High frequency waking EEG: reflection of a slow ultradian rhythm in daytime arousal

The ultradian dynamics of the human waking EEG was studied using a short vi sual fixation task repeated every 10 min throughout the daytime. The EEG sp ectra obtained from the tasks were assessed for time effect and ultradian p eriodicity. Fronto-central EEG high frequency powers (22.5-44.5 Hz) decreas ed at the time of the midafternoon vigilance dip (14.00-17.00 h) along with slight concomitant increases in parietal alpha (7.5-13.5 Hz) and delta (1- 3 Hz) powers. A slow ultradian rhythm with a 3-4 h periodicity strongly mod ulated EEG power in all frequency bands between 1 and 44.5 Hz. The high fre quency waking EEG may well reflect the activity of a brain arousal process underlying maintenance of the waking state probably throughout the 24 h cyc le. NeuroReport 11:2223-2227 (C) 2000 Lippincott Williams & Wilkins.