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Centre de Recherche en Sciences Neurologiques, Département de Physiologie, Université de Montréal, Montreal, Quebec H3C 3T8, Canada
Submitted 13 December 2002; accepted in final form 6 October 2003
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ABSTRACT |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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) are credited with a more recent proposal that the lateral and intermediate subdivisions of the cerebellum reflect separate loop circuits with different functional roles. They suggested that the lateral cerebellum, including the hemispheric cortex and dentate nucleus, together with the premotor cortex might be responsible for planning and preprogramming movements. In contrast, once a voluntary command signal was sent to the spinal cord by the motor cortex, then the intermediate region composed of the paravermal cortex and the anterior and posterior interposed nuclei would implement corrections and revisions to the motor commands based on feedback from muscle and skin afferents conveyed to the cerebellum over spinocerebellar pathways.
The concept that the control of voluntary movement involves both feedback corrections and feedforward anticipation is, in one form or another, widely accepted. More recent formulations of this idea hold that an internal model reflecting the mechanical properties of the limb and its loading conditions is stored within the CNS and in particular, the cerebellum, which is thought to play a role in motor learning (Blakemore et al. 2001; Schweighofer 1998a,b; Wolpert et al. 1998). It is further suggested that the internal models are able to simulate and anticipate the dynamic behavior of the arm and hand-held objects from prior experience (Jordan and Wolpert 1999; Wolpert et al. 2001). This internal model is necessarily subjected to constant revision from peripheral feedback.
Several studies have shown predictable force-pulse perturbations applied to a hand-held object produce reflex-like responses to the perturbations, but with repetition, preparatory increases in grip force emerge prior to the perturbation (Johansson and Westling 1988; Lacquaniti and Maioli 1989; Winstein et al. 2000). Interestingly, the capacity to make anticipatory responses in preparation for catching a ball is severely impaired in cerebellar patients (Lang and Bastian 1999). During the past several years, we demonstrated that monkeys also show similar adaptive behaviors when confronted with predictable perturbations. However, single-cell recordings of the neuronal discharge in the primary motor cortex (M1), the supplementary motor cortex (SMA), the dorsal and ventral premotor cortex (PMd and PMv), and the cingulate motor area (CMA) related to anticipation of a predictable perturbation has so far failed to find any strong evidence of activity related to these preparatory behaviors (Boudreau et al. 2001; Cadoret and Smith 1997; Picard and Smith 1992). To date, the only region where we have found specific anticipatory activity is in cells of the paravermal and hemispheric cerebellar cortex (Dugas and Smith 1992). About half the neurons recorded in the intermediate and lateral cerebellar cortex responded to the perturbation of a hand-held object with short-latency, reflex-like increases in discharge that were time-locked to the perturbation and that disappeared when the perturbation was withdrawn. About a quarter of the cells recorded in the same region demonstrated increases in discharge frequency that appeared related to preparatory increases in grip force prior to the perturbation. From the location of these neurons, it seemed clear that these neurons projected to both the interpositus and dentate.
The function of cerebro-cerebellar loops suggested by Allen and Tsukahara (1974) might predict that the shear and slip on the fingers produced by perturbing a hand-held object would provide the interpositus nucleus with a specific error signal. The anticipatory neuronal activity changes, which might be indicative of a reorganization of the adaptive motor strategy, ought to be most evident in the dentate nucleus, although Dugas and Smith (1992) reported anticipatory responses in the paravermal cortex projecting to the interpositus nuclei as well. With this hypothesis in mind, the present study was undertaken to explore unit activity of the anterior interpositus and dentate nucleus in search of neurons related to either the slip and shear error signal or to the anticipatory increase in grip force associated with an adaptation to the predictable perturbation.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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). However, in this paper, we will focus on the single-unit recordings from the dentate and interposed nuclei. This study was approved by the animal ethics committee of the Faculty of Medicine of the Université de Montréal. Apparatus
The monkeys were trained to grasp, lift, and hold the armature of a linear motor described in previous studies (Boudreau et al. 2001; Brochier et al. 1999) and shown in Fig. 1. Essentially, this 1 df armature was set in a compressed air bushing that allowed near frictionless movement in the vertical direction. The voltage applied to the coil provided a range of resistive forces opposing lifting. Flat rectangular grasping surfaces for the thumb and index finger were attached to a horizontal strut mounted at 90° to one end of the armature. A load cell measured the total compression or grip force between the index and thumb, and a second load cell mounted on the armature measured the vertical or lifting force (load force) on the armature. The vertical displacement of the armature was also measured. The three analog signals were digitized at 250 Hz and stored on a laboratory computer.
