Background: The authors sought to understand neural correlates of anesthetic-induced unconsciousness. Cortical gamma oscillations have been associated with neural processes supporting conscious perception, but the effect of general anesthesia on these oscillations is controversial. In this study, the authors examined three volatile anesthetics, halothane, isoflurane, and desflurane, and compared their effects on flash-induced gamma oscillations in terms of equivalent concentrations producing the loss of righting reflex (1 minimum alveolar concentration for the loss of righting [MAC(LR)]).
Methods: Light flashes were presented every 5 s for 5 min, and event-related potentials were recorded from primary visual cortex of 15 rats with a chronically implanted bipolar electrode at increasing anesthetic concentrations (0-2.4 MAC(LR)). Early cortical response was obtained by averaging poststimulus (0-100 ms) potentials filtered at 20-60 Hz across 60 trials. Late (100-1,000 ms) gamma power was calculated using multitaper power spectral technique. Wavelet decomposition was used to determine spectral and temporal distributions of gamma power.
Results: The authors found that (1) halothane, isoflurane, and desflurane enhanced the flash-evoked early cortical response in a concentration-dependent manner; (2) the effective concentration for this enhancement was the lowest for isoflurane, intermediate for halothane, and the highest for desflurane when compared at equal fractions of the concentration that led to a loss of righting; (3) the power of flash-induced late (> 100 ms) gamma oscillations was augmented at intermediate concentrations of all three anesthetic agents; and (4) flash-induced gamma power was not reduced below waking baseline even in deep anesthesia.
Conclusions: These findings suggest that a reduction in flash-induced gamma oscillations in rat visual cortex is not a unitary correlate of anesthetic-induced unconsciousness.