Cerebellar inhibition of inferior olivary transmission in the decerebrate ferret

The Effect of Nucleo-Olivary Stimulation on Climbing Fiber EPSPs in Purkinje Cells

Climbing fibers, connecting the inferior olive and Purkinje cells, form the nervous system's strongest neural connection. These fibers activate after critical events like motor errors or anticipation of rewards, leading to bursts of excitatory postsynaptic potentials (EPSPs) in Purkinje cells. The number of EPSPs is a crucial variable when the brain is learning a new motor skill. Yet, we do not know what determines the number of EPSPs. Here, we measured the effect of nucleo-olivary stimulation on periorbital elicited climbing fiber responses through in-vivo intracellular Purkinje cell recordings in decerebrated ferrets. The results show that while nucleo-olivary stimulation decreased the probability of a response occurring at all, it did not reduce the number of EPSPs. The results suggest that nucleo-olivary stimulation does not influence the number of EPSPs in climbing fiber bursts.

Role of cerebellum in sleep-dependent memory processes

The activities and role of the cerebellum in sleep have, until recently, been largely ignored by both the sleep and cerebellum fields. Human sleep studies often neglect the cerebellum because it is at a position in the skull that is inaccessible to EEG electrodes. Animal neurophysiology sleep studies have focussed mainly on the neocortex, thalamus and the hippocampus. However, recent neurophysiological studies have shown that not only does the cerebellum participate in the sleep cycle, but it may also be implicated in off-line memory consolidation. Here we review the literature on cerebellar activity during sleep and the role it plays in off-line motor learning, and introduce a hypothesis whereby the cerebellum continues to compute internal models during sleep that train the neocortex.

Purkinje cell BKchannel ablation induces abnormal rhythm in deep cerebellar nuclei and prevents LTD

Purkinje cells (PC) control deep cerebellar nuclei (DCN), which in turn inhibit inferior olive nucleus, closing a positive feedback loop via climbing fibers. PC highly express potassium BK channels but their contribution to the olivo-cerebellar loop is not clear. Using multiple-unit recordings in alert mice we found in that selective deletion of BK channels in PC induces a decrease in their simple spike firing with a beta-range bursting pattern and fast intraburst frequency (~200 Hz). To determine the impact of this abnormal rhythm on the olivo-cerebellar loop we analyzed simultaneous rhythmicity in different cerebellar structures. We found that this abnormal PC rhythmicity is transmitted to DCN neurons with no effect on their mean firing frequency. Long term depression at the parallel-PC synapses was altered and the intra-burst complex spike spikelets frequency was increased without modification of the mean complex spike frequency in BK-PC^−/− mice. We argue that the ataxia present in these conditional knockout mice could be explained by rhythmic disruptions transmitted from mutant PC to DCN but not by rate code modification only. This suggests a neuronal mechanism for ataxia with possible implications for human disease.

Synchronicity and Rhythmicity of Purkinje Cell Firing during Generalized Spike-and-Wave Discharges in a Natural Mouse Model of Absence Epilepsy

Absence epilepsy is characterized by the occurrence of generalized spike and wave discharges (GSWDs) in electrocorticographical (ECoG) recordings representing oscillatory activity in thalamocortical networks. The oscillatory nature of GSWDs has been shown to be reflected in the simple spike activity of cerebellar Purkinje cells and in the activity of their target neurons in the cerebellar nuclei, but it is unclear to what extent complex spike activity is implicated in generalized epilepsy. Purkinje cell complex spike firing is elicited by climbing fiber activation and reflects action potential firing in the inferior olive. Here, we investigated to what extent modulation of complex spike firing is reflected in the temporal patterns of seizures. Extracellular single-unit recordings in awake, head-restrained homozygous tottering mice, which suffer from a mutation in the voltage-gated Ca_V2.1 calcium channel, revealed that a substantial proportion of Purkinje cells (26%) showed increased complex spike activity and rhythmicity during GSWDs. Moreover, Purkinje cells, recorded either electrophysiologically or by using Ca²⁺-imaging, showed a significant increase in complex spike synchronicity for both adjacent and remote Purkinje cells during ictal events. These seizure-related changes in firing frequency, rhythmicity and synchronicity were most prominent in the lateral cerebellum, a region known to receive cerebral input via the inferior olive. These data indicate profound and widespread changes in olivary firing that are most likely induced by seizure-related activity changes in the thalamocortical network, thereby highlighting the possibility that olivary neurons can compensate for pathological brain-state changes by dampening oscillations.

