Does training with traditionally presented and virtually simulated tasks elicit differing changes in object interaction kinematics in persons with upper extremity hemiparesis?

OBJECTIVE: To contrast changes in clinical and kinematic measures of upper extremity movement in response to virtually simulated and traditionally presented rehabilitation interventions in persons with upper extremity hemiparesis due to chronic stroke. DESIGN: Non-randomized controlled trial. SETTING: Ambulatory research facility. PARTICIPANTS: Subjects were a volunteer sample of twenty one community-dwelling adults (mean age: 51 ± 12 years) with residual hemiparesis due to stroke more than 6 months before enrollment (mean: 74 ± 48 months), recruited at support groups. Partial range, against gravity shoulder movement and at least 10° of active finger extension were required for inclusion. All subjects completed the study without adverse events. INTERVENTIONS: A 2 weeks, 24-hour program of robotic/virtually simulated, arm and finger rehabilitation activities was compared to the same dose of traditionally presented arm and finger activities. RESULTS: Subjects in both groups demonstrated statistically significant improvements in the ability to interact with real-world objects as measured by the Wolf Motor Function Test (P = 0.01). The robotic/virtually simulated activity (VR) group but not the traditional, repetitive task practice (RTP) group demonstrated significant improvements in peak reaching velocity (P = 0.03) and finger extension excursion (P = 0.03). Both groups also demonstrated similar improvements in kinematic measures of reaching and grasping performance such as increased shoulder and elbow excursion along with decreased trunk excursion. CONCLUSIONS: Kinematic measurements identified differing adaptations to training that clinical measurements did not. These adaptations were targeted in the design of four of the six simulations performed by the simulated activity group. Finer grained measures may be necessary to accurately depict the relative benefits of dose matched motor interventions.

A virtual reality-based system integrated with fmri to study neural mechanisms of action observation-execution: a proof of concept study.

PURPOSE: Emerging evidence shows that interactive virtual environments (VEs) may be a promising tool for studying sensorimotor processes and for rehabilitation. However, the potential of VEs to recruit action observation-execution neural networks is largely unknown. For the first time, a functional MRI-compatible virtual reality system (VR) has been developed to provide a window into studying brain-behavior interactions. This system is capable of measuring the complex span of hand-finger movements and simultaneously streaming this kinematic data to control the motion of representations of human hands in virtual reality. METHODS: In a blocked fMRI design, thirteen healthy subjects observed, with the intent to imitate (OTI), finger sequences performed by the virtual hand avatar seen in 1st person perspective and animated by pre-recorded kinematic data. Following this, subjects imitated the observed sequence while viewing the virtual hand avatar animated by their own movement in real-time. These blocks were interleaved with rest periods during which subjects viewed static virtual hand avatars and control trials in which the avatars were replaced with moving non-anthropomorphic objects. RESULTS: We show three main findings. First, both observation with intent to imitate and imitation with real-time virtual avatar feedback, were associated with activation in a distributed frontoparietal network typically recruited for observation and execution of real-world actions. Second, we noted a time-variant increase in activation in the left insular cortex for observation with intent to imitate actions performed by the virtual avatar. Third, imitation with virtual avatar feedback (relative to the control condition) was associated with a localized recruitment of the angular gyrus, precuneus, and extrastriate body area, regions which are (along with insular cortex) associated with the sense of agency. CONCLUSIONS: Our data suggest that the virtual hand avatars may have served as disembodied training tools in the observation condition and as embodied "extensions" of the subject's own body (pseudo-tools) in the imitation. These data advance our understanding of the brain-behavior interactions when performing actions in VE and have implications in the development of observation- and imitation-based VR rehabilitation paradigms.

Innovative approaches to the rehabilitation of upper extremity hemiparesis using virtual environments.

