Mechanisms of human cerebellar dysmetria: experimental evidence and current conceptual bases

Updating functional brain units: Insights far beyond Luria

This paper reviews Luria's model of the three functional units of the brain. To meet this objective, several issues were reviewed: the theory of functional systems and the contributions of phylogenesis and embryogenesis to the brain's functional organization. This review revealed several facts. In the first place, the relationship/integration of basic homeostatic needs with complex forms of behavior. Secondly, the multi-scale hierarchical and distributed organization of the brain and interactions between cells and systems. Thirdly, the phylogenetic role of exaptation, especially in basal ganglia and cerebellum expansion. Finally, the tripartite embryogenetic organization of the brain: rhinic, limbic/paralimbic, and supralimbic zones. Obviously, these principles of brain organization are in contradiction with attempts to establish separate functional brain units. The proposed new model is made up of two large integrated complexes: a primordial-limbic complex (Luria's Unit I) and a telencephalic-cortical complex (Luria's Units II and III). As a result, five functional units were delineated: Unit I. Primordial or preferential (brainstem), for life-support, behavioral modulation, and waking regulation; Unit II. Limbic and paralimbic systems, for emotions and hedonic evaluation (danger and relevance detection and contribution to reward/motivational processing) and the creation of cognitive maps (contextual memory, navigation, and generativity [imagination]); Unit III. Telencephalic-cortical, for sensorimotor and cognitive processing (gnosis, praxis, language, calculation, etc.), semantic and episodic (contextual) memory processing, and multimodal conscious agency; Unit IV. Basal ganglia systems, for behavior selection and reinforcement (reward-oriented behavior); Unit V. Cerebellar systems, for the prediction/anticipation (orthometric supervision) of the outcome of an action. The proposed brain units are nothing more than abstractions within the brain's simultaneous and distributed physiological processes. As function transcends anatomy, the model necessarily involves transition and overlap between structures. Beyond the classic approaches, this review includes information on recent systemic perspectives on functional brain organization. The limitations of this review are discussed.

Cerebellar Mutism Syndrome and Dentato-Thalamo-Cortical Tract Disruption in Diffusion Tractography Following Surgery for Medulloblastoma

Background Cerebellar mutism syndrome (CMS), a complication following medulloblastoma surgery, has been linked to dentato-thalamo-cortical tract (DTCT) injury; the association of the degree of DTCT injury with severity of CMS-related symptoms has not been investigated. Purpose To investigate the association between severity of CMS-related symptoms and degree and patterns of DTCT injury with use of diffusion tensor imaging (DTI), and if laterality of injury influences neurologic symptoms. Materials and Methods This retrospective case-control study used prospectively collected clinical and DTI data on patients with medulloblastoma enrolled in a clinical trial (between July 2016 and February 2020) and healthy controls (between April and November 2017), matched with the age range of the participants with medulloblastoma. CMS was divided into types 1 (CMS1) and 2 (CMS2). Multivariable logistic regression was used to investigate the relationship between CMS likelihood and DTCT injury. Results Overall, 82 participants with medulloblastoma (mean age, 11.0 years ± 5.2 [SD]; 53 male) and 35 healthy controls (mean age, 18.0 years ± 3.06; 18 female) were included. In participants with medulloblastoma, DTCT was absent bilaterally (AB), absent on the right side (AR), absent on the left side (AL), or present bilaterally (PB), while it was PB in all healthy controls. Odds of having CMS were associated with higher degree of DTCT damage (AB, odds ratio = 272.7 [95% CI: 269.68, 275.75; P < .001]; AR, odds ratio = 14.40 [95% CI: 2.84, 101.48; P < .001]; and AL, odds ratio = 8.55 [95% CI: 1.15, 74.14; P < .001). Left (coefficient = -0.07, χ2 = 12.4, P < .001) and right (coefficient = -0.15, χ2 = 33.82, P < .001) DTCT volumes were negatively associated with the odds of CMS. More participants with medulloblastoma with AB showed CMS1; unilateral DTCT absence prevailed in CMS2. Lower DTCT volumes correlated with more severe ataxia. Unilateral DTCT injury caused ipsilateral dysmetria; AB caused symmetric dysmetria. PB indicated better neurologic outcome. Conclusion The severity of CMS-associated mutism, ataxia, and dysmetria was associated with DTCT damage severity. DTCT damage patterns differed between CMS1 and CMS2. © RSNA, 2024 Supplemental material is available for this article. See also the editorial by Dorigatti Soldatelli and Ertl-Wagner in this issue.

Changes in cerebellar output abnormally modulate cortical myoclonus sensorimotor hyperexcitability

Cortical myoclonus is produced by abnormal neuronal discharges within the sensorimotor cortex, as demonstrated by electrophysiology. Our hypothesis is that the loss of cerebellar inhibitory control over the motor cortex, via cerebello-thalamo-cortical connections, could induce the increased sensorimotor cortical excitability that eventually causes cortical myoclonus. To explore this hypothesis, in the present study we applied anodal transcranial direct current stimulation over the cerebellum of patients affected by cortical myoclonus and healthy controls and assessed its effect on sensorimotor cortex excitability. We expected that anodal cerebellar transcranial direct current stimulation would increase the inhibitory cerebellar drive to the motor cortex and therefore reduce the sensorimotor cortex hyperexcitability observed in cortical myoclonus. Ten patients affected by cortical myoclonus of various aetiology and 10 aged-matched healthy control subjects were included in the study. All participants underwent somatosensory evoked potentials, long-latency reflexes and short-interval intracortical inhibition recording at baseline and immediately after 20 min session of cerebellar anodal transcranial direct current stimulation. In patients, myoclonus was recorded by the means of surface EMG before and after the cerebellar stimulation. Anodal cerebellar transcranial direct current stimulation did not change the above variables in healthy controls, while it significantly increased the amplitude of somatosensory evoked potential cortical components, long-latency reflexes and decreased short-interval intracortical inhibition in patients; alongside, a trend towards worsening of the myoclonus after the cerebellar stimulation was observed. Interestingly, when dividing patients in those with and without giant somatosensory evoked potentials, the increment of the somatosensory evoked potential cortical components was observed mainly in those with giant potentials. Our data showed that anodal cerebellar transcranial direct current stimulation facilitates-and does not inhibit-sensorimotor cortex excitability in cortical myoclonus syndromes. This paradoxical response might be due to an abnormal homeostatic plasticity within the sensorimotor cortex, driven by dysfunctional cerebello-thalamo-cortical input to the motor cortex. We suggest that the cerebellum is implicated in the pathophysiology of cortical myoclonus and that these results could open the way to new forms of treatment or treatment targets.

Reduced Cerebellar Brain Inhibition and Vibrotactile Perception in Response to Mechanical Hand Stimulation at Flutter Frequency

The cerebellum is traditionally considered a movement control structure because of its established afferent and efferent anatomical and functional connections with the motor cortex. In the last decade, studies also proposed its involvement in perception, particularly somatosensory acquisition and prediction of the sensory consequences of movement. However, compared to its role in motor control, the cerebellum's specific role or modulatory influence on other brain areas involved in sensory perception, specifically the primary sensorimotor cortex, is less clear. In the present study, we explored whether peripherally applied vibrotactile stimuli at flutter frequency affect functional cerebello-cortical connections. In 17 healthy volunteers, changes in cerebellar brain inhibition (CBI) and vibration perception threshold (VPT) were measured before and after a 20-min right hand mechanical stimulation at 25 Hz. 5 Hz mechanical stimulation of the right foot served as an active control condition. Performance in a Grooved Pegboard test (GPT) was also measured to assess stimulation's impact on motor performance. Hand stimulation caused a reduction in CBI (13.16%) and increased VPT but had no specific effect on GPT performance, while foot stimulation had no significant effect on all measures. The result added evidence to the functional connections between the cerebellum and primary motor cortex, as shown by CBI reduction. Meanwhile, the parallel increase in VPT indirectly suggests that the cerebellum influences the processing of vibrotactile stimulus through motor-sensory interactions.

