and arousal on time perception. Atten. Percept. Psychophys. 75, 182–193 (2013).
Kralemann, B. et al. In vivo cardiac phase response curve elucidates human respiratory heart rate variability. Nat. Commun. 4, 2418 (2013).
Parviainen, T., Lyyra, P. & Nokia, M. S. Cardiorespiratory rhythms, brain oscillatory activity and cognition: review of evidence and proposal for significance. Neurosci. Biobehav. Rev. 142, 104908 (2022).
Larsen, P. D., Tzeng, Y. C., Sin, P. Y. & Galletly, D. C. Respiratory sinus arrhythmia in conscious humans during spontaneous respiration. Respir. Physiol. Neurobiol. 174, 111–118 (2010).
Kluger, D. S. et al. Modulatory dynamics of periodic and aperiodic activity in respiration–brain coupling. Nat. Commun. 14, 4699 (2023).
Allen, M., Varga, S. & Heck, D. H. Respiratory rhythms of the predictive mind. Psychol. Rev. 130, 1066–1080 (2023).
Kluger, D. S. & Gross, J. Respiration modulates oscillatory neural network activity at rest. PLoS Biol. 19, e3001457 (2021).
Zelano, C. et al. Nasal respiration entrains human limbic oscillations and modulates cognitive function. J. Neurosci. 36, 12448–12467 (2016).
Yang, Q. et al. Smell-induced gamma oscillations in human olfactory cortex are required for accurate perception of odor identity. PLoS Biol. 20, 1–30 (2022).
Perl, O. et al. Human non-olfactory cognition phase-locked with inhalation. Nat. Hum. Behav. 3, 501–512 (2019).
Laing, D. G. Natural sniffing gives optimum odour perception for humans. Perception 12, 99–117 (1983).
Sobel, N., Khan, R. M., Saltman, A., Sullivan, E. V. & Gabrieli, J. D. E. Olfaction — the world smells different to each nostril. Nature 402, 35–35 (1999).
Solaz, J. et al. Drowsiness detection based on the analysis of breathing rate obtained from real-time image recognition. Transp. Res. Proc. 14, 3867–3876 (2016).
Kiashari, S. E. H., Nahvi, A., Homayounfard, A. & Bakhoda, H. Monitoring the variation in driver respiration rate from wakefulness to drowsiness: a non-instrusive method for drowsiness detection using thermal imaging. J. Sleep Sci. 3, 1–9 (2018).
Sattin, D. et al. Olfactory discrimination in disorders of consciousness: a new sniff protocol. Brain Behav. 9, e01273 (2019).
Arzi, A. et al. Olfactory sniffing signals consciousness in unresponsive patients with brain injuries. Nature 581, 428–433 (2020).
Charland-Verville, V. et al. Detection of response to command using voluntary control of breathing in disorders of consciousness. Front. Hum. Neurosci. 8, 1020 (2014).
Plotkin, A. et al. Sniffing enables communication and environmental control for the severely disabled. Proc. Natl Acad. Sci. USA 107, 14413–14418 (2010).
Douglas, N. J., White, D. P., Pickett, C. K., Weil, J. V. & Zwillich, C. W. Respiration during sleep in normal man. Thorax 37, 840–844 (1982).
Bulow, K. Respiration and wakefulness in man. Acta Physiol. Scand. Suppl. 209, 1–110 (1963).
Trinder, J., Whitworth, F., Kay, A. & Wilkin, P. Respiratory instability during sleep onset. J. Appl. Physiol. 73, 2462–2469 (1992).
Saraswat, V. Effects of anaesthesia techniques and drugs on pulmonary function. Indian J. Anaesth. 59, 557–564 (2015).
Flexman, J. E., Demaree, R. G. & Simpson, D. D. Respiratory phase and visual signal-detection. Percept. Psychophys. 16, 337–339 (1974).
Kluger, D. S., Balestrieri, E., Busch, N. A. & Gross, J. Respiration aligns perception with neural excitability. eLife 10, e70907 (2021).
Zaccaro, A., Perrucci, M. G., Parrotta, E., Costantini, M. & Ferri, F. Brain-heart interactions are modulated across the respiratory cycle via interoceptive attention. Neuroimage 262, 1–15 (2022).
Molle, L. & Coste, A. The respiratory modulation of interoception. J. Neurophysiol. 127, 896–899 (2022).
Arsenault, M., Ladouceur, A., Lehmann, A., Rainville, P. & Piche, M. Pain modulation induced by respiration: phase and frequency effects. Neuroscience 252, 501–511 (2013).
Homma, I. & Masaoka, Y. Breathing rhythms and emotions. Exp. Physiol. 93, 1011–1021 (2008).
Gomez, P. & Danuser, B. Affective and physiological responses to environmental noises and music. Int. J. Psychophysiol. 53, 91–103 (2004).