AIM: Upper-extremity interventions for hemiparesis are a challenging aspect of stroke rehabilitation. Purpose of this paper is to report the feasibility of using virtual environments (VEs) in combination with robotics to assist recovery of hand-arm function and to present preliminary data demonstrating the potential of using sensory manipulations in VE to drive activation in targeted neural regions. METHODS: We trained 8 subjects for 8 three hour sessions using a library of complex VE's integrated with robots, comparing training arm and hand separately to training arm and hand together. Instrumented gloves and hand exoskeleton were used for hand tracking and haptic effects. Haptic Master robotic arm was used for arm tracking and generating three-dimensional haptic VEs. To investigate the use of manipulations in VE to drive neural activations, we created a "virtual mirror" that subjects used while performing a unimanual task. Cortical activation was measured with functional MRI (fMRI) and transcranial magnetic stimulation. RESULTS: Both groups showed improvement in kinematics and measures of real-world function. The group trained using their arm and hand together showed greater improvement. In a stroke subject, fMRI data suggested virtual mirror feedback could activate the sensorimotor cortex contralateral to the reflected hand (ipsilateral to the moving hand) thus recruiting the lesioned hemisphere. CONCLUSION: Gaming simulations interfaced with robotic devices provide a training medium that can modify movement patterns. In addition to showing that our VE therapies can optimize behavioral performance, we show preliminary evidence to support the potential of using specific sensory manipulations to selectively recruit targeted neural circuits.

Deficits in adaptive upper limb control in response to trunk perturbations in Parkinson's disease.

The ability of patients with Parkinson's disease (PD) to compensate for unexpected perturbations remains relatively unexplored. To address this issue PD subjects were required to compensate at the arm for an unexpected mechanical perturbation of the trunk while performing a trunk-assisted reach. Twelve healthy and nine PD subjects (off medication) performed trunk-assisted reaching movements without vision or knowledge of results to a remembered target in the ipsilateral (T1) or contralateral (T2) workspace. On 60% of the trials trunk motion was unrestrained (free condition). On the remaining 40% of randomly selected trials trunk motion was arrested at movement onset (blocked condition). If subjects appropriately changed arm joint angles to compensate for the trunk arrest, there should be spatial and temporal invariance in the hand trajectories and in the endpoint errors across conditions. The control group successfully changed their arm configuration in a context-dependent manner which resulted in invariant hand trajectory profiles across the free and blocked conditions. More so, they initiated these changes rapidly after the trunk perturbation (group mean 70 ms). Some PD subjects were unable to maintain invariant hand paths and movement errors across conditions. Their hand velocity profiles were also more variable relative to those of the healthy subjects in the blocked-trunk trials but not in the free-trunk trials. Furthermore, the latency of compensatory changes in arm joint angles in movements toward T1 was longer in the PD group (group mean 153 ms). Finally, PD subjects' arm and trunk were desynchronized at movement onset, confirming our previous findings and consistent with PD patients' known problems in the sequential or parallel generation of different movement components. The findings that individual PD subjects were unsuccessful or delayed in producing context-dependent responses at the arm to unexpected perturbations of the trunk suggests that the basal ganglia are important nodes in the organization of adaptive behavior.

A virtual reality based exercise system for hand rehabilitation post-stroke: transfer to function.

We present preliminary results from a virtual reality (VR)-based system for hand rehabilitation that uses a CyberGlove and a Rutgers Master II-ND haptic glove. This system trains finger range of motion, finger flexion speed, independence of finger motion and finger strength. Eight chronic post-stroke subjects participated. In keeping with variability in both the lesion site and in initial upper extremity function, each subject showed improvement on a unique combination of movement parameters in VR training. These improvements transferred to gains on clinical tests, as well as to significant reductions in task completion times for the prehension of real objects. These results are indicative of the potential feasibility of this exercise system for rehabilitation in patients with hand dysfunction resulting from neurological impairment.

Arm-trunk coordination in the absence of proprioception.

During trunk-assisted reaching to targets placed within arm's length, the influence of trunk motion on the hand trajectory is compensated for by changes in the arm configuration. The role of proprioception in this compensation was investigated by analyzing the movements of 2 deafferented and 12 healthy subjects. Subjects reached to remembered targets (placed approximately 80 degrees ipsilateral or approximately 45 degrees contralateral to the sagittal midline) with an active forward movement of the trunk produced by hip flexion. In 40% of randomly selected trials, trunk motion was mechanically blocked. No visual feedback was provided during the experiment. The hand trajectory and velocity profiles of healthy subjects remained invariant whether or not the trunk was blocked. The invariance was achieved by changes in arm interjoint coordination that, for reaches toward the ipsilateral target, started as early as 50 ms after the perturbation. Both deafferented subjects exhibited considerable, though incomplete, compensation for the effects of the perturbation. Compensation was more successful for reaches to the ipsilateral target. Both deafferented subjects showed invariance between conditions (unobstructed or blocked trunk motion) in their hand paths to the ipsilateral target, and one did to the contralateral target. For the other deafferented subject, hand paths in the two types of trials began to deviate after about 50% into the movement, because of excessive elbow extension. In movements to the ipsilateral target, when deafferented subjects compensated successfully, the changes in arm joint angles were initiated as early as 50 ms after the trunk perturbation, similar to healthy subjects. Although the deafferented subjects showed less than ideal compensatory control, they compensated to a remarkably large extent given their complete loss of proprioception. The presence of partial compensation in the absence of vision and proprioception points to the likelihood that not only proprioception but also vestibulospinal pathways help mediate this compensation.

