TONIC, PHASIC, AND TRANSIENT EEG CORRELATES OF AUDITORY AWARENESS IN DROWSINESS

During drowsiness, human performance in responding to above-threshold auditory targets tends to vary irregularly over periods of 4 min and l onger. These performance fluctuations are accompanied by distinct chan ges in the frequency spectrum of the electroencephalogram (EEG) on thr ee time scales: (1) during minute-scale and longer periods of intermit tent responding, mean activity levels in the (< 4 Hz) delta and (4-6 H z) theta bands, and at the sleep spindle frequency (14 Hz) are higher than during alert performance. (2) In most subjects, 4-6 Hz theta EEG activity begins to increase, and gamma band activity above 35 Hz begin s to decrease, about 10 s before presentations of undetected targets, while before detected targets, 4-6 Hz amplitude decreases and gamma ba nd amplitude increases. Both these amplitude differences last 15-20 s and occur in parallel with event-related cycles in target detection pr obability. In the same periods, alpha and sleep-spindle frequency ampl itudes also show prominent 15-20 a cycles, but these are not phase loc ked to performance cycles. (3) A second or longer after undetected tar gets, amplitude at intermediate (10-25 Hz) frequencies decreases brief ly, while detected targets are followed by a transient amplitude incre ase in the same latency and frequency range.