Role of the Express Visuomotor Response in Contextual Orienting
Abstract
To respond swiftly to urgent visual stimuli, the brain must balance speed and accuracy. A key part of this process is the express visuomotor response (EVR), a rapid muscle reflex occurring 80–120 ms after stimulus onset. This thesis explores the EVR’s role in visually-guided reaching in humans and offers converging evidence that it is initiated by the superior colliculus (SC).
In my first study (Chapter 2), I examined whether the EVR shares the SC’s preference for evolutionarily relevant stimuli, particularly faces. I found that the EVR is uniquely modulated within tasks involving face stimuli despite occurring earlier than face detection in the occipitotemporal cortex. In Chapter 3, I found that the EVR contributes to the initiation of short-latency arm movements, regardless of whether they are made from rest or to correct an ongoing movement. This result challenges long-held assumptions that ongoing reaching movements are corrected by a distinct neural circuit, suggesting instead a single nested neural system. In Chapter 4, I used transcranial magnetic stimulation to investigate claims that the posterior parietal cortex (PPC) is the main driver of online movement correction. I found that PPC stimulation had no impact on the muscle recruitment driving online correction, including the EVR. Instead, the results suggest that prior disruptions from PPC stimulation may reflect inadvertent activation of the corticospinal tract, causing disruptive motor evoked potentials.
Together, these three studies support the hypothesis that the SC mediates the EVR to initiate short-latency visually-guided movements.