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 reflect arousal processes and a shift toward wakefulness. This long-standing interpretation is based on physiological and behavioural arousal correlates of alpha activity presumed to have an occipital focus. In addition to the application of this interpretation to sleep/wake state determinations, there have been reports of nonrefreshing or nonrestorative sleep in clinical populations exhibiting dramatic amounts of alpha intrusion during sleep in the absence of awakening. Reports of the presence of alpha activity during sleep in normal subjects without sleep disruption or complaints of daytime sleepiness call into question the interpretation that this activity is associated with arousal. A re-examination of this literature, incorporating the results from recent investigations 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 distribution (frontal-central), and behavioural correlates (e.g. enhancement to stimulation during wakefulness, concentration in the first-half of the night during sleep, and absence of sleep disturbance) from occipital alpha activity. Such marked differences in defining characteristics imply different functional correlates for these activities. In this context, it is proposed that this fronto-central alpha activity is associated with sleep-maintaining processes which may be enhanced in response to sleep-disturbing events.