Department of Neurology and Neuroscience, Weill Medical College of Cornell University, New York, New York 10021, USA.
A brief taxonomy of neurologic disorders resulting in global impairments of consciousness is presented. Particular emphasis is placed on focal injuries of subcortical structures that may produce disorders that are otherwise associated to large bilateral cortical injuries. A distinction between subcortical arousal and "gating" systems is developed. Both clinical and experimental studies are reviewed in the context of these disorders and their possible underlying mechanisms.