Allocation of attention for dissociated visual and motor goals

In daily life, selecting an object visually is closely intertwined with processing that object as a potential goal for action. Since visual and motor goals are typically identical, it remains unknown whether attention is primarily allocated to a visual target, a motor goal, or both. Here, we dissociated visual and motor goals using a visuomotor adaptation paradigm, in which participants reached toward a visual target using a computer mouse or a stylus pen, while the direction of the cursor was rotated 45° counter-clockwise from the direction of the hand movement. Thus, as visuomotor adaptation was accomplished, the visual target was dissociated from the movement goal. Then, we measured the locus of attention using an attention-demanding rapid serial visual presentation (RSVP) task, in which participants detected a pre-defined visual stimulus among the successive visual stimuli presented on either the visual target, the motor goal, or a neutral control location. We demonstrated that before visuomotor adaptation, participants performed better when the RSVP stream was presented at the visual target than at other locations. However, once visual and motor goals were dissociated following visuomotor adaptation, performance at the visual and motor goals was equated and better than performance at the control location. Therefore, we concluded that attentional resources are allocated both to visual target and motor goals during goal-directed reaching movements.     

Visuomotor adaptation and generalization with repeated and varied training

Many studies have shown that reaching movements to visual targets can rapidly adapt to altered visual feedback of hand motion (i.e., visuomotor rotation) and generalize to new target directions. This generalization is thought to reflect the acquisition of a neural representation of the novel visuomotor environment that is localized to the particular trained direction. In these studies, participants perform movements to a small number of target locations repeatedly. However, it is unclear whether adaptation and generalization are comparable when target locations are constantly varied and participants reach to visual targets one time only. Here, we compared performance for reaches to a 30° counter-clockwise visuomotor rotation to four targets, spaced 90° apart across four areas of workspace 18 times each (repeated practice (RP)) with one time only reaching movements to 72 targets, spaced 5° apart (varied practice (VP)). For both training groups, participants performed 18 reaches to radial targets (either at the repeated or varied location) in a specific area of the workspace (i.e., one of four quadrants) before reaching in the adjacent workspace. We found that the RP group adapted more completely compared to the VP group. Conversely, the VP group generalized to new target directions more completely when reaching without cursor feedback compared to the RP group. This suggests that RP and VP follow a mainly common pattern of adaptation and generalization represented in the brain, with benefits of faster adaptation with RP and more complete generalization with VP.     

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.     

Cross talk in implicit assignment of error information during bimanual visuomotor learning

When a neural movement controller, called an "internal model," is adapted to a novel environment, the movement error needs to be appropriately associated with the controller. However, their association is not necessarily guaranteed for bimanual movements in which two controllers--one for each hand--result in two movement errors. Considering the implicit nature of the adaptation process, the movement error of one hand can be erroneously associated with the controller of the other hand. Here, we investigated this credit-assignment problem in bimanual movement by having participants perform bimanual, symmetric back-and-forth movements while displaying the position of the right hand only with a cursor. In the training session, the cursor position was gradually rotated clockwise, such that the participants were unaware of the rotation. The movement of the right hand gradually rotated counterclockwise as a consequence of adaptation. Although the participants knew that the cursor reflected the movement of the right hand, such gradual adaptation was also observed for the invisible left hand, especially when the cursor was presented on the left side of the display. Thus the movement error of the right hand was implicitly assigned to the left-hand controller. Such cross talk in credit assignment might influence motor adaptation performance, even when two cursors are presented; the adaptation was impaired when the rotations imposed on the cursors were opposite compared with when they were in the same direction. These results indicate the inherent presence of cross talk in the process of associating action with consequence in bimanual movement.     

