AbstractHumans 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 the shortest-latency reach responses? To address this question, 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, which has recently been shown to robustly elicit fast visuomotor responses. Our analysis focused on stimulus-locked responses (SLRs) on upper limb muscles. SLRs represent the first wave of muscle recruitment tied to visual target onset, appearing within <100 ms. Across 32 subjects studied in both experiments, 97% expressed SLRs in the emerging target paradigm. In comparison, 69% of these subjects expressed SLRs in a visually-guided reach paradigm. Within the emerging target paradigm, we found that target speed impacted SLR magnitude, whereas target contrast impacted SLR latency and magnitude. Thus, high contrast, faster-moving targets in the emerging target paradigm robustly recruited the circuitry mediating the most rapid visuomotor transformations for reaching, and such responses were associated with shorter latency RTs. Our results support the hypothesis that a subcortical pathway originating in the superior colliculus may be involved in the earliest wave of muscle recruitment following visual stimulus presentation. In scenarios requiring expedited responses, cortical areas may serve to prime this subcortical pathway, and elaborate subsequent phases of muscle recruitment following the SLR.Significance StatementHumans have a remarkable capacity, when necessary, to rapidly transform vision into action. But how does the brain do this? Here, by studying human subjects reaching to suddenly-appearing targets, we find that the earliest visually-guided actions are produced in response to high-contrast, moving targets. A millisecond-resolution examination of upper limb muscle recruitment shows that motor output can begin within less than 100 ms of target presentation. We surmise that this earliest recruitment arises from a phylogenetically-conserved brainstem circuit originating in the superior colliculus. Rather than being directly involved in the earliest phase of visuomotor actions, cortical areas may prime this brainstem circuit to produce initial muscle recruitment, and then elaborate subsequent phases of recruitment when time is of the essence.
Memory-guided microsaccades
