Reach adaptation and proprioceptive recalibration following exposure to misaligned sensory input.

Motor adaptation in response to a visuomotor distortion arises when the usual motor command no longer results in the predicted sensory output. In this study, we examined if exposure to a sensory discrepancy was sufficient on its own to produce changes in reaches and recalibrate the sense of felt hand position in the absence of any voluntary movements. Subjects pushed their hand out along a robot-generated fixed linear path (active exposure group) or were passively moved along the same path (passive exposure group). This fixed path was gradually rotated counterclockwise around the home position with respect to the path of the cursor. On all trials, subjects saw the cursor head directly to the remembered target position while their hand moved outwards. We found that after exposure to the visually distorted hand motion, subjects in both groups adapted their reaches such that they aimed ∼6° to the left of the intended target. The magnitude of reach adaptation was similar to the extent that subjects recalibrated their sense of felt hand position. Specifically the position at which subjects perceived their unseen hand to be aligned with a reference marker was the same as that to which they reached when allowed to move freely. Given the similarity in magnitude of these adaptive responses we propose that reach adaptation arose due to changes in subjects' sense of felt hand position. Moreover, results indicate that motor adaptation can arise following exposure to a sensory mismatch in the absence of movement related error signals.