Gomez, P., Stahel, W. A. & Danuser, B. Respiratory responses during affective picture viewing. Biol. Psychol. 67, 359–373 (2004).
Ogden, R. S., Henderson, J., Slade, K., McGlone, F. & Richter, M. The effect of increased parasympathetic activity on perceived duration. Conscious. Cogn. 76, 102829 (2019).
Mainland, J. & Sobel, N. The sniff is part of the olfactory percept. Chem. Senses 31, 181–196 (2006).
Masaoka, Y., Koiwa, N. & Homma, I. Inspiratory phase-locked alpha oscillation in human olfaction: source generators estimated by a dipole tracing method. J. Physiol. 566, 979–997 (2005).
Huijbers, W. et al. Respiration phase-locks to fast stimulus presentations: implications for the interpretation of posterior midline “deactivations”. Hum. Brain Mapp. 35, 4932–4943 (2014).
Bocca, E., Antonelli, A. R. & Mosciaro, O. Mechanical co-factors in olfactory stimulation. Acta Oto-Laryngol. 59, 243–247 (1965).
Leopold, D. Distortion of olfactory perception: diagnosis and treatment. Chem. Senses 27, 611–615 (2002).
Frank, R. A., Dulay, M. F. & Gesteland, R. C. Assessment of the sniff magnitude test as a clinical test of olfactory function. Physiol. Behav. 78, 195–204 (2003).
Bensafi, M. et al. Olfactomotor activity during imagery mimics that during perception. Nat. Neurosci. 6, 1142–1144 (2003).
Bensafi, M., Pouliot, S. & Sobel, N. Odorant-specific patterns of sniffing during imagery distinguish ‘bad’ and ‘good’ olfactory imagers. Chem. Senses 30, 521–529 (2005).
Oudiette, D. et al. Author correction: REM sleep respiratory behaviours match mental content in narcoleptic lucid dreamers. Sci. Rep. 8, 6128 (2018).
Breivik, H. et al. Assessment of pain. Br. J. Anaesth. 101, 17–24 (2008).
Daguet, I., Bouhassira, D. & Gronfier, C. Baseline pupil diameter is not a reliable biomarker of subjective sleepiness. Front. Neurol. 10, 108 (2019).
Cross, D. J. et al. Distinct neural circuits underlie assessment of a diversity of natural dangers by American crows. Proc. Biol. Sci. 280, 20131046 (2013).
Mott, R. O., Hawthorne, S. J. & McBride, S. D. Blink rate as a measure of stress and attention in the domestic horse (Equus caballus). Sci. Rep. 10, 21409 (2020).
Ungurean, G., Martinez-Gonzalez, D., Massot, B., Libourel, P. A. & Rattenborg, N. C. Pupillary behavior during wakefulness, non-REM sleep, and REM sleep in birds is opposite that of mammals. Curr. Biol. 31, 5370–5376 (2021).
Pophale, A. et al. Wake-like skin patterning and neural activity during octopus sleep. Nature 619, 129–134 (2023).
Medeiros, S. L. S. et al. Cyclic alternation of quiet and active sleep states in the octopus. iScience 24, 102223 (2021).
Alamia, A., VanRullen, R., Pasqualotto, E., Mouraux, A. & Zenon, A. Pupil-linked arousal responds to unconscious surprisal. J. Neurosci. 39, 5369–5376 (2019).
Barbur, J. L., Weiskrantz, L. & Harlow, J. A. The unseen color aftereffect of an unseen stimulus: insight from blindsight into mechanisms of color afterimages. Proc. Natl Acad. Sci. USA 96, 11637–11641 (1999).
Harlan, G. C. Pupil reflex in absolute blindness. Trans. Am. Ophthalmol. Soc. 7, 671–677 (1896).
Weiskrantz, L., Cowey, A. & Le Mare, C. Learning from the pupil: a spatial visual channel in the absence of V1 in monkey and human. Brain 121, 1065–1072 (1998).
Abouleish, E. & Taylor, F. H. Effect of morphine–diazepam on signs of anesthesia, awareness, and dreams of patients under N2O for cesarean section. Anesth. Analg. 55, 702–705 (1976).
Arzi, A., Rozenkrantz, L., Holtzman, Y., Secundo, L. & Sobel, N. Sniffing patterns uncover implicit memory for undetected odors. Curr. Biol. 24, R263–R264 (2014).
McGinnies, E. Emotionality and perceptual defense. Psychol. Rev. 56, 244–251 (1949).
Dimberg, U., Thunberg, M. & Elmehed, K. Unconscious facial reactions to emotional facial expressions. Psychol. Sci. 11, 86–89 (2000).
Wiemer, J., Gerdes, A. B. & Pauli, P. The effects of an unexpected spider stimulus on skin conductance responses and eye movements: an inattentional blindness study. Psychol. Res. 77, 155–166 (2013).
