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Muscles in the post-stroke arm commonly demonstrate abnormal reflexes that result in increased position- and velocity-dependent resistance to movement. We sought to develop a reliable way to quantify mechanical consequences of abnormal neuromuscular mechanisms throughout the reachable workspace in the hemiparetic arm post-stroke. Survivors of hemiparetic stroke (HS) and neurologically-intact (NI) control subjects were instructed to relax as a robotic device repositioned the hand of their (hemiparetic) arm between several testing locations sampling the arm’s passive range of motion. During transitions, the robot induced motions at either the shoulder or elbow joint at speeds ranging from very slow (6 °/s) to fast (90 °/s). The robot held the hand at the testing location for at least 20 seconds after each transition. We recorded and analyzed hand force and electromyographic activities from selected muscles spanning the shoulder and elbow during and after transitions. Hand forces and electromyographic activities were small at all speeds and sample times in NI control subjects, but varied systematically by transport speed during and shortly after movement in the HS subjects. Velocity-dependent resistance to stretch diminished within two seconds after movement ceased in the hemiparetic arms. Hand forces and EMGs changed very little from 2 seconds after the movement ended onward, exhibiting dependence on limb posture but no systematic dependence on movement speed or direction. Although each HS subject displayed a unique field of hand forces and EMG responses across the workspace after movement ceased, the magnitude of steady-state hand forces was generally greater near the outer boundaries of the workspace than in the center of the workspace for the HS group but not the NI group. In the HS group, electromyographic activities exhibited abnormalities consistent with stroke-related decreases in the stretch reflex thresholds post-stroke. These observations were consistent across repeated testing days. Implications of the findings are discussed.

Background: We examined the validity and reliability of a short robotic test of upper limb proprioception, the Arm Movement Detection (AMD) test, which yields a ratio-scaled, objective outcome measure to be used for evaluating the impact of sensory deficits on impairments of motor control, motor adaptation and functional recovery in stroke survivors. Methods: Subjects grasped the handle of a horizontal planar robot, with their arm and the robot hidden from view. The robot applied graded force perturbations, which produced small displacements of the handle. The AMD test required subjects to respond verbally to queries regarding whether or not they detected arm motions. Each participant completed ten, 60s trials; in five of the trials, force perturbations were increased in small increments until the participant detected motion while in the others, perturbations were decreased until the participant could no longer detect motion. The mean and standard deviation of the 10 movement detection thresholds were used to compute a Proprioceptive Acuity Score (PAS). Based on the sensitivity and consistency of the estimated thresholds, the PAS quantifies the likelihood that proprioception is intact. Lower PAS scores correspond to higher proprioceptive acuity. Thirty-nine participants completed the AMD test, consisting of 25 neurologically intact control participants (NIC), seven survivors of stroke with intact proprioception in the more affected limb (HSS+P), and seven survivors of stroke with impaired or absent proprioception in the more affected limb (HSS-P). Results: Significant group differences were found, with the NIC and HSS+P groups having lower (i.e., better) PAS scores than the HSS-P group. A subset of the participants completed the AMD test multiple times and the AMD test was found to be reliable across repetitions. Conclusions: The AMD test required less than 15 min to complete and provided an objective, ratio-scaled measure of proprioceptive acuity in the upper limb. In the future, this test could be utilized to evaluate the contributions of sensory deficits to motor recovery following stroke.

We examined elbow muscle activities and movement kinematics to determine how subjects combine elementary control actions in performing movements with one and two trajectory segments. In reaching, subjects made a rapid elbow flexion to a visual target before stabilizing the limb with either a low or a higher level of elbow flexor/extensor coactivity (CoA), which was cued by target diameter. Cursor diameter provided real-time biofeedback of actual muscle CoA. In reversing, the limb was to reverse direction within the target and return to the origin with minimal CoA. We previously reported that subjects overshoot the goal when attempting a reversal after first having learned to reach accurately to the same target. Here we test the hypothesis that this hypermetria results because reversals co-opt the initial feedforward control action from the preceding trained reach, thereby failing to account for task-dependent changes in limb impedance induced by differences in flexor/extensor coactivity as the target is acquired (higher in reaching than reversing). Instructed increases in elbow CoA began mid-reach, thus increasing elbow impedance and reducing transient oscillations present in low CoA movments. Flexor EMG alone increased at movement onset. Test reversals incorporated the initial agonist activity of previous reaches but not the increased coactivity at the target, thus leading to overshoot. Moreover, we observed elevated coactivity in reversals upon returning to the origin even though coactivity in reaching was centered at the goal target. These findings refute the idea that the brain necessarily invokes distinct unitary control actions for reaches and reversals made to the same target. Instead, reaches and reversals share a common control action that initiates trajectories toward their target and another later control action that terminates movement and stabilizes the limb about its final resting posture, which differs in the two tasks.

The ability to perform accurate sequential movements is essential to normal motor function. Learning a sequential motor behavior is comprised of two basic components: explicit identification of the order in which the sequence elements should be performed and implicit acquisition of spatial accuracy for each element. Here we investigated the time course of learning of these components for a first sequence (SEQA) and their susceptibility to interference from learning a second sequence (SEQB). We assessed explicit learning with a discrete index, the number of correct anticipatory movements, and implicit learning with a continuous variable, spatial error, which decreased during learning without subject awareness. Spatial accuracy to individual sequence elements reached asymptotic levels only when the whole sequence order was known. Interference with recall of the order of SEQA persisted even when SEQB was learned 24 h after SEQA. However, there was resistance to interference by SEQB with increased initial training with SEQA. For implicit learning of spatial accuracy, SEQB interfered at 5 min but not 24 h after SEQA. As in the case of sequence order, prolonged initial training with SEQA induced resistance to interference by SEQB. We conclude that explicit sequence learning is more susceptible to anterograde interference and implicit sequence learning is more susceptible to retrograde interference. However, both become resistant to interference with saturation training. We propose that an essential feature of motor skill learning is the process by which discrete explicit task elements are combined with continuous implicit features of movement to form flawless sequential actions.

We examined control of the hand's trajectory (direction and shape) and final equilibrium position in horizontal planar arm movements by quantifying transfer of learned visuomotor rotations between two tasks that required aiming the hand to the same spatial targets. In a trajectory-reversal task (“slicing”), the hand reversed direction within the target and returned to the origin. In a positioning task (“reaching”), subjects moved the hand to the target and held it there; cursor feedback was provided only after movement ended to isolate learning of final position from trajectory direction. We asked whether learning acquired in one task would transfer to the other. Transfer would suggest that the hand's entire trajectory, including its endpoint, was controlled using a common spatial plan. Instead we found minimal transfer, suggesting that the brain used different representations of target position to specify the hand's initial trajectory and its final stabilized position. We also observed asymmetrical practice effects on hand trajectory, including systematic curvature of reaches made after rotation training and hypermetria of untrained slice reversals after reach training. These are difficult to explain with a unified control model, but were replicated in computer simulations that specified the hand's initial trajectory and its final equilibrium position. Our results suggest that the brain uses different mechanisms to plan the hand's initial trajectory and final position in point-to-point movements, that it implements these control actions sequentially, and that trajectory planning does not account for specific impedance values to be implemented about the final stabilized posture.

