Acquisition of a new-latency conditioned nictitating membrane response--major, but not complete, dependence on the ipsilateral cerebellum

Differential Coding Strategies in Glutamatergic and GABAergic Neurons in the Medial Cerebellar Nucleus

The cerebellum drives motor coordination and sequencing of actions at the millisecond timescale through adaptive control of cerebellar nuclear output. Cerebellar nuclei integrate high-frequency information from both the cerebellar cortex and the two main excitatory inputs of the cerebellum: the mossy fibers and the climbing fiber collaterals. However, how nuclear cells process rate and timing of inputs carried by these inputs is still debated. Here, we investigate the influence of the cerebellar cortical output, the Purkinje cells, on identified cerebellar nuclei neurons in vivo in male mice. Using transgenic mice expressing Channelrhodopsin2 specifically in Purkinje cells and tetrode recordings in the medial nucleus, we identified two main groups of neurons based on the waveform of their action potentials. These two groups of neurons coincide with glutamatergic and GABAergic neurons identified by optotagging after Chrimson expression in VGLUT2-cre and GAD-cre mice, respectively. The glutamatergic-like neurons fire at high rate and respond to both rate and timing of Purkinje cell population inputs, whereas GABAergic-like neurons only respond to the mean population firing rate of Purkinje cells at high frequencies. Moreover, synchronous activation of Purkinje cells can entrain the glutamatergic-like, but not the GABAergic-like, cells over a wide range of frequencies. Our results suggest that the downstream effect of synchronous and rhythmic Purkinje cell discharges depends on the type of cerebellar nuclei neurons targeted.SIGNIFICANCE STATEMENT Motor coordination and skilled movements are driven by the permanent discharge of neurons from the cerebellar nuclei that communicate cerebellar computation to other brain areas. Here, we set out to study how specific subtypes of cerebellar nuclear neurons of the medial nucleus are controlled by Purkinje cells, the sole output of the cerebellar cortex. We could isolate different subtypes of nuclear cell that differentially encode Purkinje cell inhibition. Purkinje cell stimulation entrains glutamatergic projection cells at their firing frequency, whereas GABAergic neurons are only inhibited. These differential coding strategies may favor temporal precision of cerebellar excitatory outputs associated with specific features of movement control while setting the global level of cerebellar activity through inhibition via rate coding mechanisms.

Establishment and transfer of classical eyeblink conditioning using electrical microstimulation of the hippocampus as the conditioned stimulus

The present experiment was designed to determine whether classical eyeblink conditioning (EBC) can be established by using electrical microstimulation of the hippocampus as a conditioned stimulus (CS) paired with an air-puff unconditioned stimulus (US). We intended to examine whether EBC transfer could occur when a CS was shifted between microstimulation of the hippocampus as a CS (Hip-CS) and tone as a CS (tone-CS) and to compare the difference in transfer effectiveness between delay EBC (dEBC) and trace EBC (tEBC). Eight groups of guinea pigs, including 4 experimental groups and 4 control groups, were included in the study. First, the experimental groups received either a Hip-CS or a tone-CS paired with a US; then, these groups were exposed to a shifted CS (tone-CS or Hip-CS) paired with the US. The control groups received the corresponding Hip-CS or tone-CS, which was, however, pseudo-paired with the US. The control groups were then shifted to the tone-CS (or Hip-CS) paired with the US. The results show that EBC can be successfully established when using microstimulation of the hippocampus as a CS paired with an air-puff US, and that the acquisition rates of EBC are higher in the experimental groups than in the control groups after switching from the Hip-CS to the tone-CS or vice versa, indicating the occurrence of learning transfer between EBC established with the Hip-CS and tone-CS. The present study also demonstrated that the EBC re-acquisition rates were remarkably higher in dEBC than in tEBC with both types of transfer, which suggests that the saving effect was more evident in dEBC than tEBC. These results significantly expand our knowledge of EBC transfer as well as the functional neural circuit underlying EBC transfer.

