Within-channel selection and event-related potentials during selective auditory attention

Event-related potentials were recorded from subjects who listened selectively to stimuli in one of two input channels (ears). The stimuli were random sequences of five vowels and a tone pip. In separate experimental runs, the difficulty of within-channel selection was manipulated by designating either tone (T), one vowel (1V), or two vowels (2V) as targets. An attention-related negativity (Nd) was observed for attended nontarget vowels irrespective of the target difficulty, but the early, centrally maximal phase of Nd was substantial only for the one-vowel and two-vowel conditions. The Nd between its early and late peaks was significantly larger for the two-vowel condition than for the one-vowel and tone conditions. The 2V minus 1V difference in Nd had a closely similar distribution to the early Nd. These target difficulty effects were interpreted as suggesting that the early Nd recorded in two-selection tasks can be related to a within-channel selection process as well as a between-channel selection process. The more anteriorly distributed, later phase of Nd was suggested to be rather insensitive to the difficulty of the within-channel selection. The nature of the within-channel selection underlying the early Nd was discussed in relation to controlled memory search operations.