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Distribution of Neural Plasticity in CerebellumDependent Motor Learning

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Michael Longley, Christopher H. Yeo1 Department of Neuroscience, Physiology and Pharmacology, University College London, London, UK 1 Corresponding author: Tel: þ 442076797377, e-mail address: [email protected]

Abstract 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.

Keywords learning, memory, memory consolidation, eyeblink conditioning, classical conditioning, cerebellar cortical learning, cerebellar nuclear learning

1 INTRODUCTION The special role of the cerebellum in helping ensure that movements are accurate depends, in large part, upon a capacity for learning. The calibration of motor commands to movement metrics, the adjustment of reflex gains, and the development of Progress in Brain Research, Volume 210, ISSN 0079-6123, http://dx.doi.org/10.1016/B978-0-444-63356-9.00004-2 © 2014 Elsevier B.V. All rights reserved.

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motor skills have important dependencies upon the cerebellum. Implicit in these functions is a capacity for learning, and two examples of motor learning that provide useful experimental models are modification of the vestibulo-ocular reflex (Boyden et al., 2004; Ito, 1982; Koekkoek et al., 1997; Lisberger, 1994; Lisberger et al., 1984; Miles and Lisberger, 1981; Miles et al., 1980; Nagao, 1983) and classical conditioning of the nictitating membrane response (NMR) in the rabbit (Clark et al., 1984; McCormick and Thompson, 1984; Yeo and Hesslow, 1998; Yeo et al., 1984, 1985a,b). They are useful because they have high levels of cerebellar dependence— lesions or functional inactivations of the cerebellum have profound effects upon the expression of existing learned responses and upon the capacity to acquire new ones. Many other examples of motor learning will additionally engage and depend upon the cerebral cortex, basal ganglia, and other key elements of the motor system, but NMR conditioning and VOR modification are particularly useful for analyzing a cerebellar role in motor learning because of their special dependence upon it. There has been detailed analysis of these two behavioral models but also of conditioning of the eyelid blink reflex (Aksenov et al., 2004, 2005; Bracha, 2004; Garcia and Mauk, 1998; Garcia et al., 1999; Gruart et al., 1997; Hesslow, 1994a; Hesslow et al., 1999) which is similar in several respects to NMR conditioning but with some key differences in its control circuitry (van Ham and Yeo, 1996). Modification of the optokinetic response (OKR) has also shown cerebellar dependence (Okamoto et al., 2011a,b; Shutoh et al., 2006). The evidence is consistent with the suggestion that not only is the cerebellum essential for learning these tasks but also that the storage of their motor memories is to be found within the cerebellum and its associated brainstem circuitry (Attwell et al., 2002a; Boyden et al., 2004; Cooke et al., 2004; Kellett et al., 2010; Koekkoek et al., 1997; Medina and Lisberger, 2008; Nagao, 1983; Yeo and Hesslow, 1998). As model systems to characterize the cellular and molecular mechanisms underpinning behavioral learning, these cerebellum-dependent forms present many advantages. Foremost of these are that the microcircuitry and compartmental organization of the cerebellum are well defined. And yet, as an essential first step in characterizing cellular and molecular mechanisms, localizing the essential neural changes has been difficult. A major problem has been to establish to what extent the neural changes responsible for learning are distributed through the cerebellar cortex, the cerebellar nuclei, and/or the associated brainstem nuclei and, in addition, to establish whether this distribution is stable or changeable as memories develop and consolidate. Furthermore, a corollary to establishing useful models is that they should reveal common and representative underlying mechanisms. Thus, it might be assumed that the neural changes and their distribution must be very similar for all forms of cerebellum-dependent learning (Raymond et al., 1996). But this assumption may not be valid. Inspired by recent theoretical work (Porrill and Dean, 2007), here we discuss the suggestion that the distribution of plasticity across cortical and cerebellar nuclear (or brainstem vestibular system) levels for different learning tasks may be different and distinct, as supported by recent empirical work (Kassardjian et al., 2005; Kellett et al., 2010; Okamoto et al., 2011a,b; Shutoh et al., 2006).

2 Cerebellum-Dependent Learning—Conditioning Models

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 analyses.

2 CEREBELLUM-DEPENDENT LEARNING—EYEBLINK AND NMR CONDITIONING AS BEHAVIORAL MODELS FOR ANALYSIS Classical conditioning of the NMR or of the eyelid blink (also termed “eyeblink”) is associative learning (Gormezano et al., 1962; Schneiderman et al., 1962) of a motor response (Yeo and Hesslow, 1998). To classically condition either reflex, a behaviorally neutral conditional stimulus (CS) is consistently paired with an unconditional stimulus (US) that reliably evokes the NMR or eyeblink unconditional response (UR). Typically, auditory or visual CSs are used but tactile or electrical stimulation of various regions of the skin surface can be employed. For the US, an airpuff to the cornea or to the periocular area or electrical stimulation of the periocular skin is commonly used. These stimuli evoke a blink closure of the external eyelids. A third eyelid, or nictitating membrane (NM), is also present in some species including rabbits, and it sweeps horizontally across the eye in response to such stimulation (Gormezano et al., 1962, 1983). Thus, periocular or corneal stimulation in rabbits elicits a coordinated blink involving both eyelid and NM response systems (Hesslow and Yeo, 2002). After a sufficient number of CS and US pairings, a conditioned response (CR) to the CS develops and, in rabbits, the CR has both eyelid blink and NM response components (Attwell et al., 2002a). But it is important to recognize that the neural circuitry of these two response systems is distinct and different, conferring very different properties on the controlled lid movements (van Ham and Yeo, 1996). Closure of the external eyelids is by the orbicularis oculi muscle driven by motoneurons in the dorsal aspect of the facial nucleus (VII nerve nucleus) in all species and raising (opening) of the upper eyelid is by the levator palpebrae muscle with motoneurons in the oculomotor nucleus (III nerve nucleus). Thus, the external eyelid blink response has conventional agonist–antagonist control via the nVII and nIII, respectively. Importantly, this external eyelid blink system is characterized by relatively high spontaneous rates of responding and voluntary movements, consistent with the presence of a motor cortical drive to the lid motoneurons. In sharp contrast, the NM response is controlled by a subdivision of the abducens nucleus (the accessory abducens or AccVI nucleus) that activates the retractor bulbi muscle to retract the globe into the orbit and produces NM “closure” as a passive consequence (Disterhoft et al., 1985). Since globe position is restored by the elastic tissues of the orbit during relaxation of the retractor bulbi muscle, there is no antagonist muscle for the NM response and so NM position is controlled entirely by the AccVI motoneurons. Importantly, the NM response is characterized by very low

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spontaneous response levels with no evidence of voluntary movements (Gormezano, 1972; Thompson, 1976), consistent with very low baseline levels of AccVI activity (Delgado-Garcia et al., 1990). These qualities of low baseline and voluntary movement response rates for the NMR appealed strongly to Gormezano and his colleagues (Gormezano, 1972; Gormezano et al., 1962) seeking an optimal Pavlovian learning system. Their omission control experiments revealed that, with careful attention to the protocols, NMR conditioning is demonstrably purely classical, or Pavlovian, in its properties. This is particularly true when periocular electrical stimulation is used as the US because conditioned eyelid closure and NM responses can have no impact on the effects of the delivered stimulus. In contrast, use of an airpuff US delivered directly to the cornea can significantly change the pure Pavlovian nature of the learning. Conditioned lid closures and the NM responses can ameliorate the impact of an airpuff US on the cornea while exposing a different set of cutaneous receptors, especially those on the outer surface of the eyelids, to the stimulus. Without special measures, such as directing the airpuff to the periocular region only, these protocols can result in a substantial operant, or instrumental, component to the resultant learning. But should we be concerned about the balance between classical and operant contingencies in our learning models? For two important reasons, the answer is yes. First, there is a long-standing and extensive literature detailing the very significant differences between these forms of learning and the neural circuits and mechanisms responsible (Oakley and Russell, 1972, 1977). Second, the original studies that analyzed and defined the special dependence of rabbit NMR conditioning upon the cerebellum, notably by Thompson and his colleagues (Clark et al., 1984; Krupa et al., 1993; McCormick and Thompson, 1984) and those from our own laboratory (Yeo and Hesslow, 1998; Yeo et al., 1984, 1985a,b), were unequivocally Pavlovian in their design. When an NM response is formally conditioned under an instrumental avoidance contingency, then it remains cerebellum dependent (Polenchar et al., 1985). However, no studies have analyzed a specifically designed instrumental eyeblink conditioning task, with the appropriate yoked controls to dissociate the classically conditioned components of the learning, to test its dependency on the cerebellum. And yet there are many studies of eyeblink conditioning that allow significant attenuation of the corneal airpuff US that could be considered instrumental. For example, recent studies have used mice with selective manipulation of gene expression in candidate cerebellar pathways, and results from several suggest that the learning may be relatively independent of the cerebellar cortical circuitry and/or its candidate neural plasticities (De Zeeuw and Yeo, 2005; Schonewille et al., 2011). In these examples, it will be important to understand whether it is an instrumental component of the learning, with much greater dependency upon cerebral cortical and especially motor cortical function, which contributes to learned behavior that survives cerebellar manipulations. A key aspect of NMR and eyeblink conditioning studies that should not escape attention is that the CS is chosen to be behaviorally neutral at the start of conditioning. So, under initial conditions, the CS–CR relationship can be viewed as having a

