On the cerebellum, cutaneomuscular reflexes, movement control and the elusive engrams of memory

Noradrenergic projections regulate the acquisition of classically conditioned eyelid responses in wild-type and are impaired in kreisler mice

During embryonic development, heterozygous mutant kreisler mice undergo ectopic expression of the Hoxa3 gene in the rostral hindbrain, affecting the opioid and noradrenergic systems. In this model, we have investigated behavioral and cognitive processes in their adulthood. We confirmed that pontine and locus coeruleus neuronal projections are impaired, by using startle and pain tests and by analyzing immunohistochemical localization of tyrosine hydroxylase. Our results showed that, even if kreisler mice are able to generate eyelid reflex responses, there are differences with wild-types in the first component of the response (R1), modulated by the noradrenergic system. The acquisition of conditioned motor responses is impaired in kreisler mice when using the trace but not the delay paradigm, suggesting a functional impairment in the hippocampus, subsequently confirmed by reduced quantification of alpha2a receptor mRNA expression in this area but not in the cerebellum. Moreover, we demonstrate the involvement of adrenergic projection in eyelid classical conditioning, as clonidine prevents the appearance of eyelid conditioned responses in wild-type mice. In addition, hippocampal motor learning ability was restored in kreisler mice by administration of adrenergic antagonist drugs, and a synergistic effect was observed following simultaneous administration of idazoxan and naloxone.

The Emerging Concept of Intrinsic Plasticity: Activity-dependent Modulation of Intrinsic Excitability in Cerebellar Purkinje Cells and Motor Learning

What is memory? How does the brain process the sensory information and modify an organism's behavior? Many neuroscientists have focused on the activity- and experience-dependent modifications of synaptic functions in order to solve these fundamental questions in neuroscience. Recently, the plasticity of intrinsic excitability (called intrinsic plasticity) has emerged as an important element for information processing and storage in the brain. As the cerebellar Purkinje cells are the sole output neurons in the cerebellar cortex and the information is conveyed from a neuron to its relay neurons by forms of action potential firing, the modulation of the intrinsic firing activity may play a critical role in the cerebellar learning. Many voltage-gated and/or Ca2+-activated ion channels are involved in shaping the spiking output as well as integrating synaptic inputs to finely tune the cerebellar output. Recent studies suggested that the modulation of the intrinsic excitability and its plasticity in the cerebellar Purkinje cells might function as an integrator for information processing and memory formation. Moreover, the intrinsic plasticity might also determine the strength of connectivity to the sub-cortical areas such as deep cerebellar nuclei and vestibular nuclei to trigger the consolidation of the cerebellar-dependent memory by transferring the information.

Cerebellar fastigial nucleus: from anatomic construction to physiological functions

Fastigial nucleus (FN) is the phylogenetically oldest nucleus in the cerebellum, a classical subcortical motor coordinator. As one of the ultimate integration stations and outputs of the spinocerebellum, the FN holds a key position in the axial, proximal and ocular motor control by projecting to the medial descending systems and eye movement related nuclei. Furthermore, through topographic connections with extensive nonmotor systems, including visceral related nuclei in the brainstem, hypothalamus, as well as the limbic system, FN has also been implicated in regulation of various nonsomatic functions, such as feeding, cardiovascular and respiratory, defecation and micturition, immune, as well as emotional activities. In clinic, FN lesion or dysfunction results in motor deficits including spinocerebellar ataxias, and nonmotor symptoms. In this review, we summarize the cytoarchitecture, anatomic afferent and efferent connections, as well as the motor and nonmotor functions of the FN and the related diseases and disorders. We suggest that by bridging the motor and nonmotor systems, the cerebellar FN may help to integrate somatic motor and nonsomatic functions and consequently contribute to generate a coordinated response to internal and external environments.

Anticipatory Vibrotactile Cueing Facilitates Grip Force Adjustment during Perturbative Loading

Grip force applied to an object held between the thumb and index finger is automatically and unconsciously adjusted upon perception of an external disturbance to the object. Typically, this adjustment occurs within approximately 100 ms. Here, we investigated the effect of anticipatory vibrotactile cues prior to a perturbative force, which the central nervous system may use for rapid grip re-stabilization. We asked participants to grip and hold an instrumented, actuated handle between the thumb and index finger. Under computer control, the handle could suddenly be pulled away from a static grip and could independently provide vibration to the gripping fingers. The mean latency of corrective motor action was 139 ms. When vibrotactile stimulation was applied 50 ms before application of tractive force, the latency was reduced to 117 ms, whereas the mean latency of the conscious response to vibrotactile stimuli alone was 229 ms. This suggests that vibrotactile stimulation can influence reflex-like actions. We also examined the effects of anticipatory cues using a set of perturbative loads with different rising rates. As expected, facilitation of grip force adjustment was observed for moderate loads. In contrast, anticipatory cues had an insignificant effect on rapid loads that evoked an adjustment within 60-80 ms, which approaches the minimum latency of human grip adjustment. Understanding the facilitative effects of anticipatory cues on human reactive grip can aid the development of human-machine interfaces to enhance human behavior.

Spino-olivary projections in the rat are anatomically separate from postsynaptic dorsal column projections

The gracile nucleus (GN) and lateral part of rostral dorsal accessory olive (rDAO) are important relays for indirect, postsynaptic dorsal column, and direct ascending pathways, respectively, that terminate as climbing fibers in the “hindlimb-receiving” parts of the C1 and C3 zones in the cerebellar cortex. While the spinal cells of origin of that project to GN and rDAO are from largely separate territories in the spinal cord, previous studies have indicated that there could be an area of overlap between these two populations in the medial dorsal horn. Given the access of these two ascending tracts to sensory (thalamic) versus sensorimotor (precerebellar) pathways, the present study therefore addresses the important question of whether or not individual neurons have the potential to contribute axons to both ascending pathways. A double-fluorescent tracer strategy was used in rats (red Retrobeads and Fluoro-Ruby or green Retrobeads and Fluoro-Emerald) to map the spatial distribution of cells of origin of the two projections in the lumbar spinal cord. The two pathways were found to receive input from almost entirely separate territories within the lumbar cord (levels L3–L5). GN predominantly receives input from lamina IV, while rDAO receives its input from three cell populations: medial laminae V–VI, lateral lamina V, and medial laminae VII–VIII. Cells that had axons that branched to supply both GN and rDAO represented only about 1% of either single-labeled cell population. Overall, the findings therefore suggest functional independence of the two ascending pathways. J. Comp. Neurol. 522:2179–2190, 2014. © 2013 Wiley Periodicals, Inc.

Behavioral correlates of complex spike synchrony in cerebellar microzones

The olivo-cerebellar system is crucial for smooth and well timed execution of movements based on sensory and proprioceptive cues. The inferior olive (IO) plays a pivotal role in this process by synchronizing its activity across neurons internally through connexin36 gap junctions and providing a timing and/or learning signal to the cerebellum. Even though synchrony achieved through electrical coupling in IO cells is generally thought to be important in timing motor output, a direct relation between timing of movement and synchrony of olivary discharges has never been demonstrated within functional microcomplexes using transgenics. Here we combined in vivo, two-photon calcium imaging of complex spikes in microcomplexes of Purkinje cell (PC) dendrites with high-speed filming of tail, trunk, and limb movements in awake wild-type and connexin36-deficient mice. In wild types at rest, functional clusters of PCs were poorly defined with synchrony correlations that were relatively small and spatially limited to mediolateral distances of ∼50 μm, whereas during locomotion synchrony of the same PCs increased in strength and extended over distances spanning multiple microzones that could be correlated to specific components of sharp and well bounded movements. Instead, connexin36-deficient mice exhibited prolonged and desynchronized complex spike activity within PC microcomplexes both at rest and during behavior. Importantly, the mutants also showed concomitant abnormalities in the execution of spinocerebellar reflexes, which were significantly slower and more gradual than in wild-type littermates, particularly following sensory perturbations. Our results highlight the importance of modulation of synchronous activity within and between cerebellar microcomplexes in on-line temporal processing of motor output.

