Awareness during drowsiness: Dynamics and electrophysiological correlates

During drowsy periods, performance on tasks requiring continuous attention becomes intermittent. Previously, we have reported that during drowsy perio ds of intermittent performance, 7 of 10 participants performing an auditory detection task exhibited episodes of nonresponding lasting about 18 s (Mak eig & Jung, 1996). Further, the time patterns of these episodes were repeat ed precisely in subsequent sessions. The 18-s cycles were accompanied by co unterbalanced power changes within two frequency bands in the vertex EEG (n ear 4 Hz and circa 40 Hz). In the present experiment, performance patterns and concurrent EEG spectra were examined in four participants performing a continuous visuomotor compensatory tracking task in 15-20 minute bouts duri ng a 42-hour sleep deprivation study. During periods of good performance, p articipants made compensatory trackball movements about twice per second, a ttempting to keep a target disk near a central ring, Autocorrelations of ti me series representing the distance of the target disk from the ring centre showed that during periods of poor performance marked near-18-s cycles in performance again appeared. There were phases of poor or absent performance accompanied by an increase in EEG power that was largest at 3-4 Hz. These studies show that in drowsy humans, opening and closing of the gates of beh avioural awareness is marked not by the appearance of (12-14 Hz) sleep spin dles, but by prominent EEG amplitude changes in the low theta band. Further , both EE(; and behavioural changes during drowsiness often exhibit stereot yped 18-s cycles.