Virtual reality-enhanced stroke rehabilitation.

A personal computer (PC)-based desktop virtual reality (VR) system was developed for rehabilitating hand function in stroke patients. The system uses two input devices, a CyberGlove and a Rutgers Master II-ND (RMII) force feedback glove, allowing user interaction with a virtual environment. This consists of four rehabilitation routines, each designed to exercise one specific parameter of hand movement: range, speed, fractionation or strength. The use of performance-based target levels is designed to increase patient motivation and individualize exercise difficulty to a patient's current state. Pilot clinical trials have been performed using the above system combined with noncomputer tasks, such as pegboard insertion or tracing of two-dimensional (2-D) patterns. Three chronic stroke patients used this rehabilitation protocol daily for two weeks. Objective measurements showed that each patient showed improvement on most of the hand parameters over the course of the training. Subjective evaluation by the patients was also positive. This technical report focuses on this newly developed technology for VR rehabilitation.

The interaction of visual and proprioceptive inputs in pointing to actual and remembered targets in Parkinson's disease.

We previously reported that Parkinson's disease patients could point with their eyes closed as accurately as normal subjects to targets in three-dimensional space that were initially presented with full vision. We have now further restricted visual information in order to more closely examine the individual and combined influences of visual information, proprioceptive feedback, and spatial working memory on the accuracy of Parkinson's disease patients. All trials were performed in the dark. A robot arm presented a target illuminated by a light-emitting diode at one of five randomly selected points composing a pyramidal array. Subjects attempted to "touch" the target location with their right finger in one smooth movement in three conditions: dark, no illumination of arm or target during movement; movement was to the remembered target location after the robot arm retracted; finger, a light-emitting diode on the pointing fingertip was visible during the movement but the target was extinguished; again, movement was to the remembered target location; and target, the target light-emitting diode remained in place and visible throughout the trial but there was no vision of the arm. In the finger condition, there is no need to use visual-proprioceptive integration, since the continuously visualized fingertip position can be compared to the remembered location of the visual target. In the target condition, the subject must integrate the current visible target with arm proprioception, while in the dark condition, the subject must integrate current proprioception from the arm with the remembered visual target. Parkinson's disease patients were significantly less accurate than controls in both the dark and target conditions, but as accurate as controls in the finger condition. Parkinson's disease patients, therefore, were selectively impaired in those conditions (target and dark) which required integration of visual and proprioceptive information in order to achieve accurate movements. In contrast, the patients' normal accuracy in the finger condition indicates that they had no substantial deficits in their relevant spatial working memory. Final arm configurations were significantly different in the two subject groups in all three conditions, even in the finger condition where mean movement endpoints were not significantly different. Variability of the movement endpoints was uniformly increased in Parkinson's disease patients across all three conditions. The current study supports an important role for the basal ganglia in the integration of proprioceptive signals with concurrent or remembered visual information that is needed to guide movements. This role can explain much of the patients' dependence on visual information for accuracy in targeted movements. It also underlines what may be an essential contribution of the basal ganglia to movement, the integration of afferent information that is initially processed through multiple, discrete modality-specific pathways, but which must be combined into a unified and continuously updated spatial model for effective, accurate movement.

Hand trajectory invariance in reaching movements involving the trunk.

