Modulation of early auditory processing during selective listening to rapidly presented tones.

Two dichotic listening experiments were performed in which stimulus and task conditions were optimized for the early selection of inputs. Subjects listened selectively to sequences of rapidly presented tone pips in one ear while ignoring tone pips of a different pitch in the opposite ear. In both experiments, an enhanced positivity between 20 and 50 msec (the 'P20-50') was observed over central and frontal sites in the ERPs to the attended-channel tone pips. At longer latencies, the effects of attention appeared to include an amplitude modulation of several exogenous ERPs, including subcomponents of the central N1 (60-150 msec) and P2 (170-230 msec) waves and the temporal T complex (80-150 msec). In contrast, the attention effect prefrontally consisted of a broad negativity that appeared to be largely endogenous. A signal processing technique (Adjar) was employed to correct for distortion of mutually overlapping ERPs elicited by successive stimuli presented at short interstimulus intervals (ISIs). It was confirmed that the P20-50 attention effect was not the result of differential overlap from previous ERPs. In addition, this technique allowed an analysis to be made of the effects of the preceding stimulus type and ISI on the attention-sensitive ERPs, which provided further support for the view that highly focused selective attention can directly modulate exogenous components of the auditory ERP. Moreover, these sequence-dependent ERP modulations were paralleled by variations in target discrimination performance. Taken together, these results provide strong support for the early selection hypothesis that attention can serve to selectively bias or gate stimulus processing before full perceptual analysis has occurred.