Prism adaptation during walking generalizes to reaching and requires the cerebellum

Adaptation of arm movements to laterally displacing prism glasses is usually highly specific to body part and movement type and is known to require the cerebellum. Here, we show that prism adaptation of walking trajectory generalizes to reaching (a different behavior involving a different body part) and that this adaptation requires the cerebellum. In experiment 1, healthy control subjects adapted to prisms during either reaching or walking and were tested for generalization to the other movement type. We recorded lateral deviations in finger endpoint position and walking direction to measure negative aftereffects and generalization. Results showed that generalization of prism adaptation is asymmetric: walking generalizes extensively to reaching, but reaching does not generalize to walking. In experiment 2, we compared the performance of cerebellar subjects versus healthy controls during the prism walking adaptation. We measured rates of adaptation, aftereffects, and generalization. Cerebellar subjects had reduced adaptation magnitudes, slowed adaptation rates, decreased negative aftereffects, and poor generalization. Based on these experiments, we propose that prism adaptation during whole body movements through space invokes a more general system for visuomotor remapping, involving recalibration of higher-order, effector-independent brain regions. In contrast, prism adaptation during isolated movements of the limbs is probably recalibrated by effector-specific mechanisms. The cerebellum is an essential component in the network for both types of prism adaptation.     

Cerebellar inactivation impairs memory of learned prism gaze-reach calibrations

Three monkeys performed a visually guided reach-touch task with and without laterally displacing prisms. The prisms offset the normally aligned gaze/reach and subsequent touch. Naive monkeys showed adaptation, such that on repeated prism trials the gaze-reach angle widened and touches hit nearer the target. On the first subsequent no-prism trial the monkeys exhibited an aftereffect, such that the widened gaze-reach angle persisted and touches missed the target in the direction opposite that of initial prism-induced error. After 20-30 days of training, monkeys showed long-term learning and storage of the prism gaze-reach calibration: they switched between prism and no-prism and touched the target on the first trials without adaptation or aftereffect. Injections of lidocaine into posterolateral cerebellar cortex or muscimol or lidocaine into dentate nucleus temporarily inactivated these structures. Immediately after injections into cortex or dentate, reaches were displaced in the direction of prism-displaced gaze, but no-prism reaches were relatively unimpaired. There was little or no adaptation on the day of injection. On days after injection, there was no adaptation and both prism and no-prism reaches were horizontally, and often vertically, displaced. A single permanent lesion (kainic acid) in the lateral dentate nucleus of one monkey immediately impaired only the learned prism gaze-reach calibration and in subsequent days disrupted both learning and performance. This effect persisted for the 18 days of observation, with little or no adaptation.     

The role of the cross-sensory error signal in visuomotor adaptation

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.     

How perceived egocentric distance varies with changes in tonic vergence

According to the eye muscle potentiation (EMP) hypothesis, sustained vergence leads to changes in egocentric perceived distance. This perceptual effect has been attributed to a change in the resting or tonic state of vergence. The goal of the present study was to test the EMP hypothesis by quantifying the relationship between prism-induced changes in tonic vergence and corresponding changes in perceived distance and by measuring the dynamics of changes in perceived distance. During a 10-min exposure to 5-diopter base-out prisms that increased the vergence demand, thirteen right-handed subjects pointed to visual targets located within reaching space using their left hand, without visual feedback. Pre- and post-exposure tests assessed tonic vergence through phoria measurements and egocentric distance estimate through pointing to visual targets with each hand successively, without visual feedback. Similar distance aftereffects were observed for both hands, although only the left hand was used during exposure, indicating that these aftereffects are mediated by visual processes rather than by visuomotor interactions. The distance aftereffects were significantly correlated with prism-induced changes in phoria, demonstrating a relationship between perceived distance and the level of tonic vergence. Changes in perceived distance increased monotonically across trials during prism exposure and remained stable during the post-test, indicating a long time constant for these perceptual effects, consistent with current models of the vergence control system. Overall, these results support the hypothesis that vergence plays a role in reduced-cue distance perception. They further illustrate that variations in tonic vergence influence perceived distance by altering the sensed vergence effort.     

