Selective attention and the auditory vertex potential. I. Effects of stimulus delivery rate.

In a selective attention task, twelve subjects received random sequences of 800 and 1500 c/sec tone pips in their right and left ears, respectively. They were instructed to attend to one channel (ear) of tones, to ignore the other, and to press a button whenever occasional "targets", tones of a slightly higher pitch, were detected in the attended ear. In separate experimental conditions the randomized interstimulus intervals (ISIs) were "short" (averaging 350 msec), "medium" (960 mes) and "long" (1920 msec). The N1 component of the auditory evoked potential (latency 80--130 msec) was found to be enlarged to all stimuli in an attended channel (both targets and non-targets) but only in the short ISI condition. Thus, a high "information load" appears to be a prerequisite for producing channel-selective enhancement of the N1 wave; this high load condition was also associated with the most accurate target detectability scores (d'). The pattern of attention-related effects on N1 was dissociated from the pattern displayed by the subsequent P3 wave (300--450 msec), substantiating the view that the two waves are related to different modes of selective attention.