Medial auditory thalamic stimulation as a conditioned stimulus for eyeblink conditioning in rats

Positive feedback of efferent copy via pontine nucleus facilitates cerebellum-mediated associative learning

The cerebellum is critical for motor coordination and learning. However, the role of feedback circuitry in this brain region has not been fully explored. Here, we characterize a nucleo-ponto-cortical feedback pathway in classical delayed eyeblink conditioning (dEBC) of rats. We find that the efference copy is conveyed from the interposed cerebellar nucleus (Int) to cerebellar cortex through pontine nucleus (PN). Inhibiting or exciting the projection from the Int to the PN can decelerate or speed up acquisition of dEBC, respectively. Importantly, we identify two subpopulations of PN neurons (PN1 and PN2) that convey and integrate the feedback signals with feedforward sensory signals. We also show that the feedforward and feedback pathways via different types of PN neurons contribute to the plastic changes and cooperate synergistically to the learning of dEBC. Our results suggest that this excitatory nucleo-ponto-cortical feedback plays a significant role in modulating associative motor learning in cerebellum.

Functional properties of eyelid conditioned responses and involved brain centers

For almost a century the classical conditioning of nictitating membrane/eyelid responses has been used as an excellent and feasible experimental model to study how the brain organizes the acquisition, storage, and retrieval of new motor abilities in alert behaving mammals, including humans. Lesional, pharmacological, and electrophysiological approaches, and more recently, genetically manipulated animals have shown the involvement of numerous brain areas in this apparently simple example of associative learning. In this regard, the cerebellum (both cortex and nuclei) has received particular attention as a putative site for the acquisition and storage of eyelid conditioned responses, a proposal not fully accepted by all researchers. Indeed, the acquisition of this type of learning implies the activation of many neural processes dealing with the sensorimotor integration and the kinematics of the acquired ability, as well as with the attentional and cognitive aspects also involved in this process. Here, we address specifically the functional roles of three brain structures (red nucleus, cerebellar interpositus nucleus, and motor cortex) mainly involved in the acquisition and performance of eyelid conditioned responses and three other brain structures (hippocampus, medial prefrontal cortex, and claustrum) related to non-motor aspects of the acquisition process. The main conclusion is that the acquisition of this motor ability results from the contribution of many cortical and subcortical brain structures each one involved in specific (motor and cognitive) aspects of the learning process.

Medial Auditory Thalamus Is Necessary for Expression of Auditory Trace Eyelid Conditioning

Transforming a brief sensory event into a persistent neural response represents a mechanism for linking temporally disparate stimuli together to support learning. The cerebellum requires this type of persistent input during trace conditioning to engage associative plasticity and acquire adaptively timed conditioned responses (CRs). An initial step toward identifying the sites and mechanisms generating and transmitting persistent signals to the cerebellum is to identify the input pathway. The medial auditory thalamic nuclei (MATN) are the necessary and sufficient source of auditory input to the cerebellum for delay conditioning in rodents and a possible input to forebrain sites generating persistent signals. Using pharmacological and computational approaches, we test (1) whether the necessity of MATN during auditory eyelid conditioning is conserved across species, (2) whether the MATN are necessary for the expression of trace eyelid CRs, and if so, (3)whether this relates to the generation of persistent signals. We find that contralateral inactivation of MATN with muscimol largely abolished trace and delay CRs in male rabbits. Residual CRs were decreased in amplitude, but CR timing was unaffected. Results from large-scale cerebellar simulations are consistent with previous experimental demonstrations that silencing only CS-duration inputs does not abolish trace CRs, and instead affects their timing. Together, these results suggest that the MATN are a necessary component of both the direct auditory stimulus pathway to the cerebellum and the pathway generating task-essential persistent signals.SIGNIFICANCE STATEMENT Persistent activity is required for working memory-dependent tasks, such as trace conditioning, and represents a mechanism by which sensory information can be used over time for learning and cognition. This neuronal response entails the transformation of a discrete sensory-evoked response into a signal that extends beyond the stimulus event. Understanding the generation and transmission of this stimulus transformation requires identifying the input sources necessary for task-essential persistent signals. We report that the medial auditory thalamic nuclei are required for the expression of auditory trace conditioning and suggest that these nuclei are a component of the pathway-generating persistent signals. Our study provides a foundation for testing circuit-level mechanisms underlying persistent activity in a cerebellar learning model with identified inputs and well defined behavioral outputs.

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.

