The Role of the Thalamus in Human Consciousness

Currently human consciousness is considered to arise from activity in the neocortex or in thalamo-cortical loops. A compelling case can be made, however, that some subcortical areas, in particular the diencephalon, are also critical. I will describe a theory in which phenomenal consciousness depends on synchronous neural activity in the dorsal thalamus, a major component of the diencephalon. The theory of the thalamic dynamic core depends on four empirical pillars: anatomy and physiology of the brain, particularly of the dorsal thalamus and associated cortical areas; brain lesion and anesthetic studies; studies of neural synchronization, particularly in binocular rivalry; and results from the experimental psychology of cognition, in particular that we experience results of cortical computations and not the processes that produce them. The four empirical pillars support the idea that the neocortex computes potential contents of consciousness, and competitive laterally-inhibitory activity in the nucleus reticularis of the thalamus selects some of these potential contents to include in a thalamic dynamic core of synchronous (and thus integrated) neural activity that gives rise to phenomenal experience of those contents. Destruction of the thalamus thus abolishes conscious experience itself, whereas destruction of a particular cortical area abolishes only the experience of contents computed by that area. The implications of this theory, and others that implicate critical roles for subcortical areas of the brain in consciousness, for the concept of brain death are significant. In particular, the theory agrees with positions such as that of Shewmon that absence of cortex or cortical activity does not mean absence of consciousness. If permanent lack of consciousness is a criterion for "death", then both neocortex and diencephalon must be shown to be nonfunctional for the criterion to be satisfied.