Faster to react, slower to move: differential effects of multisensory facilitation on movement initiation and execution across the adult lifespan.

Introduction: Spanning multisensory integration, ageing, attention, and motor control, this study is, to our knowledge, the first to examine how audiovisual integration differentially influences the initiation and execution of goal-directed motor actions across the adult lifespan. Methods: Young adults (<30 years) and older adults (between 50 and 69 years and ≥70 years) completed a simple detection task and a visual discrimination task on a portable touchscreen device. In the detection task, participants tapped to indicate the perceived location of auditory, visual, or audiovisual stimuli. In the discrimination task, they responded to a visual target presented either with or without distractors, accompanied by auditory stimulation that was either spatially aligned or misaligned with the target. Results: Across both tasks, multisensory stimulation significantly reduced reaction times in younger and older adults, whereas movement speed slowed modestly, most reliably in the detection task and under audiovisual distraction during visual discrimination. Distractors disproportionately impaired performance in older adults, but spatially aligned auditory information partly alleviated this effect, especially in the oldest group. Importantly, these modest movement-speed costs did not outweigh the overall facilitation of goal-directed actions. Discussion: The results suggest that multisensory signals enhance rapid motor planning and initiation, followed by a later calibration phase that improves precision during movement execution. They also indicate that the spatial window of multisensory integration may narrow in the presence of distractors, particularly in older adults. Taken together, these findings demonstrate that multisensory facilitation remains functionally advantageous even under distracting conditions, albeit at a relatively small task-dependent cost to movement speed.