Increased Cerebellar Gray Matter Volume in Athletes: A Voxel-Wise Coordinate-Based Meta-Analysis

Purpose: The purpose of this systematic review and meta-analysis study was to investigate distinct brain structural characteristics in athletes as compared with those in non-athletes by quantifying regional gray matter (GM) volume changes using voxel-based morphometry analysis based on a whole-brain approach. Methods: The systematic literature search was conducted from November 1, 2020 to October 18, 2021 via the two search engines including the PubMed and Web of Science. We included 13 studies that reported GM volume data in 229 athletes as compared 219 non-athletes based on the whole-brain analysis with specific three-dimensional coordinates in a standard stereotactic space. Thus, we performed a coordinate-based meta-analysis using the seed-based d mapping via permutation of subject images methods. Result: The coordinate-based meta-analysis reported that the athletes significantly reveal greater regional GM volume across right cerebellar lobules IV-V and Brodmann area 37 regions than those in the non-athletes with minimal levels of heterogeneity and publication bias between the included studies. The subgroup analyses show that greater GM volume for athletes in closed-skill sports appeared across the right cerebellar hemispheric lobules VIII and the right cingulum than those for non-athletes. Conclusion: These cumulative findings from multiple brain imaging studies suggest potential brain plasticity evidence in the athletes who experienced extensive motor training.

Assessment and tailored physical rehabilitation approaches in persons with cerebellar impairments targeting mobility and walking according to the International Classification of Functioning: a systematic review of case-reports and case-series

PURPOSE

Cerebellar impairment (CI) manifests from different etiologies resulting in a heterogenic clinical presentation affecting walking and mobility. Case-reports were reviewed to provide an analytical clinical picture of persons with CI (PwCI) to differentiate cerebellar and non-cerebellar impairments and to identify interventions and assessments used to quantify impact on walking and mobility according to the International Classification of Functioning, Disability and Health (ICF).

MATERIALS AND METHODS

Literature was searched in PubMed, Web Of Science and Scopus. Case-reports conducting physical rehabilitation and reporting at least one outcome measure of ataxia, gait pattern, walking or mobility were included.

RESULTS

28 articles with a total of 38 different patients were included. Etiologies were clustered to: spinocerebellar degenerations, traumatic brain injuries, cerebellar tumors, stroke and miscellaneous. The interventions applied were activity-based, including gait and balance training. Participation based activities such as tai chi, climbing and dance-based therapy had positive outcomes on mobility. Outcomes on body function such as ataxia and gait pattern were only reported in 22% of the patients.

CONCLUSIONS

A comprehensive test battery to encompass the key features of a PwCI on different levels of the ICF is needed to manage heterogeneity. Measures on body function level should be included in interventions.

Excessive excitability of inhibitory cortical circuit and disturbance of ballistic targeting movement in degenerative cerebellar ataxia

This study aimed to investigate abnormalities in inhibitory cortical excitability and motor control during ballistic-targeting movements in individuals with degenerative cerebellar ataxia (DCA). Sixteen participants took part in the study (DCA group [n = 8] and healthy group [n = 8]). The resting motor-threshold and cortical silent period (cSP) were measured in the right-hand muscle using transcranial magnetic stimulation over the left primary motor cortex. Moreover, the performance of the ballistic-targeting task with right wrist movements was measured. The Scale for the Assessment and Rating of Ataxia was used to evaluate the severity of ataxia. The results indicated that the cSP was significantly longer in participants with DCA compared to that in healthy controls. However, there was no correlation between cSP and severity of ataxia. Furthermore, cSP was linked to the ballistic-targeting task performance in healthy participants but not in participants with DCA. These findings suggest that there is excessive activity in the gamma-aminobutyric acid-mediated cortical inhibitory circuit in individuals with DCA. However, this increase in inhibitory activity not only fails to contribute to the control of ballistic-targeting movement but also shows no correlation with the severity of ataxia. These imply that increased excitability in inhibitory cortical circuits in the DCA may not contribute the motor control as much as it does in healthy older adults under limitations associated with a small sample size. The study's results contribute to our understanding of motor control abnormalities in people with DCA and provide potential evidence for further research in this area.

A Study of Upper-Limb Motion using Kinematic Measures for Clinical Assessment of Cerebellar Ataxia

Cerebellar Ataxia (CA) is a group of diseases affecting the cerebellum, which is responsible for movement coordination. It causes uncoordinated movements and can also impact balance, speech, and eye movements. There are no approved disease-modifying medications for CA, so clinical studies to assess potential treatments are crucial. These studies require robust, objective measurements of CA severity to reflect changes in the progression of the disease due to medication. In recent years, studies have used kinematic measures to evaluate CA severity, but the current method relies on subjective clinical observations and is insufficient for telehealth. There is a need for a non-intrusive system that can monitor people with CA regularly to better understand the disease and develop an automated assessment system. In this study, we analyzed kinematic measures of upper-limb movements during a ballistic tracking test, which primarily involves movements at the shoulder joint. We aimed to understand the challenges of identifying CA and evaluating its severity when measuring such movements. Statistical features of the kinematic signals were used to develop machine learning models for classification and regression. The Gradient Boosting Classifier model had a maximum accuracy of 74%, but the models had low specificity and performed poorly in regression, suggesting that kinematic measures from shoulder-dominated movements during ballistic tracking are not as viable for CA assessment as other measures.

Cerebellar Excitability Regulates Physical Fatigue Perception

Fatigue is the subjective sensation of weariness, increased sense of effort, or exhaustion and is pervasive in neurologic illnesses. Despite its prevalence, we have a limited understanding of the neurophysiological mechanisms underlying fatigue. The cerebellum, known for its role in motor control and learning, is also involved in perceptual processes. However, the role of the cerebellum in fatigue remains largely unexplored. We performed two experiments to examine whether cerebellar excitability is affected after a fatiguing task and its association with fatigue. Using a crossover design, we assessed cerebellar inhibition (CBI) and perception of fatigue in humans before and after "fatigue" and "control" tasks. Thirty-three participants (16 males, 17 females) performed five isometric pinch trials with their thumb and index finger at 80% maximum voluntary capacity (MVC) until failure (force <40% MVC; fatigue) or at 5% MVC for 30 s (control). We found that reduced CBI after the fatigue task correlated with a milder perception of fatigue. In a follow-up experiment, we investigated the behavioral consequences of reduced CBI after fatigue. We measured CBI, perception of fatigue, and performance during a ballistic goal-directed task before and after the same fatigue and control tasks. We replicated the observation that reduced CBI after the fatigue task correlated with a milder perception of fatigue and found that greater endpoint variability after the fatigue task correlated with reduced CBI. The proportional relation between cerebellar excitability and fatigue indicates a role of the cerebellum in the perception of fatigue, which might come at the expense of motor control.SIGNIFICANCE STATEMENT Fatigue is one of the most common and debilitating symptoms in neurologic, neuropsychiatric, and chronic illnesses. Despite its epidemiological importance, there is a limited understanding of the neurophysiological mechanisms underlying fatigue. In a series of experiments, we demonstrate that decreased cerebellar excitability relates to lesser physical fatigue perception and worse motor control. These results showcase the role of the cerebellum in fatigue regulation and suggest that fatigue- and performance-related processes might compete for cerebellar resources.

