Adaptation to a nonlinear visuomotor amplitude transformation with continuous and terminal visual feedback

The control of a cursor on a computer monitor offers a simple means of exploring the limits of the plasticity of human visuomotor coordination. The authors explored the boundary conditions for adaptation to nonlinear visuomotor amplitude transformations. The authors hypothesized that only with terminal visual feedback during practice, but not with continuous visual feedback, humans might develop an internal model of the nonlinear visuomotor amplitude transformation. Thus, 2 groups were engaged in a sensorimotor adaptation task receiving either continuous or terminal visual feedback during the practice phase. In contrast to expectations, adaptive shifts and aftereffects observed in visual open-loop tests were linearly related to target amplitudes for both groups. Although the 2 feedback groups did not differ with respect to adaptive shifts and aftereffects, terminal visual feedback resulted in stable visual open-loop performance for an extended period, whereas movement errors increased after continuous visual feedback during practice. The benefit of continuous visual feedback, on the other hand, was faster closed-loop performance, indicating an optimization of visual closed-loop control.     

Visuomotor adaptation in normal aging

Visuomotor adaptation to a gradual or sudden screen cursor rotation was investigated in healthy young and elderly subjects. Both age groups were equally divided into two subgroups; one subgroup was exposed to 11.25° step increments of visual feedback rotation, every 45 trials (up to a total of 90°), whereas a second subgroup was subjected to 90° rotation from the onset of exposure. Participants performed discrete, horizontal hand movements to virtual targets in four randomized directions. Targets appeared on a computer screen in front of them, and a board prevented vision of the hand at all times. Differential effects of aging on visuomotor adaptation were found, depending on the time course of the visual distortion. In both age groups, early exposure to the sudden visual feedback distortion resulted in typical spiral-like trajectories, which became straighter by late exposure. However, the final adaptation level was reduced in the aged group, although the aftereffects were similar. When subjects were exposed to the gradual distortion, no statistically significant differences in measures of adaptation with advancing age were found. In this case, both age groups appeared to adapt equally. However, after removal of the distortion, elderly subjects showed reduced aftereffects as compared with the young group. These findings suggest differential effects of aging on adaptation to gradual versus sudden visual feedback distortions, and may help to explain the conflicting results obtained in previous visuomotor adaptation studies.     

Gait symmetric adaptation: Comparing effects of implicit visual distortion versus split-belt treadmill on aftereffects of adapted step length symmetry

Understanding gait adaptation is essential for rehabilitation, and visual feedback can be used during gait rehabilitation to develop effective gait training. We have previously shown that subjects can adapt spatial aspects of walking to an implicitly imposed distortion of visual feedback of step length. To further investigate the storage benefit of an implicit process engaged in visual feedback distortion, we compared the robustness of aftereffects acquired by visual feedback distortion, versus split-belt treadmill walking. For the visual distortion trial, we implicitly distorted the visual representation of subjects’ gait symmetry, whereas for the split-belt trial, the speed ratio of the two belts was gradually adjusted without visual feedback. After adaptation, the visual feedback or the split-belt perturbation was removed while subjects continued walking, and aftereffects of preserved asymmetric pattern were assessed. We found that subjects trained with visual distortion trial retained aftereffects longest. In response to the larger speed ratio of split-belt walking, the subjects showed an increase in the size of aftereffects compared to the smaller speed ratio, but it steeply decreased over time in all the speed ratios tested. In contrast, the visual distortion group showed much slower decreasing rate of aftereffects, which was evidence of longer storage of an adapted gait pattern. Visual distortion adaptation may involve the interaction and integration of the change in motor strategy and implicit process in sensorimotor adaptation. Although it should be clarified more clearly through further studies, the findings of this study suggest that gait control employs distinct adaptive processes during the visual distortion and split-belt walking and also the level of reliance of an implicit process may be greater in the visual distortion adaptation than the split-belt walking adaptation.     

Adaptation to a direction-dependent visuomotor gain in the young and elderly

Consistent with the widely accepted notion of separate specification of movement amplitude and direction, it has been argued that there is also a categorical difference between adaptation to novel visuomotor rotations and to novel visuomotor gains. In line with this view, ageing seems to affect rotation and gain adaptation differently in that age-related impairments are consistently found for the former, but not for the latter. In this study we ask whether the contrasting findings could also be ascribed to differences in the level of difficulty of gain and rotation adaptation tasks, respectively. In order to increase the difficulty of gain adaptation, younger and older participants had to adapt to a direction-dependent gain transformation. Results revealed direction-dependent adaptation in both groups. More importantly, we replicated the typical findings of age-related impairments of adaptation, but not of aftereffects, that were previously only reported for rotation adaptation. Younger participants also showed superior explicit knowledge regarding the novel visuomotor mapping as compared to the older participants. We show that this knowledge was used by younger participants to selectively augment adaptive shifts. Finally, our findings suggest that the difficulty of the novel visuomotor transformation and, related to this, the involvement of explicit knowledge in adaptation is critical for age-related changes to show up, but not the type of adaptation task, rotation and gain adaptation, respectively.     

