The nature of selective attention effects on auditory event-related potentials.

The purpose of the research reported here was to examine a number of issues relating to the nature of selective attention effects on auditory event-related potentials (ERPs), namely, to determine the relative contribution of N1 and slow wave (SW) to the early and late components of Nd respectively, where Nd is defined as the negative shift of attended ERPs relative to unattended ERPs; to examine whether individual differences in Nd morphology are related to performance and the strategies that subjects use; and to determine the contribution of changes in the attended and unattended ERPs to Nd. Auditory ERPs were recorded from subjects as they carried out an auditory selective attention task and a visual target detection task. The auditory selective attention task was a multidimensional task in which stimuli varied on location, pitch and duration and in which the subject's task was to pay attention to a particular location/pitch combination and respond whenever they detected a long-duration target tone. In the visual target detection task, subjects were required to respond whenever they detected a colour change in a light-emitting diode which also acted as a fixation point. Auditory ERPs recorded during the visual task were used to provide a measure of exogenous components uncontaminated by differential effects of selective processing of auditory stimuli. The results suggested that early Nd and N1 are independently generated as Nd did not exhibit the contralateral scalp focus typical of N1, and that late Nd is independent of SW. While substantial differences in Nd morphology were observed over subjects, these differences showed no consistent relationships to performance or to task strategies. Comparison of auditory ERPs during active auditory attention with auditory ERPs recorded during the visual control task indicated that there was an early negative shift of the attended ERP, a later negative shift of the attended ERP which had a frontal focus and a later positive shift of the unattended ERP. These results suggest that there are active processes involved in the processing of stimuli from both the attended and unattended source.