A novel tablet-based motor coordination test performs on par with the Beery VMI subtest and offers superior temporal metrics: findings from children with pediatric acute-onset neuropsychiatric syndrome

Neuropsychiatric and neurodevelopmental disorders are often associated with coordination problems. Pediatric Acute-onset Neuropsychiatric Syndrome (PANS) constitutes a specific example of acute and complex symptomatology that includes difficulties with motor control. The present proof-of-concept study aimed at testing a new, bespoke tablet-based motor coordination test named SpaceSwipe, providing fine-grained measures that could be used to follow-up on symptoms evolution in PANS. This test enables computationally precise and objective metrics of motor coordination, taking into account both directional and spatial features continuously. We used SpaceSwipe to assess motor coordination in a group of children with PANS (n = 12, assessed on in total of 40 occasions) and compared it against the motor coordination subtest from the Beery-Buktenica Developmental Test of Visual-Motor Integration (Beery VMI) 6th edition, traditionally used to follow-up symptomatology. Using a bivariate linear regression, we found that 33 s of the directional offset from tracking a moving target in SpaceSwipe could predict the Beery VMI motor coordination (VMI MC) raw scores (mean absolute error: 1.75 points). Positive correlations between the predicted scores and the VMI MC scores were found for initial testing (r_adj = 0.87) and for repeated testing (r_adj = 0.79). With its short administration time and its close prediction to Beery VMI scores, this proof-of-concept study demonstrates the potential for SpaceSwipe as a patient-friendly tool for precise, objective assessment of motor coordination in children with neurodevelopmental or neuropsychiatric disorders.

Digital health metrics reveal upper limb impairment profiles in ARSACS

OBJECTIVE

Adults with autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) often present with reduced upper limb coordination affecting their independence in daily life. Previous studies in ARSACS identified reduced performance in clinical assessments requiring fine and gross dexterity as well as prehension. However, the kinematic and kinetic aspects underlying reduced upper limb coordination in ARSACS have not been systematically investigated yet. In this work, we aimed to provide a detailed characterization of alterations in upper limb movement patterns and hand grip forces in 57 participants with ARSACS.

METHODS

We relied on a goal-directed technology-aided assessment task, which provides eight previously validated digital health metrics describing movement efficiency, smoothness, speed, and grip force control.

RESULTS

First, we observed that 98.3% of the participants were impaired in at least one of the metrics, that all metrics are significantly impaired on a population level, and that grip force control during precise manipulations is most commonly and strongly impaired. Second, we identified high inter-participant variability in the kinematic and kinetic impairment profiles, thereby capturing different clinical profiles subjectively observed in this population. Lastly, abnormal goal-directed task performance in ARSACS could be best explained by reduced movement speed, efficiency, and especially force control during precise manipulations, while abnormal movement smoothness did not have a significant effect.

INTERPRETATION

This work helped to refine the clinical profile of ARSACS and highlights the need for characterizing individual kinematic and kinetic impairment profiles in clinical trials in ARSACS.

The Role of the Cerebellar and Vestibular Networks in Anxiety Disorders and Depression: the Internal Model Hypothesis

Clinical data and animal studies confirmed that the cerebellum and the vestibular system are involved in emotions. Nowadays, no real consensus has really emerged to explain the clinical symptoms in humans and behavioral deficits in the animal models. We envisage here that the cerebellum and the vestibular system play complementary roles in emotional reactivity. The cerebellum integrates a large variety of exteroceptive and proprioceptive information necessary to elaborate and to update the internal model: in emotion, as in motor processes, it helps our body and self to adapt to the environment, and to anticipate any changes in such environment in order to produce a time-adapted response. The vestibular system provides relevant environmental stimuli (i.e., gravity, self-position, and movement) and is involved in self-perception. Consequently, cerebellar or vestibular disorders could generate « internal fake news» (due to lack or false sensory information and/or integration) that could, in turn, generate potential internal model deficiencies. In this case, the alterations provoke false anticipation of motor command and external sensory feedback, associated with unsuited behaviors. As a result, the individual becomes progressively unable to cope with the environmental solicitation. We postulate that chronically unsuited, and potentially inefficient, behavioral and visceral responses to environmental solicitations lead to stressful situations. Furthermore, this inability to adapt to the context of the situation generates chronic anxiety which could precede depressive states.

Long-Lasting Efficacy of Radio Electric Asymmetric Conveyer Neuromodulation Treatment on Functional Dysmetria, an Adaptive Motor Behavior

Background Fluctuating asymmetry (FA) is widely defined as the deviation from perfect bilateral symmetry and is considered an epigenetic measure of environmental stress. Rinaldi and Fontani hypothesized that the FA morpho-functional changes originate from an adaptive motor behavior determined by functional alterations in the cerebellum and neural circuits, not caused by a lesion, but induced by environmental stress. They called this phenomenon functional dysmetria (FD). On this premise, they developed the radio electric asymmetric conveyer (REAC) technology, a neuromodulation technology aimed at optimizing the best neuro-psycho-motor strategies in relation to environmental interaction. Aims Previous studies showed that specific REAC neuro postural optimization (NPO) treatment can induce stable FD recovery. This study aimed to verify the duration of the NPO effect in inducing the stable FD recovery over time. Materials and methods Data were retrospectively collected from a population of 29,794 subjects who underwent a specific semiological FD assessment and received the NPO treatment, regardless of the pathology referred. Results The analysis of the data collected by the various participants in the study led us to ascertain the disappearance of FD in 100% of the cases treated, with a stability of the result detected up to 18 years after the single administration of the REAC NPO treatment. Conclusions The REAC NPO neurobiological modulation treatment consisting of a single administration surprisingly maintains a very long efficacy in the correction of FD. This effect can be explained as the long-lasting capacity of the NPO treatment to induce greater functional efficiency of the brain dynamics as proven in previous studies.

Reliability and validity of digital health metrics for assessing arm and hand impairments in an ataxic disorder

OBJECTIVES

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is the second most frequent recessive ataxia and commonly features reduced upper limb coordination. Sensitive outcome measures of upper limb coordination are essential to track disease progression and the effect of interventions. However, available clinical assessments are insufficient to capture behavioral variability and detailed aspects of motor control. While digital health metrics extracted from technology-aided assessments promise more fine-grained outcome measures, these have not been validated in ARSACS. Thus, the aim was to document the metrological properties of metrics from a technology-aided assessment of arm and hand function in ARSACS.

METHODS

We relied on the Virtual Peg Insertion Test (VPIT) and used a previously established core set of 10 digital health metrics describing upper limb movement and grip force patterns during a pick-and-place task. We evaluated reliability, measurement error, and learning effects in 23 participants with ARSACS performing three repeated assessment sessions. In addition, we documented concurrent validity in 57 participants with ARSACS performing one session.

RESULTS

Eight metrics had excellent test-retest reliability (intraclass correlation coefficient 0.89 ± 0.08), five low measurement error (smallest real difference % 25.4 ± 5.7), and none strong learning effects (systematic change η -0.11 ± 2.5). Significant correlations (ρ 0.39 ± 0.13) with clinical scales describing gross and fine dexterity and lower limb coordination were observed.

INTERPRETATION

This establishes eight digital health metrics as valid and robust endpoints for cross-sectional studies and five metrics as potentially sensitive endpoints for longitudinal studies in ARSACS, thereby promising novel insights into upper limb sensorimotor control.

