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.     

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.     

Vergence-dependent adaptation of the vestibulo-ocular reflex

The gain of the vestibulo-ocular reflex (VOR) normally depends on the distance between the subject and the visual target, but it remains uncertain whether vergence angle can be linked to changes in VOR gain through a process of context-dependent adaptation. In this study, we examined this question with an adaptation paradigm that modified the normal relationship between vergence angle and retinal image motion. Subjects were rotated sinusoidally while they viewed an optokinetic (OKN) stimulus through either diverging or converging prisms. In three subjects the diverging prisms were worn while the OKN stimulus moved out of phase with the head, and the converging prisms were worn when the OKN stimulus moved in-phase with the head. The relationship between the vergence angle and OKN stimulus was reversed in the fourth subject. After 2 h of training, the VOR gain at the two vergence angles changed significantly in all of the subjects, evidenced by the two different VOR gains that could be immediately accessed by switching between the diverged and converged conditions. The results demonstrate that subjects can learn to use vergence angle as the contextual cue that retrieves adaptive changes in the angular VOR.     

Selective use of perceptual recalibration versus visuomotor skill acquisition

Exposure to laterally displacing prisms is characterized by systematic misreaching in the opposite direction after prisms are removed. Other learning tasks involving altered visuomotor mappings can often be mastered by the subject with minimal resulting aftereffects. One variable that may account for this difference is the nature of the feedback provided to the subject: during studies of prism exposure, subjects usually view the hand itself, whereas in many studies of visuomotor learning, subjects view a computer-generated representation of the hand position or movement. We compared the use of actual feedback of the hand with computer-generated representational feedback of its position during exposure to laterally displacing prisms. In the actual feedback condition (ACT), a light on the fingertip was illuminated immediately at the end of each reach. In the representational feedback condition (REP), a computer-generated spot of light was displayed to indicate the exact position of the fingertip at the end of each reach. Whereas the rate and magnitude of error correction were the same in both conditions, only the ACT condition produced the large adaptive aftereffect typically observed after prism exposure. These results suggest that the perception of a physical coincidence between the feedback source and the hand may be a key factor in determining whether adaptation is accomplished through perceptual recalibration or visuomotor skill acquisition.     

Lifespan changes in motor activation and inhibition during choice reactions: a Laplacian ERP study

Response speed improves from childhood to early adulthood and declines steadily with advancing age. The present event-related brain potential (ERP) study explored the contribution of the primary motor cortex (M1) to lifespan changes in response speed and accuracy using a choice reaction time (RT) task. Two groups of children (8 and 12 years) and two groups of adults (21 and 76 years) responded to left- or right-pointing arrows. RTs showed a typical U-shaped lifespan pattern. RT was segmented into pre-selection time, pre-motor time, and motor time by using the onset of the central motor command (i.e., LRP, and the negative Laplacian potential) and the onset of response-related EMG. Pre-motor time was most sensitive to age-related change. In addition, the positive Laplacian potential, assumed to be associated with inhibition of the incorrect response alternative, was absent in children. In adults, the onset of the ipsilateral positivity started before the onset of the contralateral negativity but in elderly the onsets occurred approximately at the same time. This pattern of findings is consistent with the observed differences in choice error rates between age groups. Taken together, the lifespan changes in motor potentials point to suboptimal motor response control in children and the elderly compared to young adults.     

Spatially precise bilateral arm movements are controlled by the contralateral hemisphere

Previous studies have reported that unilateral proximal arm movements are initiated more quickly in response to visual stimuli directed to the ispilateral hemifield than to the contralateral hemifield. This is thought to reflect differences in intrahemispheric and interhemispheric visuomotor integration. When bilateral movements are performed, this difference in reaction time (RT) is abolished owing to the involvement of bilaterally distributed motor pathways. However, these experiments typically use simple motor tasks that do not emphasise spatial precision. We investigated the hemispheric control of precise unilateral and bilateral arm movements in 12 subjects using a lateralized visual stimulus paradigm and found an ipsilateral RT advantage for both unilateral and bilateral movements. We conclude that the requirement to execute spatially precise movements restricts control to the contralateral hemisphere regardless of whether unilateral or bilateral movements are performed.     