Movements of different body segments may be combined in different ways to achieve the same motor goal. How this is accomplished by the nervous system was investigated by having subjects make fast pointing movements with the arm in combination with a forward bending of the trunk that was unexpectedly blocked in some trials. Subjects moved their hand above the surface of a table without vision from an initial position near the midline of the chest to remembered targets placed within the reach of the arm in either the ipsi- or contralateral workspace. In experiment 1, subjects were instructed to make fast arm movements to the target without corrections whether or not the trunk was arrested. Only minor changes were found in the hand trajectory and velocity profile in response to the trunk arrest, and these changes were seen only late in the movement. In contrast, the patterns of the interjoint coordination substantially changed in response to the trunk arrest, suggesting the presence of compensatory arm-trunk coordination minimizing the deflections from the hand trajectory regardless of whether the trunk is recruited or mechanically blocked. Changes in the arm interjoint coordination in response to the trunk arrest could be detected kinematically at a minimal latency of 50 ms. This finding suggests a rapid reflex compensatory mechanism driven by vestibular and/or proprioceptive afferent signals. In experiment 2, subjects were required, as soon as they perceived the trunk arrest, to change the hand motion to the same direction as that of the trunk. Under this instruction, subjects were able to initiate corrections only after the hand approached or reached the final position. Thus, centrally mediated compensatory corrections triggered in response to the trunk arrest were likely to occur too late to maintain the observed invariant hand trajectory in experiment 1. In experiment 3, subjects produced similar pointing movements, but to a target that moved together with the trunk. In these body-oriented pointing movements, the hand trajectories from trials in which the trunk was moving or arrested were substantially different. The same trajectories represented in a relative frame of reference moving with the trunk were virtually identical. We conclude that hand trajectory invariance can be produced in an external spatial (experiment 1) or an internal trunk-centered (experiment 3) frame of reference. The invariance in the external frame of reference is accomplished by active compensatory changes in the arm joint angles nullifying the influence of the trunk motion on the hand trajectory. We suggest that to make a transition to the internal frame of reference, control systems suppress this compensation. One of the hypotheses opened to further experimental testing is that the integration of additional (trunk) degrees of freedom into movement is based on afferent (proprioceptive, vestibular) signals stemming from the trunk motion and transmitted to the arm muscles.

Procedural motor learning in Parkinson's disease.

We have been investigating motor control and learning in parkinsonian subjects. In the current study, we sought to explore the existence of deficits in procedural motor learning, which is a form of implicit motor learning where skill improves over repetitive blocks of trials. We sought to determine, in particular, whether any such deficit is accentuated during specific types or phases of learning. We would expect that those specific learning tasks would require the greatest participation of the basal ganglia. Numerous studies have found that Parkinson's disease (PD) patients may show deficits in learning. Combined with information about basal ganglia neuronal connections and activity, this led some investigators to suggest that one of the key functions of the basal ganglia is to facilitate learning. To investigate these learning deficits, we used a robotic device to generate conservative force fields that disturbed the subjects' arm movements, thereby generating a "virtual mechanical environment" that subjects learned to manipulate. Movements were successively grouped into blocks comprising five different conditions: motor performance, early learning, late learning, negative transfer, and aftereffect motor performance. Our results with eight right-handed PD subjects and nine age-matched controls showed a relative decrease in the rate of learning for the PD patients in all blocks, but greater differences emerged between groups during novelty phases of learning. In particular, the difference in performance during the negative transfer condition reached statistical significance, suggesting that the basal ganglia might be a key center for "switching" motor patterns. Our results support the hypothesis that deficiencies in procedural motor learning are characteristic of PD. They add to existing evidence which has suggested a key role for the basal ganglia when new sensorimotor mappings are required by novel task environments. Better understanding of these deficits should facilitate the rehabilitation of PD patients.

The timing of arm-trunk coordination is deficient and vision-dependent in Parkinson's patients during reaching movements.

