Cerebellar Output Research Articles

Structured Connectivity in Cerebellar Inhibitory Networks

SummaryDefining the rules governing synaptic connectivity is key to formulating theories of neural circuit function. Interneurons can be connected by both electrical and chemical synapses, but the organization and interaction of these two complementary microcircuits is unknown. By recording from multiple molecular layer interneurons in the cerebellar cortex, we reveal specific, nonrandom connectivity patterns in both GABAergic chemical and electrical interneuron networks. Both networks contain clustered motifs and show specific overlap between them. Chemical connections exhibit a preference for transitive patterns, such as feedforward triplet motifs. This structured connectivity is supported by a characteristic spatial organization: transitivity of chemical connectivity is directed vertically in the sagittal plane, and electrical synapses appear strictly confined to the sagittal plane. The specific, highly structured connectivity rules suggest that these motifs are essential for the function of the cerebellar network.

Developmental origins of diversity in cerebellar output nuclei

BackgroundThe functional integration of the cerebellum into a number of different neural systems is governed by the connection of its output axons. In amniotes, the majority of this output is mediated by an evolutionarily diverse array of cerebellar nuclei that, in mice, are derived from the embryonic rhombic lip. To understand the origins of cerebellar nucleus diversity, we have explored how nucleus development is patterned in birds, which notably lack a dentate-like nucleus output to the dorsal thalamus.ResultsUsing targeted in ovo electoroporation of green fluorescent protein (GFP) and red fluorescent protein (RFP) in a variety of combinations and with different conditional enhancers, we show that cerebellar nuclei in chicks are produced, as in the mouse, at the rhombic lip. Furthermore, the comparison of fate-mapped neurons with molecular markers reveals a strict temporal sequence of cell fate allocation in establishing the avian lateral and medial cerebellar nuclei. In contrast to the mouse cerebellum, Lhx9 expression is confined to extracerebellar thalamic afferent nuclei corresponding to the absence, in chicks, of a dentate nucleus. Spatiotemporally targeted over-expression of Lhx9 in chick cerebellar nuclei (recapitulating in part the mammalian expression pattern) results in a loss of distinct nuclear boundaries and a change in axon initial trajectories consistent with a role for Lhx9 specifying targeting.ConclusionsOur results confirm the relationship between cell fate and a fine grain temporal patterning at the rhombic lip. This suggests that the lack of a cerebellar output to the dorsal thalamus of birds corresponds with a restricted expression of the LIM-homeodomain gene Lhx9 to earlier born rhombic lip cohorts when compared to mice. The evolution of cerebellar nucleus diversity in amniotes may hence reflect a heterochronic adaptation of gene expression with respect to the sequential production of rhombic lip derivatives resulting in altered axonal targeting.

Chapter 10 - Automatic and Controlled Processing in the Corticocerebellar System

During learning, performance changes often involve a transition from controlled processing in which performance is flexible and responsive to ongoing error feedback, but effortful and slow, to a state in which processing becomes swift and automatic. In this state, performance is unencumbered by the requirement to process feedback, but its insensitivity to feedback reduces its flexibility. Many properties of automatic processing are similar to those that one would expect of forward models, and many have suggested that these may be instantiated in cerebellar circuitry. Since hierarchically organized frontal lobe areas can both send and receive commands, I discuss the possibility that they can act both as controllers and controlled objects and that their behaviors can be independently modeled by forward models in cerebellar circuits. Since areas of the prefrontal cortex contribute to this hierarchically organized system and send outputs to the cerebellar cortex, I suggest that the cerebellum is likely to contribute to the automation of cognitive skills, and to the formation of habitual behavior which is resistant to error feedback. An important prerequisite to these ideas is that cerebellar circuitry should have access to higher order error feedback that signals the success or failure of cognitive processing. I have discussed the pathways through which such feedback could arrive via the inferior olive and the dopamine system. Cerebellar outputs inhibit both the inferior olive and the dopamine system. It is possible that learned representations in the cerebellum use this as a mechanism to suppress the processing of feedback in other parts of the nervous system. Thus, cerebellar processes that control automatic performance may be completed without triggering the engagement of controlled processes by prefrontal mechanisms.