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Details of the training procedures have been published previously (Dugas and Smith 1992), and a diagram of the task is shown in Fig. 1. Briefly, the monkeys were trained to grasp a metal tab covered by emery paper and to lift and hold it stationary between 15 and 35 mm from the starting position for 2.5 s. The linear motor generated a force of 0.5-0.6 N, simulating an object weight of 
50-60 g. To facilitate task performance, a 1.0-kHz tone signaled that the armature had been lifted to the desired position. On successful trials, the monkeys were rewarded with a small quantity of apple juice. The monkeys were required to release the tab for 2.5 s before a new trial could be initiated. The testing procedures applied to each recorded neuron were similar to those used in previous studies. That is, the monkeys performed a block of 25-40 consecutive unperturbed trials followed by a block of a similar number of perturbed trials. Finally if the cell was still well isolated, a third block of unperturbed trials was performed. Occasionally, cells were tested with different hold-phase durations of 1.5, 2.5, or 3.5 s if the firing-rate decreased immediately after receiving the reward.
Force-pulse perturbations
For the perturbed trials, a 100-ms force-pulse was given 1.5 s after the onset of the tone. On the majority of trial blocks, the perturbation was preceded by a light flash presented 0.7 s after the onset of the tone and 0.8 s prior to the force-pulse perturbation although an earlier study failed to find any evidence that the monkeys actually used this cue (Boudreau et al. 2001). The magnitude of the force-pulse perturbation was adjusted between 3.5 and 5.0 N to produce a downward displacement of the object, which usually would have prevented the animal from receiving the fruit juice reward. To ensure the reward, the monkeys had to resist the predictable force-pulse perturbation by increasing the grip force and stiffening the wrist to maintain it within the limits of the position window. As described in previous studies, the force-pulse perturbation elicited a reflex-like response, which persisted as long as the perturbation was present. With repeated trials, a preparatory increase in grip force emerged prior to the onset of the perturbation. This preparatory response extinguished slowly after the perturbation was removed.
Surgical preparation and single-unit recording procedures
When the monkeys had achieved a stable level of performance, they were anesthetized with ketamine followed by isoflurane and surgically prepared for cerebellar single-cell recording according to previously published procedure studies (Dugas and Smith 1992). An 18-mm circular chamber was implanted over the cerebellum ipsilateral to the working arm. The stereotaxic coordinates of the center of the chamber were 5.0 mm posterior to interaural zero and 6.0 mm lateral to the midline. After a postoperative recovery period, recording sessions were conducted on a daily basis (6-7 days/wk). Glass-insulated tungsten microelectrodes were vertically advanced through the cerebral cortex into the cerebellum using a Trent-Wells microdrive attached to an X-Y micropositioner. Identification of Purkinje cells in the anterior cerebellar cortex with activity modulated by task performance was the first step in identifying the nuclear region to be explored. Next, mapping of the deep cerebellar nuclei was initiated by identifying the position of the lateral limit of the dentate nucleus. Using these coordinates as a reference, the interpositus and dentate nuclei were explored searching for cells with activity specifically modulated during performance of the grasp, lift, and hold task. Whenever possible, the cells were tested for the presence of peripheral receptive fields (RFs). This examination consisted of imposing movements on the shoulder, elbow, wrist, and fingers and tapping the muscle mass of the arm or the thenar eminence. Potentially cutaneous fields were tested for responses to air puffs and stroking the skin with a camelhair brush and probing the skin with calibrated flexible monofilaments; however, these were very rare (2/56) in the cerebellar nuclei.