Stroke can lead to sensory deficits that impair functional control of arm movements. Here we describe a simple test of arm motion detection (AMD) that provides an objective, quantitative measure of movement perception related proprioceptive capabilities in the arm. Seven stroke survivors and thirteen neurologically intact control subjects performed the AMD test. In a series of ten trials that took less than 15 minutes to complete, participants used a two-button user interface to adjust the magnitude of hand displacements produced by a horizontal planar robot until the motions were just perceptible (i.e. on the threshold of detection). The standard deviation of movement detection threshold was plotted against the mean and a normative range was determined from the data collected with control subjects. Within this normative space, subjects with and without intact proprioception could be discriminated on a ratio scale that is meaningful for ongoing studies of degraded motor function. Thus, the AMD test provides a relatively fast, objective and quantitative measure of upper extremity proprioception of limb movement (i.e. kinesthesia).

A clinical triad of movement, behavioral, and cognitive disorders characterizes Huntington's disease (HD).1–3 However, the neurodegenerative changes of HD precede the onset of clinical signs and may be associated with subclinical alterations in brain physiology. For example, careful neuropsychological testing can detect abnormalities in presymptomatic carriers of the HD gene mutation (p-HD).4–7 In this study, we applied functional imaging methods with atrophy correction to elucidate the mechanisms that may underlie preclinical cognitive changes in HD gene carriers. In earlier imaging studies conducted in the resting state, we used a network approach to demonstrate that HD is associated with an abnormal spatial covariance pattern in both presymptomatic and symptomatic phases of disease.8,9 This reproducible regional topography is characterized by caudate/putamen and temporal hypometabolism associated with occipital hypermetabolism. Other investigators have found widespread cortical involvement in the early stages of symptomatic HD, in addition to the well-known subcortical features of the disease.10–12 Given metabolic abnormalities in brain regions subserving both motor and cognitive functions, we now sought to determine whether p-HD subjects acquire sequential information abnormally. We also sought to understand whether p-HD subjects use the same brain regions as control subjects while performing motor learning tasks, and if not, how the early preclinical pathology of HD alters the neural circuitry of learning-related pathways.

Background: Motor sequence learning is abnormal in PD. However, it is not known whether this defect is present during the earliest stages of the illness or whether it reflects specific limitations in dividing attention between cognitive and motor requirements. Methods: Fifteen patients with early stage PD and 10 age-matched and 9 younger normal controls moved the right dominant hand on a digitizing tablet to eight targets presented on a screen in synchrony with a tone at 1-second intervals. The tasks were as follows: 1) CCW—a timed-response task where targets appeared in a predictable counterclockwise order; 2) RAN—a reaction time task where targets were random and unpredictable; 3) SEQ—a task with multiple demands emphasizing explicit learning and target anticipation in which subjects learned a sequence while reaching for targets; and 4) VSEQ—subjects learned a visual sequence without moving. Results: CCW and RAN yielded similar results in all groups. In patients with PD, sequence learning was the same in SEQ and VSEQ and was slower compared to both control groups. In older controls, learning was faster in VSEQ than in SEQ, whereas younger controls learned equally fast in both tasks. Conclusions: Despite normal motor execution, the initial phases of sequence learning are impaired in early PD independent of task requirements, possibly reflecting reduced working memory. Learning was slower in older than younger controls only in tasks with multiple demands, presumably due to reduced attentional resources.

We used 15 O-labeled water and positron emission tomography to assess the effect of deep brain stimulation of the internal globus pallidus on motor sequence learning in Parkinson's disease. Seven right-handed patients were scanned on and off stimulation while they were performing a motor sequence learning task and a kinematically matched motor execution reference task. The scans were performed after a 12-hour medication washout. Stimulation parameters were adjusted for maximal motor improvement; experimental task parameters were held constant across stimulation conditions. Internal globus pallidus stimulation improved motor ratings by 37% (p < 0.01). During the sequence learning task, stimulation improved performance as measured by several correct anticipatory movements (p < 0.01) and by verbal report (p < 0.001). Concurrent positron emission tomography imaging during learning demonstrated significant (p < 0.01) increases in brain activation with stimulation in the left dorsolateral prefrontal cortex, bilaterally in premotor cortex, and in posterior parietal and occipital association areas. Stimulation did not affect the activity of these regions during the performance of the motor execution reference task. These findings suggest that internal globus pallidus deep brain stimulation can enhance the activity of prefrontal cortico-striato-pallidothalamic loops and related transcortical pathways. Improved sequence learning with stimulation may be directly related to these functional changes.

Neurological deficits after cerebrovascular accidents very frequently disrupt the kinematics of voluntary movements with the consequent impact in daily life activities. Robotic methodologies enable the quantitative characterization of specific control deficits needed to understand the basis of functional impairments and to design effective rehabilitation therapies. In a group of right handed chronic stroke survivors (SS) with right side hemiparesis, intact proprioception, and differing levels of motor impairment, we used a robotic manipulandum to study right arm function during discrete point-to-point reaching movements and reciprocal out-and-back movements to visual targets. We compared these movements with those of neurologically intact individuals (NI). We analyzed the presence of secondary submovements in the initial (i.e. outward) trajectory portion of the two tasks and found that the SS with severe impairment (FM < 30) presented arm submovements that differed notably not only from NI but also from those of SS with moderate arm impairment (FM 30-50). Therefore the results of this pilot study suggest that in SS arm kinematics vary significantly across differing levels of motor impairment. Our results support the development of rehabilitation therapies carefully tailored to each individual stroke survivor.

Background: Proprioception plays important roles in planning and control of limb posture and movement. The impact of proprioceptive deficits on motor function post-stroke has been difficult to elucidate due to limitations in current tests of arm proprioception. Common clinical tests only provide ordinal assessment of proprioceptive integrity (eg. intact, impaired or absent). We introduce a standardized, quantitative method for evaluating proprioception within the arm on a continuous, ratio scale. We demonstrate the approach, which is based on signal detection theory of sensory psychophysics, in two tasks used to characterize motor function after stroke. Methods: Hemiparetic stroke survivors and neurologically intact participants attempted to detect displacement- or force-perturbations robotically applied to their arm in a two-interval, two-alternative forced-choice test. A logistic psychometric function parameterized detection of limb perturbations. The shape of this function is determined by two parameters: one corresponds to a signal detection threshold and the other to variability of responses about that threshold. These two parameters define a space in which proprioceptive sensation post-stroke can be compared to that of neurologically-intact people. We used an auditory tone discrimination task to control for potential comprehension, attention and memory deficits. Results: All but one stroke survivor demonstrated competence in performing two-alternative discrimination in the auditory training test. For the remaining stroke survivors, those with clinically identified proprioceptive deficits in the hemiparetic arm or hand had higher detection thresholds and exhibited greater response variability than individuals without proprioceptive deficits. We then identified a normative parameter space determined by the threshold and response variability data collected from neurologically intact participants. By plotting displacement detection performance within this normative space, stroke survivors with and without intact proprioception could be discriminated on a continuous scale that was sensitive to small performance variations, e.g. practice effects across days. Conclusions: The proposed method uses robotic perturbations similar to those used in ongoing studies of motor function post-stroke. The approach is sensitive to small changes in the proprioceptive detection of hand motions. We expect this new robotic assessment will empower future studies to characterize how proprioceptive deficits compromise limb posture and movement control in stroke survivors.