Back to front: cerebellar connections and interactions with the prefrontal cortex

Although recent neuroanatomical evidence has demonstrated closed-loop connectivity between prefrontal cortex and the cerebellum, the physiology of cerebello-cerebral circuits and the extent to which cerebellar output modulates neuronal activity in neocortex during behavior remain relatively unexplored. We show that electrical stimulation of the contralateral cerebellar fastigial nucleus (FN) in awake, behaving rats evokes distinct local field potential (LFP) responses (onset latency ~13 ms) in the prelimbic (PrL) subdivision of the medial prefrontal cortex. Trains of FN stimulation evoke heterogeneous patterns of response in putative pyramidal cells in frontal and prefrontal regions in both urethane-anesthetized and awake, behaving rats. However, the majority of cells showed decreased firing rates during stimulation and subsequent rebound increases; more than 90% of cells showed significant changes in response. Simultaneous recording of on-going LFP activity from FN and PrL while rats were at rest or actively exploring an open field arena revealed significant network coherence restricted to the theta frequency range (5–10 Hz). Granger causality analysis indicated that this coherence was significantly directed from cerebellum to PrL during active locomotion. Our results demonstrate the presence of a cerebello-prefrontal pathway in rat and reveal behaviorally dependent coordinated network activity between the two structures, which could facilitate transfer of sensorimotor information into ongoing neocortical processing during goal directed behaviors.

Collateralization of cerebellar output to functionally distinct brainstem areas. A retrograde, non-fluorescent tracing study in the rat.

The organization of the cerebellum is characterized by a number of longitudinally organized connection patterns that consist of matching olivo-cortico-nuclear zones. These entities, referred to as modules, have been suggested to act as functional units. The various parts of the cerebellar nuclei (CN) constitute the output of these modules. We have studied to what extent divergent and convergent patterns in the output of the modules to four, functionally distinct brain areas can be recognized. Two retrograde tracers were injected in various combinations of the following nuclei: the red nucleus (RN), as a main premotor nucleus; the prerubral area, as a main supplier of afferents to the inferior olive (IO); the nucleus reticularis tegmenti pontis (NRTP), as a main source of cerebellar mossy fibers; and the IO, as the source of climbing fibers. For all six potential combinations three cases were examined. All nine cases with combinations that involved the IO did not, or hardly, resulted in double labeled neurons. In contrast, all other combinations resulted in at least 10% and up to 67% of double labeled neurons in cerebellar nuclear areas where both tracers were found. These results show that the cerebellar nuclear neurons that terminate within the studied areas represent basically two intermingled populations of projection cells. One population corresponds to the small nucleo-olivary neurons whereas the other consists of medium- to large-sized neurons which are likely to distribute their axons to several other areas. Despite some consistent differences between the output patterns of individual modules we propose that modular cerebellar output to premotor areas such as the RN provides simultaneous feedback to both the mossy fiber and the climbing fiber system and acts in concert with a designated GABAergic nucleo-olivary circuit. These features seem to form a basic characteristic of cerebellar operation.

Cerebellar hemorrhagic injury in premature infants occurs during a vulnerable developmental period and is associated with wider neuropathology

