Involvement of the amygdala in classical conditioning of eyeblink response in the rat

Central amygdala contributes to stimulus facilitation and pre-stimulus vigilance during cerebellar learning

Our previous studies found that the central amygdala (CeA) modulates cerebellum-dependent eyeblink conditioning (EBC) using muscimol inactivation. We also found that CeA inactivation decreases cerebellar neuronal activity during the conditional stimulus (CS) from the start of training. Based on these findings, we hypothesized that the CeA facilitates CS input to the cerebellum. The current study tested the CS facilitation hypothesis using optogenetic inhibition with archaerhodopsin (Arch) and excitation with channelrhodopsin (ChR2) of the CeA during EBC in male rats. Optogenetic manipulations were administered during the 400 ms tone CS or during a 400 ms pre-CS period. As predicted by the CS facilitation hypothesis CeA inhibition during the CS impaired EBC and CeA excitation during the CS facilitated EBC. Unexpectedly, CeA inhibition just prior to the CS also impaired EBC, while CeA excitation during the pre-CS pathway did not facilitate EBC. The results suggest that the CeA contributes to CS facilitation and vigilance during the pre-CS period. These putative functions of the CeA may be mediated through separate output pathways from the CeA to the cerebellum.

Amygdala central nucleus modulation of cerebellar learning in female rats

Previous studies found that inactivation of the central amygdala (CeA) severely impaired acquisition of cerebellum-dependent delay eye-blink conditioning (EBC) in male rats and rabbits. Sex differences in EBC and the effects of stress on EBC have been reported and might be related to sex differences in amygdala modulation of cerebellar learning. The current study examined the effects of CeA inactivation with muscimol on acquisition and retention of EBC in female rats. Like male rats, CeA inactivation in female rats severely impaired EBC acquisition and retention. Comparison of the female data with previously published data from males indicates no substantive sex differences in the effects of CeA inactivation on acquisition or retention of EBC. The results indicate that amygdala modulation of cerebellar learning is not sex-specific. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Inactivation of the interpositus nucleus blocks the acquisition of conditioned responses and timing changes in conditioning-specific reflex modification of the rabbit eyeblink response

Conditioning-specific reflex modification (CRM) of the rabbit eyeblink response is an associative phenomenon characterized by increases in the frequency, size, and peak latency of the reflexive unconditioned eyeblink response (UR) when the periorbital shock unconditioned stimulus (US) is presented alone following conditioning, particularly to lower intensity USs that produced minimal responding prior to conditioning. Previous work has shown that CRM shares many commonalities with the conditioned eyeblink response (CR) including a similar response topography, suggesting the two may share similar neural substrates. The following study examined the hypothesis that the interpositus nucleus (IP) of the cerebellum, an essential part of the neural circuitry of eyeblink conditioning, is also required for the acquisition of CRM. Tests for CRM occurred following delay conditioning under muscimol inactivation of the IP and also after additional conditioning without IP inactivation. Results showed that IP inactivation blocked acquisition of CRs and the timing aspect of CRM but did not prevent increases in UR amplitude and area. Following the cessation of inactivation, CRs and CRM latency changes developed similarly to controls with intact IP functioning, but with some indication that CRs may have been facilitated in muscimol rabbits. In conclusion, CRM timing and CRs both likely require the development of plasticity in the IP, but other associative UR changes may involve non-cerebellar structures interacting with the eyeblink conditioning circuitry, a strong candidate being the amygdala, which is also likely involved in the facilitation of conditioning. Other candidates worth consideration include the cerebellar cortex, prefrontal and motor cortices.

Central amygdala lesions inhibit pontine nuclei acoustic reactivity and retard delay eyeblink conditioning acquisition in adult rats

In delay eyeblink conditioning (EBC) a neutral conditioned stimulus (CS; tone) is repeatedly paired with a mildly aversive unconditioned stimulus (US; periorbital electrical shock). Over training, subjects learn to produce an anticipatory eyeblink conditioned response (CR) during the CS, prior to US onset. While cerebellar synaptic plasticity is necessary for successful EBC, the amygdala is proposed to enhance eyeblink CR acquisition. In the current study, adult Long-Evans rats received bilateral sham or neurotoxic lesions of the central nucleus of the amygdala (CEA) followed by 1 or 4 EBC sessions. Fear-evoked freezing behavior, CS-mediated enhancement of the unconditioned response (UR), and eyeblink CR acquisition were all impaired in the CEA lesion rats relative to sham controls. There were also significantly fewer c-Fos immunoreactive cells in the pontine nuclei (PN)-major relays of acoustic information to the cerebellum-following the first and fourth EBC session in lesion rats. In sham rats, freezing behavior decreased from session 1 to 4, commensurate with nucleus-specific reductions in amygdala Fos+ cell counts. Results suggest delay EBC proceeds through three stages: in stage one the amygdala rapidly excites diffuse fear responses and PN acoustic reactivity, facilitating cerebellar synaptic plasticity and the development of eyeblink CRs in stage two, leading, in stage three, to a diminution or stabilization of conditioned fear responding.