Costs of position, velocity, and force requirements in optimal control induce triphasic muscle activation during reaching movement

The nervous system activates a pair of agonist and antagonist muscles to determine the muscle activation pattern for a desired movement. Although there is a problem with redundancy, it is solved immediately, and movements are generated with characteristic muscle activation patterns in which antagonistic muscle pairs show alternate bursts with a triphasic shape. To investigate the requirements for deriving this pattern, this study simulated arm movement numerically by adopting a musculoskeletal arm model and an optimal control. The simulation reproduced the triphasic electromyogram (EMG) pattern observed in a reaching movement using a cost function that considered three terms: end-point position, velocity, and force required; the function minimised neural input. The first, second, and third bursts of muscle activity were generated by the cost terms of position, velocity, and force, respectively. Thus, we concluded that the costs of position, velocity, and force requirements in optimal control can induce triphasic EMG patterns. Therefore, we suggest that the nervous system may control the body by using an optimal control mechanism that adopts the costs of position, velocity, and force required; these costs serve to initiate, decelerate, and stabilise movement, respectively.

Dystonia and Cerebellum: From Bench to Bedside

Dystonia pathogenesis remains unclear; however, findings from basic and clinical research suggest the importance of the interaction between the basal ganglia and cerebellum. After the discovery of disynaptic pathways between the two, much attention has been paid to the cerebellum. Basic research using various dystonia rodent models and clinical studies in dystonia patients continues to provide new pieces of knowledge regarding the role of the cerebellum in dystonia genesis. Herein, we review basic and clinical articles related to dystonia focusing on the cerebellum, and clarify the current understanding of the role of the cerebellum in dystonia pathogenesis. Given the recent evidence providing new hypotheses regarding dystonia pathogenesis, we discuss how the current evidence answers the unsolved clinical questions.

A multizone cerebellar chip for bioinspired adaptive robot control and sensorimotor processing

The cerebellum is a neural structure essential for learning, which is connected via multiple zones to many different regions of the brain, and is thought to improve human performance in a large range of sensory, motor and even cognitive processing tasks. An intriguing possibility for the control of complex robotic systems would be to develop an artificial cerebellar chip with multiple zones that could be similarly connected to a variety of subsystems to optimize performance. The novel aim of this paper, therefore, is to propose and investigate a multizone cerebellar chip applied to a range of tasks in robot adaptive control and sensorimotor processing. The multizone cerebellar chip was evaluated using a custom robotic platform consisting of an array of tactile sensors driven by dielectric electroactive polymers mounted upon a standard industrial robot arm. The results demonstrate that the performance in each task was improved by the concurrent, stable learning in each cerebellar zone. This paper, therefore, provides the first empirical demonstration that a synthetic, multizone, cerebellar chip could be embodied within existing robotic systems to improve performance in a diverse range of tasks, much like the cerebellum in a biological system.

The Cerebellar Nuclei and Dexterous Limb Movements

Dexterous forelimb movements like reaching, grasping, and manipulating objects are fundamental building blocks of the mammalian motor repertoire. These behaviors are essential to everyday activities, and their elaboration underlies incredible accomplishments by human beings in art and sport. Moreover, the susceptibility of these behaviors to damage and disease of the nervous system can lead to debilitating deficits, highlighting a need for a better understanding of function and dysfunction in sensorimotor control. The cerebellum is central to coordinating limb movements, as defined in large part by Joseph Babinski and Gordon Holmes describing motor impairment in patients with cerebellar lesions over 100 years ago (Babinski, 1902; Holmes, 1917), and supported by many important human and animal studies that have been conducted since. Here, with a focus on output pathways of the cerebellar nuclei across mammalian species, we describe forelimb movement deficits observed when cerebellar circuits are perturbed, the mechanisms through which these circuits influence motor output, and key challenges in defining how the cerebellum refines limb movement.

Cerebellar patients have intact feedback control that can be leveraged to improve reaching

It is thought that the brain does not simply react to sensory feedback, but rather uses an internal model of the body to predict the consequences of motor commands before sensory feedback arrives. Time-delayed sensory feedback can then be used to correct for the unexpected—perturbations, motor noise, or a moving target. The cerebellum has been implicated in this predictive control process. Here, we show that the feedback gain in patients with cerebellar ataxia matches that of healthy subjects, but that patients exhibit substantially more phase lag. This difference is captured by a computational model incorporating a Smith predictor in healthy subjects that is missing in patients, supporting the predictive role of the cerebellum in feedback control. Lastly, we improve cerebellar patients’ movement control by altering (phase advancing) the visual feedback they receive from their own self movement in a simplified virtual reality setup.

Dysmetria and Errors in Predictions: The Role of Internal Forward Model

The terminology of cerebellar dysmetria embraces a ubiquitous symptom in motor deficits, oculomotor symptoms, and cognitive/emotional symptoms occurring in cerebellar ataxias. Patients with episodic ataxia exhibit recurrent episodes of ataxia, including motor dysmetria. Despite the consensus that cerebellar dysmetria is a cardinal symptom, there is still no agreement on its pathophysiological mechanisms to date since its first clinical description by Babinski. We argue that impairment in the predictive computation for voluntary movements explains a range of characteristics accompanied by dysmetria. Within this framework, the cerebellum acquires and maintains an internal forward model, which predicts current and future states of the body by integrating an estimate of the previous state and a given efference copy of motor commands. Two of our recent studies experimentally support the internal-forward-model hypothesis of the cerebellar circuitry. First, the cerebellar outputs (firing rates of dentate nucleus cells) contain predictive information for the future cerebellar inputs (firing rates of mossy fibers). Second, a component of movement kinematics is predictive for target motions in control subjects. In cerebellar patients, the predictive component lags behind a target motion and is compensated with a feedback component. Furthermore, a clinical analysis has examined kinematic and electromyography (EMG) features using a task of elbow flexion goal-directed movements, which mimics the finger-to-nose test. Consistent with the hypothesis of the internal forward model, the predictive activations in the triceps muscles are impaired, and the impaired predictive activations result in hypermetria (overshoot). Dysmetria stems from deficits in the predictive computation of the internal forward model in the cerebellum. Errors in this fundamental mechanism result in undershoot (hypometria) and overshoot during voluntary motor actions. The predictive computation of the forward model affords error-based motor learning, coordination of multiple degrees of freedom, and adequate timing of muscle activities. Both the timing and synergy theory fit with the internal forward model, microzones being the elemental computational unit, and the anatomical organization of converging inputs to the Purkinje neurons providing them the unique property of a perceptron in the brain. We propose that motor dysmetria observed in attacks of ataxia occurs as a result of impaired predictive computation of the internal forward model in the cerebellum.

EphA4 Is Required for Neural Circuits Controlling Skilled Reaching

Skilled forelimb movements are initiated by feedforward motor commands conveyed by supraspinal motor pathways. The accuracy of reaching and grasping relies on internal feedback pathways that update ongoing motor commands. In mice lacking the axon guidance molecule EphA4, axonal misrouting of the corticospinal tract and spinal interneurons is manifested, leading to a hopping gait in hindlimbs. Moreover, mice with a conditional forebrain deletion of EphA4, display forelimb hopping in adaptive locomotion and exploratory reaching movements. However, it remains unclear how loss of EphA4 signaling disrupts function of forelimb motor circuit and skilled reaching and grasping movements. Here we investigated how neural circuits controlling skilled reaching were affected by the loss of EphA4. Both male and female C57BL/6 wild-type, heterozygous EphA4^+/-, and homozygous EphA4^-/- mice were used in behavioral and in vivo electrophysiological investigations. We found that EphA4 knock-out (-/-) mice displayed impaired goal-directed reaching movements. In vivo intracellular recordings from forelimb motor neurons demonstrated increased corticoreticulospinal excitation, decreased direct reticulospinal excitation, and reduced direct propriospinal excitation in EphA4 knock-out mice. Cerebellar surface recordings showed a functional perturbation of the lateral reticular nucleus-cerebellum internal feedback pathway in EphA4 knock-out mice. Together, our findings provide in vivo evidence at the circuit level that loss of EphA4 disrupts the function of both feedforward and feedback motor pathways, resulting in deficits in skilled reaching.SIGNIFICANCE STATEMENT The central advances of this study are the demonstration that null mutation in the axon guidance molecule EphA4 gene impairs the ability of mice to perform skilled reaching, and identification of how these behavioral deficits correlates with discrete neurophysiological changes in central motor pathways involved in the control of reaching. Our findings provide in vivo evidence at the circuit level that loss of EphA4 disrupts both feedforward and feedback motor pathways, resulting in deficits in skilled reaching. This analysis of motor circuit function may help to understand the pathophysiological mechanisms underlying movement disorders in humans.