Consensus paper: current views on the role of cerebellar interpositus nucleus in movement control and emotion

In the present paper, we examine the role of the cerebellar interpositus nucleus (IN) in motor and non-motor domains. Recent findings are considered, and we share the following conclusions: IN as part of the olivo-cortico-nuclear microcircuit is involved in providing powerful timing signals important in coordinating limb movements; IN could participate in the timing and performance of ongoing conditioned responses rather than the generation and/or initiation of such responses; IN is involved in the control of reflexive and voluntary movements in a task- and effector system-dependent fashion, including hand movements and associated upper limb adjustments, for quick effective actions; IN develops internal models for dynamic interactions of the motor system with the external environment for anticipatory control of movement; and IN plays a significant role in the modulation of autonomic and emotional functions.

Acquisition of conditioned eyeblink responses is modulated by cerebellar tDCS

BACKGROUND

Classical conditioning of the eyeblink reflex is a simple form of motor learning which depends on the integrity of the cerebellum. Acquisition of conditioned eyeblink responses is markedly reduced in patients with cerebellar disorders. Noninvasive transcranial direct current stimulation (tDCS) has been reported to modify the excitability of the cerebellar cortex.

OBJECTIVE

The aim of the study was to assess whether acquisition of conditioned eyeblink responses (CR) is altered by cerebellar tDCS.

METHODS

A standard delay conditioning paradigm with a 540 ms tone as conditioned stimulus (CS) coterminating with a 100 ms air puff as unconditioned stimulus (US) was used in a total of 30 healthy subjects (18 female, 12 male, mean age 23.4 ± 1.9 years). One hundred paired CS-US trials and 30 extinction CS alone trials were given. tDCS (2 mA intensity, ramp like onset) was applied over the right cerebellar hemisphere ipsilaterally to the US during the acquisition phase. Subjects were randomly assigned to three groups (n = 10) using anodal, cathodal or sham stimulation. The investigator as well as the participants was blinded to the stimulation modality.

RESULTS

CR acquisition was significantly enhanced by anodal tDCS (mean total CR incidence 73.4 ± 25.2%) and significantly reduced by cathodal stimulation (12.6 ± 17.2%) compared to sham stimulation (43.8 ± 24.1%). During anodal tDCS CR onset occurred significantly earlier, that is mean onset of responses was shifted closer to CS onset.

CONCLUSION

Acquisition and timing of conditioned eyeblink responses is modified by cerebellar tDCS in a polarity dependent manner.

Comparison of the classically conditioned withdrawal reflex in cerebellar patients and healthy control subjects during stance: 2. Biomechanical characteristics

This study addresses cerebellar involvement in classically conditioned nociceptive lower limb withdrawal reflexes in standing humans. A preceding study compared electromyographic activities in leg muscles of eight patients with cerebellar disease (CBL) and eight age-matched controls (CTRL). The present study extends and completes that investigation by recording biomechanical signals from a strain-gauge-equipped platform during paired auditory conditioning stimuli (CS) and unconditioned stimuli (US) trials and during US-alone trials. The withdrawal reflex performance-lifting the stimulated limb (decreasing the vertical force from that leg, i.e. 'unloading') and transferring body weight to the supporting limb (increasing the vertical force from that leg, i.e. 'loading')-was quantified by the corresponding forces exerted onto the platform. The force changes were not simultaneous but occurred as a sequence of multiple force peaks at different times depending on the specific limb task (loading or unloading). Motor learning, expressed by the occurrence of conditioned responses (CR), is characterized by this sequence beginning already within the CSUS window. Loading and unloading were delayed and prolonged in CBL, resulting in incomplete rebalancing during the analysis period. Trajectory loops of the center of vertical pressure-derived from vertical forces-were also incomplete in CBL within the recording period. However, exposing CBL to a CS resulted in motor improvement reflected by shortening the time of rebalancing and by optimizing the trajectory loop. In summary, associative responses in CBL are not absent although they are less frequent and of smaller amplitude than in CTRL.

Cerebellum and conditioned reflexes

The central assumption of existing models of motor learning in the cerebellum is that cerebellar mossy fibres signal information about the context in which a movement is to be performed and climbing fibres signal in relation to a movement error. This leads to changes in the responsiveness of Purkinje cells, which on the next occasion will generate a corrected output in a given context. Support for this view has come mainly from work on adaptation of the vestibulo-ocular reflex. The discovery that classically conditioned eyeblink responses depend critically on the cerebellum offers the possibility to study the learning of a novel behaviour, rather than modification of an existing reflex. After repeated pairing of a neutral stimulus, such as a tone, with a blink-eliciting stimulus, the tone will acquire the ability to elicit a blink on its own. We review evidence from studies employing a wide variety of techniques that the cerebellum is critical in this type of learning as well as evidence that mossy and climbing fibres have roles assigned to them in cerebellar learning models.

What features of limb movements are encoded in the discharge of cerebellar neurons?

This review examines the signals encoded in the discharge of cerebellar neurons during voluntary arm and hand movements, assessing the state of our knowledge and the implications for hypotheses of cerebellar function. The evidence for the representation of forces, joint torques, or muscle activity in the discharge of cerebellar neurons is limited, questioning the validity of theories that the cerebellum directly encodes the motor command. In contrast, kinematic parameters such as position, direction, and velocity are widely and robustly encoded in the activity of cerebellar neurons. These findings favor hypotheses that the cerebellum plans or controls movements in a kinematic framework, such as the proposal that the cerebellum provides a forward internal model. Error signals are needed for on-line correction and motor learning, and several hypotheses postulate the need for their representations in the cerebellum. Error signals have been described mostly in the complex spike discharge of Purkinje cells, but no consensus has emerged on the exact information signaled by complex spikes during limb movements. Newer studies suggest that simple spike firing may also encode error signals. Finally, Purkinje cells located more posterior and laterally in the cerebellar cortex and dentate neurons encode nonmotor, task-related signals such as visual cues. These results suggest that cerebellar neurons provide a complement of information about motor behaviors. We assert that additional single unit studies are needed using rich movement paradigms, given the power of this approach to directly test specific hypotheses about cerebellar function.

Comparison of the classically conditioned withdrawal reflex in cerebellar patients and healthy control subjects during stance: I. electrophysiological characteristics

The aim of this study was to demonstrate the involvement of the human cerebellum in the classically conditioned lower limb withdrawal reflex in standing subjects. Electromyographic activity was recorded from the main muscle groups of both legs of eight patients with cerebellar disease (CBL) and eight control subjects (CTRL). The unconditioned stimulus (US) consisted of electrical stimulation of the tibial nerve at the medial malleolus. The conditioning stimulus (CS) was an auditory signal given via headphones. Experiments started with 70 paired conditioning stimulus-unconditioned stimulus(CSUS) trials followed by 50 US-alone trials. The general reaction consisted of lifting and flexing the stimulated (stepping) leg with accompanying activation of the contralateral (supporting) leg. In CTRL, the ipsilateral (side of stimulation) flexor and contralateral extensor muscles were activated characteristically. In CBL, the magnitudes of ipsilateral flexor and contralateral extensor muscle activation were reduced comparably. In CTRL, the conditioning process increased the incidence of conditioned responses (CR), following a typical learning curve, while CBL showed a clearly lower CR incidence with a marginal increase, albeit, at a shorter latency. Conditioning processes also modified temporal parameters by shortening unconditioned response (UR) onset latencies and UR times to peak and, more importantly in CBL, also the sequence of activation of muscles, which became similar to that of CTRL. The expression of this reflex in standing subjects showed characteristic differences in the groups tested with the underlying associative processes not being restricted exclusively to the CR but also modifying parameters of the innate UR.

