Crick and Koch’s Neural Correlates of Consciousness

The neural correlates of consciousness (NCC) are defined as the minimum activities in the brain jointly sufficient (and probably necessary) for any one specific conscious perception and, extended, for subjective experience in general, the inner awareness of qualia. Originally applied to sleep and wakefulness (i.e., the reticular activating system in the brain stem), the NCC were formally proposed by Francis Crick and Christof Koch as a scientific approach to what had been believed to be the vague, metaphysical, and somewhat discredited idea of consciousness (Crick & Koch, 1990), a program then championed by Koch (Koch, 2004, Closer To Truth) and others (though Koch has become something of a “romantic reductionist” [Koch, 2012]).

NCC Mechanisms and Early Candidates

While there are complex methodological issues, NCC mechanisms include neuronal electrophysiological action potentials (spikes), their frequencies and sequences; neurochemical transmitter flows in the synapses between neurons; and recurrent brain circuits in specific brain areas. An example is clusters of neurons that underlie wakefulness in the brainstem connecting to clusters of neurons in the thalamus, hypothalamus, basal ganglia, and cerebral cortex related to awareness/consciousness (Wong, 2023).

Similarly, a “default ascending arousal network” (dANN) has been proposed, with subcortical nodes in the brainstem, hypothalamus, thalamus, and basal forebrain (Edlow, 2024). While necessary for conscious arousal and wakefulness, the dANN is not sufficient for phenomenal consciousness and is not what this Landscape is about.

As an example of the NCC way of thinking, an early NCC candidate was the claustrum, which receives input from almost all regions of cortex and projects back to almost all regions of cortex, and which, Crick and Koch speculated, could give rise to “integrated conscious percepts.” They used the analogy of the claustrum to a “conductor” and the cortex to an “orchestra,” such that the claustrum as a conductor “coordinates a group of players in the orchestra, the various cortical regions.”

Without the conductor, as they build the analogy, “players can still play but they fall increasingly out of synchrony with each other. The result is a cacophony of sounds.” In the absence of the claustra in both cerebral hemispheres, attributes such as sensory modalities “may not be experienced in an integrated manner and the subject may fail to altogether perceive these objects or events or only be consciously aware of some isolated attribute.” This would mean, they suggest, “that different attributes of objects … are rapidly combined and bound in the claustrum” (Crick & Koch, 2005).

Ongoing Search for NCC

A more recent candidate for full and content-specific NCC is located in the posterior cerebral cortex, in a temporo-parietal-occipital hot zone (Koch et al., 2016), though no one is yelling “Eureka” and the search continues. Even so, while everyone knows that even strong correlation is not causation, strong correlation is still something. NCCs can be considered macroscopic materialism.

It was in 1998 that Christof Koch made the now legendary 25-year bet with philosopher David Chalmers—they are long-time friends—that neuroscientists would discover a “clear” NCC by 2023. No surprise that the bet paid off in Chalmers’ favor. (Koch presented Chalmers with a case of 1978 Madeira wine.) As Chalmers said, notwithstanding neuroscience's great progress, “It's clear that things are not clear,” while Koch, feigning chagrin, agreed (Horgan, 2023).

Koch was down but not out: he may have lost this consciousness battle, but the consciousness war would still be waged. Koch offered to re-up: another bet, another 25 years to achieve that “clear” NCC, another case of wine. “I hope I lose,” Chalmers said, smiling, taking the new bet, “but I suspect I'll win.”

The Question of Explanation

The smart money is again on Chalmers, although I have a different issue. What would a “clear” NCC mean? Suppose a specific group of neurons were proven to be both necessary and sufficient for a particular conscious experience, a direct correlation that no other group of neurons could claim? Koch would rightly win the bet, but would consciousness have been explained?

Still, the perennial question: How can action potentials zipping along neurons and chemicals flowing between neurons literally be the phenomenal consciousness of inner experience? By what magic?