Effects of Parkinson's disease on visuomotor adaptation

Visuomotor adaptation to a kinematic distortion was investigated in Parkinson's disease (PD) patients and age-matched controls. Participants performed pointing movements in which the visual feedback of hand movement, displayed as a screen cursor, was normal (pre-exposure condition) or rotated by 90° counterclockwise (exposure condition). Aftereffects were assessed in a post-exposure condition in which the visual feedback of hand movement was set back to normal. In pre- and early-exposure trials, both groups showed similar initial directional error (IDE) and movement straightness (RMSE, root mean square error), but the PD group showed reduced movement smoothness (normalized jerk, NJ) and primary submovement to total movement distance ratios (PTR). During late-exposure the PD subjects, compared with controls, showed larger IDE, RMSE, NJ, and smaller PTR scores. Moreover, PD patients showed smaller aftereffects than the controls during the post-exposure condition. Overall, the PD group showed both slower and reduced adaptation compared with the control group. These results are discussed in terms of reduced signal-to-noise ratio in feedback signals related to increased movement variability and/or disordered kinesthesia, deficits in movement initiation, impaired selection of initial movement direction, and deficits in internal model formation in PD patients. We conclude that Parkinson's disease impairs visuomotor adaptation. Electronic Publication     

The interference effects of non-rotated versus counter-rotated trials in visuomotor adaptation

An isometric torque-production task was used to investigate interference and retention in adaptation to multiple visuomotor environments. Subjects produced isometric flexion–extension and pronation–supination elbow torques to move a cursor to acquire targets as quickly as possible. Adaptation to a 30° counter-clockwise (CCW) rotation (task A), was followed by a period of rest (control), trials with no rotation (task B0), or trials with a 60° clockwise (CW) rotation (task B60). For all groups, retention of task A was assessed 5 h later. With initial training, all groups reduced the angular deviation of cursor paths early in the movements, indicating feedforward adaptation. For the control group, performance at commencement of the retest was significantly better than that at the beginning of the initial learning. For the B0 group, performance in the retest of task A was not dissimilar to that at the start of the initial learning, while for the B60 group retest performance in task A was markedly worse than initially observed. Our results indicate that close juxtaposition of two visuomotor environments precludes improved retest performance in the initial environment. Data for the B60 group, specifically larger angular errors upon retest compared with initial exposures, are consistent with the presence of anterograde interference. Furthermore, full interference occurred even when the visuomotor environment encountered in the second task was not rotated (B0). This latter novel result differs from those obtained for force field learning, where interference does not occur when task B does not impose perturbing forces, i.e., when B consists of a null field (Brashers-Krug et al., Nature 382:252–255, 1996). The results are consistent with recent proposals suggesting different interference mechanisms for visuomotor (kinematic) compared to force field (dynamic) adaptations, and have implications for the use of washout trials when studying interference between multiple visuomotor environments.     

On-line corrections for visuomotor errors

This study was designed to determine how visual feedback mediates error corrections during reaching. We used visuomotor rotations to dissociate a cursor, representing finger position, from the actual finger location. We then extinguished cursor feedback at different distances from the start location to determine whether corrections were based on error extrapolation from prior cursor information. Results indicated that correction amplitude varied with the extent of cursor feedback. A second experiment tested specific aspects of error information that might mediate corrections to visuomotor rotations: rotation angle, distance between the finger and cursor positions and the duration of cursor exposure. Results showed that corrections did not depend on the amplitude of the rotation angle or the amount of time the cursor was shown. Instead, participants corrected for the cursor–finger distance, at the point where cursor feedback was last-seen. These findings suggest that within-trial corrections and inter-trial adaptation might employ different mechanisms.     