3 Eyeblink/NMR Conditioning and Cortical Lobule HVI

gain of zero. Only after CS–US pairings does the CS–CR relationship develop a positive gain. Learning theory (Mackintosh, 1974) indicates that this may be an oversimplification of the initial CS salience, and detailed electrophysiological analysis could well reveal that the CS may evoke sub-threshold activity within associative and efferent components of the reflex. However, as measured by behavioral outputs, this zero gain in the overt state at the start of conditioning is in sharp contrast to that of the other commonly studied cerebellum-dependent form of learning, the modification of VOR gain or phase. Before any modification is attempted, there is a significant behavioral output response to the modifiable, vestibular signal. Thus, electrophysiological analysis of cerebellar function in classical conditioning of the NMR and/or eyelid responses should not assume that a CS representation will be present at the neural spiking level in associative elements of the network. This representation may only be detectable after CRs are established. In contrast, neural spiking associated with the vestibular drive signal is assumed, and detected, in the presumed cerebellar and brainstem associative elements and in the known output pathways for VOR modification (Ito, 1982; Lisberger and Fuchs, 1978a; Lisberger et al., 1994; Nagao, 1989).

3 LESION STUDIES REVEAL THAT NMR CONDITIONING DEPENDS UPON CEREBELLAR COMPARTMENTS WITH C1 AND C3 CORTICAL ZONES IN LOBULE HVI Early lesion studies from several laboratories revealed that NMR conditioning is cerebellum-dependent. Large cerebellar lesions (Lincoln et al., 1982), or lesions of cerebellar outputs in superior cerebellar peduncle (McCormick et al., 1982), or lesions within the cerebellar nuclei (Clark et al., 1984; McCormick and Thompson, 1984) abolished established conditioned NM responses and prevented reacquisition. Following these initial observations, more selective lesions helped identify the olivo-cortico-nuclear (OCN) compartments essential for learning and provided the framework for analyzing the distribution of the essential plasticity within them. Lesions within the anterior interpositus nucleus (AIP), but in no other nuclear region (Yeo et al., 1985a), and lesions within the rostral part of the dorsal accessory olive (DAO) (Yeo et al., 1986), where face somatosensory information is represented (Van Ham and Yeo, 1992), were sufficient to abolish existing CRs and prevent their reacquisition. This identification of regions critical for NMR conditioning in the rostral DAO and in the AIP strongly suggested, on the basis of connectivity studies in several other species (Apps and Hawkes, 2009; Voogd et al., 1981), that either or both of two cerebellar compartments, with cortical territories in the C1 and C3 zones, would be important for NMR conditioning. These are zones with climbing fiber (CF) afferents from the critical region of DAO and with efferent projections to the essential nuclear territory, in AIP. Consistent with this suggestion, lesions within a region in the depths of lobule HVI, including its medial wall,

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abolished NMR conditioning (Yeo et al., 1984, 1985b) and anatomical tracing confirmed efferent and afferent connections consistent with the lesion studies, in AIP and in medial rostral DAO, respectively (Yeo et al., 1985c). Hesslow was first to use electrophysiology to examine eyeblink control regions in the cerebellar cortex. In decerebrate cat, and later ferret, models, he discovered two small cortical territories in lobule HVI whose locations were surprisingly consistent with those in our rabbit NMR conditioning studies. They had short-latency (around 8 ms), periocular receptive field CF response activity consistent with their identity as eyeblink microzones within the C1 and/or C3 zones (Hesslow, 1994b). In a crucial experiment, he found that the effects of stimulation to these two cortical sites only and no others defined their special relationship with reflex and learned eyeblinks (Hesslow, 1994a). A train of cortical surface stimulation produced a reflex eyeblink to train offset and, even more significantly, the same train stimulation completely suppressed the production of previously trained conditioned responses for the duration of stimulation. These studies provided three crucial observations that might guide future analyses. First, the critical eyeblink C1/C3 microzones are small and they occupy only a small part of lobule HVI and, of course, an even smaller percentage of the whole cerebellar cortex. Second, the zones might be relatively inaccessible. In the cat, parts of the microzones were at the apex of the lobule but these and subsequent studies indicated that other parts may lie deeper within the fissures. The lesion work with rabbits suggested that the microzones are particularly deep in this species with little or no extents at the apex of the lobule. Third, Hesslow’s electrophysiology indicated that the microzones can be identified reliably by analysis of the receptive field properties and latency of the CF response measured at the cortical surface. It is important to note that CF responses with longer latency (around 20 ms) and much broader receptive fields, consistent with the properties of C2 zone territories, are often elicited by periocular stimulation and recorded quite widely in other cortical territories. But these regions do not have the special relationship with the reflex and learned blinks that was revealed by train stimulation of the C1/C3 eyeblink microzones. These early lesion and electrophysiology studies invited a simple hypothesis for the mechanisms underlying NMR and eyeblink conditioning. The critical cortical region receives a discrete CF input from regions of the olive with face somatosensory inputs and mossy fiber inputs from auditory and visual regions of the dorsolateral and lateral basilar pontine nuclei (Yeo et al., 1985c), so the convergence principle essential for learning was easily established (Yeo and Hesslow, 1998; Yeo et al., 1985c). These findings added weight to the suggestion that a learning rule similar to that proposed in the Marr and Albus models (Albus, 1971; Marr, 1969) of cerebellar learning could apply. In these models, the context for movement is supplied by a mossy fiber input and the teaching signal by the CF input. For classical conditioning, these models could be implemented simply with CS and US coding via the mossy and CF afferents, respectively. The information modalities suggested by the anatomical study matches this implementation very closely and so provided direct and compelling evidence in favor of a cortical learning rule as envisaged by Marr and by Albus (Yeo and Hesslow, 1998; Yeo et al., 1985c) (see Fig. 1A).

FIGURE 1 A model of NMR and eyeblink conditioning circuitry and the effects of cerebellar manipulations on excitability in the network. (A) A simplified view of olivo-cortico-nuclear circuitry involved in NMR and eyeblink conditioning with an implementation of a cortical conditioning mechanism based on that of Yeo and Hesslow (1998). The conditional stimulus (CS) is shown entering the system via mossy fibers (mf) and the unconditional stimulus (US) via climbing fibers from the rostromedial dorsal accessory olive (rmDAO) that receives face somatosensory information from the trigeminal nucleus (NV). The olivo-cortico-nuclear loop is indicated with dashed lines. There is convergence of CS- and US-related information at both the cerebellar cortex (lobule HVI) and at the cerebellar nuclei (anterior interpositus nucleus—AIP), via climbing fiber and mossy fiber collaterals. There are local inhibitory interneurons (in) in the cerebellar nuclei with influence on the excitatory projection neurons. (Continued)

