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J C Houk

Publications and source records attributed to J C Houk.

At least 19 recordsLinked to original sources
PubMed2004

Movement-related discharge in the cerebellar nuclei persists after local injections of GABA(A) antagonists.

Limb movement-related neurons in the cerebellar nuclei (CN) typically produce bursts of discharge in association with movement. Consequently, given the inhibitory nature of the Purkinje cell (PC) projection to CN, it is puzzling that only a minority of movement-related PCs pause; the majority burst. Some of the movement-related CN activity may be the result of excitation from collaterals of mossy and climbing fiber projections to the cerebellar cortex. The only other input to CN is diffuse and neuromodulatory, from locus ceruleus and raphe nuclei. To investigate the role of the excitatory mossy fiber input, single units in CN were recorded in macaque monkeys during the performance of reaching and manipulation tasks, before and after blocking the PC input with local microinjections of GABA(A) antagonists (bicuculline or SR95531). After these injections, the movement-related modulation of CN discharge was greater and began earlier, compared with the modulation in the preinjection group of neurons. These observations indicate that an important excitatory drive is provided by extracerebellar inputs to CN, most likely from collaterals of mossy fibers. PCs may serve primarily to regulate this activity, by either pausing or bursting as necessary.

Action Potentials↗
PubMed2002

Features of motor performance that drive adaptation in rapid hand movements.

In order to explore how subjects correct for errors in movement and adapt their motor programs, we studied rapid hand movements. Subjects grasped a grooved knob and made brisk turning movements to various targets, similar to tuning a radio dial. A motor attached to the knob shaft was configured to apply a destabilizing negative viscous perturbation. Following a period of practice with no perturbations, the negative viscosity was engaged, which caused a large change in overall kinematics: the peak velocity increased, the movement amplitude was too large, and discrete corrective submovements were generated to bring the pointer back onto the target. After about an hour and nearly 1000 trials, subjects learned to move accurately in the new dynamic environment, returning their overall kinematics near to previous levels. Measures of performance included the endpoint error of the primary movement (the initial movement segment), the frequency and amplitude of corrective submovements, task success rate, mean squared jerk, and deviation from a "normal" angular velocity temporal profile. Both the amplitude and frequency of corrective submovements decreased progressively during adaptation as the subjects made fewer target overshoot errors. These results are consistent with motor learning schemes in which adaptation of the motor controller is driven by an attempt to reduce the endpoint error of the primary movement. While there have been many theories regarding what is being optimized in motor control, in general, biologically plausible mechanisms for implementing these schemes have not been described. A biologically plausible optimization criterion is the minimization of the occurrence and amplitude of corrective submovements, since the latter have been proposed as realistic climbing fiber training signals for adaptive changes in the cerebellum. We postulate that the other criteria that have been proposed are instead secondary to an increased accuracy of the primary movement and a corresponding decrease in the occurrence and amplitude of corrective submovements.

Adaptation, Physiological↗
PubMed2002

The use of overlapping submovements in the control of rapid hand movements.

Rapid targeted movements are subject to special control considerations, since there may be inadequate time available for either visual or somatosensory feedback to be effective. In our experiments, subjects rapidly rotated a knob to align a pointer to one of several targets. We recognized three different types of movement segments: the primary movement, and two types of submovement, which frequently followed. The submovements were initiated either before or after the end of the primary movement. The former, or "overlapping" type of submovement altered the kinematics of the overall movement and was consequently difficult to detect. We used a direct, objective test of movement regularity to detect overlapping submovements, namely, examining the number of jerk and snap zero crossings during the second half of a movement. Any overlapping submovements were parsed from the overall movement by subtracting the velocity profile of the primary movement. The velocity profiles of the extracted submovements had near-symmetric bell shapes, similar to the shapes of both pure primary movements and nonoverlapping submovements. This suggests that the same neural control mechanisms may be responsible for producing all three types of movement segments. Overlapping submovements corrected for errors in the amplitude of the primary movement. Furthermore, they may account for the previously observed, speed-dependent asymmetry of the velocity profile. We used a nonlinear model of the musculoskeletal system to explain most of the kinematic features of these rapid hand movements, including how discrete submovements are superimposed on a primary movement. Finally, we present a plausible scheme for how the central nervous system may generate the commands to control these rapid hand movements.