Online and post-trial feedback differentially affect implicit adaptation to a visuomotor rotation

Multiple motor learning processes can be discriminated in visuomotor rotation paradigms. At least four processes have been proposed: Implicit adaptation updates an internal model based on prediction errors. Model-free reinforcement reinforces actions that achieve task success. Use-dependent learning favors repetition of prior movements, and strategic learning uses explicit knowledge about the task. The current experiment tested whether the processes involved in motor learning differ when visual feedback is altered. Specifically, we hypothesized that online and post-trial feedback would cause different amounts of implicit adaptation. Twenty subjects performed drawing movements to targets under a 45° counterclockwise visuomotor rotation while aiming at a clockwise adjacent target. Subjects received visual feedback via a cursor on a screen. One group saw the cursor throughout the movement (online feedback), while the other only saw the final position after movement execution (post-trial feedback). Both groups initially hit the target by applying the strategy. After 80 trials, subjects with online feedback had drifted in clockwise direction [mean direction error: 15.1° (SD 11.2°)], thus overcompensating the rotation. Subjects with post-trial feedback remained accurate [mean: 0.7° (SD 2.0°), TIME × GROUP: F = 3.926, p = 0.003]. We interpret this overcompensation to reflect implicit adaptation isolated from other mechanisms, because it is driven by prediction error rather than task success (model-free reinforcement) or repetition (use-dependent learning). The current findings extend previous work (e.g., Mazzoni and Krakauer in J Neurosci 26:3642–3645, 2006; Hinder et al. in Exp Brain Res 201:191–207, 2010) and suggest that online feedback promotes more implicit adaptation than does post-trial feedback.     

Sensitivity to prediction error in reach adaptation

It has been proposed that the brain predicts the sensory consequences of a movement and compares it to the actual sensory feedback. When the two differ, an error signal is formed, driving adaptation. How does an error in one trial alter performance in the subsequent trial? Here we show that the sensitivity to error is not constant but declines as a function of error magnitude. That is, one learns relatively less from large errors compared with small errors. We performed an experiment in which humans made reaching movements and randomly experienced an error in both their visual and proprioceptive feedback. Proprioceptive errors were created with force fields, and visual errors were formed by perturbing the cursor trajectory to create a visual error that was smaller, the same size, or larger than the proprioceptive error. We measured single-trial adaptation and calculated sensitivity to error, i.e., the ratio of the trial-to-trial change in motor commands to error size. We found that for both sensory modalities sensitivity decreased with increasing error size. A reanalysis of a number of previously published psychophysical results also exhibited this feature. Finally, we asked how the brain might encode sensitivity to error. We reanalyzed previously published probabilities of cerebellar complex spikes (CSs) and found that this probability declined with increasing error size. From this we posit that a CS may be representative of the sensitivity to error, and not error itself, a hypothesis that may explain conflicting reports about CSs and their relationship to error.     

The effect of visuomotor adaptation on proprioceptive localization: the contributions of perceptual and motor changes

Reaching movements are rapidly adapted following training with rotated visual feedback of the hand (motor recalibration). Our laboratory has also found that visuomotor adaptation results in changes in estimates of felt hand position (proprioceptive recalibration) in the direction of the visuomotor distortion (Cressman and Henriques 2009, 2010; Cressman et al. 2010). In the present study, we included an additional method for measuring hand proprioception [specifically, proprioceptive-guided reaches of the unadapted (left) hand to the robot-guided adapted (right) hand-target] and compared this with our original perceptual task (estimating the felt hand position of the adapted hand relative to visual reference markers/the body midline), as well as to no-cursor reaches with the adapted hand (reaching to visual and midline-targets), to better identify whether changes in reaching following adaptation to a 50° rightward-rotated cursor reflect sensory or motor processes. Results for the proprioceptive estimation task were consistent with previous findings; subjects felt their hand to be aligned with a reference marker when it was shifted approximately 4° more in the direction of the visuomotor distortion following adaptation compared with baseline conditions. Moreover, we found similar changes in the proprioceptive-guided reaching task such that subjects misreached 5° in the direction of the cursor rotation. However, these results were true only for proprioceptive-guided reaches to the adapted hand, as reaches to the body midline were not affected by adaptation. This suggests that proprioceptive recalibration is restricted to the adapted hand and does not generalize to the rest of the body; this truly reflects a change in the sensory representation of the hand rather than changes in the motor program. This is in contrast to no-cursor reaches made with the adapted hand, which show reach after-effects for both visual targets and the midline, suggesting that reaches with the adapted hand reflect more of a change in the motor system. Our results also shed light on previous studies that may have misattributed these sensory and motor changes.     