The role of the basal ganglia in the coordination of different body segments and utilization of motor synergies was investigated by analyzing reaching movements to remembered three-dimensional (3D) targets in patients with Parkinson's disease (PD). Arm movements were produced alone or in combination with a forward bending of the trunk, with or without visual feedback. Movements in PD patients were more temporally segmented, as evidenced by irregular changes in tangential velocity profiles. In addition, the relative timing in the onsets and offsets of fingertip and trunk motions were substantially different in PD patients than in control subjects. While the control subjects synchronized both onsets and offsets, the PD patients had large mean intervals between the onsets and offsets of the fingertip and trunk motions. Moreover, PD patients showed substantially larger trial-to-trial variability in these intervals. The degree of synchronization in PD patients gradually increased during the movement under the influence of visual feedback. The mean and variability of the intersegmental intervals decreased as the fingertip approached the target. This improvement in timing occurred even though the separate variability in the timing of arm and trunk motions was not reduced by vision. In combined movements, even without vision, the PD patients were able to achieve normal accuracy, suggesting they were able to use the same movement synergies as normals to control the multiple degrees of freedom involved in the movements and to compensate for the added trunk movement. However, they were unable to recruit these synergies in the stereotyped manner characteristic of healthy subjects. These results suggest that the basal ganglia are involved in the temporal coordination of movement of different body segments and that related timing abnormalities may be partly compensated by vision. Abnormal intersegmental timing may be a highly sensitive indicator of a deficient ability to assemble complex movements in patients with basal-ganglia dysfunction. This abnormality may be apparent even when the overall movement goal of reaching a target is preserved and normal movement synergies appear to be largely intact.

A novel quantitative method for 3D measurement of Parkinsonian tremor.

OBJECTIVE: To demonstrate the usefulness of a three dimensional (3D) motion analysis system for the quantitative measurement of tremor in patients with Parkinson's disease (PD). METHODS: Six PD patients with hand tremors were studied using a system that employed 3D electromagnetic position sensors to measure the actual, cumulative displacement of the tremoring finger. Patients were studied in different hand positions and activating conditions before and 30, 60, 90 and 120 min after intake of Pramipexole, a dopamine agonist known to reduce tremor. Tremor amplitude and frequency, before and after drug intake, were compared using Mann-Whitney U test and Wilcoxon rank test, respectively. RESULTS: The motion analysis system allowed discrimination of tremor related events from movement artifact and allowed the calculation of real world movement of the finger tremor despite altered hand positions and orientation. Average 3D tremor frequency ranged from 3.71 to 4.34 Hz. Median tremor amplitude (total distance traveled per 5 s interval) decreased with drug from 4.9 to 1.6 cm for resting tremor, 4.5 to 3.7 cm for postural tremor, 3.4 to 3.3 cm for precision tremor, 10.2 to 3.3 cm for tapping activation and 108.6 to 5.7 cm for counting activation. CONCLUSIONS: Our method of 3D analysis provides a robust, single quantitative measure of tremor amplitude that is intuitive and likely to reflect the functional impact of tremor. This methodology should be useful in comparing tremor across patients and in measuring the efficacy of therapeutic interventions.

Pointing in 3D space to remembered targets. II. Effects of movement speed toward kinesthetically defined targets.

The accuracy of visually guided pointing movements decreases with speed. We have shown that for movements to a visually defined remembered target, the variability of the final arm endpoint position does not depend on movement speed. We put forward a hypothesis that this observation can be explained by suggesting that movements directed at remembered targets are produced without ongoing corrections. In the present study, this hypothesis was tested for pointing movements in 3D space to kinesthetically defined remembered targets. Passive versus active acquisition of kinesthetic information was contrasted. Pointing errors, movement kinematics, and joint-angle coordination were analyzed. The movements were performed at a slow speed (average peak tangential velocity of about 1.2 m/s) and at a fast speed (2.7 m/s). No visual feedback was allowed during the target presentation or the movement. Variability in the final position of the arm endpoint did not increase with speed in either the active or the passive condition. Variability in the final values of the arm-orientation angles determining the position of the forearm and of the upper arm in space was also speed invariant. This invariance occurred despite the fact that angular velocities increased by a factor of two for all the angles involved. The speed-invariant variability supports the hypothesis that there is an absence of ongoing corrections for movements to remembered targets: in the case of a slower movement, where there is more time for movement correction, the final arm endpoint variability did not decrease. In contrast to variability in the final endpoint position, the variability in the peak tangential acceleration increased significantly with movement speed. This may imply that the nervous system adopts one of two strategies: either the final endpoint position is not encoded in terms of muscle torques or there is a special on-line mechanism that adjusts movement deceleration according to the muscle-torque variability at the initial stage of the movement. The final endpoint position was on average farther from the shoulder than the target. Constant radial-distance errors were speed dependent in both the active and the passive conditions. In the fast speed conditions, the radial distance overshoots of the targets increased. This increase in radial-distance overshoot with movement speed can be explained by the hypothesis that the final arm position is not predetermined in these experimental conditions, but is defined during the movement by a feedforward or feedback mechanism with an internal delay.

