Science
September 7, 2006
Adrian M. Owen, Martin R. Coleman, Melanie Boly et al.
1,974 citations
A patient who met the clinical criteria for a vegetative state showed brain activity indistinguishable from healthy volunteers when asked to imagine playing tennis or moving around her home. Using functional magnetic resonance imaging, the patient activated the same predicted cortical areas as conscious individuals, indicating preserved conscious awareness despite appearing unresponsive. This finding suggests that some patients diagnosed as vegetative may retain covert consciousness that standard behavioral assessments cannot detect.
Science Translational Medicine
August 14, 2013
Adenauer G. Casali, Olivia Gosseries, Mario Rosanova et al.
1,299 citations
A new index called the perturbational complexity index (PCI) can objectively measure a person's level of consciousness without requiring them to respond or interact. PCI works by using transcranial magnetic stimulation to briefly perturb the cortex and then measuring the algorithmic complexity of the resulting brain activity patterns. The index was tested on healthy individuals during wakefulness, dreaming, nonrapid eye movement sleep, and under sedation with midazolam, xenon, and propofol, as well as on patients who had emerged from coma. PCI reliably distinguished conscious from unconscious states in single individuals across all conditions, including vegetative state, minimally conscious state, and locked-in syndrome.
Anesthesiology
September 30, 2010
Pierre Boveroux, Audrey Vanhaudenhuyse, Marie‐Aurélie Bruno et al.
645 citations
Propofol-induced unconsciousness is linked to decreased connectivity within frontoparietal networks (the default-mode and executive-control networks) and between the thalamus and these networks, with a negative correlation between thalamic and cortical activity emerging during unconsciousness. In contrast, connectivity in low-level sensory cortices (auditory and visual networks) is preserved, including their thalamocortical connections. Loss of consciousness is associated with a breakdown of cross-modal interactions between visual and auditory networks. These findings suggest that unconsciousness results from disrupted communication between sensory and higher-order frontoparietal cortices, preventing conscious perception.
Science Advances
February 1, 2019
Athena Demertzi, Enzo Tagliazucchi, Stanislas Dehaene et al.
545 citations
Consciousness depends on the brain's ability to sustain rich, dynamic patterns of signal coordination. Using functional magnetic resonance imaging, a complex pattern of coordinated and anticoordinated signals characterized healthy individuals and minimally conscious patients. Unresponsive patients showed low interareal phase coherence mainly mediated by structural connectivity, with fewer transitions between patterns. This complex pattern was also seen in patients with covert cognition who could perform mental imagery tasks, validating its link to consciousness. Anesthesia increased the probability of the less complex pattern to levels seen in unresponsive patients, confirming its role in unconsciousness. These results establish generalizable fingerprints of conscious and unconscious states after brain damage.
Annals of Neurology
September 22, 2016
Silvia Casarotto, Angela Comanducci, Mario Rosanova et al.
504 citations
A brain-based measure called the Perturbational Complexity Index (PCI) can reliably distinguish conscious from unconscious individuals, even when they cannot speak or move. The index was first validated in 150 healthy and brain-injured people who could report their conscious state, achieving perfect accuracy in separating conscious from unconscious conditions. Applied to 81 noncommunicative patients, PCI correctly identified 94.7% of those in a minimally conscious state and revealed that 9 of 43 patients diagnosed as vegetative had PCI values overlapping with conscious individuals. These findings suggest that some behaviorally unresponsive patients may retain hidden conscious capacity.
Anesthesiology
February 7, 2012
Robert D. Sanders, Giulio Tononi, Steven Laureys et al.
478 citations
Consciousness—subjective experience—persists during sleep and anesthesia, as evidenced by dreaming. A defining feature of dreaming is disconnection from the environment. Anesthesia aims to prevent the experience of surgery (connected consciousness) by inducing either unconsciousness or disconnection. The isolated forearm technique reveals that consciousness, connectedness, and responsiveness can uncouple during anesthesia; under clinical conditions, a median 37% of patients demonstrate connected consciousness. Potential neurobiological constructs explain this: during light anesthesia, subcortical mechanisms for spontaneous behavioral responsiveness are disabled, but information integration within the corticothalamic network continues producing consciousness, while unperturbed norepinephrinergic signaling maintains connectedness. These concepts emphasize the need for anesthetic regimens and depth-of-anesthesia monitors targeting mechanisms of consciousness, connectedness, and responsiveness.