Transfer of classical eyeblink conditioning with electrical stimulation of mPFC or tone as conditioned stimulus in guinea pigs

Learning with a stimulus from one sensory modality can facilitate subsequent learning with a new stimulus from a different sensory modality. To date, the characteristics and mechanism of this phenomenon named transfer effect still remain ambiguous. Our previous work showed that electrical stimulation of medial prefrontal cortex (mPFC) as a conditioned stimulus (CS) could successfully establish classical eyeblink conditioning (EBC). The present study aimed to (1) observe whether transfer of EBC learning would occur when CSs shift between central (mPFC electrical stimulation as a CS, mPFC-CS) and peripheral (tone as a CS, tone CS); (2) compare the difference in transfer effect between the two paradigms, delay EBC (DEBC) and trace EBC (TEBC). A total of 8 groups of guinea pigs were tested in the study, including 4 experimental groups and 4 control groups. Firstly, the experimental groups accepted central (or peripheral) CS paired with corneal airpuff unconditioned stimulus (US); then, CS shifted to the peripheral (or central) and paired with US. The control groups accepted corresponding central (or peripheral) CS and pseudo-paired with US, and then shifted CS from central (or peripheral) to peripheral (or central) and paired with US. The results showed that the acquisition rates of EBC were higher in experimental groups than in control groups after CS switching from central to peripheral or vice versa, and the CR acquisition rate was remarkably higher in DEBC than in TEBC in both transfer ways. The results indicate that EBC transfer can occur between learning established with mPFC-CS and tone CS. Memory of CS-US association for delay paradigm was less disturbed by the sudden switch of CS than for trace paradigm. This study provides new insight into neural mechanisms underlying conditioned reflex as well as the role of mPFC.

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.

Cerebellum and conditioned reflexes

The central assumption of existing models of motor learning in the cerebellum is that cerebellar mossy fibres signal information about the context in which a movement is to be performed and climbing fibres signal in relation to a movement error. This leads to changes in the responsiveness of Purkinje cells, which on the next occasion will generate a corrected output in a given context. Support for this view has come mainly from work on adaptation of the vestibulo-ocular reflex. The discovery that classically conditioned eyeblink responses depend critically on the cerebellum offers the possibility to study the learning of a novel behaviour, rather than modification of an existing reflex. After repeated pairing of a neutral stimulus, such as a tone, with a blink-eliciting stimulus, the tone will acquire the ability to elicit a blink on its own. We review evidence from studies employing a wide variety of techniques that the cerebellum is critical in this type of learning as well as evidence that mossy and climbing fibres have roles assigned to them in cerebellar learning models.

Memory consolidation in the cerebellar cortex

Several forms of learning, including classical conditioning of the eyeblink, depend upon the cerebellum. In examining mechanisms of eyeblink conditioning in rabbits, reversible inactivations of the control circuitry have begun to dissociate aspects of cerebellar cortical and nuclear function in memory consolidation. It was previously shown that post-training cerebellar cortical, but not nuclear, inactivations with the GABA_A agonist muscimol prevented consolidation but these findings left open the question as to how final memory storage was partitioned across cortical and nuclear levels. Memory consolidation might be essentially cortical and directly disturbed by actions of the muscimol, or it might be nuclear, and sensitive to the raised excitability of the nuclear neurons following the loss of cortical inhibition. To resolve this question, we simultaneously inactivated cerebellar cortical lobule HVI and the anterior interpositus nucleus of rabbits during the post-training period, so protecting the nuclei from disinhibitory effects of cortical inactivation. Consolidation was impaired by these simultaneous inactivations. Because direct application of muscimol to the nuclei alone has no impact upon consolidation, we can conclude that post-training, consolidation processes and memory storage for eyeblink conditioning have critical cerebellar cortical components. The findings are consistent with a recent model that suggests the distribution of learning-related plasticity across cortical and nuclear levels is task-dependent. There can be transfer to nuclear or brainstem levels for control of high-frequency responses but learning with lower frequency response components, such as in eyeblink conditioning, remains mainly dependent upon cortical memory storage.