Amygdala central nucleus modulation of cerebellar learning with a visual conditioned stimulus

Previous studies found that reversible inactivation of the central amygdala (CeA) severely impairs acquisition and retention of cerebellum-dependent eye-blink conditioning (EBC) with an auditory conditioned stimulus (CS). A monosynaptic pathway between the CeA and basilar pontine nuclei (BPN) may be capable of facilitating cerebellar learning. However, given that the CeA projects to the medial auditory thalamus, a critical part of the auditory CS pathway in EBC, the CeA influence on cerebellar learning could be specific to auditory stimuli. Here we examined the generality of CeA facilitation of EBC acquisition and retention in rats using a visual CS. As in our previous studies using an auditory CS, inactivation of the CeA with muscimol severely impaired acquisition and retention of EBC with a visual CS. Extending training to 15 100-trial sessions resulted in acquisition of EBC, indicating that the CeA plays a modulatory role in cerebellar learning and is not part of the necessary neural circuitry for EBC. Tract-tracing experiments verified that axons from the CeA reach both the BPN and medial auditory thalamus (part of the necessary auditory CS pathway), but were not found in the ventral lateral geniculate (part of the necessary visual CS pathway). The neuroanatomical results suggest that the CeA most likely modulates cerebellar learning through its projection to the BPN. The findings of the current study are consistent with the hypothesis that the CeA modulates cerebellar learning by increasing CS-related sensory input to the cerebellar cortex and interpositus nucleus via the BPN. This increase in CS-related input is thought to constitute an increase in attention to the CS during EBC.

Memory consolidation within the central amygdala is not necessary for modulation of cerebellar learning

Amygdala lesions impair, but do not prevent, acquisition of cerebellum-dependent eyeblink conditioning suggesting that the amygdala modulates cerebellar learning. Two-factor theories of eyeblink conditioning posit that a fast-developing memory within the amygdala facilitates slower-developing memory within the cerebellum. The current study tested this hypothesis by impairing memory consolidation within the amygdala with inhibition of protein synthesis, transcription, and NMDA receptors in rats. Rats given infusions of anisomycin or DRB into the central amygdala (CeA) immediately after each eyeblink conditioning session were severely impaired in contextual and cued fear conditioning, but were completely unimpaired in eyeblink conditioning. Rats given the NMDA antagonist ifenprodil into the CeA before each eyeblink conditioning session also showed impaired fear conditioning, but no deficit in eyeblink conditioning. The results indicate that memory formation within the CeA is not necessary for its modulation of cerebellar learning mechanisms. The CeA may modulate cerebellar learning and retention through an attentional mechanism that develops within the training sessions.

A neural model of normal and abnormal learning and memory consolidation: adaptively timed conditioning, hippocampus, amnesia, neurotrophins, and consciousness

How do the hippocampus and amygdala interact with thalamocortical systems to regulate cognitive and cognitive-emotional learning? Why do lesions of thalamus, amygdala, hippocampus, and cortex have differential effects depending on the phase of learning when they occur? In particular, why is the hippocampus typically needed for trace conditioning, but not delay conditioning, and what do the exceptions reveal? Why do amygdala lesions made before or immediately after training decelerate conditioning while those made later do not? Why do thalamic or sensory cortical lesions degrade trace conditioning more than delay conditioning? Why do hippocampal lesions during trace conditioning experiments degrade recent but not temporally remote learning? Why do orbitofrontal cortical lesions degrade temporally remote but not recent or post-lesion learning? How is temporally graded amnesia caused by ablation of prefrontal cortex after memory consolidation? How are attention and consciousness linked during conditioning? How do neurotrophins, notably brain-derived neurotrophic factor (BDNF), influence memory formation and consolidation? Is there a common output path for learned performance? A neural model proposes a unified answer to these questions that overcome problems of alternative memory models.