Cerebellar physiology: links between microcircuitry properties and sensorimotor functions

Existing knowledge of the cerebellar microcircuitry structure and physiology allows a rather detailed description of what it in itself can and cannot do. Combined with a known mapping of different cerebellar regions to afferent systems and motor output target structures, there are several constraints that can be used to describe how specific components of the cerebellar microcircuitry may work during sensorimotor control. In fact, as described in this review, the major factor that hampers further progress in understanding cerebellar function is the limited insights into the circuitry-level function of the targeted motor output systems and the nature of the information in the mossy fiber afferents. The cerebellar circuitry in itself is here summarized as a gigantic associative memory element, primarily consisting of the parallel fiber synapses, whereas most other circuitry components, including the climbing fiber system, primarily has the role of maintaining activity balance in the intracerebellar and extracerebellar circuitry. The review explores the consistency of this novel interpretational framework with multiple diverse observations at the synaptic and microcircuitry level within the cerebellum.

The Anatomy and Physiology of Eyeblink Classical Conditioning

This chapter reviews the past research toward identifying the brain circuit and its computation underlying the associative memory in eyeblink classical conditioning. In the standard delay eyeblink conditioning paradigm, the conditioned stimulus (CS) and eyeblink-eliciting unconditioned stimulus (US) converge in the cerebellar cortex and interpositus nucleus (IPN) through the pontine nuclei and inferior olivary nucleus. Repeated pairings of CS and US modify synaptic weights in the cerebellar cortex and IPN, enabling IPN neurons to activate the red nucleus and generate the conditioned response (CR). In a variant of the standard paradigm, trace eyeblink conditioning, the CS and US are separated by a brief stimulus-free trace interval. Acquisition in trace eyeblink conditioning depends on several forebrain regions, including the hippocampus and medial prefrontal cortex as well as the cerebellar-brainstem circuit. Details of computations taking place in these regions remain unclear; however, recent evidence supports a view that the forebrain encodes a temporal sequence of the CS, trace interval, and US in a specific environmental context and signals the cerebellar-brainstem circuit to execute the CR when the US is likely to occur. Together, delay eyeblink conditioning represents one of the most successful cases of understanding the neural substrates of long-term memory in mammals, while trace eyeblink conditioning demonstrates its utility for uncovering detailed computations in the whole brain network underlying long-term memory.

Climbing fibers encode a temporal-difference prediction error during cerebellar learning in mice

Climbing fiber inputs to Purkinje cells are thought to play a teaching role by generating the instructive signals that drive cerebellar learning. To investigate how these instructive signals are encoded, we recorded the activity of individual climbing fibers during cerebellar-dependent eyeblink conditioning in mice. Our findings show that climbing fibers signal both the unexpected delivery and the unexpected omission of the periocular airpuff that serves as the instructive signal for eyeblink conditioning. In addition, we report the surprising discovery that climbing fibers activated by periocular airpuffs also respond to stimuli from other sensory modalities, if those stimuli are novel or if they predict that the periocular airpuff is about to be presented. This pattern of climbing fiber activity is strikingly similar to the responses of dopamine neurons during reinforcement learning, which have been shown to encode a particular type of instructive signal known as a temporal difference prediction error.