Differences in unconditioned and conditioned responses of the human withdrawal reflex during stance: muscle responses and biomechanical data

The aim of this study was to characterize differences between unconditioned and classically conditioned lower limb withdrawal reflexes in young subjects during standing. Electromyographic activity in the main muscle groups and biomechanical signals from a strain-gauge-equipped platform on which subjects stood were recorded from 17 healthy subjects during unconditioned stimulus (US)-alone trials and during auditory conditioning stimuli (CS) and US trials. In US-alone trials the leg muscle activation sequence was characteristic: ipsilateral, distal muscles were activated prior to proximal muscles; contralaterally the sequence was reversed. In CSUS trials latencies were shorter. Subjects unloaded the stimulated leg and shifted body weight to the supporting leg. In US-alone and in CSUS trials leg forces on each side were inversely related and asymmetric, due to preparation for unloading, whilst conditioned responses (CR), representing the unloading preparation, were symmetric. The trajectory of the center of vertical pressure during US-alone trials moved initially forward (a preparatory balance reaction) and to the stimulation side, followed by a large lateral shift to the side of the supporting limb. During CSUS trials the forwards shift was absent but the CR (early lateral shift) represented a preponed preparatory unloading. Electrophysiological and biomechanical responses of the classically conditioned lower limb withdrawal reflex in standing subjects changed significantly in CSUS trials compared to US-alone trials with higher sensitivity in the biomechanics. These findings will serve as a basis for a subsequent study on a group of patients with cerebellar diseases in whom the success of establishing procedural processes is known to be impaired.

Timing of conditioned eyeblink responses is impaired in children with attention-deficit/hyperactivity disorder

Structural changes of the cerebellum have been reported in several psychiatric diseases like schizophrenia, autism and attention-deficit/hyperactivity disorder (ADHD). Beside behavioral deficits children with ADHD often show slight motor abnormalities. Cerebellar malfunction may contribute. The cerebellum is a structure essential for motor coordination, various forms of motor learning and timing of motor responses. In the present study, eyeblink conditioning was applied to investigate learning and timing of motor responses both in children with ADHD and children with cerebellar lesions. Acquisition, timing and extinction of conditioned eyeblink responses were investigated in children with ADHD, children with chronic surgical cerebellar lesions and controls using a standard delay paradigm with two different interstimulus intervals. Timing of conditioned eyeblink responses was significantly impaired in children with ADHD in the long interstimulus interval condition. Also in children with cerebellar lesions conditioned responses (CR) tended to occur earlier than in controls. Incidences of CRs were significantly reduced in children with cerebellar lesions and tended to be less in children with ADHD than in controls. Extinction of the CRs was impaired in children with cerebellar lesions in both interstimulus interval conditions and in children with ADHD in the long interstimulus interval condition. Cerebellar malfunction may contribute to disordered eyeblink conditioning in ADHD. However, because CR abnormalities differed between ADHD and cerebellar subjects, dysfunction of non-cerebellar structures cannot be excluded.

Dynamic associations in the cerebellar-motoneuron network during motor learning

We assessed here true causal directionalities in cerebellar-motoneuron (MN) network associations during the classical conditioning of eyelid responses. For this, the firing activities of identified facial MNs and cerebellar interpositus (IP) nucleus neurons were recorded during the acquisition of this type of associative learning in alert behaving cats. Simultaneously, the eyelid conditioned response (CR) and the EMG activity of the orbicularis oculi (OO) muscle were recorded. Nonlinear association analysis and time-dependent causality method allowed us to determine the asymmetry, time delays, direction in coupling, and functional interdependences between neuronal recordings and learned motor responses. We concluded that the functional nonlinear association between the IP neurons and OO muscle activities was bidirectional and asymmetric, and the time delays in the two directions of coupling always lagged the start of the CR. Additionally, the strength of coupling depended inversely on the level of expression of eyeblink CRs, whereas causal inferences were significantly dependent on the phase information status. In contrast, the functional association between OO MNs and OO muscle activities was unidirectional and quasisymmetric, and the time delays in coupling were always of opposed signs. Moreover, information transfer in cerebellar-MN network associations during the learning process required a "driving common source" that induced the mere "modulating coupling" of the IP nucleus with the final common pathway for the eyelid motor system. Thus, it can be proposed that the cerebellum is always looking back and reevaluating its own function, using the information acquired in the process, to play a modulating-reinforcing role in motor learning.

Blocking GABAA neurotransmission in the interposed nuclei: effects on conditioned and unconditioned eyeblinks

The interposed nuclei (IN) of the intermediate cerebellum are critical components of the circuits that control associative learning of eyeblinks and other defensive reflexes in mammals. The IN, which represent the sole output of the intermediate cerebellum, receive massive GABAergic input from Purkinje cells of the cerebellar cortex and are thought to contribute to the acquisition and performance of classically conditioned eyeblinks. The specific role of deep cerebellar nuclei and the cerebellar cortex in eyeblink conditioning are not well understood. One group of studies reported that blocking GABA(A) neurotransmission in the IN altered the time profile of conditioned responses (CRs), suggesting that the main function of the cerebellar cortex is to shape the timing of CRs. Other studies reported that blocking GABA(A) neurotransmission in the IN abolished CRs, indicating a more fundamental involvement of the cerebellar cortex in CR generation. When examining this controversy, we hypothesized that the behavioral effect of GABA(A) blockers could be dose-dependent. The IN of classically conditioned rabbits were injected with high and low doses of picrotoxin and gabazine. Both GABA(A) blockers produced tonic eyelid closure. A high dose of both drugs abolished CRs, whereas a less complete block of GABA(A)-mediated inputs with substantially smaller drug doses shortened CR latencies. In addition, low doses of picrotoxin facilitated the expression of unconditioned eyeblinks evoked by trigeminal stimulation. These results suggest that the intermediate cerebellum regulates both associative and non-associative components of the eyeblink reflex, and that behavioral effects of blocking Purkinje cell action on IN neurons are related to collective changes in cerebellar signals and in the excitability of extra-cerebellar eyeblink circuits.

Implications on cerebellar function from information coding

One function of the cerebellar cortex is to process information. There are at least two types of information. Temporal information is encoded in the timing pattern of action and synaptic potentials, whereas structural information is encoded in the spatial pattern of the cerebellar synaptic circuitry. Intuitively, analysis of highly complex information in the time domain would require a cerebellar cortex with structural complexity to match. Information theory offers a way to estimate quantitatively both types of information and thereby helps to test hypotheses or advance theories of cerebellar neurobiology. These estimates suggest: (i) That the mossy-fiber-granule-cell system carries far more (temporal) information than the climbing fiber system, (ii) that Purkinje cells extract only a fraction of the (temporal) information from their afferents, and (iii) that the cerebellar cortex has a large (spatial) information coding capacity. Concerning information, one can argue that the cerebellar cortex analyzes temporal information in its afferents as a search engine, in search of coincidental mossy fiber events based on timing cues provided by climbing fiber events. Results of successive searches are continuously being converted into structural information encoded in the spatial distribution pattern of granule-cell-Purkinje-cell synapses along granule cell axons, thereby providing an adaptive and indeed self-correcting dimension to the structural information code. The search engine operation involves cellular mechanisms acting on temporal events and is part of an associative learning process. The conversion and generation of structural information involves neuroplasticity mechanisms acting at the synaptic level, with electrophysiological as well as structural consequences, and may be part of the short- and long-term memory process. These and other attributes qualify the cerebellar cortex as a dynamic information processing center, contributing to memory and learning while linking motor output with sensory events.

