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Biomedical subjects

R Shadmehr

Publications and source records attributed to R Shadmehr.

At least 19 recordsLinked to original sources
PubMed2001

Long-term adaptation to dynamics of reaching movements: a PET study.

Positron emission tomography (PET) was used to examine changes in the cerebellum as subjects learned to make movements with their right arm while holding the handle of a robot that produced a force field. Brain images were acquired during learning and then during recall at 2 and 4 weeks. We also acquired images during a control task where the force field was not learnable and subjects did not show any improvements across sessions. During the 1st day, we observed that motor errors decreased from the control condition to the learning condition. However, regional cerebral blood flow (rCBF) in the posterior region of the right cerebellar cortex initially increased from the control condition and then gradually declined with reductions in motor error. Correspondingly, rCBF in the ipsilateral deep cerebellar nuclei (DCN) initially decreased from the control condition and then increased with reductions in motor error. If measures of rCBF mainly reflect presynaptic activity of neurons, this result predicts that DCN neurons fire with a pattern that starts high in the control task then decreases as learning proceeds. Similarly, Purkinje cells should generally have their lowest activity in the control task. This pattern is consistent with neurophysiological recordings that find that cerebellar activity during early learning of a motor task may mainly reflect changes in coactivation of muscles of the limbs, rather than a learning specific signal. By the end of the first session, motor errors had reached a minimum and no further improvements were observed. However, across the weeks a region in the anterior cerebellar cortex showed gradual decreases in rCBF that could not be attributed to changes in motor performance. Because patterns of rCBF in the cortex and nuclei were highly anti-correlated, we used structural equation modeling to estimate how synaptic activity in the cerebellar cortex influenced synaptic activity in the DCN. We found a negative correlation with a strength that significantly increased during the 4 weeks. This suggests that, during long-term recall, the same input to the cerebellar cortex would produce less synaptic activity at the DCN, possibly because of reduced cerebellar cortex output to the DCN.

Adaptation, Biological↗
PubMed2001

Learning the dynamics of reaching movements results in the modification of arm impedance and long-latency perturbation responses.

Some characteristics of arm movements that humans exhibit during learning the dynamics of reaching are consistent with a theoretical framework where training results in motor commands that are gradually modified to predict and compensate for novel forces that may act on the hand. As a first approximation, the motor control system behaves as an adapting controller that learns an internal model of the dynamics of the task. It approximates inverse dynamics and predicts motor commands that are appropriate for a desired limb trajectory. However, we had previously noted that subtle motion characteristics observed during changes in task dynamics challenged this simple model and raised the possibility that adaptation also involved sensory-motor feedback pathways. These pathways reacted to sensory feedback during the course of the movement. Here we hypothesize that adaptation to dynamics might also involve a modification of how the CNS responds to sensory feedback. We tested this through experiments that quantified how the motor system's response to errors during voluntary movements changed as it adapted to dynamics of a force field. We describe a nonlinear approach that approximates the impedance of the arm, i.e., force response as a function of arm displacement trajectory. We observe that after adaptation, the impedance function changes in a way that closely matches and counters the effect of the force field. This is particularly prominent in the long-latency (> 100 ms) component of response to perturbations. Therefore, it appears that practice not only modifies the internal model with which the brain generates motor commands that initiate a movement, but also the internal model with which sensory feedback is integrated with the ongoing descending commands in order to respond to error during the movement.

Adult↗
PubMed2000

Spatial generalization from learning dynamics of reaching movements.

When subjects practice reaching movements in a force field, they learn a new sensorimotor map that associates desired trajectories to motor commands. The map is formed in the brain with elements that allow for generalization beyond the region of training. We quantified spatial generalization properties of these elements by training in one extreme of the reachable space and testing near another. Training resulted in rotations in the preferred direction (PD) of activation of some arm muscles. We designed force fields that maintained a constant rotation in muscle PDs as the shoulder joint rotated in the horizontal plane. In such fields, training in a small region resulted in generalization to near and far work spaces (80 cm). In one such field, the forces on the hand reversed directions for a given hand velocity with respect to the location of original training. Despite this, there was generalization. However, if the field was such that the change in the muscle PDs reversed as the work spaces changed, then performance was worse than performance of naive subjects. We suggest that the sensorimotor map of arm dynamics is represented in the brain by elements that globally encode the position of the arm but locally encode its velocity. The elements have preferred directions of movement but are modulated globally by the position of the shoulder joint. We suggest that tuning properties of cells in the motor system influence behavior and that this influence is reflected in the way that we learn dynamics of reaching movements.