FIGURE 1—Cont’d The unconditional reflex blink pathway is via second- and third-order trigeminal projections to the premotor and motor neurons. Abbreviations: Ba, basket cell; cf, climbing fiber; Go, Golgi cell; Grc, granule cell; mf, mossy fibers; mfc, mossy fiber collateral to the cerebellar nuclei; NV, trigeminal nucleus; PC, Purkinje cell; pf, parallel fibers; RN, red nucleus; St, Stellate cell. (B) Lesions of cerebellar cortex in the regions that include the eyeblink microzones in lobule HVI produce changes of excitability in the cerebellar nuclei through disinhibition. Raised excitability in the AIP excitatory projection neurons is indicated (↑). Such lesions abolish or impair CR expression while enhancing the amplitudes of the UR, through disinhibition of the premotor and motor neurons (Gruart and Yeo, 1995; Yeo and Hardiman, 1992; Yeo et al., 1985b). Other accounts report the loss of correctly timed CRs, with the appearance of a short-latency response driven, it is suggested, by learning-related modifications of mossy fiber collateral synapses upon cerebellar nuclear neurons and revealed by the raised excitability of the nuclear neurons (Garcia et al., 1999; Medina et al., 2000, 2001; Perrett and Mauk, 1995; Perrett et al., 1993). A two-layer learning mechanism is proposed. (C) Application of the GABAA antagonists picrotoxin or gabazine to the cerebellar nuclei affects modulation of nuclear excitatory projection neurons by Purkinje cells and local inhibitory interneurons (*). Short-latency CS-related responses have been reported, suggesting plasticity of mossy fiber collateral synaptic to nuclear neurons and consistent with two-layer learning in the cerebellum (Garcia and Mauk, 1998; Ohyama et al., 2006). We question this interpretation. If GABAA antagonist effects are partial, then residual modulation from Purkinje cells acting on cerebellar nuclear neurons already disinhibited by the GABAA antagonist effects on the local interneuron (in) synaptic inputs could generate these responses. (D) In addition to the application of the GABAA antagonist picrotoxin with effects indicated (*), the AMPA receptor antagonist NBQX and the NMDA receptor antagonist APV are additionally infused. Application of these drugs, with effects indicated (þ), tests the suggestion that the mossy fiber collateral activity drives the short-latency responses. Application of the NBQX and APV suppressed the short-latency responses but the correctly timed CRs reappear (Ohyama et al., 2006). The AMPA and NMDA receptor blocks will reduce the excitability produced by picrotoxin toward the normal state. The result suggests there was only partial block of GABAA receptors and the short-latency responses were driven by residual modulation by Purkinje cells. In this case, plasticity at the mossy fiber collateral to nuclear neuron synapses is not demonstrated and a two-layer model is not confirmed.

4 Cerebellar Cortex, Cerebellar Nuclei and Inferior Olive

Our initial cortical lesion studies of NMR conditioning were viewed as controversial. Few were able to replicate the findings, leading to suggestions that the essential memory trace was exclusively nuclear (Lavond and Steinmetz, 1989; Lavond et al., 1987; McCormick and Thompson, 1984) or that the relevant cortical territory is elsewhere, notably in anterior lobe (Green and Steinmetz, 2005; Perrett and Mauk, 1995; Perrett et al., 1993). However, our follow-up lesion studies with reduced areas and with fiber-sparing techniques (Hardiman and Yeo, 1992; Yeo and Hardiman, 1992) and then with discrete reversible inactivations (Attwell et al., 1999, 2001, 2002a,b; Hardiman et al., 1996) further confirmed the identity and localization of the lobule HVI microzone in the rabbit. Guided by the principles established in Hesslow’s recording studies (Hesslow, 1994a,b), a preliminary single electrode recording study (Ivarsson et al., 2002) and then a significantly more detailed analysis with a multi-electrode array (Mostofi et al., 2010) confirmed microzone localizations deep in the medial wall of lobule HVI in the rabbit. Their electrophysiological identity and compartmentation relative to Zebrin II immunohistochemistry (Sanchez et al., 2002) with reference to modern accounts in the rat (Apps and Hawkes, 2009; Sugihara and Shinoda, 2004) suggested territories in the C3 zone and the closely related and more recently identified D0 zone. It seems likely that reported difficulties in confirming the role of cerebellar cortex in NMR and eyeblink conditioning may relate to difficulties in overcoming the problems identified in our early lesion studies and in electrophysiological mapping work (see Yeo and Hesslow, 1998), that the relevant microzones are small and at the depths of lobule HVI, especially in the rabbit. Encouragingly, however, there is now a developing consensus that this region contains the critical cortical regions for eyeblink conditioning in rabbits and other species. Transsynaptic retrograde tracing from the eyeblink muscles has revealed a population of Purkinje cells (PCs) in lobule HVI (Gonzalez-Joekes and Schreurs, 2012), and contemporary analyses using electrophysiological (Maiz et al., 2012) and molecular (Williams et al., 2012) techniques have switched their attention to lobule HVI with positive outcomes.

4 INACTIVATION EXPERIMENTS REVEAL ESSENTIAL ROLES FOR THE CEREBELLAR NUCLEI AND INFERIOR OLIVE IN THE ACQUISITION OF NMR AND EYEBLINK CONDITIONING Lesion studies were useful in identifying which OCN compartments are essential for eyeblink and NMR conditioning but they do not easily distinguish between effects upon acquisition, storage, and performance of the learned responses. And yet to make this distinction is essential for determining where and how the memories are stored. Indeed, the observation that cerebellar nuclear lesions produce some depression of excitability in the premotor pathways through a loss of tonic excitatory drive (Welsh and Harvey, 1989) raised the possibility that a significant component of the learning may be completely outside the cerebellum, though this appeared less

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likely if cerebellar cortical lesion effects are examined. Cortical lesions disinhibit reflex activity, as revealed by significantly larger amplitude unconditioned responses to the US and yet CRs are absent or highly attenuated following lesions of lobule HVI (Hesslow and Yeo, 2002; Yeo and Hardiman, 1992), a result entirely consistent with the suggestion that memory storage is certainly cerebellar and probably cortical. Nonetheless, a technique for dissociating acquisition, storage, and performance mechanisms was needed in order to localize storage mechanisms prior to determining mechanism. Localized reversible inactivations appeared to provide one such solution. Functional inactivation by discrete infusions of local anesthetic or receptor antagonist can be given to different levels in the OCN compartment at different stages of acquisition or consolidation. Subsequent testing in the performance phase, when pharmacological effects have dissipated and normal signaling conditions return, should reveal whether the inactivated region normally made an essential contribution to learning during the targeted learning phase. The first reversible inactivation study of NMR conditioning employed unilateral infusions of lidocaine in AIP (Welsh and Harvey, 1991) during NMR conditioning to an auditory tone CS. CS testing after the lidocaine effects had worn off revealed high levels of CRs, suggesting that conditioning is independent of normal AIP function and could, indeed, be cortical. However, these subjects had previously also received conditioning to a light CS using the same interstimulus interval (ISI) between CS and US onsets as for the tone CS pairings. Although this design provided an excellent opportunity to test the efficacy of lidocaine inactivation using probe trials with the light CS as the lidocaine was infused, it simultaneously maximized the opportunity for general transfer effects, where CRs to a second CS can be established very easily indeed, sometimes in a single trial, if the ISIs are matched. Furthermore, since tones are generally far more salient than lights as a CS in eyeblink conditioning (Marlatt et al., 1966), the experimental design may have underestimated the efficacy of lidocaine block for tone CS conditioning and provided maximum opportunity for general transfer effects. Subsequent reversible inactivation experiments used local infusions of the GABAA agonist muscimol (Hardiman et al., 1996; Krupa et al., 1993; Yeo et al., 1997) or lidocaine (Nordholm et al., 1993) to suppress activity in the AIP and all produced substantial impairments of acquisition or extinction (Hardiman et al., 1996; Ramnani and Yeo, 1996). For some, these findings were evidence that the essential memory trace is stored within the cerebellar nuclei (Krupa et al., 1993). However, there were important alternative interpretations. The cerebellar cortex, cerebellar nuclei, and inferior olive are interconnected within an OCN compartment (see Fig. 1A). The inferior olive receives a GABAergic projection from projection neurons in the cerebellar nuclei (Nelson and Mugnaini, 1988) that provides an additional element within the compartment to form an OCN loop and exerts an inhibitory influence over inferior olive activity (Andersson et al., 1988; Hesslow, 1986; Svensson et al., 2006). Although its neurons are excitatory, inferior olive activity exerts a simple spike rate-reducing influence on cortical PC activity that may be related to the pause in firing that follows CF-evoked

5 Inferior Olive Function in NMR and Eyeblink Conditioning

complex spike activity in PCs, or to cortical interneuron effects (Montarolo et al., 1982). Various experimental approaches have manipulated activity around the OCN loop. Stimulating CFs suppresses simple spike firing in PCs (Rawson and Tilokskulchai, 1981), whereas silencing CF activity increases PC firing rates (Cerminara and Rawson, 2004; Montarolo et al., 1982). Blocking or stimulating the nucleo-olivary pathway decreases or increases PC excitability, respectively (Bengtsson et al., 2004; Svensson et al., 2006). Blocking glutamatergic afferent input to the olive using localized application of the AMPA receptor antagonist NBQX reduces firing rate in the cerebellar nuclei via increased PC activity (Zbarska et al., 2008). These findings confirm the dynamic nature of the connections within an OCN compartment. Excitability changes at one level in the compartment have influences upon excitability at the other levels. In analyses of cerebellar function in conditioning, it is clear that inactivation of the AIP has powerful effects upon inferior olive excitability, so these inactivations are better seen as experimental disturbances of the entire OCN compartment (Hardiman et al., 1996; Hesslow and Yeo, 2002; Ramnani and Yeo, 1996). The effects of AIP inactivation upon NMR conditioning should not be taken as evidence for a critical learning and storage mechanism in AIP but as evidence that these processes are within the OCN compartment. In most of the AIP inactivation experiments, there was always some evidence of savings in the relearning stages following training under the AIP activity block. It was difficult to be sure whether these might be general savings, facilitated by the additional exposure to the experimental situation and the training stimuli themselves, or whether there were genuine low levels of sub-threshold learning during AIP blockade. This possibility was tested with a two-stage experiment, using a different ISI before and during the AIP inactivation and counterbalanced for their order of presentation (Yeo et al., 1997). The design allowed the efficacy of inactivation to be monitored throughout, and very low levels of learning were revealed to have occurred during AIP inactivation. The findings may be taken as evidence for residual cortical function during nuclear inactivation, as evidence for a contralateral cerebellar contribution to learning or for a small, entirely extracerebellar component to NMR conditioning.