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.

Selective impairment of the cerebellar C1 module involved in rat hind limb control reduces step-dependent modulation of cutaneous reflexes

The cerebellum is divided into multiple parasagittally organized modules, which are thought to represent functional entities. How individual modules participate in cerebellar control of complex movements such as locomotion remains largely unknown. To a large extent, this is caused by the inability to study the contribution of individual modules during locomotion. Because of the architecture of modules, based on narrow, elongated cortical strips that may be discontinuous in the rostrocaudal direction, lesion of a complete module, without affecting neighboring modules, has not been possible. Here, we report on a new method for inducing a selective dysfunction of spatially separated parts of a single module using a small cortical injection of a retrogradely transported neurotoxin, cholera toxin b-subunit-saporin. We show that such a local injection into the C1 module results in climbing fiber and partial mossy fiber deafferentation of functionally related areas of this module, thereby resulting in a severe impairment of the whole module without affecting neighboring modules. A subsequent functional analysis indicates that such an impairment of the hindlimb part of the C1 module did not have a significant impact on skilled walking or overall stepping pattern. However, the modulation of cutaneously induced reflexes during stepping was severely diminished. We propose that the C1 module is specifically involved in the adaptive control of reflexes.

Trace eyeblink conditioning in patients with cerebellar degeneration: comparison of short and long trace intervals

To elucidate whether the cerebellar cortex may contribute to trace eyeblink conditioning in humans, eight patients with degenerative cerebellar disorders (four with sporadic adult onset ataxia, three with autosomal dominant cerebellar ataxia type III and one with spinocerebellar ataxia type 6) and eight age- and sex-matched healthy control subjects were investigated. Individual high resolution three-dimensional MRI data sets were acquired. As revealed by volumetric measurements of the cerebellum using ECCET software, patients showed cerebellar atrophy to various degrees. No abnormalities were observed in the control subjects. Eyeblink conditioning was performed twice using a tone of 40 ms as conditioned stimulus, followed by a short (400 ms) and a long (1,000 ms) trace interval and an air-puff of 100 ms as unconditioned stimulus. Using the short trace interval, eyeblink conditioning was significantly impaired in cerebellar patients compared to controls, even in those who fulfilled criteria of awareness. Using the long trace interval no significant group differences could be observed. The present findings of impaired trace eyeblink acquisition in patients with cortical cerebellar degeneration suggest that the cerebellar cortex in humans, in addition to the interposed nucleus, is involved in trace eyeblink conditioning, if the trace interval is relatively short. Using a long trace interval, the cerebellum appears to be less important.

The involvement of the human cerebellum in eyeblink conditioning

Besides its known importance for motor coordination, the cerebellum plays a major role in associative learning. The form of cerebellum-dependent associative learning, which has been examined in greatest detail, is classical conditioning of eyeblink responses. The much advanced knowledge of anatomical correlates, as well as cellular and molecular mechanisms involved in eyeblink conditioning in animal models are of particular importance because there is general acceptance that findings in humans parallel the animal data. The aim of the present review is to give an update of findings in humans. Emphasis is put on human lesion studies, which take advantage of the advances of high-resolution structural magnetic resonance imaging (MRI). In addition, findings of functional brain imaging in healthy human subjects are reviewed. The former helped to localize areas involved in eyeblink conditioning within the cerebellum, the latter was in particular helpful in delineating extracerebellar neural substrates, which may contribute to eyeblink conditioning. Human lesion studies support the importance of cortical areas of the ipsilateral superior cerebellum both in the acquisition and timing of conditioned eyeblink responses (CR). Furthermore, the ipsilateral cerebellar cortex seems to be also important in extinction of CRs. Cortical areas, which are important for CR acquisition, overlap with areas related to the control of the unconditioned eyeblink response. Likewise, cortical lesions are followed by increased amplitudes of unconditioned eyeblinks. These findings are in good accordance with the animal literature. Knowledge about contributions of the cerebellar nuclei in humans, however, is sparse. Due to methodological limitations both of human lesion and functional MRI studies, at present no clear conclusions can be drawn on the relative contributions of the cerebellar cortex and nuclei.

Cerebellum and cognition: viewed from philosophy of mind

Traditionally, it is believed, that the primary function of the cerebellum is to coordinate movement. During the past three decades, it has been controversially discussed, whether the cerebellum may also contribute to cognition and mental states like emotions. In this paper, no position relating to this controversy will be taken. Instead, the hypothesis of non-motor functions of the cerebellum will be viewed from the position of the philosophy of mind. The remarkably uniform microscopic structure and neuronal networks of the cerebellum have led to computer analogies by several authors. The main idea of functionalism, i.e., a theory within the philosophy of mind, is that the mental relates to the physical as computer software does to hardware. This raises the question, whether a cerebellar contribution to cognition and mental states would support functionalism in the philosophy of mind. No support of functionalism could be found in this study, investigating the classical philosophical arguments pro and con functionalism such as those of multiple realizability, the Chinese room and the explanatory gap, while taking the results of cerebellar research into account. On the other hand, philosophical reflection suggests a careful use of the phrases "cognitive dysmetria" (Andreasen et al. Proc Natl Acad Sci USA. 1996;93:9985-90) in the context of mental illness and of "dysmetria of thought" (Schmahmann Arch Neurol. 1991;48:1178-87). According to the argument of the explanatory gap there is at present little support for the assumption that the phenomenal experiencing of an altered emotion can be reduced to the dysmetria of movement.

The cerebellum and sensorimotor coupling: Looking at the problem from the perspective of vestibular reflexes

Cerebellar modules process afferent information and deliver outputs relevant for both reflex and voluntary movements. The response of cerebellar modules to a given input depends on the whole array of signals impinging on them. Studies on vestibular reflexes indicate that the response of the cerebellar circuits to the vestibular input is modified by the integration of multiple visual, vestibular and somatosensory afferent signals. In this way the cerebellum slowly adapts these reflexes when they are not adequate to the behavioural condition and allows their fast modifications when the relative position of the body segments and that of the body in space are changed. Studies on voluntary movements indicate that the cerebellum is responsible for motor learning that consists of the development of new input-output associations. Several theoretical, anatomical and clinical studies are consistent with the hypothesis that the cerebellum allows the delivery of motor commands which vary according to the condition of the motor apparatus. Finally, the cerebellum could change the relation between visual information and aimed reaching movements according to the position of the eyes in the orbit and of the neck over the body. We propose that, due to the large expansion of its cortex, an important function of the cerebellum could be that of expanding the range of sensorimotor associations according to all the factors characterizing the behavioural condition. Indeed, following cerebellar lesion, learning is often lost, the movement results impaired and requires an increased attention. In the light of the recently discovered connections of the cerebellum with the rostral regions of the frontal lobe, it can be suggested that the ability of cerebellar circuits to modify the rules of input-output coupling according to a general context is a fundamental property allowing the cerebellum to control not only motor but also cognitive functions.

