Lack of renewal effect in extinction of naturally acquired conditioned eyeblink responses, but possible dependency on physical context.

Context dependency of extinction is well known and has extensively been studied in fear conditioning, but has rarely been assessed in eyeblink conditioning. One way to demonstrate context dependency of extinction is the renewal effect. ABA paradigms are most commonly used to show the renewal effect of extinguished learned fear: if acquisition takes place in context A, and extinction takes place in context B (extinction phase), learned responses will recover in subsequent extinction trials presented in context A (renewal phase). The renewal effect of the visual threat eyeblink response (VTER), a conditioned eyeblink response, which is naturally acquired in early infancy, was examined in a total of 48 young and healthy participants with two experiments using an ABA paradigm. Twenty paired trials were performed in context A (baseline trials), followed by 50 extinction trials in context B (extinction phase) and 50 extinction trials in context A (renewal phase). In 24 participants, contexts A and B were two different rooms, and in the other 24 participants, two different background colors (orange and blue) and noises were used. To rule out spontaneous recovery, an AAA design was used for comparison. There were significant effects of extinction in both experiments. No significant renewal effects were observed. In experiment 2, however, extinction was significantly less using orange background during extinction compared to the blue background. The present findings suggest that extinction of conditioned eyeblinks depends on the physical context. Findings add to the animal literature that context can play a role in the acquisition of classically conditioned eyeblink responses. Future studies, however, need to be performed to confirm the present findings. Lack of renewal effect may be explained by the highly overlearned character of the VTER.

Age effects in storage and extinction of a naturally acquired conditioned eyeblink response.

Acquisition of conditioned eyeblink responses is known to decline with age, and age-related decline has been related to a reduction of cerebellar size and function. The aim of the present study was to investigate age-related effects on storage-related processes and extinction of visual threat eyeblink responses (VTERs), conditioned responses which are naturally acquired in early childhood. Storage and extinction of VTERs were tested in 34 healthy participants with an age range from 21 to 74 years (mean age 41.6±16.3 years). High-resolution structural magnetic resonance images (MRI) were acquired in all subjects. Conventional volumetric measures and voxel-based morphometry (VBM) were performed at the level of the cerebellum. Storage and extinction of VTERs showed a significant age-dependent decline. Likewise, cerebellar volume decreased with age. Storage, but not extinction showed a significant positive correlation with age-dependent reduction of total cerebellar volume. VBM analysis showed that gray matter volume in circumscribed areas of intermediate lobules VI, and Crus I and II bilaterally were positively correlated with VTER storage (p<0.05, FWE corrected). Considering extinction, no significant correlations with gray matter cerebellar volume were observed. The present findings show that reduction of storage of learned eyeblink responses with age is explained at least in part by age-dependent decline of cerebellar function. Future studies need to be performed to better understand which brain areas contribute to age-dependent reduction of extinction.

The cerebellum and eye-blink conditioning: learning versus network performance hypotheses.

Classical conditioning of the eye-blink reflex in the rabbit is a form of motor learning that is uniquely dependent on the cerebellum. The cerebellar learning hypothesis proposes that plasticity subserving eye-blink conditioning occurs in the cerebellum. The major evidence for this hypothesis originated from studies based on a telecommunications network metaphor of eye-blink circuits. These experiments inactivated parts of cerebellum-related networks during the acquisition and expression of classically conditioned eye blinks in order to determine sites at which the plasticity occurred. However, recent evidence revealed that these manipulations could be explained by a network performance hypothesis which attributes learning deficits to a non-specific tonic dysfunction of eye-blink networks. Since eye-blink conditioning is mediated by a spontaneously active, recurrent neuronal network with strong tonic interactions, differentiating between the cerebellar learning hypothesis and the network performance hypothesis represents a major experimental challenge. A possible solution to this problem is offered by several promising new approaches that minimize the effects of experimental interventions on spontaneous neuronal activity. Results from these studies indicate that plastic changes underlying eye-blink conditioning are distributed across several cerebellar and extra-cerebellar regions. Specific input interactions that induce these plastic changes as well as their cellular mechanisms remain unresolved.