BackgroundCerebellar hemorrhagic injury (CHI) is being recognized more frequently in premature infants. However, much of what we know about CHI neuropathology is from autopsy studies that date back to a prior era of neonatal intensive care. To update and expand our knowledge of CHI we reviewed autopsy materials and medical records of all live-born preterm infants (<37 weeks gestation) autopsied at our institution from 1999–2010 who had destructive hemorrhagic injury to cerebellar parenchyma (n = 19) and compared them to matched non-CHI controls (n = 26).ResultsCHI occurred at a mean gestational age of 25 weeks and involved the ventral aspect of the posterior lobe in almost all cases. CHI arose as a large hemorrhage or as multiple smaller hemorrhages in the emerging internal granule cell layer of the developing cortex or in the nearby white matter. Supratentorial germinal matrix hemorrhage occurred in 95% (18/19) of CHI cases compared to 54% (14/26) of control cases (p = 0.003). The cerebellar cortex frequently showed focal neuronal loss and gliosis (both 15/19, 79%) in CHI cases compared to control cases (both 1/26, 4% p < 0.0001). The cerebellar dentate had more neuronal loss (8/15, 53%) and gliosis (9/15, 60%) in CHI cases than controls (both 0/23, 0%; p < 0.0001). The inferior olivary nuclei showed significantly more neuronal loss in CHI (10/17, 59%) than in control cases (5/26, 19%) (p = 0.0077). All other gray matter sites examined showed no significant difference in the incidence of neuronal loss or gliosis between CHI and controls.ConclusionsWe favor the possibility that CHI represents a primary hemorrhage arising due to the effects of impaired autoregulation in a delicate vascular bed. The incidences of neuronal loss and gliosis in the inferior olivary and dentate nuclei, critical cerebellar input and output structures, respectively were higher in CHI compared to control cases and may represent a transsynpatic degenerative process. CHI occurs during a critical developmental period and may render the cerebellum vulnerable to additional deficits if cerebellar growth and neuronal connectivity are not established as expected. Therefore, CHI has the potential to significantly impact neurodevelopmental outcome in survivors.

Persistent activity in prefrontal cortex during trace eyelid conditioning: dissociating responses that reflect cerebellar output from those that do not.

Persistent neural activity, responses that outlast the stimuli that evoke them, plays an important role in neural computations and possibly in processes, such as working memory. Recent studies suggest that trace eyelid conditioning, which involves a temporal gap between the conditioned and unconditioned stimuli (the trace interval), requires persistent neural activity in a region of medial prefrontal cortex (mPFC). This persistent activity, which could be conveyed to cerebellum via a pathway through pons, may engage the cerebellum and allow for the expression of conditioned responses. Given the substantial reciprocity observed among many brain regions, it is essential to demonstrate that persistent responses in mPFC neurons are not simply a reflection of cerebellar feedback to the forebrain, leaving open the possibility that such responses could serve as input to the cerebellum. This concern is highlighted by studies showing that hippocampal learning-related activity is abolished by cerebellar inactivation. We inactivated the cerebellum while recording single-unit activity from the mPFC of rabbits trained with a forebrain-dependent trace eyelid conditioning procedure. We report that, whereas the responses of cells that show an onset of increased spike activity during the trace interval were abolished by cerebellar inactivation, persistent responses that begin during the conditioned stimulus and persisted into the trace interval were unaffected. Therefore, conditioned stimulus-evoked persistent responses remain the strongest candidate input pattern to support the cerebellar expression of learned responses.

Alterations in Cortical and Cerebellar Motor Processing in Subclinical Neck Pain Patients Following Spinal Manipulation

ObjectiveThe purpose of this study was investigate whether there are alterations in cerebellar output in a subclinical neck pain (SCNP) group and whether spinal manipulation before motor sequence learning might restore the baseline functional relationship between the cerebellum and motor cortex. MethodsTen volunteers were tested with SCNP using transcranial magnetic stimulation before and after a combined intervention of spinal manipulation and motor sequence learning. In a separate experiment, we tested 10 healthy controls using the same measures before and after motor sequence learning. Our transcranial magnetic stimulation measurements included short-interval intracortical inhibition, long-interval intracortical inhibition, and cerebellar inhibition (CBI). ResultsThe SCNP group showed a significant improvement in task performance as indicated by a 19% decrease in mean reaction time (P < .0001), which occurred concurrently with a decrease in CBI following the combined spinal manipulation and motor sequence learning intervention (F1,6 = 7.92, P < .05). The control group also showed an improvement in task performance as indicated by a 25% increase in reaction time (P < .001) with no changes to CBI. ConclusionsSubclinical neck pain patients have altered CBI when compared with healthy controls, and spinal manipulation before a motor sequence learning task changes the CBI pattern to one similar to healthy controls.