Comparison of the electrically evoked leg withdrawal reflex in cerebellar patients and healthy controls

The aim of this study was to analyze the contribution of the cerebellum in the performance of the lower limb withdrawal reflexes. This has been accomplished by comparing the electrically evoked responses in cerebellar patients (CBL) with those in sex- and age-matched healthy control subjects (CTRL). The stimulus was applied to the subjects' medial plantar nerve in four blocks of ten trials each with switching the stimulus from one leg to the other after each block. Responses of the main muscle groups (tibial muscle: TA; gastrocnemius muscle: GA; rectus femoris muscle: RF; biceps femoris muscle: BI) of both legs were recorded during each stimulus. The group of CBL patients consisted of both focally lesioned patients (CBLf) and patients presenting a diffuse degenerative pathology (CBLd). (1) For the withdrawal reflex in CTRL subjects, responses were observed in distal and proximal muscles of the ipsilateral side and corresponding concomitant responses on the side contralateral to the stimulation, whereas in CBL patients responses were restricted primarily to distal muscles, particularly the TA of the ipsilateral, i.e. the stimulated, side. (2) The sequence of activation of the different distal and proximal muscles ipsilateral to the stimulation, derived from latencies and times-to-peak, was for the CTRL group: TA-GA-BI-RF. This sequence was found also in the CBLf patients on their unaffected side. However, on their affected side CBLf patients showed very early GA activation, almost simultaneously with TA and RF activations and before BI activation. RF activation before BI activation was also found in CBLd. In the latter group, GA was activated after RF but before BI with all responses typically delayed. (3) The general pattern of the electrically evoked lower limb reflex consisted of an early, excitatory F1 component and a later, excitatory F2 component of larger amplitude observed in the CTRL subjects and the CBLd patients. In contrast to this pattern CBLf patients exhibited large F1 components followed by small F2 components. (4) The characteristic differences in the withdrawal reflex responses of cerebellar patients depended on the type of the lesion, providing evidence for an important involvement of the cerebellum in the control of the performance of withdrawal reflexes.

Acquisition of simple auditory and visual sequences in cerebellar patients

Previous studies suggest a role of the cerebellum in detecting and recognizing event sequences. In the present study sequences of two acoustic tones of different frequencies and sequences of two visual stimuli with different colours were presented with short, long and very long durations. Thirteen cerebellar patients and 13 controls were required to indicate whether the order of stimuli was identical or opposite to a reference by saying 'yes' or 'no'. To minimize motor requirements no time restrictions were given. In none of the test conditions a significant difference between patients and controls was observed. The results suggest that the cerebellum is not essential for acquisition of simple sensory sequences. Differences in findings between previous and the present study may be explained by differences in motor demands and the explicit nature of the tasks.

Involvement of the human cerebellum in short-term and long-term habituation of the acoustic startle response: a serial PET study

OBJECTIVE

Numerous studies have shown an involvement of the human cerebellum in motor learning, but little is known about the role of the cerebellum in learning of unspecific aversive reactions. The present study sought to distinguish which areas of the human cerebellum and brain-stem are involved in short-term habituation (STH) and long-term habituation (LTH) of the acoustic startle response.

METHODS

On 5 consecutive days 42 acoustic startle stimuli were applied each day in 8 male healthy subjects. On the first and on the fifth day of the experiment [15O]H2O PET scans were performed.

RESULTS

Electromyographic recordings revealed a significant decrease of the startle response within each day (STH) and across the 5 days of the experiment (LTH). On both days a decrease of regional cerebral blood flow (rCBF) across PET scans was found in the medial cerebellum most probably reflecting reduced sensory feedback during STH. Between days an increase of rCBF in the dorsomedial pons, in the mesencephalon and in an area of the medial cerebellum was observed. These activations may reflect increased inhibition of the startle response during LTH and correspond to previous animal lesion studies. Furthermore, during LTH an increase of rCBF within the lateral cerebellum in lobule HVI/Crus I was detected.

CONCLUSIONS

These results suggest that distinct parts of the medial and lateral cerebellum are involved in habituation of the acoustic startle response. Lobule HVI/Crus I most likely plays a more general role in implicit learning processes considering its involvement in several conditioning paradigms.

SIGNIFICANCE

The results of the present study contribute to the understanding of cerebellar involvement in learning of unspecific aversive reactions.

Hemicerebellectomy impairs the modulation of cutaneomuscular reflexes by the motor cortex following repetitive somatosensory stimulation

We examined the cutaneomuscular reflex of the plantaris muscle of rats in response to cutaneous stimulation in isolation and in conjunction with subthreshold high-frequency trains of stimuli applied on the motor cortex, prior to and following repetitive peripheral stimulation. The cutaneomuscular reflex was also investigated under the same paradigm following hemicerebellectomy. The enhancement of cutaneomuscular responses associated with subthreshold high-frequency trains of stimulation following repetitive peripheral stimulation was prevented by hemicerebellectomy. Our results suggest that the pathways passing through the cerebellum are involved in the calibration of cutaneomuscular responses.

Immediate-early gene-encoded protein Arc is associated with synaptic delivery of GluR4-containing AMPA receptors during in vitro classical conditioning

The immediate-early gene Arc is rapidly expressed in response to neuronal activity and is thought to be involved in mechanisms of synaptic plasticity. The function of Arc in these processes remains unknown. The present study demonstrates that during an in vitro neural correlate of eyeblink classical conditioning, there is a rapid and transient increase in levels of Arc protein that require activation of N-methyl-d-aspartate receptors. In the early phase of conditioning during conditioned response (CR) acquisition, there is significantly greater colocalization of Arc protein and GluR4-containing AMPA receptors at synaptic sites, however, colocalization of Arc and GluR4 was not observed after later stages of conditioning during CR expression. There was also significantly enhanced coimmunoprecipitation of Arc with GluR4 subunits and actin early in conditioning but not of Arc with NR1 subunits, and these associations declined to control levels in later stages of conditioning. These data suggest a role for Arc protein in the synaptic delivery of GluR4-containing AMPA receptors by interactions with cytoskeletal protein complexes during the acquisition phase of in vitro classical conditioning.

The lower limb flexion reflex in humans

The flexion or flexor reflex (FR) recorded in the lower limbs in humans (LLFR) is a widely investigated neurophysiological tool. It is a polysynaptic and multisegmental spinal response that produces a withdrawal of the stimulated limb and resembles (having several features in common) the hind-paw FR in animals. The FR, in both animals and humans, is mediated by a complex circuitry modulated at spinal and supraspinal level. At rest, the LLFR (usually obtained by stimulating the sural/tibial nerve and by recording from the biceps femoris/tibial anterior muscle) appears as a double burst composed of an early, inconstantly present component, called the RII reflex, and a late, larger and stable component, called the RIII reflex. Numerous studies have shown that the afferents mediating the RII reflex are conveyed by large-diameter, low-threshold, non-nociceptive A-beta fibers, and those mediating the RIII reflex by small-diameter, high-threshold nociceptive A-delta fibers. However, several afferents, including nociceptive and non-nociceptive fibers from skin and muscles, have been found to contribute to LLFR activation. Since the threshold of the RIII reflex has been shown to correspond to the pain threshold and the size of the reflex to be related to the level of pain perception, it has been suggested that the RIII reflex might constitute a useful tool to investigate pain processing at spinal and supraspinal level, pharmacological modulation and pathological pain conditions. As stated in EFNS guidelines, the RIII reflex is the most widely used of all the nociceptive reflexes, and appears to be the most reliable in the assessment of treatment efficacy. However, the RIII reflex use in the clinical evaluation of neuropathic pain is still limited. In addition to its nocifensive function, the LLFR seems to be linked to posture and locomotion. This may be explained by the fact that its neuronal circuitry, made up of a complex pool of interneurons, is interposed in motor control and, during movements, receives both peripheral afferents (flexion reflex afferents, FRAs) and descending commands, forming a multisensorial feedback mechanism and projecting the output to motoneurons. LLFR excitability, mediated by this complex circuitry, is finely modulated in a state- and phase-dependent manner, rather as we observe in the FR in animal models. Several studies have demonstrated that LLFR excitability may be influenced by numerous physiological conditions (menstrual cycle, stress, attention, sleep and so on) and pathological states (spinal lesions, spasticity, Wallenberg's syndrome, fibromyalgia, headaches and so on). Finally, the LLFR is modulated by several drugs and neurotransmitters. In summary, study of the LLFR in humans has proved to be an interesting functional window onto the spinal and supraspinal mechanisms of pain processing and onto the spinal neural control mechanisms operating during posture and locomotion.

