A historical review of consciousness and its disorders.

Concepts of consciousness and its disorders begin with the realization that both reside in the brain. Then came the realization that consciousness had various components, with two principal aspects, wakefulness and awareness. Awareness has multiple interconnected components, ranging from perception to abstract thought. These require selection of certain stimuli and processing as well as judgment and motivation, colored by emotion, before a consciously directed action is produced. The brain processes information and can influence behavior at levels below conscious awareness. Deeper insights into underlying neuronal functions and the complex interactions of various brain regions that support conscious experience have been made possible by scientific and technologic advancements. Our understanding of regional and global brain functions has been influenced by studies of various diseases and disorders, ranging from "brain death" to delirium. We now recognize that we cannot solely rely on behavioral responses to determine the conscious level, as some "unresponsive-wakeful," previously termed "vegetative," patients retain cognitive capacity, revealed by fMRI and electrophysiologic advances. There is still much to learn, especially as to how full awareness and the awareness of awareness arise from the brain and how to best assess and manage patients with various disorders of consciousness.