Optimal response of eye and hand motor systems in pointing at a visual target. II. Static and dynamic visual cues in the control of hand movement.

The closed loop situation of hand pointing at a target has been experimentally divided into its static and dynamic components. When the subjects see their hand at first (closed loop) until the start of the hand movement cuts off the vision of the hand (open loop), the pointing is significantly more accurate than when it is performed without any vision of the hand before and throughout the movement (fully open loop). This suggests that initial cues as regards hand and target position, improve the motor program by a better identification of initial and final states. As poor as it is, the extra retinal signal (encoding of eye position) improves performance when the foveation is done under closed loop; it allows a better redefinition of target position, and thus modulates the hand motor program through a direct central pathway, which is quicker than the processing of the visual feedback of the hand movement error.