Mechanisms underlying vestibulo-cerebellar motor learning in mice depend on movement direction

Purkinje cells in Crus I and II encode the visual stimulus and the impending choice as monkeys learn a reinforcement based visuomotor association task

Visuomotor association involves linking an arbitrary visual cue to a well-learned movement. Transient inactivation of Crus I/II impairs primates' ability to learn new associations and delays motor responses without affecting the kinematics of the movement. The simple spikes of Purkinje cells in the Crus regions signal cognitive errors as monkeys learn to associate specific fractal stimuli with movements of the left or right hand. Here we show that as learning progresses, the simple spike activity of individual neurons becomes more selective for stimulus-response associations, with selectivity developing closer to the appearance of visual stimuli. Initially, most neurons respond to both associations, irrespective of the identity of the stimulus and the associated movement, but as learning advances, more neurons distinguish between specific stimulus-hand associations. Using a linear decoder, it was found that in early learning stages, the visual stimulus can be decoded only when the choice can also be decoded. As learning improves, the visual stimulus is decoded earlier than the choice. A simple model can replicate the observed simple spike signals and the monkeys' behavior in both the early and late learning stages.

Behind mouse eyes: The function and control of eye movements in mice

The mouse visual system has become the most popular model to study the cellular and circuit mechanisms of sensory processing. However, the importance of eye movements only started to be appreciated recently. Eye movements provide a basis for predictive sensing and deliver insights into various brain functions and dysfunctions. A plethora of knowledge on the central control of eye movements and their role in perception and behaviour arose from work on primates. However, an overview of various eye movements in mice and a comparison to primates is missing. Here, we review the eye movement types described to date in mice and compare them to those observed in primates. We discuss the central neuronal mechanisms for their generation and control. Furthermore, we review the mounting literature on eye movements in mice during head-fixed and freely moving behaviours. Finally, we highlight gaps in our understanding and suggest future directions for research.

Impaired cerebellar plasticity hypersensitizes sensory reflexes in SCN2A-associated ASD

Children diagnosed with autism spectrum disorder (ASD) commonly present with sensory hypersensitivity or abnormally strong reactions to sensory stimuli. Such hypersensitivity can be overwhelming, causing high levels of distress that contribute markedly to the negative aspects of the disorder. Here, we identify a mechanism that underlies hypersensitivity in a sensorimotor reflex found to be altered in humans and in mice with loss of function in the ASD risk-factor gene SCN2A. The cerebellum-dependent vestibulo-ocular reflex (VOR), which helps maintain one's gaze during movement, was hypersensitized due to deficits in cerebellar synaptic plasticity. Heterozygous loss of SCN2A-encoded NaV1.2 sodium channels in granule cells impaired high-frequency transmission to Purkinje cells and long-term potentiation, a form of synaptic plasticity important for modulating VOR gain. VOR plasticity could be rescued in mice via a CRISPR-activator approach that increases Scn2a expression, demonstrating that evaluation of a simple reflex can be used to assess and quantify successful therapeutic intervention.

Whisker kinematics in the cerebellum

The whisker system is widely used as a model system for understanding sensorimotor integration. Purkinje cells in the crus regions of the cerebellum have been reported to linearly encode whisker midpoint, but it is unknown whether the paramedian and simplex lobules as well as their target neurons in the cerebellar nuclei also encode whisker kinematics and if so which ones. Elucidating how these kinematics are represented throughout the cerebellar hemisphere is essential for understanding how the cerebellum coordinates multiple sensorimotor modalities. Exploring the cerebellar hemisphere of mice using optogenetic stimulation, we found that whisker movements can be elicited by stimulation of Purkinje cells in not only crus1 and crus2, but also in the paramedian lobule and lobule simplex; activation of cells in the medial paramedian lobule had on average the shortest latency, whereas that of cells in lobule simplex elicited similar kinematics as those in crus1 and crus2. During spontaneous whisking behaviour, simple spike activity correlated in general better with velocity than position of the whiskers, but it varied between protraction and retraction as well as per lobule. The cerebellar nuclei neurons targeted by the Purkinje cells showed similar activity patterns characterized by a wide variety of kinematic signals, yet with a dominance for velocity. Taken together, our data indicate that whisker movements are much more prominently and diversely represented in the cerebellar cortex and nuclei than assumed, highlighting the rich repertoire of cerebellar control in the kinematics of movements that can be engaged during coordination. KEY POINTS: Excitation of Purkinje cells throughout the cerebellar hemispheres induces whisker movement, with the shortest latency and longest duration within the paramedian lobe. Purkinje cells have differential encoding for the fast and slow components of whisking. Purkinje cells encode not only the position but also the velocity of whiskers. Purkinje cells with high sensitivity for whisker velocity are preferentially located in the medial part of lobule simplex, crus1 and lateral paramedian. In the downstream cerebellar nuclei, neurons with high sensitivity for whisker velocity are located at the intersection between the medial and interposed nucleus.

Internal models of self-motion: neural computations by the vestibular cerebellum

The vestibular cerebellum plays an essential role in maintaining our balance and ensuring perceptual stability during activities of daily living. Here I examine three key regions of the vestibular cerebellum: the floccular lobe, anterior vermis (lobules I-V), and nodulus and ventral uvula (lobules X-IX of the posterior vermis). These cerebellar regions encode vestibular information and combine it with extravestibular signals to create internal models of eye, head, and body movements, as well as their spatial orientation with respect to gravity. To account for changes in the external environment and/or biomechanics during self-motion, the neural mechanisms underlying these computations are continually updated to ensure accurate motor behavior. To date, studies on the vestibular cerebellum have predominately focused on passive vestibular stimulation, whereas in actuality most stimulation is the result of voluntary movement. Accordingly, I also consider recent research exploring these computations during active self-motion and emerging evidence establishing the cerebellum's role in building predictive models of self-generated movement.