Adult↗
PubMed2000

Learning of action through adaptive combination of motor primitives.

Understanding how the brain constructs movements remains a fundamental challenge in neuroscience. The brain may control complex movements through flexible combination of motor primitives, where each primitive is an element of computation in the sensorimotor map that transforms desired limb trajectories into motor commands. Theoretical studies have shown that a system's ability to learn action depends on the shape of its primitives. Using a time-series analysis of error patterns, here we show that humans learn the dynamics of reaching movements through a flexible combination of primitives that have gaussian-like tuning functions encoding hand velocity. The wide tuning of the inferred primitives predicts limitations on the brain's ability to represent viscous dynamics. We find close agreement between the predicted limitations and the subjects' adaptation to new force fields. The mathematical properties of the derived primitives resemble the tuning curves of Purkinje cells in the cerebellum. The activity of these cells may encode primitives that underlie the learning of dynamics.

Brain↗
PubMed2000

Motor disorder in Huntington's disease begins as a dysfunction in error feedback control.

A steady progression of motor dysfunction takes place in Huntington's disease (HD). The origin of this disturbance with relation to the motor control process is not understood. Here we studied reaching movements in asymptomatic HD gene-carriers (AGCs) and subjects with manifest HD. We found that movement jerkiness, which characterizes the smoothness and efficiency of motion, was a sensitive indicator of presymptomatic HD progression. A large fraction of AGCs displayed elevated jerk even when more than seven years remained until predicted disease onset. Movement termination was disturbed much more than initiation and was highly variable from trial to trial. Analysis of this variability revealed that the sensitivity of end-movement jerk to subtle, self-generated early-movement errors was greater in HD subjects than in controls. Additionally, we found that HD corrective responses to externally-generated force pulses were greatly disturbed, indicating that HD subjects display aberrant responses to both external and self-generated errors. Because feedback corrections are driven by error and are delayed such that they predominantly affect movement termination, these findings suggest that a dysfunction in error correction characterizes the motor control deficit in early HD. This dysfunction may be observed years before clinical disease onset and grows worse as the disease progresses.

Age of Onset↗
PubMed1999

Electromyographic correlates of learning an internal model of reaching movements.

Theoretical and psychophysical studies have suggested that humans learn to make reaching movements in novel dynamic environments by building specific internal models (IMs). Here we have found electromyographic correlates of internal model formation. We recorded EMG from four muscles as subjects learned to move a manipulandum that created systematic forces (a "force field"). We also simulated a biomechanical controller, which generated movements based on an adaptive IM of the inverse dynamics of the human arm and the manipulandum. The simulation defined two metrics of muscle activation. The first metric measured the component of the EMG of each muscle that counteracted the force field. We found that early in training, the field-appropriate EMG was driven by an error feedback signal. As subjects practiced, the peak of the field-appropriate EMG shifted temporally to earlier in the movement, becoming a feedforward command. The gradual temporal shift suggests that the CNS may use the delayed error-feedback response, which was likely to have been generated through spinal reflex circuits, as a template to learn a predictive feedforward response. The second metric quantified formation of the IM through changes in the directional bias of each muscle's spatial EMG function, i.e., EMG as a function of movement direction. As subjects practiced, co-activation decreased, and the directional bias of each muscle's EMG function gradually rotated by an amount that was specific to the field being learned. This demonstrates that formation of an IM can be represented through rotations in the spatial tuning of muscle EMG functions. Combined with other recent work linking spatial tunings of EMG and motor cortical cells, these results suggest that rotations in motor cortical tuning functions could underlie representation of internal models in the CNS.

Arm↗
PubMed1999

Inhibitory control of competing motor memories.