Differential effects of thyrotropin releasing hormone (TRH) on motor execution and motor adaptation process in patients with spinocerebellar degeneration

BACKGROUND

The cerebellum is known to play a crucial role in sensori-motor adaptation, which includes the prism adaptation. TRH has been widely used as a treatment for cerebellar ataxia in Japan, however effects of TRH on cerebellar adaptation process have not been studied. Here, we studied effects of TRH treatment on the prism adaptation task.

METHODS

Eighteen spinocerebellar degeneration (SCD) patients participated in this study. The participants received intravenous injection of 2 mg/day protirelin tartrate once a day for 14 days. In the prism adaptation task, the participants reached to the target on the screen wearing wedge prisms. We compared the Scale for Assessment and Rating of Ataxia (SARA), baseline errors and the aftereffect (AE) of the prism adaptation task between before and after TRH therapy.

RESULTS

TRH therapy improved SARA significantly (p = .005). Multiple regression analysis revealed that improvement of SARA score was mainly due to improvement of "Stance" category score. TRH decreased baseline errors of the prism adaptation task (p = .021), while unaffected AEs (p = .252).

CONCLUSION

TRH differentially affected clinical cerebellar ataxia including baseline reaching performance in the prism adaptation task, whereas TRH did not affect the learning process of prism adaptation. Different cerebellar functional aspects may underlie the learning process of sensori-motor adaptation and simple motor execution (clinically evaluated cerebellar ataxia).

Quantitative Separation of Tremor and Ataxia in Essential Tremor

OBJECTIVE

This study addresses an important problem in neurology, distinguishing tremor and ataxia using quantitative methods. Specifically, we aimed to quantitatively separate dysmetria, a cardinal sign of ataxia, from tremor in essential tremor (ET).

METHODS

In Experiment 1, we compared 19 participants diagnosed with ET undergoing thalamic deep brain stimulation (DBS; ET_DBS ) to 19 healthy controls (HC). We quantified tremor during postural tasks using accelerometry and dysmetria with fast, reverse-at-target goal-directed movements. To ensure that endpoint accuracy was unaffected by tremor, we quantified dysmetria in selected trials manifesting a smooth trajectory to the endpoint. Finally, we manipulated tremor amplitude by switching DBS ON and OFF to examine its effect on dysmetria. In Experiment 2, we compared 10 ET participants with 10 HC to determine whether we could identify and distinguish dysmetria from tremor in non-DBS ET.

RESULTS

Three findings suggest that we can quantify dysmetria independently of tremor in ET. First, ET_DBS and ET exhibited greater dysmetria than HC and dysmetria did not correlate with tremor (R²  < 0.01). Second, even for trials with tremor-free trajectories to the target, ET exhibited greater dysmetria than HC (p < 0.01). Third, activating DBS reduced tremor (p < 0.01) but had no effect on dysmetria (p > 0.2).

INTERPRETATION

We demonstrate that dysmetria can be quantified independently of tremor using fast, reverse-at-target goal-directed movements. These results have important implications for the understanding of ET and other cerebellar and tremor disorders. Future research should examine the neurophysiological mechanisms underlying each symptom and characterize their independent contribution to disability. ANN NEUROL 2020;88:375-387.

A data-driven framework for selecting and validating digital health metrics: use-case in neurological sensorimotor impairments

Digital health metrics promise to advance the understanding of impaired body functions, for example in neurological disorders. However, their clinical integration is challenged by an insufficient validation of the many existing and often abstract metrics. Here, we propose a data-driven framework to select and validate a clinically relevant core set of digital health metrics extracted from a technology-aided assessment. As an exemplary use-case, the framework is applied to the Virtual Peg Insertion Test (VPIT), a technology-aided assessment of upper limb sensorimotor impairments. The framework builds on a use-case-specific pathophysiological motivation of metrics, models demographic confounds, and evaluates the most important clinimetric properties (discriminant validity, structural validity, reliability, measurement error, learning effects). Applied to 77 metrics of the VPIT collected from 120 neurologically intact and 89 affected individuals, the framework allowed selecting 10 clinically relevant core metrics. These assessed the severity of multiple sensorimotor impairments in a valid, reliable, and informative manner. These metrics provided added clinical value by detecting impairments in neurological subjects that did not show any deficits according to conventional scales, and by covering sensorimotor impairments of the arm and hand with a single assessment. The proposed framework provides a transparent, step-by-step selection procedure based on clinically relevant evidence. This creates an interesting alternative to established selection algorithms that optimize mathematical loss functions and are not always intuitive to retrace. This could help addressing the insufficient clinical integration of digital health metrics. For the VPIT, it allowed establishing validated core metrics, paving the way for their integration into neurorehabilitation trials.

Rapid Diagnosis of Spinocerebellar Ataxia 36 in a Three-Generation Family Using Short-Read Whole-Genome Sequencing Data

BACKGROUND

Spinocerebellar ataxias are often caused by expansions of short tandem repeats. Recent methodological advances have made repeat expansion (RE) detection with whole-genome sequencing (WGS) feasible.

OBJECTIVES

The objective of this study was to determine the genetic basis of ataxia in a multigenerational Australian pedigree with autosomal-dominant inheritance.

METHODS AND RESULTS

WGS was performed on 3 affected relatives. The sequence data were screened for known pathogenic REs using 2 RE detection tools: exSTRa and ExpansionHunter. This screen provided a clear and rapid diagnosis (<5 days from receiving the sequencing data) of spinocerebellar ataxia 36, a rare form of ataxia caused by an intronic GGCCTG RE in NOP56.

CONCLUSIONS

The diagnosis of rare ataxias caused by REs is highly feasible and cost-effective with WGS. We propose that WGS could potentially be implemented as the frontline, cost-effective methodology for the molecular testing of individuals with a clinical diagnosis of ataxia. © 2020 International Parkinson and Movement Disorder Society.

Performance Limitations in Sensorimotor Control: Trade-Offs Between Neural Computation and Accuracy in Tracking Fast Movements

The ability to move fast and accurately track moving objects is fundamentally constrained by the biophysics of neurons and dynamics of the muscles involved. Yet the corresponding trade-offs between these factors and tracking motor commands have not been rigorously quantified. We use feedback control principles to quantify performance limitations of the sensorimotor control system (SCS) to track fast periodic movements. We show that (1) linear models of the SCS fail to predict known undesirable phenomena, including skipped cycles, overshoot and undershoot, produced when tracking signals in the "fast regime," while nonlinear pulsatile control models can predict such undesirable phenomena, and (2) tools from nonlinear control theory allow us to characterize fundamental limitations in this fast regime. Using a validated and tractable nonlinear model of the SCS, we derive an analytical upper bound on frequencies that the SCS model can reliably track before producing such undesirable phenomena as a function of the neurons' biophysical constraints and muscle dynamics. The performance limitations derived here have important implications in sensorimotor control. For example, if the primary motor cortex is compromised due to disease or damage, the theory suggests ways to manipulate muscle dynamics by adding the necessary compensatory forces using an assistive neuroprosthetic device to restore motor performance and, more important, fast and agile movements. Just how one should compensate can be informed by our SCS model and the theory developed here.