Adult↗
PubMed2002

Cerebellar input to magnocellular neurons in the red nucleus of the mouse: synaptic analysis in horizontal brain slices incorporating cerebello-rubral pathways.

We studied the synaptic input from the nucleus interpositus of the cerebellum to the magnocellular division of the red nucleus (RNm) in the mouse using combined electrophysiological and neuroanatomical methods. Whole-cell patch-clamp recordings were made from brain slices (125-150 microm) cut in a horizontal plane oriented to pass through both red nucleus and nucleus interpositus. Large cells that were visually selected and patched were injected with Lucifer Yellow and identified as RNm neurons. Using anterograde tracing from nucleus interpositus in vitro, we examined the course of interposito-rubral axons which are dispersed in the superior cerebellar peduncle. In vitro monosynaptic responses in RNm were elicited by an electrode array placed contralaterally in this pathway but near the midline. Mixed excitatory post-synaptic potentials (EPSPs)/inhibitory post-synaptic potentials (IPSPs) were observed in 48 RNm neurons. Excitatory components of the evoked potentials were studied after blocking inhibitory components with picrotoxin (100 microM) and strychnine (5 microM). All RNm neurons examined continued to show monosynaptic EPSPs after non-N-methyl-D-aspartate (NMDA) glutamate receptor components were blocked with 10 microM 6,7-dinitroquinoxaline-2,3-dione or 5 microM 2,3-dihydro-6-nitro-7-sulfamoyl-benzo(f)-quinoxaline (NBQX; n=12). The residual potentials were identified as NMDA receptor components since they (i) were blocked by the addition of the NMDA receptor antagonist, D,L-2-amino-5-phosphonovaleric acid (APV), (ii) were voltage-dependent, and (iii) were enhanced by Mg(2+) removal. Inhibitory components of the evoked potentials were studied after blocking excitatory components with NBQX and APV. Under these conditions, all RNm neurons studied continued to show IPSPs. Blockade of GABA(A) receptors reduced but did not eliminate the IPSPs. These were eliminated when GABA(A) receptor blockade was combined with strychnine to eliminate glycine components of the IPSPs. Thus, IPSPs evoked by midline stimulation of the superior cerebellar peduncle, while blocking alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and NMDA receptors, raise the possibility of direct inhibitory inputs to RNm from the cerebellum. In summary we propose that the special properties of the NMDA receptor components are considered important for the generation of RNm motor commands: their slow time course will contribute a steady driving force for sustained discharge and their voltage dependency will facilitate abrupt transitions from a resting state of quiescence to an active state of intense motor command generation.

Animals↗
PubMed2002

The role of the cerebellum in modulating voluntary limb movement commands.

We recorded the activity of cerebellar Purkinje cells (PCs), primary motor cortical (M1) neurons, and limb EMG signals while monkeys executed a sequential reaching and button pressing task. PC simple spike discharge generally correlated well with the activity of one or more forelimb muscles. Surprisingly, given the inhibitory projection of PCs, only about one quarter of the correlations were negative. The largest group of neurons burst during movement and were positively correlated with EMG signals, while another significant group burst and were negatively correlated. Among the PCs that paused during movement most were negatively correlated with EMG. The strength of these various correlations was somewhat weaker, on average, than equivalent correlations between M1 neurons and EMG signals. On the other hand, there were no significant differences in the timing of the onset of movement related discharge among these groups of PCs, or between the PCs and M1 neurons. PC discharge was modulated largely in phase, or directly out of phase, with muscle activity. The nearly synchronous activation of PCs and muscles yielded positive correlations, despite the fact that the synaptic effect of the PC discharge is inhibitory. The apparent function of this inhibition is to restrain activity in the limb premotor network, shaping it into a spatiotemporal pattern that is appropriate for controlling the many muscles that participate in this task. The observed timing suggests that the cerebellar cortex learns to modulate PC discharge predictively. Through the cerebellar nucleus, this PC signal is combined with an underlying cerebral cortical signal. In this manner the cerebellum refines the descending command as compared with the relatively crude version generated when the cerebellum is damaged.

Action Potentials↗
PubMed2001

Context dependency in the globus pallidus internal segment during targeted arm movements.

