No abstract

Reaching to targets with misaligned visual feedback of the hand leads to changes in proprioceptive estimates of hand position and reach aftereffects. In such tasks, subjects are able to make use of two error signals: the discrepancy between the desired and actual movement, known as the sensorimotor error signal, and the discrepancy between visual and proprioceptive estimates of hand position, which we refer to as the cross-sensory error signal. We have recently shown that mere exposure to a sensory discrepancy in the absence of goal-directed movement (i.e. no sensorimotor error signal) is sufficient to produce similar changes in felt hand position and reach aftereffects. Here, we sought to determine the extent that this cross-sensory error signal can contribute to proprioceptive recalibration and movement aftereffects by manipulating the magnitude of this signal in the absence of volitional aiming movements. Subjects pushed their hand out along a robot-generated linear path that was gradually rotated clockwise relative to the path of a cursor. On all trials, subjects viewed a cursor that headed directly towards a remembered target while their hand moved out synchronously. After exposure to a 30° rotated hand-cursor distortion, subjects recalibrated their sense of felt hand position and adapted their reaches. However, no additional increases in recalibration or aftereffects were observed following further increases in the cross-sensory error signal (e.g. up to 70°). This is in contrast to our previous study where subjects freely reached to targets with misaligned visual hand position feedback, hence experiencing both sensorimotor and cross-sensory errors, and the distortion magnitude systematically predicted increases in proprioceptive recalibration and reach aftereffects. Given these findings, we suggest that the cross-sensory error signal results in changes to felt hand position which drive partial reach aftereffects, while larger aftereffects that are produced after visuomotor adaptation (and that vary with the size of distortion) are related to the sensorimotor error signal.

Previous studies have shown that both young and older subjects adapt their reaches in response to a visuomotor distortion. It has been suggested that one's continued ability to adapt to a visuomotor distortion with advancing age is due to the preservation of implicit learning mechanisms, where implicit learning mechanisms include processes that realign sensory inputs (i.e. shift one's felt hand position to match the visual representation). The present study examined this proposal by determining if changes in sense of felt hand position (i.e. proprioceptive recalibration) follow visuomotor adaptation in older subjects. As well, we examined the influence of age on proprioceptive recalibration by comparing young and older subjects' estimates of the position at which they felt their hand was aligned with a visual reference marker before and after aiming with a misaligned cursor that was gradually rotated 30 degrees clockwise of the actual hand location. On estimation trials, subjects moved their hand along a robot-generated constrained pathway. At the end of the movement, a reference marker appeared and subjects indicated if their hand was left or right of the marker. Results indicated that all subjects adapted their reaches at a similar rate and to the same extent across the reaching trials. More importantly, we found that both young and older subjects recalibrated proprioception, such that they felt their hand was aligned with a reference marker when it was approximately 6 degrees more left (or counterclockwise) of the marker following reaches with a rotated cursor. The leftward shift in both young and older subjects' estimates was in the same direction and a third of the extent of adapted movement. Given that the changes in the estimate of felt hand position were only a fraction of the changes observed in the reaching movements, it is unlikely that sensory recalibration was the only source driving changes in reaches. Thus, we propose that proprioceptive recalibration combines with adapted sensorimotor mappings to produce changes in reaching movements. From the results of the present study, it is clear that changes in both sensory and motor systems are possible in older adults and could contribute to the preserved visuomotor adaptation.

Previous studies have demonstrated that after reaching with misaligned visual feedback of the hand, one adapts his or her reaches and partially recalibrates proprioception, such that sense of felt hand position is shifted to match the seen hand position. However, to date, this has only been demonstrated in the right (dominant) hand following reach training with a visuomotor distortion in which the rotated cursor distortion was introduced gradually. As reach adaptation has been shown to differ depending on how the distortion is introduced (gradual vs. abrupt), we sought to examine proprioceptive recalibration following reach training with a cursor that was abruptly rotated 30° clockwise relative to hand motion. Furthermore, because the left and right arms have demonstrated selective advantages when matching visual and proprioceptive targets, respectively, we assessed proprioceptive recalibration in right-handed subjects following training with either the right or the left hand. On average, we observed shifts in felt hand position of approximately 7.6° following training with misaligned visual feedback of the hand, which is consistent with our previous findings in which the distortion was introduced gradually. Moreover, no difference was observed in proprioceptive recalibration across the left and right hands. These findings suggest that proprioceptive recalibration is a robust process that arises symmetrically in the two hands following visuomotor adaptation regardless of the initial magnitude of the error signal.

Reaching with visual feedback that is misaligned with respect to the actual hand's location leads to changes in reach trajectories (i.e., visuomotor adaptation). Previous studies have also demonstrated that when training to reach with misaligned visual feedback of the hand, the opposite hand also partially adapts, providing evidence of intermanual transfer. Moreover, our laboratory has shown that visuomotor adaptation to a misaligned hand cursor, either translated or rotated relative to the hand, also leads to changes in felt hand position (what we call proprioceptive recalibration), such that subjects' estimate of felt hand position relative to both visual and non-visual reference markers (e.g., body midline) shifts in the direction of the visuomotor distortion. In the present study, we first determined the extent that motor adaptation to a translated cursor leads to transfer to the opposite hand, and whether this transfer differs across the dominant and non-dominant hands. Second, we looked to establish whether changes in hand proprioception that occur with the trained hand following adaptation also transfer to the untrained hand. We found intermanual motor transfer to the left untrained (non-dominant) hand after subjects trained their right (dominant) hand to reach with translated visual feedback of their hand. Motor transfer from the left trained to the right untrained hand was not observed. Despite finding changes in felt hand position in both trained hands, we did not find similar evidence of proprioceptive recalibration in the right or left untrained hands. Taken together, our results suggest that unlike visuomotor adaptation, proprioceptive recalibration does not transfer between hands and is specific only to the arm exposed to the distortion.