Sound speeds vision through preparation, not integration.

In manual choice reaction time (RT) tasks, people respond faster to a visual target stimulus when it is accompanied by a task-irrelevant tone than when it is presented alone. This intersensory facilitation effect is often attributed to multisensory integration, but here we show it to be a reflection of temporal preparation. According to this view, the more rapidly processed tone serves as a warning signal (S1), which initiates preparation for the more sluggish visual target (S2). To test this view, we varied the delay between S1 and S2 in conjunction with the modality of S1 (auditory or visual). For brief delays, responses to S2 were faster when S1 was auditory than when it was visual. Crucially, however, this intersensory facilitation effect disappeared after correction for the difference in S1-detection time, equating the effective preparation period. This shows that sound speeds response to a visual target only through preparation.