Molecular layer disinhibition unlocks climbing-fiber-instructed motor learning in the cerebellum

Climbing fibers supervise cerebellar learning by providing signals to Purkinje cells (PCs) that instruct adaptive changes to mistakenly performed movements. Yet, climbing fibers are regularly active, even during well performed movements, suggesting that a mechanism dynamically regulates the ability of climbing fibers to induce corrective plasticity in response to motor errors. We found that molecular layer interneurons (MLIs), whose inhibition of PCs powerfully opposes climbing-fiber-mediated excitation, serve this function. Optogenetically suppressing the activity of floccular MLIs in mice during the vestibulo-ocular reflex (VOR) induces a learned increase in gain despite the absence of performance errors. Suppressing MLIs when the VOR is mistakenly underperformed reveled that their inhibitory output is necessary to orchestrate gain-increase learning by conditionally permitting climbing fibers to instruct plasticity induction during ipsiversive head turns. Ablation of an MLI circuit for PC disinhibition prevents gain-increase learning during VOR performance errors which was rescued by re-imposing PC disinhibition through MLI activity suppression. Our findings point to a decisive role for MLIs in gating climbing-fiber-mediated learning through their context-dependent inhibition of PCs.

Purkinje cell microzones mediate distinct kinematics of a single movement

The classification of neuronal subpopulations has significantly advanced, yet its relevance for behavior remains unclear. The highly organized flocculus of the cerebellum, known to fine-tune multi-axial eye movements, is an ideal substrate for the study of potential functions of neuronal subpopulations. Here, we demonstrate that its recently identified subpopulations of 9+ and 9- Purkinje cells exhibit an intermediate Aldolase C expression and electrophysiological profile, providing evidence for a graded continuum of intrinsic properties among PC subpopulations. By identifying and utilizing two Cre-lines that genetically target these floccular domains, we show with high spatial specificity that these subpopulations of Purkinje cells participate in separate micromodules with topographically organized connections. Finally, optogenetic excitation of the respective subpopulations results in movements around the same axis in space, yet with distinct kinematic profiles. These results indicate that Purkinje cell subpopulations integrate in discrete circuits and mediate particular parameters of single movements.

Impaired cerebellar plasticity hypersensitizes sensory reflexes in SCN2A-associated ASD

Children diagnosed with autism spectrum disorder (ASD) commonly present with sensory hypersensitivity, or abnormally strong reactions to sensory stimuli. Such hypersensitivity can be overwhelming, causing high levels of distress that contribute markedly to the negative aspects of the disorder. Here, we identify the mechanisms that underlie hypersensitivity in a sensorimotor reflex found to be altered in humans and in mice with loss-of-function in the ASD risk-factor gene SCN2A. The cerebellum-dependent vestibulo-ocular reflex (VOR), which helps maintain one's gaze during movement, was hypersensitized due to deficits in cerebellar synaptic plasticity. Heterozygous loss of SCN2A-encoded NaV1.2 sodium channels in granule cells impaired high-frequency transmission to Purkinje cells and long-term potentiation, a form of synaptic plasticity important for modulating VOR gain. VOR plasticity could be rescued in adolescent mice via a CRISPR-activator approach that increases Scn2a expression, highlighting how evaluation of simple reflexes can be used as quantitative readout of therapeutic interventions.

cAMP-EPAC-PKCε-RIM1α signaling regulates presynaptic long-term potentiation and motor learning

The cerebellum is involved in learning of fine motor skills, yet whether presynaptic plasticity contributes to such learning remains elusive. Here, we report that the EPAC-PKCε module has a critical role in a presynaptic form of long-term potentiation in the cerebellum and motor behavior in mice. Presynaptic cAMP-EPAC-PKCε signaling cascade induces a previously unidentified threonine phosphorylation of RIM1α, and thereby initiates the assembly of the Rab3A-RIM1α-Munc13-1 tripartite complex that facilitates docking and release of synaptic vesicles. Granule cell-specific blocking of EPAC-PKCε signaling abolishes presynaptic long-term potentiation at the parallel fiber to Purkinje cell synapses and impairs basic performance and learning of cerebellar motor behavior. These results unveil a functional relevance of presynaptic plasticity that is regulated through a novel signaling cascade, thereby enriching the spectrum of cerebellar learning mechanisms.

Lobule-Related Action Potential Shape- and History-Dependent Current Integration in Purkinje Cells of Adult and Developing Mice

Purkinje cells (PCs) are the principal cells of the cerebellar cortex and form a central element in the modular organization of the cerebellum. Differentiation of PCs based on gene expression profiles revealed two subpopulations with distinct connectivity, action potential firing and learning-induced activity changes. However, which basal cell physiological features underlie the differences between these subpopulations and to what extent they integrate input differentially remains largely unclear. Here, we investigate the cellular electrophysiological properties of PC subpopulation in adult and juvenile mice. We found that multiple fundamental cell physiological properties, including membrane resistance and various aspects of the action potential shape, differ between PCs from anterior and nodular lobules. Moreover, the two PC subpopulations also differed in the integration of negative and positive current steps as well as in size of the hyperpolarization-activated current. A comparative analysis in juvenile mice confirmed that most of these lobule-specific differences are already present at pre-weaning ages. Finally, we found that current integration in PCs is input history-dependent for both positive and negative currents, but this is not a distinctive feature between anterior and nodular PCs. Our results support the concept of a fundamental differentiation of PCs subpopulations in terms of cell physiological properties and current integration, yet reveals that history-dependent input processing is consistent across PC subtypes.