Effect of visuomotor-map uncertainty on visuomotor adaptation

Vision and proprioception contribute to generating hand movement. If a conflict between the visual and proprioceptive feedback of hand position is given, reaching movement is disturbed initially but recovers after training. Although previous studies have predominantly investigated the adaptive change in the motor output, it is unclear whether the contributions of visual and proprioceptive feedback controls to the reaching movement are modified by visuomotor adaptation. To investigate this, we focused on the change in proprioceptive feedback control associated with visuomotor adaptation. After the adaptation to gradually introduce visuomotor rotation, the hand reached the shifted position of the visual target to move the cursor to the visual target correctly. When the cursor feedback was occasionally eliminated (probe trial), the end point of the hand movement was biased in the visual-target direction, while the movement was initiated in the adapted direction, suggesting the incomplete adaptation of proprioceptive feedback control. Moreover, after the learning of uncertain visuomotor rotation, in which the rotation angle was randomly fluctuated on a trial-by-trial basis, the end-point bias in the probe trial increased, but the initial movement direction was not affected, suggesting a reduction in the adaptation level of proprioceptive feedback control. These results suggest that the change in the relative contribution of visual and proprioceptive feedback controls to the reaching movement in response to the visuomotor-map uncertainty is involved in visuomotor adaptation, whereas feedforward control might adapt in a manner different from that of the feedback control.     

Trajectories of reaches to prismatically-displaced targets: evidence for “automatic” visuomotor recalibration

Summary The present study examined the kinematics of unrestricted reaches to prismaticallydisplaced targets. The kinematic analysis allowed us (1) to document how and where in the reach adjustments were made to compensate for the prismatic displacement, (2) to detail the changes that occur in the characteristics of reaches during the course of adaptation to the prisms, and (3) to look at the effects of providing information (or not) to the subject about the presence and nature of the prismatic distortion. The experiment differed from classic studies of prism adaptation in that subjects were permitted full visual feedback of their moving limb at all times, and entire reaching movements were recorded in addition to terminal errors. Experimental subjects were tested either with large-displacement prisms of the sort typically used in such experiments (20 diopters) or with small-displacement prisms (5 diopters) the properties of which went undetected in uninformed subjects. By using small displacements, it was possible to examine the process of visuomotor recalibration directly, free of contamination by conscious correction strategies. There were no differences in the terminal accuracies of the reaches made by subjects in any of the conditions. The availability of visual feedback allowed subjects to place their finger accurately on the target, despite the fact that in some cases their vision was displaced by as much as 11.4° to the right. When the entire reach was examined, however, it was found that the amount of curvature in the path increased when large or small diopter prisms were unexpectedly introduced, with the subjects showing large deviations to the right. This rightward deviation was corrected in the final approach with a larger terminal correction. On some occasions, nonetheless, corrections were observed very early in the course of the reaching movement and appeared to be part of a natural process of trajectory finetuning. Uninformed subjects exposed to either large or small prismatic displacements also showed evidence of adaptation through an increased number of on-line corrections which compensated for a tendency to reach into the side of space opposite to the direction of the displacement (a negative aftereffect in the path of the reach). Moreover, when questioned after the experiment, it became clear that uninformed subjects exposed to small prismatic displacements had apparently failed to detect any visual displacement whatsoever. Taken together, these results suggest that visuomotor recalibration can take place automatically without feedback from terminal errors and without the use of conscious strategies. In fact, making subjects aware of the distortion by providing them with explicit information about the prisms led to reduced levels of adaptation. These informed subjects showed more smoothly generated reaches during prism exposure, while post-exposure reaches showed less evidence of a negative aftereffect. In fact, postexposure reaches of subjects informed of the presence of the 5 diopter prismatic displacement were indistinguishable from reaches of control subjects.     

Visuomotor adaptation and proprioceptive recalibration in older adults

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° 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° 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.     

Dissociation between vergence and binocular disparity cues in the control of prehension

Binocular vision provides important advantages for controlling reach-to-grasp movements. We examined the possible source(s) of these advantages by comparing prehension proficiency under four different binocular viewing conditions, created by randomly placing a neutral lens (control), an eight dioptre prism (Base In or Base Out) or a low-power (2.00–3.75 dioptre) Plus lens over the eye opposite the moving limb. The Base In versus Base Out prisms were intended to selectively alter vergence-specified distance (VSD) information, such that the targets appeared beyond or closer than their actual physical position, respectively. The Plus lens was individually tailored to reduce each subject’s disparity sensitivity (to 400–800 arc s), while minimizing effects on distance estimation. In pre-testing, subjects pointed (without visual feedback) to mid-line targets at different distances, and produced the systematic directional errors expected of uncorrected movements programmed under each of the perturbed conditions. For the prehension tasks, subjects reached and precision grasped (with visual feedback available) cylindrical objects (two sizes and three locations), either following a 3 s preview in which to plan their actions or immediately after the object became visible. Viewing condition markedly affected performance, but the planning time allowed did not. Participants made the most errors suggesting premature collision with the object (shortest ‘braking’ times after peak deceleration; fastest velocity and widest grip at initial contact) under Base In prism viewing, consistent with over-reaching movements programmed to transport the hand beyond the actual target due to its ‘further’ VSD. Conversely, they produced the longest terminal reaches and grip closure times, with multiple corrections just before and after object contact, under the Plus lens (reduced disparity) condition. Base Out prism performance was intermediate between these two, with significant increases in additional forward movements during the transport end-phase, indicative of initial under-reaching in response to the target’s ‘nearer’ VSD. Our findings suggest dissociations between the role of vergence and binocular disparity in natural prehension movements, with vergence contributing mainly to reach planning and high-grade disparity cues providing particular advantages for grasp-point selection during grip programming and application.     