Knowledge of Performance is Insufficient for Implicit Visuomotor Rotation Adaptation

The ability to adapt is a fundamental and vital characteristic of the motor system. The authors altered the visual environment and focused on the ability of humans to adapt to a rotated environment in a reaching task, in the absence of continuous visual information about their hand location. Subjects could not see their arm but were provided with post trial knowledge of performance depicting hand path from movement onset to final position. Subjects failed to adapt under these conditions. The authors sought to find out whether the lack of adaptation is related to the number of target directions presented in the task, and planned 2 protocols in which subjects were gradually exposed to 22.5° visuomotor rotation. These protocols differed only in the number of target directions: 8 and 4 targets. The authors found that subjects had difficulty adapting without the existence of continuous visual feedback of their performance regardless of the number of targets presented in task. In the 4-target protocol, some of the subjects noticed the rotation and explicitly aimed to the correct direction. The results suggest that real-time feedback is required for motor adaptation to visual rotation during reaching movements.     

Adaptation to visuomotor rotations in younger and older adults

Adaptation to a visuomotor rotation is known to be impaired at older adult age. The authors examined whether the impairment is present already at preretirement age and whether it depends on the difficulty of the adaptation task. Moreover, the authors tested predictions of the hypothesis that the age-related impairment pertains primarily to strategic corrections and the explicit knowledge on which they are based but not to the acquisition of an (implicit) internal model of the novel visuomotor transformation. The authors found an age-related impairment of adaptation and explicit knowledge already at preretirement age but no age-related change of aftereffects. With an incremental simplification of the adaptation task, age-related changes were able to be eliminated. Individual differences of the quality of explicit knowledge were associated with differences of adaptation, but not of aftereffects. When age groups were matched by explicit knowledge, age-related impairments of adaptation largely disappeared. However, a reliable difference remained in one of the experiments, suggesting that other processes of adjustment to a visuomotor rotation might be affected by aging as well.     

Snoddy (1926) Revisited: Time Scales of Motor Learning

A new 3-stage model based on neuroimaging evidence is proposed by Chein and Schneider (2012). Each stage is associated with different brain regions, and draws on cognitive abilities: the first stage on creativity, the second on selective attention, and the third on automatic processing. The purpose of the present study was to scrutinize the validity of this model for 1 popular learning paradigm, visuomotor adaptation. Participants completed tests for creativity, selective attention and automated processing before attending in a pointing task with adaptation to a 60° rotation of visual feedback. To examine the relationship between cognitive abilities and motor learning at different times of practice, associations between cognitive and adaptation scores were calculated repeatedly throughout adaptation. The authors found no benefit of high creativity for adaptive performance. High levels of selective attention were positively associated with early adaptation, but hardly with late adaptation and de-adaptation. High levels of automated execution were beneficial for late adaptation, but hardly for early and de-adaptation. From this we conclude that Chein and Schneider's first learning stage is difficult to confirm by research on visuomotor adaptation, and that the other 2 learning stages rather relate to workaround strategies than to actual adaptive recalibration.     

Visuomotor error augmentation affects mediolateral head and trunk stabilization during walking

Prior work demonstrates that humans spontaneously synchronize their head and trunk kinematics to a broad range of driving frequencies of perceived mediolateral motion prescribed using optical flow. Using a closed-loop visuomotor error augmentation task in an immersive virtual environment, we sought to understand whether unifying visual with vestibular and somatosensory feedback is a control goal during human walking, at least in the context of head and trunk stabilization. We hypothesized that humans would minimize visual errors during walking – i.e., those between the visual perception of movement and actual movement of the trunk. We found that subjects did not minimize errors between the visual perception of movement and actual movement of the head and trunk. Rather, subjects increased mediolateral trunk range of motion in response to error-augmented optical flow with positive feedback gains. Our results are more consistent with our alternative hypothesis – that visual feedback can override other sensory modalities and independently compel adjustments in head and trunk position. Also, aftereffects following exposure to error-augmented optical flow included longer, narrower steps and reduced mediolateral postural sway, particularly in response to larger amplitude positive feedback gains. Our results allude to a recalibration of head and trunk stabilization toward more tightly regulated postural control following exposure to error-augmented visual feedback. Lasting reductions in mediolateral postural sway may have implications for using error-augmented optical flow to enhance the integrity of walking balance control through training, for example in older adults.     