Neural correlates of fast stimulus discrimination and response selection in top-level fencers

Flexible adaptation of behaviour is highly required in some sports, such as fencing. In particular, stimulus discrimination and motor response selection and inhibition processes are crucial. We investigated the neural mechanisms responsible for fencers' fast and flexible behaviour recording event-related potentials (ERPs) in discriminative reaction task (DRT, Go/No-go task) and simple reaction task (SRT) to visual stimuli. In the DRT, in addition to faster RTs measured in fencers with respect to control subjects, three main electrophysiological differences were found. First, attentional modulation of the visual processing taking place in the occipital lobes and reaching a peak at 170 ms was enhanced in the athletes group. Second, the activity in the posterior cingulate gyrus, associated with the stimulus discrimination stage, started earlier in fencers than controls (150 ms versus 200 ms) and the peak had larger amplitude. Third, the activity at the level of the prefrontal cortex (time range: 250-350 ms), associated with response selection stage and particularly with motor inhibition process, was stronger in fencers. No differences between athletes and controls were found in the SRT for both ERPs and RTs. Concluding, the fencers' ability to cope to the opponent feint switching quickly from an intended action to a new more appropriate action is likely due to a faster stimulus discrimination facilitated by higher attention and by stronger inhibition activity in prefrontal cortex.     

Sensory prediction errors drive cerebellum-dependent adaptation of reaching

The cerebellum is an essential part of the neural network involved in adapting goal-directed arm movements. This adaptation might rely on two distinct signals: a sensory prediction error or a motor correction. Sensory prediction errors occur when an initial motor command is generated but the predicted sensory consequences do not match the observed values. In some tasks, these sensory errors are monitored and result in on-line corrective motor output as the movement progresses. Here we asked whether cerebellum-dependent adaptation of reaching relies on sensory or on-line motor corrections. Healthy controls and people with hereditary cerebellar ataxia reached during a visuomotor perturbation in two conditions: "shooting" movements without on-line corrections and "pointing" movements that allowed for on-line corrections. Sensory (i.e., visual) errors were available in both conditions. Results showed that the addition of motor corrections did not influence adaptation in control subjects, suggesting that only sensory errors were needed for learning. Cerebellar subjects were comparably impaired in both adaptation conditions relative to controls, despite abnormal and inconsistent on-line motor correction. Specifically, poor on-line motor corrections were unrelated to cerebellar subjects' adaptation deficit (i.e., adaptation did not worsen), further suggesting that only sensory prediction errors influence this process. Therefore adaptation to visuomotor perturbations depends on the cerebellum and is driven by the mismatch between predicted and actual sensory outcome of motor commands.     

Motor response thresholds to electrical stimulation at the wrist in human newborns

Two studies of motor response thresholds (RTs) to electricalstimulation at the wrist in newborns were done. Adult controls were employed. Strength-duration curves for infant and adult RTs and adult sensory thresholds (STs) were plotted. RTs of infants are approximately 1.75 times higher than those for adults. Adult RTs are consistently 3 to 4 times higher than STs. Test-retest reliabilities were satisfactory. RTs of half of the newborns were notably variable. No relationship between RT variability and stages of the neonatal wakefulness-sleep cycle could be established, but RTs were significantly, although not markedly, higher during rapid-eye-movement (REM) episodes than during nREM. For evoked potential studies with newborns it has been decided to use as a stimulus a constant-current square-wave electrical pulse of 0.5 msec duration at RT during nREM sleep.     

Aging reduces asymmetries in interlimb transfer of visuomotor adaptation

Hemispheric asymmetry reduction in older adults (HAROLD) has been reported in previous imaging studies that employed not only cognitive, but also motor tasks. However, whether age-related reductions in asymmetry of hemispheric activations affect the symmetry of motor behavior in older adults remains largely untested. We now examine the effect of aging on lateralization of motor adaptation and transfer by investigating adaptation to novel visuomotor transformations in both old and young age groups. We have previously reported substantial asymmetries in interlimb transfer of learning these transformations in young adults, and attributed these asymmetries in transfer to hemispheric lateralization for motor control, as detailed by our dynamic dominance hypothesis. Based on the HAROLD model, we reasoned that older adults should recruit more symmetrical hemispheric activity, and thus show more symmetrical transfer of adaptation across the arms. Half of the subjects in each age group first adapted to a rotated visual display with the left arm, then with the right arm; and the other half in the reversed order. Naïve performance with one arm and the same-arm performance following opposite arm adaptation were compared to determine the extent of transfer in each age group. Our results showed that interlimb transfer of initial direction information only occurred from the nondominant to dominant arm in young adults, whereas it occurred in both directions in older adults. Our findings clearly indicate substantially reduced asymmetry in visuomotor adaptation in older adults, and suggest that this reduced motor asymmetry might be related to diminished hemispheric lateralization for motor control.     