Long-lasting aftereffects of prism adaptation in the monkey

The errors in target-reaching that are produced by laterally displacing vision with wedge prisms decrease with trials (prism adaptation). When the prisms are removed, errors in the opposite direction are observed (aftereffect). We investigated the size of the aftereffect 24 h and 72 h after a monkey had adapted to a visual displacement (30 mm), with rapid reaching movements. The aftereffect more than half of the size of the displacement was observed when the effect was tested immediately after the monkey had been exposed to the displacement for 50 trials. In contrast, the aftereffect was not observed at 24 h even when the monkey had been exposed to the displacement for 250 trials. However, when the monkey had been exposed for 500 trials, significant aftereffects more than half of the size of the displacement were observed at 24 h and 72 h. When both arms were adapted to opposite prism displacements, the long-lasting aftereffect was further shown to be specific for the arm used during the exposure. The results indicate that the aftereffects of prism adaptation last for at least 3 days, though more than 200 trials of additional repetition are required to consolidate the short-term effects into long-lasting ones.     

Proprioceptive recalibration in the right and left hands following abrupt 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.     

The effect of target modality on visual and proprioceptive contributions to the control of movement distance

The goal of this study was to determine whether the sensory nature of a target influences the roles of vision and proprioception in the planning of movement distance. Two groups of subjects made rapid, elbow extension movements, either toward a visual target or toward the index fingertip of the unseen opposite hand. Visual feedback of the reaching index fingertip was only available before movement onset. Using a virtual reality display, we randomly introduced a discrepancy between actual and virtual (cursor) fingertip location. When subjects reached toward the visual target, movement distance varied with changes in visual information about initial hand position. For the proprioceptive target, movement distance varied mostly with changes in proprioceptive information about initial position. The effect of target modality was already present at the time of peak acceleration, indicating that this effect include feedforward processes. Our results suggest that the relative contributions of vision and proprioception to motor planning can change, depending on the modality in which task relevant information is represented.     

Asymmetric generalization between the arm and leg following prism-induced visuomotor adaptation

We have previously shown an asymmetric generalization following a prism-induced visuomotor adaptation. Subjects who adapt to laterally deviating prism lenses during walking show a broad generalization to an arm pointing task, while subjects who adapt to prisms during arm pointing do not show generalization to walking. It is not known whether this broad generalization persists with other movements outside of walking or what specific features of the walking task, e.g. lower extremity involvement, allow it to be so broadly generalizable. In the current study, we tested healthy adult subjects performing one of three forms of prism adaptation and subsequently measured generalization. In Experiment 1 we tested whether a seated arm pointing prism adaptation would generalize to the leg. In Experiment 2 we tested whether a seated leg pointing prism adaptation would generalize to the arm. In Experiment 3 we tested whether standing influenced the extent of generalization from leg to arm. Results were surprising. We found a clear and consistent generalization from arm to leg, but much less so from leg to arm during either the seated or the standing task. These findings indicate that prism adaptations during arm movements are not limb-specific, as has been previously suggested. Further, the lack of generalization from leg to arm suggests that neither the adaptation of leg movements specifically, nor standing posture, nor the bilateral component of walking could be the salient feature allowing for its broad generalization across body parts.     