The ability to inhibit previously learned visuomotor associations is essential for efficient learning of novel behaviors. While the neural basis of the system that might control interactions between competing motor memories is not known, it has been demonstrated that animals with ventral and orbital prefrontal cortex (PFC) deficits have particular difficulties in learning to withhold responses to previously conditioned sensory stimuli. Here we measured regional cerebral blood flow (rCBF), using positron emission tomography, during learning of a novel motor task that required inhibition of a previously learned motor memory. Subjects (n=24) learned reaching movements in a force field (field A). After a variable time interval, some subjects (n=15) learned to reach in a field with a reversed pattern of forces (field B). When the time interval was short (10 min), learning in field B was coincident with a reactivation of regions that had become initially activated during learning in field A: the left putamen and bilaterally in the dorsolateral PFC. Behaviorally, this was accompanied with perseveration that lasted for hundreds of movements, suggesting an instantiation of the internal model for field A during learning in field B. Neither the reactivation nor the perseveration were observed in a different group of subjects that learned field B at 5.5 h. We found that the regions which significantly differentiated the two groups during learning of B were in the ventrolateral PFC (bilaterally): there were sharp decreases in rCBF here in the 5.5 group but not in the 10-min group. At 5.5 h motor learning again involved the striatum, but this time in the left caudate. Neither the caudate nor the ventral PFC had exhibited learning-related activity in field A. Instead, they showed changes in rCBF during the reversal of the learning problem when the previously acquired motor memory was successfully gated. The results demonstrate that: (1) perseveration of a competing motor memory may be linked to reactivation of the neural circuit that participated in acquiring that memory, and (2) the ventral PFC may play an important role in the inhibitory control of the competing motor memory.

Adult↗
PubMed1999

Computational nature of human adaptive control during learning of reaching movements in force fields.

Learning to make reaching movements in force fields was used as a paradigm to explore the system architecture of the biological adaptive controller. We compared the performance of a number of candidate control systems that acted on a model of the neuromuscular system of the human arm and asked how well the dynamics of the candidate system compared with the movement characteristics of 16 subjects. We found that control via a supra-spinal system that utilized an adaptive inverse model resulted in dynamics that were similar to that observed in our subjects, but lacked essential characteristics. These characteristics pointed to a different architecture where descending commands were influenced by an adaptive forward model. However, we found that control via a forward model alone also resulted in dynamics that did not match the behavior of the human arm. We considered a third control architecture where a forward model was used in conjunction with an inverse model and found that the resulting dynamics were remarkably similar to that observed in the experimental data. The essential property of this control architecture was that it predicted a complex pattern of near-discontinuities in hand trajectory in the novel force field. A nearly identical pattern was observed in our subjects, suggesting that generation of descending motor commands was likely through a control system architecture that included both adaptive forward and inverse models. We found that as subjects learned to make reaching movements, adaptation rates for the forward and inverse models could be independently estimated and the resulting changes in performance of subjects from movement to movement could be accurately accounted for. Results suggested that the adaptation of the forward model played a dominant role in the motor learning of subjects. After a period of consolidation, the rates of adaptation in the internal models were significantly larger than those observed before the memory had consolidated. This suggested that consolidation of motor memory coincided with freeing of certain computational resources for subsequent learning.

Adaptation, Physiological↗
PubMed1998

Time-dependent motor memory processes in amnesic subjects.

Functional properties of motor memory change with the passage of time. The time-dependent nature of memories in humans has also been demonstrated for certain "declarative" memories. When the declarative memory system is damaged, are the time-dependent properties associated with motor memories intact? To approach this question, we examined five subjects with global amnesia (AMN), including subject H.M., and a group of age-matched control subjects. The task was to make reaching movements to visually presented targets. We found that H.M. (but not the other subjects) was significantly impaired in the ability to perform the visuomotor kinematic transformations required in this task, to accurately move the hand in the direction specified by a target. With extensive practice, H.M.'s performance improved significantly. At this point, a force field was imposed on the hand. With practice in field A, H.M. and other AMN subjects developed aftereffects and maintained these aftereffects for 24 h. To quantify postpractice properties associated with motor memories, subjects learned field B on day 2 and at 5 min were retested in field A. In both subject groups, performance in field A was significantly worse than their own naive performance a day earlier. The aftereffects indicated persistence of the just-learned but now inappropriate motor memory. After 4 h of rest, subjects were retested in B. Performance was now at naive levels. The aftereffects at 4 h indicated a reduced influence of the memory of field A. The time-dependent patterns of motor memory perseveration, as measured at 5 min and 4 h, were not different in the AMN and normal control groups.

Aged↗
PubMed1997

Neural correlates of motor memory consolidation.