P 50. Anodal cerebellar tDCS impairs verbal working memory

Introduction The cerebellum is not only engaged in sensory-motor function but also cognitive processes like attention and perception (Stoodley and Schmahmann, 2009). The verbal working memory is one of these cognitive processes. Growing evidence suggests that the right cerebellum interacts with Broca's area and left frontal premotor regions during the articulatory control process and contributes to phonological storage (Baddeley, 2003; Paulesu et al., 1993; Desmond et al., 1997). We previously showed that cathodal transcranial direct current stimulation (tDCS) applied to the right cerebellum impairs verbal working memory. We found that cathodal tDCS reduced forward digit spans and blocked practice dependent increase in backward digit span suggesting that the cerebellum contributes to memory recall (Boehringer et al., in press). To address the question on how the right cerebellum contributes to memory recognition we here used cathodal and anodal tDCS before subjects participated in the Sternberg item recognition task. Methods We used a sham-controlled, randomized, blind, crossover design. We investigated 19 young healthy right-handed subjects (age=26±4years, 10 females). Cathodal, anodal or sham tDCS was applied over the right cerebellum with 2mA for 25min. For the Sternberg task subjects were listening to a set of digits. After a break of 7s subjects were listening to the target digit and decided whether the target digit occurred in the set of digits or not. The task was specifically adapted to the individual memory capacities with three different memory load levels (i.e., easy, medium, hard). Results We found impaired item recognition after anodal tDCS ( p =.008). This effect was only present on the medium memory load level while for the easy or hard condition we found no such influence. For cathodal tDCS we found no significant influence on item recognition for any of the three memory load levels. Discussion We found impaired item recognition after anodal tDCS. This effect was only present in the medium memory load level. This suggests that the cerebellum is only involved if individuals are moderately engaged in verbal memory, but not when the memory load is too low or too high. This findings extends previous observations of the involvement of the cerebellum in verbal working memory and suggest that the cerebellum is not generally involved in recognition but for specific demands (Pope and Miall, 2012). Here anodal tDCS may impair the cerebellar output to the PFC. These results extend our previous tDCS study (Boehringer et al., in press) and suggests that the cerebellum contributes not only to memory recall but also to recognition.

P 191. Modulation of the functional connectivity between the cerebellum and the cortico-striatal loops

Question Artificial modulation of the cerebellar output with repetitive transcranial magnetic stimulation could directly influence the plastic response of the motor cortex that depends on the peripheral input ( Popa et al., 2012 ). Here we investigate the change of the resting-state functional connectivity between the cerebellum, the basal ganglia, the thalamus, and the cortical motor areas before and after the inhibitory rTMS or sham stimulation of the right cerebellar hemisphere. Methods The resting state fMRI of seventeen right-handed healthy subjects (age 26.6 ± 9 years, range: 18–53) was acquired twice at three time points around two types of intervention. The intervention types were either real or sham continuous theta-burst stimulation (cTBS; Huang et al., 2005 ) of the right cerebellar hemisphere, delivered in two different days. The scanning time-points for each day were: baseline, before the intervention, and at 10 min and 60 min after the end of the intervention. The stimulation was aimed at the VIII lobule of the right cerebellar hemisphere ( Popa et al., 2010 ) and delivered at 90% of the active motor threshold. The MRI data was preprocessed with SPM5 ( SPM5, xxxx ) and time series of the signal in selected regions of interest were extracted. The regions of interest were selected in FSLview ( FSLview, xxxx ) as 4 mm-radius spheres around specific coordinates previously defined for circuits involved in motor-skill learning, which are both distinct and connected ( Doyon et al., 2009 ): the cerebello-thalamo-cortical (CTC) loop and the striatio-thalamo-cortical (STC) loop. The integration within a circuit was defined as Ix = −1/2 ln∣Rx∣ and between circuits as Ix/y = 1/2 ln(∣Rx∣. ∣Ry∣/∣Rxy∣), where Rx is the correlation matrix of network X and ∣·∣ stands for the determinant function ( Merrelec et al., 2008 ). The integration was computed for each subject, each stimulation type, each time-point, and every combination of regions/circuits. Results We have found that despite the lack of effect on the integration of the whole motor network, there were significant changes within and between sub-networks: an increase in integration from baseline to10 min after the intervention between the left cortico-cerebellar and the left and right cortico-striatal loops, as well as within the cerebello-thalamo-striatal loops bilaterally. All parameters returned to baseline levels at 60 min after the intervention. Conclusions This demonstrates that inhibitory cerebellar stimulation is actively influencing the connectivity within the motor loops, involving both cortical and subcortical structures. It suggests that the stimulation of one area can potentially change the flow of information throughout the brain, and that the inhibition of cerebellar cortex in particular can enhance the strength of the communication between the cortical motor areas and the basal ganglia.

