Multisensory integration in orienting behavior: Pupil size, microsaccades, and saccades.

Signals from different sensory modalities are integrated in the brain to optimize behavior. Although multisensory integration has been demonstrated in saccadic eye movements, its influence on other orienting responses, including pupil size and microsaccades, is still poorly understood. We examined human gaze orienting responses following presentation of visual, auditory, or combined audiovisual stimuli. Transient pupil dilation and microsaccade inhibition were evoked shortly after the appearance of a salient stimulus. Audiovisual stimuli evoked larger pupil dilation, greater microsaccade inhibition, and faster saccade reaction times compared to unimodal conditions. Trials with faster saccadic reaction times were accompanied with greater pupil dilation responses. Similar modulation of pre-stimulus pupil-size-change rate was observed between stimulus-evoked saccadic and pupillary responses. Thus, multisensory integration impacts multiple components of orienting, with coordination between saccade and pupil responses, implicating the superior colliculus in coordinating these responses because of its central role in both orienting behavior and multisensory integration.