Cerebellar inactivation impairs cross modal savings of eyeblink conditioning

Eyeblink conditioning using a conditioned stimulus (CS) from one sensory modality (e.g., an auditory CS) is greatly enhanced when the subject is previously trained with a CS from a different sensory modality (e.g., a visual CS). The enhanced acquisition to the second modality CS results from cross modal savings. The current study was designed to examine the role of the cerebellum in establishing cross modal savings in eyeblink conditioning with rats. In the first experiment rats were given paired or unpaired presentations with a CS (tone or light) and an unconditioned stimulus. All rats were then given paired training with a different modality CS. Only rats given paired training showed cross modal savings to the second modality CS. Experiment 2 showed that cerebellar inactivation during initial acquisition to the first modality CS completely prevented savings when training was switched to the second modality CS. Experiment 3 showed that cerebellar inactivation during initial cross modal training also prevented savings to the second modality stimulus. These results indicate that the cerebellum plays an essential role in establishing cross modal savings of eyeblink conditioning.

Eyeblink conditioning during an interstimulus interval switch in rabbits (Oryctolagus cuniculus) using picrotoxin to disrupt cerebellar cortical input to the interpositus nucleus

The role of the cerebellar cortex in eyeblink classical conditioning remains unclear. Experimental manipulations that disrupt the normal function impair learning to various degrees, and task parameters may be important factors in determining the severity of impairment. This study examined the role of cerebellar cortex in eyeblink conditioning under conditioned stimulus?unconditioned stimulus intervals known to be optimal or nonoptimal for learning. Using infusions of picrotoxin to the interpositus nucleus of the rabbit cerebellum, the authors pharmacologically disrupted input from the cerebellar cortex while training with an interstimulus interval (ISI)-switch procedure. One group of rabbits (Oryctolagus cuniculus) was 1st trained with a 250-ms ISI (optimal) and then switched to a 750-ms ISI (nonoptimal). A 2nd group was trained in the opposite order. The most striking effect was that picrotoxin-treated rabbits initially trained with a 250-ms ISI learned comparably to controls, but those initially trained with a 750-ms ISI were severely impaired. These results suggest that functional input from cerebellar cortex becomes increasingly important for the interpositus nucleus to learn delay eyeblink conditioning as the ISI departs from an optimal interval.

Involvement of the ipsilateral and contralateral cerebellum in the acquisition of unilateral classical eyeblink conditioning in guinea pigs

AIM

The aim of this study was to evaluate the relative contributions of the ipsilateral and contralateral cerebellum to the acquisition of unilateral classical eyeblink conditioning (EBCC).

METHODS

The unilateral EBCC was achieved using a binaural tone conditioned stimulus (CS) paired with a left airpuff unconditioned stimulus (US). A high-resolution potentiometer was used to monitor eyeblink responses. Guinea pigs received one CS-US session followed by three CS-US sessions (sessions 2 to 4), during which microinjections of muscimol, a GABA(A) receptor agonist, were performed to reversibly inactivate the cerebellum unilaterally prior to training. To test whether any learning had occurred during these inactivation sessions, training was continued for six more CS-US sessions (sessions 5 to 10) without any inactivation.

RESULTS

Animals with inactivation of the left cerebellum had no signs of left conditioned response (CR) during sessions 2 to 4, and their CR acquisition during sessions 5 to 10 was not distinguishable from that of control animals during sessions 2 to 7. In contrast, animals with inactivation of the right cerebellum acquired left CRs during sessions 2 to 4, although their CR acquisition was significantly retarded during session 2. In addition, microinjections of muscimol into the right cerebellum did not affect left neuro-behavioral activity. Finally, microinjections of muscimol into either the left or the right cerebellum did not affect the performance of tone-airpuff evoked unconditioned response (UR).

CONCLUSION

In contrast to the essential role of the ipsilateral cerebellum, the contralateral cerebellum is potentially involved in the acquisition of unilateral EBCC during the early stage of training.