5 INFERIOR OLIVE FUNCTION IN NMR AND EYEBLINK CONDITIONING Central to many theories of cerebellar learning is that the inferior olive conveys a teaching signal (Albus, 1971; Gilbert, 1974; Ito, 1982; Marr, 1969). The role of the inferior olive in NMR conditioning was established by inactivating it using lidocaine infusions during training trials (Welsh and Harvey, 1998). Learning was prevented, confirming the idea that normal olivary activity is essential in acquisition of learning and, as had been suggested earlier based on evidence from an olivary lesion study (Yeo et al., 1986), it is also essential for the expression of learned responses.

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Therefore, these findings lead to an important question. Is the inferior olive so dominant in its regulatory role on cerebellar cortical and nuclear activity, as indicated in earlier studies (Manil et al., 1975; Montarolo et al., 1982), that acquisition deficits need not reflect the transmission of critical teaching instructions, as suggested in the early and contemporary theories of cerebellar function (Albus, 1971; Dean and Porrill, 2010; Hesslow and Yeo, 2002; Ito, 1982; Marr, 1969; Miles and Lisberger, 1981; Ohyama et al., 2002)? Instead, does olivary inactivation simply close down the cerebellar associative network and prevent acquisition through a general loss of function? These questions were addressed in a key paper that used a different approach (Medina et al., 2002). Instead of directly inactivating the olive, the AMPA receptor antagonist NBQX was used to block glutamatergic transmission to the olivary neurons from its afferents. For the DAO and its engagement in eyeblink conditioning, these would include second- and third-order trigeminal information related to the cornea and periocular regions (Van Ham and Yeo, 1992). Such an inactivation would spare intrinsic activity of the olivary neurons and permit relatively normal levels of feedback inhibition via the nucleo-olivary pathway. Importantly, this pharmacological inactivation of olivary afferents prevented acquisition and, in previously trained subjects, produced extinction during paired CS–US trials that would, under normal conditions, sustain CR production at asymptotic levels. In effect, the treatment was equivalent to turning off the reinforcing, or teaching, signal while maintaining some baseline level of activity in the CF inputs to the cortex. The finding is entirely consistent with the suggestion that the inferior olive does, indeed, supply a teaching signal to the cerebellum (see Fig. 1A). The idea that a CF teaching signal might be downregulated as learning reached asymptote was first suggested by Hesslow and colleagues when they originally described nucleo-olivary inhibition mediated via the nucleo-olivary pathway (Andersson et al., 1988; Hesslow, 1986; Svensson et al., 2006). They concluded that not only would CR-related activity in the deep cerebellar nuclei drive an inhibitory modulation at the olive around the time of US delivery, but it would also provide a mechanism for Kamin blocking, a key inhibitory mechanism in classical conditioning. Hesslow and Ivarsson (1996) then formally demonstrated CR-related downregulation of the CF response in the eyeblink microzones during eyeblink conditioning, confirming their earlier predictions. Additional support for the suggested regulatory role of the nucleo-olivary pathway came from related inactivation experiments. Infusing the GABAA antagonist picrotoxin into the inferior olive prevented Kamin blocking (Kim et al., 1998) and prevented extinction during unpaired CS presentations (Medina et al., 2002). In summary, electrophysiology and reversible inactivation or pharmacological manipulations of the inferior olive have given some important insights into the nature of the teaching signal in cerebellar learning and its modulation. However, as with understanding the effects of cerebellar nuclear inactivations, the interpretation of this work is not without difficulties because of activity changes around the OCN loop. This has been clearly revealed in recent analyses of olivary inactivation and disturbance of glutamatergic transmission to the olive with simultaneous

6 Cerebellar Cortex Function in NMR and Eyeblink Conditioning

electrophysiological monitoring of nuclear activity and motor output (Bracha et al., 2009; Zbarska et al., 2008). Such analyses reveal that pharmacological manipulations such as these can produce tonic shifts in activity that must be carefully monitored and considered when drawing conclusions from experiments that rely entirely upon behavioral output measures.

6 CEREBELLAR CORTEX FUNCTION IN NMR AND EYEBLINK CONDITIONING The essence of early influential theories was that the unique anatomy and functional properties of the cerebellar cortex make it a prime candidate for learning (Albus, 1971; Gilbert, 1974; Marr, 1969). All of these models propose a modification of granule cell synaptic input upon PCs, under the instructive influence of the CF input, with later studies identifying parallel fiber to PC long-term depression as a leading candidate mechanism. More recent work has characterized a large number of other candidate plasticities for learning (De Zeeuw and Yeo, 2005; Hansel et al., 2001) that new models can incorporate (Dean et al., 2010). In order to try to establish which of these plasticities established in vitro have a real relationship with identified behavioral changes in vivo, some preliminary analysis is needed. A critical question is whether normal cortical function is essential for learning. There are potential problems in the interpretation of the results of manipulating cerebellar nuclei and inferior olive function during learning because of effects upon the OCN loop. However, it was important to establish whether inactivation of the relevant cortical eyeblink microzones would also prevent acquisition. If, in the unlikely event, it did not, then cortical mechanisms of plasticity would be less important and plasticity within the cerebellar nuclei would become a strong candidate mechanism for behavioral learning. Our earlier lesion studies had identified a region in the depths of medial lobule HVI as critical for NMR conditioning but that finding was controversial because few had replicated the findings. In order to verify the localization of the essential cortical territories for an acquisition study, local infusions of the AMPA receptor antagonist CNQX were applied to lobule HVI and complete suppression of established CRs was obtained (Attwell et al., 1999), verifying that these cortical microzones are essential for CR expression. Autoradiography of tritiated CNQX confirmed the location and spread of the drug and ruled out effects upon the cerebellar nuclei. A more recent study using much smaller infusion volumes targeted to an eyeblink microzone by simultaneous electrophysiological recording showed similar suppressions of established CRs with accompanying reduction of CS-related PC activity (Mostofi et al., 2010). CNQX infusions will block excitatory transmission to PCs in the cerebellar cortex but they maintain baseline activity due to intrinsic properties. Thus, general effects due to changes in OCN loop activity might be less than with some other manipulations. It would be inappropriate to conclude that the loss of CR production was entirely due to manipulation of cortical function only, but it was possible.

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Importantly, acquisition of NMR conditioning was also prevented by CNQX infusions confined to critical regions in lobule HVI (Attwell et al., 2002a). This experiment completes a set of observations on the functions of the cerebellum in NMR conditioning and allows us to conclude that all levels in the OCN loop must function normally for NMR conditioning to be established. What types of analysis could now reveal how the plasticities underlying learning are distributed within the OCN loop?