Journal of Cognitive Neuroscience
June 1, 2010
Audrey Vanhaudenhuyse, Athena Demertzi, Manuel Schabus et al.
460 citations
Resting brain activity reveals two anticorrelated cortical systems linked to conscious awareness: an extrinsic system (lateral fronto-parietal areas) associated with external awareness and an intrinsic system (medial brain areas) associated with internal awareness. In 31 healthy volunteers, external and internal awareness were significantly anticorrelated, with a mean switching frequency of 0.05 Hz, similar to BOLD fMRI slow oscillations. In 22 volunteers, fMRI showed that precuneus/posterior cingulate, anterior cingulate/mesiofrontal cortices, and parahippocampal areas (intrinsic system) correlated with internal awareness, while lateral fronto-parietal cortices (extrinsic system) correlated with external awareness.
NeuroImage
January 11, 2019
Michele Colombo, Martino Napolitani, Melanie Boly et al.
359 citations
During anesthesia, people may still be conscious even though they do not respond. A marker of consciousness based on the decay rate of the power spectral density (PSD) of resting EEG—measured by the spectral exponent β—was tested in healthy participants under xenon, propofol, or ketamine anesthesia (n=5 per group). Delayed reports indicated whether consciousness was present or absent. Xenon and propofol, which abolish consciousness, caused a steeper PSD decay (more negative β) compared to wakefulness. Ketamine, which preserves consciousness, showed a PSD decay similar to wakefulness overall but a flattening in high frequencies (20–40 Hz). The spectral exponent correlated strongly with the Perturbational Complexity Index (PCI), supporting its use as a marker of consciousness.
Frontiers in Psychology
January 1, 2013
Melanie Boly, Anil K. Seth, Melanie Wilke et al.
295 citations
Consciousness research over the last decade has advanced through evidence that fronto-parietal connectivity and top-down processes enable information to travel across distant cortical areas, and through numerous dissociations between consciousness and cognitive functions such as attention in humans. Mental imagery paradigms now allow identification of covert awareness in non-responsive subjects. In non-human animals, technological enhancements have clarified the roles of cortical areas and higher-order thalamus for consciousness, and progress has been made in understanding non-vertebrate cognition relevant to possible conscious states. Major advances have also occurred in theories of consciousness and their comparison with available evidence.
Frontiers in Psychology
August 15, 2018
Christopher Timmermann, Leor Roseman, L. Williams et al.
228 citations
Near-death experiences (NDEs) share striking phenomenological similarities with the effects of the psychedelic drug DMT. In a placebo-controlled, within-subjects study, 13 healthy participants received DMT and placebo, then completed a standard NDE measure. DMT significantly increased NDE-like features compared to placebo. NDE scores were linked to DMT-induced ego-dissolution and mystical experiences, as well as baseline traits of absorption and delusional ideation. Nearly all NDE features overlapped between DMT-induced experiences and a matched group of actual NDE experiencers. These results indicate a remarkable similarity between the DMT state and NDEs, warranting further research.
Journal of The Royal Society Interface
January 1, 2016
Enzo Tagliazucchi, Dante R. Chialvo, Michael Siniatchkin et al.
210 citations
Loss of consciousness from propofol sedation reduces long-range temporal correlations in frontothalamic brain activity and weakens the link between functional connectivity and anatomical structure. A model based on phase transitions in complex systems reproduces these patterns and also explains the cortex's reduced sensitivity to external stimuli during unconsciousness. The findings suggest that these neural changes are universal across different causes of unconsciousness.
Anesthesiology
August 9, 2016
Vincent Bonhomme, Audrey Vanhaudenhuyse, Athena Demertzi et al.
206 citations
Ketamine alters consciousness by disrupting connectivity within and between specific resting-state brain networks, particularly the default mode network (DMn) and salience network (SALn), while leaving sensory and motor networks largely intact. In healthy volunteers given stepwise ketamine infusions until they lost responsiveness, DMn connectivity between the medial prefrontal cortex and other network regions decreased (from 0.20 to 0.07), and the normal anticorrelated activity between the DMn and sensory regions reversed (e.g., right sensory cortex shifted from -0.07 to 0.04). SALn connectivity was also suppressed but nonuniformly. These specific changes, including preserved sensory network connectivity, are shared with propofol-induced unconsciousness.
Proceedings of the National Academy of Sciences
March 26, 2012
Melanie Boly, Vincent Perlbarg, Guillaume Marrelec et al.