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 the lack of visuomotor feedback affects even the early stages of goal-directed pointing movements

Pointing movements made with a hidden cursor from the center of gaze to a stationary, visible target overshot the actual target location. The systematic error decreased when the final cursor location from the previous trial was shown, which likely led to the creation of an internal sensorimotor model of movement. However, the putative model had a short memory, and could not substitute for on-line visuomotor feedback on subsequent trials. Contrary to common belief, the effect of a lack of visuomotor feedback was seen even in the early acceleration stage of the movement trajectory. Unchecked in the absence of visual monitoring, the acceleration stage of the movement lasted longer, as was evidenced by the significantly larger value of the peak cursor speed. Moreover, the speed peaked much later in the course of the movement. Speed declined more rapidly thereafter. Consequently, the delayed deceleration stage lasted far less than the acceleration stage. In the absence of visual feedback, the shift rightward in time of the peak speed position (PSP) in relation to total movement duration and other changes in the trajectory imply that visual feedback must play a significant role in determining when acceleration ceases (d V/d t=0), and argue against the traditional notion that visuomotor feedback is unavailable until the later stages of movement. Moreover, our data suggest that non-visual modalities, e.g., proprioception, may be too slow to make up for the absence of vision.     

The efficacy of colour cues in facilitating adaptation to opposing visuomotor rotations

We investigated visuomotor adaptation using an isometric, target-acquisition task. Following trials with no rotation, two participant groups were exposed to a random sequence of 30° clockwise (CW) and 60° counter-clockwise (CCW) rotations, with (DUAL-CUE), or without (DUAL-NO CUE), colour cues that enabled each environment (non-rotated, 30° CW and 60° CCW) to be identified. A further three groups experienced only 30° CW trials or only 60° CCW trials (SINGLE rotation groups) in which each visuomotor mapping was again associated with a colour cue. During training, all SINGLE groups reduced angular deviations of the cursor path during the initial portion of the movements, indicating feedforward adaptation. Consistent with the view that the adaptation occurred automatically via recalibration of the visuomotor mapping (Krakauer et al. 1999), post-training aftereffects were observed, despite colour cues that indicated that no rotation was present. For the DUAL-CUE group, angular deviations decreased with training in the 60° trials, but were unchanged in the 30° trials, while for the DUAL-NO CUE group angular deviations decreased for the 60° CW trials but increased for the 30° CW trials. These results suggest that in a dual adaptation paradigm a colour cue can permit delineation of the two environments, with a subsequent change in behaviour resulting in improved performance in at least one of these environments. Increased reaction times within the training block, together with the absence of aftereffects in the post-training period for the DUAL-CUE group suggest an explicit cue-dependent strategy was used in an attempt to compensate for the rotations.     

Learning a visuomotor rotation: simultaneous visual and proprioceptive information is crucial for visuomotor remapping

Visuomotor adaptation is mediated by errors between intended and sensory-detected arm positions. However, it is not clear whether visual-based errors that are shown during the course of motion lead to qualitatively different or more efficient adaptation than errors shown after movement. For instance, continuous visual feedback mediates online error corrections, which may facilitate or inhibit the adaptation process. We addressed this question by manipulating the timing of visual error information and task instructions during a visuomotor adaptation task. Subjects were exposed to a visuomotor rotation, during which they received continuous visual feedback (CF) of hand position with instructions to correct or not correct online errors, or knowledge-of-results (KR), provided as a static hand-path at the end of each trial. Our results showed that all groups improved performance with practice, and that online error corrections were inconsequential to the adaptation process. However, in contrast to the CF groups, the KR group showed relatively small reductions in mean error with practice, increased inter-trial variability during rotation exposure, and more limited generalization across target distances and workspace. Further, although the KR group showed improved performance with practice, after-effects were minimal when the rotation was removed. These findings suggest that simultaneous visual and proprioceptive information is critical in altering neural representations of visuomotor maps, although delayed error information may elicit compensatory strategies to offset perturbations.     