Expression of a Form of Cerebellar Motor Memory Requires Learned Alterations to the Activity of Inhibitory Molecular Layer Interneurons

Procedural memories formed in the cerebellum in response to motor errors depend on changes to Purkinje cell (PC) spiking patterns that correct movement when the erroneous context is repeated. Because molecular layer interneurons (MLIs) inhibit PCs, learning-induced changes to MLI output may participate in reshaping PC spiking patterns. However, it remains unclear whether error-driven learning alters MLI activity and whether such changes are necessary for the memory engram. We addressed this knowledge gap by measuring and manipulating MLI activity in the flocculus of both sexes of mice before and after vestibulo-ocular reflex (VOR) adaptation. We found that MLIs are activated during vestibular stimuli and that their population response exhibits a phase shift after the instantiation of gain-increase VOR adaptation, a type of error-driven learning thought to require climbing-fiber-mediated instructive signaling. Although acute optogenetic suppression of MLI activity did not affect baseline VOR performance, it negated the expression of gain-increase learning, demonstrating a specific role of MLI activity changes in motor memory expression. This effect was transitory; after a multiday consolidation period, the expression of VOR gain-increase learning was no longer sensitive to MLI activity suppression. Together, our results indicate that error-driven alteration of MLI activity is necessary for labile, climbing-fiber-induced motor memory expression.SIGNIFICANCE STATEMENT In the cerebellum, motor learning induces an associative memory of the sensorimotor context of an erroneous movement that, when recalled, results in a new pattern of output that improves subsequent trials of performance. Our study shows that error-driven motor learning induces changes to the activity pattern of cerebellar molecular layer interneurons (MLIs) and that this new pattern of activity is required to express the corrective motor memory.

Postsynaptic plasticity of Purkinje cells in mice is determined by molecular identity

Cerebellar learning is expressed as upbound or downbound changes in simple spike activity of Purkinje cell subpopulations, but the underlying mechanism remains enigmatic. By visualizing murine Purkinje cells with different molecular identities, we demonstrate that the potential for induction of long-term depression is prominent in downbound and minimal in the upbound subpopulation. These differential propensities depend on the expression profile, but not on the synaptic inputs, of the individual Purkinje cell involved, highlighting the functional relevance of intrinsic properties for memory formation.

A review of motor neural system robotic modeling approaches and instruments

In this review, we are considering an actively developing tool in neuroscience-robotic modeling. The new perspective and existing application fields, tools, and methods are discussed. We try to determine starting positions and approaches that are useful at the beginning of new research in this field. Among multiple directions of the research is robotic modeling on the level of muscles fibers and their afferents, skin surface sensors, muscles, and joints proprioceptors. Some examples of technical implementation for physical modeling are reviewed. They are software and hardware tools like event-related modeling algorithms, reduced neuron models, robotic drives constructions. We observe existing drives technologies and prospective electric motor types: switched reluctance and transverse flux motors. Next, we look at the existing examples and approaches for robotic modeling of the cerebellum and spinal cord neural networks. These examples show practical methods for the model neural network architecture and adaptation. Those methods allow the use of cortical and spinal cord reflexes for the network training and apply additional artificial blocks for data processing in other brain structures that transmit and receive data from biologically realistic models.

Olivocerebellar control of movement symmetry

Coordination of bilateral movements is essential for a large variety of animal behaviors. The olivocerebellar system is critical for the control of movement, but its role in bilateral coordination has yet to be elucidated. Here, we examined whether Purkinje cells encode and influence synchronicity of left-right whisker movements. We found that complex spike activity is correlated with a prominent left-right symmetry of spontaneous whisker movements within parts, but not all, of Crus1 and Crus2. Optogenetic stimulation of climbing fibers in the areas with high and low correlations resulted in symmetric and asymmetric whisker movements, respectively. Moreover, when simple spike frequency prior to the complex spike was higher, the complex spike-related symmetric whisker protractions were larger. This finding alludes to a role for rebound activity in the cerebellar nuclei, which indeed turned out to be enhanced during symmetric protractions. Tracer injections suggest that regions associated with symmetric whisker movements are anatomically connected to the contralateral cerebellar hemisphere. Together, these data point toward the existence of modules on both sides of the cerebellar cortex that can differentially promote or reduce the symmetry of left and right movements in a context-dependent fashion.

Input and output organization of the mesodiencephalic junction for cerebro-cerebellar communication

Most studies investigating the impact of the cerebral cortex (CC) onto the cerebellum highlight the role of the pons, which provides the mossy fibers to the cerebellum. However, cerebro‐cerebellar communication may also be mediated by the nuclei of the mesodiencephalic junction (MDJ) that project to the inferior olive (IO), which in turn provides the climbing fibers to the molecular layer. Here, we uncover the precise topographic relations of the inputs and outputs of the MDJ using multiple, classical, and transneuronal tracing methods as well as analyses of mesoscale cortical injections from Allen Mouse Brain. We show that the caudal parts of the CC predominantly project to the principal olive via the rostral MDJ and that the rostral parts of the CC predominantly project to the rostral medial accessory olive via the caudal MDJ. Moreover, using triple viral tracing technology, we show that the cerebellar nuclei directly innervate the neurons in the MDJ that receive input from CC and project to the IO. By unraveling these topographic and prominent, mono‐ and disynaptic projections through the MDJ, this work establishes that cerebro‐cerebellar communication is not only mediated by the pontine mossy fiber system, but also by the climbing fiber system.

