Abstract
Humans have the remarkable ability to react quickly to sudden changes in the visual environment, presumably via a fast, subcortical pathway. However, how these rapid orienting responses are affected by stimuli from mixed modalities, such as light and sound, is less well known. Here, we study the fast inhibition of smooth pursuit eye movements, occurring reflexively in response to the sudden onset of auditory, visual, or audiovisual distractors. Unlike saccades, smooth pursuit provides a continuous and graded measure of sensorimotor control, allowing us to reveal mechanisms of multisensory integration for fast sensorimotor control. Observers (n=16) tracked a horizontally moving disc while eye movements were recorded. After a random delay (0.5-1s) an auditory, visual, or audiovisual distractor was presented for 50 ms in 60% of trials; no distractor was presented in the remaining 40% of trials (control trials). Distractor presentation elicited a transient inhibition in smooth pursuit velocity ~100 ms after distractor onset, followed by an increase in pursuit velocity above baseline (rebound). Pursuit inhibition revealed graded velocity changes to the auditory (-2.7%), visual (-5.5%), and audiovisual distractors (-9.1 %; all p<.01 compared to control trials). Similarly, the increase in pursuit velocity during the rebound period scaled with distractor type (auditory: +3.7%, visual: +6.2%, and audiovisual: +9.6%; all p<.01). The audiovisual responses were significantly stronger than the visual-only (p=.004) or auditory-only conditions (p<.001). Strikingly, the linear sum of the unisensory conditions predicted the outcome of the multisensory condition, indicating an additive effect of multisensory stimulation on oculomotor inhibition. Our findings demonstrate strong effects of multisensory stimulation on fast sensorimotor control (smooth pursuit inhibition). The short latency of these responses suggests the involvement of a fast sensorimotor system, presumably mediated via the superior colliculus—a midbrain structure involved in both oculomotor orienting and multisensory integration.