What is a Neural Correlate of Consciousness?

The search for neural correlates of consciousness (or NCCs) is arguably the cornerstone in the recent resurgence of the science of consciousness. The search poses many difficult empirical problems, but it seems to be tractable in principle, and some ingenious studies in recent years have led to considerable progress. A number of proposals have been put forward concerning the nature and location of neural correlates of consciousness. A few of these include: • 40-hertz oscillations in the cerebral cortex (Crick and Koch 1990) • Intralaminar nuclei in the thalamus (Bogen 1995) • Re-entrant loops in thalamocortical systems (Edelman 1989) • 40-hertz rhythmic activity in thalamocortical systems (Llinas et al 1994) • Extended reticular-thalamic activation system (Newman and Baars 1993) • Neural assemblies bound by NMDA (Flohr 1995) • Certain neurochemical levels of activation (Hobson 1997) • Certain neurons in inferior temporal cortex (Sheinberg and Logothetis 1997) • Neurons in extrastriate visual cortex projecting to prefrontal areas (Crick and Koch 1995) • Visual processing within the ventral stream (Milner and Goodale 1995) (A longer list can be found in Chalmers 1998. Review articles on neural correlates of consciousness, especially visual consciousness, can be found in Crick and Koch 1998 and Milner 1995.)