Impact of Purkinje Cell Simple Spike Synchrony on Signal Transmission from Flocculus

Purkinje cells (PCs) in the cerebellar flocculus carry rate-coded information that ultimately drives eye movement. Floccular PCs lying nearby each other exhibit partial synchrony of their simple spikes (SS). Elsewhere in the cerebellum, PC SS synchrony has been demonstrated to influence activity of the PCs' synaptic targets, and some suggest it constitutes another vector for information transfer. We investigated in the cerebellar flocculus the extent to which the rate code and PC synchrony interact. One motivation for the study was to explain the cerebellar deficits in ataxic mice like tottering; we speculated that PC synchrony has a positive effect on rate code transmission that is lost in the mutants. Working in transgenic mice whose PCs express channelrhodopsin, we exploited a property of optogenetics to control PC synchrony: pulsed photostimulation engenders stimulus-locked spiking, whereas continuous photostimulation engenders spiking whose timing is unconstrained. We photoactivated flocculus PCs using pulsed stimuli with sinusoidally varying timing vs. continuous stimuli with sinusoidally varying intensity. Recordings of PC pairs confirmed that pulsed stimuli engendered greater PC synchrony. We quantified the efficiency of transmission of the evoked PC firing rate modulation from the amplitudes of firing rate modulation and eye movement. Rate code transmission was slightly poorer in the conditions that generated greater PC synchrony, arguing against our motivating speculation regarding the origin of ataxia in tottering. Floccular optogenetic stimulation prominently augmented a 250-300 Hz local field potential oscillation, and we demonstrate relationships between the oscillation power and the evoked PC synchrony.

Population calcium responses of Purkinje cells in the oculomotor cerebellum driven by non-visual input

The climbing fiber input to the cerebellum conveys instructive signals that can induce synaptic plasticity and learning by triggering complex spikes accompanied by large calcium transients in Purkinje cells. In the cerebellar flocculus, which supports oculomotor learning, complex spikes are driven by image motion on the retina, which could indicate an oculomotor error. In the same neurons, complex spikes also can be driven by non-visual signals. It has been shown that the calcium transients accompanying each complex spike can vary in amplitude, even within a given cell, therefore, we compared the calcium responses associated with the visual and non-visual inputs to floccular Purkinje cells. The calcium indicator GCaMP6f was selectively expressed in Purkinje cells, and fiber photometry was used to record the calcium responses from a population of Purkinje cells in the flocculus of awake behaving mice. During visual (optokinetic) stimuli and pairing of vestibular and visual stimuli, the calcium level increased during contraversive retinal image motion. During performance of the vestibulo-ocular reflex in the dark, calcium increased during contraversive head rotation and the associated ipsiverse eye movements. The amplitude of this non-visual calcium response was comparable to that during conditions with retinal image motion present that induce oculomotor learning. Thus, population calcium responses of Purkinje cells in the cerebellar flocculus to visual and non-visual input are similar to what has been reported previously for complex spikes, suggesting that multimodal instructive signals control the synaptic plasticity supporting oculomotor learning.

Environmental Enrichment Improves Vestibular Oculomotor Learning in Mice

We assessed the behavioral effects of environmental enrichment on contrast sensitivity, reflexive eye movements and on oculomotor learning in mice that were housed in an enriched environment for a period of 3 weeks. Research has shown that a larger cage and a more complex environment have positive effects on the welfare of laboratory mice and other animals held in captivity. It has also been shown that environmental enrichment affects various behavior and neuroanatomical and molecular characteristics. We found a clear effect on oculomotor learning. Animals that were housed in an enriched environment learned significantly faster than controls that were housed under standard conditions. In line with existing literature, the enriched group also outperformed the controls in behavioral tests for explorative behavior. Meanwhile, both visual and reflexive oculomotor performance in response to visual and vestibular stimuli was unaffected. This points toward an underlying mechanism that is specific for motor learning, rather than overall motor performance.

Autonomous Purkinje cell activation instructs bidirectional motor learning through evoked dendritic calcium signaling

The signals in cerebellar Purkinje cells sufficient to instruct motor learning have not been systematically determined. Therefore, we applied optogenetics in mice to autonomously excite Purkinje cells and measured the effect of this activity on plasticity induction and adaptive behavior. Ex vivo, excitation of channelrhodopsin-2-expressing Purkinje cells elicits dendritic Ca²⁺ transients with high-intensity stimuli initiating dendritic spiking that additionally contributes to the Ca²⁺ response. Channelrhodopsin-2-evoked Ca²⁺ transients potentiate co-active parallel fiber synapses; depression occurs when Ca²⁺ responses were enhanced by dendritic spiking. In vivo, optogenetic Purkinje cell activation drives an adaptive decrease in vestibulo-ocular reflex gain when vestibular stimuli are paired with relatively small-magnitude Purkinje cell Ca²⁺ responses. In contrast, pairing with large-magnitude Ca²⁺ responses increases vestibulo-ocular reflex gain. Optogenetically induced plasticity and motor adaptation are dependent on endocannabinoid signaling, indicating engagement of this pathway downstream of Purkinje cell Ca²⁺ elevation. Our results establish a causal relationship among Purkinje cell Ca²⁺ signal size, opposite-polarity plasticity induction, and bidirectional motor learning.

