Tonic, phasic, and transient EEG correlates of auditory awareness in drowsiness

Brain Res Cogn Brain Res. 1996 Jul;4(1):15-25. doi: 10.1016/0926-6410(95)00042-9.


During drowsiness, human performance in responding to above-threshold auditory targets tends to vary irregularly over periods of 4 min and longer. These performance fluctuations are accompanied by distinct changes in the frequency spectrum of the electroencephalogram (EEG) on three time scales: (1) during minute-scale and longer periods of intermittent responding, mean activity levels in the (< 4 Hz) delta and (4-6 Hz) 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 begins to decrease, about 10 s before presentations of undetected targets, while before detected targets, 4-6 Hz amplitude decreases and gamma band amplitude increases. Both these amplitude differences last 15-20 s and occur in parallel with event-related cycles in target detection probability. In the same periods, alpha and sleep-spindle frequency amplitudes also show prominent 15-20 s cycles, but these are not phase locked to performance cycles. (3) A second or longer after undetected targets, amplitude at intermediate (10-25 Hz) frequencies decreases briefly, while detected targets are followed by a transient amplitude increase in the same latency and frequency range.

Publication types

  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Acoustic Stimulation
  • Adult
  • Auditory Threshold*
  • Awareness*
  • Electroencephalography*
  • Humans
  • Reproducibility of Results
  • Sleep / physiology
  • Sleep Stages*
  • Theta Rhythm
  • Time Factors