Toward a unified theory of narcosis: Brain imaging evidence for a thalamocortical switch as the neurophysiologic basis of anesthetic-induced unconsciousness

A unifying theory of general anesthetic-induced unconsciousness must explai n the common mechanism through which various anesthetic agents produce unco nsciousness. Functional-brain-imaging data obtained from II volunteers juri ng general anesthesia showed specific suppression of regional thalamic and midbrain reticular formation activity across two different commonly used vo latile agents. These findings are discussed in relation to findings from sl eep neurophysiology and the implications of this work for consciousness res earch. It is hypothesized that the essential common neurophysiologic mechan ism underlying anesthetic-induced unconsciousness is, as with sleep-induced unconsciousness, a hyperpolarization block of thalamocortical neurons. A m odel of anesthetic-induced unconsciousness is introduced to explain how the plethora of effects anesthetics have on cellular functioning ultimately al l converge on a single neuroanatomic/neurophysiologic system, thus providin g for a unitary physiologic theory of narcosis related to consciousness. (C ) 2000 Academic Press.