The experiments described in this proposal investigate the brain mechanisms of selective information processing in man as revealed in the properties of the sensory-evoked electrical responses recorded from the scalp. In recent studies we have developed a procedure whereby auditory stimuli are delivered from multiple sound sources concurrently at very high rates while the subjects attention is confined to only one of the sources. Using this paradigm we showed for the first time that selective attention towards one of several stimulus channels (sources) was reliably indexed by the amplitude of a specific component of the auditory evoked potential-- a negative wave with a latency of some 100 msec. A subsequent brain potential-- a positive wave at 300 msec-- was selectively elicited only by the occasional "target" signals occurring in the attended channel. The proposed experiments test our working hypothesis that these two evoked potential components index the operation of separate, heirarchically organized stages of selective attention with different principles of selection, which correspond closely to the "stimulus set" and "response set" modes of attention that were distinguished by Broadbent. It will be determined if these two modes of attention and their physiological indices are common across many varieties of tasks involving spatial and pitch discriminations, cross-modality discriminations, pattern recognition, threshold signal detection, and dichotic listening to speech sounds. The quantitative relation will be established between the amplitudes of these components and measures of selective attention derived from signal detection theory (d', beta, and d'2 equals information transfer). Early evoked responses from the brainstem (auditory) and primary cortex (somatosensory) will be scrutinized to see if selective attention influences transmission at these levels.