7 DISTRIBUTION OF PLASTICITY AT CEREBELLAR CORTICAL AND CEREBELLAR NUCLEAR, OR BRAINSTEM, LEVELS There is now strong evidence for a distribution of plasticities across the cortical flocculus and brainstem vestibular levels in adaptation of the vestibulo-ocular reflex. Instructive signals from the CF input at the cortical level and from the PC input at the vestibular level can contribute to a distribution of learning (Boyden et al., 2004; Ito, 1982; Koekkoek et al., 1997; Lisberger, 1994; Lisberger and Fuchs, 1978a,b; Lisberger et al., 1984; Miles and Lisberger, 1981; Miles et al., 1980; Nagao, 1983, 1989). It is suggested that the distribution of vestibular afferents to both cortical and brainstem levels allows each to enter into an association dealing with different aspects of the task (Porrill and Dean, 2007). The short-latency, brainstem vestibular pathway can deal with the relatively linear plant demands that obtain at high movement frequencies through a simple gain change whereas the high capacity of the cortical network can deal with the complex plant properties that obtain for low movement frequencies, where the latency penalties for the longer path length are less important. But is this two-layer model representative of all forms of cerebellumdependent learning, as has been suggested (Raymond et al., 1996)? Two early cerebellar lesion studies claimed that the two-layer model may also apply for eyeblink conditioning in the rabbit (Perrett and Mauk, 1995; Perrett et al., 1993). Large lesions of the anterior lobe produced a significant change in the topography of the CRs. The normal, adaptively timed CR with a delayed onset and a peak amplitude timed to match the ISI (and thus occur around US onset) was lost. Instead, the subjects gave responses with short onset latency and a considerably shorter latency-to-peak. It was suggested that the cortical lesion had removed a cortical associative network that is specially responsible for the adaptive timing of the CR and revealed an underlying plasticity at the mossy fiber collateral input to the cerebellar nuclei—analogous to that at vestibular afferent inputs to the vestibular nuclei for the VOR (Medina et al., 2000, 2001, 2002; Perrett and Mauk, 1995; Perrett et al., 1993) (see Fig. 1B). Other studies using lesions directed to the lobule HVI microzones had not revealed these short-latency responses (SLRs) (Gruart and Yeo, 1995; Yeo and Hardiman, 1992). Further analysis of the SLRs was pharmacological. Gradually incrementing doses of the GABAA antagonists picrotoxin or gabazine applied to the AIP were shown, at sufficient level, to reveal SLRs quite similar to those seen with anterior lobe lesions. It was suggested that the GABAA antagonists provided a functional disconnection of

7 Plasticity at Cerebellar Cortical and Cerebellar Nuclear Levels

the cerebellar cortex from the cerebellar nuclei, and the SLRs were produced by the same putative cellular substrate: potentiated mossy fiber collateral synaptic drive to the cerebellar nuclei (Garcia and Mauk, 1998; Ohyama and Mauk, 2001; Ohyama et al., 2003, 2006) (see Fig. 1C). Again, it proved difficult to replicate these findings. In the main, infusions of picrotoxin into the cerebellar nuclei completely abolished conditioned eyeblink responses (Aksenov et al., 2004; Parker et al., 2009) and conditioned NMRs (Attwell et al., 2002a; Bao et al., 2002) as well as conditioned eyelid EMG responses (Attwell et al., 2002a). Where there were residual responses, or as the drug effects wore off, CR amplitudes were reduced and onset latencies were correspondingly longer (Attwell et al., 2002a). If, however, the unmasking of SLRs driven by a putative mossy fiber collateral plasticity were especially sensitive to the exact level of GABAA antagonist application, then this might explain the failure to replicate. Would it be possible to reveal some aspect of the mossy fiber collateral drive to the cerebellar nuclei without cortical disconnection? Surprisingly, conditioned NM responses and eyelid EMG responses were completely unaffected by intranuclear application of the AMPA receptor antagonist CNQX (Attwell et al., 2002b) or the fast glutamatergic transmission antagonist DGG (Aksenov et al., 2005). However, NMDA receptors have been shown to be important for dynamic synaptic signaling at cerebellar nuclear neurons (Anchisi et al., 2001) so CNQX or DGG infusions alone would be insufficient fully to prevent glutamatergic signaling. Consistent with the view that mossy fiber signaling to the cerebellar nuclei depends upon AMPA and NMDA receptor-mediated transmission, intranuclear infusions of the NMDA receptor antagonist AP5 only mildly impaired acquisition and performance of NMR conditioning in rabbits (Chen and Steinmetz, 2000). A full analysis of the genesis of SLRs, as a means to demonstrate that there is a two-level plasticity in NMR and eyeblink conditioning, requires that SLRs are first revealed by GABAA antagonist application to the cerebellar nuclei and then blocked by combined application of AMPA receptor and NMDA receptor antagonists. This exact procedure was reported by Ohyama and colleagues (Ohyama et al., 2006) with startling results (see Fig. 1D). Picrotoxin application revealed SLRs as before and the combined AMPA/NMDA receptor antagonist application clearly blocked them. This evidence seems to support the suggestion that there is, indeed, a learning-related modified mossy fiber drive to the cerebellar nuclei, as claimed. However, there were additional effects seen in the study that are entirely inconsistent with this claim. Although SLRs were removed following the AMPA/NMDA receptor antagonist application, correctly timed, adaptive CRs reemerged as the SLRs declined (Ohyama et al., 2006, fig. 3). The return of normal CRs needs explanation. If all of the blocks are complete, then the normal CRs must be generated from outside the cerebellum— a suggestion at odds with all of the previous work leading up to this study. Alternatively, the original picrotoxin block might be incomplete, so the returning CRs are generated by PC activity. This suggestion is consistent with findings in a study (Parker et al., 2009) in which picrotoxin or gabazine was incrementally infused into

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the AIP. Low doses of either drug led to short-latency eyeblink responses, while higher doses abolished the long-latency CRs and the SLRs. What conditions could lead to the generation of SLRs from low-dose GABAA antagonist application in the cerebellar nuclei? Importantly, there are several types of neuron in the cerebellar nuclei, including local GABAergic interneurons (Uusisaari and Knopfel, 2012) (see Fig. 1A, C, and D). Infusions of picrotoxin or gabazine would also affect this local inhibition, leading to increased excitability of the excitatory projection CN neurons that are also modulated by PC input. Tonic increases in nuclear excitability following picrotoxin or gabazine infusions are well known (Aksenov et al., 2004; Chen and Evinger, 2006; Parker et al., 2009). We suggest that there is a window of excitability, seen following small doses of a GABAA antagonist, where nuclear activity may be raised by effects upon local interneurons while PC to cerebellar nuclear synapses are only partially blocked. It might appear unlikely that the significantly shorter latency of SLRs could be accounted for by learning-related PC activity that, under normal circumstances, would be generating behavioral responses 100 ms or more later than the onset of SLRs. However, electrophysiological recording studies of identified, eyeblink microzone PC activity during conditioning trials (Jirenhed et al., 2007) show that the conditioning-related pause in simple spike activity begins at a remarkably short latency of around 50 ms. This pause onset is fast enough to act as a disinhibitory signal upon a cerebellar nuclear pool already made hyperexcitable through disinhibition of local inhibitory interneurons and so to generate the SLRs with reported latencies of around 100 ms. If raised levels of excitability in the cerebellar nuclei coupled with incomplete block of PC modulation explain the SLRs that have formed the main experimental support for a two-layer plasticity mechanism in NMR and eyeblink conditioning, then this would be evidence that such learning depends significantly upon cortical plasticity with little or no plasticity at the level of the nuclei (see Fig. 1D).

8 CORTICAL PLASTICITY IN NMR AND EYEBLINK CONDITIONING Recent work from our own laboratory has tested the hypothesis that cortical mechanisms are dominant in NMR conditioning and that a significant component of the memory trace is stored within the cortex. To localize memory storage, we have used posttraining inactivations of the cerebellar cortex or the cerebellar nuclei to target consolidation processes in a time window when they are expected to occur. Muscimol inactivation of the eyeblink microzones in lobule HVI immediately after training sessions substantially blocked consolidation of NMR conditioning but similar inactivations of the cerebellar nuclei were without effect upon consolidation (Attwell et al., 2002a). By adjusting the delay between the end of the training sessions and cortical inactivation, it was shown that consolidation processes began after a short delay but were substantially complete within 1–2 h (Cooke et al., 2004). It was

References

important to establish that the consolidation losses directly reflected the experimental block of cortical function, rather than the raised nuclear excitability that the cortical inactivation produces. A double inactivation, of cortex and nuclei, controlled for this possibility and confirmed that consolidation processes for NMR conditioning are substantially cortical (Kellett et al., 2010).

9 CONCLUSIONS This review of evidence has focused on a simple question: Where are cerebellumdependent memories stored? We suggest that there should be no single answer to this question, as the distribution of the essential plasticity will be strongly related to the nature of the task. In line with a model proposed by Porrill and Dean (2007) that plasticity in the VOR distributes at cortical or brainstem level dependent upon the frequency components of the learned movement, our analysis of conditioned NM responses (Kellett et al., 2010; Lepora et al., 2010) suggests that NMR and eyeblink conditioning has relatively low frequency components and would, therefore, depend primarily upon cortical mechanisms. None of the evidence presented rules out a subsidiary cerebellar nuclear component to NMR eyeblink conditioning, and there is recent structural evidence that some may be present (Boele et al., 2013; Kleim et al., 2002). We suggest that NMR conditioning presents a special case of cerebellar cortical dependence in motor learning because the learned responses are of relatively low frequency and the behavior receives little or no motor cortical influence. Other learning models, especially in other species, may show progressive departures from relative independence from motor cortical modulation and cerebellar nuclear plasticity.

