Neuronal activity in the superior colliculus related to saccade initiation during coordinated gaze-reach movements

One must be quick and precise when foveating targets to be reached, because the eyes have to guide the hand trajectory by visual feedback, and we may miss a rapidly moving target if our grasping is not fast and accurate enough. To this end, our brains developed mechanisms coordinating gaze and hand movements to optimize the way in which we foveate and reach. One of these mechanisms is the facilitation of the primary saccade--proven in humans and confirmed here in monkeys--which allows the generation of short-latency gaze movements when reaching towards visual targets. Here we tested whether the neuronal activity in the superior colliculus (SC) accounts for this mechanism; alternatively, cortical saccade-related areas could play a major role in the fast initiation of saccades during such elaborated behaviours bypassing the SC. Upon presentation of a target, neurons located at the rostral pole of the SC started the saccade-related pause in their activity earlier in tasks involving coordinated gaze-reach movements than in tasks in which the saccades were made in isolation. In the same tasks neurons located at the caudal SC reached peak firing rates earlier in coordinated gaze-reach movements than with isolated saccades, confirming the tight coupling between their burst activity latencies and the saccadic reaction times. In sum, our results extend the role of the SC in saccade initiation to coordinated gaze-reach movements, identifying its activity as an important part of the distributed neural system for eye-hand coordination.