Chronic 30-Hz Deep Cerebellar Stimulation Coupled With Training Enhances Post-ischemia Motor Recovery and Peri-infarct Synaptophysin Expression in Rodents

Over 500,000 Americans have strokes every year, making stroke the leading cause for disability in the United States and in the industrialized world. New treatments to improve poststroke motor recovery are needed. To investigate a novel approach for enhancing motor recovery that involves chronic, electrical stimulation of ascending cerebellar output combined with motor training. Adult Sprague-Dawley rats underwent unilateral endothelin-1 injections in the dominant cerebral cortex and placement of a chronic stimulating electrode in the contralateral lateral cerebellar nucleus. After 1 week, the animals were separated into 2 groups (STIM+ and STIM-), matched for poststroke motor performance in the pasta matrix task. At 2 weeks post-ischemia, the treatment phase was initiated, with animals in the STIM+ group receiving pulsed, 30-Hz stimulation for 12 hours/day. Motor training continued for both groups over 3 to 5 weeks. A total of 23 animals were examined after 3 weeks of treatment. STIM+ animals showed a significant improvement in motor function compared with post-ischemia baseline performance as well as in comparison with the STIM- group. Immunohistochemistry revealed a significant increase in the perilesional expression of synaptophysin for the STIM+ vs the STIM- animals. These results indicate that chronic activation of ascending cerebellofugal pathways enhances motor recovery after focal cortical ischemia. The recovery was associated with an increase in perilesional cortical plasticity relative to nontreated controls.

Diffusion tensor imaging of the superior cerebellar peduncle identifies patients with posterior fossa syndrome

Posterior fossa tumors are the most common brain tumor of children. Aggressive resection correlates with long-term survival. A high incidence of posterior fossa syndrome (PFS), impairing the quality of life in many survivors, has been attributed to damage to bilateral dentate nucleus or to cerebellar output pathways. Using diffusion tensor imaging (DTI), we examined the involvement of the dentothalamic tracts, specifically the superior cerebellar peduncle (SCP), in patients with posterior fossa tumors and the association with PFS. DTI studies were performed postoperatively in patients with midline (n = 12), lateral cerebellar tumors (n = 4), and controls. The location and visibility of the SCP were determined. The postoperative course was recorded, especially with regard to PFS, cranial nerve deficits, and oculomotor function. The SCP travels immediately adjacent to the lateral wall of the fourth ventricle and just medial to the middle cerebellar peduncle. Patients with midline tumors that still had observable SCP did not develop posterior fossa syndrome (N = 7). SCPs were absent, on either preoperative (N = 1, no postoperative study available) or postoperative studies (N = 4), in the five patients who developed PFS. Oculomotor deficits of tracking were observed in patients independent of PFS or SCP involvement. PFS can occur with bilateral injury to the outflow from dentate nuclei. In children with PFS, this may occur due to bilateral injury to the superior cerebellar peduncle. These tracts sit immediately adjacent to the wall of the ventricle and are highly vulnerable when an aggressive resection for these tumors is performed.