Retention of eyeblink conditioning in periweanling and adult rats

Eyeblink conditioning is a well-established model for studying the developmental neurobiology of associative learning and memory, though nothing is known regarding developmental differences in retention between periweanling and adult rats. The present study examined retention of eyeblink conditioning in periweanling (postnatal day 24 at the start of training) and adult rats 1, 7, or 28 days after acquisition. Retention was assessed by (1) a conditional stimulus (CS)-alone test session followed by (2) CS-unconditional stimulus (US) reacquisition tests. Conditional response (CR) levels at acquisition were comparable in most respects between ages, and robust CR levels were present at the start of retention tests for both ages in the 1 day group, with CR percentages at block 1 of reacquisition higher in periweanlings relative to adults. At the 7 day retention test there was a trend toward significance for higher CR percentages at the CS-alone test in adults relative to periweanlings, though there were no age differences at reacquisition testing. When testing occurred 28 days after acquisition, however, periweanlings showed fewer CRs relative to adults during reacquisition despite low CR levels in both ages throughout the CS-alone test. Furthermore, periweanlings in the 28 day group required more trials at reacquisition than all other groups to exceed CR levels from their first acquisition session. These findings are consistent with rapid forgetting in the young commonly referred to as "infantile amnesia." The well-characterized eyeblink preparation may be useful for future studies investigating neural mechanisms responsible for rapid forgetting in developing animals.

Medial auditory thalamus is necessary for acquisition and retention of eyeblink conditioning to cochlear nucleus stimulation

Associative learning tasks commonly involve an auditory stimulus, which must be projected through the auditory system to the sites of memory induction for learning to occur. The cochlear nucleus (CN) projection to the pontine nuclei has been posited as the necessary auditory pathway for cerebellar learning, including eyeblink conditioning. However, the medial auditory thalamic nuclei (MATN), consisting of the medial division of the medial geniculate, suprageniculate, and posterior interlaminar nucleus have also been implicated as a critical auditory relay to the pontine nuclei for cerebellum-dependent motor learning. The MATN also conveys auditory information to the amygdala necessary for avoidance and fear conditioning. The current study used CN stimulation to increase activity in the pontine nuclei, relative to a tone stimulus, and possibly provide sufficient input to the cerebellum for acquisition or retention of eyeblink conditioning during MATN inactivation. Primary and secondary effects of CN stimulation and MATN inactivation were examined using 2-deoxy-glucose autoradiography. Stimulation of CN increased activity in the pontine nuclei, however, this increase was not sufficient for cerebellar learning during MATN inactivation. Results of the current experiment provide additional evidence indicating the MATN may be the critical auditory relay for many associative learning tasks.

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

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

Cerebellar-dependent expression of motor learning during eyeblink conditioning in head-fixed mice

Eyeblink conditioning in restrained rabbits has served as an excellent model of cerebellar-dependent motor learning for many decades. In mice, the role of the cerebellum in eyeblink conditioning is less clear and remains controversial, partly because learning appears to engage fear-related circuits and lesions of the cerebellum do not abolish the learned behavior completely. Furthermore, experiments in mice are performed using freely moving systems, which lack the stability necessary for mapping out the essential neural circuitry with electrophysiological approaches. We have developed a novel apparatus for eyeblink conditioning in head-fixed mice. Here, we show that the performance of mice in our apparatus is excellent and that the learned behavior displays two hallmark features of cerebellar-dependent eyeblink conditioning in rabbits: (1) gradual acquisition; and (2) adaptive timing of conditioned movements. Furthermore, we use a combination of pharmacological inactivation, electrical stimulation, single-unit recordings, and targeted microlesions to demonstrate that the learned behavior is completely dependent on the cerebellum and to pinpoint the exact location in the deep cerebellar nuclei that is necessary. Our results pave the way for using eyeblink conditioning in head-fixed mice as a platform for applying next-generation genetic tools to address molecular and circuit-level questions about cerebellar function in health and disease.

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

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

A hypothetical universal model of cerebellar function: reconsideration of the current dogma

The cerebellum is commonly studied in the context of the classical eyeblink conditioning model, which attributes an adaptive motor function to cerebellar learning processes. This model of cerebellar function has quite a few shortcomings and may in fact be somewhat deficient in explaining the myriad functions attributed to the cerebellum, functions ranging from motor sequencing to emotion and cognition. The involvement of the cerebellum in these motor and non-motor functions has been demonstrated in both animals and humans in electrophysiological, behavioral, tracing, functional neuroimaging, and PET studies, as well as in clinical human case studies. A closer look at the cerebellum's evolutionary origin provides a clue to its underlying purpose as a tool which evolved to aid predation rather than as a tool for protection. Based upon this evidence, an alternative model of cerebellar function is proposed, one which might more comprehensively account both for the cerebellum's involvement in a myriad of motor, affective, and cognitive functions and for the relative simplicity and ubiquitous repetitiveness of its circuitry. This alternative model suggests that the cerebellum has the ability to detect coincidences of events, be they sensory, motor, affective, or cognitive in nature, and, after having learned to associate these, it can then trigger (or "mirror") these events after having temporally adjusted their onset based on positive/negative reinforcement. The model also provides for the cerebellum's direction of the proper and uninterrupted sequence of events resulting from this learning through the inhibition of efferent structures (as demonstrated in our lab).

