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How consciousness (experience) arises from and relates to material brain processes (the "mind-body problem") has been pondered by thinkers for centuries, and is regarded as among the deepest unsolved problems in science, with wide-ranging theoretical, clinical, and ethical implications. Until the last few decades, this was largely seen as a philosophical topic, but not widely accepted in mainstream neuroscience. Since the 1980s, however, novel methods and theoretical advances have yielded remarkable results, opening up the field for scientific and clinical progress. Since a seminal paper by Crick and Koch (1998) claimed that a science of consciousness should first search for its neural correlates (NCC), a variety of correlates have been suggested, including both content-specific NCCs, determining particular phenomenal components within an experience, and the full NCC, the neural substrates supporting entire conscious experiences. In this review, we present recent progress on theoretical, experimental, and clinical issues. Specifically, we (1) review methodological advances that are important for dissociating conscious experience from related enabling and executive functions, (2) suggest how critically reconsidering the role of the frontal cortex may further delineate NCCs, (3) advocate the need for general, objective, brain-based measures of the capacity for consciousness that are independent of sensory processing and executive functions, and (4) show how animal studies can reveal population and network phenomena of relevance for understanding mechanisms of consciousness. ; European Union's Horizon 2020 Research and Innovation Programme ; Hermann and Lilly Schilling Foundation ; German Research Foundation ; Center for Nanoscale Microscopy and Molecular Physiology of the Brain ; National Institutes of Health/National Institute of Neurological Disorders and Stroke ; Sao Paulo Research Foundation ; James S. McDonnell Foundation Scholar Award ; EU Grant H2020-FETOPEN ; Canadian Institute for Advanced Research ; Azrieli Program in Brain, Mind and Consciousness ; FLAG-ERA JTC project CANON ; Norwegian Research Council ; Netherlands Organization for Scientific Research ; Univ Oslo, Inst Basal Med Sci, Div Physiol, Dept Mol Med, POB 1103 Blindern, N-0317 Oslo, Norway ; Univ Wisconsin, Dept Neurol, Madison, WI 53705 USA ; Univ Wisconsin, Dept Psychiat, Madison, WI 53719 USA ; Univ Fed Sao Paulo, Inst Sci & Technol, BR-12231280 Sao Jose Dos Campos, SP, Brazil ; Univ Milan, Dept Biomed & Clin Sci Luigi Sacco, I-20157 Milan, Italy ; Fdn Don Carlo Gnocchi ONLUS, Ist Ricovero & Cura Carattere Sci, I-20162 Milan, Italy ; Univ Amsterdam, Swammerdam Inst Life Sci, Cognit & Syst Neurosci Grp, NL-1098 XH Amsterdam, Netherlands ; Univ Amsterdam, Res Prior Program Brain & Cognit, NL-1098 XH Amsterdam, Netherlands ; Univ Med Goettingen, Dept Cognit Neurol, D-37075 Gottingen, Germany ; Leibniz Inst Primate Res, German Primate Ctr, D-37077 Gottingen, Germany ; Leibniz Sci Campus Primate Cognit, D-37077 Gottingen, Germany ; Univ Fed Sao Paulo, Inst Sci & Technol, BR-12231280 Sao Jose Dos Campos, SP, Brazil ; European Union's Horizon 2020 Research and Innovation Programme: 720270 ; German Research Foundation: WI 4046/1-1 ; National Institutes of Health/National Institute of Neurological Disorders and Stroke: 1R03NS096379 ; FAPESP: 2016/08263-9 ; EU Grant H2020-FETOPEN: RIA 686764 ; Web of Science

Funding Information: This work was supported by the Institute for Information and Communications Technology Planning and Evaluation (IITP) funded by the Korean government (Nos. 2017-0-00451; 2017-0-01779; 2019-0-00079; 2019-0-01371; and 2021-0-02068), the University and University Hospital of Liège, Belgian National Fund for Scientific Research (F.R.S-FNRS), the Italian Ministry of Health, GR-2016–02361494 (to S.C.), the Canadian Institute for Advanced Research (CIFAR) (to M.M.), European Union's Horizon 2020 Framework Program for Research and Innovation under the Specific Grant Agreement (No. 945539, Human Brain Project SGA3) (to M.M. and S.L.), BIAL Foundation, AstraZeneca Foundation, Fund Generate, King Baudouin Foundation, DOCMA project [EU-H2020-MSCA-RISE-–778234], James McDonnell Foundation, Mind Science Foundation, Fondazione Europea di Ricerca Biomedica, National Institutes of Health (No. R01MH064498), Academy of Finland (Nos. 265680 and 294625), Tiny Blue Dot Foundation (to M.M.), and grant EraPerMed JTC 2019 "PerBrain" (to M.R.). L.R.D.S. and R.P. are PhD fellows, O.G. and A. T. are research associates, and S.L. is research director at the F.R.S.