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W T Thach

Publications and source records attributed to W T Thach.

At least 19 recordsLinked to original sources
PubMed2000

Cerebellar ataxia: torque deficiency or torque mismatch between joints?

Prior work has shown that cerebellar subjects have difficulty adjusting for interaction torques that occur during multi-jointed movements. The purpose of this study was to determine whether this deficit is due to a general inability to generate sufficient levels of phasic torque inability or due to an inability to generate muscle torques that predict and compensate for interaction torques. A second purpose was to determine whether reducing the number of moving joints by external mechanical fixation could improve cerebellar subjects' targeted limb movements. We studied control and cerebellar subjects making elbow flexion movements to touch a target under two conditions: 1) a shoulder free condition, which required only elbow flexion, although the shoulder joint was unconstrained and 2) a shoulder fixed condition, where the shoulder joint was mechanically stabilized so it could not move. We measured joint positions of the arm in the sagittal plane and electromyograms (EMGs) of shoulder and elbow muscles. Elbow and shoulder torques were estimated using inverse dynamics equations. In the shoulder free condition, cerebellar subjects made greater endpoint errors (primarily overshoots) than did controls. Cerebellar subjects' overshoot errors were largely due to unwanted flexion at the shoulder. The excessive shoulder flexion resulted from a torque mismatch, where larger shoulder muscle torques were produced at higher rates than would be appropriate for a given elbow movement. In the shoulder fixed condition, endpoint errors of cerebellar subjects and controls were comparable. The improved accuracy of cerebellar subjects was accompanied by reduced shoulder flexor muscle activity. Most of the correct cerebellar trials in the shoulder fixed condition were movements made using only muscles that flex the elbow. Our findings suggest that cerebellar subjects' poor shoulder control is due to an inability to generate muscle torques that predict and compensate for interaction torques, and not due to a general inability to generate sufficient levels of phasic torque. In addition, reducing the number of muscles to be controlled improved cerebellar ataxia.

Adult↗
PubMed1998

Posterior vermal split syndrome.

We have studied a battery of movements in 5 children (age, 6-15 years) after transection of the posterior inferior cerebellar vermis. In each case, the surgery destroyed the midline vermis only (ranging from lobules VI-X). Tandem gait was badly impaired in all subjects. No subjects had impairments of kicking, reaching, pinching, or speech. Regular gait, standing, and hopping on one leg were relatively unimpaired. Cutting the parallel fibers that cross the midline may be the critical variable causing incoordination in tandem gait.

Adolescent↗
PubMed1998

Combination, complementarity and automatic control: a role for the cerebellum in learning movement coordination.

We have examined several different paradigms of adaptation and of 'acquisition of skill'--skill defined as a movement specialized to meet a certain goal and gained through practice. In each paradigm, change occurs through trial-and-error performance. In some of the tasks, damage of cerebellar cortex impairs adaptation and not performance. The deficits in performance cannot explain the deficits in adaptation. In some of the tasks, the discharge of Purkinje cells and, by inference, the discharge of inferior olive cells and mossy fibres has behaved in a manner consistent with the Marr-Albus theory of motor learning. We extend the theory to show how parallel fibres could implement both the coordination of complex movements and the learning of new movements. The size of the response combinations would be proportionate to the length of parallel fibres. The mechanism proposed here would permit optimized complex movement behaviours to respond to specific behavioural contexts rapidly, stereotypically and automatically. The mechanism would permit storage of many context-response couplings, and many complex responses. The mechanism would permit privacy, individuality and a large number of behavioural responses.

Animals↗
PubMed1997

Context-response linkage.

Brindley (1969) proposed that we initially generate movements "consciously," under higher cerebral control. As the movement is practiced, the cerebellum learns to link within itself the context in which the movement is made to the lower level movement generators. Marr, (1969) Albus (1971), and Ito (1972) proposed that the linkage is established by a special input from the inferior olive, which plays upon an input-output element within the cerebellum during the period of the learning. When the linkage is complete, the occurrence of the context (represented by a certain input to the cerebellum) will trigger (through the cerebellum) the appropriate motor response. The "learned" movement is distinguished from the "unlearned" conscious movement by being automatic, rapid, and stereotyped. Another important variable can be added to the idea of the context-response linkage: novel combinations of downstream elements. With regard to the motor system, this could explain how varied combinations of muscles may become active in precise time-amplitude specifications so as to produce coordinated movements appropriate to specific contexts. This chapter further extends this idea to the premotor parts of the brain and their role in cognition. These areas receive influences from the cerebellum and are active both in planning movements that are to be executed and in thinking about movements that are not be executed. Evidence shows that the cerebellar output extends even to what has been characterized as the ultimate frontal planning area, the "prefrontal" cortex, area 46. The cerebellum thus may be involved in context-response linkage and in response combination even at these higher levels. The implication would be that, through practice, an experiential context would automatically evoke a certain mental action plan. The plan would be in the realm of thought and could-but need not-lead to execution. The specific cerebellar contribution would be one of the context linkage and the shaping of the response through trial and error learning. The prefrontal and premotor areas could still plan without the help of the cerebellum, but not so automatically, rapidly, stereotypically, so precisely linked to context, or so free or error. Nor would their activities improve optimally with mental practice.