Purkinje cell activity during classical conditioning with different conditional stimuli explains central tenet of Rescorla–Wagner model [corrected]

A central tenet of Rescorla and Wagner's model of associative learning is that the reinforcement value of a paired trial diminishes as the associative strength between the presented stimuli increases. Despite its fundamental importance to behavioral sciences, the neural mechanisms underlying the model have not been fully explored. Here, we present findings that, taken together, can explain why a stronger association leads to a reduced reinforcement value, within the context of eyeblink conditioning. Specifically, we show that learned pause responses in Purkinje cells, which trigger adaptively timed conditioned eyeblinks, suppress the unconditional stimulus (US) signal in a graded manner. Furthermore, by examining how Purkinje cells respond to two distinct conditional stimuli and to a compound stimulus, we provide evidence that could potentially help explain the somewhat counterintuitive overexpectation phenomenon, which was derived from the Rescorla-Wagner model.

A neuro-inspired model-based closed-loop neuroprosthesis for the substitution of a cerebellar learning function in anesthetized rats

Neuroprostheses could potentially recover functions lost due to neural damage. Typical neuroprostheses connect an intact brain with the external environment, thus replacing damaged sensory or motor pathways. Recently, closed-loop neuroprostheses, bidirectionally interfaced with the brain, have begun to emerge, offering an opportunity to substitute malfunctioning brain structures. In this proof-of-concept study, we demonstrate a neuro-inspired model-based approach to neuroprostheses. A VLSI chip was designed to implement essential cerebellar synaptic plasticity rules, and was interfaced with cerebellar input and output nuclei in real time, thus reproducing cerebellum-dependent learning in anesthetized rats. Such a model-based approach does not require prior system identification, allowing for de novo experience-based learning in the brain-chip hybrid, with potential clinical advantages and limitations when compared to existing parametric "black box" models.

Inhibition of the amygdala central nucleus by stimulation of cerebellar output in rats: a putative mechanism for extinction of the conditioned fear response

The amygdala and the cerebellum serve two distinctively different functions. The amygdala plays a role in the expression of emotional information, whereas the cerebellum is involved in the timing of discrete motor responses. Interaction between these two systems is the basis of the two-stage theory of learning, according to which an encounter with a challenging event triggers fast classical conditioning of fear-conditioned responses in the amygdala and slow conditioning of motor-conditioned responses in the cerebellum. A third stage was hypothesised when an apparent interaction between amygdala and cerebellar associative plasticity was observed: an adaptive rate of cerebellum-dependent motor-conditioned responses was associated with a decrease in amygdala-dependent fear-conditioned responses, and was interpreted as extinction of amygdala-related fear-conditioned responses by the cerebellar output. To explore this hypothesis, we mimicked some components of classical eyeblink conditioning in anesthetised rats by applying an aversive periorbital pulse as an unconditioned stimulus and a train of pulses to the cerebellar output nuclei as a cerebellar neuronal-conditioned response. The central amygdala multiple unit response to the periorbital pulse was measured with or without a preceding train to the cerebellar output nuclei. The results showed that activation of the cerebellar output nuclei prior to periorbital stimulation produced diverse patterns of inhibition of the amygdala response to the periorbital aversive stimulus, depending upon the nucleus stimulated, the laterality of the nucleus stimulated, and the stimulus interval used. These results provide a putative extinction mechanism of learned fear behavior, and could have implications for the treatment of pathologies involving abnormal fear responses by using motor training as therapy.