Plastic reweighting of parallel fiber synaptic strength is a mechanism for the acquisition of cerebellum-dependent motor learning. Here, the authors found that optogenetic activation of PCs generates dendritic Ca2+ signals that induce plasticity in vitro and instruct learned changes to coincident eye movements in vivo.

Diversity and dynamism in the cerebellum

The past several years have brought revelations and paradigm shifts in research on the cerebellum. Historically viewed as a simple sensorimotor controller with homogeneous architecture, the cerebellum is increasingly implicated in cognitive functions. It possesses an impressive diversity of molecular, cellular and circuit mechanisms, embedded in a dynamic, recurrent circuit architecture. Recent insights about the diversity and dynamism of the cerebellum provide a roadmap for the next decade of cerebellar research, challenging some old concepts, reinvigorating others and defining major new research directions.

Bidirectional learning in upbound and downbound microzones of the cerebellum

Over the past several decades, theories about cerebellar learning have evolved. A relatively simple view that highlighted the contribution of one major form of heterosynaptic plasticity to cerebellar motor learning has given way to a plethora of perspectives that suggest that many different forms of synaptic and non-synaptic plasticity, acting at various sites, can control multiple types of learning behaviour. However, there still seem to be contradictions between the various hypotheses with regard to the mechanisms underlying cerebellar learning. The challenge is therefore to reconcile these different views and unite them into a single overall concept. Here I review our current understanding of the changes in the activity of cerebellar Purkinje cells in different 'microzones' during various forms of learning. I describe an emerging model that indicates that the activity of each microzone is bound to either increase or decrease during the initial stages of learning, depending on the directional and temporal demands of its downstream circuitry and the behaviour involved.

Ocular Reflex Adaptation as an Experimental Model of Cerebellar Learning -- In Memory of Masao Ito -

Masao Ito proposed a cerebellar learning hypothesis with Marr and Albus in the early 1970s. He suggested that cerebellar flocculus (FL) Purkinje cells (PCs), which directly inhibit the vestibular nuclear neurons driving extraocular muscle motor neurons, adaptively control the horizontal vestibulo-ocular reflex (HVOR) through the modification of mossy and parallel fiber-mediated vestibular responsiveness by visual climbing fiber (CF) inputs. Later, it was suggested that the same FL PCs adaptively control the horizontal optokinetic response (HOKR) in the same manner through the modification of optokinetic responsiveness in rodents and rabbits. In 1982, Ito and his colleagues discovered the plasticity of long-term depression (LTD) at parallel fiber (PF)-PC synapses after conjunctive stimulation of mossy or parallel fibers with CFs. Long-term potentiation (LTP) at PF-PC synapses by weak PF stimulation alone was found later. Many lines of experimental evidence have supported their hypothesis using various experimental methods and materials for the past 50 years by many research groups. Although several controversial findings were presented regarding their hypothesis, the reasons underlying many of them were clarified. Today, their hypothesis is considered as a fundamental mechanism of cerebellar learning. Furthermore, it was found that the memory of adaptation is transferred from the FL to vestibular nuclei for consolidation by repetition of adaptation through the plasticity of vestibular nuclear neurons. In this article, after overviewing their cerebellar learning hypothesis, I discuss possible roles of LTD and LTP in gain-up and gain-down HVOR/HOKR adaptations and refer to the expansion of their hypothesis to cognitive functions.

World Statistics Drive Learning of Cerebellar Internal Models in Adaptive Feedback Control: A Case Study Using the Optokinetic Reflex

The cerebellum is widely implicated in having an important role in adaptive motor control. Many of the computational studies on cerebellar motor control to date have focused on the associated architecture and learning algorithms in an effort to further understand cerebellar function. In this paper we switch focus to the signals driving cerebellar adaptation that arise through different motor behavior. To do this, we investigate computationally the contribution of the cerebellum to the optokinetic reflex (OKR), a visual feedback control scheme for image stabilization. We develop a computational model of the adaptation of the cerebellar response to the world velocity signals that excite the OKR (where world velocity signals are used to emulate head velocity signals when studying the OKR in head-fixed experimental laboratory conditions). The results show that the filter learnt by the cerebellar model is highly dependent on the power spectrum of the colored noise world velocity excitation signal. Thus, the key finding here is that the cerebellar filter is determined by the statistics of the OKR excitation signal.

SK2 channels in cerebellar Purkinje cells contribute to excitability modulation in motor-learning-specific memory traces

Neurons store information by changing synaptic input weights. In addition, they can adjust their membrane excitability to alter spike output. Here, we demonstrate a role of such "intrinsic plasticity" in behavioral learning in a mouse model that allows us to detect specific consequences of absent excitability modulation. Mice with a Purkinje-cell-specific knockout (KO) of the calcium-activated K+ channel SK2 (L7-SK2) show intact vestibulo-ocular reflex (VOR) gain adaptation but impaired eyeblink conditioning (EBC), which relies on the ability to establish associations between stimuli, with the eyelid closure itself depending on a transient suppression of spike firing. In these mice, the intrinsic plasticity of Purkinje cells is prevented without affecting long-term depression or potentiation at their parallel fiber (PF) input. In contrast to the typical spike pattern of EBC-supporting zebrin-negative Purkinje cells, L7-SK2 neurons show reduced background spiking but enhanced excitability. Thus, SK2 plasticity and excitability modulation are essential for specific forms of motor learning.