We measured regional cerebral blood flow with H2 15O and positron emission tomography (PET) scanning at rest and during a motor task to study the mechanism of motor improvement induced by deep brain stimulation of the internal globus pallidus in Parkinson's disease. Six right-handed patients with Parkinson's disease were scanned while performing a predictable paced sequence of reaching movements and while observing the same screen displays and tones. PET studies were performed ON and OFF stimulation in a medication-free state. Internal globus pallidus deep brain stimulation improved off-state United Parkinson's Disease Rating Scale motor ratings (37%, p < 0.002) and reduced timing errors (movement onset time, 55%, p < 0.01) as well as spatial errors (10%, p < 0.02). Concurrent regional cerebral blood flow recordings revealed a significant enhancement of motor activation responses in the left sensorimotor cortex (Brodmann area [BA] 4), bilaterally in the supplementary motor area (BA 6), and in the right anterior cingulate cortex (BA 24/32). Significant correlations were evident between the improvement in motor performance and the regional cerebral blood flow changes mediated by stimulation. With internal globus pallidus deep brain stimulation, improved movement initiation correlated with regional cerebral blood flow increases in the left sensorimotor cortex and ventrolateral thalamus and in the contralateral cerebellum. By contrast, improved spatial accuracy correlated with regional cerebral blood flow increases in both cerebellar hemispheres and in the left sensorimotor cortex. These results suggest that internal globus pallidus deep brain stimulation may selectively improve different aspects of motor performance. Multiple, overlapping neural pathways may be modulated by this intervention.

We previously showed that inactivating the anterior interpositus nucleus in cats disrupts prehension; paw paths, normally straight and accurate, become curved, hypometric, and more variable. In the present study, we determined the joint kinematic and dynamic origins of this impairment. Animals were restrained in a hammock and trained to reach and grasp a cube of meat from a narrow food well at varied heights; movements were monitored using the MacReflex analysis system. The anterior interpositus nucleus was inactivated by microinjection of the GABA agonist muscimol (0.25–0.5 μg in 0.5 μL saline). For each joint, we computed the torque due to gravity, inertial resistance (termed self torque), interjoint interactions (termed interaction torque), and the combined effects of active muscle contraction and passive soft tissue stretch (termed generalized muscle torque). Inactivation produced significant reductions in the amplitude, velocity, and acceleration of elbow flexion. However, these movements continued to scale normally with target height. Shoulder extension was reduced by inactivation but wrist angular displacement and velocity were not. Inactivation also produced changes in the temporal coordination between elbow, shoulder, and wrist kinematics. Dynamic analysis showed that elbow flexion both before and during inactivation was produced by the combined action of muscle and interaction torque, but that the timing depended on muscle torque. Elbow interaction and muscle torques were scaled to target height both before and during inactivation. Inactivation produced significant reductions in elbow flexor interaction and muscle torques. The duration of elbow flexor muscle torque was prolonged to compensate for the reduction in flexor interaction torque. Shoulder extension was produced by extensor interaction and muscle torques both before and during inactivation. Inactivation produced a reduction in shoulder extension, primarily by reduced interaction torque, but without compensation. Wrist plantarflexion, which occurred during elbow flexion, was driven by plantarflexor interaction and gravitational torques both before and during inactivation. Muscle torque acted in the opposite direction with a phase lead to restrain the plantarflexor interaction torque. During inactivation, there was a reduction in plantarflexor interaction torque and a loss of the phase lead of the muscle torque. Our findings implicate the C1/C3 anterior interpositus zone of the cerebellum in the anticipatory control of intersegmental dynamics during reaching, which zone is required for coordinating the motions of the shoulder and wrist with those of the elbow. In contrast, this cerebellar zone does not play a role in scaling the movement to match a target.

Movement accuracy in normal subjects depends on feedforward commands based on representation in memory of spatial and biomechanical features. Here we ask whether memory deficits in Alzheimer's disease (AD) interfere with movement planning and execution. Nine AD patients and nine age-matched controls moved a cursor to targets without seeing their limb. Starting and target positions were always visible on a screen, while, during movement, cursor position was either visible or blanked. Patients' paths showed discontinuous segments and prolonged movement time; movement inaccuracy, which increased without visual feedback, correlated significantly with scores of disease severity, working memory and attention.

We have previously demonstrated that, in preparing themselves to aim voluntary impulses of isometric elbow force to unpredictable targets, subjects selected default values for amplitude and direction according the range of targets that they expected. Once a specific target appeared, subjects specified amplitude and direction through parallel processes. Amplitude was specified continuously from an average or central default; direction was specified stochastically from one of the target directions. Using the same timed response paradigm, we now report three experiments to examine how the time available for processing target information influences trajectory characteristics in two-degree-of-freedom forces and multijoint movements. We first sought to determine whether the specification of force direction could also take the form of a discrete stochastic process in pulses of wrist muscle force, where direction can vary continuously. With four equiprobable targets (two force amplitudes in each of two directions separated by 22 degrees or 90 degrees), amplitude was specified from a central default value for both narrow and wide target separations as a continuous variable. Direction, however, remained specified as a discrete variable for wide target separations. For narrow target separations, the directional distribution of default responses suggested the presence of both discrete and central values. We next examined point-to-point movements in a multijoint planar hand movement task with targets at two distances and two directions but at five directional separations (from 30 degrees to 150 degrees separation). We found that extent was again specified continuously from a central default. Direction was specified discretely from alternative default directions when target separation was wide and continuously from a central default when separation was narrow. The specification of both extent and direction evolved over a 200-ms time period beginning about 100 ms after target presentation. As in elbow force pulses, extent was specified progressively in both correct and wrong direction responses through a progressive improvement in the scaling of acceleration and velocity peaks to the target. On the other hand, movement time and hand path straightness did not change significantly in the course of specification. Thus, the specification of movement time and linearity, global features of the trajectories, are given priority over the specific values of extent and direction. In a third experiment, we varied the distances between unidirectional target pairs and found that movement extent is specified discretely, like direction, when the disparity in distances is large. The implications of these findings for contextual effects on trajectory planning are discussed. The independence of extent and direction specification and the prior setting of response duration and straightness provide critical support for the hypothesis that point-to-point movements are planned vectorially.