The cerebellum may implement the appropriate coupling of sensory inputs and motor responses: evidence from vestibular physiology

Starting from a survey of current ideas on the role of the cerebellum in sensorimotor transformations, the present review summarizes the results of recent experiments showing that (a) somatosensory signals modify the spatial organization of the postural reflexes, thus leading to body stability, and (b) otolith input changes the plane of reflex eye movements, by keeping it perpendicular to the gravito-inertial vector. Evidence will be given that both transformations require the integrity of specific cerebellar regions. These data indicates that the cerebellum allows an optimal input-output coupling in relation to the ultimate behavioural goal of the motor activity.

Timing of conditioned eyeblink responses is impaired in cerebellar patients

In the present study, timing of conditioned eyeblink responses (CRs) was investigated in cerebellar patients and age-matched controls using a standard delay paradigm. Findings were compared with previously published data of CR incidences in the same patient population (Gerwig et al., 2003; Timmann et al., 2005). Sixteen patients with pure cortical cerebellar degeneration (spinocerebellar ataxia type 6 and idiopathic cerebellar ataxia), 14 patients with lesions within the territory of the superior cerebellar artery, and 13 patients with infarctions within the territory of the posterior inferior cerebellar artery were included. The affected cerebellar lobules and possible involvement of cerebellar nuclei were determined by three-dimensional magnetic resonance imaging (MRI) in patients with focal lesions (n = 27). Based on a voxel-by-voxel analysis, MRI lesion data were related to eyeblink conditioning data. CR incidence was significantly reduced, and CRs occurred significantly earlier in patients with cortical cerebellar degeneration and lesions of the superior cerebellum compared with controls. Incidence and timing of CRs was not impaired in patients with lesions restricted to the posterior and inferior cerebellum. Voxel-based MRI analysis revealed that cortical areas within the anterior lobe (Larsell lobule HV) were most significantly related to timing deficits, whereas reduced CR incidences were related to more caudal parts (lobule HVI) of the superior cerebellar cortex. The present data suggest that different parts of the superior cerebellar cortex may be involved in the formation of the stimulus association and appropriate timing of conditioned eyeblink responses in humans. Extracerebellar premotoneuronal disinhibition, however, is another possible explanation for changes in CR timing.

Acquisition, consolidation, reconsolidation, and extinction of eyelid conditioning responses require de novo protein synthesis

Memory, as measured by changes in an animal's behavior some time after learning, is a reflection of many processes. Here, using a trace paradigm, in mice we show that de novo protein synthesis is required for acquisition, consolidation, reconsolidation, and extinction of classically conditioned eyelid responses. Two critical periods of protein synthesis have been found: the first, during training, the blocking of which impaired acquisition; and the second, lasting the first 4 h after training, the blocking of which impaired consolidation. The process of reconsolidation was sensitive to protein synthesis inhibition if anisomycin was injected before or just after the reactivation session. Furthermore, extinction was also dependent on protein synthesis, following the same temporal course as that followed during acquisition and consolidation. This last fact reinforces the idea that extinction is an active learning process rather than a passive event of forgetting. Together, these findings demonstrate that all of the different stages of memory formation involved in the classical conditioning of eyelid responses are dependent on protein synthesis.

Modulation of cutaneous reflexes in hindlimb muscles during locomotion in the freely walking rat: A model for studying cerebellar involvement in the adaptive control of reflexes during rhythmic movements

This study aims to demonstrate stepphase-dependent modulation in the gain of cutaneously triggered reflexes in the freely locomoting rat. Electromyographic recordings of biceps femoris (mainly involved in knee flexion) and gastrocnemius (mainly involved in ankle extension) muscles were continuously monitored during locomotion and cutaneous reflexes were induced by subcutaneously placed stimulation electrodes in the lateral malleolal region. The results show that the reflex responses in both muscles during locomotion were generally reduced compared to reflexes induces in rest. For the biceps femoris reduction of reflex gain was highest during the stance phase whereas for the gastrocnemius the period of highest depression was found during the swing phase. We conclude that stepphase-dependent modulation of peripheral reflexes can be measured in freely locomoting rats and generally concur with previous studies in cat and man that this type of modulation may be functionally important for maintaining and adjusting gait. Moreover, although the mechanism of inducing and maintaining this modulation is not fully known, it is now open to experimental investigation in rodents.

Pathways controlling trigeminal and auditory nerve-evoked abducens eyeblink reflexes in pond turtles

An in vitro brain stem preparation from turtles exhibits a neural correlate of eyeblink classical conditioning during pairing of auditory (CS) and trigeminal (US) nerve stimulation while recording from the abducens nerve. The premotor neuronal circuits controlling abducens nerve-mediated eyeblinks in turtles have not been previously described, which is a necessary step for understanding cellular mechanisms of conditioning in this preparation. The purpose of the present study was to neuroanatomically define the premotor pathways that underlie the trigeminal and auditory nerve-evoked abducens eyeblink responses. The results show that the principal sensory trigeminal nucleus forms a disynaptic pathway from both the trigeminal and auditory nerves to the principal and accessory abducens motor nuclei. Additionally, the principal abducens nucleus receives vestibular inputs, whereas the accessory nucleus receives input from the cochlear nucleus. The late R2-like component of abducens nerve responses is mediated by the spinal trigeminal nucleus in the medulla. Both the principal sensory trigeminal nucleus and the abducens motor nuclei receive CS-US convergence and therefore both, or either, might be considered potential sites of synapse modification during in vitro abducens conditioning. Further data are required to determine the role of the principal sensory trigeminal nucleus in in vitro conditioning.

Classically conditioned postural reflex in cerebellar patients

The aim of the current study was to compare postural responses to repetitive platform-evoked perturbations in cerebellar patients with those of healthy subjects using a classical conditioning paradigm. The perturbations consisted of tilting of the platform (unconditioned stimulus: US) at random time intervals, preceded by an auditory signal that represented the conditioning stimulus (CS). Physiological reactions were recorded biomechanically by measuring the vertical ground forces, yielding the center of vertical pressure (CVP), and electrophysiologically by EMG measurements of the main muscle groups of both legs. The recording session consisted of a control section with US-alone trials, a testing section with paired stimuli and a brief final section with US-alone trials. Healthy control subjects were divided into those establishing conditioned responses (CR) in all muscles tested (strategy I) and those with CR in the gastrocnemius muscles only (strategy II), suggesting an associative motor-related process is involved. Patients with a diffuse, non-localized disease were almost unable to establish CR. This was also true for a patient with a focal surgical lesion with no CR on the affected side but who, simultaneously, showed an essentially normal CR incidence on the intact side. During US-alone trials healthy controls exhibited a remarkable decay of the UR amplitude due to a non-associative motor-related process such as habituation. The decay was most prominent in the paired trials section. In contrast, patients showed no significant differences in the UR amplitude throughout the entire recording session. Analysis of the CVP supported the electrophysiological findings, showing CR in the controls only. The differences between the responses of control subjects and those of the cerebellar patients imply strongly that the cerebellum is involved critically in controlling associative and non-associative motor-related processes.