Histological analysis
Before the conclusion of recording sessions, small electrolytic lesions were made to identify the limits of the recording area. The animals were killed with an overdose of pentobarbital and perfused transcardially with 0.9% saline followed by 4% paraformaldehyde. The brains were immersed in a solution of 20% sucrose at 4°C for 24 h for cryoprotection before freezing at -80°C. The cerebellum was cut into 40-µm frozen coronal sections on a cryostat and the sections were stained with cresyl violet.
Quantitative and statistical analysis
According to previous studies (Boudreau et al. 2001; Dugas and Smith 1992), the cell discharge in the lift-and-hold task was judged to be significantly modulated if a change in the firing rate deviated by >2 SDs for 
200 ms from a mean baseline activity occurring 1 s before the grip force onset. An anticipatory response to the perturbation was defined by comparing the mean activity histograms with and without the perturbation. If an activity change of >2 SDs from an equivalent period of time in unperturbed control trials occurred, then the cell was identified as demonstrating a preparatory response. If a similar change began after the perturbation, the cell was considered as demonstrating a reflex-like response. Some cells had both preparatory and reflex-like responses. To identify the time at which preparatory activity changes occurred, mean activity on perturbed and unperturbed trials was compared every 50 ms for preparatory responses and every 5 ms for the reflex-like responses. A t-test or an ANOVA (followed by Tukey's HSD multiple comparison test) determined whether the perturbation had any significant influence on cellular activity (P < 0.05).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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The initial recording sessions were used to map the extent of the dentate and interposed nuclei to reliably determine the location of modulated cell activity. Once the lateral and caudal limits of the dentate nucleus had been established, 150 cells with activity related to performance of the precision grip were recorded in two monkeys. Examination of the gliosis associated with repeated electrode penetrations in cresyl-violetstained sections indicated that the greatest number of penetrations were concentrated in the anterior interpositus nucleus. Figure 2 shows coronal cerebellar sections from both monkeys. On the basis of the stereotaxic coordinates relative to the position of the lateral and caudal borders of the dentate nucleus and the nuclear contours described by Courville and Cooper (1970), it appears that the exploration of the posterior interpositus and dentate nucleus was less extensive than the anterior interpositus. Figure 3 indicates the approximate locations of the electrode penetrations in both monkeys. Cells with taskmodulated activity and cells additionally responding specifically to the perturbations were all located in the same region of the anterior interpositus nucleus, near, but not within, the dorsomedial border of the dentate nucleus.
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One-hundred-fifty task-related neurons were recorded in the anterior interpositus nucleus in two monkeys. Activity generally increased before grip onset, and almost half the cells (72/150 or 47%) were active only during the dynamic lifting phase of the task (e.g., the cell shown in Fig. 6A). Slightly more than 20% (33/150) of the cells had both phasic-tonic activity patterns (Fig. 6B) and only 5/150 had purely tonic activity during maintained grasping. In addition, 17/150 neurons had discharge patterns that simply decreased with the pinching whereas 13/150 had complex modulations, which could not be classified. Finally, we noted a particular variation of phasictonic activity in which the neuronal discharge stopped as soon as the fruit juice reward was delivered although the monkey continued to grip the tab without a change in position (Figs. 4A and 7).
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Clear receptive fields were established for 56/150 (37%) of the cells. As with the cerebellar cortex (Dugas and Smith 1992), we did not find any single cells receiving convergent proprioceptive and cutaneous afferents. During receptive-field testing, care was taken to prevent joint motion in the stimulated areas when examining for cutaneous responses, and cutaneous responses were identified only if cellular discharge increased in response to air puffs, light brushing, or punctate pressure with monofilaments. As a result, the vast majority of receptive fields were proprioceptive (54/56) and only 2/56 were identified as clearly cutaneous. Generally the receptive fields appeared to originate from forearm muscles and the neurons responded to stretches applied to the wrist and fingers. Some receptive fields were larger and included arms or both the ipsilateral arm and leg.
