Patterns of transfer of adaptation among body segments

Different generalization of fast and slow visuomotor adaptation across locomotion and pointing tasks

Sensorimotor adaptation can involve multiple learning processes with different time courses, and these processes may have different patterns of transfer. In this study, we tested how slow learning and fast learning transfer across tasks, and the specificity of transfer. We tested two natural goal-directed tasks: pointing and walking toward a visible target. We also tested a novel "hand locomotion" task in which subjects used pointing movements to cause simulated self-motion in virtual reality. The hand locomotion task used the same physical movement as pointing, but performed the same function as stepping. During an experimental block, subjects performed alternating training trials with perturbed visual feedback and test trials with no feedback. The test trials were either the same task to measure adaptation, or a different task to measure transfer. Perturbations on adaptation trials varied over time as a sum of sinusoids with different frequencies. Fast learning would be expected to produce equal responses to fast and slow perturbations, while slower learning would dampen responses to higher frequency perturbations. Subjects were generally not aware of the smoothly varying perturbations, but showed detectable adaptation for all three tasks. Only pointing produced significantly different responses to high- and low-frequency perturbations, consistent with slow learning. Adaptation of pointing produced more transfer to the hand locomotion task, which shared the same effector and motor actions, than to the stepping task. The other tasks showed fast learning but little or no slow learning, and equal transfer to tasks with different effector or function. Our results suggest that the slower components of sensorimotor adaptation are more movement specific, while faster learning is more generalizable.

Challenging balance during sensorimotor adaptation increases generalization

From reaching to walking, real-life experience suggests that people can generalize between motor behaviors. One possible explanation for this generalization is that real-life behaviors often challenge our balance. We propose that the exacerbated body motions associated with balance-challenged whole body movements increase the value to the nervous system of using a comprehensive internal model to control the task. Because it is less customized to a specific task, a more comprehensive model is also a more generalizable model. Here we tested the hypothesis that challenging balance during adaptation would increase generalization of a newly learned internal model. We encouraged participants to learn a new internal model using prism lenses that created a new visuomotor mapping. Four groups of participants adapted to prisms while performing either a standing-based reaching or precision walking task, with or without a manipulation that challenged balance. To assess generalization after the adaptation phase, participants performed a single trial of each of the other groups' tasks without prisms. We found that both the reaching and walking balance-challenged groups showed significantly greater generalization to the equivalent, nonadapted task than the balance-unchallenged groups. Additionally, we found some evidence that all groups generalized across tasks, for example, from walking to reaching and vice versa, regardless of balance manipulation. Overall, our results demonstrate that challenging balance increases the degree to which a newly learned internal model generalizes to untrained movements.NEW & NOTEWORTHY Real-life experience indicates that people can generalize between motor behaviors. Here we show that challenging balance during the learning of a new internal model increases the degree of generalization to untrained movements for both reaching and walking tasks. These results suggest that the effects of challenging balance are not specific to the task but instead apply to motor learning more broadly.

Nonlinear visuomotor transformations: Locus and modularity

Participants in two experiments moved a mouse-like device to the right to move a cursor on a computer screen to a target position. The cursor was invisible during motion but reappeared at the end of each movement. The relationship between the amplitudes of the cursor movement and the mouse movement was exponential in Experiment 1 and logarithmic in Experiment 2 for two groups of participants, while it was linear for the control groups in both experiments. The results of both experiments indicate that participants adjusted well to the external transformation by developing an internal model that approximated the inverse of the external transformation. We introduce a method to determine the locus of the internal model. It indicates that the internal model works at a processing level that either preceded specification of movement amplitude, or had become part of movement amplitude specification. Limited awareness of the nonlinear mouse–cursor relationship and the fact that a working-memory task had little effect on performance suggest that the internal model is modular and not dependent on high-level cognitive processes.

Properties of intermodal transfer after dual visuo- and auditory-motor adaptation

Previous work documented that sensorimotor adaptation transfers between sensory modalities: When subjects adapt with one arm to a visuomotor distortion while responding to visual targets, they also appear to be adapted when they are subsequently tested with auditory targets. Vice versa, when they adapt to an auditory-motor distortion while pointing to auditory targets, they appear to be adapted when they are subsequently tested with visual targets. Therefore, it was concluded that visuomotor as well as auditory-motor adaptation use the same adaptation mechanism. Furthermore, it has been proposed that sensory information from the trained modality is weighted larger than sensory information from an untrained one, because transfer between sensory modalities is incomplete. The present study tested these hypotheses for dual arm adaptation. One arm adapted to an auditory-motor distortion and the other either to an opposite directed auditory-motor or visuomotor distortion. We found that both arms adapted significantly. However, compared to reference data on single arm adaptation, adaptation in the dominant arm was reduced indicating interference from the non-dominant to the dominant arm. We further found that arm-specific aftereffects of adaptation, which reflect recalibration of sensorimotor transformation rules, were stronger or equally strong when targets were presented in the previously adapted compared to the non-adapted sensory modality, even when one arm adapted visually and the other auditorily. The findings are discussed with respect to a recently published schematic model on sensorimotor adaptation.

