High-contrast, moving targets in an emerging target paradigm promote fast visuomotor responses during visually guided reaching.

Humans have a remarkable capacity to rapidly interact with the surrounding environment, often by transforming visual input into motor output on a moment-to-moment basis. But what visual features promote rapid reaching? High-contrast, fast-moving targets elicit strong responses in the superior colliculus (SC), a structure associated with express saccades and implicated in rapid electromyographic (EMG) responses on upper limb muscles. To test the influence of stimulus properties on rapid reaches, we had human subjects perform visually guided reaches to moving targets varied by speed (experiment 1) or speed and contrast (experiment 2) in an emerging target paradigm that has recently been shown to robustly elicit fast visuomotor responses. Our analysis focused on stimulus-locked responses (SLRs) on upper limb muscles. SLRs appear within <100 ms of target presentation, and as the first wave of muscle recruitment they have been hypothesized to arise from the SC. Across 32 subjects studied in both experiments, 97% expressed SLRs in the emerging target paradigm, whereas only 69% expressed SLRs in an immediate response paradigm toward static targets. Faster-moving targets (experiment 1) evoked large-magnitude SLRs, whereas high-contrast fast-moving targets (experiment 2) evoked short-latency, large-magnitude SLRs. In some instances, SLR magnitude exceeded the magnitude of movement-aligned activity. Both large-magnitude and short-latency SLRs were correlated with short-latency reach reaction times. Our results support the hypothesis that, in scenarios requiring expedited responses, a subcortical pathway originating in the SC elicits the earliest wave of muscle recruitment, expediting reaction times.NEW & NOTEWORTHY How does the brain rapidly transform vision into action? Here, by recording upper limb muscle activity, we find that high-contrast and fast-moving targets are highly effective at evoking rapid visually guided reaches. We surmise that a brain stem circuit originating in the superior colliculus contributes to the most rapid reaching responses. When time is of the essence, cortical areas may serve to prime this circuit and elaborate subsequent phases of recruitment.