The neuronal correlates of consciousness (NCC) are the minimum neuronal mechanisms jointly sufficient for any one specific conscious experience. It is important to distinguish full NCC (the neural substrate supporting experience in general, irrespective of its specific content), content-specific NCC (the neural substrate supporting a particular content of experience — for example, faces, whether seen, dreamt or imagined) and background conditions (factors that enable consciousness, but do not contribute directly to the content of experience — for example, arousal systems that ensure adequate excitability of the NCC).
The no-report paradigm allows the NCC to be distinguished from events or processes — such as selective attention, memory and response preparation — that are associated with, precede or follow conscious experience. In such paradigms, trials with explicit reports are included along with trials without explicit reports, during which indirect physiological measures are used to infer what the participant is perceiving.
The best candidates for full and content-specific NCC are located in the posterior cerebral cortex, in a temporo-parietal-occipital hot zone. The content-specific NCC may be any particular subset of neurons within this hot zone that supports specific phenomenological distinctions, such as faces.
The two most widely used electrophysiological signatures of consciousness — gamma range oscillations and the P3b event-related potential — can be dissociated from conscious experiences and are more closely correlated with selective attention and novelty, respectively.
New electroencephalography- or functional MRI-based variables that measure the extent to which neuronal activity is both differentiated and integrated across the cortical sheet allow the NCC to be identified more precisely. Moreover, a combined transcranial magnetic stimulation–electroencephalography procedure can predict the presence or absence of consciousness in healthy people who are awake, deeply sleeping or under different types of anaesthesia, and in patients with disorders of consciousness, at the single-person level.
Extending the NCC derived from studies in people who can speak about the presence and quality of consciousness to patients with severe brain injuries, fetuses and newborn infants, non-mammalian species and intelligent machines is more challenging. For these purposes, it is essential to combine experimental studies to identify the NCC with a theoretical approach that characterizes in a principled manner what consciousness is and what is required of its physical substrate.
There have been a number of advances in the search for the neural correlates of consciousness — the minimum neural mechanisms sufficient for any one specific conscious percept. In this Review, we describe recent findings showing that the anatomical neural correlates of consciousness are primarily localized to a posterior cortical hot zone that includes sensory areas, rather than to a fronto-parietal network involved in task monitoring and reporting. We also discuss some candidate neurophysiological markers of consciousness that have proved illusory, and measures of differentiation and integration of neural activity that offer more promising quantitative indices of consciousness.
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The authors thank laboratory members and colleagues for their various contributions to the work presented here. This work was supported by the Templeton World Charity Foundation, the McDonnell Foundation and the Distinguished Chair in Consciousness Science (University of Wisconsin) to G.T. and by the EU project 686764 'Luminous' to M.M. C.K. thanks the Allen Institute for Brain Science founders, P. G. Allen and J. Allen, for their vision, encouragement and support.
The authors declare no competing financial interests.
- Neural correlates of consciousness
(NCC). The minimum neural mechanisms jointly sufficient for any one specific conscious experience.
- No-report paradigm
A paradigm in which trials with explicit report are included along with trials without explicit report, during which indirect physiological measures are used to infer what the participant is perceiving. This paradigm allows the neural correlates of consciousness to be distinguished from events or processes that are associated with, precede or follow conscious experience.
- Hot zone
A temporo-parietal-occipital zone of the posterior cerebral cortex where the best current anatomical candidates for full and content-specific neural correlates of consciousness in the human brain are located. The content-specific neural correlates of consciousness may be any particular subset of neurons within this hot zone that supports specific phenomenological distinctions, such as faces.
- Vegetative state
A disorder of consciousness that occurs in some patients with brain injury. The patients remain unresponsive and show no purposeful behaviour, but retain the ability to spontaneously open their eyes and maintain autonomic reflexes.
- Content-specific NCC
(Content-specific neural correlates of consciousness). The neural substrate supporting a particular content of experience (for example, faces) whether seen, dreamt or imagined.
- Transcranial magnetic stimulation
(TMS). A method of non-invasive brain stimulation, in which a magnetic field is induced by an electrical current in a coil placed onto the skull to induce neuronal activity in the underlying cortex.
- Full NCC
(Full neural correlates of consciousness). The neural substrate supporting experience in general, irrespective of its specific content.
- Background conditions
Factors that enable consciousness, but do not contribute directly to the content of experience.
- Binocular rivalry
An experimental paradigm used to evoke bi-stable percepts in which one image is presented to the left eye of the participant and a different image is presented to the right eye; rather than seeing a juxtaposition or fusion of both images, participants see one or the other object alternately.
A disorder of consciousness occurring in some patients with brain injury, in which the patient remains in an enduring, sleep-like state of immobility and unresponsiveness, with their eyes closed, from which they cannot be aroused.
A state associated with impairment in decision making and loss of self-initiated actions.
A visual experience, in particular featuring flashes of light, that occurs in response to direct mechanical, electrical or magnetic stimulation of the visual cortex.
- Auditory oddball paradigm
A sequence of auditory stimuli in which the last stimulus differs from the preceding stimuli; for example, a sequence of low-frequency tones followed by a single high-frequency tone
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Koch, C., Massimini, M., Boly, M. et al. Neural correlates of consciousness: progress and problems. Nat Rev Neurosci 17, 307–321 (2016). https://doi.org/10.1038/nrn.2016.22
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