The ontogeny of associative cerebellar learning

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

Amygdala inactivation impairs eyeblink conditioning in developing rats

The amygdala facilitates acquisition of eyeblink conditioning in adult animals by enhancing conditioned stimulus (CS) inputs to the cerebellum and the unconditioned response circuitry. Ontogenetic changes in amygdala modulation of eyeblink conditioning have not been investigated directly. We examined the effects of amygdala inactivation on the ontogeny of eyeblink conditioning and conditioned freezing in rat pups. Rat pups received bilateral infusions of saline or bupivacaine into the central nucleus of the amygdala before each of the first five training sessions, which consisted of paired CS-US trials on postnatal days (P) 17-19, P21-23, or P24-26. The final session consisted of CS-alone test trials to assess the effect of amygdala inactivation during training on conditioned freezing. Amygdala inactivation impaired acquisition of eyeblink conditioning in all of the age groups and impaired freezing to the context during the extinction test. The results indicate that the amygdala modulates cerebellar learning as soon as it begins to emerge ontogenetically.

Amygdala's involvement in facilitating associative learning-induced plasticity: a promiscuous role for the amygdala in memory acquisition

It is widely accepted that the amygdala plays a critical role in acquisition and consolidation of fear-related memories. Some of the more widely employed behavioral paradigms that have assisted in solidifying the amygdala's role in fear-related memories are associative learning paradigms. With most associative learning tasks, a neutral conditioned stimulus (CS) is paired with a salient unconditioned stimulus (US) that elicits an unconditioned response (UR). After multiple CS-US pairings, the subject learns that the CS predicts the onset or delivery of the US, and thus elicits a learned conditioned response (CR). Most fear-related associative paradigms have suggested that an aspect of the fear association is stored in the amygdala; however, some fear-motivated associative paradigms suggest that the amygdala is not a site of storage, but rather facilitates consolidation in other brain regions. Based upon various learning theories, one of the most likely sites for storage of long-term memories is the neocortex. In support of these theories, findings from our laboratory, and others, have demonstrated that trace-conditioning, an associative paradigm where there is a separation in time between the CS and US, induces learning-specific neocortical plasticity. The following review will discuss the amygdala's involvement, either as a site of storage or facilitating storage in other brain regions such as the neocortex, in fear- and non-fear-motivated associative paradigms. In this review, we will discuss recent findings suggesting a broader role for the amygdala in increasing the saliency of behaviorally relevant information, thus facilitating acquisition for all forms of memory, both fear- and non-fear-related. This proposed promiscuous role of the amygdala in facilitating acquisition for all memories further suggests a potential role of the amygdala in general learning disabilities.

Developmental changes in medial auditory thalamic contributions to associative motor learning

Eyeblink conditioning (EBC) was used in the current study to examine the mechanisms underlying the ontogeny of associative motor learning in rats. Eyeblink conditioning emerges ontogenetically between postnatal day 17 (P17) and P24 in rats. Previous studies used electrical stimulation to show that the ontogeny of EBC is influenced by developmental changes in input from the medial auditory thalamus to the pontine nuclei, which in turn affects input to the cerebellum. The current study used tetrode recordings to examine the ontogeny of medial auditory thalamic sensory responses to the conditioned stimulus (CS) and learning-related activity during EBC. Rat pups were implanted with multiple tetrodes in the medial nucleus of the medial geniculate (MGm) and suprageniculate (SG) and trained on delay EBC on P17-P19, P24-P26, or P31-P33 while recording spike activity. Developmental changes in MGm and SG sensory-related activity were found during a pretraining session with unpaired presentations of the auditory CS and periorbital stimulation unconditioned stimulus (US). Substantial developmental changes were observed in learning-related activity in the MGm and SG during CS-US paired training. The ontogenetic changes in learning-related activity may be related to developmental changes in input to the medial auditory thalamus from the amygdala and cerebellum. The findings suggest that the ontogeny of associative motor learning involves developmental changes in sensory input to the thalamus, amygdala input to the thalamus, thalamic input to the pontine nuclei, and cerebellar feedback to the thalamus.

