Complex-spike activity of cerebellar Purkinje cells related to wrist tracking movement in monkey

Four rhesus monkeys were trained to perform visually guided wrist tracking movements (50). While they performed tasks by wrist flexion or extension from a neutral position, simple-spike (SS) and complex-spike (CS) discharges of a single Purkinje cell (P-cell) were recorded from intermediate and lateral parts of cerebellar hemispheres (lobules IV to VI) ipsilateral to the task-performing wrist. Of approximately 400 P-cells observed, 215 (54%) significantly increased or decreased their SS discharge rate during task performance (task-related P-cells). Of these, 161 were selected for analysis of CS activity; in these P-cells, we could reliably discriminate between CS and background SS by a spike discriminator. The 161 P-cells were further classified into response locked (n = 65) and poorly locked (n = 96) cells according to temporal coupling of the SS frequency modulation to the onset of wrist movements. About 60% of the response-locked P-cells showed a phasic increase (statistical significance level: P less than 0.01) of CS firing rate at the onset of wrist tracking movement. In a few P-cells, a phasic decrease (statistically insignificant) of CS firing rate was observed with the wrist movement. In most P-cells, an increase of CS firing rate was observed with both rapid- and slow-tracking wrist movements. The increase was larger with faster step-tracking movement than with slower ramp-tracking movement. In most P-cells, the CS activity increased with both wrist flexion and extension; in some cells, however, it increased only with either flexion or extension. In most of the response-locked P-cells, the increase of CS firing rate occurred during motor time, i.e., after the onset of the EMG change in prime movers and before the beginning of wrist tracking movement. The increase occurred phasically at the onset and/or at the recovery phase of SS frequency modulation. At neutral wrist position, the maintained frequency of the CS was 0.72 +/- 0.29 CS/s (mean and SD for 161 task-related P-cells). Compared with the frequency at neutral position, the CS frequency did not change tonically during maintained flexed or extended wrist position in any response-locked P-cells. There was no increase of CS firing rate when the monkey returned the handle to center position after completing the tracking task, even in P-cells that had shown a significant increase of CS activity during tracking.(ABSTRACT TRUNCATED AT 400 WORDS)     

Purkinje cell complex and simple spike changes during a voluntary arm movement learning task in the monkey.

1. To evaluate the role of the cerebellum during improvement of voluntary motor performance over time, the discharge of 88 Purkinje cells in the intermediate and lateral cerebellum of two primates (Macaca mulatta) was investigated during a motor learning task involving visually guided arm movements. The animals were trained to move a draftsman's style manipulandum over a horizontally placed video screen. The animals were required to move a cursor from the start box to one of four target boxes by movement of the manipulandum. Errors were introduced into the movement by altering the visual feedback loop, changing the gain between the cursor movement and the hand movement. When a novel gain was presented over 100-200 movement trials, the animals adapted the movements to the new gain. The animals used a strategy of scaling the amplitude and velocity of the initial phase of the movement while keeping the time to peak velocity constant. 2. The learning paradigm consisted of an initial control phase with 35-100 trials at the gain of 1.0. The next 100-200 trials, the learning phase, were presented at one of four gains (0.6, 0.75, 1.5, 2.0). Lastly, a testing phase involved 80% of 100 trials at the learned gain and 20% of the trials randomly interspersed at the control gain of 1.0. An additional "distance control" was used in most experiments to control for the movement scaling associated with learning. In this series of movements using a gain of 1.0, the target box was placed at the distance and direction the hand would have to move in the adapted state. Two aspects of the kinematics were the same for the distance control and the movement at the learned gain: movement amplitude and time to peak velocity. There were, however, slight differences in the peak velocity attained. For gains < 1.0, the peak velocity of the learned task was 14-20% lower than the distance controls, and for gains > 1.0, it was 10-18% higher. 3. After implantation of chronic unit recording hardware, Purkinje cell simple and complex spike discharge was recorded extracellularly during the learning task. The cells were located primarily in the ipsilateral intermediate zone or nearby hemisphere of lobules V and VI. Simple and complex spike histograms, as well as averages of the hand displacement and velocity profiles, were calculated for each phase of the paradigm. To determine the time course of any changes, the learning trials were subdivided into three equal phases.(ABSTRACT TRUNCATED AT 400 WORDS)     