–FNRS. We thank S. Lapuschkin for sharing the code; further, we thank all the healthy participants, patients, and their families who participated in this study. Publisher Copyright: © 2022, The Author(s). ; Consciousness can be defined by two components: arousal (wakefulness) and awareness (subjective experience). However, neurophysiological consciousness metrics able to disentangle between these components have not been reported. Here, we propose an explainable consciousness indicator (ECI) using deep learning to disentangle the components of consciousness. We employ electroencephalographic (EEG) responses to transcranial magnetic stimulation under various conditions, including sleep (n = 6), general anesthesia (n = 16), and severe brain injury (n = 34). We also test our framework using resting-state EEG under general anesthesia (n = 15) and severe brain injury (n = 34). ECI simultaneously quantifies arousal and awareness under physiological, pharmacological, and pathological conditions. Particularly, ketamine-induced anesthesia and rapid eye movement sleep with low arousal and high awareness are clearly distinguished from other states. In addition, parietal regions appear most relevant for quantifying arousal and awareness. This indicator provides insights into the neural correlates of altered states of consciousness. ; Peer reviewed

ObjectiveValidating objective, brain-based indices of consciousness in behaviorally unresponsive patients represents a challenge due to the impossibility of obtaining independent evidence through subjective reports. Here we address this problem by first validating a promising metric of consciousnessthe Perturbational Complexity Index (PCI)in a benchmark population who could confirm the presence or absence of consciousness through subjective reports, and then applying the same index to patients with disorders of consciousness (DOCs). MethodsThe benchmark population encompassed 150 healthy controls and communicative brain-injured subjects in various states of conscious wakefulness, disconnected consciousness, and unconsciousness. Receiver operating characteristic curve analysis was performed to define an optimal cutoff for discriminating between the conscious and unconscious conditions. This cutoff was then applied to a cohort of noncommunicative DOC patients (38 in a minimally conscious state [MCS] and 43 in a vegetative state [VS]). ResultsWe found an empirical cutoff that discriminated with 100% sensitivity and specificity between the conscious and the unconscious conditions in the benchmark population. This cutoff resulted in a sensitivity of 94.7% in detecting MCS and allowed the identification of a number of unresponsive VS patients (9 of 43) with high values of PCI, overlapping with the distribution of the benchmark conscious condition. InterpretationGiven its high sensitivity and specificity in the benchmark and MCS population, PCI offers a reliable, independently validated stratification of unresponsive patients that has important physiopathological and therapeutic implications. In particular, the high-PCI subgroup of VS patients may retain a capacity for consciousness that is not expressed in behavior. Ann Neurol 2016;80:718-729 ; Prin "Connage" (Italian Government) ; European Union (EU) ; James S. McDonnell Foundation Scholar Award ; Belgian National Fund for Scientific Research ; Templeton World Charity Foundation ; McDonnell Foundation ; Distinguished Chair in Consciousness Science at the University of Wisconsin ; Univ Milan, Dept Biomed & Clin Sci L Sacco, Via GB Grassi 74, I-20157 Milan, Italy ; Fdn Europea Ric Biomed, Milan, Italy ; Univ Milano Bicocca, Sch Med & Surg, Dept Hlth Sci, Monza, Italy ; Univ Fed Sao Paulo, Inst Sci & Technol, Sao Jose Dos Campos, Brazil ; Fdn Don Gnocchi Onlus, Ist Ricovero & Cura Carattere Sci, Milan, Italy ; Univ Wisconsin, Dept Psychiat, Madison, WI 53706 USA ; Univ Wisconsin, Dept Neurol, Madison, WI 53706 USA ; Univ Liege, Coma Sci Grp, GIGA, Liege, Belgium ; Univ Liege, Dept Neurol, Liege, Belgium ; Univ Hosp Liege, Liege, Belgium ; Grande Osped Metropolitano Niguarda Ca Granda, Neurocrit Care Unit, Azienda Sociosanit Terr, Dept Neurosci, Milan, Italy ; Institute of Science and Technology, Universidade Federal de São Paulo (UNIFESP), São José dos Campos, Brazil ; EU: 600806 ; EU: 686764 ; EU: 720270 ; Web of Science