Changes in complex spike activity during classical conditioning

The cerebellar cortex is necessary for adaptively timed conditioned responses (CRs) in eyeblink conditioning. During conditioning, Purkinje cells acquire pause responses or "Purkinje cell CRs" to the conditioned stimuli (CS), resulting in disinhibition of the cerebellar nuclei (CN), allowing them to activate motor nuclei that control eyeblinks. This disinhibition also causes inhibition of the inferior olive (IO), via the nucleo-olivary pathway (N-O). Activation of the IO, which relays the unconditional stimulus (US) to the cortex, elicits characteristic complex spikes in Purkinje cells. Although Purkinje cell activity, as well as stimulation of the CN, is known to influence IO activity, much remains to be learned about the way that learned changes in simple spike firing affects the IO. In the present study, we analyzed changes in simple and complex spike firing, in extracellular Purkinje cell records, from the C3 zone, in decerebrate ferrets undergoing training in a conditioning paradigm. In agreement with the N-O feedback hypothesis, acquisition resulted in a gradual decrease in complex spike activity during the conditioned stimulus, with a delay that is consistent with the long N-O latency. Also supporting the feedback hypothesis, training with a short interstimulus interval (ISI), which does not lead to acquisition of a Purkinje cell CR, did not cause a suppression of complex spike activity. In contrast, observations that extinction did not lead to a recovery in complex spike activity and the irregular patterns of simple and complex spike activity after the conditioned stimulus are less conclusive.

Distribution of neural plasticity in cerebellum-dependent motor learning

The cerebellum is essential for some forms of motor learning. Two examples that provide useful experimental models are modification of the vestibulo-ocular reflex and classical conditioning of the nictitating membrane response (NMR) in the rabbit. There has been considerable analysis of these behavioral models and of conditioning of the eyelid blink reflex, which is similar in several respects to NMR conditioning but with some key differences in its control circuitry. The evidence is consistent with the suggestion that storage of these motor memories is to be found within the cerebellum and its associated brainstem circuitry. The cerebellum presents many advantages as a model system to characterize the cellular and molecular mechanisms underpinning behavioral learning. And yet, localizing the essential synaptic changes has proven to be difficult. A major problem has been to establish to what extent these neural changes are distributed through the cerebellar cortex, cerebellar nuclei, and associated brainstem nuclei. Inspired by recent theoretical work, here we review evidence that the distribution of plasticity across cortical and cerebellar nuclear (or brainstem vestibular system) levels for different learning tasks may be different and distinct. Our primary focus is on classical conditioning of the NMR and eyelid blink, and we offer comparisons with mechanisms for modifications of the vestibulo-ocular reflex. We describe a view of cerebellar learning that satisfies theoretical and empirical analysis.

The ontogeny of associative cerebellar learning

Ontogenetic changes in associative cerebellar learning have been examined extensively using eyeblink conditioning in infant humans and rats. The cerebellum is essential for eyeblink conditioning in adult and infant animals. The cerebellum receives input from the conditional stimulus (CS) through the pontine mossy fiber projection and unconditional stimulus (US) input through the inferior olive climbing fiber projection. Coactivation of the CS and US pathways induces synaptic plasticity in the cerebellum, which is necessary for the conditional response. Ontogenetic changes in eyeblink conditioning are driven by developmental changes in the projections of subcortical sensory nuclei to the pontine nuclei and in the inhibitory projection from the cerebellar deep nuclei to the inferior olive. Developmental changes in the CS and US pathways limit the induction of learning-related plasticity in the cerebellum and thereby limit acquisition of eyeblink conditioning.