Timing and direction selectivity of subthalamic and pallidal neurons in patients with Parkinson disease

Current models of basal ganglia function suggest that some manifestations of Parkinson disease (PD) arise from abnormal activity and decreased selectivity of neurons in the subthalamic nucleus (STN) and globus pallidus internus (Gpi). Our goal was to examine the timing and direction selectivity of neuronal activity relative to visually guided movements in the STN and Gpi of patients with PD. Recordings were made from 152 neurons in the STN and 33 neurons in the Gpi of awake subjects undergoing surgery for PD. Corresponding EMG data were obtained for half the cells. We employed a structured behavioral task in which the subjects used a joystick to guide a cursor to one of four targets displayed on a monitor. Each direction was tested over multiple trials. Movement-related modulation of STN activity began on average 264±10 ms before movement initiation and 92±13 ms before initial EMG activity, while modulation of Gpi activity began 204±21 ms before overt movement initiation. In the STN, 40% of cells demonstrated perimovement activity, and of these 64% were directionally selective. In Gpi, 45% of cells showed perimovement activity of which 80% were selective. In both nuclei, directionally selective cells had significantly lower baseline firing rates than nonselective cells (41±5 vs 59±4 spikes/s in STN, and 50±9 vs 74±15 spikes/s in Gpi). These results suggest that STN activity occurs earlier than previously reported, and that higher neuronal firing rates maybe associated with decreased direction selectivity in PD patients.     

Motor cortical activity during drawing movements: single-unit activity during sinusoid tracing.

1. This study examines the neuronal activity of motor cortical cells associated with the production of arm trajectories during drawing movements. Three monkeys were trained to perform two tasks. The first task ("center----out" task) required the animal to move its arm in different directions from a center start position to one of eight targets spaced at equal angular intervals and equal distances from the origin. Movements to each target were in a constant direction, and the average rate of neuronal discharge with movements to different targets varied in a characteristic pattern. A cosine tuning function was used to map each cell's discharge rate to the direction of arm movement. This function spanned all movement directions, with a peak firing rate in the cell's preferred direction. 2. The second task ("tracing" task) required the animal to trace curved figures consisting of sine waves of different spatial frequencies and amplitudes. Both the speed and direction changed continuously throughout these movements. The cosine tuning function derived from the center----out task was used to model the activity of the cell during the tracing of sinusoids in the second task. Sinusoidal data were divided into 20-ms bins; instantaneous direction, speed, and discharge rate were analyzed bin by bin. This provided a way to compare directly the tuning parameters during a task with constant direction to a task where the direction varied continuously. 3. Movement direction as it changed during the tracing task was an important factor in the discharge pattern of cells that had discharge patterns that could be represented by the cosine tuning function. 4. The modulation of discharge rate during figure tracing depended on both the cell's preferred direction and the orientation of the figure. The activity of cells with preferred directions perpendicular to the axis of the sinusoidal figure was most modulated, whereas the activity of those cells with preferred directions aligned to the figure's axis was least modulated. 5. The cells with modulated activity tended to have firing rates that differed from the predicted cosine tuning function during the sinusoidal movements for those portions of the trajectory where the movement direction was in the cell's preferred direction. 6. Finger speed during figure tracing varied inversely with path curvature with the same relation that has been found during human drawing. To assess the relation of instantaneous speed to discharge rate, the component of the discharge pattern related to direction was subtracted from the total discharge.(ABSTRACT TRUNCATED AT 400 WORDS)     

Role of the cerebellum in visuomotor coordination

The initiation of coupled eye and arm movements was studied in six patients with mild cerebellar dysfunction and in six age-matched control subjects. The experimental paradigm consisted of 40 deg step-tracking elbow movements made under different feedback conditions. During tracking with the eyes only, saccadic latencies in patients were within normal limits. When patients were required to make coordinated eye and arm movements, however, eye movement onset was significantly delayed. In addition, removal of visual information about arm versus target position had a pronounced differential effect on movement latencies. When the target was extinguished for 3 s immediately following a step change in target position, both eye and arm onset times were further prolonged compared to movements made to continuously visible targets. When visual information concerning arm position was removed, onset times were reduced. Eye and arm latencies in control subjects were unaffected by changes in visual feedback. The results of this study clearly demonstrate that, in contrast to earlier reports of normal saccadic latencies associated with cerebellar dysfunction, initiation of both eye and arm movements is prolonged during coordinated visuomotor tracking thus supporting a coordinative role for the cerebellum during oculo-manual tracking tasks.     

