Chloral hydrate anesthesia: EEG power spectrum analysis and effects on VEPs in the rat.

This paper describes the use of EEG power spectrum analysis as a tool in identifying and controlling the state of an animal, a major source of variability in evoked potential (EP) data. Also described is a procedure of anesthetic administration combined with on-line EEG power spectrum analysis that permits an animal to be maintained in a stable, moderately anesthetized state during which VEP's are qualitatively comparable to those seen in the unanesthetized animal. Anesthesia level in acute preparations was varied by IV injections of chloral hydrate, and the EEG and light-flash VEP's were recorded from visual cortex. Under deep anesthesia, EEG activity was concentrated in the 0.1 Hz to 3 Hz band with very little contribution from frequencies higher than 10 Hz. After this period of deep anesthesia, EEG frequency distribution became bimodal with peaks in the 0.1-3 Hz and 5-7 Hz ranges, and a minor frequency band between 10 and 30 Hz. This moderate anesthetic state persisted for about 30 min. Two distinct forms of VEP's were highly correlated with these EEG frequency distributions. VEP's associated with the moderate anesthetic state were comparable to those reported from awake, chronic preparations. In VEP's associated with deep anesthesia, negative components were greatly attenuated or entirely absent; amplitudes of the positive components were increased as were the latencies of all components.