Journal Article

Neural correlates of consciousness: progress and problems

Tononi, Giulio, Koch, Christof, Boly, Melanie, Massimini, Marcello

2016

Overview

Key Points 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. Several brain regions and physiological processes have been proposed to constitute the neural correlates of consciousness. In this Review, Koch and colleagues discuss studies that distinguish the neural correlates of consciousness from other neural processes that precede, accompany or follow it, and suggest that the neural correlates of consciousness are localized to posterior cortical regions. 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.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 59/36 / 59/78 / 631/378/2649/1398 / 692/617/375/1399 / Analysis / Animal Genetics and Genomics / Animals / Behavioral Sciences / Biological Techniques / Biomedicine / Brain - physiology / Brain research / Consciousness / Consciousness - physiology / Humans / Neural Pathways / Neurobiology / Neurons - physiology / Neurophysiology / Neurosciences / Physiology / review-article / Sleep