201 citations
Consciousness is reduced during nonrapid eye movement (NREM) sleep due to changes in brain function that are still poorly understood. Here, we tested the hypothesis that impaired consciousness during NREM sleep is associated with an increased modularity of brain activity. Cerebral connectivity was quantified in resting-state functional magnetic resonance imaging times series acquired in 13 healthy volunteers during wakefulness and NREM sleep. The analysis revealed a modification of the hierarchical organization of large-scale networks into smaller independent modules during NREM sleep, independently from EEG markers of the slow oscillation. Such modifications in brain connectivity, possibly driven by sleep ultraslow oscillations, could hinder the brain's ability to integrate information and account for decreased consciousness during NREM sleep.
Clinical EEG and Neuroscience
January 1, 2014
Simone Sarasso, Mario Rosanova, Adenauer G. Casali et al.
164 citations
Consciousness requires both functional integration and differentiation in the brain, a property termed brain complexity. Transcranial Magnetic Stimulation combined with electroencephalography (TMS/EEG) can quantify this complexity. Studies consistently show that the complexity of the cortical response to TMS collapses during loss of consciousness in deep sleep, anesthesia, and vegetative state after severe brain injury. Complexity recovers when consciousness returns during wakefulness, dreaming, the minimally conscious state, or locked-in syndrome. This approach may help understand the pathophysiology of disorders of consciousness and requires careful methodological attention.
Trends in Cognitive Sciences
December 22, 2022
Christopher Timmermann, Prisca R. Bauer, Olivia Gosseries et al.
145 citations
A neurophenomenological framework is proposed for studying non-ordinary states of consciousness (NSCs) such as hypnosis, meditation, and psychedelics. NSCs involve shifts in what appears to the experiencer or how it appears, enabling investigation of the plastic and dynamic nature of experience across mind, brain, body, and context. The approach highlights NSCs as catalysts of transformation in clinical practice by refining understanding of relationships between subjective experience and neural dynamics. Ethical implications for standard conceptions of health and pathology are outlined, along with the crucial role of experience-based know-how in NSC research and application.
PLoS ONE
March 27, 2013
Marie Thonnard, Vanessa Charland-Verville, Serge Brédart et al.
140 citations
Near-Death Experiences (NDEs) are not well explained. Because NDEs are sometimes considered imagined, researchers compared the phenomenological characteristics of NDE memories with memories of real and imagined events. Coma survivors with NDEs (8), with coma memories but no NDE (6), and without coma memories (7), plus 18 healthy volunteers, completed the Memory Characteristics Questionnaire. NDE memories had more characteristics than memories of imagined or real events, and more self-referential and emotional information and better clarity than coma memories. These findings suggest NDE memories cannot be considered imagined events; they may be perceived as real despite not being lived in physical reality.
PLoS ONE
February 14, 2018
Héléna Cassol, Benoît Pétré, Sophie Degrange et al.
131 citations
Near-death experiences (NDEs) are profound psychological events that can significantly affect experiencers' lives. Previous research shows NDE memories are phenomenologically rich. This study analyzed the narratives of 34 cardiac arrest survivors to extract common themes. A qualitative thematic analysis identified 10 'time-bounded' themes—isolated events during the NDE—and 1 'transversal' theme that characterizes the entire narrative, often as a retrospective self-reflective comment. The division into themes provides detailed vocabulary used by experiencers and enables a rigorous description of the phenomenon, ensuring all self-reported manifestations are included.
Annals of the New York Academy of Sciences
March 1, 2009
Athena Demertzi, Charlene Liew, Didier Ledoux et al.
125 citations
Attitudes about whether mind and brain are separate or the same thing vary by age, gender, and religious belief. Two surveys—one of university students in Edinburgh (250 people) and another of health-care workers and the public in Liège (1,858 people)—found that dualistic views (seeing mind and brain as distinct) were common. In the Liège survey, younger participants, women, and those with religious beliefs were more likely to endorse separation of mind and brain, survival of a spiritual part after death, and the existence of a soul distinct from the body. Religious belief was the strongest predictor of dualism. Over a third of medical and paramedical professionals also regarded mind and brain as separate, even though most health-care workers rejected that distinction.
Nature Communications
February 25, 2022
Minji Lee, Leandro Sanz, Alice Barra et al.