Connection control implications in a distributed plasticity cerebellar model

The cerebellum is one of the parts of the brain which has aroused more curiosity amongst neuroscientists. Indeed, several forms of plasticity mechanisms have been reported at several sites of the cerebellar model circuitry, thus including plasticity mechanisms not just at parallel fibers (a well-accepted plasticity site) but also at synaptic inputs of deep cerebellar nuclei (DCN) (from mossy fibers (MFs) [1,2]and Purkinje cells (PCs) [3,4]). The Marr and Albus model already hypothesized that parallel fiber (PFs) →Purkinje cell synapses presented both long-term potentiation (LTP) [5,6] and long-term depression (LTD) [5-7] plasticity so as to correlate the activity at parallel fibers with the incoming error signal through climbing fibers. Nevertheless, in subsequent studies, it has been demonstrated that many sites in the cerebellum show traces of plasticity [8-10]. But the way in which those distributed plasticity mechanisms may improve the operational capabilities of the cerebellum is still an open issue. In this work we propose that the synaptic plasticity of mossy-fiber-to-deep-cerebellar-nucleus-cell and Purkinje-cell-to-deep-cerebellar-nucleus-cell may regulate the effect of Purkinje-cell activity on the cerebellar output, behaving as a distributed homeostatic mechanism [11]. The plasticity at the DCN afferents helps to keep the Purkinje-cell activity in an adequate range independently of the magnitude required for the cerebellar output, thus improving the precision of this output signal. Since these plasticity mechanisms are capable of adapting the cerebellar behavior in the long-term, it is of necessity the presence of fast feedback for motor activities in order to perform precise movements. Thus, the presented work also explores the control implication that the inferior-olive-to-deep-cerebellar-nucleus-cell connection may possess in conjunction with the previously suggested plasticity mechanisms. As it is widely assumed, the climbing fiber activity that our cerebellar model implements is considered to be a teaching signal (targeting Purkinje cells). But we also explore its potential role as a control signal over the cerebellar output (targeting the deep cerebellar nuclei). To investigate all these proposals, we have embedded the cerebellar model in a feed-forward control loop which is connected to a simulated 3-degree-of-freedom robot model.

Acquisition and execution of motor sequences by a computational model of the cerebellum

Although there is evidence that the cerebellum is sufficient for the execution of certain action sequences [1], its role in motor sequence acquisition is still unclear. Motor sequence acquisition is studied with complex tasks (e.g., serial reaction-time) that involve several brain areas, such as the cortex and the striatum, in addition to the cerebellum. Due to these interactions it is hard to disentangle the specific function performed by the cerebellum alone. With a computational study, here we address whether the cerebellum alone is sufficient for the learning of action sequences in a simple motor acquisition task. To achieve this we extend an existing cerebellar-based control architecture [2] by adding the Nucleo-Pontine Projections (NPPs). Such NPPs establish the so-called cerebello-pontine loops, such that cerebellar output at a given time can be fed back to the cerebellar input layer. Hypothetically, given the recurrence established by the NPPs, the cerebellum could control action sequences, chaining the next action execution to the current one. We test this prediction with a variation of the classical conditioning paradigm, where a Conditioning Stimulus (CS) is followed not just by a single Uncoditioned Stimulus (US), but by multiple USs. Provided that the inter-stimulus intervals are sufficiently long, only the first Conditioned Response (CR) should be triggered by the CS and all the following CRs should be recurrently triggered. To confirm that learning in the cerebellum is sufficient to acquire action sequences within the above-mentioned paradigm, we apply our model to two different scenarios: in the first, the model acquires a series of CRs in the classical conditioning of the eye-blink, a paradigm commonly used to study learning in the cerebellum; whereas in the second, the model controls a real robot that traverses a track with multiple turns. In the latter scenario, collision with walls acts as the US and the visual stimulus is the CS. After learning, the CS alone triggers a sequence of well-timed turns. This work leads to two applications. On one hand, we demonstrate a control system that can autonomously acquire motor responses encompassing multiple linked actions. This system is strongly based in the known cerebellar circuitry and can be embedded in robotic platforms. On the other hand, these results provide a set of experimental predictions that can guide the study of cerebellar function in the acquisition of action sequences.