Pointing in 3D space to remembered targets. I. Kinesthetic versus visual target presentation.

This study investigated the influence of different modalities of target information (visual, kinesthetic) on the accuracy, kinematics, and interjoint coordination of pointing movements to remembered targets. The targets were presented by a robot arm in five locations in three-dimensional (3D) space, either as a point of light in a dark room ("visual" condition), or kinesthetically. Relative pointing accuracy in the visual compared with kinesthetic conditions was influenced by the target location: pointing errors were the largest for the visual targets most eccentric relative to the subject's head. In addition, for the two most lateral targets, the final arm positions were, on average, closer to the center than the targets in the visual condition and farther from the center than the targets in the kinesthetic conditions. This result suggests that the pattern of errors in the visual condition described elsewhere ("range effect") may derive from visual processing rather than motor planning and implementation. Two modes of kinesthetic target presentation were utilized. During "passive" kinesthetic presentation of the target, the experimenter moved the subject's relaxed arm. Alternately, in "active" kinesthetic presentation of the target, the subject actively (with minimal help from the experimenter) moved his arm. No visual feedback was allowed in either kinesthetic condition. The variability in the final fingertip position was significantly smaller in the active condition than in the passive condition. In contrast, variability in the final values of arm orientation angles did not differ significantly in the active and passive conditions. This apparent contradiction may be resolved by the fact that, for the given target location, the influence of the deviation of these angles in the given trial from their average values on the position of the fingertip tended to be mutually compensated, and this tendency was stronger in the active condition. Our analysis of the correlations among the arm orientation angles and of the relationship between the initial and final arm configurations suggests that the kinesthetic conditions enabled the implementation of a mixture of strategies for achieving accuracy. The first strategy is to use a specific memory of an adequate arm configuration (that assumed during target presentation), such that accuracy is achieved by using this memory as a template. The second strategy is to use synergistically coordinating joint angles, such that accuracy is achieved by focusing on a specific endpoint that can be reached by a range of equivalent arm positions. The latter strategy was better utilized in the active condition. In conclusion, our results indicate that human subjects can use diverse sensory information to achieve comparable final accuracy, but that the details of the strategies employed differ with the kind of information available.

Control processes underlying elbow flexion movements may be independent of kinematic and electromyographic patterns: experimental study and modelling.

Using a non-linear dynamic model based on the lambda version of the equilibrium-point hypothesis, we investigated the shape and duration of the control patterns underlying discrete elbow movements. The model incorporates neural control variables, time-, position- and velocity-dependent intrinsic muscle and reflex properties. Two control variables (R and C) specify a positional frame of reference for activation of flexor and extensor motoneurons. The variable R (reciprocal command) specifies the referent joint angle (R) at which the transition of net flexor to extensor active torque or vice versa can be observed during changes in the actual joint angle elicited by an external force. The variable C (coactivation command) surrounds the transition angle by an angular range in which flexor and extensor muscles may be simultaneously active (if C > 0) or silent (if C < or = 0). An additional, time-dimensional control variable (mu command) influences the dependency of the threshold of the stretch reflex on movement velocity. This control variable is responsible for the reflex damping. Changes in the R command result in shifts in the equilibrium state of the system, a dynamical process leading to electromyographic modifications and movement production. Commands C and mu provide movement stability and effective energy dissipation preventing oscillations at the end of movement. A comparison of empirical and model data was carried out. A monotonic ramp-shaped pattern of the R command can account for the empirical kinematic and electromyographic patterns of the fastest elbow flexion movements made with or without additional inertia, as well as of self-paced movements. The rate of the shifts used in simulation was different for the three types of movements but independent of movement distance (20-80 degrees). This implies that, for a given type of movement, the distance is encoded by the duration of shift in the equilibrium state. The model also reproduces the kinematic and electromyographic patterns of the fastest uncorrected movements opposed in random trials by a high load (80-90% of the maximal) generated by position feedback to a torque motor. The following perturbation effects were simulated: a substantial decrease in the arm displacement (from 60-70 degrees to 5-15 degrees) and movement duration (to about 100 ms) so that these movements ended near the peak velocity of those which were not perturbed; a prolongation of the first agonist electromyographic burst as long as the load was applied; the suppression of the antagonist burst during the dynamic and static phases of movements: the reappearance of the antagonist burst in response to unloading accompanied by a short-latency suppression of agonist activity. These kinematic and electromyographic features of both perturbed and non-perturbed movements were reproduced by using the same control patterns which elicited a monotonic shift in the equilibrium state of the system ending before the peak velocity of non-perturbed movements. Our results suggest that the neural control processes underlying the fastest unopposed changes in the arm position are completed long before the end of the movement and phasic electromyographic activity. Neither the timing nor the amplitude of electromyographic bursts are planned but rather they represent the long-lasting dynamic response of central, reflex and mechanical components of the system to a monotonic, short-duration shift in the system's equilibrium state.