Feedback control of learning by the cerebello-olivary pathway

The ability to anticipate future events and to modify erroneous anticipatory actions is crucial for the survival of any organism. Both theoretical and empirical lines of evidence implicate the cerebellum in this ability. It is often suggested that the cerebellum acquires "expectations" or "internal models." However, except in a metaphorical sense, the cerebellum, which consists of a set of interconnected nerve cells, cannot contain "internal models" or "have expectations." In this chapter, we try to untangle these metaphors by translating them back into neurophysiological cause and effect relationships. We approach this task from within the paradigm of classical conditioning, in which a subject, through repeated presentations of a conditional stimulus, followed by an unconditional stimulus, acquires a conditioned response. Importantly, the conditioned response is timed so that it anticipates the unconditioned response. Available neurophysiological evidence suggests that Purkinje cells, in the cerebellar cortex, generate the conditioned response. In addition, Purkinje cells provide negative feedback to the inferior olive, which is a relay for the unconditional stimulus, via the nucleo-olivary pathway. Purkinje cells can therefore regulate the intensity of the signal derived from the unconditional stimulus, which, in turn, decides subsequent plasticity. Hence, as learning progresses, the olivary signal will become weaker and weaker due to increasing negative feedback from Purkinje cells. Thus, in an important sense, learning-induced changes in Purkinje cell activity constitute an "expectation" or "anticipation" of a future event (the unconditional stimulus), and, consistent with theoretical models, future learning depends on the accuracy of this expectation.

Stimulation within the cuneate nucleus suppresses synaptic activation of climbing fibers

Several lines of research have shown that the excitability of the inferior olive is suppressed during different phases of movement. A number of different structures like the cerebral cortex, the red nucleus, and the cerebellum have been suggested as candidate structures for mediating this gating. The inhibition of the responses of the inferior olivary neurons from the red nucleus has been studied extensively and anatomical studies have found specific areas within the cuneate nucleus to be target areas for projections from the magnocellular red nucleus. In addition, GABA-ergic cells projecting from the cuneate nucleus to the inferior olive have been found. We therefore tested if direct stimulation of the cuneate nucleus had inhibitory effects on a climbing fiber field response, evoked by electrical stimulation of the pyramidal tract, recorded on the surface of the cerebellum. When the pyramidal tract stimulation was preceded by weak electrical stimulation (5–20 μA) within the cuneate nucleus, the amplitude of the climbing fiber field potential was strongly suppressed (approx. 90% reduction). The time course of this suppression was similar to that found after red nucleus stimulation, with a peak suppression occurring at 70 ms after the cuneate stimulation. Application of CNQX (6-cyano-7-nitroquinoxaline-2,3-dione, disodium salt) on the cuneate nucleus blocked the suppression almost completely. We conclude that a relay through the cuneate nucleus is a possible pathway for movement-related suppression of climbing fiber excitability.

Properties of the nucleo-olivary pathway: an in vivo whole-cell patch clamp study

The inferior olivary nucleus (IO) forms the gateway to the cerebellar cortex and receives feedback information from the cerebellar nuclei (CN), thereby occupying a central position in the olivo-cerebellar loop. Here, we investigated the feedback input from the CN to the IO in vivo in mice using the whole-cell patch-clamp technique. This approach allows us to study how the CN-feedback input is integrated with the activity of olivary neurons, while the olivo-cerebellar system and its connections are intact. Our results show how IO neurons respond to CN stimulation sequentially with: i) a short depolarization (EPSP), ii) a hyperpolarization (IPSP) and iii) a rebound depolarization. The latter two phenomena can also be evoked without the EPSPs. The IPSP is sensitive to a GABA_A receptor blocker. The IPSP suppresses suprathreshold and subthreshold activity and is generated mainly by activation of the GABA_A receptors. The rebound depolarization re-initiates and temporarily phase locks the subthreshold oscillations. Lack of electrotonical coupling does not affect the IPSP of individual olivary neurons, nor the sensitivity of its GABA_A receptors to blockers. The GABAergic feedback input from the CN does not only temporarily block the transmission of signals through the IO, it also isolates neurons from the network by shunting the junction current and re-initiates the temporal pattern after a fixed time point. These data suggest that the IO not only functions as a cerebellar controlled gating device, but also operates as a pattern generator for controlling motor timing and/or learning.

