When an irrelevant ＇accessory＇ stimulus is presented at about the same time as the imperative signal in a choice reaction time-task, the latency of the voluntary response is markedly reduced(1), The most prominent cognitive theories agree that this effect-is attributable to a brief surge in arousal (＇automatic alerting＇), but they disagree over whether the facilitation is localized to a late, low-level motoric process(2) or to an earlier stage, the process of orienting to and then perceptually categorizing the reaction stimulus(3,4). To lest these alternative hypotheses, we used the onset of the lateralized readiness potential (a movement-related brain potential) as a temporal landmark to partition mean reaction time into two time segments. The first segment included the time required to perceive the visual stimulus and decide which hand to react with; the second included only motoric processes. Presentation of an irrelevant acoustic stimulus shortened the first interval but had no effect on the second. We therefore rejected the motoric hypothesis.