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.     

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.     

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.     

The effects of aging on the asymmetry of inter-limb transfer in a visuomotor task

The direction of the asymmetry of inter-limb transfer has been suggested to identify the specialization of each hemisphere when performing a motor task. In an earlier study, we showed that trajectory information is only transferred from the right to the left hand, while final movement outcome-associated parameters transferred in both directions when right-hand-dominant individuals perform a motor task with visual distorted feedback. In the current study, we try to replicate this finding in young adults and test whether the asymmetry of inter-limb transfer in visuomotor task reduces in older adults, suggesting that hemispheric lateralization reduces with age. Young and older adults (all right-hand-dominant) performed a multidirectional point-to-point drawing task in which the visual feedback was rotated and the gain was increased. Half of the participants in each age group trained with the right hand and the other half trained with the left hand. Performances of both hands with non-distorted and distorted visual feedback were collected from all participants before and after the training session. The results showed that the pattern of inter-limb transfer was similar between young and older adults, i.e., inter-limb transfer is asymmetric for initial direction and symmetric for movement time and trajectory length. The results suggest that older adults retain the specialized functions of the non-dominant (right) hemisphere allowing them to program movement direction of a graphic aiming task when visual feedback is distorted.     

Beside the point: motor adaptation without feedback-based error correction in task-irrelevant conditions

Adaptation of movement may be driven by the difference between planned and actual motor performance, or the difference between expected and actual sensory consequences of movement. To identify how the nervous system differentially uses these signals, we asked: does motor adaptation occur when movement errors are irrelevant to the task goal? Participants reached on a digitizing tablet from a fixed start location to one of three targets: a point, an arc, or a ray. For the arc, reaches could be in any direction, but to a specific extent. For the ray, reaches could be to any distance, but in a targeted direction. After baseline reaching to the point, the direction or extent of continuous visual feedback was perturbed during training with either a cursor rotation or gain, respectively, while reaching to either the ray (goal = direction) or the arc (goal = extent). The perturbation, therefore, was either relevant or irrelevant to the task goal, depending on target type. During interspersed catch trials, the perturbation was removed and the target switched back to the point, identical to baseline. Although the goal of baseline and catch trials was the same, significant aftereffects in catch trials indicated behavioral adaptation in response to the perturbation. Adaptation occurred regardless of whether the perturbation was relevant to the task, and it was independent of feedback control. The presence of adaptation orthogonal to task demands supports the hypothesis that the nervous system can rely on sensory prediction to drive motor learning that can generalize across tasks.     

Preferred directions of arm movements are independent of visual perception of spatial directions

Directional preferences have previously been demonstrated during horizontal arm movements. These preferences were characterized by a tendency to exploit interaction torques for movement production at the shoulder or elbow, indicating that the preferred directions depend on biomechanical, and not on visual perception-based factors. We directly tested this hypothesis by systematically dissociating visual information from arm biomechanics. Sixteen subjects performed a free-stroke drawing task that required performance of fast strokes from the circle center toward the perimeter, while selecting stroke directions in a random order. Hand position was represented by a cursor displayed in the movement plane. The free-stroke drawing was performed twice, before and after visuomotor adaptation to a 30° clockwise rotation of the perceived hand path. The adaptation was achieved during practicing pointing movements to eight center-out targets. Directional preferences during performance of the free-stroke drawing task were revealed in ten out of the sixteen subjects. The orientation and strength of these preferences were largely the same in both conditions, showing no significant effect of the visuomotor adaptation. In both conditions, the major preferred directions were characterized by higher contribution of interaction torque to net torque at the shoulder as well as by relatively low inertial resistance and the sum of squared shoulder and elbow muscle torques. These results support the hypothesis that directional preferences are largely determined by biomechanical factors. However, this biomechanical effect can decrease or even disappear in some subjects when movements are performed in special conditions, such as the virtual environment used here.