The Motor Cortex Is Involved in the Generation of Classically Conditioned Eyelid Responses in Behaving Rabbits

Classical blink conditioning is a well known model for studying neural generation of acquired motor responses. The acquisition of this type of associative learning has been related to many cortical, subcortical, and cerebellar structures. However, until now, no one has studied the motor cortex (MC) and its possible role in classical eyeblink conditioning. We recorded in rabbits the activity of MC neurons during blink conditioning using a delay paradigm. Neurons were identified by their antidromic activation from facial nucleus (FN) or red nucleus (RN). For conditioning, we used a tone as a conditioned stimulus (CS) followed by an air puff as an unconditioned stimulus (US) that coterminated with it. Conditioned responses (CRs) were determined from the electromyographic activity of the orbicularis oculi muscle and/or from eyelid position recorded with the search coil technique. Type A neurons increased their discharge rates across conditioning sessions and reached peak firing during the CS-US interval, while type B cells presented a second peak during US presentation. Both of them project to the FN. Type C cells increased their firing across the CS-US interval, reaching peak values at the time of US presentation, and were activated from the RN. These three types of neurons fired well in advance of the beginning of CRs and changed with them. Reversible inactivation of the MC during conditioning evoked a decrease in learning curves and in the amplitude of CRs, while train stimulation of the MC simulated the profile and kinematics of conditioned blinks. In conclusion, MC neurons are involved in the acquisition and expression of CRs.

SIGNIFICANCE STATEMENT

Classical blink conditioning is a popular experimental model for studying neural mechanisms underlying the acquisition of motor skills. The acquisition of this type of associative learning has been related to many cortical, subcortical, and cerebellar structures. However, until now, no one has studied the motor cortex (MC) and its possible role in classical eyeblink conditioning. Here, we report that the firing activities of MC neurons, recorded in behaving rabbits, are related to and preceded the initiation of conditioned blinks. MC neurons were identified as projecting to the red or facial nuclei and encoded the kinematics of conditioned eyelid responses. The timed stimulation of recording sites simulated the profile of conditioned blinks. MC neurons play a role in the acquisition and expression of these acquired motor responses.

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.

Amygdala Modulation of Cerebellar Learning

Previous studies showed that amygdala lesions or inactivation slow the acquisition rate of cerebellum-dependent eyeblink conditioning, a type of associative motor learning. The current study was designed to determine the behavioral nature of amygdala-cerebellum interactions, to identify the neural pathways underlying amygdala-cerebellum interactions, and to examine how the amygdala influences cerebellar learning mechanisms in rats. Pharmacological inactivation of the central amygdala (CeA) severely impaired acquisition and retention of eyeblink conditioning, indicating that the amygdala continues to interact with the cerebellum after conditioning is consolidated (Experiment 1). CeA inactivation also substantially reduced stimulus-evoked and learning-related neuronal activity in the cerebellar anterior interpositus nucleus during acquisition and retention of eyeblink conditioning (Experiment 2). A very small proportion of cerebellar neurons responded to the conditioned stimulus (CS) during CeA inactivation. Finally, retrograde and anterograde tracing experiments identified the basilar pontine nucleus at the confluence of outputs from CeA that may support amygdala modulation of CS input to the cerebellum (Experiment 3). Together, these results highlight a role for the CeA in the gating of CS-related input to the cerebellum during motor learning that is maintained even after the conditioned response is well learned.

SIGNIFICANCE STATEMENT

The current study is the first to demonstrate that the amygdala modulates sensory-evoked and learning-related neuronal activity within the cerebellum during acquisition and retention of associative learning. The findings suggest a model of amygdala-cerebellum interactions in which the amygdala gates conditioned stimulus inputs to the cerebellum through a direct projection from the medial central nucleus to the basilar pontine nucleus. Amygdala gating of sensory input to the cerebellum may be an attention-like mechanism that facilitates cerebellar learning. In contrast to previous theories of amygdala-cerebellum interactions, the sensory gating hypothesis posits that the gating mechanism continues to be necessary for retrieval of cerebellar memory after learning is well established.

Listening to another sense: somatosensory integration in the auditory system

Conventionally, sensory systems are viewed as separate entities, each with its own physiological process serving a different purpose. However, many functions require integrative inputs from multiple sensory systems and sensory intersection and convergence occur throughout the central nervous system. The neural processes for hearing perception undergo significant modulation by the two other major sensory systems, vision and somatosensation. This synthesis occurs at every level of the ascending auditory pathway: the cochlear nucleus, inferior colliculus, medial geniculate body and the auditory cortex. In this review, we explore the process of multisensory integration from (1) anatomical (inputs and connections), (2) physiological (cellular responses), (3) functional and (4) pathological aspects. We focus on the convergence between auditory and somatosensory inputs in each ascending auditory station. This review highlights the intricacy of sensory processing and offers a multisensory perspective regarding the understanding of sensory disorders.

