Present study was examined the spatiotemporal changes of EEG during hypnagogic period or waking-sleeping transition. EEG (F3, F4, C3, C4, P3, P4. O1, O2), EOG and EMG were recorded from waking to manually scored stage 3. Spectral analysis of four scalp EEGs (F3, C3, P3, O1) were carried out. Power and coherence spectra (F3-C3, C3-P3, P3-O1, F3-P3, C3-O1, F3-O1) were obtained every 30 s (25 ms X 200 points X 5). Coherence spectra were determined for the four frequency bands (δ: 0.6-3.4 Hz, θ: 3.6-7.4 Hz, α: 7.6-12.4 Hz, σ: 12.6-14.4 Hz).
The results were as follows:
(1) In the contour mapping type of display, the coherence spectra which were connected with O1 showed clear differences among all pairs.
(2) Comparisons of temporal variations among four EEG coherence band showed that the delta band had regional differences and the sigma band had no regional differences.
(3) The delta band coherence which were connected with O1 (F3-O1, C3-O1, P3-O1) showed a sharp drop immediately before the disappearance of alpha wave, while the other delta bands coherence did not any change.
These results suggest that if the sharp drop of the delta band coherence is characteristic in the hypnagogic period, the period starts before the disappearance of alpha wave.