Computational studies suggest that acquisition of a motor skill involves learning an internal model of the dynamics of the task, which enables the brain to predict and compensate for mechanical behavior. During the hours that follow completion of practice, representation of the internal model gradually changes, becoming less fragile with respect to behavioral interference. Here, functional imaging of the brain demonstrates that within 6 hours after completion of practice, while performance remains unchanged, the brain engages new regions to perform the task; there is a shift from prefrontal regions of the cortex to the premotor, posterior parietal, and cerebellar cortex structures. This shift is specific to recall of an established motor skill and suggests that with the passage of time, there is a change in the neural representation of the internal model and that this change may underlie its increased functional stability.

Adult↗
PubMed1997

Functional stages in the formation of human long-term motor memory.

Previous research has demonstrated that the primate CNS has the ability to learn and store multiple and conflicting visuo-motor maps. Here we studied the ability of human subjects to learn to make reaching movements while interacting with one of two conflicting mechanical environments as produced by a robotic manipulandum. We demonstrate that two motor maps may be learned and retained, but only if the training sessions in the tasks are separated by an interval of approximately 5 hr. If the interval is shorter, learning of the second map begins with an internal model appropriate for the first task and performance in the second task is significantly impaired. Analysis of the after-effects suggests that with a short temporal distance, learning of the second task leads to an unlearning of the internal model for the first. With the longer temporal distance, learning of the second task starts with an unbiased internal model, and performance approaches that of naives. Furthermore, the memory of the consolidated skill lasts for at least 5 months after training. These results argue for a distinct change in the state of resistance of motor memory (to disruption) within a few hours after acquisition. We suggest that motor practice results in memories that have at least two functional components: soon after completion of practice, one component fades while another is strengthened. A further experiment suggests that the hypothetical first stage is not merely a gateway to long-term memory, but also temporary storage for items of information, whether new or old, for use in the near-term. Our results raise the possibility that there are distinct neuronal mechanisms for representation of the two functional stages of motor memory.

Adult↗
PubMed1997

Augmented Feedback Presented in a Virtual Environment Accelerates Learning of a Difficult Motor Task.

One can use a number of techniques (e.g., from videotaping to computer enhancement of the environment) to augment the feedback that a subject usually receives during training on a motor task. Although some forms of augmented feedback have been shown to enhance performance on isolated isometric tasks during training, when the feedback has been removed subjects have sometimes not been able to perform as well in the "real-world" task as controls. Indeed, for realistic, nonisometric motor tasks, improved skill acquisition because of augmented feedback has not been demonstrated. In the present experiments, subjects (Experiment 1, N = 42; Experiment 2, N = 21) performed with a system that was designed for teaching a difficult multijoint movement in a table tennis environment. The system was a fairly realistic computer animation of the environment and included paddles for the teacher and subject, as well as a virtual ball. Each subject attempted to learn a difficult shot by matching the pattern of movements of the expert teacher. Augmented feedback focused the attention of the subject on a minimum set of movement details that were most relevant to the task; feedback was presented in a form that required the least perceptual processing. Effectiveness of training was determined by measuring their performance in the real task. Subjects who received the virtual environment training performed significantly better than subjects who received a comparable amount of real-task practice or coaching. Kinematic analysis indicated that practice with the expert's trajectory served as a basis for performance on the real-world task and that the movements executed after training were subject-specific modifications of the expert's trajectory. Practice with this trajectory alone was not sufficient for transfer to the real task, however: When a critical component of the virtual environment was removed, subjects showed no transfer to the real task.

augmented feedback↗
PubMed1996

Consolidation in human motor memory.

Learning a motor skill sets in motion neural processes that continue to evolve after practice has ended, a phenomenon known as consolidation. Here we present psychophysical evidence for this, and show that consolidation of a motor skill was disrupted when a second motor task was learned immediately after the first. There was no disruption if four hours elapsed between learning the two motor skills, with consolidation occurring gradually over this period. Previous studies in humans and other primates have found this time-dependent disruption of consolidation only in explicit memory tasks, which rely on brain structures in the medial temporal lobe. Our results indicate that motor memories, which do not depend on the medial temporal lobe, can be transformed by a similar process of consolidation. By extending the phenomenon of consolidation to motor memory, our results indicate that distinct neural systems share similar characteristics when encoding and storing new information.

Adult↗
PubMed1995

Sense of muscular effort and somesthetic afferent information in humans.