Interlimb differences in visuomotor and dynamic adaptation during targeted reaching in children

While a number of studies have focused on movement (a)symmetries between the arms in adults, less is known about movement asymmetries in typically developing children. The goal of this study was to examine interlimb differences in children when adapting to novel visuomotor and dynamic conditions while performing a center-out reaching task. We tested 13 right-handed children aged 9–11 years old. Prior to movement, one of eight targets arranged radially around the start position was randomly displayed. Movements were made either with the right (dominant) arm or the left (nondominant) arm. The children participated in two experiments separated by at least one week. In one experiment, subjects were exposed to a rotated visual display (30° about the start circle); and in the other, a 1 kg mass (attached eccentrically to the forearm axis). Each experiment consisted of three blocks: pre-exposure, exposure and post-exposure. Three measures of task performance were calculated from hand trajectory data: hand-path deviation from the straight target line, direction error at peak velocity and final position error. Results showed that during visuomotor adaptation, no interlimb differences were observed for any of the three measures. During dynamic adaptation, however, a significant difference between the arms was observed at the first cycle during dynamic adaptation. With regard to the aftereffects observed during the post-exposure block, direction error data indicate considerably large aftereffects for both arms during visuomotor adaptation; and there was a significant difference between the arms, resulting in substantially larger aftereffects for the right arm. Similarly, dynamic adaptation results also showed a significant difference between the arms; and post hoc analyses indicated that aftereffects were present only for the right arm. Collectively, these findings indicate that the dominant arm advantage for developing an internal model associated with a novel visuomotor or dynamic transform, as previously shown in young adults, may already be apparent at 9 to 11-year old children.     

Effects of situational cues on prism-induced aftereffects

A test was made of the hypothesis that external stimuli present during exposure to lateral displacement of the visual field can serve as situational cues whose presence or absence will influence the magnitude of aftereffects manifested subsequent to adaptation resulting from the exposure. The results indicated that the relative aftereffects were significantly greater when thenondisplacing goggles were worn during the periods in which aftereffect measurements were taken than was the case when they were removed during these test periods. The finding that manipulation of certain cues, i.e., the restriction of the visual field, weight, etc., of the goggles, associated with the adaptation period can in part determine the size of observed aftereffects provides evidence in support of the notion that aftereffects can be conditioned to precisely given constellations of stimuli In addition, the need for caution in conceptualizing aftereffects as simply the persistence of adaptive shifts once visual displacement has been terminated is suggested.     

Adaptive coordination and alignment of eye and hand

Under spatial misalignment of eye and hand induced by laterally displacing prisms (11.4 degrees in the rightward direction), subjects pointed 60 times (once every 3 s) at a visually implicit target (straight ahead of nose, Experiment 1) or a visually explicit target (an objectively straight-ahead target, Experiment 2). For different groups in each experiment, the hand became visible early in the sagittal pointing movement (early visual feedback). Adaptation to the optical misalignment during exposure (direct effects) was rapid, especially with early feedback; complete compensation for the misalignment was achieved within about 30 trials, and overcompensation occurred in later trials, especially with an explicit target. In contrast, adaptation measured with the misalignment removed and without visual feedback after blocks of 10 pointing trials (aftereffects) was slow to develop, especially with delayed feedback and an implicit target; at most, about 40% compensation for the misalignment occurred after 60 trials. This difference between direct effects and aftereffects is discussed in terms of separable adaptive mechanisms that are activated by different error signals. Adaptive coordination is activated by error feedback and involves centrally located, strategically flexible, short-latency processes to correct for sudden changes in operational precision that normally occur with short-term changes in coordination tasks. Adaptive alignment is activated automatically by spatial discordance between misaligned systems and involves distributed, long-latency processes to correct for slowly developing shifts in alignment among perceptual-motor components that normally occur with long-term drift. The sudden onset of misalignment in experimental situations activates both mechanisms in a complex and not always cooperative manner, which may produce overcompensatory behavior during exposure (i.e., direct effects) and which may limit long-term alignment (i.e., aftereffects).     