TRPC3 is a major contributor to functional heterogeneity of cerebellar Purkinje cells

Despite the canonical homogeneous character of its organization, the cerebellum plays differential computational roles in distinct sensorimotor behaviors. Previously, we showed that Purkinje cell (PC) activity differs between zebrin-negative (Z–) and zebrin-positive (Z+) modules (Zhou et al., 2014). Here, using gain-of-function and loss-of-function mouse models, we show that transient receptor potential cation channel C3 (TRPC3) controls the simple spike activity of Z–, but not Z+ PCs. In addition, TRPC3 regulates complex spike rate and their interaction with simple spikes, exclusively in Z– PCs. At the behavioral level, TRPC3 loss-of-function mice show impaired eyeblink conditioning, which is related to Z– modules, whereas compensatory eye movement adaptation, linked to Z+ modules, is intact. Together, our results indicate that TRPC3 is a major contributor to the cellular heterogeneity that introduces distinct physiological properties in PCs, conjuring functional heterogeneity in cerebellar sensorimotor integration.

Corticotropin-releasing factor depolarizes rat lateral vestibular nuclear neurons through activation of CRF receptors 1 and 2

Corticotropin-releasing factor (CRF) is a neuropeptide mainly synthesized in the hypothalamic paraventricular nucleus and has been traditionally implicated in stress and anxiety. Intriguingly, genetic or pharmacological manipulation of CRF receptors affects locomotor activity as well as motor coordination and balance in rodents, suggesting an active involvement of the central CRFergic system in motor control. Yet little is known about the exact role of CRF in central motor structures and the underlying mechanisms. Therefore, in the present study, we focused on the effect of CRF on the lateral vestibular nucleus (LVN) in the brainstem vestibular nuclear complex, an important center directly contributing to adjustment of muscle tone for both postural maintenance and the alternative change from the extensor to the flexor phase during locomotion. The results show that CRF depolarizes and increases the firing rate of neurons in the LVN. Tetrodotoxin does not block the CRF-induced depolarization and inward current on LVN neurons, suggesting a direct postsynaptic action of the neuropeptide. The CRF-induced depolarization on LVN neurons was partly blocked by antalarmin or antisauvagine-30, selective antagonists for CRF receptors 1 (CRFR1) and 2 (CRFR2), respectively. Furthermore, combined application of antalarmin and antisauvagine-30 totally abolished the CRF-induced depolarization. Immunofluorescence results show that CRFR1 and CRFR2 are co-localized in the rat LVN. These results demonstrate that CRF excites the LVN neurons by co-activation of both CRFR1 and CRFR2, suggesting that via the direct modulation on the LVN, the central CRFergic system may actively participate in the central vestibular-mediated postural and motor control.

New advances regarding adaptation of the vestibulo-ocular reflex

This is a review summarizing the development of vestibulo-ocular reflex (VOR) adaptation behavior with relevance to rehabilitation over the last 10 years and examines VOR adaptation using head-on-body rotations, specifically the influence of training target contrast, position and velocity error signal, active vs. passive head rotations, and sinusoidal vs. head impulse rotations. This review discusses optimization of the single VOR adaptation training session, consolidation between repeated training sessions, and dynamic incremental VOR adaptation. Also considered are the effects of aging and the roles of the efferent vestibular system, cerebellum, and otoliths on angular VOR adaptation. Finally, this review examines VOR adaptation findings in studies using whole body rotations.

Neurons of the inferior olive respond to broad classes of sensory input while subject to homeostatic control

KEY POINTS

Purkinje cells in the cerebellum integrate input from sensory organs with that from premotor centres. Purkinje cells use a variety of sensory inputs relaying information from the environment to modify motor control. Here we investigated to what extent the climbing fibre inputs to Purkinje cells signal mono- or multi-sensory information, and to what extent this signalling is subject to recent history of activity. We show that individual climbing fibres convey multiple types of sensory information, together providing a rich mosaic projection pattern of sensory signals across the cerebellar cortex. Moreover, firing probability of climbing fibres following sensory stimulation depends strongly on the recent history of activity, showing a tendency to homeostatic dampening.

ABSTRACT

Cerebellar Purkinje cells integrate sensory information with motor efference copies to adapt movements to behavioural and environmental requirements. They produce complex spikes that are triggered by the activity of climbing fibres originating in neurons of the inferior olive. These complex spikes can shape the onset, amplitude and direction of movements and the adaptation of such movements to sensory feedback. Clusters of nearby inferior olive neurons project to parasagittally aligned stripes of Purkinje cells, referred to as 'microzones'. It is currently unclear to what extent individual Purkinje cells within a single microzone integrate climbing fibre inputs from multiple sources of different sensory origins, and to what extent sensory-evoked climbing fibre responses depend on the strength and recent history of activation. Here we imaged complex spike responses in cerebellar lobule crus 1 to various types of sensory stimulation in awake mice. We find that different sensory modalities and receptive fields have a mild, but consistent, tendency to converge on individual Purkinje cells, with climbing fibres showing some degree of input-specificity. Purkinje cells encoding the same stimulus show increased events with coherent complex spike firing and tend to lie close together. Moreover, whereas complex spike firing is only mildly affected by variations in stimulus strength, it depends strongly on the recent history of climbing fibre activity. Our data point towards a mechanism in the olivo-cerebellar system that regulates complex spike firing during mono- or multi-sensory stimulation around a relatively low set-point, highlighting an integrative coding scheme of complex spike firing under homeostatic control.