The Primate Cerebellum Selectively Encodes Unexpected Self-Motion

The ability to distinguish sensory signals that register unexpected events (exafference) from those generated by voluntary actions (reafference) during self-motion is essential for accurate perception and behavior. The cerebellum is most commonly considered in relation to its contributions to the fine tuning of motor commands and sensorimotor calibration required for motor learning. During unexpected motion, however, the sensory prediction errors that drive motor learning potentially provide a neural basis for the computation underlying the distinction between reafference and exafference. Recording from monkeys during voluntary and applied self-motion, we demonstrate that individual cerebellar output neurons encode an explicit and selective representation of unexpected self-motion by means of an elegant computation that cancels the reafferent sensory effects of self-generated movements. During voluntary self-motion, the sensory responses of neurons that robustly encode unexpected movement are canceled. Neurons with vestibular and proprioceptive responses to applied head and body movements are unresponsive when the same motion is self-generated. When sensory reafference and exafference are experienced simultaneously, individual neurons provide a precise estimate of the detailed time course of exafference. These results provide an explicit solution to the longstanding problem of understanding mechanisms by which the brain anticipates the sensory consequences of our voluntary actions. Specifically, by revealing a striking computation of a sensory prediction error signal that effectively distinguishes between the sensory consequences of self-generated and externally produced actions, our findings overturn the conventional thinking that the sensory errors coded by the cerebellum principally contribute to the fine tuning of motor activity required for motor learning.

Diversity of Neural Responses in the Brainstem during Smooth Pursuit Eye Movements Constrains the Circuit Mechanisms of Neural Integration

Neural integration converts transient events into sustained neural activity. In the smooth pursuit eye movement system, neural integration is required to convert cerebellar output into the sustained discharge of extraocular motoneurons. We recorded the expression of integration in the time-varying firing rates of cerebellar and brainstem neurons in the monkey during pursuit of step-ramp target motion. Electrical stimulation with single shocks in the cerebellum identified brainstem neurons that are monosynaptic targets of inhibition from the cerebellar floccular complex. They discharge in relation to eye acceleration, eye velocity, and eye position, with a stronger acceleration signal than found in most other brainstem neurons. The acceleration and velocity signals can be accounted for by opponent contributions from the two sides of the cerebellum, without integration; the position signal implies participation in the integrator. Other neurons in the vestibular nucleus show a wide range of blends of signals related to eye velocity and eye position, reflecting different stages of integration. Neurons in the abducens nucleus discharge homogeneously in relation mainly to eye position, and reflect almost perfect integration of the cerebellar outputs. Average responses of neural populations and the diverse individual responses of large samples of individual neurons are reproduced by a hierarchical neural circuit based on a model suggested the anatomy and physiology of the larval zebrafish brainstem. The model uses a combination of feedforward and feedback connections to support a neural circuit basis for integration in monkeys and other species.

Multiple Layer 5 Pyramidal Cell Subtypes Relay Cortical Feedback from Secondary to Primary Motor Areas in Rats

Higher-order motor cortices, such as the secondary motor area (M2) in rodents, select future action patterns and transmit them to the primary motor cortex (M1). To better understand motor processing, we characterized "top-down" and "bottom-up" connectivities between M1 and M2 in the rat cortex. Somata of pyramidal cells (PCs) in M2 projecting to M1 were distributed in lower layer 2/3 (L2/3) and upper layer 5 (L5), whereas PCs projecting from M1 to M2 had somata distributed throughout L2/3 and L5. M2 afferents terminated preferentially in upper layer 1 of M1, which also receives indirect basal ganglia output through afferents from the ventral anterior and ventromedial thalamic nuclei. On the other hand, M1 afferents terminated preferentially in L2/3 of M2, a zone receiving indirect cerebellar output through thalamic afferents from the ventrolateral nucleus. While L5 corticopontine (CPn) cells with collaterals to the spinal cord did not participate in corticocortical projections, CPn cells with collaterals to the thalamus contributed preferentially to connections from M2 to M1. L5 callosal projection (commissural) cells participated in connectivity between M1 and M2 bidirectionally. We conclude that the connectivity between M1 and M2 is directionally specialized, involving specific PC subtypes that selectively target lamina receiving distinct thalamocortical inputs.