Inferior olivary inactivation abolishes conditioned eyeblinks: extinction or cerebellar malfunction?

The inferior olive (IO) is a required component of neural circuits controlling the classical conditioning of eyeblink responses. Previous reports indicated that lesioning or inactivating the IO abolishes conditioned eyeblinks (CRs), but there was disagreement regarding the timing of the CR performance deficit. As a result, it was not clear whether IO inactivation produces unlearning of CRs or a non-specific dysfunction of cerebellar circuits. Since most of these studies used methods that could block unrelated axons passing through the IO region, additional experiments are required to further elucidate IO function, using inactivating agents that act selectively on cell bodies. In the present study, the IO was inactivated using the glutamate receptor antagonist DGG and the GABA-A receptor agonist muscimol in rabbits performing well-learned CRs. Effects of inactivating the IO on CR expression and on neuronal activity in the anterior cerebellar interposed nucleus (IN) were examined. We found that either blocking excitatory glutamate inputs or activating inhibitory GABA inputs to the IO abolished CRs. This effect occurred with variable delay following drug injections. Additional experiments, in which post-injection testing was delayed to allow for drug diffusion, revealed invariably immediate suppression of CRs. This demonstrated that suppressing IO activity using DGG or muscimol does not induce unlearning of CRs. Single-unit recording during DGG injections revealed that CR suppression was paralleled by a dramatic suppression of IN neuronal activity. We concluded that inactivating the rostral parts of the IO complex abolishes CRs by producing a tonic malfunction of cerebellar eyeblink conditioning circuits.

Inactivation of cerebellar output axons impairs acquisition of conditioned eyeblinks.

Acquisition of classically conditioned eyeblink responses (CRs) in the rabbit critically depends on intermediate cerebellum-related neural circuits. A highly efficient method for determining possible sites of plasticity within eyeblink circuits is the reversible inactivation of circuit components during learning. Inactivation of either the HVI region of the cerebellar cortex or the cerebellar interposed nuclei (IN) during learning is known to prevent CR acquisition. In contrast, inactivating cerebellar efferent axons in the brachium conjunctivum (BC) with small injections of tetrodotoxin (TTX) has been reported to have no effect on CR acquisition. This suggested that the intermediate cerebellum is essential for learning CRs and that activity mediated by the BC is not required for this process. Since we previously found that BC inactivation blocks CR extinction we re-examined its role in CR acquisition. To ensure complete and long-lasting inactivation of the BC, we injected before each training session doses of TTX that were larger than those in the previous acquisition study. Contrary to the previous negative findings, we found that this temporary block of axons in the brachium conjunctivum prevented normal acquisition of CRs. Injecting TTX directly in the adjacent lateral lemniscus, which could possibly influence CR acquisition, had no effect on learning. In addition, a functional test of TTX diffusion around the BC indicated that the inactivation did not affect other known parts of eyeblink circuits, such as the cerebellar interposed nuclei, the middle cerebellar peduncle or the contralateral red nucleus. We conclude that this form of associative learning in the rabbit eyeblink system requires extra-cerebellar learning and/or cerebellar learning that depends on the operation of cerebellar feedback loops.

Glutamate neurotransmission in the cerebellar interposed nuclei: involvement in classically conditioned eyeblinks and neuronal activity.

The cerebellar interposed nuclei (IN) are critical components of a neural network that controls the expression of classically conditioned eyeblinks. The IN receive 2 major inputs: the massive, gamma-aminobutyric acid (GABA)-mediated input from the Purkinje cells of the cerebellar cortex and the relatively weaker, glutamate-mediated input from collaterals of mossy and climbing fiber cerebellar afferent systems. To elucidate the role of IN glutamate neurotransmission in conditioned response (CR) expression, effects of blocking fast glutamatergic neurotransmission in the IN with gamma-d-glutamylglycine (DGG) on the expression of conditioned eyeblinks and on cerebellar nuclear neuronal activity were examined. Surprisingly, blocking fast glutamate receptors in the IN did not abolish CRs. DGG decreased CR incidence and slightly increased CR latency. In contrast, identical amounts of DGG applied to the cerebellar cortex abolished CRs. Similar to the behavioral effects, DGG had unexpectedly mild effects on IN neurons. At the population level, the baseline firing frequency of IN cells was not affected. After DGG injections, the incidence of excitatory modulation of cell activity in the interstimulus interval decreased but was not abolished. A combined block of fast glutamate and GABA(A) neurotransmission using a mixture of DGG and picrotoxin dramatically reduced CR incidence, increased the firing frequency of all cell types, and virtually abolished all modulation of neuronal activity. These results indicate that fast glutamate neurotransmission in the IN plays only an accessory role both in the expression of behavioral CRs and in the generation of associated neuronal activity in the IN.