Potentiation of cerebellar Purkinje cells facilitates whisker reflex adaptation through increased simple spike activity

Cerebellar plasticity underlies motor learning. However, how the cerebellum operates to enable learned changes in motor output is largely unknown. We developed a sensory-driven adaptation protocol for reflexive whisker protraction and recorded Purkinje cell activity from crus 1 and 2 of awake mice. Before training, simple spikes of individual Purkinje cells correlated during reflexive protraction with the whisker position without lead or lag. After training, simple spikes and whisker protractions were both enhanced with the spiking activity now leading behavioral responses. Neuronal and behavioral changes did not occur in two cell-specific mouse models with impaired long-term potentiation at their parallel fiber to Purkinje cell synapses. Consistent with cerebellar plasticity rules, increased simple spike activity was prominent in cells with low complex spike response probability. Thus, potentiation at parallel fiber to Purkinje cell synapses may contribute to reflex adaptation and enable expression of cerebellar learning through increases in simple spike activity.

Rodents use their whiskers to explore the world around them. When the whiskers touch an object, it triggers involuntary movements of the whiskers called whisker reflexes. Experiencing the same sensory stimulus multiple times enables rodents to fine-tune these reflexes, e.g., by making their movements larger or smaller. This type of learning is often referred to as motor learning.

A part of the brain called cerebellum controls motor learning. It contains some of the largest neurons in the nervous system, the Purkinje cells. Each Purkinje cell receives input from thousands of extensions of small neurons, known as parallel fibers. It is thought that decreasing the strength of the connections between parallel fibers and Purkinje cells can help mammals learn new movements. This is the case in a type of learning called Pavlovian conditioning. It takes its name from the Russian scientist, Pavlov, who showed that dogs can learn to salivate in response to a bell signaling food. Pavlovian conditioning enables animals to optimize their responses to sensory stimuli.

But Romano et al. now show that increasing the strength of connections between parallel fibers and Purkinje cells can also support learning. To trigger reflexive whisker movements, a machine blew puffs of air onto the whiskers of awake mice. After repeated exposure to the air puffs, the mice increased the size of their whisker reflexes. At the same time, their Purkinje cells became more active and the connections between Purkinje cells and parallel fibers grew stronger. Artificially increasing Purkinje cell activity triggered the same changes in whisker reflexes as the air puffs themselves.

Textbooks still report that only weakening of connections within the cerebellum enables animals to learn and modify movements. The data obtained by Romano al. thus paint a new picture of how the cerebellum works in the context of whisker learning. They show that strengthening these connections can also support movement-related learning.

Cerebellar Modules and Their Role as Operational Cerebellar Processing Units: A Consensus paper [corrected]

The compartmentalization of the cerebellum into modules is often used to discuss its function. What, exactly, can be considered a module, how do they operate, can they be subdivided and do they act individually or in concert are only some of the key questions discussed in this consensus paper. Experts studying cerebellar compartmentalization give their insights on the structure and function of cerebellar modules, with the aim of providing an up-to-date review of the extensive literature on this subject. Starting with an historical perspective indicating that the basis of the modular organization is formed by matching olivocorticonuclear connectivity, this is followed by consideration of anatomical and chemical modular boundaries, revealing a relation between anatomical, chemical, and physiological borders. In addition, the question is asked what the smallest operational unit of the cerebellum might be. Furthermore, it has become clear that chemical diversity of Purkinje cells also results in diversity of information processing between cerebellar modules. An additional important consideration is the relation between modular compartmentalization and the organization of the mossy fiber system, resulting in the concept of modular plasticity. Finally, examination of cerebellar output patterns suggesting cooperation between modules and recent work on modular aspects of emotional behavior are discussed. Despite the general consensus that the cerebellum has a modular organization, many questions remain. The authors hope that this joint review will inspire future cerebellar research so that we are better able to understand how this brain structure makes its vital contribution to behavior in its most general form.

The basal interstitial nucleus (BIN) of the cerebellum provides diffuse ascending inhibitory input to the floccular granule cell layer

The basal interstitial nucleus (BIN) in the white matter of the vestibulocerebellum has been defined more than three decades ago, but has since been largely ignored. It is still unclear which neurotransmitters are being used by BIN neurons, how these neurons are connected to the rest of the brain and what their activity patterns look like. Here, we studied BIN neurons in a range of mammals, including macaque, human, rat, mouse, rabbit, and ferret, using tracing, immunohistological and electrophysiological approaches. We show that BIN neurons are GABAergic and glycinergic, that in primates they also express the marker for cholinergic neurons choline acetyl transferase (ChAT), that they project with beaded fibers to the glomeruli in the granular layer of the ipsilateral floccular complex, and that they are driven by excitation from the ipsilateral and contralateral medio-dorsal medullary gigantocellular reticular formation. Systematic analysis of codistribution of the inhibitory synapse marker VIAAT, BIN axons, and Golgi cell marker mGluR2 indicate that BIN axon terminals complement Golgi cell axon terminals in glomeruli, accounting for a considerable proportion ( > 20%) of the inhibitory terminals in the granule cell layer of the floccular complex. Together, these data show that BIN neurons represent a novel and relevant inhibitory input to the part of the vestibulocerebellum that controls compensatory and smooth pursuit eye movements.