Effects of altering initial position on movement direction and extent

The purpose of this study was to examine the relative influence of initial hand location on the direction and extent of planar reaching movements. Subjects performed a horizontal-plane reaching task with the dominant arm supported above a table top by a frictionless air-jet system. A start circle and a target were reflected from a horizontal projection screen onto a horizontally positioned mirror, which blocked the subject's view of the arm. A cursor, representing either actual or virtual finger location, was only displayed between each trial to allow subjects to position the cursor in the start circle. Prior to occasional "probe trials," we changed the start location of the finger relative to the cursor. Subjects reported being unaware of the discrepancy between cursor and finger. Our results indicate that regardless of initial hand location, subjects did not alter the direction of movement. However, movement distance was systematically adjusted in accord with the baseline target position. Thus when the hand start position was perpendicularly displaced relative to the target direction, neither the direction nor the extent of movement varied relative to that of baseline. However, when the hand was displaced along the target direction, either anterior or posterior, movements were made in the same direction as baseline trials but were shortened or lengthened, respectively. This effect was asymmetrical such that movements from anterior displaced positions showed greater distance adjustment than those from posterior displaced positions. Inverse dynamic analysis revealed substantial changes in elbow and shoulder muscle torque strategies for both right/left and anterior/posterior pairs of displacements. In the case of right/left displacements, such changes in muscle torque compensated changes in limb configuration such that movements were made in the same direction and to the same extent as baseline trials. Our results support the hypothesis that movement direction is specified relative to an origin at the current location of the hand. Movement extent, on the other hand, appears to be affected by the workspace learned during baseline movement experience.     

Role of primate basal ganglia and frontal cortex in the internal generation of movements

Summary This study is a part of a project investigating neuronal activity in the basal ganglia and frontal cortex and describes externally and internally induced preparatory activity in the supplementary motor area (SMA), which forms a closed neuronal loop with the striatum. Monkeys made self-initiated arm reaching movements toward a constant target in the absence of phasic external stimuli. In separate blocks of trials, animals performed in a delayed go no-go task in which an instruction cue prepared for subsequent movement or no-movement to a trigger stimulus. A total of 328 neurons were tested in the delay task. Of these, 91 responded transiently to the instruction light with a median latency of 262 ms. Three quarters of these responses were restricted to the instruction preparing for arm movement, as opposed to with-holding it, and thus may be involved in movement preparation processes. Sustained activation during the instruction-trigger interval was found for 67 neurons and occurred nearly exclusively in movement trials. Activation usually increased gradually after the cue and ended abruptly upon movement onset and thus could be related to the setting and maintenance of processes underlying the preparation of movement. Time-locked responses to the trigger stimulus were found in 38 neurons and were usually restricted to movement trials (median latency 80 ms). Activity time-locked to movement execution occurred in 67 neurons, beginning up to 252 ms before movement onset. A total of 266 neurons were tested with self-initiated arm movements. Of these, 43 showed premovement activity beginning 610–3030 ms before movement onset (median 1430 ms). The activity increased slowly and reached its peak at 370 ms before movement onset. It ended before movement onset or continued until the arm began to move or reached the target. This activity appears to reflect neuronal processes related to the internal generation of movements. Two thirds of activations preceding self-initiated movements occurred in neurons not activated before externally instructed movements, suggesting a selectivity for the internal generation process. Activity related to the execution of self-initiated movements occurred in 67 neurons: it began during and up to 420 ms before movement onset and was usually not associated with pre-movement activity. Most of these neurons were also activated with stimulus-triggered movements, suggesting a lack of selectivity for the execution of self-initiated movements. In comparison with the striatum, more SMA neurons showed preparatory activity preceding externally instructed movements (transient 27% vs 16%, sustained 20% vs 12%) and self-initiated movements (16% vs 11%). Whereas transient responses showed similar latencies and durations in the two structures, sustained preparatory activity preceding externally instructed or self-initiated movements began and reached its peak earlier in SMA compared to striatal neurons. However, due to the long durations, sustained activation largely overlapped in the two structures, and thus essentially occurred simultaneously. Instruction-induced or internally generated preparatory activity may originate outside of the SMA and striatum or may derive from activity reverberating in cortico-basal ganglia loops, possibly in conjunction with other, closely associated cortical and subcortical structures. These data would favor a conjoint role for SMA and striatum in the internal generation of individual behavioral acts and the preparation of behavioral reactions.     