GABA neurotransmission in the cerebellar interposed nuclei: involvement in classically conditioned eyeblinks and neuronal activity.

The cerebellar interposed nuclei (IN) are an essential part of circuits that control classically conditioned eyeblinks in the rabbit. The function of the IN is under the control of GABAergic projections from Purkinje cells of the cerebellar cortex. The exact involvement of cerebellar cortical input into the IN during eyeblink expression is not clear. While it is known that the application of gamma-aminobutyric acid-A (GABA(A)) agonists and antagonists affects the performance of classically conditioned eyeblinks, the effects of these drugs on IN neurons in vivo are not known. The purpose of the present study was to measure the effects of muscimol and picrotoxin on the expression of conditioned eyeblinks and the activity of IN cells simultaneously. Injections of muscimol abolished conditioned responses and either silenced or diminished the activity of IN cells. Two injections were administered in each picrotoxin experiment. The first injection of picrotoxin slightly modified the timing and amplitude of the eyeblink, produced mild tonic eyelid closure, increased tonic activity of IN cells, and reduced the amplitude of the neural responses. The second injection of picrotoxin abolished conditioned responses, further increased tonic eyelid closure, dramatically elevated the tonic activity of IN cells, and in most cases, abolished neuronal responses. These results demonstrate that both GABA(A)-mediated inactivation and tonic up-regulation of IN cells can interrupt the expression of conditioned eyeblinks and that this behavioral effect is accompanied by the suppression of the neuronal activity correlates of the conditioned stimulus and response.

Intermediate cerebellum and conditioned eyeblinks. Parallel involvement in eyeblinks and tonic eyelid closure.

The intermediate cerebellum (the intermediate cerebellar cortex and interposed nuclei) and associated brainstem circuits are essential for the acquisition and expression of classically conditioned eyeblinks in the rabbit. The purpose of the present experiment was to determine whether these circuits are also involved in adaptive eyelid closure learned in an instrumental paradigm. For that purpose, rabbits with unrestrained eyelids were trained in two tasks: (1) classical conditioning of the eyeblink; and (2) a new instrumental task in which they avoided delivery of an aversive stimulus by maintaining tonic eyelid closure. To examine the involvement of the intermediate cerebellum in these two types of learned behavior, the cerebellar interposed nuclei were injected with the GABAA agonist muscimol and with the GABAA antagonist picrotoxin. Inactivating the interposed nuclei with muscimol abolished classically conditioned eyeblinks and severely affected the rabbit's capacity to maintain tonic eyelid closure. On the other hand, reducing inhibition with picrotoxin failed to interrupt the learned responses and increased the amplitude of eyelid closure. These data indicate that the cerebellar interposed nuclei control both phasic classically conditioned eyeblinks and tonic instrumental eyelid closure. To account for this new finding, a "hybrid" hypothesis combining the cerebellar learning hypothesis and the performance hypothesis is proposed.

Effects of accuracy constraints on reach-to-grasp movements in cerebellar patients.