Graded Control of Climbing-Fiber-Mediated Plasticity and Learning by Inhibition in the Cerebellum

Purkinje cell dendrites convert excitatory climbing fiber input into signals that instruct plasticity and motor learning. Modulation of instructive signaling may increase the range in which learning is encoded, yet the mechanisms that allow for this are poorly understood. We found that optogenetic activation of molecular layer interneurons (MLIs) that inhibit Purkinje cells suppressed climbing-fiber-evoked dendritic Ca²⁺ spiking. Inhibitory suppression of Ca²⁺ spiking depended on the level of MLI activation and influenced the induction of associative synaptic plasticity, converting climbing-fiber-mediated potentiation of parallel fiber-evoked responses into depression. In awake mice, optogenetic activation of floccular climbing fibers in association with head rotation produced an adaptive increase in the vestibulo-ocular reflex (VOR). However, when climbing fibers were co-activated with MLIs, adaptation occurred in the opposite direction, decreasing the VOR. Thus, MLIs can direct a continuous spectrum of plasticity and learning through their influence on Purkinje cell dendritic Ca²⁺ signaling.

Multiple components in direction learning in smooth pursuit eye movements of monkeys

We analyzed behavioral features of smooth pursuit eye movements to characterize the course of acquisition and expression of multiple neural components of motor learning. Monkeys tracked a target that began to move in an initial "pursuit" direction and suddenly, but predictably, changed direction after a fixed interval of 250 ms. As the trial is repeated, monkeys learn to make eye movements that predict the change in target direction. Quantitative analysis of the learned response revealed evidence for multiple, dynamic, parallel processes at work during learning. 1) The overall learning followed at least two trial courses: a fast component grew and saturated rapidly over tens of trials, and a slow component grew steadily over up to 1,000 trials. 2) The temporal specificity of the learned response within each trial was crude during the first 100 trials but then improved gradually over the remaining trials. 3) External influences on the gain of pursuit initiation modulate the expression but probably not the acquisition of learning. The gain of pursuit initiation and the expression of the learned response decreased in parallel, both gradually through a 1,000-trial learning block and immediately between learning trials with different gains in the initiation of pursuit. We conclude that at least two distinct neural mechanisms drive the acquisition of pursuit learning over 100 to 1,000 trials (3 to 30 min). Both mechanisms generate underlying memory traces that are modulated in relation to the gain of pursuit initiation before expression in the final motor output. NEW & NOTEWORTHY We show that cerebellum-dependent direction learning in smooth pursuit eye movements grows in at least two components over 1,100 behavioral learning repetitions. One component grows over tens of trials and the other over hundreds. Within trials, learned temporal specificity gradually improves over hundreds of trials. The expression of each learning component on a given trial can be modified by external factors that do not affect the underlying memory trace.

Modulation of complex spike activity differs between zebrin-positive and -negative Purkinje cells in the pigeon cerebellum

The cerebellum is organized into parasagittal zones defined by its climbing and mossy fiber inputs, efferent projections, and Purkinje cell (PC) response properties. Additionally, parasagittal stripes can be visualized with molecular markers, such as heterogeneous expression of the isoenzyme zebrin II (ZII), where sagittal stripes of high ZII expression (ZII+) are interdigitated with stripes of low ZII expression (ZII-). In the pigeon vestibulocerebellum, a ZII+/- stripe pair represents a functional unit, insofar as both ZII+ and ZII- PCs within a stripe pair respond best to the same pattern of optic flow. In the present study, we attempted to determine whether there were any differences in the responses between ZII+ and ZII- PCs within a functional unit in response to optic flow stimuli. In pigeons of either sex, we recorded complex spike activity (CSA) from PCs in response to optic flow, marked recording sites with a fluorescent tracer, and determined the ZII identity of recorded PCs by immunohistochemistry. We found that CSA of ZII+ PCs showed a greater depth of modulation in response to the preferred optic flow pattern compared with ZII- PCs. We suggest that these differences in the depth of modulation to optic flow stimuli are due to differences in the connectivity of ZII+ and ZII- PCs within a functional unit. Specifically, ZII+ PCs project to areas of the vestibular nuclei that provide inhibitory feedback to the inferior olive, whereas ZII- PCs do not. NEW & NOTEWORTHY Although the cerebellum appears to be a uniform structure, Purkinje cells (PCs) are heterogeneous and can be categorized on the basis of the expression of molecular markers. These phenotypes are conserved across species, but the significance is undetermined. PCs in the vestibulocerebellum encode optic flow resulting from self-motion, and those that express the molecular marker zebrin II (ZII+) exhibit more sensitivity to optic flow than those that do not express zebrin II (ZII-).

Impact of NMDA Receptor Overexpression on Cerebellar Purkinje Cell Activity and Motor Learning

In many brain regions involved in learning NMDA receptors (NMDARs) act as coincidence detectors of pre- and postsynaptic activity, mediating Hebbian plasticity. Intriguingly, the parallel fiber (PF) to Purkinje cell (PC) input in the cerebellar cortex, which is critical for procedural learning, shows virtually no postsynaptic NMDARs. Why is this? Here, we address this question by generating and testing independent transgenic lines that overexpress NMDAR containing the type 2B subunit (NR2B) specifically in PCs. PCs of the mice that show larger NMDA-mediated currents than controls at their PF input suffer from a blockage of long-term potentiation (LTP) at their PF-PC synapses, while long-term depression (LTD) and baseline transmission are unaffected. Moreover, introducing NMDA-mediated currents affects cerebellar learning in that phase-reversal of the vestibulo-ocular reflex (VOR) is impaired. Our results suggest that under physiological circumstances PC spines lack NMDARs postsynaptically at their PF input so as to allow LTP to contribute to motor learning.