The Output Signal of Purkinje Cells of the Cerebellum and Circadian Rhythmicity

Measurement of clock gene expression has recently provided evidence that the cerebellum, like the master clock in the SCN, contains a circadian oscillator. The cerebellar oscillator is involved in anticipation of mealtime and possibly resides in Purkinje cells. However, the rhythmic gene expression is likely transduced into a circadian cerebellar output signal to exert an effective control of neuronal brain circuits that are responsible for feeding behavior. Using electrophysiological recordings from acute and organotypic cerebellar slices, we tested the hypothesis whether Purkinje cells transmit a circadian modulated signal to their targets in the brain. Extracellular recordings from brain slices revealed the typical discharge pattern previously described in vivo in single cell recordings showing basically a tonic or a trimodal-like firing pattern. However, in acute sagittal cerebellar slices the average spike rate of randomly selected Purkinje cells did not exhibit significant circadian variations, irrespective of their specific firing pattern. Also, frequency and amplitude of spontaneous inhibitory postsynaptic currents and the amplitude of GABA- and glutamate-evoked currents did not vary with circadian time. Long-term recordings using multielectrode arrays (MEA) allowed to monitor neuronal activity at multiple sites in organotypic cerebellar slices for several days to weeks. With this recording technique we observed oscillations of the firing rate of cerebellar neurons, presumably of Purkinje cells, with a period of about 24 hours which were stable for periods up to three days. The daily renewal of culture medium could induce circadian oscillations of the firing rate of Purkinje cells, a feature that is compatible with the behavior of slave oscillators. However, from the present results it appears that the circadian expression of cerebellar clock genes exerts only a weak influence on the electrical output of cerebellar neurons.

Nucleo-olivary inhibition balances the interaction between the reactive and adaptive layers in motor control

In the acquisition of adaptive motor reflexes to aversive stimuli, the cerebellar output fulfills a double purpose: it controls a motor response and it relays a sensory prediction. However, the question of how these two apparently incompatible goals might be achieved by the same cerebellar area remains open. Here we propose a solution where the inhibition of the Inferior Olive (IO) by the cerebellar Deep Nuclei (DN) translates the motor command signal into a sensory prediction allowing a single cerebellar area to simultaneously tackle both aspects of the problem: execution and prediction. We demonstrate that having a graded error signal, the gain of the Nucleo-Olivary Inhibition (NOI) balances the generation of the response between the cerebellar and the reflexive controllers or, in other words, between the adaptive and the reactive layers of behavior. Moreover, we show that the resulting system is fully autonomous and can either acquire or erase adaptive responses according to their utility.

When during horizontal saccades in monkey does cerebellar output affect movement?

The caudal part of the cerebellar fastigial nucleus (CFN) influences the horizontal component of saccades. Previous reports show that activity in the CFN contralateral to saccade direction aids saccade acceleration and that activity in the ipsilateral CFN aids saccade deceleration. Here we refine this description by characterizing how blocking CFN activity changes the distance that the eye rotates during each of 4 phases of saccades, the increasing and decreasing saccade acceleration (phases 1 and 2) and deceleration (3 and 4). We found that unilateral CFN inactivation increases total eye rotation to ∼1.8× normal. This resulted from rotation increases in all four phases of ipsiversive saccades. Rotation during phases 1 and 2 increases slightly, more during phase 3, and most during phase 4, to ∼4.4× normal. Thus, the ipsilateral CFN normally reduces eye rotation throughout a saccade but reduces it the most near saccade end. After unilateral CFN inactivation, rotation during contraversive saccades was ∼0.8× normal. This resulted from decreased rotation during phases 1–3, to ∼0.7× normal, and then normal rotation during phase 4. Thus the CFN contraversive to saccade direction normally increases eye rotation during acceleration and the first phase of deceleration. These data indicate that the influences of the CFNs on saccades overlap extensively and that there is a smooth shift from predominance of the contralateral CFN early in a saccade to the ipsilateral CFN later. The pathway from the CFN to contralateral IBNs and then to the abducens nucleus can account for these effects.