Unconsciousness.

This paper reviews a theory on the physiological conditions of consciousness. The theory consists of four hypotheses: (1) The occurrence of states of consciousness depends on the formation of higher-order representations that represent the internal state of the brain itself. (2) Higher-order representations are instantiated by the spatio-temporal activity pattern of large-scale neuronal assemblies. (3) The N-methyl-D-aspartate (NMDA) synapse plays a crucial role in the generation of conscious states by implementing the binding mechanism that the brain uses to produce large-scale assemblies. (4) The activation state of the NMDA receptor determines the rate at which representational structures can be built up. Unconsciousness or altered states of consciousness occur if, and only if, NMDA-dependent binding processes are inhibited.