Different visuomotor processes maturation rates in children support dual visuomotor learning systems

Different processes are involved during visuomotor learning, including an error-based procedural and a strategy based cognitive mechanism. Our objective was to analyze if the changes in the adaptation or the aftereffect components of visuomotor learning measured across development, reflected different maturation rates of the aforementioned mechanisms. Ninety-five healthy children aged 4–12 years and a group of young adults participated in a wedge prism and a dove prism throwing task, which laterally displace or horizontally reverse the visual field respectively. The results show that despite the age-related differences in motor control, all children groups adapted in the error-based wedge prisms condition. However, when removing the prism, small children showed a slower aftereffects extinction rate. On the strategy-based visual reversing task only the older children group reached adult-like levels. These results are consistent with the idea of different mechanisms with asynchronous maturation rates participating during visuomotor learning.     

Prism exposure aftereffects and direct effects for different movement and feedback times

The effects of movement time and time to visual feedback (feedback time) on prism exposure aftereffects and direct effects were studied. In Experiment 1, the participants' (N = 60) pointing limb became visible early in the movement (.2-s feedback time), and eye-head aftereffects increased with increasing movement time (.5 to 3.0 s), but larger hand-head aftereffects showed little change. Direct effects (terminal error during exposure) showed near-perfect compensation for the prismatic displacement (11.4 diopters) when movement time was short but decreasing compensation with longer movement times. In Experiment 2, participants' (N = 48) eye-head aftereffects increased and their larger hand-head aftereffects decreased with increasing movement time (2.0 and 3.0 s), especially when feedback time increased (.25 and 1.5 s). Direct effects showed increasing overcompensation for longer movement and feedback times. Those results suggest that aftereffects and direct effects measure distinct adaptive processes, namely, spatial realignment and strategic control, respectively. Differences in movement and feedback times evoke different eye-hand coordination strategies and consequent direct effects. Realignment aftereffects also depend upon the coordination strategy deployed, but not all strategies support realignment. Moreover, realignment is transparent to strategic control and, when added to strategic correction, may produce nonadaptive performance.     

Influence of divergent and convergent thinking on visuomotor adaptation in young and older adults

Visuomotor adaptation declines in older age. This has been attributed to cognitive impairments. One relevant cognitive function could be creativity, since creativity is implicated as mediator of early learning. The present study therefore evaluates whether two aspects of creativity, divergent and convergent thinking, are differentially involved in the age-dependent decline of visuomotor adaptation.In 25 young and 24 older volunteers, divergent thinking was assessed by the alternative-uses-task (AUT), convergent thinking by the Intelligenz-Struktur-Test-2000 (IST), and sensorimotor-adaptation by a pointing task with 60° rotated visual feedback.Young participants outperformed older participants in all three tasks. AUT scores were positively associated with young but not older participants’ adaptive performance, whereas IST scores were negatively associated with older but not young participants’ adaptive performance. This pattern of findings could be attributed to a consistent relationship between AUT, IST and adaptation; taking this into account, adaptation deficits of older participants were no longer significant.We conclude that divergent thinking supports workaround-strategies during adaptation, but doesn’t influence visuomotor recalibration. Furthermore, the decay of divergent thinking in older adults may explain most of age-related decline of adaptive strategies. When the age-related decay of divergent thinking coincides with well-preserved convergent thinking, adaptation suffers most.     

Prism Exposure Aftereffects and Direct Effects for Different Movement and Feedback Times

The effects of movement time and time to visual feedback (feedback time) on prism exposure aftereffects and direct effects were studied. In Experiment 1, the participants' (N = 60) pointing limb became visible early in the movement (.2-s feedback time, and eye-head aftereffects increased with increasing movement time (.5 to 3.0 s), but larger hand-head aftereffects showed little change. Direct effects (terminal error during exposure) showed near-perfect compensation for the prismatic displacement (11.4 diopters) when movement time was short but decreasing compensation with longer movement times. In Experiment 2, participants' (N = 48) eye-head aftereffects increased and their larger hand-head aftereffects decreased with increasing movement time (2.0 and 3.0 s), especially when feedback time increased (.25 and 1.5 s). Direct effects showed increasing overcompensation for longer movement and feedback times. Those results suggest that aftereffects and direct effects measure distinct adaptive processes, namely, spatial realignment and strategic control, respectively. Differences in movement and feedback times evoke different eye -hand coordination strategies and consequent direct effects. Realignment aftereffects also depend upon the coordination strategy deployed, but not all strategies support realignment. Moreover, realignment is transparent to strategic control and, when added to strategic correction, may produce nonadaptive performance.     