120 citations
A deep-learning-based explainable consciousness indicator (ECI) uses EEG responses to transcranial magnetic stimulation and resting-state EEG to separately quantify arousal and awareness. Tested during sleep (n=6), general anesthesia (n=16), and severe brain injury (n=34), ECI distinguishes states such as ketamine-induced anesthesia and rapid eye movement sleep, which combine low arousal with high awareness. Parietal brain regions are most relevant for these measurements. The indicator offers a way to disentangle the two components of consciousness across physiological, pharmacological, and pathological conditions.
Frontiers in Human Neuroscience
May 27, 2014
Vanessa Charland-Verville, Jean-Pierre Jourdan, Marie Thonnard et al.
116 citations
Near death experiences (NDEs) reported after a non-life-threatening event (NDE-like) are similar in intensity and content to those occurring after a pathological coma (real NDE). In a retrospective analysis of 190 reports meeting the Greyson NDE scale threshold (score >7/32), the most common feature was peacefulness (89-93%), and only 1% recounted a negative experience. The intensity and features did not differ between NDE-like and real NDE groups, nor among coma causes (anoxic, traumatic, other). However, the core features were more frequently reported in this retrospective anoxic cohort compared to historical prospective data, suggesting that retrospective recall may shape the experience's content.
PLoS ONE
June 30, 2014
Enrico Amico, Francisco Gómez, Carol di Perri et al.
115 citations
Functional connectivity of brain areas fluctuates over time, even during anesthesia. By applying point process analysis to resting-state fMRI data from 18 healthy subjects during propofol-induced wakefulness, sedation, unconsciousness, and recovery, the study identified eight distinct co-activation patterns centered on the posterior cingulate cortex (PCC). The core of these patterns remained stable across consciousness levels, but propofol caused region-specific reductions in co-activation, including disconnections of the prefrontal cortex, thalamus, auditory cortex, motor cortex, and visual area. The methodology helps refine characterization of local functional brain changes associated with propofol-induced modulation of consciousness.
Frontiers in Systems Neuroscience
August 14, 2019
Vincent Bonhomme, Cécile Staquet, Javier Montupil et al.
109 citations
General anesthesia reversibly alters consciousness without globally shutting down the brain. Depending on the agent and dose, it can produce a complete absence of subjective experience (unconsciousness), a conscious experience without environmental perception (disconnected consciousness, like during dreaming), or oriented consciousness with environmental awareness (connected consciousness). Each state may be followed by explicit or implicit memories. Progress in brain function exploration has improved understanding of neural correlates of consciousness and their alterations during anesthesia, including changes in functional and effective brain connectivity, consciousness network topology, and spatio-temporal dynamics.
Human Brain Mapping
July 1, 2013
Zirui Huang, Rui Dai, Xuehai Wu et al.
104 citations
Patients with disorders of consciousness (DOC) show reduced brain activity in midline regions (anterior and posterior cingulate cortex) during self-referential thinking tasks compared to healthy individuals. The degree of signal reduction in the perigenual anterior cingulate cortex predicts their level of consciousness. These same midline regions also display abnormal resting-state functional connectivity and low-frequency fluctuations. The findings link self-referential processing, resting-state brain activity, and consciousness, suggesting that neural abnormalities in these areas contribute to DOC.
Consciousness and cognition
March 1, 2019
Charlotte Martial, Héléna Cassol, Vanessa Charland-Verville et al.
98 citations
Near-death experiences (NDEs) share consistent features across cultures, suggesting a common neurobiological basis. Analyzing semantic similarity between about 15,000 reports from 165 psychoactive substances and 625 NDE narratives, the NMDA receptor antagonist ketamine produced reports most similar to NDEs, followed by Salvia divinorum and serotonergic psychedelics like DMT. The similarity was driven by concepts of self and environmental consciousness, as well as therapeutic, ceremonial, and religious aspects of drug use. Ketamine may serve as a safe experimental model for NDE phenomenology, and endogenous NMDA antagonists might be released near death.
Trends in Cognitive Sciences
January 22, 2020
Charlotte Martial, Héléna Cassol, Steven Laureys et al.
90 citations
Near-death experiences (NDEs) occur during comatose states, challenging the common view that consciousness disappears when the brain shuts down. A new framework distinguishing awareness, wakefulness, and connectedness is needed to understand the phenomenon. Classical NDEs represent internal awareness experienced in unresponsive conditions, constituting an episode of disconnected consciousness. This proposal points toward new directions for NDE research and consciousness science more broadly.