Developmental changes in medial auditory thalamic contributions to associative motor learning

Eyeblink conditioning (EBC) was used in the current study to examine the mechanisms underlying the ontogeny of associative motor learning in rats. Eyeblink conditioning emerges ontogenetically between postnatal day 17 (P17) and P24 in rats. Previous studies used electrical stimulation to show that the ontogeny of EBC is influenced by developmental changes in input from the medial auditory thalamus to the pontine nuclei, which in turn affects input to the cerebellum. The current study used tetrode recordings to examine the ontogeny of medial auditory thalamic sensory responses to the conditioned stimulus (CS) and learning-related activity during EBC. Rat pups were implanted with multiple tetrodes in the medial nucleus of the medial geniculate (MGm) and suprageniculate (SG) and trained on delay EBC on P17-P19, P24-P26, or P31-P33 while recording spike activity. Developmental changes in MGm and SG sensory-related activity were found during a pretraining session with unpaired presentations of the auditory CS and periorbital stimulation unconditioned stimulus (US). Substantial developmental changes were observed in learning-related activity in the MGm and SG during CS-US paired training. The ontogenetic changes in learning-related activity may be related to developmental changes in input to the medial auditory thalamus from the amygdala and cerebellum. The findings suggest that the ontogeny of associative motor learning involves developmental changes in sensory input to the thalamus, amygdala input to the thalamus, thalamic input to the pontine nuclei, and cerebellar feedback to the thalamus.

Neural circuitry and plasticity mechanisms underlying delay eyeblink conditioning

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

The effects of two forms of physical activity on eyeblink classical conditioning

Voluntary exercise, in the form of free access to a running wheel in the home cage, has been shown to improve several forms of learning and memory. Acrobatic training, in the form of learning to traverse an elevated obstacle course, has been shown to induce markers of neural plasticity in the cerebellar cortex in rodents. In three experiments, we examined the effects of these two forms of physical activity on delay eyeblink conditioning in rats. In Experiment 1, exercising rats were given 17 days of free access to a running wheel in their home cage prior to 10 days of delay eyeblink conditioning. Rats that exercised conditioned significantly better and showed a larger reflexive eyeblink unconditioned response to the periocular stimulation unconditioned stimulus than rats that did not exercise. In Experiment 2, exercising rats were given 17 days of free access to a running wheel in their home cage prior to 10 days of explicitly unpaired stimulus presentations. Rats that exercised responded the same to tone, light, and periocular stimulation as rats that did not exercise. In Experiment 3, acrobatic training rats were given 15 days of daily training on an elevated obstacle course prior to 10 days of eyeblink conditioning. Activity control rats underwent 15 days of yoked daily running in an open field. Rats that underwent acrobatic training did not differ in eyeblink conditioning from activity control rats. The ability to measure the learned response precisely, and the well-mapped neural circuitry of eyeblink conditioning offer some advantages for the study of exercise effects on learning and memory.

Hippocampal ripple-contingent training accelerates trace eyeblink conditioning and retards extinction in rabbits

There are at least two distinct oscillatory states of the hippocampus that are related to distinct behavioral patterns. Theta (4-12 Hz) oscillation has been suggested to indicate selective attention during which the animal concentrates on some features of the environment while suppressing reactivity to others. In contrast, sharp-wave ripples ( approximately 200 Hz) can be seen in a state in which the hippocampus is at its most responsive to any kind of afferent stimulation. In addition, external stimulation tends to evoke and reset theta oscillation, the phase of which has been shown to modulate synaptic plasticity in the hippocampus. Theoretically, training on a hippocampus-dependent learning task contingent upon ripples could enhance learning rate due to elevated responsiveness and enhanced phase locking of the theta oscillation. We used a brain-computer interface to detect hippocampal ripples in rabbits to deliver trace eyeblink conditioning and extinction trials selectively contingent upon them. A yoked control group was trained regardless of their ongoing neural state. Ripple-contingent training expedited acquisition of the conditioned response early in training and evoked stronger theta-band phase locking to the conditioned stimulus. Surprisingly, ripple-contingent training also resulted in slower extinction in well trained animals. We suggest that the ongoing oscillatory activity in the hippocampus determines the extent to which a stimulus can induce a phase reset of the theta oscillation, which in turn is the determining factor of learning rate in trace eyeblink conditioning.

