Quantitative topographic electroencephalographic mapping during drowsiness and sleep onset.

The topographic EEG features of drowsiness and sleep onset are much less well documented than are their temporal aspects. A commercial topographical mapping system was used to assess the main EEG features employing all 19 international 10/20 system electrode sites referenced to linked ears during wakefulness, stages 1A and 1B drowsiness, stage 2 sleep, and sleep onset REM periods in 19 patients. All patients had been referred for a diagnostic EEG or a Multiple Sleep Latency Test and had essentially normal EEGs. Anterior alpha of drowsiness seldom represented frontal spread of the occipital alpha rhythm but usually was a distinct activity of apparent separate origin. Theta activities of drowsiness were maximum at CZ and FZ. Vertex sharp waves and sawtooth waves of rapid eye movement (REM) sleep had similar fields maximum at the midline with a steep decrease laterally. Isolated anterior mainly negative waves were identified. Sleep spindles were usually maximal in fronto central, occasionally centro parietal, or even parietal areas.