The natural discharges of Purkinje cells in paravermal regions of lobules V and VI of the monkey's cerebellum.

1. Conscious monkeys were trained with food rewards to perform movement tasks with the left hand and to accept manipulation of the joints and muscles and natural non-noxious stimulation of the skin of both forelimbs.2. Recordings were made from 230 Purkinje cells situated in the paravermal region of lobules V and VI or immediately adjacent folia of the left cerebellum in a region from 2 to 7 mm from the mid line. These neurones were all in a zone which was demonstrated to receive inputs from the ipsilateral hand and which is known to receive projections, via the pontine nuclei from the ;arm area' of motor cortex in the right hemisphere.3. Modulation of the natural activity of 182 of these 230 Purkinje cells (79%) occurred in a reproducible manner in temporal association, each with a particular phase of the self-paced movement tasks performed by the animal using the ipsilateral arm and hand. The patterns of modulation of Purkinje cell firing in this limited zone of cerebellar cortex could be classified into one of four groups, and each cell's discharge was associated with a particular aspect of movement such as general arm flexion, shoulder retraction, elbow extension or elbow flexion whenever it occurred.4. The cells were spontaneously active at rest. Most commonly, marked accelerations of the discharge were related to one direction of the particular aspect of movement and a reduction of activity or even total silence accompanied movement in the opposite direction.5. Variation of the amount of discharge demonstrated during a movement performance with which this discharge was characteristically associated could be related to the range of the movement or its duration, more activity being characteristic of more prolonged movement performance through larger angles of joint displacement.6. Both simple spikes and complex spikes of some cells showed characteristic modulation of their activity during the monkey's self-initiated movements. Cells whose simple spikes did not change in frequency during the movement task, also showed no modification of complex spike discharge.7. Of the 182 neurones whose discharges changed during active movement performance, 105 (roughly 60%) were demonstrated to be in receipt of an input from peripheral receptors in the hand which could be activated by brisk tapping of the skin or brushing of hairs. In contrast, none of the Purkinje cells whose discharges were unchanged during arm movements could be demonstrated to receive such an input.8. Movement of joints through their full range and prodding of muscles were completely ineffective stimuli for causing changes in Purkinje cell firing in this zone of the cerebellar cortex while the animal was passive and relaxed. Imposed perturbations of movement performance injected unexpectedly during the execution of a movement task were also ineffective in modifying the discharge of these Purkinje cells in relation to the task.     

Cerebellar neuronal activity related to whole-arm reaching movements in the monkey

1. Three monkeys were trained to make whole-arm reaching movements from a common central starting position toward eight radially arranged targets disposed at 45 degrees intervals. A sample of 312 cerebellar neurons with proximal-arm receptive fields or discharge related to shoulder or elbow movements was studied in the task. The sample included 69 Purkinje cells, 115 unidentified cortical cells, 65 interpositus neurons, and 63 dentate units. 2. The reaching task was divided into three movement-related epochs: a reaction time, a movement time, and holding over the target. All neurons demonstrated significant changes in discharge during one or more of these three epochs. Almost all of the cells (95%) showed a significant change in activity during the movement, whereas 68-69% of the cells showed significant changes from premovement activity during the reaction time and holding periods. 3. During the combined reaction time-movement period, 231/312 cells were strongly active in the task. Of these, 151 cells (65.4%) demonstrated unimodal directional responses. Sixty-three had a reciprocal relation to movement direction, whereas 88 showed only graded increases or decreases in activity. A further 37 cells (16.0%) were nondirectional, with statistically uniform changes in discharge in all eight directions. The remaining 43 cells (18.6%) showed significant differences in activity for different directions of movement, but their response patterns were not readily classifiable. 4. The proportion of directional versus nondirectional cells was consistent across the four cell populations. However, graded response patterns were more common and reciprocal responses less common among Purkinje and dentate neurons than among unidentified cortical cells and interpositus neurons. 5. The distribution of preferred directions of the population of cerebellar neurons covered all possible movement directions away from the common central starting position in the horizontal plane. When the preferred direction of each cell in the sample population was aligned, the mean direction-related activity of the cerebellar population formed a bell-shaped tuning curve for the activity recorded during both the reaction time and the movement, as well as during the time the arm maintained a fixed posture over the targets. A vector representation also showed that the overall activity of the cerebellar population during normal reaching arm movements generated a signal that varied with movement direction. 6. These results demonstrate that the cerebellum generates a signal that varies with the direction of movement of the proximal arm during normal aimed reaching movements and is consistent with a role in the control of the activity of muscles or muscle groups generating these movements.     