TO further understand visuomotor transformations in reaching, we compared adaptation to display rotation and altered gain in planar movements. Healthy subjects moved a cursor on a screen by moving an indicator on a horizontal digitizing tablet with their unseen hand. Adaptation to rotation was less complete and was accompanied by markedly increased directional variability. Adaptation training on a single target generalized broadly for gain change, but poorly for rotation. We propose that the difficulty in adapting to rotation arises from the substantial demands on short-term working memory imposed by the need to determine the new reference direction. Adaptation to gain change makes more modest demands on short-term memory to recalibrate the visuomotor scaling factor.

1. The aim of this study was to determine how vision of a cursor indicating hand position on a computer screen or vision of the limb itself improves the accuracy of reaching movements in patients deprived of limb proprioception due to large-fiber sensory neuropathy. In particular, we wished to ascertain the contribution of such information to improved planning rather than to feedback corrections. We analyzed spatial errors and hand trajectories of reaching movements made by subjects moving a hand-held cursor on a digitizing tablet while viewing targets displayed on a computer screen. The errors made when movements were performed without vision of their arm or of a screen cursor were compared with errors made when this information was available concurrently or prior to movement. 2. Both monitoring the screen cursor and seeing their limb in peripheral vision during movement improved the accuracy of the patients' movements. Improvements produced by seeing the cursor during movement are attributable simply to feedback corrections. However, because the target was not present in the actual workspace, improvements associated with vision of the limb must involve more complex corrective mechanisms. 3. Significant improvements in performance also occurred in trials without vision that were performed after viewing the limb at rest or during movements. In particular, prior vision of the limb in motion improved the ability of patients to vary the duration of movements in different directions so as to compensate for the inertial anisotropy of the limb. In addition, there were significant reductions in directional errors, path curvature, and late secondary movements. Comparable improvements in extent, direction, and curvature were produced when subjects could see the screen cursor during alternate movements to targets in different directions. 4. The effects of viewing the limb were transient and decayed during a period of minutes once vision of the limb was no longer available. 5. It is proposed that the improvements in performance produced after vision of the limb were mediated by the visual updating of internal models of the limb. Vision of the limb at rest may provide configuration information while vision of the limb in motion provides additional dynamic information. Vision of the cursor and the resulting ability to correct ongoing movements, however, is considered primarily to provide information about the dynamic properties of the limb and its response to neural commands.

1. This paper introduces a series of studies in which we analyze the impairments in a planar reaching task in human patients with severe proprioceptive deficits resulting from large-fiber sensory neuropathy. We studied three patients, all of whom showed absence of discriminative tactile sensation, position sense, and stretch reflexes in the upper extremities. Muscle strength was normal. We compared the reaching movements of the patients with those of normal control subjects. The purpose of this first paper was no characterize the spatial errors in these patients that result primarily from impairments in the planning and execution of movement rather than in feedback control. This was done by using a task in which visual feedback of errors during movement was prevented. 2. Subjects were instructed to move their hand from given starting positions of different targets on a horizontal digitizing tablet. Hand position and targets were displayed on a computer screen. Subjects could not see their hand, and the screen display of hand position was blanked at the signal to move. Thus visual feedback during movement could not be used to achieve accuracy. Movement paths were displayed as knowledge of results after each trial. 3. Compared with controls, the patients made large spatial errors in both movement direction and extent. Directional errors were evident from movement onset, suggesting that they resulted from improper planning. In addition, patients' hand paths showed large curves and secondary movements after initial stops. 4. The overall control strategy used by patients appeared the same as that used by controls. Hand trajectories were approximately bell shaped, and movement extent was controlled by scaling a trajectory waveform in amplitude and time. However, both control subjects and patients showed systematic errors in movement extent that depended on the direction of hand movement. In control subjects, these systematic dependencies of extent on direction were small, but in patients they produced large and prominent errors. Analysis of the hand trajectories revealed that errors were associated with differences in velocity and acceleration for movements in different directions. In an earlier study, we showed that in subjects with normal sensation that the dependence of acceleration and velocity on direction results from a failure to take the inertial properties of the limb into account in programming the initial trajectory. In control subjects, these differences in initial acceleration are partially compensated by direction-dependent variations in movement time.(ABSTRACT TRUNCATED AT 400 WORDS)

Stroke often results in both motor and sensory deficits, which may interact in the manifested functional impairment. Proprioception is known to play important roles in the planning and control of limb posture and movement; however, the impact of proprioceptive deficits on motor function has been difficult to elucidate due in part to the qualitative nature of available clinical tests. We present a quantitative and standardized method for evaluating proprioception in tasks directly relevant to those used to assess motor function. Using a robotic manipulandum that exerted controlled displacements of the hand, stroke participants were evaluated, and compared with a control group, in their ability to detect such displacements in a 2-alternative, forced-choice paradigm. A psychometric function parameterized the decision process underlying the detection of the hand displacements. The shape of this function was determined by a signal detection threshold and by the variability of the response about this threshold. Our automatic procedure differentiates between participants with and without proprioceptive deficits and quantifies functional proprioceptive sensation on a magnitude scale that is meaningful for ongoing studies of degraded motor function in comparable horizontal movements.

Effects of dual-responding on tracking performance after 49-hr of sleep deprivation (SD) were evaluated behaviorally and with functional magnetic resonance imaging (fMRI). Continuous visuomotor tracking was performed simultaneously with an intermittent color-matching visual detection task in which a pair of color-matched stimuli constituted a target and non-matches were non-targets. Tracking error means were binned time-locked to stimulus onset of the detection task in order to observe changes associated with dual-responding by comparing the error during targets and non-targets. Similar comparison was made with fMRI data. Our result showed that despite a significant increase in the overall tracking error post SD, from 20 pixels pre SD to 45 pixels post SD, error decreased to a minimum of about 25 pixels 0 to 6 s after dual-response. Despite an overall reduced activation post SD, greater activation difference between targets and non-targets was found post SD in task-related regions, such as the left cerebellum, the left somatosensory cortex, the left extrastriate cortex, bilateral precuneus, the left middle frontal gyrus, and the left motor cortex. Our results suggest that dual-response helps to alleviate performance impairment usually associated with SD. The duration of the alleviation effect was on the order of seconds after dual-responding.

This study examined the variability in movement end points in a task in which human subjects reached to targets in different locations on a horizontal surface. The primary purpose was to determine whether patterns in the variable errors would reveal the nature and origin of the coordinate system in which the movements were planned. Six subjects moved a hand-held cursor on a digitizing tablet. Target and cursor positions were displayed on a computer screen, and vision of the hand and arm was blocked. The screen cursor was blanked during movement to prevent visual corrections. The paths of the movements were straight and thus directions were largely specified at the onset of movement. The velocity profiles were bell-shaped, and peak velocities and accelerations were scaled to target distance, implying that movement extent was also programmed in advance of the movement. The spatial distributions of movement end points were elliptical in shape. The major axes of these ellipses were systematically oriented in the direction of hand movement with respect to its initial position. This was true for both fast and slow movements, as well as for pointing movements involving rotations of the wrist joint. Using principal components analysis to compute the axes of these ellipses, we found that the eccentricity of the elliptical dispersions was uniformly greater for small than for large movements: variability along the axis of movement, representing extent variability, increased markedly but nonlinearly with distance. Variability perpendicular to the direction of movement, which results from directional errors, was generally smaller than extent variability, but it increased in proportion to the extent of the movement. Therefore, directional variability, in angular terms, was constant and independent of distance. Because the patterns of variability were similar for both slow and fast movements, as well as for movements involving different joints, we conclude that they result largely from errors in the planning process. We also argue that they cannot be simply explained as consequences of the inertial properties of the limb. Rather they provide evidence for an organizing mechanism that moves the limb along a straight path. We further conclude that reaching movements are planned in a hand-centered coordinate system, with direction and extent of hand movement as the planned parameters. Since the factors which influence directional variability are independent of those that influence extent errors, we propose that these two variables can be separately specified by the brain.