Effects of angular gain transformations between movement and visual feedback on coordination performance in unimanual circling

Tool actions are characterized by a transformation (of spatio-temporal and/or force-related characteristics) between movements and their resulting consequences in the environment. This transformation has to be taken into account, when planning and executing movements and its existence may affect performance. In the present study we investigated how angular gain transformations between movement and visual feedback during circling movements affect coordination performance. Participants coordinated the visual feedback (feedback dot) with a continuously circling stimulus (stimulus dot) on a computer screen in order to produce mirror symmetric trajectories of them. The movement angle was multiplied by a gain factor (0.5-2; nine levels) before it was presented on the screen. Thus, the angular gain transformations changed the spatio-temporal relationship between the movement and its feedback in visual space, and resulted in a non-constant mapping of movement to feedback positions. Coordination performance was best with gain = 1. With high gains the feedback dot was in lead of the stimulus dot, with small gains it lagged behind. Anchoring (reduced movement variability) occurred when the two trajectories were close to each other. Awareness of the transformation depended on the deviation of the gain from 1. In conclusion, the size of an angular gain transformation as well as its mere presence influence performance in a situation in which the mapping of movement positions to visual feedback positions is not constant. When designing machines or tools that involve transformations between movements and their external consequences, one should be aware that the mere presence of angular gains may result in performance decrements and that there can be flaws in the representation of the transformation.

Age effects on controlling tools with sensorimotor transformations

Controlling tools in technical environments bears a lot of challenges for the human information processing system, as locations of tool manipulation and effect appearance are spatially separated, and distal action effects are often not generated in a 1:1 manner. In this study we investigated the susceptibility of older adults to distal action effects. Younger and older participants performed a Fitts' task on a digitizer tablet without seeing their hand and the tablet directly. Visual feedback was presented on a display in that way, that cursor amplitude and visual target size varied while the pre-determined hand amplitude remained constant. In accordance with distal action effects being predominant in controlling tool actions we found an increase in hand movement times and perceptual errors as a function of visual task characteristics. Middle-aged adults more intensely relied on visual feedback than younger adults. Age-related differences in speed-accuracy trade-off are not likely to account for this finding. However, it is well known that proprioceptive acuity declines with age. This might be one reason for middle-aged adults to stronger rely on the visual information instead of the proprioceptive information. Consequently, design and application of tools for elderly should account for this.

Neural correlates of motor learning, transfer of learning, and learning to learn

Recent studies on the neural bases of sensorimotor adaptation demonstrate that the cerebellar and striatal thalamocortical pathways contribute to early learning. Transfer of learning involves a reduction in the contribution of early learning networks and increased reliance on the cerebellum. The neural correlates of learning to learn remain to be determined but likely involve enhanced functioning of the general aspects of early learning.

Degree of handedness affects intermanual transfer of skill learning

Intermanual transfer of skill learning has often been used as a paradigm to study functional specialization and hemispheric interactions in relation to handedness. This literature has not evaluated whether degree of handedness impacts learning and intermanual transfer. Because handedness scores are related to factors that might influence intermanual transfer, such as engagement of the ipsilateral hemisphere during movement (Dassonville et al. in Proc Natl Acad Sci USA 94:14015-14018, 1997) and corpus callosum volume (Witelson in Science 229:665-668, 1985; Brain 112:799-835, 1989), we tested whether degree of handedness is correlated with transfer magnitude. We had groups of left and right handed participants perform a sensorimotor adaptation task and a sequence learning task. Following learning with either the dominant or nondominant hand, participants transferred to task performance with the other hand. We evaluated whether the magnitude of learning and intermanual transfer were influenced by either direction and/or degree of handedness. Participants exhibited faster sensorimotor adaptation with the right hand, regardless of whether they were right or left handed. In addition, less strongly left handed individuals exhibited better intermanual transfer of sensorimotor adaptation, while less strongly right handed individuals exhibited better intermanual transfer of sequence learning. The findings suggest that involvement of the ipsilateral hemisphere during learning may influence intermanual transfer magnitude.

Asymmetrical after-effects of prism adaptation during goal oriented locomotion

In healthy subjects, sensorimotor after-effects of prism adaptation are known to be symmetric (they appear after using leftward and rightward optical deviations), whereas cognitive after-effects are asymmetric (they appear after using a leftward optical deviation) and rightward oriented. Sensorimotor and cognitive after-effects have been classically studied using different specific tasks. The purpose of the current study was to investigate whether both after-effects may be involved in a same visuo-spatial task. Therefore we compared the amplitude of after-effects following adaptation to a rightward or leftward optical deviation. After-effects were assessed by manual pointing or goal oriented locomotor task. The main result showed a greater amplitude for rightward locomotor after-effects (after adaptation to a leftward deviation) than for leftward locomotor after-effects (after adaptation to a rightward deviation). This means that cognitive after-effects may add to sensorimotor after-effects following adaptation to a leftward optical deviation. This asymmetry challenges the classical distinction between sensorimotor and cognitive after-effects of prism adaptation. Implications for the functional mechanisms and the neuroanatomical substrate of prism adaptation are discussed.

