The objective of these studies is to investigate brain mechanisms of selective attention in man by recording event-related brain potentials from the scalp during the execution of sensory-motor performance tasks. By examining changes in stimulus-evoked brain potentials as subjects shift their attention between different categories of stimuli (visual or auditory), we hope to determine the timing and localization of the selective routing of information in the brain. These data will be used to evaluate the validity of competing psychological models of attention which hypothesize separate "early" and "late" stages of stimulus selection. We have tentatively identified the auditory-evoked N1 wave (onset latency at 50-80 msec) as a correlate of an early stage of selection between different channels of input while the P3 wave (onset at 150-200 msec) reflects the selection of a significant target stimulus at a later stage. We plan to test the generality of these waves as signs of attention in a series of visual and auditory discrimination tasks. We anticipate that these waves will provide objective measures of a person's attentive capabilities and will become useful in the characterization and diagnosis of the disorders of attention which accompany different psychopathological syndromes.