Internally driven control of reaching movements: a study on a proprioceptively deafferented subject

We investigated the possibility of controlling reaching movements on the sole basis of central mechanisms, i.e., without peripheral feedback on hand and target positions. A deafferented subject (GL) and control subjects reached with the unseen hand for a straight-ahead target that could be displaced laterally at movement onset. The shifted target was continuously or briefly lit, or not visible. In this latter condition, a beep from either side of subjects' head single-handedly signaled the change in the movement goal, so that movements could only be controlled through an internal representation of the memorised target position. Compared to controls, GL showed quantitatively similar corrections (77% of the target displacement, on an average) and similar reaction times to the target shift (mean = 516 ms), regardless of target visual information. These results highlight a remarkable capacity for controlling reaching movements on the sole basis of internally driven processes. On the other hand, trajectories in double-step trials differed drastically between GL and controls. Controls' trajectories were composed of two segments, the second of which brought the hand directly toward the displaced target. The patient produced three-segment, stair-like trajectories. The first and third segments were mainly in the sagittal plane and the second segment was a vector-image of the lateral target shift. A control experiment showed that GL's trajectories were not the result of a voluntary strategy used to adjust movement trajectory in the absence of peripheral information on hand position. We suggest that GL's trajectories reflect a deficit in interjoint coordination in the absence of proprioception.