Neural mechanisms underlying reaching for remembered targets cued kinesthetically or visually in left or right hemispace

Reaching for a target involves integrative coordinate transformation processes between the representation of the target location, the sensorimotor information of limb of reach, and body space. Although right hemisphere dominance for visuospatial information processing is well established, corresponding right hemisphere dominance for kinesthetic spatial information processing remains to be demonstrated. We explored neural mechanisms of encoding target locations using 15O-butanol positron emission tomography (PET) in normal volunteers in a factorial experiment, where modality (visual/kinesthetic) and hemispace of target presentation (left/right of midsagittal plane) were varied systematically. After target presentation, subjects reached to the encoded target location. PET data analysis using SPM99 showed increased neural activity (P < 0.05, corrected) associated with left hemispace target presentation in right hemisphere areas (sensorimotor, anterior cingulate, insular, and temporo-occipital cortex) only. By contrast, right hemispace target presentation activated bilateral temporo-occipital cortex, which extended into the right temporo-parietal cortex and left sensorimotor cortex. A significant interaction of hemispace and modality of target presentation observed in right temporo-parietal cortex resulted from an increase in neural activity with kinesthetic target presentation in right hemispace. The data support an important role for the right temporo-parietal area in visuospatial processing and suggest a specific role of the right hemisphere in kinesthetic spatial processing.

Amplitude changes of unconditioned eyeblink responses in patients with cerebellar lesions

Timing and amplitude parameters of unconditioned eyeblink responses were investigated in 24 patients with unilateral cerebellar lesions following infarcts within the territory of the superior cerebellar artery (SCA, n=12) and of the posterior inferior cerebellar artery (PICA, n=12). The extent of cortical cerebellar lesions, i.e., which lobules were affected and possible involvement of cerebellar nuclei, was determined by three-dimensional magnetic resonance imaging (3D MRI). Amplitude parameters of eyeblink responses were normalized and expressed as percentage of the unaffected side in patients and the second tested side in age-matched controls. Normalized peak amplitudes, burst area and burst duration were significantly increased in SCA patients with lesions restricted to cortical areas. Burst onset and time to peak were not significantly different compared with controls. Temporal and amplitude parameters of eyeblink responses were unchanged in SCA patients with additional involvement of cerebellar nuclei and in patients with lesions of the PICA territory. Consistent with animal lesion and recording studies and a recent human functional magnetic resonance imaging (fMRI) study, the present data suggest that cortical areas of the superior cerebellum are of importance in eyeblink control in humans. These areas partly overlap with areas known to be critical in eyeblink conditioning.

Task-dependent role of the cerebellum in motor learning

This chapter reviews several findings from our laboratory supporting the hypothesis that the cerebellum's role in motor learning is task-dependent. Namely, its contribution is dependent on the specific task being learned. Several studies are reviewed to demonstrate that the effect of temporary or permanent cerebellar lesions on a specific process such as storage varies depending on the behavior. Furthermore, this task-dependency is reflected also in the modulation of Purkinje cells and nuclear neurons recorded during the learning process. The behavioral correlates of this modulation are very paradigm specific. These observations support the above hypothesis and emphasize the importance of paradigm selection in designing experiments focused on elucidating the cerebellum's role in learning a specific motor behavior.

Integration of multiple motor segments for the elaboration of locomotion: role of the fastigial nucleus of the cerebellum

This chapter provides a conceptual overview of the role and operation of higher structures of the central nervous system (CNS) in the control of posture and locomotion in the mammal, including the nonhuman primate and the human. Both quadrupedal and bipedal locomotion require the integrated neural control of multiple body segments against gravity. During development, and in selected instances in the adult, motor learning is required, particularly for merging anticipatory and reactive CNS processes, the latter being necessary after tripping and stumbling. We have recently found that the fastigial nucleus (FN) of the cerebellum in the cat plays a particularly important role in the control of locomotion, by virtue of its critical position in uniting the cerebro-cerebellar and the spino-cerebellar loops of neural activity that participate in the integrated control of multiple body segments. Further understanding of the CNS structures that achieve this integration has come from our recent study of an intact nonhuman primate, the Japanese monkey, Macaca fuscata, as it learns to elaborate bipedal locomotion rather than its normal quadrupedal fashion. Based on findings from these two animal species, we now present a model of the overall integrated control of posture and locomotion that features the combined operation of parallel and distributed neural circuitry throughout the CNS.

Targeting of GLUR4-containing AMPA receptors to synaptic sites during in vitro classical conditioning

The synaptic delivery of GluR4-containing AMPA receptors during in vitro classical conditioning of a neural correlate of an eyeblink response was examined by fluorescence imaging of punctate staining for glutamate receptor subunits and the presynaptic marker synaptophysin. There was a significant increase in GluR4-containing AMPA receptors to synaptic sites after conditioning as determined by colocalization of GluR4 subunit puncta with synaptophysin. Moreover, the trafficking of these receptor subunits requires NMDA receptor activation as it was blocked by D,L-2-amino-5-phosphonovaleric acid (AP-5). In contrast, colocalization of NR1 subunits with synaptophysin was stable regardless of whether the preparations had undergone conditioning or had been treated by AP-5. The enhanced colocalization of GluR4 and synaptophysin was accompanied by an increase in both the total number and size of puncta for both proteins, suggesting greater synthesis and aggregation during conditioning. Western blot analysis confirmed upregulation of synaptophysin and GluR4 following conditioning. These data support the hypothesis that GluR4-containing AMPA receptors are delivered to synaptic sites during conditioning. Further, they suggest coordinate presynaptic and postsynaptic modifications during in vitro classical conditioning.

On-line compensation for perturbations of a reaching movement is cerebellar dependent: support for the task dependency hypothesis

Although the cerebellum has been shown to be critical for the acquisition and retention of adaptive modifications in certain reflex behaviors, this structure's role in the learning of motor skills required to execute complex voluntary goal-directed movements still is unclear. This study explores this issue by analyzing the effects of inactivating the interposed and dentate cerebellar nuclei on the adaptation required to compensate for an external elastic load applied during a reaching movement. We show that cats with these nuclei inactivated can adapt to predictable perturbations of the forelimb during a goal-directed reach by including a compensatory component in the motor plan prior to movement initiation. In contrast, when comparable compensatory modifications must be triggered on-line because the perturbations are applied in randomized trials (i.e., unpredictably), such adaptive responses cannot be executed or reacquired after the interposed and dentate nuclei are inactivated. These findings provide the first demonstration of the condition-dependent nature of the cerebellum's contribution to the learning of a specific volitional task.