Central modifications of reflex parameters may underlie the fastest arm movements.

Descending and reflex pathways usually converge on common interneurons and motoneurons. This implies that active movements may result from changes in reflex parameters produced by control signals conveyed by descending systems. Specifically, according to the lambda-model, a fast change in limb position is produced by a rapid change in the threshold of the stretch reflex. Consequently, external perturbations may be ineffective in eliciting additional reflex modifications of electromyographic (EMG) patterns unless the perturbations are relatively strong. In this way, the model accounts for the relatively weak effects of perturbations on the initial agonist EMG burst (Ag1) usually observed in fast movements. On the other hand, the same model permits robust reflex modifications of the timing and shape of the Ag1 in response to strong perturbations even in the fastest movements. To test the model, we verified the suggestion that the onset time of the Ag1, even in the fastest movements, depends on proprioceptive feedback in a manner consistent with a stretch reflex. In control trials, subjects (n = 6) made fast unopposed elbow flexion movements of approximately 60 degrees (peak velocity 500-700 degrees/s) in response to an auditory signal. In random test trials, a brief (50 ms) torque of 8-15 Nm either assisting or opposing the movement was applied 50 ms after this signal. Subjects had no visual feedback and were instructed not to correct arm deflections in case of perturbations. In all subjects, the onset time of the Ag1 depended on the direction of perturbation: it was 25-60 ms less in opposing compared with assisting load conditions. Assisting torques caused, at a short latency of 37 ms, an additional antagonist EMG burst preceding the Ag1. The direction-dependent effects of the perturbation persisted when cutaneous feedback was suppressed. It was concluded that the direction-dependent changes in the onset time and duration of the Ag1 as well as the antagonist activation preceding the Ag1 resulted from stretch reflex activity elicited by the perturbations rather than from a change in the control strategy or cutaneous reflexes. The results support the hypothesis on the hierarchical scheme of sensorimotor integration in which EMG patterns and movement emerge from the modification of the thresholds and other parameters of proprioceptive reflexes by control systems.

The interaction of visual and proprioceptive inputs in pointing to actual and remembered targets.

Errors in pointing to actual and remembered targets presented in three-dimensional (3D) space in a dark room were studied under various conditions of visual feedback. During their movements, subjects either had no vision of their arms or of the target, vision of the target but not of their arms, vision of a light-emitting diode (LED) on their moving index fingertip but not of the target, or vision of an LED on their moving index fingertip and of the target. Errors depended critically upon feedback condition. 3D errors were largest for movements to remembered targets without visual feedback, diminished with vision of the moving fingertip, and diminished further with vision of the target and vision of the finger and the target. Moreover, the different conditions differentially influenced the radial distance, azimuth, and elevation errors, indicating that subjects control motion along all three axes relatively independently. The pattern of errors suggest that the neural systems that mediate processing of actual versus remembered targets may have different capacities for integrating visual and proprioceptive information in order to program spatially directed arm movements.

The relationship between control, kinematic and electromyographic variables in fast single-joint movements in humans.