Animals↗
PubMed1997

No clock signal in the discharge of neurons in the deep cerebellar nuclei.

We examined the spike activity of deep cerebellar nuclear cells recorded from awake, behaving monkeys to determine if there was a tendency for periodic discharge at or near 10 Hz. Data were obtained from four Rhesus monkeys trained to perform either targeted flexions and extensions of the wrist in relation to a visual cue (2 monkeys) or instrumented digit movements and natural reaches (2 monkeys). We determined the interspike intervals of 274 isolated cells. We looked for periodicity by autocorrelating the interval data and Fourier transforming the resulting autocorrelation function. The autocorrelograms and the Fourier transforms failed to reveal periodicity at or near 10 Hz for any cell. This lack of oscillatory discharge in deep nuclear cells of the cerebellum is consistent with our previously reported results that the complex spike of the Purkinje cell is aperiodic. Our failure to observe a clocklike timing signal in awake, behaving animals in either the Purkinje cell complex spike or the deep nuclear cell discharge argues against a popular idea that the inferior olive may act through the cerebellum as a motor clock.

Animals↗
PubMed1996

Throwing while looking through prisms. I. Focal olivocerebellar lesions impair adaptation.

Normal human subjects and patients with lesions of the olivocerebellar system threw balls of clay at a visual target while wearing wedge prism spectacles. Normal subjects initially threw in the direction of prism-bent gaze, but with repeated throws adapted to hit the target. Patients with generalized cerebellar atrophy, inferior olive hypertrophy, or focal infarcts in the distribution of the posterior inferior cerebellar artery, in the ipsilateral inferior peduncle, in the contralateral basal pons or in the ipsilateral middle cerebellar peduncle had impaired or absent prism adaptation. Patients with infarcts in the distribution of the posterior inferior cerebellar artery usually had impaired or absent adaptation but little or no ataxia. By contrast, patients with damage in the distribution of the superior cerebellar artery or in cerebellar thalamus usually had ataxia but preserved adaptation. These results implicate climbing fibres from the contralateral inferior olive via the ipsilateral inferior cerebellar peduncle, mossy fibres from the contralateral pontocerebellar nuclei via the ipsilateral middle cerebellar peduncle, and posterior inferior cerebellar artery territory cortex as being critical for this adaptation. The dentatothalamic projection and the superior cerebellar artery territory cortex are not necessary for this adaptation.

Adult↗
PubMed1996

Throwing while looking through prisms. II. Specificity and storage of multiple gaze-throw calibrations.

Human subjects threw balls of clay at a visual target while looking through wedge prism spectacles. In studies of short-term adjustment, subjects threw in the direction of their prism-bent gaze, missing the target to that side. Within 10-30 throws, they gradually adapted with a wider gaze-throw angle and hit the target. Immediately after removal of the prisms the wide gaze-throw angle persisted and throws missed the target to the opposite side, the so-called 'negative after effect'. Repeated throws were required to adapt back to the normal gaze-throw angle and hit the target. The adaptation was specific both to the body parts trained and the type of throw trained: training with the right hand did not generalize to throwing with the left; overhand training seldom generalized to underhand throwing. In a study of long-term adjustment, two subjects threw with the same hand (right) and the same type of throw (overhand) alternately, with and without prisms, over a period of 6 weeks. They gradually learned to hit the target on the first throw, with and without prisms. The two gaze-throw calibrations (prism and no-prism) were retained for > 27 months. The long-term adjustment was shown to consist of a coordinated relationship of eye-in-head, head-on-trunk and trunk-on-arm angles.

Adult↗
PubMed1996

Cerebellar ataxia: abnormal control of interaction torques across multiple joints.