Visuomotor adaptation and intermanual transfer under different viewing conditions

Does the brain use a separate internal model for cursor mechanics during visuomotor adaptation? We compared the amount of adaptation and transfer to the opposite arm when subjects reached the targets under different viewing conditions of the arm during reaching. If the brain forms separate models, we predict a difference in the amount of adaptation and transfer for each viewing condition. If the brain forms one model, we predict equivalent amounts of adaptation and transfer between the two hands for each viewing condition. Separate groups of subjects performed a reaching task with either a rotated view of cursor motion representing their unseen hand or a rotated view of their actual hand. The two groups were further divided so that the magnitude of the rotation was either 45° or 75° counter-clockwise. After adapting to the rotation with one hand, subjects reached the same targets under the same viewing condition but with the opposite hand. Similar amounts of adaptation and intermanual transfer were found across the different magnitudes of rotation and across patterns of hand-order. Our results suggest that the brain may not be learning a distinct model for cursor mechanics, or if it is, it must be equivalent or overlapping with the arm model.     

Absence of after-effects for observers after watching a visuomotor adaptation

We tested whether observational practice would elicit after-effects in a normal environment following observation of an actor performing in a perturbed visuomotor environment. Two actor groups (with and without vision of the hand) practised reaching to visual targets with the cursor rotated 30° to the actual hand movement. An observer group viewed this adaptation. Observers demonstrated significant learning when they subsequently performed the aiming task in the perturbed environment. However, different from both actor groups, observers did not show after-effects in the normal visuomotor condition. Our findings imply that there is a qualitative difference in the processes between observational and physical practice and suggest that physical exposure is required to update an internal model of the visuomotor environment.     

Differential contributions of vision and proprioception to movement accuracy

We examined the relative roles of visual and proprioceptive information about initial hand position on movement accuracy. A virtual reality environment was employed to dissociate visual information about hand position from the actual hand position. Previous studies examining the effects of such dissociations on perception of hand location have indicated a bias toward the visually displayed position. However, an earlier study, which employed optical prisms to dissociate visual and proprioceptive information prior to targeted movements, suggested a bias in movement direction toward that defined by the actual hand position. This implies that visual and proprioceptive information about hand position may be differentially employed for perceptual judgments and movement planning, respectively. We now employ a virtual reality environment to systematically manipulate the visual display of the hand start position from the actual hand position during movements made to a variety of directions. We asked whether subjects would adjust their movements in accord with the virtual or the actual hand location. Subjects performed a series of baseline movements toward one of three targets in each of three blocks of trials. Interspersed among these trials were "probe" trials in which the cursor location, but not the hand location, was displaced relative to the baseline start position. In all cases, cursor feedback was blanked at movement onset. Our findings indicated that subjects systematically adjusted the direction of movement in accord with the virtual, not the actual, start location of the hand. These findings support the hypothesis that visual information about hand position predominates in specifying movement direction.     

A motor signal and “visual” size perception

Recent models of the visual system in primates suggest that the mechanisms underlying visual perception and visuomotor control are implemented in separate functional streams in the cerebral cortex. However, a little-studied perceptual illusion demonstrates that a motor-related signal representing arm position can contribute to the visual perception of size. The illusion consists of an illusory size change in an afterimage of the hand when the hand is moved towards or away from the subject. The motor signal necessary for the illusion could be specified by feedforward and/or feedback sources (i.e. efference copy and/or proprioception/kinesthesis). We investigated the nature of this signal by measuring the illusion's magnitude when subjects moved their own arm (active condition, feedforward and feedback information available), and when arm movement was under the control of the experimenter (passive condition, feedback information available). Active and passive movements produced equivalent illusory size changes in the afterimages. However, the illusion was not obtained when an afterimage of subject's hand was obtained prior to movement of the other hand from a very similar location in space. This evidence shows that proprioceptive/kinesthetic feedback was sufficient to drive the illusion and suggests that a specific three-dimensional registration of proprioceptive input and the initial afterimage is necessary for the illusion to occur.     

Separate adaptive mechanisms for controlling trajectory and final position in reaching

We examined control of the hand's trajectory (direction and shape) and final equilibrium position in horizontal planar arm movements by quantifying transfer of learned visuomotor rotations between two tasks that required aiming the hand to the same spatial targets. In a trajectory-reversal task ("slicing"), the hand reversed direction within the target and returned to the origin. In a positioning task ("reaching"), subjects moved the hand to the target and held it there; cursor feedback was provided only after movement ended to isolate learning of final position from trajectory direction. We asked whether learning acquired in one task would transfer to the other. Transfer would suggest that the hand's entire trajectory, including its endpoint, was controlled using a common spatial plan. Instead we found minimal transfer, suggesting that the brain used different representations of target position to specify the hand's initial trajectory and its final stabilized position. We also observed asymmetrical practice effects on hand trajectory, including systematic curvature of reaches made after rotation training and hypermetria of untrained slice reversals after reach training. These are difficult to explain with a unified control model, but were replicated in computer simulations that specified the hand's initial trajectory and its final equilibrium position. Our results suggest that the brain uses different mechanisms to plan the hand's initial trajectory and final position in point-to-point movements, that it implements these control actions sequentially, and that trajectory planning does not account for specific impedance values to be implemented about the final stabilized posture.     