Spectral analysis of single-unit discharges, multi-unit EMG and muscle force during voluntary quasi-sinusoidal isometric contractions of two hand muscles revealed corresponding modulations of the firing rates of motor units at the frequency of the force oscillation. These rate modulations were correlated; and they showed a phase advance over the force oscillation, which is consistent with a cause-effect relationship between changes in firing rate and variations in force. These effects, observed over wide ranges of modulation amplitudes and frequencies, confirm the role of rate coding in the generation of time-varying muscle contractions; and they support the idea that during voluntary contraction of a given muscle, the motoneuron pool is subject to a common drive.

1. We analyzed the performance of a simple pantomimed gesture in 2 patients with large-fiber sensory neuropathy and 11 control subjects to determine how proprioceptive deafferentation disrupts unconstrained multijoint movements. Both patients had near-total loss of joint position, vibration, and discriminative touch sensation in the upper extremities. Muscle strength remained intact. 2. Subjects performed a gesture similar to slicing a loaf of bread. In this gesture, the hand first moves outward from the body, reverses direction sharply, and then moves back toward the body. Accurate performance requires precise coordination between the shoulder and elbow joints during movement reversals. Movements were performed under two conditions: with eyes open and with eyes closed. Three dimensional shoulder, elbow, wrist, and hand trajectories were recorded on a WATSMART system. 3. When control subjects performed the gesture with their eyes closed, their wrist trajectories were relatively straight and individual cycles of motion were planar. Movements reversed direction sharply, such that outward and inward portions of the wrist path were closely aligned. Corresponding to this spatial profile, the reversals in movement direction at the shoulder joint, from flexion to extension, and at the elbow joint, from extension to flexion, were synchronous. 4. In contrast, when deafferented patients performed the gesture with their eyes closed, their wrist trajectories were highly curved and individual cycles were severely nonplanar. The wrist paths showed a characteristic anomaly during the reversal in movement direction, when elbow joint movement became transiently locked. Correspondingly, the movement reversals at the shoulder and elbow joints were severely temporally decoupled. 5. When patients were able to view their limbs during performance of this gesture there was significant improvement in the linearity and planarity of movements. However, the patients remained unable to synchronize the movements at the shoulder and elbow joints to produce spatially precise wrist paths. 6. We conclude that loss of proprioception disrupts interjoint coordination and discuss the hypothesis that this interjoint coordination deficit results from a failure to control the interaction forces that arise between limb segments during multijoint movements.

Subjects performed a continuous tracking concurrently with an intermittent visual detection task to investigate the existence of competition for a capacity-limited stage (a bottleneck stage). Both perceptual and response-related processes between the two tasks were examined behaviorally and the changes in brain activity during dual-tasking relative to single-task were also assessed. Tracking error and joystick speed were analyzed for changes that were time-locked to visual detection stimuli. The associated brain activations were examined with functional magnetic resonance imaging (fMRI). These were analyzed using mixed block and event-related models to tease apart sustained neural activity and activations associated with individual events. Increased tracking error and decreased joystick speed were observed relative to the target stimuli in the dual-task condition only, which supports the existence of a bottleneck stage in response-related processes. Neuroimaging data show decreased activation to target relative to non-target stimuli in the dual-task condition in the left primary motor and somatosensory cortices controlling right-hand tracking, consistent with the tracking interference observed in behavioral data. Furthermore, the ventral attention system, rather than the dorsal attention system, was found to mediate task coordination between tracking and visual detection.

There have recently been a number of advances in our knowledge of the organization of complex, multi-joint movements. Promising starts have been made in our understanding of how the motor system translates information about the location of external targets into motor commands encoded in a body-based coordinate system. Two simplifying strategies for trajectory control that are discussed are parallel specification of response features and the programming of equilibrium trajectories. New insights have also been gained into how neural systems process sensory information to plan and assist with task performance. A number of recent papers emphasize the feedforward use of sensory input, which is mediated through models of the external world, the body's physical plant, and the task structure. These models exert their influence at both reflex and higher levels and permit the preparation of predictive default parameters of trajectories as well as strategies for resolving task demands.

We report changes in the performance of a prehension task in the cat following focal inactivation of the rostrolateral subregion of the distal forelimb area of motor cortex (MCx) produced by muscimol microinjection. Animals reached into a cylindrical target to retrieve a morsel of food. Movements consisted of distinct lift and forward thrust phases following which the food was grasped and retrieved. In separate blocks of trials an obstacle was inserted in the path of the limb. Impact evoked an immediate compensatory trajectory change to bypass the obstruction and, on subsequent trials, an adaptive trajectory change to avoid impact. Inactivation produced three major defects: (1) uncompensated aiming biases to a location above the target; (2) loss of coordination of the grasp and food retrieval; and (3) impairment in trajectory adaptation to avoid impact of the limb with an obstacle. Thus, focal inactivation of the distal forelimb area of MCx produced disordered control of all forelimb joints. The impairment in trajectory adaptation and failure to compensate for aiming biases suggests that the MCx is important in motor learning.

We previously reported that the kinematics of reaching movements reflect the superimposition of two separate control mechanisms specifying the hand's spatial trajectory and its final equilibrium position. We now asked whether the brain maintains separate representations of the spatial goals for planning hand trajectory and final position. One group of subjects learned a 30° visuomotor rotation about the hand's starting point while performing a movement reversal task (“slicing”) in which they reversed direction at one target and terminated movement at another. This task required accuracy in acquiring a target mid-movement. A second group adapted while moving to—and stabilizing at—a single target (“reaching”). This task required accuracy in specifying an intended final position. We examined how learning in the two tasks generalized both to movements made from untrained initial positions and to movements directed toward untrained targets. Shifting initial hand position had differential effects on the location of reversals and final positions: Trajectory directions remained unchanged and reversal locations were displaced in slicing whereas final positions of both reaches and slices were relatively unchanged. Generalization across directions in slicing was consistent with a hand-centered representation of desired reversal point as demonstrated previously for this task whereas the distributions of final positions were consistent with an eye-centered representation as found previously in studies of pointing in three-dimensional space. Our findings indicate that the intended trajectory and final position are represented in different coordinate frames, reconciling previous conflicting claims of hand-centered (vectorial) and eye-centered representations in reach planning.