Reach-to-grasp movements of patients with pathology restricted to the cerebellum were compared with those of normal controls. Two types of paradigms with different accuracy constraints were used to examine whether cerebellar impairment disrupts the stereotypic relationship between arm transport and grip aperture and whether the variability of this relationship is altered when greater accuracy is required. The movements were made to either a vertical dowel or to a cross bar of a small cross. All subjects were asked to reach for either target at a fast but comfortable speed, grasp the object between the index finger and thumb, and lift it a short distance off the table. In terms of the relationship between arm transport and grip aperture, the control subjects showed a high consistency in grip aperture and wrist velocity profiles from trial to trial for movements to both the dowel and the cross. The relationship between the maximum velocity of the wrist and the time at which grip aperture was maximal during the reach was highly consistent throughout the experiment. In contrast, the time of maximum grip aperture and maximum wrist velocity of the cerebellar patients was quite variable from trial to trial, and the relationship of these measurements also varied considerably. These abnormalities were present regardless of the accuracy requirement. In addition, the cerebellar patients required a significantly longer time to grasp and lift the objects than the control subjects. Furthermore, the patients exhibited a greater grip aperture during reach than the controls. These data indicate that the cerebellum contributes substantially to the coordination of movements required to perform reach-to-grasp movements. Specifically, the cerebellum is critical for executing this behavior with a consistent, well-timed relationship between the transport and grasp components. This contribution is apparent even when accuracy demands are minimal.

The human cerebellum and associative learning: dissociation between the acquisition, retention and extinction of conditioned eyeblinks.

The present paper is part of a systematic exploration of the neural substrates of conditioned eyeblink responses in humans. Normal subjects and patients with lesions restricted to the cerebellum were examined for their ability to acquire new classically conditioned eyeblinks to an auditory conditioned stimulus and whether they were able to perform and extinguish a previously learned natural anticipatory eyeblink response - the kinesthetic threat eyeblink response (KTER). In classical conditioning to an auditory conditioned stimulus, cerebellar patients failed to acquire new conditioned responses. In contrast to this impairment, in the KTER task both cerebellar patients and control subjects exhibited a high incidence of anticipatory eyeblinks which were initiated before the forehead tap. These results indicate that the cerebellar circuits, which are critical for the acquisition of new conditioned responses, are not essential for the storage and expression of naturally acquired conditioned responses. In the extinction experiment, cerebellar patients failed to extinguish their KTERs. This finding suggests that in humans, the acquisition of new and the extinction of previously learned conditioned responses depends on a similar set of cerebellar circuits.

The kinesthetic threat eyeblink: a new type of anticipatory eyeblink response.

The present paper is part of a systematic exploration of naturally acquired conditioned eyeblink responses in human subjects. Normal human subjects were examined for the presence of anticipatory eyeblinks in a new paradigm. They were instructed to move their hand quickly toward their face and tap their forehead. In this situation, subjects generated anticipatory eyeblinks which were initiated before the forehead tap. Additional experiments revealed that visual stimuli and internal movement-planning cues are not required for the initiation of this response. The kinesthetic information from the passively moving arm, however, was sufficient to trigger this kinesthetic threat eyeblink response (KTER). The KTER extinguished when the forehead tap did not reinforce it. These data indicate that the KTER is a unique type of naturally acquired conditioned response system which is maintained by aversive reinforcing events.

Inactivation of interposed nuclei in the cat: classically conditioned withdrawal reflexes, voluntary limb movements and the action primitive hypothesis.

The cerebellar interposed nuclei are considered critical components of circuits controlling the classical conditioning of eyeblink responses in several mammalian species. The main purpose of the present experiments was to examine whether the interposed nuclei are also involved in the control of classically conditioned withdrawal responses in other skeletomuscular effector systems. To achieve this objective, a unique learning paradigm was developed to examine classically conditioned withdrawal responses in three effector systems (the eyelid, forelimb and hindlimb) in individual cats. Trained animals were injected with muscimol in the cerebellar interposed nuclei, and the effects on the three conditioned responses (CRs) were examined. Although the effects of muscimol were less dramatic than previously reported in the rabbit eyeblink preparation, the inactivation of the cerebellar nuclei affected the performance of CRs in all three effector systems. In additional experiments, animals were injected with muscimol at the sites affecting classically conditioned withdrawal responses to determine the effects of these injections on reaching and locomotion behaviors. These tests demonstrated that the same regions of the cerebellar interposed nuclei which control withdrawal reflexes are also involved in the control of limb flexion and precision placement of the paw during both locomotion and reaching tasks. The obtained data indicate that the interposed nuclei are involved in the control of ipsilateral action primitives and that inactivating the interposed nuclei affects several modes of action of these functional units.