Cerebellar responses evoked by nociceptive leg withdrawal reflex as revealed by event-related FMRI

The aim of the present study was to examine nociceptive leg withdrawal reflex-related areas in the human cerebellum using event-related functional brain imaging (fMRI). Knowledge about cerebellar areas involved in unconditioned limb withdrawal reflex control has some relevance in understanding data of limb withdrawal reflex conditioning studies. Sixteen healthy adult subjects participated. Nociceptive leg withdrawal reflexes were evoked by electrical stimulation of the left tibial nerve behind the medial malleolus. An event-related fMRI paradigm was applied with a total of 30 stimuli being delivered pseudorandomly during 500 consecutive MR scans. Surface electromyographic (EMG) recordings were performed from the left anterior tibial muscle. Only trials with significant reflex EMG activity were used as active events in fMRI statistical analysis. The specified contrasts compared the active event condition with rest. Leg withdrawal reflex-related areas were located within the vermis, paravermis, and lateral posterior cerebellar hemispheres bilaterally. Vermal and paravermal areas in lobules III/IV in the anterior lobe and in lobule VIII in the posterior lobe agree with the cerebellar representation of climbing and mossy fiber hindlimb afferents and voluntary leg movements. They are likely related to efferent modulation of the leg withdrawal reflex and/or sensory processing of afferent inputs from the reflex and/or the noxious stimulus. Additional activation within vermal lobule VI and hemispheral lobules VI/Crus I may be related to other pain-related processes (e.g., facial grimacing, fear, and startlelike reactions).

In vitro classical conditioning of the turtle eyeblink reflex: approaching cellular mechanisms of acquisition

The classically conditioned eyeblink reflex is the best studied model for understanding the neural mechanisms that underlie learning and memory. Here, data from an in vitro model of the conditioned eyeblink reflex are summarized with the aim of shedding some light on potential cellular mechanisms that may underlie eyeblink classical conditioning. An isolated brainstem-cerebellum preparation from turtles was developed in which to study the synaptic circuitry of pathways involving the cerebellum, red nucleus and brainstem nuclei. A neural correlate of an eyeblink response recorded in the abducens nerve can be conditioned entirely in vitro by pairing trigeminal and auditory nerve stimulation. Conditioned abducens nerve responses (CRs) are not generated or sustained by unpaired stimuli and their long latencies, on the order of hundreds of milliseconds, support the interpretation that the CRs are not unconditioned responses. Ablation experiments show that CRs can be generated in brainstem preparations lacking a cerebellum or the medulla. However, the timing of the CRs are disrupted by removal of the cerebellar circuitry. Thus, a highly reduced in vitro brainstem preparation demonstrates acquisition of CRs but poor timing features. Recent experiments have focused on elucidating cellular mechanisms for CR acquisition in the brainstem blink circuitry. These studies show that NMDA-mediated synaptic mechanisms are required to generate CRs and that the level of conditioning is associated with the upregulation of GluR4-containing AMPA receptors in the abducens motor nuclei. Data from immunocytochemistry and physiological experiments using the calcium/calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-93 suggest that CaMKII does not have a key role in mediating the induction or expression of abducens nerve CRs. It is hypothesized that GluR4-containing AMPA receptors in the abducens motor nuclei are targeted to auditory nerve synapses by an NMDA receptor-dependent process to strengthen the CS input during conditioning which results in the generation of CRs. Future studies will examine the synaptic localization of GluR4 and potential signal transduction pathways involved in in vitro conditioning. Moreover, the role feedback loops through the cerebellum and their role in CR timing will be a key issue to address using this preparation.

Comparison of eyeblink conditioning in patients with superior and posterior inferior cerebellar lesions

The aim of the present study was to compare eyeblink conditioning in cerebellar patients with lesions including the territory of the superior cerebellar artery (SCA) and in patients with lesions restricted to the territory of the posterior inferior cerebellar artery (PICA). The cerebellar areas known to be most critical in eyeblink conditioning based on animal data (i.e. Larsell lobule H VI and interposed nucleus) are commonly supplied by the SCA. Eyeblink conditioning was expected to be impaired in SCA, but not in PICA patients. A total of 27 cerebellar patients and 25 age-matched controls were tested. Cerebellar lesions were primarily unilateral (n = 20). Most patients suffered from ischaemic infarctions of the SCA (n = 11) or the PICA (n = 13). The other patients presented with cerebellar tumours (n = 2) and cerebellar agenesis (n = 1). The extent of the cortical lesion (i.e. which lobuli were affected) and possible involvement of the cerebellar nuclei was determined by 3D-MRI. As expected, the ability to acquire classically conditioned eyeblink responses was significantly reduced in the group of all cerebellar patients compared with the controls. In the patients with unilateral cerebellar lesions, conditioning deficits were present ipsilaterally. In SCA patients with lesions including hemispheral lobules VI and Crus I, eyeblink conditioning was significantly reduced on the affected side compared with the unaffected side. No significant difference between the affected and unaffected sides was present in patients with lesions restricted to the common PICA territory (i.e. Crus II and below). Conditioning deficits were neither significantly different in SCA patients with pure cortical lesions compared with SCA patients with additional nuclear impairment nor in SCA patients with unilateral lesions compared with SCA patients with bilateral lesions. To summarize, unilateral cortical lesions of the superior cerebellum appear to be sufficient to reduce eyeblink conditioning in humans significantly.

Limb flexion reflex-related areas in human cerebellum

Numerous studies have shown that the cerebellum is involved in classical conditioning of specific aversive reactions. Only few studies, however, have examined the exact localization of cerebellar areas involved in the control of unconditioned reflex responses. The present study investigated cerebellar areas, which are activated during eliciting of the limb flexion reflex in healthy controls using [15O]H2O PET. Limb flexion reflex related areas were found in vermal lobules III-VI with the local maximum in vermal lobule V. Areas of activation were more widespread compared to a voluntary foot movement and a sensory foot stimulation condition. Limb flexion reflex areas probably reflect efferent reflex modulation as well as sensory inputs from the stimulus itself and/or the reflex response.

The olivocerebellar system as a generator of temporal patterns

The large number of diverse functions attributed to the cerebellum appears to be inconsistent with its simple, homogeneous and evolutionary preserved structure. A homogeneous structure that participates in a variety of functions implies that a common denominator underlies all of them. Since the concept of precise timing can be recognized in almost all cerebellar functions, it is likely, therefore, that the basic cerebellar circuit is capable of generating temporal patterns. Of the different mechanisms that can generate temporal patterns, two are suggested by the functional anatomy of the cerbellum: transmission lines or oscillators. Our recent experimental observations indicate that the olivary oscillatory property is more likely to serve this function. We propose that interactions between the cerebellum and the inferior olive endow the system with the ability to generate complex temporal patterns. These temporal patterns can be used for fine adjustment of motor output, sensory expectation, or shifting attentions.

Eyeblink-related areas in human cerebellum as shown by fMRI

Classical eyeblink conditioning is used frequently to study the role of the cerebellum in associative learning. To understand the mechanisms involved in learning, the neural circuits that generate the eyeblink response should be identified. The goal of the present study was to examine cerebellar regions that are likely to control the human eyeblink response using event-related functional magnetic resonance imaging (fMRI). In 14 healthy volunteers eyeblinks were evoked by unilateral air-puff stimulation (total of 30 stimuli, inter-trial interval 27-44 sec). With eyes closed throughout the experiment, eyeblinks were recorded using a video-based system with infrared reflecting markers being attached to the upper eyelids. From each subject 500 scans were taken (TR = 2.2 sec, 22 slices per scan, slice thickness = 3 mm) using an echo planar imaging sequence (EPI). The statistical parametric maps of the experimental volume images were estimated with SPM99 specifying an appropriate event-related design matrix. Two main regions of significant activation were found in the ipsilateral posterior lobe of the cerebellar hemisphere. In the more anterior region the maxima of activation were located in hemispheral lobules VI and Crus I, and in the more posterior region in hemispheral lobules VIIb, Crus II and VIIIa (nomenclature according to Schmahmann et al. [2000]: MRI Atlas of the Human Cerebellum). Although less pronounced, activity was found also in corresponding areas of the contralateral cerebellar hemisphere. These eyeblink-related areas agree with trigeminal projection areas and blink reflex control areas shown in previous animal studies.