1. We studied seven subjects with cerebellar lesions and seven control subjects as they made reaching movements in the sagittal plane to a target directly in front of them. Reaches were made under three different conditions: 1) "slow-accurate," 2) "fast-accurate," and 3) "fast as possible." All subjects were videotaped moving in a sagittal plane with markers on the index finger, wrist, elbow, and shoulder. Marker positions were digitized and then used to calculate joint angles. For each of the shoulder, elbow and wrist joints, inverse dynamics equations based on a three-segment limb model were used to estimate the net torque (sum of components) and each of the component torques. The component torques consisted of the torque due to gravity, the dynamic interaction torques induced passively by the movement of the adjacent joint, and the torque produced by the muscles and passive tissue elements (sometimes called "residual" torque). 2. A kinematic analysis of the movement trajectory and the change in joint angles showed that the reaches of subjects with cerebellar lesions were abnormal compared with reaches of control subjects. In both the slow-accurate and fast-accurate conditions the cerebellar subjects made abnormally curved wrist paths; the curvature was greater in the slow-accurate condition. During the slow-accurate condition, cerebellar subjects showed target undershoot and tended to move one joint at a time (decomposition). During the fast-accurate reaches, the cerebellar subjects showed target overshoot. Additionally, in the fast-accurate condition, cerebellar subjects moved the joints at abnormal rates relative to one another, but the movements were less decomposed. Only three subjects were tested in the fast as possible condition; this condition was analyzed only to determine maximal reaching speeds of subjects with cerebellar lesions. Cerebellar subjects moved more slowly than controls in all three conditions. 3. A kinetic analysis of torques generated at each joint during the slow-accurate reaches and the fast-accurate reaches revealed that subjects with cerebellar lesions produced very different torque profiles compared with control subjects. In the slow-accurate condition, the cerebellar subjects produced abnormal elbow muscle torques that prevented the normal elbow extension early in the reach. In the fast-accurate condition, the cerebellar subjects produced inappropriate levels of shoulder muscle torque and also produced elbow muscle torques that did not very appropriately with the dynamic interaction torques that occurred at the elbow. Lack of appropriate muscle torque resulted in excessive contributions of the dynamic interaction torque during the fast-accurate reaches. 4. The inability to produce muscle torques that predict, accommodate, and compensate for the dynamic interaction torques appears to be an important cause of the classic kinematic deficits shown by cerebellar subjects during attempted reaching. These kinematic deficits include incoordination of the shoulder and the elbow joints, a curved trajectory, and overshoot. In the fast-accurate condition, cerebellar subjects often made inappropriate muscle torques relative to the dynamic interaction torques. Because of this, interaction torques often determined the pattern of incoordination of the elbow and shoulder that produced the curved trajectory and target overshoot. In the slow-accurate condition, we reason that the cerebellar subjects may use a decomposition strategy so as to simplify the movement and not have to control both joints simultaneously. From these results, we suggest that a major role of the cerebellum is in generating muscle torques at a joint that will predict the interaction torques being generated by other moving joints and compensate for them as they occur.

Adult↗
PubMed1995

Cerebellar outflow lesions: a comparison of movement deficits resulting from lesions at the levels of the cerebellum and thalamus.

Previous work has shown that lesions in the lateral cerebellum involving the dentate nucleus impair both reaching and pinching movements in humans and monkeys. This study addressed the question of whether disruption of the cerebellar-thalamo-cortical pathway at the level of the thalamus would produce behavioral deficits similar to those seen after dentate damage. We compared the performance of both reaching and pinching movements in patients with lateral cerebellar lesions and in patients with discrete lesions of the ventrolateral thalamus. The patients with thalamic lesions had minimal or no sensory loss and no corticospinal signs, suggesting that the abnormal movements were due to disruption of the cerebellar projection to the thalamus. We found that lesions of the ventrolateral thalamus resulted in impaired pinching movements, but remarkably normal reaching movements with the exception of a slight tremor. This is in contrast to the profound pinching and reaching impairments of patients with lateral cerebellar lesions involving the dentate nucleus. Implications about the functional organization of cerebellar output are discussed.

Adult↗
PubMed1995

Nonclock behavior of inferior olive neurons: interspike interval of Purkinje cell complex spike discharge in the awake behaving monkey is random.

