Neural correlates of consciousness

Christof Koch (2004) "Figure 1.1: Neuronal correlates of consciousness " in The Quest for Consciousness: A Neurobiological Approach, Englewood, Colorado: Roberts & Company Publishers, p. 16

The neural correlates of consciousness (NCC) are the minimal set of neuronal events and mechanisms sufficient for a specific conscious experience. Neuroscientists do not yet understand enough about processes in the brain to determine exactly how consciousness arises. The brain is sufficient to give rise to any particular conscious experience so we must strive understand which of its specific parts are necessary to produce it. We can start by using neurosciencific tools and experiments to link distinct aspects of consciousness to specific brain mechanisms such as the electical and chemical signals of neurons. This will provide information as to which active neuronal processes in our head correlate with consciousness, and which processes do not. What is the difference between these processes? Dr. Christof Koch consistently points out that discovering the NCC and its properties will mark a major milestone in any scientific theory of consciousness and would be a big step in the right direction.

One popular belief is that consciousness arises as an emergent property of a very large collection of interacting neurons; this is either a holistic or quantitative approach. In this view, it would seem unnecessary to attempt to locate consciousness at the level of individual neurons. Koch proposes an alternative hypothesis: that there are distinct sets of 'consciousness' neurons distributed throughout the cerebral cortex and its associated areas. It is thought that these coalitions of neurons represent the NCC. One subtype of these NCC neurons could very well be characterized by a unique combination of molecular, biophysical, and anatomical traits. A specific site,  "layer 5 cortical pyridmidal neurons," will be further discussed in the section entitled "Where do we look for consciousness".  From an experimental point of view, it would be convenient if the unique set of traits could be something such as a strong synaptic interconnection, a unique cellular morphology, or a particular complement of ion channels that could be altered using molecular biology experimental techniques. This specificity would provide scientists with strategies for deliberately interfering with stimulus awareness by switching these NCC neuron coalitions on or off. There is no guarantee that nature is simple and such local approaches may fail but Koch believes we should start with this type of simple and straightforward hypotheses. It is possible that all 16 billion cortical neurons  are capable of participating in the conscious experience or representation of an experience, though not necessarily doing so for all conscious experiences.