Temporal but not spatial dysmetria relates to disease severity in FA

Friedreich's ataxia (FA) is an inherited disease that causes degeneration of the nervous system. Features of FA include proprioceptive and cerebellar deficits leading to impaired muscle coordination and, consequently, dysmetria in force and time of movement. The aim of this study is to characterize dysmetria and its association to disease severity. Also, we examine the neural mechanisms of dysmetria by quantifying the EMG burst area, duration, and time-to-peak of the agonist muscle. Twenty-seven individuals with FA and 13 healthy controls (HCs) performed the modified Functional Ataxia Rating Scale and goal-directed movements with the ankle. Dysmetria was quantified as position and time error during dorsiflexion. FA individuals exhibited greater time but not position error than HCs. Moreover, time error correlated with disease severity and was related to increased agonist EMG burst. Temporal dysmetria is associated to disease severity, likely due to altered activation of the agonist muscle.NEW & NOTEWORTHY For the first time, we quantified spatial and temporal dysmetria and its relation to disease severity in Friedreich's ataxia (FA). We found that FA individuals exhibit temporal but not spatial dysmetria relative to healthy controls. Temporal dysmetria correlated to disease severity in FA and was predicted from an altered activation of the agonist muscle. Therefore, these results provide novel evidence that FA exhibit temporal but not spatial dysmetria, which is different from previous findings on SCA6.

Temporal Invariance in SCA6 Is Related to Smaller Cerebellar Lobule VI and Greater Disease Severity

Regulating muscle force and timing are fundamental for accurate motor performance. In spinocerebellar ataxia type 6 (SCA6), there is evidence that individuals have greater force dysmetria but display better temporal accuracy during fast goal directed contractions. Here, we test whether greater temporal accuracy occurs in all individuals with SCA6, and can be explained by lesser temporal variability. Further we examine whether it is linked to disease severity and specific degenerative changes in the cerebellum. Nineteen human participants with SCA6 (13 woman) and 18 healthy controls performed fast goal-directed ankle dorsiflexion contractions aiming at a spatiotemporal target. We quantified the endpoint control of these contractions, gray matter (GM) integrity of the cerebellum, and disease severity using the International Cooperative Ataxia Rating Scale (ICARS). SCA6 individuals exhibited lower temporal endpoint error and variability than the healthy controls (p = 0.008). Statistically, SCA6 clustered into two distinct groups for temporal variability. A group with low temporal variability ranging from 10 to 19% (SCA6a) and a group with temporal variability similar to healthy controls (SCA6b; 19-40%).SCA6a exhibited greater disease severity than SCA6b, as assessed with ICARS (p < 0.001). Lower temporal variability, which was not associated with disease duration (R ² = 0.1, p > 0.2), did correlate with both greater ICARS (R ² = 0.3) and reduced GM volume in cerebellar lobule VI (R ² = 0.35). Other cerebellar lobules did not relate to temporal variability. We provide new evidence that a subset of SCA6 with greater loss of GM in cerebellum lobule VI exhibit temporal invariance and more severe ataxia than other SCA6 individuals.SIGNIFICANCE STATEMENT Variability is an inherent feature of voluntary movement, and traditionally more variability in the targeted output infers impaired performance. For example, cerebellar patients present exacerbated temporal variability during multijoint movements, which is thought to contribute to their motor deficits. In the current work, we show that in a subgroup of spinocerebellar ataxia type 6 individuals, temporal variability is lower than that of healthy controls when performing single-joint fast-goal directed movements. This invariance related to exacerbated atrophy of lobule VI of the cerebellum and exacerbated disease severity. The relation between invariance and disease severity suggests that pathological motor variability can manifest not only as an exacerbation but also as a reduction relative to healthy controls.

Consensus paper: Decoding the Contributions of the Cerebellum as a Time Machine. From Neurons to Clinical Applications

Time perception is an essential element of conscious and subconscious experience, coordinating our perception and interaction with the surrounding environment. In recent years, major technological advances in the field of neuroscience have helped foster new insights into the processing of temporal information, including extending our knowledge of the role of the cerebellum as one of the key nodes in the brain for this function. This consensus paper provides a state-of-the-art picture from the experts in the field of the cerebellar research on a variety of crucial issues related to temporal processing, drawing on recent anatomical, neurophysiological, behavioral, and clinical research.The cerebellar granular layer appears especially well-suited for timing operations required to confer millisecond precision for cerebellar computations. This may be most evident in the manner the cerebellum controls the duration of the timing of agonist-antagonist EMG bursts associated with fast goal-directed voluntary movements. In concert with adaptive processes, interactions within the cerebellar cortex are sufficient to support sub-second timing. However, supra-second timing seems to require cortical and basal ganglia networks, perhaps operating in concert with cerebellum. Additionally, sensory information such as an unexpected stimulus can be forwarded to the cerebellum via the climbing fiber system, providing a temporally constrained mechanism to adjust ongoing behavior and modify future processing. Patients with cerebellar disorders exhibit impairments on a range of tasks that require precise timing, and recent evidence suggest that timing problems observed in other neurological conditions such as Parkinson's disease, essential tremor, and dystonia may reflect disrupted interactions between the basal ganglia and cerebellum.The complex concepts emerging from this consensus paper should provide a foundation for further discussion, helping identify basic research questions required to understand how the brain represents and utilizes time, as well as delineating ways in which this knowledge can help improve the lives of those with neurological conditions that disrupt this most elemental sense. The panel of experts agrees that timing control in the brain is a complex concept in whom cerebellar circuitry is deeply involved. The concept of a timing machine has now expanded to clinical disorders.

A simple saccadic reading test to assess ocular motor function in cerebellar ataxia

Cerebellar ataxia is a neurological disorder due to dysfunction of the cerebellum that affects coordination of fine movement, gait, and balance. Although ataxic patients commonly exhibit abnormal eye movement and have difficulties with saccadic reading, quantification of ocular motor abilities during reading in the clinical setting is rarely done. In this study, we assess visual performance with simple reading tests that can be used in the clinical setting and performed video infrared oculography in 11 patients with hereditary or acquired cerebellar ataxia and 11 age-matched controls. We found that compared with controls, ataxic patients read significantly slower on regularly and irregularly spaced 120 single-digit number reading tasks (read aloud) (p = 0.02 for both) but not on a word reading task (read silently), although there was large variability on the word reading task. Among the 3 reading tasks, the regularly spaced number reading task had the greatest difference (44%) between ataxic patients and controls. Analysis of oculography revealed that ataxic patients had slower reading speeds on the regularly spaced number reading task because of significantly higher saccade and fixation counts, impairment of small amplitude progressive saccades as well as large amplitude, line-changing saccades, greater fixation dispersion, and irregularity of scan paths and staircase gaze patterns. Our findings show that infrared oculography remains the gold standard in assessment of ocular motor difficulties during reading in ataxic patients. In the absence of this capability in the clinical setting, a simple 120 regularly spaced single-digit saccadic number reading test, which most patients can perform in less than 2 minutes, can be a possible biomarker for ocular motor abilities necessary for reading.

Tandem internal models execute motor learning in the cerebellum

In performing skillful movement, humans use predictions from internal models formed by repetition learning. However, the computational organization of internal models in the brain remains unknown. Here, we demonstrate that a computational architecture employing a tandem configuration of forward and inverse internal models enables efficient motor learning in the cerebellum. The model predicted learning adaptations observed in hand-reaching experiments in humans wearing a prism lens and explained the kinetic components of these behavioral adaptations. The tandem system also predicted a form of subliminal motor learning that was experimentally validated after training intentional misses of hand targets. Patients with cerebellar degeneration disease showed behavioral impairments consistent with tandemly arranged internal models. These findings validate computational tandemization of internal models in motor control and its potential uses in more complex forms of learning and cognition.