Neurotransmitter release during delay eyeblink classical conditioning: role of norepinephrine in consolidation and effect of age

Delay classical eyeblink conditioning (EBC) is an important model of associative, cerebellar-dependent learning. Norepinephrine (NE) plays a significant modulatory role in the acquisition of learning; however, other neurotransmitters are also involved. The goal was to determine whether NE, gamma-aminobutyric acid (GABA) and glutamate (GLU) release are observed in cerebellar cortex during EBC, and whether such release was selectively associated with training. Further studies examined the role of the beta-noradrenergic receptor in consolidation of the learned response by local infusion of propranolol at 5-120 min following training into the cerebellar cortex. In vivo microdialysis coupled to EBC was performed to examine neurotransmitter release. An increase in the extracellular level of NE was observed during EBC and was maximal on day 1 and diminished in amplitude with subsequent days of training. No changes in baseline NE release were observed in pseudoconditioning indicating that NE release is directly related to the associative learning process. The extracellular levels of GABA were also increased selectively during paired training however, the magnitude of GABA release increased over days of training. GLU release was observed to increase during both paired and unpaired training, suggesting that learning does not occur prior to the information arriving in the cerebellum. When propranolol was administered at either 5-, 60-, or 120-min post-training, there was an inhibition of conditioned responses, these data support the hypothesis that NE is important for consolidation of learning. In another set of experiments we demonstrate that the timing of release of NE, GABA and glutamate are significantly delayed in onset and lengthened in duration in the 22-month-old F344 rats. Over days of training the timing of release becomes closer to the timing of training and this is associated with increased learning of conditioned responses in the aged rats.

Evidence from retractor bulbi EMG for linearized motor control of conditioned nictitating membrane responses

Classical conditioning of nictitating membrane (NM) responses in rabbits is a robust model learning system, and experimental evidence indicates that conditioned responses (CRs) are controlled by the cerebellum. It is unknown whether cerebellar control signals deal directly with the complex nonlinearities of the plant (blink-related muscles and peripheral tissues) or whether the plant is linearized to ensure a simple relation between cerebellar neuronal firing and CR profile. To study this question, the retractor bulbi muscle EMG was recorded with implanted electrodes during NM conditioning. Pooled activity in accessory abducens motoneurons was estimated from spike trains extracted from the EMG traces, and its temporal profile was found to have an approximately Gaussian shape with peak amplitude linearly related to CR amplitude. The relation between motoneuron activity and CR profiles was accurately fitted by a first-order linear filter, with each spike input producing an exponentially decaying impulse response with time constant of order 0.1 s. Application of this first-order plant model to CR data from other laboratories suggested that, in these cases also, motoneuron activity had a Gaussian profile, with time-of-peak close to unconditioned stimulus (US) onset and SD proportional to the interval between conditioned stimulus and US onsets. These results suggest that for conditioned NM responses the cerebellum is presented with a simplified "virtual" plant that is a linearized version of the underlying nonlinear biological system. Analysis of a detailed plant model suggests that one method for linearising the plant would be appropriate recruitment of motor units.

The involvement of the human cerebellum in eyeblink conditioning

Besides its known importance for motor coordination, the cerebellum plays a major role in associative learning. The form of cerebellum-dependent associative learning, which has been examined in greatest detail, is classical conditioning of eyeblink responses. The much advanced knowledge of anatomical correlates, as well as cellular and molecular mechanisms involved in eyeblink conditioning in animal models are of particular importance because there is general acceptance that findings in humans parallel the animal data. The aim of the present review is to give an update of findings in humans. Emphasis is put on human lesion studies, which take advantage of the advances of high-resolution structural magnetic resonance imaging (MRI). In addition, findings of functional brain imaging in healthy human subjects are reviewed. The former helped to localize areas involved in eyeblink conditioning within the cerebellum, the latter was in particular helpful in delineating extracerebellar neural substrates, which may contribute to eyeblink conditioning. Human lesion studies support the importance of cortical areas of the ipsilateral superior cerebellum both in the acquisition and timing of conditioned eyeblink responses (CR). Furthermore, the ipsilateral cerebellar cortex seems to be also important in extinction of CRs. Cortical areas, which are important for CR acquisition, overlap with areas related to the control of the unconditioned eyeblink response. Likewise, cortical lesions are followed by increased amplitudes of unconditioned eyeblinks. These findings are in good accordance with the animal literature. Knowledge about contributions of the cerebellar nuclei in humans, however, is sparse. Due to methodological limitations both of human lesion and functional MRI studies, at present no clear conclusions can be drawn on the relative contributions of the cerebellar cortex and nuclei.

