Anesthesia, neural information processing, and conscious awareness

Possible systemic effects of general anesthetic agents on neural informatio n processing are discussed in the context of the thalamocortical suppressio n hypothesis presented by Drs. Alkire, Haier, and Fallen (this issue) in th eir PET study of the anesthetized state. Accounts of the neural requisites of consciousness fail into two broad categories. Neuronal-specificity theor ies postulate that activity in particular neural populations is sufficient for conscious awareness, while process-coherence theories postulate that pa rticular organizations of neural activity are sufficient. Accounts of anest hetic narcosis, on the other hand, explain losses of consciousness in terms of neural signal-suppressions, transmission blocks, and the disruptions of signal interpretation. While signal-suppression may account for the action s of some anesthetic agents, the existence of anesthetics, such as choralos e, that cause both loss of consciousness and elevated discharge rates, is p roblematic for a general theory of narcosis that is based purely on signal suppression and transmission-block. However, anesthetic agents also alter r elative firing rates and temporal discharge patterns that may disrupt the c oherence of neural signals and the functioning of the neural networks that interpret them. It is difficult at present, solely on the basis of regional brain metabolic rates, to test process-coherence hypotheses regarding orga nizational requisites for conscious awareness. While these pioneering PET s tudies have great merit as panoramic windows of mind-brain con-elates, wide r ranges of theory and empirical evidence need to be brought into the formu lation of truly comprehensive theories of consciousness and anesthesia. (C) 2000 Academic Press.