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Temporal dynamics of human auditory selective attention.

Event-related potentials (ERPs) were recorded from human subjects in response to short trains of rapidly presented tone pips of two frequencies. Subjects listened selectively to tones of one frequency as directed by a visual cue prior to each train and responded to occasional longer-duration target tones of that frequency. The early negative difference (Nd) between ERPs elicited by the tones when attended versus unattended was minimal for the first stimulus in each train but developed rapidly, attaining stable values by about the third stimulus. The development of this early Nd was most rapid when two attended tones began a train. Long time-base averages revealed the concomitant development of a standing negativity over the course of each train. In contrast, late positive ERPs distinguished attended from unattended target tones even when they occurred in the first position of a train, prior to the development of Nd. The latency of this late positivity together with the reaction time to the targets decreased substantially as the Nd developed. It was concluded that subjects can select stimuli at various stages of analysis, but employment of the early selection mechanism indexed by Nd enables an economy of processing and faster and more accurate task performance. This mode of early selection is not pre-set in advance of a stimulus train, however, at least under the present conditions.