The paradigm task A→task B→task A, which varies the time interval between task A and task B, has been used extensively to investigate the consolidation of motor memory. Consolidation is defined as resistance to retrograde interference (interference by task B on initial learning of task A). Consolidation has been demonstrated for simple skills, motor sequencing, and learning of force fields. In contrast, evidence to date suggests that visuomotor learning does not consolidate. We have shown previously that adaptation to a 30° screen-cursor rotation is faster and more complete on relearning 24 hr later. This improvement is prevented if a 30° counter-rotation is learned 5 min after the original rotation. Here, we sought to identify conditions under which rotation learning becomes resistant to interference by a counter-rotation. In experiment 1, we found that interference persists even when the counter-rotation is learned 24 hr after the initial rotation. In experiment 2, we removed potential anterograde interference (interference by task B on relearning of task A) by introducing washout blocks before all of the learning blocks. In contrast to experiment 1, we found resistance to interference (i.e., consolidation) when the counter-rotation was learned after 24 hr but not after 5 min. In experiment 3, we doubled the amount of initial rotation learning and found resistance to interference even after 5 min. Our results suggest that persistent interference is attributable to anterograde effects on memory retrieval. When anterograde effects are removed, rotation learning consolidates both over time and with increased initial training.

Previous studies suggest that horizontal reaching movements are planned vectorially with independent specification of direction and extent. The transformation from visual to hand-centered coordinates requires the learning of a task-specific reference frame and scaling factor. We studied learning of a novel reference frame by imposing a screen-cursor rotation and learning of a scaling factor by imposing a novel gain. Previous work demonstrates that rotation and gain learning have different time courses and patterns of generalization. Here we used PET to identify and compare brain areas activated during rotation and gain learning, with a baseline motor-execution task as the subtracted control. Previous work has shown that the time courses of rotation and gain adaptation have a short rapid phase followed by a longer slow phase. We therefore also sought to compare activations associated with the rapid and slower phases of adaptation. We isolated the rapid phase by alternating opposite values of the rotation or gain every 16 movements. The rapid phase of rotation adaptation activated the preSMA. More complete adaptation to the rotation activated right ventral premotor cortex, right posterior parietal cortex, and the left lateral cerebellum. The rapid phase of gain learning only activated subcortical structures: bilateral putamen and left cerebellum. More complete gain learning failed to show any significant activation. We conclude that the time course of rotation adaptation is paralleled by a frontoparietal shift in activated cortical regions. In contrast, early gain adaptation involves only subcortical structures, which we suggest reflects a more automatic process of contextual recalibration of a scaling factor.

Previous positron emission tomography (PET) studies have shown that nonmanifesting carriers of the DYT1 dystonia mutation express an abnormal pattern of resting glucose metabolism. To determine whether motor behavior is impaired in these subjects, we compared movement and sequence learning in 12 clinically unaffected DYT1 carriers with 12 age-matched controls. Regional differences in brain function during task performance were assessed with simultaneous H2 15 O/PET. We found that motor performance was similar in the DYT1 and control groups, with no significant differences in movement time and spatial accuracy measured during each of the tasks. In contrast, sequence learning was reduced in gene carriers relative to controls (p < 0.01). PET imaging during motor execution showed increased activation in gene carriers (p < 0.001, uncorrected) in the left premotor cortex and right supplementary motor area, with concomitant reduction in the posterior medial cerebellum. During sequence learning, activation responses in DYT1 carriers were increased in the left ventral prefrontal cortex, and lateral cerebellum. These findings suggest that abnormalities in motor behavior and brain function exist in clinically nonmanifesting DYT1 carriers. Although localized increases in neural activity may enable normal movement execution in these subjects, this mechanism may not compensate for their defect in sequence learning. Ann Neurol 2003;54:102–109 Primary torsion dystonia is a hyperkinetic movement disorder associated with several genetic variants. 1 One genetic variant is the GAG deletion in the DYT1 gene at 9q34. This mutation commonly causes early limb onset primary torsion dystonia and is inherited as an autosomal dominant condition with incomplete (30 – 40%) clinical penetrance. 2 Using 18 F-fluorodeoxyglucose and positron emission tomography (PET), we had identified previously abnormalities in resting glucose utilization in the putamen/globus pallidus, supplementary motor area (SMA), and cerebellum of DYT1 mutation carriers, including clinically nonpenetrant subjects in whom signs and symptoms of dystonia are lacking. 3,4 Given the presence of metabolic abnormalities in key elements of motor control pathways, we sought to determine whether clinically unaffected mutation carriers also display abnormalities in the trajectory control of reaching movements and in learning motor sequences. To this end, we utilized a series of tasks with similar and welldefined kinematics to assess performance during motor execution and learning. 5,6 Simultaneous PET recordings were acquired during task performance to determine whether specific abnormalities in brain activation responses were present within corticostriatopallidothalamocortical loops and related pathways. 7

In a previous H(2) (15)O/PET study of motor sequence learning, we used principal components analysis (PCA) of region of interest (ROI) data to identify performance‐related activation patterns in normal subjects and patients with Parkinson's disease (PD). In the present study, we determined whether these patterns predicted learning performance in subsequent normal and untreated PD cohorts. Using a voxel‐based PCA approach, we correlated the changes in network activity that occurred during antiparkinsonian treatment and their relationship to learning performance. We found that the previously identified ROI‐based patterns correlated with learning performance in the prospective normal (P < 0.01) and untreated PD (P < 0.05) cohorts. Voxel analysis revealed that target retrieval was related to a network characterized by bilateral activation of the dorsolateral prefrontal, premotor and anterior cingulate cortex, the precuneus, and the occipital association areas as well as the right ventral prefrontal and inferior parietal regions. Target acquisition was associated with a different network involving activation of the caudate, putamen, and right dentate nucleus, as well as the left ventral prefrontal and inferior parietal areas. Antiparkinsonian therapy gave rise to changes in retrieval performance that correlated with network modulation (P < 0.01). Increases in network activation and learning performance occurred with internal pallidal deep brain stimulation (GPi DBS); decrements in these measures were present with levodopa. Our findings suggest that network analysis of activation data can provide stable descriptors of learning performance. Network quantification can provide an objective means of assessing the effects of therapy on cognitive functioning in neurodegenerative disorders. Hum. Brain Mapping 11:197–211, 2003. © 2003 Wiley‐Liss, Inc.

To examine the variations in regional cerebral blood flow during execution and learning of reaching movements, we employed a family of kinematically and dynamically controlled motor tasks in which cognitive, mnemonic and executive features of performance were differentiated and characterized quantitatively. During 15O-labeled water positron emission tomography (PET) scans, twelve right-handed subjects moved their dominant hand on a digitizing tablet from a central location to equidistant targets displayed with a cursor on a computer screen in synchrony with a tone. In the preceding week, all subjects practiced three motor tasks: 1) movements to a predictable sequence of targets; 2) learning of new visuomotor transformations in which screen cursor motion was rotated by 30 degrees -60 degrees; 3) learning new target sequences by trial and error, by using previously acquired routines in a task placing heavy load on spatial working memory. The control condition was observing screen and audio displays. Subtraction images were analyzed with Statistical Parametric Mapping to identify significant brain activation foci. Execution of predictable sequences was characterized by a modest decrease in movement time and spatial error. The underlying pattern of activation involved primary motor and sensory areas, cerebellum, basal ganglia. Adaptation to a rotated reference frame, a form of procedural learning, was associated with decrease in the imposed directional bias. This task was associated with activation in the right posterior parietal cortex. New sequences were learned explicitly. Significant activation was found in dorsolateral prefrontal and anterior cingulate cortices. In this study, we have introduced a series of flexible motor tasks with similar kinematic characteristics and different spatial attributes. These tasks can be used to assess specific aspects of motor learning with imaging in health and disease.

