Hemispheric Asymmetry and Regional Differences in Electroencephalographic Alpha Activity at the Wake‐Sleep Transition

Electroencephalographic (EEG) alpha activity at the wake‐sleep transition was studied in six right‐handed adults in terms of hemispheric asymmetry and regional differences. Twelve‐channel EEGs with linked mastoid references were recorded together with horizontal and vertical electro‐oculograms (EOGs). Two types of alpha coefficient were obtained every 5.12 s by computing the relative proportion of right vs. left alpha band power and of anterior vs. posterior alpha band power. Four stages were scored using EEG sleep patterns (theta waves, vertex sharp waves, spindles, and K complex) and slow eye movement (SEM): stage W had neither EEG sleep patterns nor SEM; stage D1 had SEM and no EEG sleep patterns; stage D2 had theta waves or vertex sharp waves and SEM; stage S had spindles or K complex without SEM. It was found that the two types of alpha coefficient changed as a function of EEG‐EOG stage and were correlated. Right‐decreased and anterior‐shifted alpha activities were manifest at stages D2 and S. Drowsiness was considered to be a heterogeneous state, exhibiting different spatial changes in alpha activity between stages D1 and D2.