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Humans operate in a dynamic and uncertain world; sudden changes in an object’s position require rapid changes in motor control. Previous research in humans has shown that on-going visually-guided actions can be altered at latencies that approach minimal afferent and efferent conduction delays. The contribution of such a fast visuomotor system to overall motor control has long been recognized, but fundamental questions remain about the underlying neural substrates, and about the comparative contribution of cortical versus subcortical pathways.
My talk will focus on converging work on express responses in humans and non-human primates (NHPs). One type of express response is the express saccade, and another type is what we term an “express visuomotor response”, which can appear on upper-limb muscles in humans in less than 100 ms, and in less than 65 ms in animal models. Such remarkably short latencies limit the opportunity for extensive cortical processing, leading to the hypothesis that express visuomotor responses arise via tecto-reticulo-spinal signalling through the superior colliculus. Ongoing experiments establish that visually-related reaching signals arise within ~50 ms in the movement related layers of the superior colliculus, which appears early enough to generate express visuomotor responses. These results set the stage for comparative analysis of signal timing in frontal cortical areas. In the final part of my talk, I will discuss recent findings which show that express visuomotor responses persist in Parkinson’s Disease, despite degradation of signals related to deliberative reaching. Across all studies, we are seeing compelling similarities between the properties of express responses and the visual transient in the superior colliculus (SC). Such findings lead us to suggest that a subcortical tecto-reticulo-spinal circuit, which itself can be primed by cortical inputs, distributes the earliest visuomotor response that is shared across multiple body effectors.