1. Complex spikes of cerebellar Purkinje cells recorded from awake, behaving monkeys were studied to determine the extent to which their discharge could be quantified as periodic. Three Rhesus monkeys were trained to perform up to five different tasks involving rotation of the wrist in relation to a visual cue. Complex spike activity was recorded during task performance and intertrial time. Interspike intervals were determined from the discharge of each of 89 Purkinje cells located throughout lobules IV, V, and VI. Autocorrelation and Fourier transform of the autocorrelation function were performed on the data. In addition, the activity from one cell was transformed so that the discharge occurred on the beat of a 10-Hz clock, and in a further transformation, on the beat of a noisy 10-Hz clock. These transformed data were then analyzed as described above. 2. Fourier transform of the autocorrelogram function of the data that had been transformed to a 10-Hz clock, and that of the noisy 10-Hz clock, both showed a prominent peak at 10 Hz. However, the autocorrelograms and the Fourier transforms of the autocorrelogram functions failed to reveal a prominent periodicity for the actual discharge of any of cells, at any frequency up to 100 Hz: the discharge appeared random with respect to the interspike interval. The discharge was not random with respect to behavior. Complex spike activity was commonly time locked to the start of wrist movement. We examined this discharge to see whether oscillatory discharge could be seen after alignment of the data on the start of wrist movement, or after alignment of the data on the complex spike occurring peri-start of wrist movement. No oscillation was seen for either alignment. 3. The inferior olive, which sends its climbing fibers to the cerebellum, has been implicated in such different activities as 1) pathological tremor of the soft palate, 2) physiological tremor, 3) the normal initiation of all bodily movement, and 4) motor learning. Previous work in pharmacologically or surgically treated animals has shown that, under some conditions, the discharge of these neurons is periodic and synchronous. This firing pattern has been interpreted to support a role in the first two activities. But measurements reported here in the awake monkey show just the opposite: the discharge is aperiodic to the extent of being random. As such, the inferior olive cannot be a "motor clock" in the general role that has been proposed.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1993

Basal ganglia intrinsic circuits and their role in behavior.

There have been significant recent advances in the understanding of basal ganglia circuitry and its role in behavior. Important areas of work in the past year include, firstly, the role of striatal neurons in early phases of movement and, secondly, further characterization of the intrinsic circuitry with emphasis on the importance of the subthalamic nucleus and its connections. A conceptual model of basal ganglia inhibition of competing motor programs is discussed.

Animals↗
PubMed1993

Preserved simple and impaired compound movement after infarction in the territory of the superior cerebellar artery.

A patient with an infarct in the distribution of the right superior cerebellar artery was studied with regard to his ability to make simple movements (visually triggered, self-terminated ballistic wrist movements), and compound movements (reaching to a visual target and precision pinch of a seen object). Movements on the right side of the body alone were affected. Control movements were made by the normal left upper extremity. Wrist movement on the right side was normal in reaction time, direction, peak velocity, and end-point position control as compared to the left. By contrast, both reaching and pinching movements on the right were impaired. Reaching movements showed marked decomposition of the compound elbow-shoulder movement into seriatim simple movements made alternately at elbow and shoulder. Pinching movements were not made, and instead winkling movements (a movement of index alone) were substituted. These results are compared to similar results of controlled inactivation of the cerebellar dentate nucleus in monkeys. We conclude that one function of the cerebellum may be to combine elements in the movement repertoires of downstream movement generators. When that ability is lost, a strategy may be voluntarily adopted of using the preserved simple movements in place of the impaired compound movements.

Arteries↗
PubMed1993

Cerebellar nuclei: rapid alternating movement, motor somatotopy, and a mechanism for the control of muscle synergy.

Monkeys were trained to make rapid alternating flexion and extension movements seriatim at each of five body parts--foot (toe grasp and release), shoulder, elbow, wrist, and thumb. "Facial movements"--mouth, tongue, jaw and pharynx--were also made to drink the fruit juice reward. Movement at a given joint could be performed simply by alternate activation of the prime mover muscle groups, and EMG analysis indicated that these primary muscles were active during movement of that joint. No muscle within any one large body part (leg or arm) was strongly active in relation to movement of another body part. Yet, within a body part (arm), synergist muscles were often more active than the primaries during movement of a given joint, only to become less active during movement of the joint at which their action was primary. Neurons in dentate and interpositus discharged in relation to these movements. In the antero-posterior dimension of both dentate and interposed nuclei, there was a significant tendency for neural modulation to be somatotopically arranged according to the preferred movement: hindlimb anteriormost, forelimb in the middle, and head posteriorly. In the medio-lateral dimension, no such localization was seen for the different movements of the upper limb. Like muscle activity, neural discharge modulation usually occurred in strong relation to a number of movements in a single body part (arm), but not to the movements of different body parts (leg, face). Lesion of the middle third of dentate and of a portion of lateral interpositus had little effect upon the movements at the single joints, and thus upon the prime movers. However, the pattern of activity of agonist, antagonist, and synergist muscles was changed. These results are consistent with the view that the dentate controls muscle synergy and movement coordination more than the prime movers per se.