Amygdala conditioning modulates sensory input to the cerebellum

Localization of emotional learning in the amygdala and discrete motor learning in the cerebellum provides empirical means to study the mechanisms mediating the interaction between fast emotional and slow motor learning. Behavioral studies have demonstrated that fear conditioning facilitates the motor conditioning. The present study tests the hypothesis that the amygdala output induces this facilitation by increasing the salience of the conditioned stimulus (CS) representation in the pontine nucleus (PN) input to the cerebellum. Paired trials of CS-US (unconditioned stimulus) were applied to anesthetized rats, a condition that allows for amygdala-based fear conditioning but not cerebellar-based motor conditioning. Multiple unit recordings in the PN served to assess the salience of the CS. Results showed that CS-US conditioning increased the PN-reactivity to the CS. Lidocaine-induced reversible inactivation of the amygdala prevented the facilitatory effect of conditioning on the PN-reactivity to the CS. These findings suggest that the amygdala-based conditioned responses reach the PN and increase the salience of the CS signal there, perhaps facilitating cerebellar conditioning. This facilitatory effect of the amygdala may be conceptualized under the 'two-stage theory of learning', which predicts that emotional learning in the first stage accelerates the motor learning in the second stage. We hereby demonstrate the physiological mechanism through which fast emotional learning in the first stage facilitates slow cerebellar learning in the second stage.

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

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

The basolateral nucleus of the amygdala is necessary to induce the opposing effects of stressful experience on learning in males and females

The basolateral nucleus of the amygdala (BLA) has been implicated in the modulation of learning after stress. Acute inescapable stress enhances classical eyeblink conditioning in male rats, whereas the same stressor impairs eyeblink conditioning in female rats. The experiments here directly assessed whether inactivation of the BLA during stress exposure would block both the stress-induced facilitation in males and the retardation of eyeblink conditioning in females. To this end, the BLA was temporarily inactivated by infusion of the GABA agonist muscimol before acute stressor exposure. All rats were trained in a different context 24 h later. Males infused with muscimol before the stressful event did not exhibit facilitated eyeblink conditioning, whereas those infused with the vehicle emitted more conditioned responses than unstressed males. Females infused with muscimol before stress did not express a deficit in conditioning, whereas those infused with vehicle and stressed emitted fewer conditioned responses than unstressed vehicle controls. These data demonstrate that neuronal activity within the BLA during stress exposure is necessary to modulate learning 24 h later in a new context. Thus, the BLA is necessary to induce the long-term effect of stressful experience on conditioning regardless of sex and direction of modulation.

Perirhinal cortex lesions impair feature-negative discrimination

The role of the perirhinal cortex in inhibitory eyeblink conditioning was examined. In Experiment 1, rats were given lesions of the perirhinal cortex or control surgery and subsequently trained with a feature-negative discrimination procedure followed by summation and retardation tests for conditioned inhibition. Perirhinal cortex lesions impaired, but did not prevent acquisition of feature-negative discrimination. Results from the summation test showed that rats with perirhinal cortex lesions could not generalize feature-negative discrimination to a new stimulus. There were no group differences during the retardation test. Experiment 2 showed that lesions of the perirhinal cortex did not impair simple excitatory conditioning. Experiment 3 showed that perirhinal cortex lesions had no effect on acquisition of a simple tone-light discrimination. The results suggest that the perirhinal cortex plays a role in eyeblink conditioning when using discrimination procedures involving overlapping stimuli.

Molecular evidence for two-stage learning and partial laterality in eyeblink conditioning of mice

The anterior interpositus nucleus (AIN) is the proposed site of memory formation of eyeblink conditioning. A large part of the underlying molecular events, however, remain unknown. To elucidate the molecular mechanisms, we examined transcriptional changes in the AIN of mice trained with delay eyeblink conditioning using microarray, quantitative real-time RT-PCR, and in situ hybridization techniques. Microarray analyses suggested that transcriptionally up-regulated gene sets were largely different between early (3-d training) and late (7-d) stages. Quantitative real-time RT-PCR aided by laser microdissection indicated that the expression of representative EARLY genes (Sgk, IkBa, and Plekhf1) peaked at 1-d training in both the paired and unpaired conditioning groups, and was maintained at a higher level in the paired group than in the unpaired group after 3-d training. In situ hybridization revealed increased expression of these genes in broad cerebellar areas, including the AIN, with no hemispheric preferences. In contrast, the expression of representative LATE genes (Vamp1, Camk2d, and Prkcd) was selectively increased in the AIN of the 7-d paired group, with dominance in the ipsilateral AIN. Increased Vamp1 mRNA expression was restricted to the ipsilateral dorsolateral hump, a subregion of the AIN. These expression patterns of two distinct subsets of genes fit well with the two-stage learning theory, which proposes emotional and motor learning phases, and support the notion that AIN has a crucial role in memory formation of eyeblink conditioning.