Current concepts of climbing fiber function.

This review examines several of the current postulates regarding the function of one of the most intriguing afferent systems in the brain, the climbing fiber system. The fact that these afferents are activated under a variety of conditions has contributed substantially to the diversity of postulates that have been proposed. In part because of the unique anatomical relationship between individual climbing fibers and the dendritic tree of Purkinje cells, these afferents have been proposed as a key input in establishing long-term plastic changes in the cerebellar cortex. This concept is contrasted with other postulates proposing that the heterosynaptic action of this system produces a short-lasting enhancement rather than a long-term depression of Purkinje cell responsiveness. Although a generally accepted view regarding climbing fiber function does not exist, this review emphasizes the extensive functional insights that have been reported and supports the notion that progress toward a complete understanding of these afferents will require an integration of their morphological characteristics with the fundamental physiological properties of their responses assessed in a variety of contexts and conditions.

Ataxia reflected in the simulated movements of patients with cerebellar lesions.

Previous studies demonstrated that the time required to simulate mentally a complex movement is highly correlated with the time required to execute the same task. The purpose of this experiment was to examine whether this relationship exists when execution times are prolonged as a consequence of the motor abnormalities exhibited by patients with substantial cerebellar pathology. The paradigm required subjects to alternate between moving a hand-held stylus horizontally on a digitizing tablet through a four-segment template and imagining the same movement through the same template. These two modes of performance were compared based on the times required to complete the two types of trials. Performance using both upper extremities was assessed using templates with two different levels of difficulty. Difficulty was varied by interposing gates that narrowed the path through the template. Using a MANOVA, measurements of actual and simulated movement times were compared between the group of cerebellar patients and a group of age- and sex-matched controls. The results showed that: (1) both movement times and mental-simulation times were greater for cerebellar patients than for control subjects under all experimental conditions, (2) both the movement times and the mental-simulation times of the patients were greater on the more-affected side than on the less-affected side, and (3) on the more-affected side, there was no significant difference between the patients' simulation and movement times for either the more difficult or less difficult condition. Thus, the consequence of cerebellar dysfunction on the time required to execute a volitional movement is reflected in the time needed to simulate the same behavior.

Microinjections of anisomycin into the intermediate cerebellum during learning affect the acquisition of classically conditioned responses in the rabbit.

The purpose of this study was to examine the effects of protein synthesis inhibition in the intermediate cerebellum on the acquisition and expression of classically conditioned nictitating membrane responses in the rabbit. Animals were conditioned for three days in a standard delay paradigm. Before each training session, either a solution of anisomycin (a protein synthesis inhibitor) or vehicle was bilaterally injected into the interposed cerebellar nuclear. Following these three training sessions, rabbits were tested to determine whether the previous training under the influence of anisomycin or vehicle resulted in the acquisition of conditioned responses. In this test, animals that were injected previously with the protein synthesis inhibitor exhibited significantly less retention of conditioned responses than rabbits injected with vehicle. Additional experiments demonstrated that anisomycin does not block the expression of conditioned responses during conditioning or in well-trained animals. Microinjections of muscimol at the same sites of the previous drug infusions suppressed the expression of conditioned responses, indicating that the protein synthesis inhibitor was applied to the eyeblink-related parts of cerebellar circuits. The obtained data are the first to demonstrate that a manipulation of cerebellar circuits, which does not affect the performance of learned behavior, can affect the process of learning. These results suggest that the synthesis of new proteins in the intermediate cerebellum participates in the formation of plastic changes responsible for eyeblink conditioning.

Effects of cerebellar nuclear inactivation on the learning of a complex forelimb movement in cats.

