It has been hypothesized that one function of the startle response is to interrupt ongoing activity. The capacity of startling auditory stimuli to interrupt performance of three different button-pressing tasks was examined to test and refine this hypothesis. Various physiologic indices of the startle response were measured and compared to effects on reaction time (RT).;Startle was not effective in altering performance in a task requiring subjects to reproduce time intervals of three to six seconds. However, startle reliably delayed RTs in a four-choice serial RT (SRT) task and in a two-choice motor programming (MP) task. Thus response selection and motor programming processing stages show susceptibility to startle interrupt.;Subjects in the SRT task who showed the largest overall heart rate acceleration to startle also showed the greatest RT delay. Among trials, RT delay was positively related to the amplitude of the P300 event-related brain potential (ERP) elicited by the startling stimulus.;In the MP task, the subjects who showed larger electromyographic (EMG) responses to startle in the orbicularis oculi (OO) and sternocleidomastoid (SCM) muscles showed larger RT delays. Among trials, RT delay in this task was related to the amplitude of the N100 ERP elicited by the startling stimulus.;Previously established indices of startle show reliable covariation with measures of motor interrupt, although task demands influence which response systems best express this relationship. Overall, these results are consistent with task interrupt as a basic function of startle, best elicited when an organism's uncertainty of action is high, and when it is committed to temporally extended actions.
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