Responses to predictable force-pulse perturbations
Like cells in the cerebellar cortex, cells in the interpositus nucleus had two distinct responses to the predictable perturbation. Reflex-like responses occurred within 80 ms after the perturbation and disappeared as soon as the perturbations had been suspended. Preparatory responses occurred prior to the perturbation and emerged gradually with repeated trials. Also these preparatory responses extinguished slowly once the perturbations ceased. In all, 127/150 task-related cells were tested with the predictable force-pulse perturbation. Table 1 presents the results of the 127 tested cells with the perturbations. The preparatory responses were thought to be associated with adaptive increases in grip force because the perturbations were highly predictable allowing the monkeys to develop an adaptive strategy such as stiffening the wrist and the increasing grip force. In addition, the monkeys sometimes used another behavioral adaptation, which consisted of raising the grasping tab higher within the position window. This additional motor strategy did not seem to affect either the preparatory or reflex-like responses.
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The activity associated with the preparatory behavior generally appeared after a few perturbed trials. However, this rarely appeared as a clear and systematic increase such as would be apparent from the activity rasters. Figure 4 shows examples of a preparatory response in one neuron (Fig. 4A) and a combined preparatory and reflex-like response in another neuron shown on (Fig. 4B). The uninterrupted traces correspond to the averages on perturbed trials and the preparatory increase in grip force can be distinguished clearly from the unperturbed average. For the neuron shown on the left of the Fig. 4A, a slight, but progressive increase in the discharge frequency of the neuron can also be seen to occur at the same time as the grip force increased. Figure 4B illustrates an example of both a preparatory and a reflex-like response in another interpositus neuron. This cell also had an increase in activity prior to the perturbation, but in addition, it showed the sharp onset of a reflex-like response to the perturbation occurring 40 ms after the perturbation. The firing frequency of 39/127 single cells related to the precision-grip task increased their activity significantly (P < 0.05) prior to the onset of the force-pulse perturbation, and Fig. 5A shows the latency distribution of these preparatory responses. The onset of preparatory response ranged from 1,500 to 70 ms prior the perturbation; however, the distribution of the latencies was not normal or Gaussian. The average onset time of the preparatory response was almost 650 ms before the perturbation. By comparison the onset of preparatory responses in cerebellar cortex was closer to 500 ms prior to the force-pulse perturbation (Dugas and Smith 1992), but the duration of the static phase was also shorter than in the present study.
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These activity changes appeared and disappeared immediately with the perturbation. A t-test was used to compare the mean firing rate of the nuclear cells 100 ms before the perturbation with those occurring 
100 ms after the perturbation. For 75/127 neurons, the force-pulse perturbation elicited a significant (P < 0.05) reflex-like grip force increase at relatively short latencies. The distribution of cerebellar cortical response latencies, shown in Fig. 5B had a mean of 36 ± 22 ms. The reflex-like responses were mainly (57/75) increases in activity but decreases in activity were also observed. Figure 6A shows a typical reflex-like response associated with an activity increase, whereas Fig. 6B illustrates a decrease in neuronal discharge after the perturbation. No significant difference was found between latencies of activity increases or decreases.
Activity cessation on reward
For a few neurons (11/127), the tonic activity during the maintained precision grip ceased once the reward was signaled by an audible click of the juice-delivery solenoid. The cell activity, which up to that point appeared to be tightly associated with the grip and load force traces, was suddenly dissociated from the motor activity. We varied the duration of the static phase to test this response in 3/11 of these cells. Figure 7 illustrates this type of activity where the neuronal discharge stopped at the reward even though the monkeys continued to grip the tab without change in either force or position for another 0.5 to 1.0 s. This was most clearly shown by the cell in Fig. 7, which was tested with static phase durations of 1.5, 2.5, and 3.5 s. However, a similar pattern can be seen in the same cell shown in Fig. 4A. It can be seen from Fig. 7 that the activity stopped as soon as the reward was given to the monkey although the monkey continued the static holding for some time after the reward.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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), found cerebellar cortical cells with modulated activity over a rather wide area extending well beyond the paravermal, intermediate zone into the lateral hemisphere where Purkinje cells very likely projected to the dentate nucleus. In contrast, the nuclear cell activity that seemed specifically related to the grasp, lift, and hold movement was restricted to the dorsal anterior interpositus, and, in spite of several penetrations in the dentate nucleus, no cell activity related to performance of the lift-and-hold task was found in this area. These data would seem to agree with those of Van Kan et al. (1994), who noted much more activity related to grasping in the anterior interpositus than the dentate. Moreover their task involved whole-arm reaching as well as grasping movements. Again, contrary to what we had expected from Allen and Tsukahara's hypothesis, the cells with anticipatory activity were found in the anterior interpositus but were not observed in our recordings within the dentate nucleus. Our interpretation of the Allen and Tsukahara's hypothesis was that feedback from the perturbation would be conveyed to the cerebellum over spinocerebellar pathways, whereas preparatory commands in anticipation of the perturbation would be transmitted to the cerebellum from the cerebral cortex through the pontine nuclei. Instead, cells with anticipatory activity or reflex-like responses as well as cells with both activity patterns were all found in the anterior interpositus. It would appear that the same cerebellar nuclear neurons that respond to feedback from slip on the fingers are also implicated in the preparatory responses that prevent these same slips.