The purpose of this study was to determine the effects of inactivating concurrently the cerebellar interposed and dentate nuclei on the capacity of cats to acquire and retain a complex, goal-directed forelimb movement. To assess the effects on acquisition, cats were required to learn to move a vertical manipulandum bar through a two-segment template with a shape approximating an inverted "L" after the injection of muscimol (saline for the control group) in the interposed and dentate cerebellar nuclei. During training periods, they were exposed progressively to more difficult templates, which were created by decreasing the angle between the two segments of the template. After determining the most difficult template the injected animals could learn within the specified time and performance constraints, the retraining phase of the experiment was initiated in which the cats were required to execute the same sequence of templates in the absence of any injection. This stage of the experiment assessed retention and determined the extent of any relearning required to execute the task at criterion levels. Next, the animals were overtrained without any injection on the most difficult template they could perform. Finally, to determine the effects of nuclear inactivation on retention after extensive retraining, their capacity to perform the same template was determined after muscimol injection in the interposed and dentate nuclei. The findings show that during the inactivation of the dentate and interposed nuclei the animals could learn to execute the more difficult templates. However, when required to execute the most difficult template learned under muscimol on the day after injections were discontinued, the cats had to "relearn" (reacquire) the movement. Finally, when the cerebellar nuclei were inactivated after the animals learned the task in the absence of any injections during the retraining phase, retention was not blocked. The data indicate that the intermediate and lateral cerebellum are not required either for learning this type of complex voluntary movement or for retaining the capacity to perform the task once it is learned. Nevertheless, when the cerebellum becomes available for executing a task learned in the absence of this structure, reacquisition of the behavior usually is necessary. It is hypothesized that the relearning observed after acquisition during muscimol inactivation reflects the tendency of the system to incorporate the cerebellum into the interactions responsible for the learning and performance of a motor sequence that is optimal for executing the task.

Patients with cerebellar lesions cannot acquire but are able to retain conditioned eyeblink reflexes.

The purpose of these experiments was to examine the role of the human cerebellum in the acquisition and retention of conditioned reflexes. Normal human subjects and patients with cerebellar lesions were tested for their capacity to acquire, retain and express conditioned eyeblink responses. In acquisition tests, subjects were trained in a delay classical conditioning paradigm using a tone conditioned stimulus and a midline forehead tap as an unconditioned stimulus. While normal subjects developed anticipatory eyeblinks to the tone in one session, patients with cerebellar lesions failed to acquire conditioned responses in four consecutive training sessions. The conditioning deficit was bilateral even in patients with a unilateral cerebellar pathology. The same groups of subjects were tested for the presence of eyeblinks to a visual threat. In these experiments, both normal subjects and patients with cerebellar lesions exhibited a high level of responding when they saw an object approaching their face. These eyeblinks to the visual threat are probably naturally acquired conditioned responses because they extinguish in normal subjects if they are not reinforced by the unconditioned cutaneous stimulus. In addition, the stimulus of seeing an approaching object blocks the acquisition of classically conditioned eyeblinks to a new conditioned stimulus in normal subjects. These data imply that patients with cerebellar lesions who cannot acquire new classically conditioned responses are able to retain and express conditioned eyeblinks which were acquired before the onset of the pathology. Consequently, cerebellum-dependent neural substrates which are involved in learning new conditioned reflexes do not seem to be required for the storage of naturally learned conditioned responses.

Effects of inactivating individual cerebellar nuclei on the performance and retention of an operantly conditioned forelimb movement.