Acknowledgment This work was supported by a BBSRC studentship to M. L.

References Aksenov, D., Serdyukova, N., Irwin, K., Bracha, V., 2004. GABA neurotransmission in the cerebellar interposed nuclei: involvement in classically conditioned eyeblinks and neuronal activity. J. Neurophysiol. 91, 719–727. Aksenov, D.P., Serdyukova, N.A., Bloedel, J.R., Bracha, V., 2005. Glutamate neurotransmission in the cerebellar interposed nuclei: involvement in classically conditioned eyeblinks and neuronal activity. J. Neurophysiol. 93, 44–52. Albus, J.S., 1971. A theory of cerebellar function. Math. Biosci. 10, 25–61. Anchisi, D., Scelfo, B., Tempia, F., 2001. Postsynaptic currents in deep cerebellar nuclei. J. Neurophysiol. 85, 323–331.

95

96

CHAPTER 4 Distribution of Plasticity in Motor Learning

Andersson, G., Garwicz, M., Hesslow, G., 1988. Evidence for a GABA-mediated cerebellar inhibition of the inferior olive in the cat. Exp. Brain Res. 72, 450–456. Apps, R., Hawkes, R., 2009. Cerebellar cortical organization: a one-map hypothesis. Nat. Rev. Neurosci. 10, 670–681. Attwell, P.J., Rahman, S., Ivarsson, M., Yeo, C.H., 1999. Cerebellar cortical AMPA-kainate receptor blockade prevents performance of classically conditioned nictitating membrane responses. J. Neurosci. 19, RC45. Attwell, P.J., Rahman, S., Yeo, C.H., 2001. Acquisition of eyeblink conditioning is critically dependent on normal function in cerebellar cortical lobule HVI. J. Neurosci. 21, 5715–5722. Attwell, P.J., Cooke, S.F., Yeo, C.H., 2002a. Cerebellar function in consolidation of a motor memory. Neuron 34, 1011–1020. Attwell, P.J., Ivarsson, M., Millar, L., Yeo, C.H., 2002b. Cerebellar mechanisms in eyeblink conditioning. Ann. N.Y. Acad. Sci. 978, 79–92. Bao, S., Chen, L., Kim, J.J., Thompson, R.F., 2002. Cerebellar cortical inhibition and classical eyeblink conditioning. Proc. Natl. Acad. Sci. U.S.A. 99, 1592–1597. Bengtsson, F., Svensson, P., Hesslow, G., 2004. Feedback control of Purkinje cell activity by the cerebello-olivary pathway. Eur. J. Neurosci. 20, 2999–3005. Boele, H.J., Koekkoek, S.K., De Zeeuw, C.I., Ruigrok, T.J., 2013. Axonal sprouting and formation of terminals in the adult cerebellum during associative motor learning. J. Neurosci. 33, 17897–17907. Boyden, E.S., Katoh, A., Raymond, J.L., 2004. Cerebellum-dependent learning: the role of multiple plasticity mechanisms. Annu. Rev. Neurosci. 27, 581–609. Bracha, V., 2004. Role of the cerebellum in eyeblink conditioning. Prog. Brain Res. 143, 331–339. Bracha, V., Zbarska, S., Parker, K., Carrel, A., Zenitsky, G., Bloedel, J.R., 2009. The cerebellum and eye-blink conditioning: learning versus network performance hypotheses. Neuroscience 162, 787–796. Cerminara, N.L., Rawson, J.A., 2004. Evidence that climbing fibers control an intrinsic spike generator in cerebellar Purkinje cells. J. Neurosci. 24, 4510–4517. Chen, F.P., Evinger, C., 2006. Cerebellar modulation of trigeminal reflex blinks: interpositus neurons. J. Neurosci. 26, 10569–10576. Chen, G., Steinmetz, J.E., 2000. Intra-cerebellar infusion of NMDA receptor antagonist AP5 disrupts classical eyeblink conditioning in rabbits. Brain Res. 887, 144–156. Clark, G.A., McCormick, D.A., Lavond, D.G., Thompson, R.F., 1984. Effects of lesions of cerebellar nuclei on conditioned behavioral and hippocampal neuronal responses. Brain Res. 291, 125–136. Cooke, S.F., Attwell, P.J., Yeo, C.H., 2004. Temporal properties of cerebellar-dependent memory consolidation. J. Neurosci. 24, 2934–2941. Dean, P., Porrill, J., 2010. The cerebellum as an adaptive filter: a general model? Funct. Neurol. 25, 173–180. Dean, P., Porrill, J., Ekerot, C.F., Jorntell, H., 2010. The cerebellar microcircuit as an adaptive filter: experimental and computational evidence. Nat. Rev. Neurosci. 11, 30–43. Delgado-Garcia, J.M., Evinger, C., Escudero, M., Baker, R., 1990. Behavior of accessory abducens and abducens motoneurons during eye retraction and rotation in the alert cat. J. Neurophysiol. 64, 413–422. De Zeeuw, C.I., Yeo, C.H., 2005. Time and tide in cerebellar memory formation. Curr. Opin. Neurobiol. 15, 667–674.

References

Disterhoft, J.F., Quinn, K.J., Weiss, C., Shipley, M.T., 1985. Accessory abducens nucleus and conditioned eye retraction/nictitating membrane extension in rabbit. J. Neurosci. 5, 941–950. Garcia, K.S., Mauk, M.D., 1998. Pharmacological analysis of cerebellar contributions to the timing and expression of conditioned eyelid responses. Neuropharmacology 37, 471–480. Garcia, K.S., Steele, P.M., Mauk, M.D., 1999. Cerebellar cortex lesions prevent acquisition of conditioned eyelid responses. J. Neurosci. 19, 10940–10947. Gilbert, P.F., 1974. A theory of memory that explains the function and structure of the cerebellum. Brain Res. 70, 1–18. Gonzalez-Joekes, J., Schreurs, B.G., 2012. Anatomical characterization of a rabbit cerebellar eyeblink premotor pathway using pseudorabies and identification of a local modulatory network in anterior interpositus. J. Neurosci. 32, 12472–12487. Gormezano, I., 1972. Investigations of defense and reward conditioning in the rabbit. In: Black, A.H., Prokasy, W.F. (Eds.), Classical Conditioning II: Current Research and Theory. Appleton-Century-Crofts, New York, pp. 151–181. Gormezano, I., Schneiderman, N., Deaux, E., Fuentes, I., 1962. Nictitating membrane: classical conditioning and extinction in the albino rabbit. Science 138, 33–34. Gormezano, I., Kehoe, E.J., Marshall-Goodell, B., 1983. Twenty years of classical conditioning research with the rabbit. In: Sprague, J.M., Epstein, A.N. (Eds.), Progress in Psychobiology and Physiological Psychology. Academic Press, New York, pp. 197–275. Green, J.T., Steinmetz, J.E., 2005. Purkinje cell activity in the cerebellar anterior lobe after rabbit eyeblink conditioning. Learn. Mem. 12, 260–269. Gruart, A., Yeo, C.H., 1995. Cerebellar cortex and eyeblink conditioning: bilateral regulation of conditioned responses. Exp. Brain Res. 104, 431–448. Gruart, A., Pastor, A.M., Armengol, J.A., Delgado-Garcia, J.M., 1997. Involvement of cerebellar cortex and nuclei in the genesis and control of unconditioned and conditioned eyelid motor responses. Prog. Brain Res. 114, 511–528. Hansel, C., Linden, D.J., D’Angelo, E., 2001. Beyond parallel fiber LTD: the diversity of synaptic and non-synaptic plasticity in the cerebellum. Nat. Neurosci. 4, 467–475. Hardiman, M.J., Yeo, C.H., 1992. The effect of kainic acid lesions of the cerebellar cortex on the conditioned nictitating membrane response in the rabbit. Eur. J. Neurosci. 4, 966–980. Hardiman, M.J., Ramnani, N., Yeo, C.H., 1996. Reversible inactivations of the cerebellum with muscimol prevent the acquisition and extinction of conditioned nictitating membrane responses in the rabbit. Exp. Brain Res. 110, 235–247. Hesslow, G., 1986. Inhibition of inferior olivary transmission by mesencephalic stimulation in the cat. Neurosci. Lett. 63, 76–80. Hesslow, G., 1994a. Inhibition of classically conditioned eyeblink responses by stimulation of the cerebellar cortex in the decerebrate cat. J. Physiol. 476, 245–256. Hesslow, G., 1994b. Correspondence between climbing fibre input and motor output in eyeblink-related areas in cat cerebellar cortex. J. Physiol. Lond. 476, 229–244. Hesslow, G., Ivarsson, M., 1996. Inhibition of the inferior olive during conditioned responses in the decerebrate ferret. Exp. Brain Res. 110, 36–46. Hesslow, G., Yeo, C.H., 2002. The functional anatomy of skeletal conditioning. In: Moore, J.W. (Ed.), A Neuroscientist’s Guide to Classical Conditioning. Springer, New York, pp. 86–146. Hesslow, G., Svensson, P., Ivarsson, M., 1999. Learned movements elicited by direct stimulation of cerebellar mossy fiber afferents. Neuron 24, 179–185.