Temporally specific extinction of conditioned responses in the rabbit (Oryctolagus cuniculus) nictitating membrane preparation

Extinguishing a conditioned response (CR) has entailed separating the conditioned stimulus (CS) from the unconditioned stimulus (US). This research reveals that elimination of the rabbit (Oryctolagus cuniculus) nictitating membrane response occurred during continuous CS-US pairings. Initial training contained a mixture of 2 CS-US interstimulus intervals (ISIs): 200 ms and 1,200 ms. The CRs showed double peaks, one for each ISI. When 1 ISI was removed, its CR peak showed the hallmarks of extinction: a decline across sessions, spontaneous recovery between sessions, and rapid reacquisition when the absent ISI was reintroduced. These results support real-time models of conditioning that segment the CS into microstimuli while challenging theories that rely on contextual control, US representations, CS processing, and response inhibition.

Time and tide in cerebellar memory formation

The notion that the olivocerebellar system is crucial for motor learning is well established. In recent years, it has become evident that there can be many forms of both synaptic and non-synaptic plasticity within this system and that each might have a different role in developing and maintaining motor learning across a wide range of tasks. There are several possible molecular and cellular mechanisms that could underlie adaptation of the vestibulo-ocular reflex and eyeblink conditioning. Although causal relationships between particular cellular processes and individual components of a learned behaviour have not been demonstrated unequivocally, an overall picture is emerging that the different types and sites of cellular plasticity relate importantly to the stage of learning and/or its temporal specifics.

A biophysical model of synaptic delay learning and temporal pattern recognition in a cerebellar Purkinje cell

It has been suggested that information in the brain is encoded in temporal spike patterns which are decoded by a combination of time delays and coincidence detection. Here, we show how a multi-compartmental model of a cerebellar Purkinje cell can learn to recognise temporal parallel fibre activity patterns by adapting latencies of calcium responses after activation of metabotropic glutamate receptors (mGluRs). In each compartment of our model, the mGluR signalling cascade is represented by a set of differential equations that reflect the underlying biochemistry. Phosphorylation of the mGluRs changes the concentration of receptors which are available for activation by glutamate and thereby adjusts the time delay between mGluR stimulation and voltage response. The adaptation of a synaptic delay as opposed to a weight represents a novel non-Hebbian learning mechanism that can also implement the adaptive timing of the classically conditioned eye-blink response.

Single-cue delay and trace classical conditioning in schizophrenia

BACKGROUND

Classical conditioning provides a means of addressing mechanisms of learning and can therefore help understand the pathophysiology of memory alteration in schizophrenia.

METHODS

Single cue delay and trace eyeblink conditioning were used in patients with schizophrenia and matched normal control subjects to explore, respectively, cerebellar and hippocampal integrity during learning. We measured percent of conditioned (CRs) and unconditioned responses (URs), their amplitude, and onset and peak latencies. We also accounted for spontaneous blink rates and stimulus-induced responses before learning.

RESULTS

During delay conditioning, patients showed CRs with longer onset and peak latencies and improved efficiency compared to normal volunteers without there being differences between patients and normal control subjects in the percentage of CRs. During trace conditioning, neither group showed an increase in CRs as a function of conditioned stimulus-unconditioned stimulus pairings, in part because the level of spontaneous blink rates exceeded the level of CRs; however, patients with schizophrenia showed increased responding 150-400 msec after the conditioned stimulus and in the last 100-150 msec before the unconditioned stimulus, whereas normal control subjects showed only the latter type of responses. The former type of response was more frequent in patients with schizophrenia even before either trace or delay conditioning.

CONCLUSIONS

These results suggest integrity of cerebellar mechanisms underlying conditioning, although the altered timing of CRs in patients may indicate differences in the modulation of such responses. Both the greater CR onset latency during delay and the presence of early nonadaptive responses during trace are compatible with the pattern of responding seen in animals with hippocampal damage.