These experiments were designed to examine the effects of inactivating separately each of the major cerebellar nuclear regions in cats on the execution and retention of a previously learned, operantly conditioned volitional forelimb movement. The experiments test the postulates that the cerebellar nuclei, and particularly the interposed nuclei, contribute substantially to the spatial and temporal features of the interjoint coordination required to execute the task and that the engram necessary for the retention of this task is not located in any one of the cerebellar nuclei. All cats were trained to perform a task in which they were required to reach for and grasp a vertical bar at the sound of a tone and move the bar to a reward zone through a template consisting of two straight grooves in the shape of an inverted "L." After the task was learned, the effects of inactivating separately each nuclear region (the fastigial, interposed, and dentate nuclei) using muscimol microinjections were determined. Data were analyzed by quantifying several features of the movement's kinematics and by determining changes in the organization of the reaching component of the movement using an application of dimensionality analysis, an analysis that examines the correlation among the changes in joint angles and limb segment positions during the task. The retention of the previously learned task also was assessed after each injection. Injections of each nuclear region affected temporal and spatial features of the learned movement. However, the largest effects resulted from inactivating the interposed nuclei. These effects included an increased length of the reach trajectory, an accentuated deviation of the wrist trajectory from a straight line, cyclic movement of the distal extremity as the target was approached, a difficulty in grasping the bar, altered temporal features of the movement, and a highly characteristic change in the dimensionality measurements. The changes in dimensionality reflected a decreased correlation (linear interdependence) of the joint angular velocities coupled with an increased correlation among the linear velocities of markers located on the joints themselves. Related but less consistent changes in dimensionality resulted from fastigial injections. The motor sequence required to negotiate the template could be executed after the nuclear microinjections, indicating that retention of the motor sequence was not affected by the inactivation of any of the cerebellar nuclei. However, in two of the five animals, some decreases in performance were observed after dentate injection that were not characteristic of changes related to an effect on retention. These data suggest that the cerebellum plays an important role in regulating the consistent, stereotypic organization of complex goal-directed movements, including the temporal correlation among joint angle velocities. The data also indicate that the retention of the task is not dependent on any of the individual cerebellar nuclear regions. Consequently, these structures are unlikely to be critical storage sites for the engram established during the learning of this task.

Impaired capacity of cerebellar patients to perceive and learn two-dimensional shapes based on kinesthetic cues.

This study addresses the issue of the role of the cerebellum in the processing of sensory information by determining the capability of cerebellar patients to acquire and use kinesthetic cues received via the active or passive tracing of an irregular shape while blindfolded. Patients with cerebellar lesions and age-matched healthy controls were tested on four tasks: (1) learning to discriminate a reference shape from three others through the repeated tracing of the reference template; (2) reproducing the reference shape from memory by drawing blindfolded; (3) performing the same task with vision; and (4) visually recognizing the reference shape. The cues used to acquire and then to recognize the reference shape were generated under four conditions: (1) "active kinesthesia," in which cues were acquired by the blindfolded subject while actively tracing a reference template; (2) "passive kinesthesia," in which the tracing was performed while the hand was guided passively through the template; (3) "sequential vision," in which the shape was visualized by the serial exposure of small segments of its outline; and (4) "full vision," in which the entire shape was visualized. The sequential vision condition was employed to emulate the sequential way in which kinesthetic information is acquired while tracing the reference shape. The results demonstrate a substantial impairment of cerebellar patients in their capability to perceive two-dimensional irregular shapes based only on kinesthetic cues. There also is evidence that this deficit in part relates to a reduced capacity to integrate temporal sequences of sensory cues into a complete image useful for shape discrimination tasks or for reproducing the shape through drawing. Consequently, the cerebellum has an important role in this type of sensory information processing even when it is not directly associated with the execution of movements.

Conditioned and unconditioned forelimb reflex systems in the cat: involvement of the intermediate cerebellum.

Temporary inactivation of the cerebellar interposed nuclei was used to assess the role of the intermediate cerebellum in the performance of forelimb cutaneo-muscular reflexes in the cat. The following types of reflexive responses were evaluated: the classically conditioned and unconditioned forelimb withdrawal responses and the forelimb tactile placing, hopping and magnet responses. The experiments tested the hypothesis that the intermediate cerebellum is involved in the performance of all the above forelimb reflexes. The forelimb withdrawal reflex was classically conditioned in a newly developed paradigm in which animals were first operantly conditioned to stand on four elevated platforms. Trained animals were microinjected with a gamma-aminobutyric acid (GABA) agonist, muscimol, in the interposed nuclei, and the effects of inactivation of the intermediate cerebellar output on the forelimb reflexes were examined. The main findings of these experiments are that unilateral muscimol inactivation of the interposed nuclei in the cat abolished the expression of the classically conditioned limb flexion reflex, suppressed the performance of the unconditioned withdrawal reflex and, in parallel, down-regulated the tactile placing, hopping and magnet postural responses in the ipsilateral forelimb. These observations are inconsistent with concepts indicating exclusive involvement of the intermediate cerebellum in the classically conditioned reflexes elicited by aversive stimuli. On the contrary, they support the hypothesis of a more global involvement of this structure in learned and unlearned defensive flexion reflexes and in automatic postural response systems.