Amygdalar unit activity during three learning tasks: eyeblink classical conditioning, Pavlovian fear conditioning, and signaled avoidance conditioning

Neural activity in central and basolateral amygdala nuclei (CeA and BLA, respectively) was recorded during delay eyeblink conditioning, Pavlovian fear conditioning, and signaled barpress avoidance. During paired training, the CeA exhibited robust learning-related excitatory activity during all 3 tasks. By contrast, the BLA exhibited minimal activity during eyeblink conditioning, while demonstrating pronounced increases in learning-related excitatory responsiveness during fear conditioning and barpress avoidance. In addition, the relative amount of amygdalar activation observed appeared to be related to the relative intensity of the unconditioned stimulus and somatic requirements of the task. Results suggest the CeA mediates the Pavlovian association between sensory stimuli and the BLA mediates the modulation of instrumental responding through the assignment of motivational value to the unconditioned stimulus.

The effects of amygdala lesions on hippocampal activity and classical eyeblink conditioning in rats

The hippocampus and the amygdala have long been associated with memory, emotion, and motivated behaviors. Although the role of these two brain areas in learning a simple, discrete motor response has been well studied, a definitive theory concerning their functions remains elusive. The present experiment involved selective lesions of the central nucleus (CE) or the basolateral nucleus (BA) of the amygdala in rats followed by single-unit analyses of hippocampal CA1 subfield activity during classical eye blink conditioning. Removal of CE or BA adversely affected the development of conditioned responding. Differences between groups in the patterns of hippocampal activity were observed. Similar to previous rabbit studies, hippocampal activity recorded from sham rats showed that CA1 cells became active during the CS-US period as conditioning progressed with activity especially prevalent just prior to US onset. Increased activity over training was seen during the CS-US interval in CE-lesioned rats, but the pattern differed from control rats-uniform excitation was seen across the entire CS-US period. BA-lesioned rats initially showed uniform CS-US period activation in early phases of training, but then showed patterns of hippocampal activity that resembled control rats in later stages of conditioning. The data suggest that the amygdala may play a modulatory role in the acquisition of conditioned eye blink responses and also in the formation of learning-related activity in the hippocampus.

Balance dysfunction in childhood anxiety: findings and theoretical approach

A recent special issue of the Journal of Anxiety Disorders, reviewed the experimental and clinical findings related to comorbidity of balance disorders and anxiety [J. Anxiety Disord. 15 (2001) 1.]. The studies mentioned in that issue were based mostly on adult subjects but prevalence of balance disorders in childhood anxiety is yet to be established. We have tested a small sample of children diagnosed for general or separation anxiety disorder and a control group of normal children. Extensive neurological examination revealed no clinically relevant vestibular impairment. Nevertheless, detailed questionnaires and balance tests confirmed an excessive sensitivity of anxiety disordered children to balance-challenging situations. Moreover, balance-challenging tasks triggered more balance mistakes and slower performance in anxiety versus control children. These findings support the notion of subclinical balance disorder in childhood anxiety. Results are discussed in terms of the two-stage theory of learning, which predicts that anxiety disorder may be an offshoot of lasting balance dysfunction.

Differential effects of cerebellar, amygdalar, and hippocampal lesions on classical eyeblink conditioning in rats

Eyeblink conditioning has been hypothesized to engage two successive stages of nonspecific emotional (fear) and specific musculature (eyelid) learning, during which the nonspecific component influences the acquisition of the specific component. Here we test this notion by investigating the relative contributions of the cerebellum, the amygdala, and the hippocampus to the emergence of conditioned eyelid and fear responses during delay eyeblink conditioning in freely moving rats. Periorbital electromyography (EMG) and 22 kHz ultrasonic vocalization (USV) activities were measured concurrently from the same subjects and served as indices of conditioned eyeblink and fear responses, respectively. In control animals, conditioned EMG responses increased across training sessions, whereas USV responses were initially robust but decreased across training sessions. Animals with electrolytic lesions to their cerebellum (targeting the interpositus nucleus) were completely unable to acquire conditioned EMG responses but exhibited normal USV behavior, whereas animals with lesions to the amygdala showed decelerated acquisition of conditioned EMG responses and displayed practically no USV behavior. In contrast, hippocampal lesioned rats demonstrated facilitated acquisition of conditioned EMG responses, whereas the USV behavior was unaffected. The amygdalar involvement in eyeblink conditioning was examined further by applying the GABA(A) agonist muscimol directly into the amygdala either before or immediately after training sessions. Although pretraining muscimol infusions impaired conditioned EMG responses, post-training infusions did not. Together, these results suggest that, even during a simple delay eyeblink conditioning, animals learn about different aspects associated with the behavioral task that are subserved by multiple brain-memory systems that interact to produce the overall behavior.