Preparation for movement: neural representations of intended direction in three motor areas of the monkey

1. The purpose of this study was to compare the functional properties of neurons in three interrelated motor areas that have been implicated in the planning and execution of visually guided limb movements. All three structures, the supplementary motor area (SMA), primary motor cortex (MC), and the putamen, are components of the basal ganglia-thalamocortical "motor circuit." The focus of this report is on neuronal activity related to the preparation for movement. 2. Five rhesus monkeys were trained to perform a visuomotor step-tracking task in which elbow movements were made both with and without prior instruction concerning the direction of the forthcoming movement. To dissociate the direction of preparatory set (and limb movement) from the task-related patterns of tonic (and phasic) muscular activation, some trials included the application of a constant torque load that either opposed or assisted the movements required by the behavioral paradigm. Single-cell activity was recorded from the arm regions of the SMA, MC, and putamen contralateral to the working arm. 3. A total of 741 task-related neurons were studied, including 222 within the SMA, 202 within MC, and 317 within the putamen. Each area contained substantial proportions of neurons that manifested preparatory activity, i.e., cells that showed task-related changes in discharge rate during the postinstruction (preparatory) interval. The SMA contained a larger proportion of such cells (55%) than did MC (37%) or the putamen (33%). The proportion of cells showing only preparatory activity was threefold greater in the SMA (32%) than in MC (11%). In all three areas, cells that showed only preparatory activity tended to be located more rostrally than cells with movement-related activity. Within the arm region of the SMA, the distribution of sites from which movements were evoked by microstimulation showed just the opposite tendency: i.e., microexcitable sites were largely confined to the caudal half of this region. 4. The majority of cells with task-related preparatory activity showed selective activation in anticipation of elbow movements in a particular direction (SMA, 86%; MC, 87%; putamen, 78%), and in most cases the preparatory activity was found to be independent of the loading conditions (80% in SMA, 83% in MC, and 84% in putamen).(ABSTRACT TRUNCATED AT 400 WORDS)     

Spike firing in the lateral cerebellar cortex correlated with movement and motor parameters irrespective of the effector limb

Neuronal signals in the lateral aspect of the macaque cerebellar cortex were studied during a visually guided reaching task. During the performance of this task, the firing rate of most neurons was significantly modulated when reaching with either the ipsilateral or the contralateral arm. In some of these reach-modulated cells, we found that spike firing was correlated with the direction and speed of the reach. These correlations with motor parameters were present during reaching with either the ipsilateral or the contralateral arm. Based on these observations we suggest that spike firing in the lateral cerebellum was correlated with movement and motor parameters irrespective of the effector limb.     

Role of primate basal ganglia and frontal cortex in the internal generation of movements

