Context effects on target detection during wake and sleep

The brain is very sensitive to changes in sensory stimulation. This dissertation examines target detection when subjects were alert, drowsy, and sleeping. Subjects were required to detect and respond to target stimuli which differed from non-targets. High-density recording from scalp electrodes were used to study the brain processes involved in target detection.;Experiment 1 examines whether the sleeping brain retains the ability to discriminate infrequent tones in the presence of a second, distracting stimulus dimension. During wakefulness, infrequent tones produce a large, positive electrical field at the top of the head beginning about 300 milliseconds after the tone is presented (P300). When awake, this positivity occurs both in the absence (simple task) and the presence (complex task) of irrelevant distracting context. During sleep, the P300 was evident only in the simple condition. The presence of the distracter dimension eliminated cortical evidence for discrimination of target from background tones. Apparently, the sleeping brain was unable to separate the critical target dimension from the interfering context.;Experiment 2 examined the role of prior context in creating expectations about the target. Two auditory tones were presented in random sequences when subjects attended, rested awake, became drowsy, and slept. Reaction times became slower as arousal state changed toward sleep. The repetition effect on the event-related potential (ERP) is present in all arousal conditions although the topography of the P300 differs. Long range context effects on the P300 are present in the alert state and gradually reduce as arousal moves toward and into sleep.;The ability to identify targets and create expectations based on prior context is a basic aspect of human performance. In both experiments, sleepiness and sleep appear to preferentially impair memory and control processes while automatic processes remain intact. Naturally occurring changes in arousal state appear to gradually reduce the capacity of the brain for attentional and memory processes.;This dissertation includes previously unpublished, co-authored materials.