Extracellular discharges from single neurons in the internal segment of the globus pallidus (GPi) were recorded and analyzed for rate changes associated with visually guided forearm rotations to four different targets. We sought to examine how GPi neurons contribute to movement preparation and execution. Unit discharge from 108 GPi neurons recorded in 35 electrode penetrations was aligned to the time of various behavioral events, including the onset of cued and return movements. In total, 39 of 108 GPi neurons (36%) were task-modulated, demonstrating statistically significant changes in discharge rate at various times between the presentation of visual cues and movement generation. Most often, strong modulation in discharge rate occurred selectively during either the cued (n = 32) or return (n = 2) phases of the task, although a few neurons (n = 5) were well-modulated during both movement phases. Of the 34 neurons that were modulated exclusively during cued or return movements, 50% (n = 17) were modulated similarly in association with movements to any target. The remaining 17 neurons exhibited considerable diversity in their discharge properties associated with movements to each target. Cued phases of behavior were always rewarded if executed correctly, whereas return phases were never rewarded. Overall, these data reveal that many GPi neurons discharged in a context-dependent manner, being modulated during cued, rewarded movements, but not during similar self-paced, unrewarded movements. When considered in the light of other observations, the context-dependence we have observed seems likely to be influenced by the animal's expectation of reward.

Acoustic Stimulation↗
PubMed2000

Kinematic properties of rapid hand movements in a knob turning task.

In order to understand how the central nervous system controls the kinematics of rapid finger and hand movements, we studied the motions of subjects turning a knob to light-emitting diode targets, similar to tuning a radio dial. On many trials, subjects turned the knob with a single, smooth, and regular motion as revealed by the angular position and velocity trajectories, but on others, subjects produced irregularities in the kinematics. Like many past studies, we interpreted these irregularities as discrete corrective submovements. Unlike other studies, we used a direct, objective algorithm to identify overlapping submovements, detecting appreciable inflections in the acceleration traces by examining zero crossings in their derivatives, jerk and snap. The movements without overlapping submovements on average had a near symmetric, bell-shaped velocity profile that was independent of speed, and which matched the theoretical minimum jerk velocity very closely. We proposed three plausible mechanisms for altering the shape of movement kinematics, and implemented a mass-spring model with nonlinear damping to explore the possibilities. Although there was relatively little variability in the shape and symmetry of movements across trials, there was a fair amount of variability in their amplitude. We show that subjects attempted to eliminate the need for corrective submovements by making more accurate primary movements with practice, but that the variability inherent in rapid movements dictated the need for corrective submovements. Subjects used corrective submovements to improve final endpoint accuracy while reducing endpoint variability, resulting in higher task success rates.

Adult↗
PubMed2000

Functional connectivity between cerebellum and primary motor cortex in the awake monkey.

Simultaneous single neuron and local field potential (LFP) recordings were made in arm-related areas of the cerebellar nuclei (CN) and primary motor cortex (M1) of two monkeys during a reaching and button pressing task. Microstimulation of focal sites in CN caused short latency (median = 3.0 ms) increases in discharge in 25% of 210 M1 neurons. Suppressive effects were less common (13%) and observed at longer latencies (median = 9.9 ms). Stimulation in CN also caused reciprocal facilitation and suppression in averages of antagonist muscle electromyograms (EMGs). The latency of these effects was approximately 8-11 ms. In contrast to the selectivity of unit and EMG effects, stimulation-evoked changes in LFP occurred over a broad range of sites. There were no significant short-latency effects detected in cross-correlation histograms between single neurons in CN and M1. However, CN spike-triggered averages of M1 LFPs were observed in a few cases (10% of 126 cases). In one-half of these, there were effects both before and after the CN spikes, which may reflect causal effects from M1 to CN, as well as from CN to M1. Overall, these results demonstrate a spatially specific, short latency, primarily excitatory pathway from CN to M1. The relatively rare effects at the single neuron level may have resulted from the difficulty in achieving optimal alignment between cerebellar and cerebral sites because of the specificity of these connections.

Action Potentials↗
PubMed1999

A cerebellar model of timing and prediction in the control of reaching.

A simplified model of the cerebellum was developed to explore its potential for adaptive, predictive control based on delayed feedback information. An abstract representation of a single Purkinje cell with multistable properties was interfaced, using a formalized premotor network, with a simulated single degree-of-freedom limb. The limb actuator was a nonlinear spring-mass system based on the nonlinear velocity dependence of the stretch reflex. By including realistic mossy fiber signals, as well as realistic conduction delays in afferent and efferent pathways, the model allowed the investigation of timing and predictive processes relevant to cerebellar involvement in the control of movement. The model regulates movement by learning to react in an anticipatory fashion to sensory feedback. Learning depends on training information generated from corrective movements and uses a temporally asymmetric form of plasticity for the parallel fiber synapses on Purkinje cells.