Task-specific sensorimotor adaptation to reversing prisms

We tested between three levels of visuospatial adaptation (global map, parallel feature modules, and parallel sensorimotor transformations) by training subjects to reach and grasp virtual objects viewed through a left-right reversing prism, with either visual location or orientation feedback. Even though spatial information about the global left-right reversal was present in every training session, subjects trained with location feedback reached to the correct location but with the wrong (reversed) grasp orientation. Subjects trained with orientation feedback showed the opposite pattern. These errors were task-specific and not feature-specific; subjects trained to correctly grasp visually reversed-oriented bars failed to show knowledge of the reversal when asked to point to the end locations of these bars. These results show that adaptation to visuospatial distortion--even global reversals--is implemented through learning rules that operate on parallel sensorimotor transformations (e.g., reach vs. grasp).     

Cerebellar lesions and prism adaptation in macaque monkeys

If a laterally displacing prism is placed in front of one eye of a person or monkey with the other eye occluded, they initially will point to one side of a target that is located directly in front of them. Normally, people and monkeys adapt easily to the displaced vision and correct their aim after a few trials. If the prism then is removed, there is a postadaptation shift in which the subject misses the target and points in the opposite direction for a few trials. We tested five Macaque monkeys for their ability to adapt to a laterally displacing prism and to show the expected postadaptation shift. When tested as normals, all five animals showed the typical pattern of adaptation and postadaptation shift. Like human subjects, the monkeys also showed complete interocular transfer of the adaptation but no transfer of the adaptation between the two arms. When preoperative training and testing was complete, we made lesions of various target areas on the cerebellar cortex. A cerebellar lesion that included the dorsal paraflocculus and uvula abolished completely the normal prism adaptation for the arm ipsilateral to the lesion in one of the five monkeys. The other four animals retained the ability to prism-adapt normally and showed the expected postadaptation shift. In the one case in which the lesion abolished prism adaptation, the damage included Crus I and II, paramedian lobule and the dorsal paraflocculus of the cerebellar hemispheres as well as lobule IX, of the vermis. Thus in this case, the lesion included virtually all the cerebellar cortex that receives mossy-fiber visual information relayed via the pontine nuclei from the cerebral cortex. The other four animals had damage to lobule V, the classical anterior lobe arm area and/or vermian lobules VI/VII, the oculomotor region. When tested postoperatively, some of these animals showed a degree of ataxia equivalent to that of the case in which prism adaptation was affected, but prism adaptation and the postadaptation shift remained normal. We conclude that in addition to its role in long-term motor learning and reflex adaptation, the region of the cerebellum that was ablated also may be a critical site for a short-term motor memory. Prism adaptation seems to involve a region of the cerebellum that receives a mossy-fiber visual error signal and probably a corollary discharge of the movement.     

The role of proprioception and attention in a visuomotor adaptation task

The role of proprioception in the control and adaptation of visuomotor relationships is still unclear. We have studied a deafferented subject, IW, and control subjects in a task in which they used single joint elbow extension to move to a visual target, with visual feedback of the terminal position provided by a cursor displayed in the plane of their movements. We report the differences in movement accuracy between the deafferented subject and controls in the normal task and when challenged with a cognitive load, counting backwards. All subjects were less accurate when counting; this was a small effect for the controls (<10% change) but much greater for the deafferented subject (>60% change). We also examined changes in movement kinematics when the instructed amplitude was altered via a changed gain between final arm position and presentation of the feedback cursor. The deafferented subject maintained temporal movement parameters stable and altered amplitude by scaling force (i.e. changed peak velocity), whereas the controls scaled both movement velocity and duration. Finally, we compared the subjects' adaptation of movement amplitude after a period of exposure to the changed visuomotor gain. The deafferented subject was able to adapt, but his adaptation was severely impaired by the counting task. These results suggest that proprioception is not an absolute requirement for adaptation to occur. Instead, proprioception has a more subtle role to play in the adjustment to visuomotor perturbations. It has an important role in the control of reaching movements, while in the absence of proprioception, attention appears necessary to monitor movements.