The purpose of these studies was to investigate neuronal activity in the basal ganglia and frontal cortex in relation to the internal generation of goal-directed movements. Monkeys performed goal-directed arm movements at a self-chosen moment in the absence of phasic stimuli providing external temporal reference. They were rewarded with a small morsel of food for each movement, although automatic or repetitive behavior was not reinforced. For reasons of comparison, animals were also trained in a delayed go no-go task in which visual cues instructed them to perform or refrain from an arm movement reaction to a subsequent trigger stimulus. This report describes the activity of neurons in the head of the caudate nucleus and rostral putamen preceding self-initiated arm movements and compares it with instruction-induced preparatory activity preceding movements in the delay task. A total of 497 caudate and 354 putamen neurons were tested in the delay task. Two types of preparatory activity were observed: (1) transient responses to the instruction cue, and (2) sustained activity preceding the trigger stimulus or movement onset. Transient responses were found in 48 caudate and 50 putamen neurons, occurring twice as often in movement ('go') as compared to no-movement ('no-go') trials, but rarely in both. These responses may code the information contained in the instruction relative to the forthcoming behavioral reaction. Sustained activity began after instruction onset and lasted until the trigger stimulus or the arm movement occurred, this being for periods of 2-7 s, 12-35 s, or up to 80 s, depending on the task requirements. This activity was seen in 47 caudate and 45 putamen neurons, was largely confined to go trials, and was unrelated to the preparation of saccadic eye movements. In some cases, this activity began as direct responses to the instruction stimulus, but in the majority of cases developed more gradually before the movement. Thus, both transient and sustained activations appear to be related to the preparation of movements. A total of 390 caudate and 293 putamen neurons were tested during self-initiated movements. Activity preceding earliest movement-related muscle activity was found in 32 caudate and 42 putamen neurons. This premovement activity began 0.5-5.0 s before movement onset (median 1160 ms), increased slowly, reached its peak close to movement onset, and subsided rapidly thereafter. It was unrelated to the preparation of saccadic eye movements. Comparisons between the two tasks were made on 53 neurons.(ABSTRACT TRUNCATED AT 400 WORDS)     

Influence of joint interactional effects on the coordination of planar two-joint arm movements

We have examined EMG-movement relations in two-joint planar arm movements to determine the influence of interactional torques on movement coordination. Explicitly defined combinations of elbow movements (ranging from 20 to 70°) and wrist movements (ranging from 20 to 40°) were performed during a visual, step-tracking task in which subjects were specifically required to attend to the initial and final angles at each joint. In all conditions the wrist and elbow rotated in the same direction, that is, flexion-flexion or extension-extension. Elbow movement kinematics were only slightly influenced by motion about the wrist. In contrast, the trajectory of the wrist movement was significantly influenced by uncompensated reaction torques resulting from movement about the elbow joint. At any given wrist amplitude, wrist movement duration increased and peak velocity decreased as elbow amplitude increased. In addition, as elbow amplitude increased, wrist movement on-set was progressively delayed relative to this elbow movement. Surprisingly, the changes between joint movement onsets were not accompanied by corresponding changes between agonist EMG onsets at the elbow and wrist joints. The mean difference in onset times between elbow and wrist agonists (22–30 ms) remained unchanged across conditions. In addition, a basic pattern of muscle activation that scaled with movement amplitude was observed at each joint. Phasic agonist activity at the wrist and elbow joints remained remarkably similar across conditions and thus the changes in joint movement onset could not be attributed to changes in the motor commands. Rather, the calculated torques from the averaged data showed that the difference in timing of joint movement onsets was influenced by joint interactional torques. These findings suggest that during simple two-joint planar movements of the elbow and the wrist joint, the central nervous system does not alter the basic motor commands at each joint and as a result the actual trajectory of each joint is determined by interactional torques.     

Arm-movement-related neurons in the primate superior colliculus and underlying reticular formation: comparison of neuronal activity with EMGs of muscles of the shoulder, arm and trunk during reaching