Neural Substrates of Eyeblink Conditioning: Acquisition and Retention

Classical conditioning of the eyeblink reflex to a neutral stimulus that predicts an aversive stimulus is a basic form of associative learning. Acquisition and retention of this learned response require the cerebellum and associated sensory and motor pathways and engage several other brain regions including the hippocampus, neocortex, neostriatum, septum, and amygdala. The cerebellum and its associated circuitry form the essential neural system for delay eyeblink conditioning. Trace eyeblink conditioning, a learning paradigm in which the conditioned and unconditioned stimuli are noncontiguous, requires both the cerebellum and the hippocampus and exhibits striking parallels to declarative memory formation in humans. Identification of the neural structures critical to the development and maintenance of the conditioned eyeblink response is an essential precursor to the investigation of the mechanisms responsible for the formation of these associative memories. In this review, we describe the evidence used to identify the neural substrates of classical eyeblink conditioning and potential mechanisms of memory formation in critical regions of the hippocampus and cerebellum. Addressing a central goal of behavioral neuroscience, exploitation of this simple yet robust model of learning and memory has yielded one of the most comprehensive descriptions to date of the physical basis of a learned behavior in mammals.

Impaired conditional discrimination learning in social phobia

Eyelid conditional discrimination learning (ECDL) is a test of discriminative aversive conditioning. It places minimal demands on motivation and was shown to selectively test temporal lobe function. Twenty-five unmedicated social phobia (SP) patients (mean age 29.5 +/- 7.0 years), diagnosed according to DSM-IV criteria, and 25 age- and gender-matched healthy controls (HC, mean age 34.0 +/- 8.6 years) were examined with an ECDL paradigm. In the experiment two differently colored stimuli are randomly presented. Only one of the stimuli (reinforced trial) is followed by an aversive airpuff to the cornea, as opposed to unreinforced trials not followed by an airpuff. Conditioned responses (CRs) consist of reflex eyelid closures already upon light presentation. HC as well as SP patients showed a significant difference between reinforced and unreinforced trials. In SP patients, CR frequency did not increase during the ECDL task, while HCs showed appropriate conditional discrimination ability. Thus the results indicate an impairment of adequate behavior modification in an aversive conditioning task in SP.

Monitoring kinetic and frequency-domain properties of eyelid responses in mice with magnetic distance measurement technique

Classical eye-blink conditioning in mutant mice can be used to study the molecular mechanisms underlying associative learning. To measure the kinetic and frequency domain properties of conditioned (tone - periorbital shock procedure) and unconditioned eyelid responses in freely moving mice, we developed a method that allows adequate, absolute, and continuous determination of their eyelid movements in time and space while using an electrical shock as the unconditioned stimulus. The basic principle is to generate a local magnetic field that moves with the animal and that is picked up by either a field-sensitive chip or coil. With the use of this magnetic distance measurement technique (MDMT), but not with the use of electromyographic recordings, we were able to measure mean latency, peak amplitude, velocity, and acceleration of unconditioned eyelid responses, which equaled 7.9 +/- 0.2 ms, 1.2 +/- 0.02 mm, 28.5 +/- 1 mm/s, and 637 +/- 22 mm/s(2), respectively (means +/- SD). During conditioning, the mice reached an average of 78% of conditioned responses over four training sessions, while animals that were subjected to randomly paired conditioned and unconditioned stimuli showed no significant increases. The mean latency of the conditioned responses decreased from 222 +/- 40 ms in session 2 to 127 +/- 6 ms in session 4, while their mean peak latency increased from 321 +/- 45 to 416 +/- 67 ms. The mean peak amplitudes, peak velocities, and peak acceleration of these responses increased from 0.62 +/- 0.02 to 0.77 +/- 0.02 mm, from 3.9 +/- 0.3 to 7.7 +/- 0.5 mm/s, and from 81 +/- 7 to 139 +/- 10 mm/s(2), respectively. Power spectra of acceleration records illustrated that both the unconditioned and conditioned responses of mice had oscillatory properties with a dominant peak frequency close to 25 Hz that was not dependent on training session, interstimulus interval, or response size. These data show that MDMT can be used to measure the kinetics and frequency domain properties of conditioned eyelid responses in mice and that these properties follow the dynamic characteristics of other mammals.

Imaging learning and memory: classical conditioning

The search for the biological basis of learning and memory has, until recently, been constrained by the limits of technology to classic anatomic and electrophysiologic studies. With the advent of functional imaging, we have begun to delve into what, for many, was a "black box." We review several different types of imaging experiments, including steady state animal experiments that image the functional labeling of fixed tissues, and dynamic human studies based on functional imaging of the intact brain during learning. The data suggest that learning and memory involve a surprising conservation of mechanisms and the integrated networking of a number of structures and processes.