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Consciousness and Anesthesia
Science 7 November 2008: Vol. 322. no. 5903, pp. 876 - 880 Consciousness and Anesthesia Michael T. Alkire,1 Anthony G. Hudetz,2 Giulio Tononi3 1 Department of Anesthesiology and the Center for the Neurobiology of Learning and Memory, University of California, Irvine, CA 92868, USA. Unconsciousness is likely to ensue when a complex of brain regions in the posterior parietal area is inactivated. Consciousness vanishes when anesthetics produce functional disconnection in this posterior complex, interrupting cortical communication and causing a loss of integration; or when they lead to bistable, stereotypic responses, causing a loss of information capacity. Thus, anesthetics seem to cause unconsciousness when they block the brain's ability to integrate information. Anesthetic-induced unconsciousness is usually associated with deactivation of mesial parietal cortex, posterior cingulate cortex, and precuneus. These same areas are deactivated in vegetative patients but are the first to reactivate in those who recover. These mesial cortical areas are strategically located at the main hub of the brain's connectional core. Loss of consciousness may not necessarily require that neurons in these posterior brain areas be inactivated. Instead, it may be sufficient that dynamic aspects of neural activity change, especially if these affect the brain's ability to integrate information. Information can be loosely defined as the number of discriminable activity patterns. When the repertoire of discriminable firing patterns available to the corticothalamic system shrinks, neural activity becomes less informative, even though it may be globally integrated Loss of consciousness is associated with a breakdown of cortical connectivity and thus of integration, or with a collapse of the repertoire of cortical activity patterns and thus of information. A corticothalamic complex is essential for consciousness and is thus the proper target for anesthesia: By conjoining functional specialization (each cortical area and neuronal group within each area is exquisitely specialized) with functional integration (thanks to extensive corticocortical and corticothalamocortical connectivity), a corticothalamic complex is well suited to behave as a single dynamic entity endowed with a large number of discriminable states. By contrast, parts of the brain made up of small, quasi-independent modules, such as the cerebellum, and parallel loops through the basal ganglia, are not sufficiently integrated, which is perhaps why they can be lesioned without loss of consciousness.
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