Effect of visuomotor calibration and uncertainty on the perception of peripersonal space

Target selection for action depends not only on the egocentric location of objects estimated from retinal and extraretinal variables, but also on the assessment of current action possibilities. In the present study, we investigated the effect of altering sensorimotor anticipation processes on subsequent perceptual estimates of reachability. To do so, we conducted two experiments in which we changed the relation between visual distance and movement amplitude. Experiment 1 showed that iterative visuomotor adaptation to distorted visual feedback (in steps of ±15?mm, up to a total adaptation of ±75?mm) led to a congruent variation of perceived reachable space, although the first introduction of the shifted visual feedback produced a reduction of perceived reachable space whatever the direction of the feedback shift. Experiment 2 showed that increasing uncertainty about visuomotor performances, by providing a visual feedback randomly shifted in depth (±7.5?mm), produced the same reduction of perceived reachable space in the absence of visuomotor adaptation. Taken together, these data suggest that the visual perception of reachable space depends on a motor-related perceptual system, which is affected by both visuomotor recalibration and reliability of the visuomotor system.     

Implicit and explicit components of dual adaptation to visuomotor rotations

Concurrent adaptation to two different visuomotor transformations has been shown to be possible as long as discriminative contextual cues are available. The authors examined explicit and implicit components of visually cued dual adaptation in younger and older adults. They found that only young adults, but not old adults, produced appropriate adaptive shifts of hand-movement direction to compensate for the visuomotor rotations. Aftereffects, conceived as a measure of implicit knowledge, were only poorly developed. Furthermore, only participants in the younger group exhibited systematic explicit knowledge of the visuomotor rotations. Subsequent analyses revealed strong correlations between the quality of explicit knowledge and the overall visuomotor adaptation. Thus, visually cued dual adaptation to two opposite visuomotor rotations is primarily mediated by conscious strategic corrections based on explicit knowledge of the transformations, a process, which is selectively impaired in older adults.     

Calibration and Alignment are Separable: Evidence From Prism Adaptation

In 2 prism adaptation experiments, the authors investigated the effects of limb starting position visibility (visible or not visible) and visual feedback availability (early or late in target pointing movements). Thirty-two students participated in Experiment 1 and 24 students participated in Experiment 2. Independent of visual feedback availability, constant error was larger and variable error was smaller for target pointing when limb starting position was visible during prism exposure. Independent of limb starting position visibility, aftereffects of prism exposure were determined by visual feedback availability. Those results support the hypothesis that calibration is determined by limb starting position visibility, whereas alignment is determined separately by visual feedback availability.     

Adaptation to direction-dependent visuo-motor rotations and its decay in younger and older adults

We studied adaptation to a direction-dependent visuo-motor rotation in adults of early and late working age. For hand movements to the right, visual motion of the cursor on a monitor was rotated clockwise, for forward movements rotation of the cursor motion was zero, and for directions in-between rotation was intermediate. In contrast to previous studies, in which adaptation was more difficult (larger visuo-motor rotation, larger number of targets during practice) and the older age group was of higher age, we found no age-related deficit of adaptation. However, consistent with previous studies we found an age-related impairment of explicit knowledge of the visuo-motor rotation and no age-related differences of aftereffects. Across periods of not performing the task for 24 h and of performing the task for a prolonged period of time without visual feedback, we observed a decay of adaptation which did not depend on age. The present findings cast doubts on the prevalent interpretation of age-related impairments of adaptation in the absence of age-related changes of aftereffects as resulting from intentional strategic corrections, which become less efficient at higher age.     

Not letting the left leg know what the right leg is doing: limb-specific locomotor adaptation to sensory-cue conflict

We investigated the phenomenon of limb-specific locomotor adaptation in order to adjudicate between sensory-cue-conflict theory and motor-adaptation theory. The results were consistent with cue-conflict theory in demonstrating that two different leg-specific hopping aftereffects are modulated by the presence of conflicting estimates of self-motion from visual and nonvisual sources. Experiment 1 shows that leg-specific increases in forward drift during attempts to hop in place on one leg while blindfolded vary according to the relationship between visual information and motor activity during an adaptation to outdoor forward hopping. Experiment 2 shows that leg-specific changes in performance on a blindfolded hopping-to-target task are similarly modulated by the presence of cue conflict during adaptation to hopping on a treadmill. Experiment 3 shows that leg-specific aftereffects from hopping additionally produce inadvertent turning during running in place while blindfolded. The results of these experiments suggest that these leg-specific locomotor aftereffects are produced by sensory-cue conflict rather than simple motor adaptation.