[Intersensory facilitation: also concomitant visual stimuli can decrease reaction time].

Subjects respond faster to a visual response signal when an auditory accessory is presented at the same time. In contrast, a visual accessory does not reduce reaction time to an auditory response signal. Previous studies suggest that this asymmetrical effect is due to different properties of visual and auditory accessories (i.e., arousal and preparation enhancement). We assessed whether this asymmetrical effect would disappear when subjects were to process the accessory stimulus in a secondary task while responding to the response signal in the main task. This dual task method formed the basis for two experiments. Each experiment employed both visual and auditory accessories, as well as auditory and visual response signals. Experiment 1 manipulated factorially the intensity levels of the accessory and response signal. Experiment 2 was similar to Experiment 1 but manipulated the time interval between the accessory and the response signal instead of stimulus intensities. Response force was recorded in addition to reaction time. In both experiments the previously reported asymmetrical effect disappeared showing that auditory and visual accessories are functionally equivalent when the task demands more central processing of these stimuli. The analysis of response force suggested that both the visual and auditory accessory produce nonspecific arousal effects that may facilitate the generation of the response.