Correlation of high-frequency oscillations with the sleep-wake cycle and cognitive activity in humans

While several authors have suggested that high-frequency electroencephalogr am activity (gamma, >30 Hz) correlates with conscious thought, others have suggested that electroencephalogram activity >30Hz shows the same relations hips to cognitive activity and sleep as activity in the conventional beta f requency band. The existence of coherence of gamma over large distances als o remains controversial. We studied quantitatively the relationship of gamm a activity to the sleep-wake cycle and cognitive tasks during wakefulness i n humans using intracranial electroencephalogram. Gamma activity made up le ss than 1% of the total power spectrum. A significant relationship was obse rved between gamma activity and the sleep-wake cycle such that gamma was hi ghest during wakefulness, intermediate during light and rapid eye movement sleep, and lowest during slow-wave sleep. As well, gamma was higher during rapid eye movement sleep with eye movements than during rapid eye movement sleep without eye movements. During a cognitive task experiment, while lowe r frequencies, including beta, showed a stepwise reduction with increasing task difficulty, gamma was observed to increase during cognitive tasks as c ompared to the resting state. The relationship between gamma and the sleep- wake cycle and cognitive tasks was independent of brain region and hemisphe re. Coherence of gamma activity at distances of 5 mm and greater was not ob served. Our data support previously reported findings that gamma activity has a sig nificant relationship to the sleep-wake cycle. The findings of differences in gamma during REM sleep with and without eye movements suggest that the p resence or absence of eye movements may reflect two different states of bra in activity. Our findings of differences in the relationships of the beta a nd gamma bands to both the sleep-wake cycle and cognitive tasks demonstrate that various components of the high-frequency spectrum behave differently in some situations. (C) 1999 IBRO. Published by Elsevier Science Ltd.