This study examined the effects of selective inactivation of the cerebellar nuclei in the cat on the control of multijoint trajectories and trajectory adaptation to avoid obstacles. Animals were restrained in a hammock and trained to perform a prehension task in which they reached to grasp a small cube of meat from a narrow food well. To examine trajectory adaptation, reaching was obstructed by placing a horizontal bar in the limb's path. Inactivation was produced by microinjection of the GABA agonist muscimol (0.25–1.0 μg in 1 μL saline). Fastigial nucleus inactivation produced a severe impairment in balance and in head and trunk control but no effect on reaching and grasping. Dentate inactivation slowed movements significantly and produced a significant increase in tip path curvature but did not impair reaching and grasping. Selective inactivation of the anterior and posterior interpositus nuclei did not impair grasping but severely decreased the accuracy of reaching movements and produced different biases in wrist and paw paths. Anterior interpositus inactivation produced movement slowing (wrist speed) and under-reaching to the food well. Wrist and tip paths showed anterior biases and became more curved. Also animals could no longer make anticipatory adjustments in limb kinematics to avoid obstructions but sensory-evoked corrective responses were preserved. Posterior interpositus inactivation produced a significant increase in wrist speed and overreaching. Wrist and tip paths showed a posterior bias and became more curved, although in a different way than during anterior interpositus inactivation. Posterior interpositus inactivation did not impair trajectory adaptation to reach over the obstacle. During inactivation of either interpositus nucleus, all measures of kinematic temporal and spatial variability increased with somewhat greater effects being produced by anterior interpositus inactivation. We discuss our results in relation to the hypothesis that anterior and posterior interpositus have different roles in trajectory control, related possibly to feed-forward use of cutaneous and proprioceptive inputs, respectively. The loss of adaptive reprogramming during anterior interpositus inactivation further suggests a role in motor learning. Comparison with results from our earlier motor cortical study shows that the distinctive impairments produced by inactivation of these two nuclei are similar to those produced by selective inactivation of different zones in the forelimb area of rostral motor cortex. Our findings are consistent with the hypothesis that there are separate functional output channels from the anterior and posterior interpositus nuclei to rostral motor cortex for distinct aspects of trajectory control and, from anterior interpositus alone, for trajectory adaptation.

The planning of visually guided reaches is accomplished by independent specification of extent and direction. We investigated whether this separation of extent and direction planning for well practiced movements could be explained by differences in the adaptation to extent and directional errors during motor learning. We compared the time course and generalization of adaptation with two types of screen cursor transformation that altered the relationship between hand space and screen space. The first was a gain change that induced extent errors and required subjects to learn a new scaling factor. The second was a screen cursor rotation that induced directional errors and required subjects to learn new reference axes. Subjects learned a new scaling factor at the same rate when training with one or multiple target distances, whereas learning new reference axes took longer and was less complete when training with multiple compared with one target direction. After training to a single target, subjects were able to transfer learning of a new scaling factor to previously unvisited distances and directions. In contrast, generalization of rotation adaptation was incomplete; there was transfer across distances and arm configurations but not across directions. Learning a rotated reference frame only occurred after multiple target directions were sampled during training. These results suggest the separate processing of extent and directional errors by the brain and support the idea that reaching movements are planned as a hand-centered vector whose extent and direction are established via learning a scaling factor and reference axes.

Psychophysical studies of reaching movements suggest that hand kinematics are learned from errors in extent and direction in an extrinsic coordinate system, whereas dynamics are learned from proprioceptive errors in an intrinsic coordinate system. We examined consolidation and interference to determine if these two forms of learning were independent. Learning and consolidation of two novel transformations, a rotated spatial reference frame and altered intersegmental dynamics, did not interfere with each other and consolidated in parallel. Thus separate kinematic and dynamic models were constructed simultaneously based on errors computed in different coordinate frames, and possibly, in different sensory modalities, using separate working-memory systems. These results suggest that computational approaches to motor learning should include two separate performance errors rather than one.

Intersegmental dynamics are controlled by sequential anticipatory, error correction, and positional control mechanisms. The purpose of this study is to examine the mechanisms underlying control of intersegmental dynamics during reaching movements. Two experiments were conducted to determine the relative contributions of anticipatory and somatosensory feedback mechanisms in controlling intersegmental dynamics and whether adaptation to novel intersegmental dynamics generalizes across a range of movement directions. The mechanisms used to control interaction torques were examined by altering the inertial load of the forearm. Movements were restricted to the shoulder and elbow and supported on a horizontal plane by a frictionless air-jet system. Subjects made rapid out-and-back movements over a target line presented on a computer screen. The screen cursor disappeared at movement onset, and hand paths were displayed after each movement. After subjects adapted to a novel inertial configuration, the position of an attached mass was changed on pseudorandom trials. During these “surprise” trials, movements were initiated with the torque patterns appropriate to the previously learned inertial condition. As a result, characteristic errors in initial movement direction were predicted by an open-looped forward simulation. After these errors occurred, feedback mediated changes in torque emerged that, surprisingly, further decreased the accuracy of movement reversals. Nevertheless at the end of movement, the hand consistently returned to the starting position. It is plausible that the final position was determined completely by feedback-mediated changes in torque. In a second experiment, adaptation to a novel inertial load during movements made in a single direction showed limited transfer across a range of directions. These findings support and extend those of previous reports, which indicated combined anticipatory and postural mechanisms to coordinate rapid reaching movements. The current results indicate a three-stage control system that sequentially links anticipatory, error correction, and postural mechanisms to control intersegmental dynamics. Our results, showing limited generalization across directions, are consistent with previous reports examining adaptation to externally applied forces and extend those findings to indicate that the nervous system uses sensory information to recalibrate internal representations of the musculoskeletal apparatus itself.

Publisher Summary The kinematic features of grasp depend primarily on the shape and orientation of the object. Preparation for grasp is initiated during transport and preshapes the hand to the contours of the object before reaching it. Taken as a whole, analyses of hand trajectories and reaching errors in humans support the idea that planning takes place in a neural representation that preserves the spatial relationships of hand and target, as well as the location of potential obstacles that could constrain hand path. In controlling the motions of a multiarticulated limb, however, muscle contraction acts in concert with additional forces, which vary during movement in still more complex ways. At rotating joints, the actions of these forces are expressed as torques. The chapter elaborates the regulation of distal joint angles in preparation for target entry and grasping and suggests that the loads imposed by interaction torques during reaching might be countered automatically, simply by stiffness of the joints, as the hand traverses a planned equilibrium trajectory.