Anatomical connections of dentate and interpositus
An important question raised by these data is whether they are consistent with the established anatomical connections of the dentate and interpositus nuclei. The hind limb spinal afferents to the cerebellar cortex have been the subject of much more research than the connections with the cerebellar nuclei (see review by Bloedel and Courville 1981). There are few studies of the spinal projections from the forelimb to the deep cerebellar nuclei. A physiological study by Bantli and Bloedel (1977) described converging afferents from many regions of the body to single dentate neurons, whereas other investigators have described more specific and focused projections from forelimb muscle afferents to anterior interpositus neurons (Armstrong et al. 1973, 1975; Eccles et al. 1974; MacKay and Murphy 1974). Although it was generally assumed that these responses were conveyed by cuneo-cerebellar projections, an additional anatomical pathway has been shown from the lateral reticular nucleus to both the anterior interpositus and dentate nuclei. (Dietrichs 1983; Matsushita and Ikeda 1976; Qvist 1989; Wu et al. 1999).
The efferent targets of both the anterior interpositus and dentate are better established than the excitatory afferent sources. Both nuclei send axons out of the cerebellum through the superior cerebellar peduncle. The interpositus and dentate axons establish en passant connections with the magnocellular and parvocellular red nucleus respectively on their way to thalamic nuclei including ventralis posterior lateralis, pars oralis (VPLo), which in turn, projects to area 4 motor cortex. Using retrograde trans-synaptic tracers, Wiesendanger and Wiesendanger (1985) and Hoover and Strick (1999) found that both the dorsal dentate and dorsal anterior interpositus nuclei project to the arm and hand area of the primary motor cortex. In contrast, the ventral dentate projects to a variety of premotor and prefrontal area targets (Middleton and Strick 2001; Wiesendanger and Wiesendanger 1985).
Cerebro-cerebellar interactions
Some authors have linked the phylogenetic expansion of the lateral cerebellar cortex and the dentate nucleus with the emergence and development of the cerebral cortical association areas and suggested a role in the initiation of voluntary limb movements (Evarts and Thach 1969). Alternatively, Massion (1973) suggested that the hypertrophy of the lateral cerebellum seen in primates might be associated with the use of brachiation for locomotion. According to this view, the expansion of the lateral cerebellum might reflect an increased range of motion about the shoulder in a context of reaching to grasp. More recently, Thach et al. (1992) and Mason et al. (1998) have described deficits in reaching and grasping movements after inactivation of the dentate-interpositus region by intracerebellar injections of muscimol. However, reaching and grasping was not impaired after dentate inactivation in the cat (Martin et al. 2000) nor did dentate inactivation in a monkey impair an over-trained grasp and lifting movement (see Monzée et al. 2004). It would seem that the inactivation evidence is equivocal with respect to what parts of the arm and hand are controlled by the dentate and interpositus nuclei.