Animals↗
PubMed1992

The cerebellum and the adaptive coordination of movement.

Based on a review of cerebellar anatomy, neural discharge in relation to behavior, and focal ablation syndromes, we propose a model of cerebellar function that we believe is both comprehensive as to the available information (at these levels) and unique in several respects. The unique features are the inclusion of new information on (a) cerebellar output--its replicative representation of body maps in each of the deep nuclei, each coding a different type and context of movement, and each appearing to control movement of multiple body parts more than of single body parts; and (b) the newly assessed long length of the parallel fiber. The parallel fiber, by virtue of its connection through Purkinje cells to the deep nuclei, appears optimally designed to combine the actions at several joints and to link the modes of adjacent nuclei into more complex coordinated acts. We review the old question of whether the cerebellum is responsible for the coordination of body parts as opposed to the tuning of downstream executive centers, and conclude that it is both, through mechanisms that have been described in the cerebellar cortex. We argue that such a mechanism would require an adaptive capacity, and support the evidence and interpretation that it has one. We point out that many parts of the motor system may be involved in different types of motor learning for different purposes, and that the presence of the many does not exclude an existence of the one in the cerebellar cortex. The adaptive role of the cerebellar cortex would appear to be specialized for combining simpler elements of movement into more complex synergies, and also in enabling simple, stereotyped reflex apparatus to respond differently, specifically, and appropriately under different task conditions. Speed of learning and magnitude of memory for both novel synergies and task-specific performance modifications are other attributes of the cerebellar cortex.

Adaptation, Physiological↗
PubMed1991

Basal ganglia motor control. I. Nonexclusive relation of pallidal discharge to five movement modes.

1. To evaluate the various hypotheses that the basal ganglia preferentially control one mode of movement to the exclusion of others, we recorded the discharge of single neurons in the globus pallidus (GP) in rhesus monkeys during their performance of five trained wrist-movement tasks. The tasks were designed to dissociate several modes and parameters of movement to see whether pallidal neurons would discharge in relation to one and not the others. All tasks were performed by flexing and extending the wrist with opposing or assisting torque loads (0.2 Nm). The five tasks included 1) VisStep, a visually cued step tracking task; 2) VisRamp, a visually guided hold-ramp-hold tracking task; 3) VisSine, a visually guided rapid sinusoidal tracking task; 4) SelfRamp, a self-paced hold-ramp-hold task with delayed alternation, trained velocity, and no visual feedback of wrist position; and 5) SelfSine, a self-paced rapid sinusoidal movement without visual feedback of wrist position. Wrist position and velocity were monitored during all recordings; and wrist, arm, shoulder, and back electromyographs (EMGs) were monitored periodically. Unit discharge was recorded extracellularly from both segments of the GP. The results were similar in the present analysis and are considered together. As a control, units were also recorded in the dentate nucleus of the lateral cerebellum, and the EMGs of many muscles were recorded in limbs, neck and trunk. 2. For 100 GP neurons [41 in the internal segment (GPi) and 59 in the external segment (GPe)], the activity of which changed with task performance, the discharge patterns varied greatly across tasks. The discharge of 96/97 neurons (99%) changed during VisStep, 66/91 neurons (73%) changed during VisRamp, 41/81 neurons (51%) changed during VisSine, 7/34 neurons (21%) changed during SelfRamp, and 25/80 neurons (31%) changed during SelfSine. Of 74 neurons that were fully tested in four or more tasks, 16 (21%) were related only to one task; only 17 cells (23%) were related to all tasks; and, for the remaining 41 (55%) neurons, the relation of the discharge of a given neuron to one task did not predict its relation to other tasks. These task-dependent differences in the discharge of pallidal neurons were not correlated with differences in wrist position, velocity, load, or muscle activity (see also the following paper--Mink and Thach, 1991a). 3. From these data, we conclude that no one task engaged all pallidal neurons to the exclusion of other tasks.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Basal ganglia motor control. II. Late pallidal timing relative to movement onset and inconsistent pallidal coding of movement parameters.