1. The dependence of directional biases in reaching movements on the initial position of the hand was studied in normal human subjects moving their unseen hand on a horizontal digitizing tablet to visual targets displayed on a vertical computer screen. 2. When initial hand positions were to the right of midline, movements were systematically biased clockwise. Biases were counterclockwise for starting points to the left. Biases were unaffected by the screen location of the starting and target positions. 3. Vision of the hand in relation to the target before movement, as well as practice with vision of the cursor during the movement, temporarily eliminated these biases. The spatial organization of the biases suggests that, without vision of the limb, the nervous system underestimates the distance of the hand from an axis or plane that includes its most common operating location. 4. To test the hypothesis that such an underestimate might represent an adaptation to a local area of work space or range effect, subjects were trained to reach accurately from right or left positions. After training, movements initiated from other locations, including ones that were previously error free, showed new biases that again represented underestimates of the distance of the initial hand position from the new trained location. 5. We conclude that hand path planning is dependent on learned representations of the location of the hand in the work space.

1. We recently showed that patients lacking proprioceptive input from their limbs have particular difficulty performing multijoint movements. In a pantomimed slicing gesture requiring sharp reversals in hand path direction, patients showed large hand path distortions at movement reversals because of failure to coordinate the timing of the separate reversals at the shoulder and elbow joints. We hypothesized that these reversal errors resulted from uncompensated effects of inertial interactions produced by changes in shoulder joint acceleration that were transferred to the elbow. We now test this hypothesis and examine the role of proprioceptive input by comparing the motor performance of five normal subjects with that of two patients with large-fiber sensory neuropathy. 2. Subjects were to trace each of six template lines presented randomly on a computer screen by straight overlapping out-and-back movements of the hand on a digitizing tablet. The lines originated from a common starting position but were in different directions and had different lengths. Directions and lengths were adjusted so that tracing movements would all require the same elbow excursion, whereas shoulder excursion would vary. The effects of varying interaction torques on elbow kinematics were then studied. The subject's dominant arm was supported in the horizontal plane by a low-inertia brace equipped with ball bearing joints and potentiometers under the elbow and shoulder. Hand position was monitored by a magnetic pen attached to the brace 1 cm above a digitizing tablet and could be displayed as a screen cursor. Vision of the subject's arm was blocked and the screen cursor was blanked at movement onset to prevent visual feedback during movement. Elbow joint torques were calculated from joint angle recordings and compared with electromyographic recordings of elbow joint musculature. 3. In control subjects, outward and inward paths were straight and overlapped the template lines regardless of their direction. As prescribed by the task, elbow kinematics remained the same across movement directions, whereas interaction torques varied substantially. The timing of the onsets of biceps activity and the offsets of triceps activity during elbow flexion varied systematically with direction-dependent changes in interaction torques. Controls exploited or dampened these interaction torques as needed to meet the kinematic demands of the task. 4. In contrast, the patients made characteristic errors at movement reversals that increased systematically across movement directions. These reversal errors resulted from improper timing of elbow and shoulder joint reversals.(ABSTRACT TRUNCATED AT 400 WORDS)

This study analyzed changes in the performance of a reaching task and its adaptive modification produced by reversible inactivation of three sites within the forelimb representation of the motor cortex (MCx, area 4 gamma) in five cats by microinjections of muscimol. Two sites were located in the lateral MCx, rostral (RL-MCx) and caudal (CL-MCx) to the end of the cruciate sulcus, where intracortical microstimulation (ICMS) produced contraction of the most distal muscles. The third site was located more medially, in the anterior sigmoid gyrus (RM-MCx) where ICMS primarily produced contraction of more proximal muscles. The task required the animals to reach into a horizontal target well, located in front of them at one of three possible heights, to grasp and retrieve a small piece of food. The height of the reach was primarily achieved by elbow flexion. Grasping consisted primarily of digit flexion, and food retrieval consisted of forearm supination and shoulder extension. In some blocks of trials, an obstacle was placed in the path of the limb to assess the animal's ability to adaptively adjust the kinematic characteristics of their response trajectory. In normal animals, contact with the bar on the first trial triggered a corrective response at short latency that allowed the paw to circumvent the bar. On all subsequent trials, the trajectory was adapted to prevent contact with the obstacle, with a safety margin of about 1 cm. Inactivation at all sites produced a slowing of movement, a protracted and extended forelimb posture, and increased variability of initial limb position. In addition, inactivation of RL-MCx immediately produced systematic reaching errors, consisting of hypermetric movements, as well as impaired grasping and food retrieval. The degree of hypermetria was similar for all target heights and was not associated with alterations in trajectory control. During inactivation, animals did not compensate for the hypermetria by reducing paw path elevation, suggesting a defect in kinematic planning or in adaptive control. This was confirmed by finding that trajectory adaptation to avoid bar contact was impaired during RL-MCx inactivation. The short latency corrective response, triggered by contact of the limb with the obstacle was, however, preserved. Inactivation of CL-MCx did not impair aiming, grasping, or adaptation immediately after injection. However, impairments occurred after about 1 h postinjection, and at that time mimicked the effects of RL-MCx inactivation.(ABSTRACT TRUNCATED AT 400 WORDS)

This study examined changes in the performance of a single-joint, elbow task produced by reversible inactivation of local regions within the proximal forelimb representation in area 4 gamma of motor cortex (MCx) and the red nucleus (RN) of the cat. Inactivation was carried out by microinjecting lidocaine, gamma-aminobutyric acid, or muscimol into sites where microstimulation evoked contraction of elbow muscles. Reaction time, amplitude, and speed (velocity or dF/dt) of position and force responses elicited during inactivation were compared to control values obtained immediately prior to inactivation. In addition, we assessed qualitatively the effects of inactivation on reaching, placing reactions, and proprioceptive responses to imposed limb displacement. In the single-joint task, injections in MCx did not increase reaction time (simple or choice) and produced modest and inconsistent reductions in response amplitude (mean -8%) and speed (mean -19%). In contrast, injections of the same amounts of inactivating agents in the forelimb representation of RN consistently increased reaction time (34.4%), and increased the reaction time coefficient of variability (32%). There were small reductions in response amplitude (-4%) and speed (-10%) which were less than those produced by MCx inactivation. During reaching, however, these same injections in MCx and RN produced a substantial loss of accuracy. For MCx, this was due, in part, to systematic hypometria: for RN, inaccuracy resulted from increased variability in paw paths. Placing reactions and corrective responses to imposed limb displacements were also depressed by the cortical and rubral injections. Our results suggest that the forelimb representation in RN plays a role in the initiation of the single-joint, elbow tracking response examined here. The RN may mediate cerebellar regulation of response timing, a function that is likely to be important for interjoint coordination. Although neurons in the forelimb representations of MCx may contribute to force generation in single-joint movements, their contribution to multijoint control appears to be more important and is examined in the subsequent report (Martin and Ghez 1993).