Adaptation, Physiological↗
PubMed1999

Relationship between modulation of the cerebellorubrospinal system in the in vitro turtle brain and changes in motor behavior in rats: effects of novel sigma ligands.

Saturation and competition binding studies showed that the turtle brain contains sigma sites labeled by both [3H]di-o-tolylguanidine (DTG) and [3H](+)-pentazocine. There was a significant correlation between the IC50 values of sigma ligands for [3H]DTG sites in the turtle vs. rat brain, suggesting that the sites are comparable in the two species. In contrast, [3H](+)-pentazocine, which primarily labels sigma1 sites in the rodent brain, labels a heterogeneity of sites in the turtle brain. In extracellular recordings from the in vitro turtle brainstem, some sigma ligands enhanced the burst responses of red nucleus (RN) neurons (DTG, haloperidol, BD1031, BD1052, BD1069) while other sigma ligands decreased the burst responses (BD1047, BD1063). Control compounds (turtle Ringer vehicle control, opiate antagonist naloxone, atypical neuroleptic sulpiride) had no significant effects on the RN burst responses recorded from the in vitro turtle brain. The ED50s of the ligands for altering the burst responses in RN neurons from the turtle brain were correlated with their IC50s for turtle brain sites labeled with [3H]DTG, but not [3H](+)-pentazocine; this pattern is identical to that previously reported in rats, where there is a correlation between the potencies of sigma ligands for producing dystonic postures after microinjection into the rat RN and their binding to rat brain sites labeled with [3H]DTG, but not [3H](+)-pentazocine. When the novel sigma ligands were microinjected into the rat RN, dystonic postures were produced by ligands that increased the burst duration of RN neurons in the turtle brain. Novel sigma ligands that reduced the burst responses in the in vitro turtle brain have previously been reported to have no effects on their own when microinjected into the rat RN, but to block the dystonic postures produced by other sigma ligands. Taken together, the data suggest that the opposite effects of the novel ligands in the turtle electrophysiological studies represent the actions of agonists vs. antagonists, and that the directionality of the effects has predictive value for the expected motor effects of the drugs.

Animals↗
PubMed1999

Emergence of symmetric, modular, and reciprocal connections in recurrent networks with Hebbian learning.

While learning and development are well characterized in feedforward networks, these features are more difficult to analyze in recurrent networks due to the increased complexity of dual dynamics - the rapid dynamics arising from activation states and the slow dynamics arising from learning or developmental plasticity. We present analytical and numerical results that consider dual dynamics in a recurrent network undergoing Hebbian learning with either constant weight decay or weight normalization. Starting from initially random connections, the recurrent network develops symmetric or near-symmetric connections through Hebbian learning. Reciprocity and modularity arise naturally through correlations in the activation states. Additionally, weight normalization may be better than constant weight decay for the development of multiple attractor states that allow a diverse representation of the inputs. These results suggest a natural mechanism by which synaptic plasticity in recurrent networks such as cortical and brainstem premotor circuits could enhance neural computation and the generation of motor programs.

Animals↗
PubMed1998

Organization of ascending pathways to the forelimb area of the dorsal accessory olive in the cat.

The purpose of these experiments was to define the topography of cuneate and spinal projections to the forelimb representation in the rostral dorsal accessory olive (rDAO). We were interested in determining whether the spinal and cuneate inputs constitute a homogeneous afferent source, and whether there is evidence that they serve different functional roles. We were also interested in determining whether the somatotopy of rDAO is the result of a point-to-point projection from its afferent sources, or whether the projection suggests a reorganization of afferents at the olive. Single unit recording was used to identify specific regions of rDAO, and the topography of inputs to the identified regions was determined by using wheat germ agglutinin-horseradish peroxidase (WGA-HRP) as a tracer. The results from retrograde tracing were confirmed by using WGA-HRP as an anterograde tracer from input sources. The cuneate and spinal neurons providing input to rDAO constitute two distinct neural populations. One consists of cells in the caudal cuneate nucleus and lamina VI of the rostral two cervical segments, the other consists of cells in the rostral cuneate nucleus. The cells in the caudal cuneate nucleus and the rostral cervical segments are large, multipolar neurons that form a single column of rDAO input cells. The column of cells projects to the contralateral rDAO in a topographic fashion with rostral regions of the column projecting to rostral rDAO, which contains cells that respond to somatosensory stimulation of the contralateral shoulder, trunk, and proximal forelimb. Caudal regions of the column project to caudal rDAO, which contains cells that respond to stimulation of the distal forelimb. Despite this topography, there is a large degree of overlap in the terminations from neighboring regions of the input column, indicating that a major reorganization occurs at the rDAO. The projection from the rostral cuneate nucleus arises from small neurons that project bilaterally to rDAO, and the input from the rostral cuneate nucleus lacks a clear topography. We propose that input from the cell column is responsible for the somatosensory sensitivity of rDAO neurons, whereas input from rostral cuneate is most likely modulatory, probably inhibitory, in nature.