Neural circuitry and plasticity mechanisms underlying delay eyeblink conditioning

Pavlovian eyeblink conditioning has been used extensively as a model system for examining the neural mechanisms underlying associative learning. Delay eyeblink conditioning depends on the intermediate cerebellum ipsilateral to the conditioned eye. Evidence favors a two-site plasticity model within the cerebellum with long-term depression of parallel fiber synapses on Purkinje cells and long-term potentiation of mossy fiber synapses on neurons in the anterior interpositus nucleus. Conditioned stimulus and unconditioned stimulus inputs arise from the pontine nuclei and inferior olive, respectively, converging in the cerebellar cortex and deep nuclei. Projections from subcortical sensory nuclei to the pontine nuclei that are necessary for eyeblink conditioning are beginning to be identified, and recent studies indicate that there are dynamic interactions between sensory thalamic nuclei and the cerebellum during eyeblink conditioning. Cerebellar output is projected to the magnocellular red nucleus and then to the motor nuclei that generate the blink response(s). Tremendous progress has been made toward determining the neural mechanisms of delay eyeblink conditioning but there are still significant gaps in our understanding of the necessary neural circuitry and plasticity mechanisms underlying cerebellar learning.

Multiple sites of extinction for a single learned response

Most learned responses can be diminished by extinction, a process that can be engaged when a conditioned stimulus (CS) is presented but not reinforced. We present evidence that plasticity in at least two brain regions can mediate extinction of responses produced by trace eyelid conditioning, where the CS and the reinforcing stimulus are separated by a stimulus-free interval. We observed individual differences in the effects of blocking extinction mechanisms in the cerebellum, the structure that, along with several forebrain structures, mediates acquisition of trace eyelid responses; in some rabbits extinction was prevented, whereas in others it was largely unaffected. We also show that cerebellar mechanisms can mediate extinction when noncerebellar mechanisms are bypassed. Together, these observations indicate that trace eyelid responses can be extinguished via processes operating at more than one site, one in the cerebellum and one upstream in forebrain. The relative contributions of these sites may vary from animal to animal and situation to situation.

The mysterious microcircuitry of the cerebellar nuclei

The microcircuitry of cerebellar cortex and, in particular, the physiology of its main element, the Purkinje neuron, has been extensively investigated and described. However, activity in Purkinje neurons, either as single cells or populations, does not directly mediate the cerebellar effects on the motor effector systems. Rather, the result of the entire cerebellar cortical computation is passed to the relatively small cerebellar nuclei that act as the final, integrative processing unit in the cerebellar circuitry. The nuclei ultimately control the temporal and spatial features of the cerebellar output. Given this key role, it is striking that the internal organization and the connectivity with afferent and efferent pathways in the cerebellar nuclei are rather poorly known. In the present review, we discuss some of the many critical shortcomings in the understanding of cerebellar nuclei microcircuitry: the extent of convergence and divergence of the cerebellar cortical pathway to the various cerebellar nuclei neurons and subareas, the possible (lack of) conservation of the finely-divided topographical organization in the cerebellar cortex at the level of the nuclei, as well as the absence of knowledge of the synaptic circuitry within the cerebellar nuclei. All these issues are important for predicting the pattern-extraction and encoding capabilities of the cerebellar nuclei and, until resolved, theories and models of cerebellar motor control and learning may err considerably.

Sensory prediction or motor control? Application of marr-albus type models of cerebellar function to classical conditioning

Marr-Albus adaptive filter models of the cerebellum have been applied successfully to a range of sensory and motor control problems. Here we analyze their properties when applied to classical conditioning of the nictitating membrane response in rabbits. We consider a system-level model of eyeblink conditioning based on the anatomy of the eyeblink circuitry, comprising an adaptive filter model of the cerebellum, a comparator model of the inferior olive and a linear dynamic model of the nictitating membrane plant. To our knowledge, this is the first model that explicitly includes all these principal components, in particular the motor plant that is vital for shaping and timing the behavioral response. Model assumptions and parameters were systematically investigated to disambiguate basic computational capacities of the model from features requiring tuning of properties and parameter values. Without such tuning, the model robustly reproduced a range of behaviors related to sensory prediction, by displaying appropriate trial-level associative learning effects for both single and multiple stimuli, including blocking and conditioned inhibition. In contrast, successful reproduction of the real-time motor behavior depended on appropriate specification of the plant, cerebellum and comparator models. Although some of these properties appear consistent with the system biology, fundamental questions remain about how the biological parameters are chosen if the cerebellar microcircuit applies a common computation to many distinct behavioral tasks. It is possible that the response profiles in classical conditioning of the eyeblink depend upon operant contingencies that have previously prevailed, for example in naturally occurring avoidance movements.