97

98

CHAPTER 4 Distribution of Plasticity in Motor Learning

Ito, M., 1982. Cerebellar control of the vestibulo-ocular reflex—around the flocculus hypothesis. Annu. Rev. Neurosci. 5, 275–296. Ivarsson, M., Millar, L., Yeo, C.H., 2002. Electrophysiological mapping of eyeblink microzones in the rabbit. Ann. N.Y. Acad. Sci. 978, 520. Jirenhed, D.A., Bengtsson, F., Hesslow, G., 2007. Acquisition, extinction, and reacquisition of a cerebellar cortical memory trace. J. Neurosci. 27, 2493–2502. Kassardjian, C.D., Tan, Y.F., Chung, J.Y., Heskin, R., Peterson, M.J., Broussard, D.M., 2005. The site of a motor memory shifts with consolidation. J. Neurosci. 25, 7979–7985. Kellett, D.O., Fukunaga, I., Chen-Kubota, E., Dean, P., Yeo, C.H., 2010. Memory consolidation in the cerebellar cortex. PLoS One 5, e11737. Kim, J.J., Krupa, D.J., Thompson, R.F., 1998. Inhibitory cerebello-olivary projections and blocking effect in classical conditioning. Science 279, 570–573. Kleim, J.A., Freeman, J.H., Bruneau, R., Nolan, B.C., Cooper, N.R., Zook, A., Walters, D., 2002. Synapse formation is associated with memory storage in the cerebellum. Proc. Natl. Acad. Sci. U.S.A. 99, 13228–13231. Koekkoek, S.K., v Alphen, A.M., vd Burg, J., Grosveld, F., Galjart, N., De Zeeuw, C.I., 1997. Gain adaptation and phase dynamics of compensatory eye movements in mice. Genes Funct. 1, 175–190. Krupa, D.J., Thompson, J.K., Thompson, R.F., 1993. Localization of a memory trace in the mammalian brain. Science 260, 989–991. Lavond, D.G., Steinmetz, J.E., 1989. Acquisition of classical conditioning without cerebellar cortex. Behav. Brain Res. 33, 113–164. Lavond, D.G., Steinmetz, J.E., Yokaitis, M.H., Thompson, R.F., 1987. Reacquisition of classical conditioning after removal of cerebellar cortex. Exp. Brain Res. 67, 569–593. Lepora, N.F., Porrill, J., Yeo, C.H., Dean, P., 2010. Sensory prediction or motor control? Application of marr-albus type models of cerebellar function to classical conditioning. Front. Comput. Neurosci. 4, 140. Lincoln, J.S., McCormick, D.A., Thompson, R.F., 1982. Ipsilateral cerebellar lesions prevent learning of the classically conditioned nictitating membrane/eyelid response. Brain Res. 242, 190–193. Lisberger, S.G., 1994. Neural basis for motor learning in the vestibuloocular reflex of primates. III. Computational and behavioral analysis of the sites of learning. J. Neurophysiol. 72, 974–998. Lisberger, S.G., Fuchs, A.F., 1978a. Role of primate flocculus during rapid behavioral modification of vestibuloocular reflex. I. Purkinje cell activity during visually guided horizontal smooth-pursuit eye movements and passive head rotation. J. Neurophysiol. 41, 733–763. Lisberger, S.G., Fuchs, A.F., 1978b. Role of primate flocculus during rapid behavioral modification of vestibuloocular reflex. II. Mossy fiber firing patterns during horizontal head rotation and eye movement. J. Neurophysiol. 41, 764–777. Lisberger, S.G., Miles, F.A., Zee, D.S., 1984. Signals used to compute errors in monkey vestibuloocular reflex: possible role of flocculus. J. Neurophysiol. 52, 1140–1153. Lisberger, S.G., Pavelko, T.A., Bronte-Stewart, H.M., Stone, L.S., 1994. Neural basis for motor learning in the vestibuloocular reflex of primates. II. Changes in the responses of horizontal gaze velocity Purkinje cells in the cerebellar flocculus and ventral paraflocculus. J. Neurophysiol. 72, 954–973. Mackintosh, N.J., 1974. Classical conditioning: basic operations. In: Mackintosh, N.J. (Ed.), The Psychology of Animal Learning. Academic Press, Hertfordshire, UK, pp. 8–77.

References

Maiz, J., Karakossian, M.H., Pakaprot, N., Robleto, K., Thompson, R.F., Otis, T.S., 2012. Prolonging the postcomplex spike pause speeds eyeblink conditioning. Proc. Natl. Acad. Sci. U.S.A. 109, 16726–16730. Manil, J., Colin, F., Voogd, J., 1975. Proceedings: role of the long ascending pathways in the averaged cerebral somatosensory evoked potentials in the unrestrained dutch rabbit. Arch. Int. Physiol. Biochim. 83, 335–336. Marlatt, G.A., Lilie, D., Selvidge, B.D., Sipes, M.D., Gormezano, I., 1966. Cross-modal generalization to tone and light in human eyelid conditioning. Psychon. Sci. 5, 59–60. Marr, D., 1969. A theory of cerebellar cortex. J. Physiol. 202, 437–470. McCormick, D.A., Thompson, R.F., 1984. Cerebellum: essential involvement in the classically conditioned eyelid response. Science 223, 296–299. McCormick, D.A., Guyer, P.E., Thompson, R.F., 1982. Superior cerebellar peduncle lesions selectively abolish the ipsilateral classically conditioned nictitating membrane/eyelid response of the rabbit. Brain Res. 244, 347–350. Medina, J.F., Lisberger, S.G., 2008. Links from complex spikes to local plasticity and motor learning in the cerebellum of awake-behaving monkeys. Nat. Neurosci. 11, 1185–1192. Medina, J.F., Nores, W.L., Ohyama, T., Mauk, M.D., 2000. Mechanisms of cerebellar learning suggested by eyelid conditioning. Curr. Opin. Neurobiol. 10, 717–724. Medina, J.F., Garcia, K.S., Mauk, M.D., 2001. A mechanism for savings in the cerebellum. J. Neurosci. 21, 4081–4089. Medina, J.F., Nores, W.L., Mauk, M.D., 2002. Inhibition of climbing fibres is a signal for the extinction of conditioned eyelid responses. Nature 416, 330–333. Miles, F.A., Lisberger, S.G., 1981. Plasticity in the vestibulo-ocular reflex: a new hypothesis. Annu. Rev. Neurosci. 4, 273–299. Miles, F.A., Braitman, D.J., Dow, B.M., 1980. Long-term adaptive changes in primate vestibuloocular reflex. IV. Electrophysiological observations in flocculus of adapted monkeys. J. Neurophysiol. 43, 1477–1493. Montarolo, P.G., Palestini, M., Strata, P., 1982. The inhibitory effect of the olivocerebellar input on the cerebellar Purkinje cells in the rat. J. Physiol. Lond. 332, 187–202. Mostofi, A., Holtzman, T., Grout, A.S., Yeo, C.H., Edgley, S.A., 2010. Electrophysiological localization of eyeblink-related microzones in rabbit cerebellar cortex. J. Neurosci. 30, 8920–8934. Nagao, S., 1983. Effects of vestibulocerebellar lesions upon dynamic characteristics and adaptation of vestibulo-ocular and optokinetic responses in pigmented rabbits. Exp. Brain Res. 53, 36–46. Nagao, S., 1989. Behavior of floccular Purkinje cells correlated with adaptation of vestibuloocular reflex in pigmented rabbits. Exp. Brain Res. 77, 531–540. Nelson, B.J., Mugnaini, E., 1988. The rat inferior olive as seen with immunostaining for glutamate decarboxylase. Anat. Embryol. (Berl.) 179, 109–127. Nordholm, A.F., Thompson, J.K., Dersarkissian, C., Thompson, R.F., 1993. Lidocaine infusion in a critical region of cerebellum completely prevents learning of the conditioned eyeblink response. Behav. Neurosci. 107, 882–886. Oakley, D.A., Russell, I.S., 1972. Neocortical lesions and Pavlovian conditioning. Physiol. Behav. 8, 915–926. Oakley, D.A., Russell, I.S., 1977. Subcortical storage of Pavlovian conditioning in the rabbit. Physiol. Behav. 18, 931–937.

