A RECONCEPTUALIZATION OF EEG ALPHA-ACTIVITY AS AN INDEX OF AROUSAL DURING SLEEP - ALL ALPHA-ACTIVITY IS NOT EQUAL

Alpha activity occurring during sleep is generally considered to refle ct arousal processes and a shift toward wakefulness. This long-standin g interpretation is based on physiological and behavioural arousal cor relates of alpha activity presumed to have an occipital focus. In addi tion to the application of this interpretation to sleep/wake state det erminations, there have been reports of nonrefreshing or nonrestorativ e sleep in clinical populations exhibiting dramatic amounts of alpha i ntrusion during sleep in the absence of awakening. Reports of the pres ence of alpha activity during sleep in normal subjects without sleep d isruption or complaints of daytime sleepiness call into question the i nterpretation that this activity is associated with arousal. A re-exam ination of this literature, incorporating the results from recent inve stigations employing multi-site EEG recordings, electronic processing and source dipole analyses of this activity, suggests the existence of alpha activity which differs in generation site (thalamus), scalp dis tribution (frontal-central), and behavioural correlates (e.g. enhancem ent to stimulation during wakefulness, concentration in the first-half of the night during sleep, and absence of sleep disturbance) from occ ipital alpha activity. Such marked differences in defining characteris tics imply different functional correlates for these activities. In th is context, it is proposed that this fronto-central alpha activity is associated with sleep-maintaining processes which may be enhanced in r esponse to sleep-disturbing events.