Electrophysiological correlates of interval timing in the Stop-Reaction-Time task

Scalp-recorded event-related potentials were recorded while participants completed an auditory or a visual Stop-Reaction-Time task (Stop-RT task). Participants were asked to listen to a sequence of tones (Experiment 1) or view a sequence of LED flashes (Experiment 2) that had a constant stimulus onset asynchrony (SOA) of either 470 or 770 ms, and to respond as soon as the sequence ended. Sequence length varied across trials, so counting the tones or light flashes did not permit determination of sequence end. Instead, participants had to time the SOA, even though they were not explicitly instructed to do so, and respond when the SOA had been exceeded. At sequence end, a biphasic negative-positive ERP response was emitted following the point in time when a stimulus would have occurred had the sequence continued. The negative component of this omission potential (OP) had a right fronto-central focus, whereas the positive component had a parietal focus; a pattern that held across SOA duration and signal modality. Moreover, the mean peak latency of the negative OP was equivalent in the visual and auditory modalities, even though the behavioral Stop-RTs obtained in the auditory experiment were 200 ms faster than those obtained in the visual experiment. The amodal negative OP appears to be a correlate of an interval-timing process that precedes response initiation, whereas the positive OP is most likely a target P300.