Animals↗
PubMed1998

Organization of reaching and grasping movements in the primate cerebellar nuclei as revealed by focal muscimol inactivations.

Two monkeys were trained to point to targets and to retrieve fruit bits from a Kluver board, bottles, and tubes. Once proficient in the tasks, the macaques underwent aseptic surgical implantation of a recording chamber over the cerebellar nuclei on the side of their preferred hand. After recovery from surgery, a series of mapping penetrations were completed to identify task-related areas within the cerebellar nuclei. Muscimol (4- 16 microgram; 1-2 microgram/microliter) was pressure injected at different sites within the forelimb zone, and the resultant deficits were observed as the monkeys performed the behavioral tasks. Quantitative measures of task performance were supplemented by direct observation of live and videotaped performance. The locations of nuclear inactivation sites were reconstructed from marking lesions and tracks visible in histological sections. Injections placed in the cerebellar interpositus nucleus and adjacent regions of dentate caused a variety of deficits in forelimb function. A prominent anteroposterior specialization was apparent within the forelimb zone of this intermediate nuclear region. Injections into the anterior interpositus nucleus and adjacent dentate impaired preshaping of the hand and the manipulation of objects, whereas injections placed more posteriorly in posterior interpositus nucleus and adjacent dentate produced deficits in the aiming of reach and the stability of the arm. During anterior injections, the monkeys failed to adequately extend their fingers in preparation for target contact, as documented for >85% of the reaches in the pointing task of monkey J. Up to 38% of the fruit bits it attempted to retrieve from the Kluver board were dropped. In comparison, during posterior inactivations, 15% were dropped and during control conditions 3% were dropped. The monkeys made significantly greater pointing errors during posterior inactivations (11 times for monkey J and 4 times for monkey C) than during anterior inactivations (8 times for monkey J and 2 times for monkey C). We refer to the region producing hand deficits as the anterior hand zone and the region producing reaching deficits as the posterior reach zone. These results are discussed in relation to the problem of achieving spatiotemporal coordination in the large population of nuclear cells that participate in any given movement. The results do not favor the hypothesis that coordination is achieved through a selection of Purkinje cells along beams of parallel fibers. Instead, it is proposed that distal and proximal musculature is coordinated by the adaptive influences of climbing fiber input to Purkinje cells. We envision a relatively nonspecific recruitment of anterior and posterior nuclear cells due to positive feedback in the limb premotor network, which then is shaped into an appropriate spatiotemporal pattern of discharge through the inhibitory input from Purkinje cells.

Animals↗
PubMed1998

Model of cortical-basal ganglionic processing: encoding the serial order of sensory events.

Several lines of evidence suggest that the prefrontal (PF) cortex and basal ganglia are important in cognitive aspects of serial order in behavior. We present a modular neural network model of these areas that encodes the serial order of events into spatial patterns of PF activity. The model is based on the topographically specific circuits linking the PF with the basal ganglia. Each module traces a pathway from the PF, through the basal ganglia and thalamus, and back to the PF. The complete model consists of an array of modules interacting through recurrent corticostriatal projections and collateral inhibition between striatal spiny units. The model's architecture positions spiny units for the classification of cortical contexts and events and provides bistable cortical-thalamic loops for sustaining a representation of these contextual events in working memory activations. The model was tested with a simulated version of a delayed-sequencing task. In single-unit studies, the task begins with the presentation of a sequence of target lights. After a short delay, the monkey must touch the targets in the order in which they were presented. When instantiated with randomly distributed corticostriatal weights, the model produces different patterns of PF activation in response to different target sequences. These patterns represent an unambiguous and spatially distributed encoding of the sequence. Parameter studies of these random networks were used to compare the computational consequences of collateral and feed-forward inhibition within the striatum. In addition, we studied the receptive fields of 20,640 model units and uncovered an interesting set of cue-, rank- and sequence-related responses that qualitatively resemble responses reported in single unit studies of the PF. The majority of units respond to more than one sequence of stimuli. A method for analyzing serial receptive fields is presented and utilized for comparing the model units to single-unit data.

