Online control of the direction of rapid reaching movements.

Online visual control of the direction of rapid reaching movements was assessed by evaluating how human subjects reacted to shifts in seen hand position near movement onsets. Participants ( N=10) produced saccadic eye and rapid arm movements (mean duration = 328 ms) towards a peripheral visual target in complete darkness. During the saccade, visual feedback of hand position could be shifted by 1, 2, 3 or 4 cm perpendicularly to the main movement direction. The resulting discrepancies between visual and proprioceptive information about hand position were never consciously perceived by the subjects. Following the shifts, hand trajectories deviated from those produced in a control condition (without shift) in order to bring seen hand position closer to the target. Globally, the deviations corresponded to 45% of the shifts, regardless of their magnitude or movement duration. This finding highlights not only the efficiency of visual feedback processing in online motor control but also underlines the significant contribution of limb proprioception.