Evidence for effector independent and dependent representations and their differential time course of acquisition during motor sequence learning

To investigate the representation of motor sequence, we tested transfer effects in a motor sequence learning paradigm. We hypothesize that there are two sequence representations, effector independent and dependent. Further, we postulate that the effector independent representation is in visual/spatial coordinates, that the effector dependent representation is in motor coordinates, and that their time courses of acquisition during learning are different. Twelve subjects were tested in a modified 2x10 task. Subjects learned to press two keys (called a set) successively on a keypad in response to two lighted squares on a 3x3 display. The complete sequence to be learned was composed of ten such sets, called a hyperset. Training was given in the normal condition and sequence recall was assessed in the early, intermediate, and late stages in three conditions, normal, visual, and motor. In the visual condition, finger-keypad mapping was rotated 90 degrees while the keypad-display mapping was kept identical to normal. In the motor condition, the keypad-display mapping was also rotated 90 degrees, resulting in an identical finger-display mapping as in normal. Subjects formed two groups with each group using a different normal condition. One group learned the sequence in a standard keypad-hand setting and subsequently recalled the sequence using a rotated keypad-hand setting in the test conditions. The second group learned the sequence with a rotated keypad-hand setting and subsequently recalled the sequence with a standard keypad-hand setting in the test conditions. Response time (RT) and sequencing errors during recall were recorded. Although subjects committed more sequencing errors in both testing conditions, visual and motor, as compared to the normal condition, the errors were below chance level. Sequencing errors did not differ significantly between visual and motor conditions. Further, the sequence recall accuracy was over 70% even by the early stage when the subjects performed the sequence for the first time with the altered conditions, visual and motor. There were parallel improvements thereafter in all the conditions. These results of positive transfer of sequence knowledge across conditions that use dissimilar finger movements point to an effector independent sequence representation, possibly in visual/spatial coordinates. Initially the RTs were similar in the visual and the motor conditions, but with training RTs in the motor condition became significantly shorter than in the visual condition, as revealed by significant interaction for the testing stage and condition term in the repeated measures ANOVA. Moreover, using RTs for single key pressing in the three conditions as baseline indices, it was again observed that RTs in the visual and motor conditions were not significantly different in the early stage, but motor RTs became significantly shorter by the late testing stage. These results support the hypothesis that the motor condition benefits more than the visual because it uses identical effector movements to the normal condition. Further, these results argue for the existence of effector dependent sequence representation, in motor coordinates, which is acquired relatively slowly. The difference in the time course of learning of these two representations may account for the differential involvement of brain areas in early and late learning phases found in lesion and imaging studies.     

Primacy of the negative aftereffect over positive adaptation in prism adaptation with newly hatched chicks

Negative afteraffects (NAE) to prism displacement should not exist without prior positive adaptation (PA) during initial prism exposure. In 4 experiments, chicks wearing hoods containing 8.5° wedge prisms from the day of hatching showed: (a) significant NAE (pecking overcompensation in the direction opposite to their initial displacement error when matched O-degree clear plates were substituted for the prisms), but not significant PA (reduction in the lateral error of pecking while wearing the prisms) at the sixth day; (b) NAE exceeding the theoretically predicted PA on the sixteenth day; (c) significant short and long duration NAE, and significant short and long duration reversal overcompensation effects (ROE) (left-right pecking asymmetry following reversal of the prism base directions) on the seventh and eighth days, without prior PA on the seventh day; (d) neither significant NAE nor significant PA on the fourth day. Both significant NAE and significant PA had been reported previously on the eighth day. Geometrical analyses suggest that variables latent in unrestricted application of adult sensory rearrangement techniques to rapidly growing chicks of low sensory-motor plasticity produce an ontogenetic-development-related reciprocity paradox.     

