Control of the spatial orientation of the hand is an important component of reaching and grasping movements. We studied the contribution of vision and proprioception to the perception and control of hand orientation in orientation-matching and letter-posting tasks. In the orientation-matching task, subjects aligned a "match" handle to a "target" handle that was fixed in different orientations. In letter-posting task 1, subjects simultaneously reached and rotated the right hand to insert a match handle into a target slot fixed in the same orientations. Similar sensory conditions produced different error patterns in the two tasks. Furthermore, without vision of the hand, final hand-orientation errors were smaller overall in letter-posting task 1 than in the orientation-matching task. In letter-posting task 2, subjects first aligned their hand to the angle of the target and then reached to it with the instruction not to change their initial hand orientation. Nevertheless, hand orientation changed during reaching in a way that reduced the initial orientation errors. This did not occur when there was no explicitly defined target toward which the subjects reached (letter-posting task 3). The reduction in hand-orientation errors during reach, even when told not to change it, suggests the engagement of an automatic error correction mechanism for hand orientation during reaching movements toward stationary targets. The correction mechanism was engaged when the task involved transitive actions directed at the target object. The on-line adjustments can occur without vision of the hand and even when target orientation is defined only by proprioceptive inputs.

The contribution of visual experience to the perception and sensorimotor control of spatial orientation of the hand was investigated in blind subjects. In "orientation-matching" tasks, subjects aligned a match handle held in their right hand to a target handle held in their left hand and fixed in different orientations, with both arms outstretched. In "letter-posting" task 1, the same subjects reached out and simultaneously oriented their right hand to insert the match handle into a target slot fixed in the same range of orientations. Orientations were signaled proprioceptively by a reference handle held in the left hand. Final hand orientation errors were smaller when blind subjects simultaneously reached out and rotated their hand to insert the match handle into the target slot in letter-posting task 1 than when they held their arm extended and aligned the handles in the orientation-matching task. In letter-posting task 2, blind subjects first aligned their hand to the orientation of the target and then subsequently reached to the target with the instruction to not change hand orientation during reaching. Despite the instruction, subjects showed a reduction in absolute hand orientation error from the beginning to the end of the reach. In all tasks, performance of blind subjects was very similar to that of blindfolded normally sighted subjects. These findings provide the first evidence of an automatic on-line error-correction mechanism for hand orientation guided only by proprioceptive inputs during reaching in blind subjects, and reveal that the on-line mechanism does not depend on prior visual experience.