Two versions of the hypothesis that discrete movements are produced by shifts in the system's equilibrium point are considered. The first suggests that shifts are monotonic and end near the peak velocity of movement, and the second presumes that they are nonmonotonic ("N-shaped") and proceed until the end of movement. The first version, in contrast to the second, predicts that movement time may be significantly reduced by opposing loads without changes in the control pattern. The purpose of the present study was to test the two hypotheses about the duration and shape of the shift in the equilibrium point based on their respective predictions concerning the effects of perturbations on kinematic and EMG patterns in fast elbow flexor movements. Subjects performed unopposed flexions of about 55-70 degrees (control trials) and, in random test trials, movements were opposed by spring-like loads generated by a torque motor. Subjects had no visual feedback and were instructed not to correct arm deflections in case of perturbations. After the end of the movement, the load was removed leading to a secondary movement to the same final position as that in control trials (equifinality). When the load was varied, the static arm positions before unloading and associated joint torques (ranging from 0 to 80-90% of maximum voluntary contraction) had a monotonic relationship. Test movements opposed by a high load (80-90% of maximal voluntary contraction) ended near the peak velocity of control movements. Phasic and tonic electromyographic patterns were load-dependent. In movements opposed by high loads, the first agonist burst was significantly prolonged and displayed a high level of tonic activity for as long as the load was maintained. In the same load conditions, the antagonist burst was suppressed during the dynamic and static phases of movement. The findings of suppression of the antagonist burst does not support the hypothesis of an N-shaped control signal. Equally, the substantial reduction in movement time by the introduction of an opposing load cannot be reconciled in this model. Instead, our data indicate that the shifts in the equilibrium point underlying fast flexor movements are of short duration, ending near the peak velocity of unopposed movement. This suggests that kinematic and electromyographic patterns represent a long-lasting oscillatory response of the system to the short-duration monotonic control pattern, external forces and proprioceptive feedback.

Merging different motor patterns: coordination between rhythmical and discrete single-joint movements.

Subjects made fast, discrete elbow flexion movements while simultaneously producing rhythmical oscillations about initial and final visual targets embedded on a horizontal surface. Based on kinematic and electromyographic (EMG) analysis, we found that the discrete movement could start at any phase of the cyclical movement. The most likely onset time occurred when the first agonist burst started at the same moment as a rhythmical burst would have appeared. This resulted in a smooth conjugation between discrete and rhythmical movements. The initiation of the discrete movement was associated with the resetting of the phase of the rhythmical movements. Thus, the time characteristics of the two motor tasks were interdependent. A subset of trials with a uniform distribution of discrete movement onset phases could be selected in most subjects and was averaged to eliminate the cyclical component from the combined movement. Mean kinematic and EMG traces up until the peak velocity were practically identical to those of the discrete movement made alone. The averaging procedure was ineffective in eliminating the rhythmical component following the discrete movement because of the resetting of the phase of oscillation. Using the same procedure it has been shown that initiating the rhythmical movement at the same time as beginning the discrete movement did not affect the initial part of discrete movement. The whole discrete movement was not affected when subjects simultaneously terminated the ongoing rhythmical movements. Our findings are consistent with the hypothesis that although the rhythmical movement constrains the onset time of discrete movement, the latter, once initiated, proceeds independently of the ongoing rhythmical movement. We also subtracted the discrete component from the combined movement to see how the former affected the rhythmical movement. The residual pattern showed that the rhythmical movements rapidly attenuated when the discrete movement started and then apparently resumed after the peak velocity of the discrete movement. The results corroborate the hypothesis that the control signals underlying the two motor tasks cannot be applied simultaneously, since they may be associated with conflicting stability requirements. Instead, these control signals may be generated sequentially, but the resulting kinematic responses may outlast them and be superposed.

[A model of central regulation of movement parameters]. / Model' tsentral'noi reguliatsii parametrov dvigatel'nykh traektorii.

The central processes responsible for a gradation of muscle torques or joint angles are suggested on the basis of the mass-spring hypothesis. Two fundamental commands (reciprocal and co- activative ) involved in the control over antagonist muscles are defined in terms of shifts of the so-called invariant characteristics (muscle torque vs joint angle). Each of the commands is graded by a neuronal ensemble arranged in line. Excitation propagates along the line at a centrally established rate. As the wave front moves, the output ensemble neurons are tonically recruited, and they discretely contribute to the respective command according to the superposition principle. The terminal position of the wave front of the reciprocal command is responsible for the final angular limb position, whereas the wave velocity--for the movement speed. The coactivation command just enhances muscle stiffness for a time of the movement. The theory presented is sufficiently well-defined to yield a variety of specific and testable predictions. After insignificant modifications the theory may be referred to the generation of the eye and head movements, both slow and fast ones.