Real-time error detection but not error correction drives automatic visuomotor adaptation

We investigated the role of visual feedback of task performance in visuomotor adaptation. Participants produced novel two degrees of freedom movements (elbow flexion–extension, forearm pronation–supination) to move a cursor towards visual targets. Following trials with no rotation, participants were exposed to a 60° visuomotor rotation, before returning to the non-rotated condition. A colour cue on each trial permitted identification of the rotated/non-rotated contexts. Participants could not see their arm but received continuous and concurrent visual feedback (CF) of a cursor representing limb position or post-trial visual feedback (PF) representing the movement trajectory. Separate groups of participants who received CF were instructed that online modifications of their movements either were, or were not, permissible as a means of improving performance. Feedforward-mediated performance improvements occurred for both CF and PF groups in the rotated environment. Furthermore, for CF participants this adaptation occurred regardless of whether feedback modifications of motor commands were permissible. Upon re-exposure to the non-rotated environment participants in the CF, but not PF, groups exhibited post-training aftereffects, manifested as greater angular deviations from a straight initial trajectory, with respect to the pre-rotation trials. Accordingly, the nature of the performance improvements that occurred was dependent upon the timing of the visual feedback of task performance. Continuous visual feedback of task performance during task execution appears critical in realising automatic visuomotor adaptation through a recalibration of the visuomotor mapping that transforms visual inputs into appropriate motor commands.     

The organization of patterns of multilimb coordination as revealed through reaction time measures

Simple visual reaction time (RT) during the performance of sagittal movements of the upper and/or lower limbs was investigated. Experiment 1 demonstrated that RTs increased when more limbs were to be moved simultaneously. This effect was more apparent for the upper than for the lower limbs. Experiment 2 allowed a separation of RT into premotor time (PMT) and motor time (MOT) components through analysis of electromyographic activity, and showed that these longer response delays were associated with increased PMTs. This suggests that the time required for the central organization of movements increased as more limbs were to be controlled simultaneously. Compared to single-limb performance conditions, the increases in RT were much larger in the upper limbs (up to 16%) than in the lower limbs (up to 5%) when limb segments were added. During single-limb conditions, RTs in the upper limbs tended to be smaller than in the lower limbs, in accordance with efferent nerve conduction time estimates. Conversely, the lower limb(s) was (were) initiated before the upper limb(s) when both effector types were moved simultaneously. This pattern of activation is reminiscent of the organization of postural control during upright standing, where goal-directed arm activity is preceded by (bilateral) leg activity to anticipate for the upcoming postural destabilization. Finally, hemifield manipulations in experiment 2 revealed faster RTs and PMTs for stimuli presented in the right visual field in comparison with the left field. This advantage was evident for ipsilateral as well as contralateral responses and supports the pre-eminence of the left hemisphere in the complex organization of gross motor responses.     

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.     

Triggering of preprogrammed movements as reactions to masked stimuli

1. Visual stimuli were presented to normal human subjects to test simple and more complex voluntary motor responses. Large and small visual stimuli were presented. In some trials, the small stimulus was followed 50 ms later by the large stimulus, so that the small stimulus was not perceived; this is the phenomenon of "backward masking." 2. Although subjects were not able to detect the masked, visual stimulus on forced-choice testing, they performed motor, reaction-time (RT) tasks in response to it. The RTs for responses to the masked stimulus were the same as those for responses to the easily perceived, nonmasked stimulus. 3. This result confirms and extends the findings of Fehrer and Biederman and was demonstrated with both simple and more complex motor responses. 4. Discussion of the findings focuses on their implications for motor control, particularly with respect to the preprogramming of voluntary movement.     

