Role of the Rostral Superior Colliculus in Gaze Anchoring During Reach Movements

When reaching for an object, primates usually look at their target before touching it with the hand. This gaze movement prior to the arm movement allows target fixation, which is usually prolonged until the target is reached. In this manner, a stable image of the object is provided on the fovea during the reach, which is crucial for guiding the final part of the hand trajectory by visual feedback. Here we investigated a neural substrate possibly responsible for this behavior. In particular we tested the influence of reaching movements on neurons recorded at the rostral pole of the superior colliculus (rSC), an area classically related to fixation. Most rSC neurons showed a significant increase in their activity during reaching. Moreover, this increase was particularly high when the reaching movements were preceded by corresponding saccades to the targets to be reached, probably revealing a stronger coupling of the oculo-manual neural system during such a natural task. However, none of the parameters tested-including movement kinematics and target location-was found to be closely related to the observed increase in neural activity. Thus the increase in activity during reaching was found to be rather nonspecific except for its dependence on whether the reach was produced in isolation or in combination with a gaze movement. These results identify the rSC as a neural substrate sufficient for gaze anchoring during natural reaching movements, placing its activity at the core of the neural system dedicated to eye-hand coordination.