Attention effects on auditory EPs as a function of inter-stimulus interval.

The present study focuses on the effect of the duration of the inter-stimulus interval (ISI) on the 'processing negativity', the endogeneous negative brain potential elicited by attended stimuli in various selective-attention paradigms. Three constant ISIs are used, 250 msec. 800 msec, and 2000 msec, in a selective dichotic-listening situation in which the subject's task is to detect occasional slight pitch changes in the input to a designated ear. Frontal (Fz), vertex (Cz), and parietal (Pz) evoked potentials (EP) are recorded. The processing negativity is observed in each condition, even in that involving the 2 sec ISI, which is in contrast with the commonly held view that this attention effect is only obtained with very short ISIs. The processing negativity was observed to have two components: (1) an early, central component of small amplitude which appears to be independent of the ISI: (2) a later, frontal component of large amplitude and long duration which is certainly elicited with long ISIs such as the 800 msec and 2000 msec ISIs used here, but also appears to be present even in the short-ISI condition overlapping the central component. This suggests that it is the frontal component which is sensitive to the ISI.