PT719. Asymmetrical electroencephalographic change of human brain during sleep onset period

Objective: The human cerebral hemispheres are known to function asymmetrically with daytime left hemisphere superiority in most right-handed persons. It may have relevance to localization of specific function of the brain. This study attempted to reveal whether or not the functional cerebral asymmetry of wakeful state holds in the sleep onset period. Methods: Thirty channel EEG was recorded in 61 healthy subjects (male: female = 34:27, age = 27.2 ± 3.0 years). EEG power spectra in seven frequency bands (delta, theta, alpha-1, alpha-2, beta-1, beta-2 and beta-3) were compared between two kinds of 30 second states such as wakeful stage (WS) and late sleep stage 1 (LSS1). These two stages were selected by the distribution of the alpha and the theta in O1 and O2 electrodes. WS was determined as the first 30 seconds having 100 % of alpha pattern. LSS1 was defined as the last 30 seconds of sleep stage 1. The left-right power asymmetry, the asymmetry index, that was defined as log(power of the left lead)-log(power of the right lead) was compared between WS and LSS1 stages. Results: The asymmetry indices of LSS1 at some frontocentral leads were decreased in delta, theta, alpha2 and all beta bands. On the contrary, at parts of parietooccipital leads showed increased indices in theta, alphas, beta1 and beta2 bands. Any frontocentral leads did not show the increase of the index, neither did any parietooccipital leads the decrease of it. CONCLUSIONS: During sleep onset period, power spectral asymmetry of the hemispheres showed a different pattern compared to wakeful stage. This asymmetrical pattern of EEG powers may suggest the reversal of left hemispheric dominance during sleep.