Interstimulus contingency facilitates saccadic responses in a bimodal go/no-go task

The saccadic response to a suddenly appearing visual target stimulus is faster when an accessory auditory stimulus is presented in its spatiotemporal proximity. This multisensory facilitation of reaction time is usually considered a mandatory bottom-up process. Here, we report that it can be modulated by the predictability of the target location provided by an accessory stimulus, thereby indicating a form of top-down processing. Subjects were asked to make a saccade in the direction of a visual target randomly appearing left or right from fixation. An accessory auditory stimulus was presented either at the same location or opposite to the target, with the probability varying over blocks of presentation. Thus, the auditory stimulus contained probabilistic information about the target location (interstimulus contingency). A certain percentage of the trials were catch trials in which the auditory accompanying stimulus (Experiment 1) or the visual target (Experiment 2) was presented alone and the subjects were asked to withhold their response. In particular with visual catch trials, varying the predictability of target location resulted in reaction time facilitation in the bimodal trials, with both high (80%) and low predictability (20%), but only when both stimuli were presented within a small time window (40 ms). As subjects could not possibly follow the task instructions in this short period explicitly, we conclude that they utilized the interstimulus contingency information implicitly, thus revealing an extremely fast involuntary top-down control on saccadic eye movements. (c) 2005 Elsevier B.V. All rights reserved.