Comparison of eyeblink conditioning in patients with superior and posterior inferior cerebellar lesions

The aim of the present study was to compare eyeblink conditioning in cerebellar patients with lesions including the territory of the superior cerebellar artery (SCA) and in patients with lesions restricted to the territory of the posterior inferior cerebellar artery (PICA). The cerebellar areas known to be most critical in eyeblink conditioning based on animal data (i.e. Larsell lobule H VI and interposed nucleus) are commonly supplied by the SCA. Eyeblink conditioning was expected to be impaired in SCA, but not in PICA patients. A total of 27 cerebellar patients and 25 age-matched controls were tested. Cerebellar lesions were primarily unilateral (n = 20). Most patients suffered from ischaemic infarctions of the SCA (n = 11) or the PICA (n = 13). The other patients presented with cerebellar tumours (n = 2) and cerebellar agenesis (n = 1). The extent of the cortical lesion (i.e. which lobuli were affected) and possible involvement of the cerebellar nuclei was determined by 3D-MRI. As expected, the ability to acquire classically conditioned eyeblink responses was significantly reduced in the group of all cerebellar patients compared with the controls. In the patients with unilateral cerebellar lesions, conditioning deficits were present ipsilaterally. In SCA patients with lesions including hemispheral lobules VI and Crus I, eyeblink conditioning was significantly reduced on the affected side compared with the unaffected side. No significant difference between the affected and unaffected sides was present in patients with lesions restricted to the common PICA territory (i.e. Crus II and below). Conditioning deficits were neither significantly different in SCA patients with pure cortical lesions compared with SCA patients with additional nuclear impairment nor in SCA patients with unilateral lesions compared with SCA patients with bilateral lesions. To summarize, unilateral cortical lesions of the superior cerebellum appear to be sufficient to reduce eyeblink conditioning in humans significantly.

Cerebellar mechanisms in eyeblink conditioning

A recent model of cerebellar learning in eyeblink conditioning predicts two sites of plasticity, the cerebellar cortex and cerebellar nuclei, which store information relating to timing and driving the movement, respectively. Consistent with this idea, lesions of the cortex or reversible "disconnections" of Purkinje cell output to the nuclei have been shown to disrupt response timing to produce short-latency conditioned eyeblinks. To better characterize potential cortical and nuclear plasticities, we analyzed the effects upon nictitating membrane (NM) and eyeblink conditioned responses (CRs) of different drugs administered to the cortex and to the nuclei. When either excitatory or inhibitory inputs to the cerebellar cortical lobule HVI were blocked by infusions of the AMPA receptor antagonist CNQX or the GABA-A receptor antagonists picrotoxin or SR95531, CRs were abolished. Similarly GABA-A receptor antagonists in the cerebellar nuclei abolished CRs. CR latencies were never shortened. However, blockade of AMPA/kainate receptor-mediated excitatory transmission to the nuclei had no effect upon CR frequencies or latencies. These results suggest that normal cortical and nuclear function is required for performance of NM and eyeblink CRs. We saw no evidence that CRs can be driven by AMPA/kainate receptor-mediated transmission from mossy fiber afferents to the cerebellar nuclei. So, although plasticity in the cerebellar nuclei is not ruled out, it is unlikely that a long-term change in AMPA receptor-mediated transmission from mossy fiber inputs to the nuclei is an essential mechanism in eyeblink conditioning. Our findings indicate that a fully functional olivo-cortico-nuclear loop is required to express all characteristics of associatively conditioned responses.

Cerebellar function in consolidation of a motor memory

Several forms of motor learning, including classical conditioning of the eyeblink and nictitating membrane response (NMR), are dependent upon the cerebellum, but it is not known how motor memories are stored within the cerebellar circuitry. Localized infusions of the GABA(A) agonist muscimol were used to target putative consolidation processes by producing reversible inactivations after NMR conditioning sessions. Posttraining inactivations of eyeblink control regions in cerebellar cortical lobule HVI completely prevented conditioning from developing over four sessions. In contrast, similar inactivations of eyeblink control regions in the cerebellar nuclei allowed conditioning to develop normally. These findings provide evidence that there are critical posttraining memory consolidation processes for eyeblink conditioning mediated by the cerebellar cortex.