Laboratory and clinical observations of patients with a large-fiber somatic sensory neuropathy indicate a dramatic inability of these patients to set accurate tonic or phasic levels of muscle activity needed to maintain static postures and to reproduce simple movements. These observations suggest that somatic sensation contributes to sensations of motor output, previously thought to be mediated by central mechanisms of corollary discharge. We review data describing psychophysical performance on weight-matching tasks and discuss new experiments on reaching tasks done by patients with a large-fiber sensory neuropathy and normal controls. In combination, the data show that patients with peripheral sensory deficits exhibit an impaired sense of muscular effort and the consequences of active movement. In addition, the data on weight matching indicate that the basis of disrupted effort sense relates to an inability to correlate psychophysical decisions with concomitant muscle activity. In new experiments, accuracy to match actively achieved arm end points by pointing was decreased in patients with large-fiber sensory neuropathy. The collective results suggest that appreciation of motor output is mediated in part by peripheral return from somatic sensory afferent systems.

Afferent Pathways↗
PubMed1994

Adaptive representation of dynamics during learning of a motor task.

We investigated how the CNS learns to control movements in different dynamical conditions, and how this learned behavior is represented. In particular, we considered the task of making reaching movements in the presence of externally imposed forces from a mechanical environment. This environment was a force field produced by a robot manipulandum, and the subjects made reaching movements while holding the end-effector of this manipulandum. Since the force field significantly changed the dynamics of the task, subjects' initial movements in the force field were grossly distorted compared to their movements in free space. However, with practice, hand trajectories in the force field converged to a path very similar to that observed in free space. This indicated that for reaching movements, there was a kinematic plan independent of dynamical conditions. The recovery of performance within the changed mechanical environment is motor adaptation. In order to investigate the mechanism underlying this adaptation, we considered the response to the sudden removal of the field after a training phase. The resulting trajectories, named aftereffects, were approximately mirror images of those that were observed when the subjects were initially exposed to the field. This suggested that the motor controller was gradually composing a model of the force field, a model that the nervous system used to predict and compensate for the forces imposed by the environment. In order to explore the structure of the model, we investigated whether adaptation to a force field, as presented in a small region, led to aftereffects in other regions of the workspace. We found that indeed there were aftereffects in workspace regions where no exposure to the field had taken place; that is, there was transfer beyond the boundary of the training data. This observation rules out the hypothesis that the subject's model of the force field was constructed as a narrow association between visited states and experienced forces; that is, adaptation was not via composition of a look-up table. In contrast, subjects modeled the force field by a combination of computational elements whose output was broadly tuned across the motor state space. These elements formed a model that extrapolated to outside the training region in a coordinate system similar to that of the joints and muscles rather than end-point forces. This geometric property suggests that the elements of the adaptive process represent dynamics of a motor task in terms of the intrinsic coordinate system of the sensors and actuators.

Adaptation, Physiological↗
PubMed1993

Postural force fields of the human arm and their role in generating multijoint movements.

When a perturbation displaces the human hand from equilibrium, arm muscles respond by producing restoring forces. When a set of displacements are given at various directions from the same equilibrium position, the resulting restoring forces form a "postural force field." It is not known whether these postural forces are related to those generated when a reaching movement is executed. However, if a movement is a consequence of a shift of the equilibrium position of the hand toward the target, then, from the postural force field, predictions can be made regarding the nature of the elastic forces acting on the hand during the movement. We have taken the first steps in testing this hypothesis by measuring the postural force field of a subject's arm over relatively large distances, and comparing these forces with the static forces generated at the hand while the subject attempted a reaching movement. Using a robot manipulandum, the hand was displaced at various directions from an equilibrium position. The measured restoring forces were fitted to a nonlinear model to define a postural force field for that equilibrium position. This field was used to predict elastic forces generated when the subject attempted to move the manipulandum from a point on the circumference of a circle to a target at its center--the center corresponded to the equilibrium position at which the postural field was measured. In some of the movement trials, the manipulandum was locked during approximately the first 120 msec of the program for motion and the resulting static "evoked" forces measured. We found that (1) the evoked forces did not point to the target, but were a function of the configuration of the arm and rotated with the shoulder joint, and (2) the magnitude of the evoked forces varied systematically, even though the movements were of the same magnitude. These patterns were remarkably similar to those observed in the postural forces. Our results provide experimental evidence linking maintenance of posture in a multijoint system to that of generating a movement. The evidence is consistent with the hypothesis that the CNS programs a reaching movement by shifting the equilibrium position of the hand toward the target.

Adult↗