 Neuronal activity was recorded from the superior colliculus (SC) and the underlying reticular formation in two monkeys during an arm reaching task. Of 744 neurons recorded, 389 (52%) clearly modulated their activity with arm movements. The temporal activity patterns of arm-movement-related neurons often had a time course similar to rectified electromyograms (EMGs) of particular muscles recorded from the shoulder, arm or trunk. These reach cells, as well as the muscles investigated, commonly exhibited mono- or biphasic (less frequently tri- or polyphasic) excitatory bursts of activity, which were related to the (pre-)movement period, the contact phase and/or the return movement. The vast majority of reach cells exhibited a consistent activity pattern from trial to trial as did most of the muscles of the shoulder, arm and trunk. Similarities between the activity patterns of the neurons and the muscles were sometimes very strong and were especially notable with the muscles of the shoulder girdle (e.g. trapezius descendens, supraspinatus, infraspinatus or the anterior and medial deltoids). This high degree of co-activation suggests a functional linkage, though not direct, between the collicular reach cells and these muscles. Neuronal activity onset was compared with that of 25 muscles of the arms, shoulders and trunk. The majority of cells (78.5%) started before movement onset with a mean lead time of 149±90 ms, and 36.5% were active even before the earliest EMG onset. The neurons exhibited the same high degree of correlation (r=0.97, Spearman rank) between activity onset and the beginning of the arm movement as did the muscles (r=0.98) involved in the task. The mean neuronal reach activity (background subtracted) ranged between 7 and 193 impulses/s (mean 40.5±24.2). The mean modulation index calculated [(reach activity −background activity)/reach activity+background activity)] was 0.75±0.23 for neurons (n=358) and 0.87±0.14 for muscles (n=25). As the monkeys fixated the reach target constantly during an arm movement, neuronal activity which was modulated in this period was not related to eye movements. The three neck muscles investigated in the reach task exhibited no reach-related activity modulation comparable to that of either the reach cells or the muscles of the shoulder, arm and trunk. However, tonic neck muscle EMG was monotonically related to horizontal eye position. The clear skeletomotor discharge characteristics of arm-movement-related SC neurons revealed in this study agree with those already known from other sensorimotor regions (for example the primary motor, the premotor and parietal cortex, the basal ganglia or the cerebellum) and are consistent with the possible role of this population of reach cells in the control of arm movements. Received: 17 June 1996 / Accepted: 24 December 1996     

Accuracy of planar reaching movements

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.     

Walking delays anticipatory postural adjustments but not reaction times in a choice reaction task

During standing, anticipatory postural adjustments (APAs) and focal movements are delayed while performing a choice reaction task, compared with a simple reaction task. We hypothesized that APAs and focal movements of a choice reaction task would be similarly delayed during walking. Furthermore, reaction times are delayed during walking compared with standing. We further hypothesized that APAs and focal movements would be delayed during walking, compared with standing, for both simple and choice reaction tasks. Subjects either walked or stood on a treadmill while holding on to stable handles. They were asked to push or pull on the handles in response to a visual cue. Muscle activity was recorded from muscles of the leg (APA) and arm (RT). Our results were in agreement with previous work showing APA onset was delayed in the choice reaction task compared with the simple reaction task. In addition, the interval between the onset of APA and focal movement activity increased with choice reaction tasks. The task of walking did not delay the onset of focal movement for either the simple or choice reaction tasks. Walking did delay the onset of the APA, but only during choice reaction tasks. The results suggest the added demand of walking does not significantly modify the control of focal arm movements. However, additional attentional demands while walking may compromise anticipatory postural control.     

Single cell studies of the primate putamen

The major goal of this study was to determine whether the activity of single cells in the primate putamen was better related to the direction of limb movement or to the underlying pattern of muscular activity. In addition, the neural responses to load application were studied in order to determine whether the same neurons were also responsive to somatosensory stimuli. Two rhesus monkeys were trained to perform a visuomotor arm tracking task which required elbow flexion/extension movements with assisting and opposing loads in order to dissociate the direction of elbow movement from the pattern of muscular activity required for the movement. Neurons in the putamen were selected for study only if they were related both to the task and to arm movements outside the task. Most (96%) of the cells studied responded to load application: 36% of these showed short-latency (less than 50 ms), "sensory" responses. Forty-four percent of neurons had significant relations to the level of static load as the animal held the arm stationary against the steady loads: in general, static load effects were relatively weak. During the elbow flexion/extension movements in the task, 76% of cells had significant relations to the direction of movement, and 52% of neurons had significant dynamic relations to the level of load. Half of all neurons studied were primarily related to the direction of movement independent of the load. Only thirteen percent of cells in the putamen had a pattern of activity similar to that of muscles. These results indicate that neuronal activity in the putamen is predominantly related to the direction of limb movement rather than to the activity of particular muscles and that the basal ganglia may play a role in the specification of parameters of movement independent of the activity of specific muscles. These results also indicate that the basal ganglia receive proprioceptive input which may be used in the control of ongoing movement.     