Energy exchanges at contact events guide sensorimotor integration

The brain must consider the arm’s inertia to predict the arm's movements elicited by commands impressed upon the muscles. Here, we present evidence suggesting that the integration of sensory information leading to the representation of the arm's inertia does not take place continuously in time but only at discrete transient events, in which kinetic energy is exchanged between the arm and the environment. We used a visuomotor delay to induce cross-modal variations in state feedback and uncovered that the difference between visual and proprioceptive velocity estimations at isolated collision events was compensated by a change in the representation of arm inertia. The compensation maintained an invariant estimate across modalities of the expected energy exchange with the environment. This invariance captures different types of dysmetria observed across individuals following prolonged exposure to a fixed intermodal temporal perturbation and provides a new interpretation for cerebellar ataxia.

Cerebellum-from J. E. Purkyně up to Contemporary Research

Jan. Evangelista Purkyně, the most famous among Czech physiologists, was the first who identified and described the largest nerve cells in the cerebellum. The most distinguished researchers of the nervous system then recommended naming these neurons Purkinje cells in his honor. Through experiments by Purkinje and his followers, the function of the cerebellum was properly attributed to the precision of motor movements and skills. This traditional concept was valid until early 1990s, when it was readjusted and replenished with new and important findings. It was discovered that the cerebellar cortex contains more neurons than the cerebral cortex and shortly thereafter was gradually revealed that such enormous numbers of neural cells are not without impact on brain functions. It was shown that the cerebellum, in addition to its traditional role, also participates in higher nervous activity. These new findings were obtained thanks to the introduction of modern methods of examination into the clinical praxis, and experimental procedures using animal models of cerebellar disorders described in this work.

Restoration of Central Programmed Movement Pattern by Temporal Electrical Stimulation-Assisted Training in Patients with Spinal Cerebellar Atrophy

Disrupted triphasic electromyography (EMG) patterns of agonist and antagonist muscle pairs during fast goal-directed movements have been found in patients with hypermetria. Since peripheral electrical stimulation (ES) and motor training may modulate motor cortical excitability through plasticity mechanisms, we aimed to investigate whether temporal ES-assisted movement training could influence premovement cortical excitability and alleviate hypermetria in patients with spinal cerebellar ataxia (SCA). The EMG of the agonist extensor carpi radialis muscle and antagonist flexor carpi radialis muscle, premovement motor evoked potentials (MEPs) of the flexor carpi radialis muscle, and the constant and variable errors of movements were assessed before and after 4 weeks of ES-assisted fast goal-directed wrist extension training in the training group and of general health education in the control group. After training, the premovement MEPs of the antagonist muscle were facilitated at 50 ms before the onset of movement. In addition, the EMG onset latency of the antagonist muscle shifted earlier and the constant error decreased significantly. In summary, temporal ES-assisted training alleviated hypermetria by restoring antagonist premovement and temporal triphasic EMG patterns in SCA patients. This technique may be applied to treat hypermetria in cerebellar disorders. (This trial is registered with NCT01983670.).

Plasticity of cerebellar Purkinje cells in behavioral training of body balance control

Neural responses to sensory inputs caused by self-generated movements (reafference) and external passive stimulation (exafference) differ in various brain regions. The ability to differentiate such sensory information can lead to movement execution with better accuracy. However, how sensory responses are adjusted in regard to this distinguishability during motor learning is still poorly understood. The cerebellum has been hypothesized to analyze the functional significance of sensory information during motor learning, and is thought to be a key region of reafference computation in the vestibular system. In this study, we investigated Purkinje cell (PC) spike trains as cerebellar cortical output when rats learned to balance on a suspended dowel. Rats progressively reduced the amplitude of body swing and made fewer foot slips during a 5-min balancing task. Both PC simple (SSs; 17 of 26) and complex spikes (CSs; 7 of 12) were found to code initially on the angle of the heads with respect to a fixed reference. Using periods with comparable degrees of movement, we found that such SS coding of information in most PCs (10 of 17) decreased rapidly during balance learning. In response to unexpected perturbations and under anesthesia, SS coding capability of these PCs recovered. By plotting SS and CS firing frequencies over 15-s time windows in double-logarithmic plots, a negative correlation between SS and CS was found in awake, but not anesthetized, rats. PCs with prominent SS coding attenuation during motor learning showed weaker SS-CS correlation. Hence, we demonstrate that neural plasticity for filtering out sensory reafference from active motion occurs in the cerebellar cortex in rats during balance learning. SS-CS interaction may contribute to this rapid plasticity as a form of receptive field plasticity in the cerebellar cortex between two receptive maps of sensory inputs from the external world and of efference copies from the will center for volitional movements.

The Equine Movement Disorder “Shivers” Is Associated With Selective Cerebellar Purkinje Cell Axonal Degeneration

“Shivers” is a progressive equine movement disorder of unknown etiology. Clinically, horses with shivers show difficulty walking backward, assume hyperflexed limb postures, and have hind limb tremors during backward movement that resembles shivering. At least initially, forward movements are normal. Given that neither the neurophysiologic nor the pathologic mechanisms of the disease is known, nor has a neuroanatomic locus been identified, we undertook a detailed neuroanatomic and neuropathologic analysis of the complete sensorimotor system in horses with shivers and clinically normal control horses. No abnormalities were identified in the examined hind limb and forelimb skeletal muscles nor the associated peripheral nerves. Eosinophilic segmented axonal spheroids were a common lesion. Calretinin-positive axonal spheroids were present in many regions of the central nervous system, particularly the nucleus cuneatus lateralis; however, their numbers did not differ significantly from those of control horses. When compared to controls, calretinin-negative, calbindin-positive, and glutamic acid decarboxylase–positive spheroids were increased 80-fold in Purkinje cell axons within the deep cerebellar nuclei of horses with shivers. Unusual lamellar or membranous structures resembling marked myelin decompaction were present between myelin sheaths of presumed Purkinje cell axons in the deep cerebellar nuclei of shivers but not control horses. The immunohistochemical and ultrastructural characteristics of the lesions combined with their functional neuroanatomic distribution indicate, for the first time, that shivers is characterized by end-terminal neuroaxonal degeneration in the deep cerebellar nuclei, which results in context-specific hypermetria and myoclonus.

Predicting and correcting ataxia using a model of cerebellar function

Cerebellar damage results in uncoordinated, variable and dysmetric movements known as ataxia. Here we show that we can reliably model single-joint reaching trajectories of patients (n = 10), reproduce patient-like deficits in the behaviour of controls (n = 11), and apply patient-specific compensations that improve reaching accuracy (P < 0.02). Our approach was motivated by the theory that the cerebellum is essential for updating and/or storing an internal dynamic model that relates motor commands to changes in body state (e.g. arm position and velocity). We hypothesized that cerebellar damage causes a mismatch between the brain's modelled dynamics and the actual body dynamics, resulting in ataxia. We used both behavioural and computational approaches to demonstrate that specific cerebellar patient deficits result from biased internal models. Our results strongly support the idea that an intact cerebellum is critical for maintaining accurate internal models of dynamics. Importantly, we demonstrate how subject-specific compensation can improve movement in cerebellar patients, who are notoriously unresponsive to treatment.

