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.