Acquisition of eyeblink conditioning is critically dependent on normal function in cerebellar cortical lobule HVI

Classical conditioning of the nictitating membrane response (NMR)/eyeblink response of rabbits is a simple form of cerebellar-dependent, associative motor learning. Reversible inactivations of the cerebellar nuclei and inferior olive have implicated the olivo-cortico-nuclear loop in the acquisition of nictitating membrane conditioning, but the role of the cerebellar cortex in acquisition has not been tested directly. Here we have used local infusions of the water-soluble, disodium salt of 6-cyano-7-nitroquinoxaline-2,3-dione reversibly to block cerebellar cortical AMPA/kainate receptors in lobule HVI during acquisition training. After the drug effects dissipated, there was no evidence that acquisition had taken place; the subjects behaved as if naive. Further training without inactivation then allowed normal acquisition, and further inactivations during performance of conditioned responses abolished these established responses. There was a strong correlation between the inactivation effects on acquisition and subsequent inactivation effects on performance, indicating that the same eyeblink-control cortical microzones are engaged in learning and expressing this behavior. The cortical component of the olivo-cortico-nuclear loop is essential for acquisition of classically conditioned nictitating membrane response learning, and eyeblink control areas in HVI are critical. Our findings are consistent with models of cerebellar learning that assign essential plasticity to the cortex or to a distribution between levels in olivo-cortico-nuclear modules.

Latent acquisition of timed responses in cerebellar cortex

Evidence indicates that rabbit eyelid conditioning is mediated by plasticity in the interpositus cerebellar nucleus and in cerebellar cortex. Although the relative contributions of these sites are not fully characterized, evidence suggests that plasticity in the cerebellar cortex influences conditioned response amplitude and timing, whereas plasticity in the interpositus nucleus is necessary or permissive for conditioned response expression. Recent empirical and computational analyses suggest that, during training, plasticity is initially established in the cerebellar cortex, whereas conditioned response expression begins later as plasticity is induced in the interpositus nucleus. We used the dependence of response timing on the interstimulus interval (ISI) to test this latent learning hypothesis. Rabbits were initially trained using a tone conditioned stimulus (CS) with a relatively long ISI to a low-criterion threshold. The relative absence of plasticity in the interpositus nucleus was then examined via reversible disconnection of the cerebellar cortex. Later, to induce plasticity in the interpositus nucleus, subjects were trained to robust levels of conditioned response expression using a shorter ISI. Reversible disconnection of the cerebellar cortex at this time confirmed the presence of robust interpositus nucleus plasticity after the second phase. Subsequent probe trials with the long CS alone then revealed double-peaked responses whose peaks were appropriately timed to the two ISIs. The results are consistent with the hypothesis that temporally specific learning occurs first in the cerebellar cortex before the appearance of conditioned responses. This latent learning is expressed only after plasticity is induced in the interpositus nucleus.

Learning- and expectation-related changes in the human brain during motor learning

We have studied a simple form of motor learning in the human brain so as to isolate activity related to motor learning and the prediction of sensory events. Whole-brain, event-related functional magnetic resonance imaging (fMRI) was used to record activity during classical discriminative delay eyeblink conditioning. Auditory conditioned stimulus (CS+) trials were presented either with a corneal airpuff unconditioned stimulus (US, paired), or without a US (unpaired). Auditory CS- trials were never reinforced with a US. Trials were presented pseudorandomly, 66 times each. The subjects gradually produced conditioned responses to CS+ trials, while increasingly differentiating between CS+ and CS- trials. The increasing difference between hemodynamic responses for unpaired CS+ and for CS- trials evolved slowly during conditioning in the ipsilateral cerebellar cortex (Crus I/Lobule HVI), contralateral motor cortex and hippocampus. To localize changes that were related to sensory prediction, we compared trials on which the expected airpuff US failed to occur (Unpaired CS+) with trials on which it occurred as expected (Paired CS+). Error-related signals in the contralateral cerebellum and somatosensory cortex were seen to increase during learning as the sensory prediction became stronger. The changes seen in the ipsilateral cerebellar cortex may be due either to the violations of sensory predictions, or to learning-related increases in the excitability of cerebellar neurons to presentations of the CS+.