Neuroscience and Consciousness

Neural Correlates of Consciousness : progress and problems

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


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.

Being conscious means that one is having an experience- the subjective, phenomenal

‘what it is like’ to see an image, hear a sound, think a thought or feel an emotion. Although
our waking experiences usually refer to the external world, we continue to be conscious when we daydream and during those periods of sleep when we dream1. Consciousness only vanishes during dreamless sleep or under general anaesthesia when, from our own intrinsic perspective, everything disappears and we experience nothing1. Understanding the origin of consciousness, how it fits into a physical account of the universe and its relationship with the body are long-standing questions in philosophy, psychology and brain science. It has been known for a long time that being conscious requires the proper functioning of midline brain structures and that the particular contents of an experience are supported by the activity of neurons in parts of the cerebral cortex2.
The research strategy to identify the neural correlates of consciousness (NCC) involves relating behavioural correlates of consciousness to the neural mechanisms underlying them. There has been considerable progress in this area since the subject was last reviewed in this journal 3. In this Review, we start by outlining some contemporary approaches used to characterize the NCC, including the no‑report paradigm. This work has led to a shift in our understanding of the location of the NCC, away from a broad fronto parietal network towards a more restricted posterior cortical hot zone.
We discuss how two popular candidate physiological markers of consciousness — gamma activity and the P3b wave — have not shown any predictive power and how other promising quantitative indices of consciousness have been developed. We outline the inherent limits of such an empirical research programme.

This Review focuses on visual and auditory studies; for accounts of the NCC for metacognition, body, tactile and olfactory experiences, see REFS 4–7.

Behavioural correlates of consciousness

Although experiences are private, we can usually infer that people are conscious if they are awake and act purposefully, in particular if they can report what they experience and if that report accords with what is experienced by others. In a clinical setting, simple behavioural criteria are often used to infer consciousness, such as the ability to respond to a command (for example, patients may be asked to squeeze the observer’s hand twice if they feel pain). The level of consciousness2,8 is typically assessed by assigning a rating to a subject’s auditory, visual, verbal and motor functions using standardized scales (such as REF. 9). However, consciousness can be present even in the absence of reliable behavioural responses. For example, minimally conscious patients can be misclassified as being in a

non-conscious, vegetative state, with significant clinical, practical and emotional
consequences, if careful and repeated assessments are not carried out10,11.


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