Fast visuomotor processing made faster by sound

Reaction time to a visual event can be dramatically reduced if the visual stimulus is accompanied by a startling sound. The mechanism may involve a motor programme being stored and triggered early by the sound. However, in a choice reaction task the required response is not known in advance, and so cannot be stored. In this case startling sound does not usually speed up the reaction and may even be detrimental to performance. Here we show that the reaction time of a special type of visually evoked movement can be substantially reduced by startling sound, even though the movement requires choice. The task involved stepping onto an illuminated target that sometimes moved mid-step left or right, requiring a foot trajectory adjustment. These adjustments occur at much shorter latency than conventional visuomotor reaction tasks and are thought to involve subcortical brain areas. The presence of the sound, which carried no information, shortened the already fast mean response time of 134 ms by similar to 20 ms. We attribute this to auditory-visual interaction since sound alone had no effect. Although we observed startle responses, the quickening effect was not contingent upon their presence. Given minimum motor and sensory conduction time, we estimate that the loud sound reduced the central visuomotor processing time by at least 30%.