Reaction Time in a Visual 4-Choice Reaction Time Task: ERP Effects of Motor Preparation and Hemispheric Involvement

Reaction time (RT), the most common measure of CNS efficiency, shows intra- and inter-individual variability. This may be accounted for by hemispheric specialization, individual neuroanatomy, and transient functional fluctuations between trials. To explore RT on these three levels, ERPs were measured in a visual 4-choice RT task with lateralized stimuli (left lateral, left middle, right middle, and right lateral) in 28 healthy right-handed subjects. We analyzed behavioral data, ERP microstates (MS), N1 and P3 components, and trial-by-trial variance. Across subjects, the N1 component was contralateral to the stimulation side. N1-MSs were stronger over the left hemisphere, and middle stimulation evoked stronger activation than lateral stimulation in both hemispheres. The P3 was larger for the right visual field stimulation. RTs were shorter for the right visual hemifield stimulation/right hand responses. Within subjects, covariance analysis of single trial ERPs with RTs showed consistent lateralized predictors of RT over the motor cortex (MC) in the 112–248 ms interval. Decreased RTs were related to negativity over the MC contralateral to the stimulation side, an effect that could be interpreted as the lateralized readiness potential (LRP), and which was strongest for right side stimulation. The covariance analysis linking individual mean RTs and individual mean ERPs showed a frontal negativity and an occipital positivity correlating with decreased RTs in the 212–232 ms interval. We concluded that a particular RT is a composite measure that depends on the appropriateness of the motor preparation to a particular response and on stimulus lateralization that selectively involves a particular hemisphere.     

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.     

Long lasting aftereffect of a single prism adaptation: directionally biased shift in proprioception and late onset shift of internal egocentric reference frame

We aimed to dissociate components in prism adaptation and its aftereffect by using prism adaptation training in healthy humans. Arm proprioceptive aftereffects are usually measured by indicating the subjective straight ahead direction with eyes closed (S). This measure however could be affected by other components besides proprioception, such as an efferent motor component and internal egocentric reference frame. Here we report a very long lasting proprioceptive shift, detected by two measuring methods, that is a component of the adaptation aftereffects to left wedge prism glasses. In order to minimize possible active motor components, arm passive proprioceptive midsagittal judgment was measured (P). The subject’s arm was passively brought from the right or left lateral position, and stopped by subjects’ verbal order. The results from these different measurements of midsagittal judgment were compared for 7 days after prism adaptation. Surprisingly, we found two distinctly separate aftereffects of proprioceptive shift depending on the directions of the passive arm movement. The shift of the midsagittal plane appeared only when tested from the left (Pl). This indicates that our strong prism adaptation procedure affected proprioception in a directionally biased way and not a spatially ubiquitous way. Further, the early aftereffect seen in active straight ahead pointing (S) was mostly similar to this biased shift in proprioception (Pl). However the long lasting aftereffect in straight ahead pointing was independently maintained up to day 7, when the passive proprioception had returned to pretest level. These results indicate that active straight ahead pointing (S) involves other components in addition to the passively measurable proprioceptive component. We suggest a late onset shift in the internal egocentric reference frame is involved in S. Possible neural mechanisms for these phenomena are discussed.R. Chris Miall and Yves Rossetti contributed equally to this work.     

Incomplete inactivation and rapid recovery of voltage-dependent sodium channels during high-frequency firing in cerebellar Purkinje neurons

The ability of individuals to adapt locomotion to constraints associated with the complex environments normally encountered in everyday life is paramount for survival. Here, we tested the ability of 24 healthy young adults to adapt to a rightward prism shift (～11.3°) while either walking and stepping to targets (i.e., precision stepping task) or stepping over an obstacle (i.e., obstacle avoidance task). We subsequently tested for generalization to the other locomotor task. In the precision stepping task, we determined the lateral end-point error of foot placement from the targets. In the obstacle avoidance task, we determined toe clearance and lateral foot placement distance from the obstacle before and after stepping over the obstacle. We found large, rightward deviations in foot placement on initial exposure to prisms in both tasks. The majority of measures demonstrated adaptation over repeated trials, and adaptation rates were dependent mainly on the task. On removal of the prisms, we observed negative aftereffects for measures of both tasks. Additionally, we found a unilateral symmetric generalization pattern in that the left, but not the right, lower limb indicated generalization across the 2 locomotor tasks. These results indicate that the nervous system is capable of rapidly adapting to a visuomotor mismatch during visually demanding locomotor tasks and that the prism-induced adaptation can, at least partially, generalize across these tasks. The results also support the notion that the nervous system utilizes an internal model for the control of visually guided locomotion.