Algorithms↗
PubMed1997

Network models of the basal ganglia.

Over the past year, a number of conceptual and mathematical models of the basal ganglia and their interactions with other areas of the brain have appeared in the literature. Even though the models each differ in significant ways, several computational principles, such as convergence, recurrence and competition, appear to have emerged as common themes of information processing in the basal ganglia. Simulation studies of these models have provoked new types of questions at the many levels of inquiry linking biophysics to behavior.

Animals↗
PubMed1997

Prediction of complex two-dimensional trajectories by a cerebellar model of smooth pursuit eye movement.

A neural network model based on the anatomy and physiology of the cerebellum is presented that can generate both simple and complex predictive pursuit, while also responding in a feedback mode to visual perturbations from an ongoing trajectory. The model allows the prediction of complex movements by adding two features that are not present in other pursuit models: an array of inputs distributed over a range of physiologically justified delays, and a novel, biologically plausible learning rule that generated changes in synaptic strengths in response to retinal slip errors that arrive after long delays. To directly test the model, its output was compared with the behavior of monkeys tracking the same trajectories. There was a close correspondence between model and monkey performance. Complex target trajectories were created by summing two or three sinusoidal components of different frequencies along horizontal and/or vertical axes. Both the model and the monkeys were able to track these complex sum-of-sines trajectories with small phase delays that averaged 8 and 20 ms in magnitude, respectively. Both the model and the monkeys showed a consistent relationship between the high- and low-frequency components of pursuit: high-frequency components were tracked with small phase lags, whereas low-frequency components were tracked with phase leads. The model was also trained to track targets moving along a circular trajectory with infrequent right-angle perturbations that moved the target along a circle meridian. Before the perturbation, the model tracked the target with very small phase differences that averaged 5 ms. After the perturbation, the model overshot the target while continuing along the expected nonperturbed circular trajectory for 80 ms, before it moved toward the new perturbed trajectory. Monkeys showed similar behaviors with an average phase difference of 3 ms during circular pursuit, followed by a perturbation response after 90 ms. In both cases, the delays required to process visual information were much longer than delays associated with nonperturbed circular and sum-of-sines pursuit. This suggests that both the model and the eye make short-term predictions about future events to compensate for visual feedback delays in receiving information about the direction of a target moving along a changing trajectory. In addition, both the eye and the model can adjust to abrupt changes in target direction on the basis of visual feedback, but do so after significant processing delays.

Animals↗
PubMed1996

Somatosensory and movement-related properties of red nucleus: a single unit study in the turtle.

Extracellular recordings were performed from turtle red nucleus neurons to examine their responsiveness to peripheral somatic stimulation and to study differences between rubral sensory and movement-related responses. In pentobarbital sodium-anesthetized or decerebrate turtles, red nucleus neurons could be divided into two categories based on their response characteristics. The first group, which included 87% of neurons studied, had low spontaneous rates of activity and responded with excitation to electrical stimulation of the spinal cord or the cerebellum, or during active movement of the contralateral limbs. Neurons in this category were likely to be rubrospinal cells. The remaining 13% of cells studied had higher rates of spontaneous discharge and were inhibited by electrical stimulation or during active movement. These cells might be rubral GABAergic interneurons. Single red nucleus neurons responded with excitation and/or inhibition to somatosensory stimulation. Unlike the motor fields, which were restricted to a single contralateral limb, red nucleus sensory receptive fields were wide and often bilaterally distributed. Rubral responsiveness to sensory stimulation was found to be significantly diminished during active limb movements, thereby suggesting that sensory inputs to the red nucleus are not used for the on-line modification of motor commands. Inactivation of the cerebellar cortex enhanced the sensory responsiveness of rubral neurons and expanded the size of red nucleus receptive fields. These results suggest that the red nucleus receives substantial sensory input, and that the cerebellar cortex can modify the flow of sensory information to the red nucleus.

Animals↗