A hypothetical universal model of cerebellar function: reconsideration of the current dogma

The cerebellum is commonly studied in the context of the classical eyeblink conditioning model, which attributes an adaptive motor function to cerebellar learning processes. This model of cerebellar function has quite a few shortcomings and may in fact be somewhat deficient in explaining the myriad functions attributed to the cerebellum, functions ranging from motor sequencing to emotion and cognition. The involvement of the cerebellum in these motor and non-motor functions has been demonstrated in both animals and humans in electrophysiological, behavioral, tracing, functional neuroimaging, and PET studies, as well as in clinical human case studies. A closer look at the cerebellum's evolutionary origin provides a clue to its underlying purpose as a tool which evolved to aid predation rather than as a tool for protection. Based upon this evidence, an alternative model of cerebellar function is proposed, one which might more comprehensively account both for the cerebellum's involvement in a myriad of motor, affective, and cognitive functions and for the relative simplicity and ubiquitous repetitiveness of its circuitry. This alternative model suggests that the cerebellum has the ability to detect coincidences of events, be they sensory, motor, affective, or cognitive in nature, and, after having learned to associate these, it can then trigger (or "mirror") these events after having temporally adjusted their onset based on positive/negative reinforcement. The model also provides for the cerebellum's direction of the proper and uninterrupted sequence of events resulting from this learning through the inhibition of efferent structures (as demonstrated in our lab).

The contributions of the cerebellum in sensorimotor control: what are the prevailing opinions which will guide forthcoming studies?

Although considerable progress has been made in developing models of cerebellar function in sensorimotor control, the exact nature of the basic operations performed by the cerebellum remain elusive. Several major theories have emerged these last decades. According to the hypothesis of Marr and Albus, the climbing fiber input carries an error signal weakening the strength of a subset of parallel fibers/Purkinje neurons synapses in the cerebellar cortex. Cerebellar circuits would gain the control of movement through trial and error. The hypothesis of internal models emulating movements is currently highly cited. There is a general agreement that (1) the central nervous system has to cope with an intrinsic time delay of sensory feedback related to motor activities and (2) estimations of future motor states are essential to perform fast and accurate movements. According to this second theory, cerebellar dysmetria, one of the cardinal cerebellar deficits, would result from a distorted predictive control. A third popular theory relates to the inverse models that would be stored in the cerebellum. Acquisition of a motor act would require forward models, and the acquisition process itself would generate an inverse model to allow an unconscious coordinated movement. Recently, an international panel of experts from various disciplines discussed the prevailing opinions in a consensus statement and tried to extract their clinical relevance in terms of pathogenesis of the clinical symptoms. Although a consensus is still not reached, the prevailing opinions provide a sound framework to conduct novel studies and try to discover the secrets of cerebellar circuits.

Lesion in the lateral cerebellum specifically produces overshooting of the toe trajectory in leading forelimb during obstacle avoidance in the rat

During locomotion, stepping over an obstacle under visual guidance is crucial to continuous safe walking. Studies of the role of the central nervous system in stepping movements have focused on cerebral cortical areas such as the primary motor cortex and posterior parietal cortex. There is speculation that the lateral cerebellum, which has strong anatomical connections with the cerebral cortex, also plays a key role in stepping movements over an obstacle, although this function of the lateral cerebellum has not yet been elucidated. Here we investigated the role of the lateral cerebellum during obstacle avoidance locomotion in rats with a lateral cerebellar lesion. A unilateral lesion in the lateral cerebellum did not affect limb movements during overground locomotion. Importantly, however, the lesioned animals showed overshooting of the toe trajectory specific to the leading forelimb ipsilateral to the lesion when stepping over an obstacle, and the peak toe position, in which the toe is maximally raised during stepping, shifted away from the upper edge of the obstacle. Recordings of EMG activity from elbow flexor and extensor muscles suggested that the overshooting toe trajectory in the ipsilateral leading forelimb possibly resulted from sustained elbow flexion and delayed elbow extension following prolonged activity of the biceps brachii. These results suggest that the lateral cerebellum specifically contributes to generating appropriate toe trajectories in the ipsilateral leading forelimb and to controlling related muscle activities in stepping over an obstacle, especially when accurate control of the distal extremity is achieved under visual guidance.

Topography of cerebellar deficits in humans

The cerebellum is a key-piece for information processing and is involved in numerous motor and nonmotor activities, thanks to the anatomical characteristics of the circuitry, the enormous computational capabilities and the high connectivity to other brain areas. Despite its uniform cytoarchitecture, cerebellar circuitry is segregated into functional zones. This functional parcellation is driven by the connectivity and the anatomo-functional heterogeneity of the numerous extra-cerebellar structures linked to the cerebellum, principally brain cortices, precerebellar nuclei and spinal cord. Major insights into cerebellar functions have been gained with a detailed analysis of the cerebellar outputs, with the evidence that fundamental aspects of cerebrocerebellar operations are the closed-loop circuit and the predictions of future states. Cerebellar diseases result in disturbances of accuracy of movements and lack of coordination. The cerebellar syndrome includes combinations of oculomotor disturbances, dysarthria and other speech deficits, ataxia of limbs, ataxia of stance and gait, as well as often more subtle cognitive/behavioral impairments. Our understanding of the corresponding anatomo-functional maps for the human cerebellum is continuously improving. We summarize the topography of the clinical deficits observed in cerebellar patients and the growing evidence of a regional subdivision into motor, sensory, sensorimotor, cognitive and affective domains. The recently described topographic dichotomy motor versus nonmotor cerebellum based upon anatomical, functional and neuropsychological studies is also discussed.

Childhood cerebellar ataxia

Childhood presentations of ataxia, an impairment of balance and coordination caused by damage to or dysfunction of the cerebellum, can often be challenging to diagnose. Presentations tend to be clinically heterogeneous, but key considerations may vary based on the child's age at onset, the course of illness, and subtle differences in phenotype. Systematic investigation is recommended for efficient diagnosis. In this review, we outline common etiologies and describe a comprehensive approach to the evaluation of both acquired and genetic cerebellar ataxia in children.

Consensus paper: roles of the cerebellum in motor control--the diversity of ideas on cerebellar involvement in movement

Considerable progress has been made in developing models of cerebellar function in sensorimotor control, as well as in identifying key problems that are the focus of current investigation. In this consensus paper, we discuss the literature on the role of the cerebellar circuitry in motor control, bringing together a range of different viewpoints. The following topics are covered: oculomotor control, classical conditioning (evidence in animals and in humans), cerebellar control of motor speech, control of grip forces, control of voluntary limb movements, timing, sensorimotor synchronization, control of corticomotor excitability, control of movement-related sensory data acquisition, cerebro-cerebellar interaction in visuokinesthetic perception of hand movement, functional neuroimaging studies, and magnetoencephalographic mapping of cortico-cerebellar dynamics. While the field has yet to reach a consensus on the precise role played by the cerebellum in movement control, the literature has witnessed the emergence of broad proposals that address cerebellar function at multiple levels of analysis. This paper highlights the diversity of current opinion, providing a framework for debate and discussion on the role of this quintessential vertebrate structure.

Preliminary pilot fMRI study of neuropostural optimization with a noninvasive asymmetric radioelectric brain stimulation protocol in functional dysmetria

PURPOSE

This study assessed changes in functional dysmetria (FD) and in brain activation observable by functional magnetic resonance imaging (fMRI) during a leg flexion-extension motor task following brain stimulation with a single radioelectric asymmetric conveyer (REAC) pulse, according to the precisely defined neuropostural optimization (NPO) protocol.

POPULATION AND METHODS

Ten healthy volunteers were assessed using fMRI conducted during a simple motor task before and immediately after delivery of a single REAC-NPO pulse. The motor task consisted of a flexion-extension movement of the legs with the knees bent. FD signs and brain activation patterns were compared before and after REAC-NPO.

RESULTS

A single 250-millisecond REAC-NPO treatment alleviated FD, as evidenced by patellar asymmetry during a sit-up motion, and modulated activity patterns in the brain, particularly in the cerebellum, during the performance of the motor task.

CONCLUSION

Activity in brain areas involved in motor control and coordination, including the cerebellum, is altered by administration of a REAC-NPO treatment and this effect is accompanied by an alleviation of FD.