A quantitative analysis of pallidal discharge during targeted reaching movement in the monkey

Summary Neurons in the globus pallidus have been studied during reaching movements of the arm made at varying speeds. The reaching task is similar to one used in earlier experiments, in which disruption of normal pallidal output caused changes in movement time. The pallidal cells studied were those that showed task-related changes in activity and a modification of discharge when the arm was manipulated outside of the task. Neuronal discharge was assessed to evaluate two possible models, one in which the timing of task-related discharge varied as a function of movement time and the other in which the amplitude (mean firing rate) of the change in discharge varied as movement time varied. The relation between neuronal discharge and movement time was examined quantitatively on a trial-by-trial basis using a statistical algorithm that identified each phase of the change in neuronal discharge on each trial. A nonparametric statistic was used to determine the correlation between movement time and the duration or latency of changes in neural firing or the mean discharge during each phase of the cell's response. For fifty-five percent of the 40 neurons studied, the timing of the cell's response (duration or latency) varied as a function of movement time. For only 10 cells (25%) was there a significant correlation between movement time and the mean firing rate during one or more phases of the cell's response. Both timing and frequency modulation with movement time were limited to cells responsive to manipulation at the wrist or the shoulder.     

Cortical mechanisms related to the direction of two-dimensional arm movements: relations in parietal area 5 and comparison with motor cortex

The relations between the direction of two-dimensional arm movements and single cell discharge in area 5 were investigated during 49 penetrations into the superior parietal lobule of 3 monkeys. A significant variation of cell discharge with the direction of movement was observed in 182 of 212 cells that were related to arm movements. In 151/182 of these cells the frequency of discharge was highest during movements in a preferred direction, and decreased in an orderly fashion with movements made in directions farther and farther away from the preferred one; in 112/151 cells this variation in discharge was a sinusoidal function of the direction of movement. Preferred directions differed for different cells so that directional tuning curves overlapped partially. These results are similar to those described for cells in the motor cortex (Georgopoulos et al. 1982): this suggests that directional information may be processed in a similar way in these structures. Many cells in area 5 changed activity before the onset of movement, and several did so before the earliest electromyographic changes (63% and 35%, respectively, of the cells that showed an increase in activity with movements in the preferred direction). However, the distribution of onset times of the parietal cells lagged the corresponding one of the motor cortical cells by about 60 ms. This suggests that the early changes observed in the parietal cortex might represent a corollary discharge from the precentral motor fields, whereas later activity might reflect peripheral as well as central events.     

Scaling of the metrics of visually-guided arm movements during motor learning in primates

Summary Hand trajectory, tangential velocity and acceleration, time and distance until peak velocity and reaction time were analyzed during the process of learning a skilled, visually-guided arm movement. Primates were trained to move a cursor with a manipulandum from a start box to target boxes displayed on a horizontal video screen during control conditions and when the relationship (gain) between the cursor and manipulandum was altered. The animals adapted to the altered feedback over 100–200 trials. A subsequent testing phase with randomly interspersed trials using the control gain demonstrated that the animals had modified their movements appropriately for the novel gain. Examination of the kinematics revealed that in adapting to a novel gain, primates scaled movement amplitude, tangential velocity, acceleration, and duration appropriately for the distance the hand needed to travel. Yet time to peak velocity was kept constant. Reaction time also remained unchanged for three of the four animals. Movements were performed in two phases, the first from movement onset to peak velocity and the second from peak velocity until the end of the movement. During the first phase the shape of the trajectory and velocity profile were stereotypic and without evidence of any corrections, consistent with this phase being essentially open loop. However, corrections occurred in the second phase and we propose visual feedback was used to correct for the difference in hand/cursor position. Learning appeared to involve utilizing the errors from previous trials to modify the early feedforward phase of subsequent trials. Peak tangential velocity, total movement duration and distance reached at peak tangential velocity all scaled linearly with the total movement distance required at each gain. Based on regression analyses, for none of these variables were the changes in learning completely adequate to compensate for total distance required. However, distance to peak velocity scaled with peak velocity in relation to the control gain. The results show that non-human primates adopt a consistent strategy when learning to scale a multi-joint movement. The metrics of the movement scaled yet the time to peak velocity remained constant, suggesting independent control of time and amplitude. Keeping time to peak velocity constant as well as the scaling of peak velocity with distance to peak velocity are viewed as ways to simplify the learning process.