1. We have tested the hypothesis that the basal ganglia initiate some one or several modes of movement by recording the change in discharge frequency of pallidal neurons during visually triggered step and visually paced ramp moves in relation to the visual stimulus onset, the change in the electromyograph (EMG), and the movement onset of trained rhesus monkeys. 2. The modal times of change for globus pallidus pars interna (GPi) were significantly later than those for forearm agonist muscle EMG. By contrast, the modal time of change for the cerebellar dentate nucleus preceded that for wrist agonist EMG. 3. The direction of change in discharge frequency of the GPi cells was for 71% an increase and for 29% a decrease. 4. Because of the relatively late change of activity of GPi neurons, we propose that GPi neurons cannot initiate these movements, as others have also suggested. The commands for the initiation of these movements may instead be generated by structures that include the lateral cerebellum and the anterior cerebral cortex. 5. We have also tested the hypothesis that the pallidum of the basal ganglia or the dentate of the lateral cerebellum may control the direction and other parameters of the trajectory by recording from both structures to see whether cell discharge correlated with the parameter and whether the correlation was consistent across tasks. Two rhesus monkeys were trained to perform hold-ramp-hold and hold-step-hold visually guided movements in opposite directions by flexing and extending the wrist with and against uniform oppositely directed torque loads (0.2 Nm). Wrist position, velocity, force, and EMG were recorded simultaneously. Movement amplitudes and directional intent were computed and inferred, respectively. 6. Task related neurons were classified as follows: 1) directional, if the discharge rate was reciprocal for opposite movements or if it increased or decreased during movement in one direction only; 2) bidirectional, if the discharge rate increased or decreased during movement in both directions; and 3) "other," if it was directional under one load and bidirectional under the other. During step tracking, 34 GPi, 47 globus pallidus pars externa (GPe), and 44 cerebellar dentate nuclear neurons were related to the task. Of the GPi cells, 14 (41%) were directional, 6 (18%) bidirectional, and 14 (41%) other. Of the GPe neurons, 13 (28%) were directional, 19 (40%) bidirectional, and 15 (32%) other. Of the dentate cerebellar nuclear cells, 5 (11%) were bidirectional, 31 (70%) bidirectional, and 8 (18%) other.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1991

Basal ganglia motor control. III. Pallidal ablation: normal reaction time, muscle cocontraction, and slow movement.

1. Inactivation of the portions of globus pallidus pars interna (GPi) containing the greatest concentration of wrist-related neurons was achieved in two rhesus monkeys with microinjections of muscimol (temporary) and kainic acid (permanent). 2. After muscimol injection, there was onset within 30 s of 1) tonic and phasic coactivation of wrist flexors and extensors; 2) slightly greater activation of the flexors, giving a flexor bias in postural holds and the endpoint of movements; and 3) slowness of all movements with a prolonged movement time. Nevertheless, 4) movements made by lessening prior loaded muscle activity (to move in the direction of the load) were slower than movement made by increasing muscle activity (to move against the direction of the load). Despite marked slowing of all movements, there was 5) a normal reaction time for movement onset. Finally, there was 6) a reduced amplitude of most movements. Open room behavior included 7) spiraling contralateral to the lesion while walking. Effects were reproducible (12 injections), were apparent for 7-8 h and were usually completely gone by the next day's testing. 3. After kainic acid injection, there was a period of mixed effects, followed by a period of permanent defects (observed for up to 24 days) that duplicated the temporary effects of muscimol. 4. By contrast, muscimol inactivation of the cerebellar dentate nucleus resulted in 1) a prolonged reaction time and 2) an increased variability of movement trajectory, but 3) without change in movement time or peak velocity. Open room behavior included overshoot in reaching for fruit with the forelimb ipsilateral to the injection. 5. From the facts that normal pallidal neurons fire constantly, that pallidal neurons inhibit their target neurons, and that the muscimol effect was immediate, we conclude that the release of the target neurons from the tonic inhibition allowed them to fire in patterns that promoted a maintained state of cocontraction of agonist and antagonist muscles. From the fact that movement time was prolonged, we conclude that the maintained state of neural activity that caused the muscle cocontraction interfered with the commands for voluntary movement, which were generated by other mechanisms. From the fact that reaction time for movement onset was normal, we conclude that the pallidal neurons may play little or no role in the voluntary initiation of these movements, which are instead generated by other structures that include the anterior cerebral cortex and the lateral cerebellum.

Animals↗