Antero-posterior EEG changes during the wakefulness-sleep transition

Objectives: To investigate the brain topography of the human sleep EEG alon g the antero-posterior axis during the wakefulness-sleep transition, by mea ns of both a single Hz analysis and a grouped-frequency analysis of EEG cha nges. Methods: EEG power values were calculated across a 1-28 Hz frequency range in a 1 Hz resolution during the wakefulness-sleep transition of 7 normal su bjects. Topographical changes were assessed from C3-A2, C4-A1, Fpz-A1, Fz-A l, Cz-Al, Pz-Al, Oz-Al recordings, after averaging individual time series, aligned with respect to the onset of stage 2. Results: The single Hz analysis showed that before sleep onset (SO), the <7 Hz slow frequencies were more prominent at the more anterior scalp locatio ns; this anterior prominence was counterbalanced by a reciprocal prevalence across the >8 Hz frequencies of EEG activity from the occipital areas; whi le the > 13 Hz fast frequencies were not characterized by significant anter o-posterior differences. After SO, more EEG power was found in the range of slow frequencies at the centro-frontal scalp locations and a second peak o f EEG activity was also revealed within the range of the sigma frequency, h igher at the centro-parietal scalp locations. No consistent topographical c hanges were observed within the range of faster EEG frequencies. Grouped-fr equency analysis confirmed these results, also pointing to different change s in the alpha frequency as a function of the SO point. Conclusions: The results suggest that: (a) the alpha rhythm spreads anterio rly as the transition progresses; b) several anterior areas first synchroni ze EEG activity; (c) the functional meaning of the EEG bands during the SO period should be partially revised with regard at least to alpha rhythm; (d ) SO coincides with the start of stage 2. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.