Cerebellar and extracerebellar involvement in mouse eyeblink conditioning: the ACDC model

Over the past decade the advent of mouse transgenics has generated new perspectives on the study of cerebellar molecular mechanisms that are essential for eyeblink conditioning. However, it also appears that results from eyeblink conditioning experiments done in mice differ in some aspects from results previously obtained in other mammals. In this review article we will, based on studies using (cell-specific) mouse mutants and region-specific lesions, re-examine the general eyeblink behavior in mice and the neuro-anatomical circuits that might contribute to the different peaks in the conditioned eyeblink trace. We conclude that the learning process in mice has at least two stages: An early stage, which includes short-latency responses that are at least partly controlled by extracerebellar structures such as the amygdala, and a later stage, which is represented by well-timed conditioned responses that are mainly controlled by the pontocerebellar and olivocerebellar systems. We refer to this overall concept as the Amygdala-Cerebellum-Dynamic-Conditioning Model (ACDC model).

Effect of conditioned stimulus parameters on timing of conditioned Purkinje cell responses

Pavlovian eyeblink conditioning is a useful experimental model for studying adaptive timing, an important aspect of skilled movements. The conditioned response (CR) is precisely timed to occur just before the onset of the expected unconditioned stimulus (US). The timing can be changed immediately, however, by varying parameters of the conditioned stimulus (CS). It has previously been shown that increasing the intensity of a peripheral CS or the frequency of a CS consisting of a train of stimuli to the mossy fibers shortens the latency of the CR. The adaptive timing of behavioral CRs probably reflects the timing of an underlying learned inhibitory response in cerebellar Purkinje cells. It is not known how the latency of this Purkinje cell CR is controlled. We have recorded form Purkinje cells in conditioned decerebrate ferrets while increasing the intensity of a peripheral CS or the frequency of a mossy fiber CS. We observe changes in the timing of the Purkinje cell CR that match the behavioral effects. The results are consistent with the effect of CS parameters on behavioral CR latency being caused by corresponding changes in Purkinje cell CRs. They suggest that synaptic temporal summation may be one of several mechanisms underlying adaptive timing of movements.

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

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

Interaction between Purkinje cells and inhibitory interneurons may create adjustable output waveforms to generate timed cerebellar output

We develop a new model that explains how the cerebellum may generate the timing in classical delay eyeblink conditioning. Recent studies show that both Purkinje cells (PCs) and inhibitory interneurons (INs) have parallel signal processing streams with two time scales: an AMPA receptor-mediated fast process and a metabotropic glutamate receptor (mGluR)-mediated slow process. Moreover, one consistent finding is an increased excitability of PC dendrites (in Larsell's lobule HVI) in animals when they acquire the classical delay eyeblink conditioning naturally, in contrast to in vitro studies, where learning involves long-term depression (LTD). Our model proposes that the delayed response comes from the slow dynamics of mGluR-mediated IP3 activation, and the ensuing calcium concentration change, and not from LTP/LTD. The conditioned stimulus (tone), arriving on the parallel fibers, triggers this slow activation in INs and PC spines. These excitatory (from PC spines) and inhibitory (from INs) signals then interact at the PC dendrites to generate variable waveforms of PC activation. When the unconditioned stimulus (puff), arriving on the climbing fibers, is coupled frequently with this slow activation the waveform is amplified (due to an increased excitability) and leads to a timed pause in the PC population. The disinhibition of deep cerebellar nuclei by this timed pause causes the delayed conditioned response. This suggested PC-IN interaction emphasizes a richer role of the INs in learning and also conforms to the recent evidence that mGluR in the cerebellar cortex may participate in slow motor execution. We show that the suggested mechanism can endow the cerebellar cortex with the versatility to learn almost any temporal pattern, in addition to those that arise in classical conditioning.