Anticipatory activity elsewhere in the brain
When we first published the anticipatory responses of cerebellar cortical neurons to predictable perturbations (Dugas and Smith 1992), we fully expected to find similar activity in the premotor regions of the cerebral cortex. This expectation was surprisingly not confirmed. Using a similar paradigm of predictable perturbations, we explored the supplementary and cingulate motor areas (Cadoret and Smith 1997), the dorsal and ventral premotor areas (Boudreau et al. 2001), and the motor cortex itself (Picard and Smith 1992). Although all of these areas yielded modulated activity patterns related to grasping and lifting as well as reflex-like responses to the perturbation, none of these same areas had any significant amount of anticipatory activity resembling neurons in the cerebellar nuclei and the cerebellar cortex. This is indeed puzzling in view of the well-known projections from the anterior interpositus to the motor cortex through the thalamus. It would certainly be of interest to examine the responses of red nucleus neurons to predictable perturbations given the strong rubral projections from the anterior interpositus and its proposed role in synaptic plasticity suggested by Pananceau et al. (1996).
Cerebellum and inverse dynamics and forward models
Although the cerebellum has long been thought to play a critical role in motor learning, an extensive review is beyond the scope of the present study. However, a number of hypothetical mechanisms have been proposed to explain the adaptive contribution of the cerebellum to motor learning (Hansel et al. 2001 Houk et al. 1996: Ito 2001; Smith 1996; Thach 1996). Recent theoretical concepts about sensorimotor control suggest that the CNS may acquire activity patterns, termed neural internal models that are able to simulate and anticipate the dynamic behavior of the arm and of the hand-held object from prior experience (Jordan and Wolpert 1999; Kawato 1999). Feedback from previous experience would be employed to generate an internal inverse dynamics model of the limb, hand, and object. The appropriate motor command necessary to compensate for a predictable perturbation could be calculated in a feedforward manner from an inverse model of the arm, hand, and object dynamics. The motor command sent to the arm muscles to stiffen the wrist and fingers to absorb the perturbation would also be sent to a forward dynamics model as an efference copy. From the current state of the limb and hand and from the efference copy of the command to muscles, the dynamics forward model can estimate both the required joint stiffness and the expected reafferent feedback. Just such a mechanism has been proposed for the cerebellum (Blakemore et al. 2001; Schweighofer 1998a,b; Wolpert et al. 1998), and the present study suggests that the interpositus nucleus and the intermediate zone of the cerebellar cortex are involved with both the generation of an inverse dynamics model and forward control commands in grasping and object manipulation.
Increased activity in both Purkinje and nuclear cells
Dugas and Smith (1992) reported that although perturbations evoked both increases and decreases in Purkinje cell activity, the anticipatory activity consisted entirely of increased activity. Given that Purkinje cells are known to inhibit the cerebellar nuclei, it is puzzling that all the anticipatory activity recorded in the present study was also excitatory. Do Purkinje cells simply serve to limit the excitation of nuclear cells or do Purkinje and nuclear show reciprocally organized excitation and inhibition related to different muscle groups? Although we did not observe many inhibited nuclear cells, it is possible that decreased activity was actually present but failed to reach the statistical significance because of an inadequate sample size or the absence of a clearly defined onset and duration. Alternatively, mossy fiber afferents may well be driving simultaneous excitation of both Purkinje cells through the parallel fiber system and the nuclear cells through mossy fiber collaterals (Dietrichs 1983; Matsushita and Ikeda 1976; Qvist 1989; Wu et al. 1999). This is a long-standing paradox that was first described and discussed in behaving animals by Thach (1970) for wrist movements, and later a similar observation was made by Armstrong and Edgley (1988) during locomotion in the cat. Hansel et al. (2001) recently proposed that both nuclear and Purkinje cells can demonstrate use-dependent long-term depression and potentiation. However, further experiments are needed to determine the simultaneous actions of connected Purkinje and nuclear cells during motor performance and learning.
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ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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GRANTS
This research was supported by a grant to Groupe de Recherche en Sciences Neurologiques from the Canadian Institutes for Health Research Council and to the Groupe de Recherche sur le Système Nerveux Central from the Fonds pour la Formation des Chercheurs et l'Aide à la Recherche and the FRSQ-FCAR Santé program.
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FOOTNOTES |
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Address reprint requests and other correspondence to: A. M. Smith, Centre de Recherche en Sciences Neurologiques, Dépt. de Physiologie, Université de Montréal, C.P. 6128 Succursale Centre ville, Montréal, Québec H3C 3T8, Canada (E-mail: allan.smith{at}umontreal.ca).
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