99

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CHAPTER 4 Distribution of Plasticity in Motor Learning

Ohyama, T., Mauk, M., 2001. Latent acquisition of timed responses in cerebellar cortex. J. Neurosci. 21, 682–690. Ohyama, T., Medina, J.F., Nores, W.L., Mauk, M.D., 2002. Trying to understand the cerebellum well enough to build one. Ann. N.Y. Acad. Sci. 978, 425–438. Ohyama, T., Nores, W.L., Mauk, M.D., 2003. Stimulus generalization of conditioned eyelid responses produced without cerebellar cortex: implications for plasticity in the cerebellar nuclei. Learn. Mem. 10, 346–354. Ohyama, T., Nores, W.L., Medina, J.F., Riusech, F.A., Mauk, M.D., 2006. Learning-induced plasticity in deep cerebellar nucleus. J. Neurosci. 26, 12656–12663. Okamoto, T., Endo, S., Shirao, T., Nagao, S., 2011a. Role of cerebellar cortical protein synthesis in transfer of memory trace of cerebellum-dependent motor learning. J. Neurosci. 31, 8958–8966. Okamoto, T., Shirao, T., Shutoh, F., Suzuki, T., Nagao, S., 2011b. Post-training cerebellar cortical activity plays an important role for consolidation of memory of cerebellum-dependent motor learning. Neurosci. Lett. 504, 53–56. Parker, K.L., Zbarska, S., Carrel, A.J., Bracha, V., 2009. Blocking GABAA neurotransmission in the interposed nuclei: effects on conditioned and unconditioned eyeblinks. Brain Res. 1292, 25–37. Perrett, S.P., Mauk, M.D., 1995. Extinction of conditioned eyelid responses requires the anterior lobe of cerebellar cortex. J. Neurosci. 15, 2074–2080. Perrett, S.P., Ruiz, B.P., Mauk, M.D., 1993. Cerebellar cortex lesions disrupt learningdependent timing of conditioned eyelid responses. J. Neurosci. 13, 1708–1718. Polenchar, B.E., Patterson, M.M., Lavond, D.G., Thompson, R.F., 1985. Cerebellar lesions abolish an avoidance response in rabbit. Behav. Neural Biol. 44, 221–227. Porrill, J., Dean, P., 2007. Cerebellar motor learning: when is cortical plasticity not enough? PLoS Comput. Biol. 3, 1935–1950. Ramnani, N., Yeo, C.H., 1996. Reversible inactivations of the cerebellum prevent the extinction of conditioned nictitating membrane responses in rabbits. J. Physiol. 495 (Pt. 1), 159–168. Rawson, J.A., Tilokskulchai, K., 1981. Suppression of simple spike discharges of cerebellar Purkinje cells by impulses in climbing fibre afferents. Neurosci. Lett. 25, 125–130. Raymond, J.L., Lisberger, S.G., Mauk, M.D., 1996. The cerebellum: a neuronal learning machine? Science 272, 1126–1131. Sanchez, M., Sillitoe, R.V., Attwell, P.J., Ivarsson, M., Rahman, S., Yeo, C.H., Hawkes, R., 2002. Compartmentation of the rabbit cerebellar cortex. J. Comp. Neurol. 444, 159–173. Schneiderman, N., Fuentes, I., Gormezano, I., 1962. Acquisition and extinction of the classically conditioned eyelid response in the albino rabbit. Science 136, 650–652. Schonewille, M., Gao, Z., Boele, H.J., Veloz, M.F., Amerika, W.E., Simek, A.A., De Jeu, M.T., Steinberg, J.P., Takamiya, K., Hoebeek, F.E., Linden, D.J., Huganir, R.L., De Zeeuw, C.I., 2011. Reevaluating the role of LTD in cerebellar motor learning. Neuron 70, 43–50. Shutoh, F., Ohki, M., Kitazawa, H., Itohara, S., Nagao, S., 2006. Memory trace of motor learning shifts transsynaptically from cerebellar cortex to nuclei for consolidation. Neuroscience 139, 767–777. Sugihara, I., Shinoda, Y., 2004. Molecular, topographic, and functional organization of the cerebellar cortex: a study with combined aldolase C and olivocerebellar labeling. J. Neurosci. 24, 8771–8785. Svensson, P., Bengtsson, F., Hesslow, G., 2006. Cerebellar inhibition of inferior olivary transmission in the decerebrate ferret. Exp. Brain Res. 168, 241–253.

References

Thompson, R.F., 1976. The search for the engram. Am. Psychol. 31, 209–227. Uusisaari, M.Y., Knopfel, T., 2012. Diversity of neuronal elements and circuitry in the cerebellar nuclei. Cerebellum 11, 420–421. Van Ham, J.J., Yeo, C.H., 1992. Somatosensory trigeminal projections to the inferior olive, cerebellum and other precerebellar nuclei in rabbits. Eur. J. Neurosci. 4, 302–317. van Ham, J.J., Yeo, C.H., 1996. Trigeminal inputs to eyeblink motoneurons in the rabbit. Exp. Neurol. 142, 244–257. Voogd, J., Bigare, F., Gerrits, N.M., Marani, E., 1981. Structure and fiber connections of the cerebellum. Prog. Clin. Biol. Res. 59A, 259–268. Welsh, J.P., Harvey, J.A., 1989. Cerebellar lesions and the nictitating membrane reflex: performance deficits of the conditioned and unconditioned response. J. Neurosci. 9, 299–311. Welsh, J.P., Harvey, J.A., 1991. Pavlovian conditioning in the rabbit during inactivation of the interpositus nucleus. J. Physiol. Lond. 444, 459–480. Welsh, J.P., Harvey, J.A., 1998. Acute inactivation of the inferior olive blocks associative learning. Eur. J. Neurosci. 10, 3321–3332. Williams, M.R., Fuchs, J.R., Green, J.T., Morielli, A.D., 2012. Cellular mechanisms and behavioral consequences of Kv1.2 regulation in the rat cerebellum. J. Neurosci. 32, 9228–9237. Yeo, C.H., Hardiman, M.J., 1992. Cerebellar cortex and eyeblink conditioning: a reexamination. Exp. Brain Res. 88, 623–638. Yeo, C.H., Hesslow, G., 1998. Cerebellum and conditioned reflexes. Trends Cogn. Sci. 2, 322–330. Yeo, C.H., Hardiman, M.J., Glickstein, M., 1984. Discrete lesions of the cerebellar cortex abolish the classically conditioned nictitating membrane response of the rabbit. Behav. Brain Res. 13, 261–266. Yeo, C.H., Hardiman, M.J., Glickstein, M., 1985a. Classical-conditioning of the nictitatingmembrane response of the rabbit. I. Lesions of the cerebellar nuclei. Exp. Brain Res. 60, 87–98. Yeo, C.H., Hardiman, M.J., Glickstein, M., 1985b. Classical-conditioning of the nictitatingmembrane response of the rabbit. II. Lesions of the cerebellar cortex. Exp. Brain Res. 60, 99–113. Yeo, C.H., Hardiman, M.J., Glickstein, M., 1985c. Classical-conditioning of the nictitatingmembrane response of the rabbit. III. Connections of cerebellar lobule HVI. Exp. Brain Res. 60, 114–126. Yeo, C.H., Hardiman, M.J., Glickstein, M., 1986. Classical conditioning of the nictitating membrane response of the rabbit. IV. Lesions of the inferior olive. Exp. Brain Res. 63, 81–92. Yeo, C.H., Lobo, D.H., Baum, A., 1997. Acquisition of a new-latency conditioned nictitating membrane response–major, but not complete, dependence on the ipsilateral cerebellum. Learn. Mem. 3, 557–577. Zbarska, S., Bloedel, J.R., Bracha, V., 2008. Cerebellar dysfunction explains the extinctionlike abolition of conditioned eyeblinks after NBQX injections in the inferior olive. J. Neurosci. 28, 10–20.

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