Extinction of conditioned blink responses by cerebello-olivary pathway stimulation

Learning of classically conditioned eyeblink responses depends on mechanisms within the cerebellum. It has been suggested that climbing fibres from the inferior olive transmit the unconditioned stimulus signal to the cerebellum. We have previously shown that the pathway from the deep cerebellar nuclei to the inferior olive inhibits olivary activity. It is known that repeated presentation of the conditioned stimulus on its own leads to extinction of the conditioned response. If the unconditioned stimulus signal is transmitted to the cerebellum via the inferior olive - climbing fibre system then stimulation of the nucleo-olivary pathway just before the unconditioned stimulus in a trained animal should lead to extinction. The results from this investigation confirm this.

Acquisition, extinction, and reacquisition of a cerebellar cortical memory trace

Associative learning in the cerebellum underlies motor memories and probably also cognitive associations. Pavlovian eyeblink conditioning, a widely used experimental model of such learning, depends on the cerebellum, but the memory locus within the cerebellum as well as the underlying mechanisms have remained controversial. To date, crucial information on how cerebellar Purkinje cells change their activity during learning has been ambiguous and contradictory, and there is no information at all about how they behave during extinction and reacquisition. We have now tracked the activity of single Purkinje cells with microelectrodes for up to 16 h in decerebrate ferrets during learning, extinction, and relearning. We demonstrate that paired peripheral forelimb and periocular stimulation, as well as paired direct stimulation of cerebellar afferent pathways (mossy and climbing fibers) consistently causes a gradual acquisition of an inhibitory response in Purkinje cell simple spike firing. This conditioned cell response has several properties that matches known features of the behavioral conditioned response. The response latency varies with the interstimulus interval, and the response maximum is adaptively timed to precede the unconditioned stimulus. Across training trials, it matches behavioral extinction to unpaired stimulation and also the substantial savings that occur when paired stimulation is reinstated. These data suggest that many of the basic behavioral phenomena in eyeblink conditioning can be explained at the level of the single Purkinje cell.

Cerebellar control of the inferior olive

A subpopulation of neurones in the cerebellar nuclei projects to the inferior olive, the source of the climbing fibre input to the cerebellum. This nucleo-olivary projection follows the zonal and, probably also, the microzonal arrangement of the cerebellum so that closed loops are formed between the neurones in the olive, the cerebellar cortex and the nuclei. The nucleo-olivary pathway is GABAergic, but several investigators argue that its main effect is to regulate electrotonic coupling between cells in the inferior olive rather than inhibit the olive. However, there is now strong evidence that the nucleo-olivary fibres do inhibit the olive. Three functions have been suggested for this inhibition: (i) feedback control of background activity in Purkinje cells, (ii) feedback control of learning, and (iii) gating of olivary input in general. Evidence is consistent with (i) and (ii). Activity in the nucleo-olivary pathway suppresses both synaptic transmission and background activity in the olive. When learned blink responses develop, the blink related part of the olive is inhibited while blinks are produced. When the nucleo-olivary pathway is interrupted, there is a corresponding increase in complex spike discharge in Purkinje cells followed by a strong suppression of simple spike firing. Stimulation of the pathway has the opposite results. It is concluded that the nucleo-olivary fibres are inhibitory and that they form a number of independent feedback loops, each one specific for a microcomplex, that regulate cerebellar learning as well as spontaneous activity in the olivo-cerebellar circuit.