Adaptation of Voluntary Saccades, But Not of Reactive Saccades, Transfers to Hand Pointing Movements

Studying the transfer of visuomotor adaptation from a given effector (e.g., the eye) to another (e.g., the hand) allows us to question whether sensorimotor processes influenced by adaptation are common to both effector control systems and thus to address the level where adaptation takes place. Previous studies have shown only very weak transfer of the amplitude adaptation of reactive saccades—i.e., produced automatically in response to the sudden appearance of visual targets—to hand pointing movements. Here we compared the amplitude of hand pointing movements recorded before and after adaptation of either reactive or voluntary saccades, produced either in a saccade sequence task or in a single saccade task. No transfer to hand pointing movements was found after adaptation of reactive saccades. In contrast, a substantial transfer to the hand was obtained following adaptation of voluntary saccades produced in sequence. Large amounts of transfer between the two saccade types were also found. These results demonstrate that the visuomotor processes influenced by saccadic adaptation depend on the type of saccades and that, in the case of voluntary saccades, they are shared by hand pointing movements. Implications for the neurophysiological substrates of the adaptation of reactive and voluntary saccades are discussed.

Older adults can learn to learn new motor skills

Many studies have demonstrated that aging is associated with declines in skill acquisition. In the current study, we tested whether older adults could acquire general, transferable knowledge about skill learning processes. Older adult participants learned five different motor tasks. Two older adult control groups performed the same number of trials, but learned only one task. The experimental group exhibited faster learning than that seen in the control groups. These data demonstrate that older adults can learn to learn new motor skills.

The locus of adaptation to altered gain in aimed movements

Altered gain settings cause a mismatch between the actual movement amplitude across the surface and the distance covered on a real time visual display. The present study pursued three objectives of how adaptation to altered gain affects aimed motor behavior. First, we replicated findings of an earlier study reporting a negative linear relation between gain and both target acquisition time and end-point variability. This means that our data do not agree with the classic U-shaped relation between gain and acquisition time. Second, our results proved to be robust when we manipulated movement difficulty by varying target distance. And third, dividing a movement into four successive sections on the basis of key kinematic events revealed the locus of the adaptation to altered gain within movement execution. Time differences between gain conditions proved to start at a very early part of the movement, but appeared to be absent in the final movement section. In contrast, differences between gain conditions regarding the use of online feedback were also present in the last part of the movement during which the final target approach takes place.

Influence of different modes of real time visual information on single aimed movements

This study shows that the difference in appearance between two types of computer generated real time visual information namely, cursor and trace feedback, becomes visible in the execution of aimed stylus movements. Movement execution was scrutinized by division into four relevant subsequent constituents on the basis of key kinematic events. As expected, the final section of a movement took more time and showed a larger number of adjustments in the cursor condition. Instead, the influence of trace feedback on movement execution became discernible in earlier sections of movement execution. When a combination of feedback types was employed, trace feedback seemed to dominate the impact on execution. Discussion focuses on the contribution of the present study to understanding the impact of specific features inherent to types of real time visual feedback, and the relevance of the present study's result to future research.

Multiple motor learning experiences enhance motor adaptability

Traditional motor learning theory emphasizes that skill learning is specific to the context and task performed. Recent data suggest, however, that subjects exposed to a variety of motor learning paradigms may be able to acquire general, transferable knowledge about skill learning processes. I tested this idea by having subjects learn five different motor tasks, three that were similar to each other and two that were not related. A group of experimental subjects first performed a joystick-aiming task requiring adaptation to three different visuomotor rotations, with a return to the null conditions between each exposure. They then performed the same joystick-aiming task but had to adapt to a change in display gain instead of rotation. Lastly, the subjects used the joystick-aiming task to learn a repeating sequence of movements. Two groups of control subjects performed the same number of trials, but learned only the gain change or the movement sequence. Experimental subjects showed generalization of learning across the three visuomotor rotations. Experimental subjects also exhibited transfer of learning ability to the gain change and the movement sequence, resulting in faster learning than that seen in the control subjects. However, transient perturbations affected the movements of the experimental subjects to a greater extent than those of the control subjects. These data demonstrate that humans can acquire a general enhancement in motor skill learning capacity through experience, but it comes with a cost. Although movement becomes more adaptable following multiple learning experiences, it also becomes less stable to external perturbation.