Startling acoustic stimuli hasten choice reaching tasks by strengthening, but not changing the timing of, express visuomotor responses

ABSTRACT

Responding to an external stimulus takes ∼200 ms, but this can be shortened to as little as ∼120 ms with the additional presentation of a startling acoustic stimulus. This StartReact phenomenon is hypothesized to arise from the involuntary release of a fully prepared movement. However, a startling acoustic stimulus also expedites rapid mid-flight, reactive adjustments to unpredictably displaced targets which could not have been prepared in advance. We surmise that for such rapid visuomotor transformations, intersensory facilitation may occur between auditory signals arising from the startling acoustic stimulus and visual signals relayed along a fast subcortical network. To explore this, we examined the StartReact phenomenon in a task that produces express visuomotor responses, which are brief bursts of muscle activity that arise from a fast tectoreticulospinal network. We measured express visuomotor responses on upper limb muscles in humans as they reached either toward or away from a stimulus in blocks of trials where movements could either be fully prepared or not, occasionally pairing stimulus presentation with a startling acoustic stimulus. The startling acoustic stimulus reliably produced larger but fixed-latency express visuomotor responses in a target-selective manner. Consistent with the StartReact phenomenon, it shortened reaction times, which were as fast for prepared and unprepared movements. Our results confirm that the StartReact phenomenon can be elicited for reactive movements without any motor preparation, consistent with intersensory facilitation. We propose the reticular formation to be the likely node for intersensory convergence during the most rapid transformations of vision into targeted reaching actions.