Tonic, Phasic, and Transient EEG Correlates of Auditory Awareness in Drowsiness.
NAVAL HEALTH RESEARCH CENTER SAN DIEGO CA
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During drowsiness, human perfoimance in responding to above-threshold auditory targets tends to vary irregularly over periods of 4 min and longer. These pefformance fluctuations are accompanied by distinct changes in the frequency spectrum of the electroencephalo- gram EEG on three time scales I during minute-scale and longer periods of intermittent responding mean activity levels in the 4 Hz delta and 4-6 Hz fleta 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 hegins to decrease, about 10 5 hefore pr-sentations of undetected targets, while hefore detected targets, 4-6 Hz amplitude decreases and gamma hand amplitude increases. Both these amplitude differences last 15-20 5 and occur in parallel with event-related cycles in target detection prohahility. In the same periods, alpha and sleep-spindle frequency amplitudes also show prominent 15-20 5 cycles, but these are not phase locked to pefformance cycles. 3 A second or longer after undetected targets, amplitude at intermediate 10-25 Hz frequencies decreases hriefly, while detected targets are followed by a transient amplitude increase in the same latency and frequency range.
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