Chechko, N. et al. Progressively analogous evidence of covert face recognition from functional magnetic resonance imaging and skin conductance responses studies involving a patient with dissociative amnesia. Eur. J. Neurosci. 48, 1964–1975 (2018).
Tamietto, M. et al. Unseen facial and bodily expressions trigger fast emotional reactions. Proc. Natl Acad. Sci. USA 106, 17661–17666 (2009).
Gallegos-Ayala, G. et al. Brain communication in a completely locked-in patient using bedside near-infrared spectroscopy. Neurology 82, 1930–1932 (2014).
Grujic, N., Polania, R. & Burdakov, D. Neurobehavioral meaning of pupil size. Neuron 112, 3381–3395 (2024).
Meissner, S. N. et al. Self-regulating arousal via pupil-based biofeedback. Nat. Hum. Behav. https://doi.org/10.1038/s41562-023-01729-z (2023).
Rainey, S., Martin, S., Christen, A., Megevand, P. & Fourneret, E. Brain recording, mind-reading, and neurotechnology: ethical issues from consumer devices to brain-based speech decoding. Sci. Eng. Ethics 26, 2295–2311 (2020).
Drew, L. Mind-reading machines are coming — how can we keep them in check? Nature 620, 18–19 (2023).
Mecacci, G. & Haselager, P. Identifying criteria for the evaluation of the implications of brain reading for mental privacy. Sci. Eng. Ethics 25, 443–461 (2019).
Istace, T. Neurorights: the debate about new legal safeguards to protect the mind. Issues Law Med. 37, 95–114 (2022).
Sim, I. Mobile devices and health. N. Engl. J. Med. 381, 956–968 (2019).
Zheng, N. S. et al. Sleep patterns and risk of chronic disease as measured by long-term monitoring with commercial wearable devices in the all of us research program. Nat. Med. 30, 2648–2656 (2024).
Kronemer, S. I. et al. Cross-species real-time detection of trends in pupil size fluctuation. Behav. Res. Methods 57, 9 (2025).
Meegan, D. V. Neuroimaging techniques for memory detection: scientific, ethical, and legal issues. Am. J. Bioeth. 8, 9–20 (2008).
Shao, D. D., Liu, C. B. & Tsow, F. Noncontact physiological measurement using a camera: a technical review and future directions. ACS Sens. 6, 321–334 (2021).
Fultz, N. E. et al. Coupled electrophysiological, hemodynamic, and cerebrospinal fluid oscillations in human sleep. Science 366, 628–631 (2019).
Gottschalk, L. A., Stone, W. N., Gleser, G. C. & Iacono, J. M. Anxiety levels in dreams: relation to changes in plasma free fatty acids. Science 153, 654–657 (1966).
Wooley, S. C. & Wooley, O. W. Salivation to the sight and thought of food: a new measure of appetite. Psychosom. Med. 35, 136–142 (1973).
Jenkins, G. N. & Dawes, C. The psychic flow of saliva in man. Arch. Oral. Biol. 11, 1203–1204 (1966).
Wilhelm, B., Jordan, M. & Birbaumer, N. Communication in locked-in syndrome: effects of imagery on salivary pH. Neurology 67, 534–535 (2006).
Acknowledgements
This research was made possible by support from the National Institute of Mental Health Intramural Research Program (ZIAMH002783). The authors thank members of the Section on Functional Imaging Methods for their constructive feedback. The views expressed in this article do not necessarily represent the views of the National Institutes of Health, the Department of Health and Human Services, or the United States Government.
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S.I.K. researched data for the article, provided a substantial contribution to discussion of its content, wrote the article and reviewed/edited the manuscript before submission. P.A.B. and J.G.-C. provided substantial contributions to discussion of the article’s content and reviewed/edited the manuscript before submission.
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Glossary
- Conscious content
-
The subject or category of subjective experience; also called qualia, phenomenology or phenomenal experience.
- Conscious level
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The likelihood of being or becoming conscious, closely linked with the physiological states of arousal, alertness and vigilance.
- Conscious state
-
The unique combination of conscious level and content characteristic of specific states of consciousness.
- Covert measures of consciousness
-
Physiological signals linked to conscious level, state or content (for example, pupil size, eye movements and skin conductance).
- Enabling factor of consciousness
-
Prerequisite or precursor physiological states (for example, arousal and attention) that facilitate consciousness. These factors are often necessary but not sufficient for consciousness.
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Kronemer, S.I., Bandettini, P.A. & Gonzalez-Castillo, J. Sleuthing subjectivity: a review of covert measures of consciousness. Nat. Rev. Neurosci. 26, 476–496 (2025). https://doi.org/10.1038/s41583-025-00934-1
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DOI: https://doi.org/10.1038/s41583-025-00934-1