Optogenetic stimulation of mPFC pyramidal neurons as a conditioned stimulus supports associative learning in rats

It is generally accepted that the associative learning occurs when a behaviorally neutral conditioned stimulus (CS) is paired with an aversive unconditioned stimulus (US) in close temporal proximity. Eyeblink conditioning (EBC) is a simple form of associative learning for motor responses. Specific activation of a population of cells may be an effective and sufficient CS for establishing EBC. However, there has been no direct evidence to support this hypothesis. Here, we show in rats that optogenetic activation of the right caudal mPFC pyramidal neurons as a CS is sufficient to support the acquisition of delay eyeblink conditioning (DEC). Interestingly, the associative memory was not stably expressed during the initial period of daily conditioning session even after the CR acquisition reached the asymptotic level. Finally, the intensity and consistency of the CS were found to be crucial factors in regulating the retrieval of the associative memory. These results may be of importance in understanding the neural cellular mechanisms underlying associative learning and the mechanisms underlying retrieval process of memory.

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.

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.

Cerebellar learning mechanisms

The mechanisms underlying cerebellar learning are reviewed with an emphasis on old arguments and new perspectives on eyeblink conditioning. Eyeblink conditioning has been used for decades a model system for elucidating cerebellar learning mechanisms. The standard model of the mechanisms underlying eyeblink conditioning is that there two synaptic plasticity processes within the cerebellum that are necessary for acquisition of the conditioned response: (1) long-term depression (LTD) at parallel fiber-Purkinje cell synapses and (2) long-term potentiation (LTP) at mossy fiber-interpositus nucleus synapses. Additional Purkinje cell plasticity mechanisms may also contribute to eyeblink conditioning including LTP, excitability, and entrainment of deep nucleus activity. Recent analyses of the sensory input pathways necessary for eyeblink conditioning indicate that the cerebellum regulates its inputs to facilitate learning and maintain plasticity. Cerebellar learning during eyeblink conditioning is therefore a dynamic interactive process which maximizes responding to significant stimuli and suppresses responding to irrelevant or redundant stimuli. This article is part of a Special Issue entitled SI: Brain and Memory.

Sensory system development influences the ontogeny of eyeblink conditioning

A rate-limiting factor in the ontogeny of auditory eyeblink conditioning (EBC) is the development of sensory inputs to the pontine nucleus. One possible way to facilitate the emergence of EBC would be to use a conditioned stimulus (CS) that activates an earlier-developing sensory system. The goal of the current study was to investigate whether using a vibration CS would facilitate the ontogeny of delay EBC relative to an auditory CS. Rat pups received six sessions of delay EBC or unpaired training using either a tone or vibration CS on postnatal day (P)14-15, 17-18, 21-22, or 24-25. Conditioning with a vibration CS resulted in rapid learning as early as P17-18, whereas conditioning with a tone CS did not result in rapid conditioning until after P17-18. Control experiments verified that the differences in EBC were due to CS-specific sensory properties. The results suggest that the ontogeny of EBC depends on sensory system development.

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.

Inactivation of the ventral lateral geniculate and nucleus of the optic tract impairs retention of visual eyeblink conditioning

Reversible inactivation was used to examine the conditioned stimulus (CS) pathway for visual eyeblink conditioning (EBC). Previous research has shown that the ventral lateral geniculate (LGNv) and nucleus of the optic tract (NOT) could play a role in visual EBC through ipsilateral projections to the medial pontine nuclei. Rats were given visual EBC followed by inactivation of the ventral lateral geniculate (LGNv), nucleus of the optic tract (NOT), or both nuclei contralateral to the conditioned eye. Muscimol infusions into the NOT or LGNv impaired retention. Combined inactivation of LGNv/NOT produced the most severe impairment. Rats given inactivation of these visual nuclei after training with a vibration CS showed no impairment. The findings indicate that a parallel pathway of visual CS information projects from the LGNv and NOT to the medial pontine nuclei.

Reevaluating the role of the hippocampus in delay eyeblink conditioning

The role of the hippocampus in delay eyeblink conditioning (DEC) remains controversial. Here, we investigated the involvement of the hippocampus in DEC with a soft tone as the conditioned stimulus (CS) by using electrolytic lesions or muscimol inactivation of guinea pig dorsal hippocampus. Interestingly, when a soft tone was used as a CS, electrolytic lesions of the hippocampus significantly retarded acquisition of the conditioned response (CR), and muscimol infusions into hippocampus distinctly inhibited the acquisition and expression of CR, but had no significant effect on consolidation of well-learned CR. In contrast, both electrolytic lesions and muscimol inactivation of hippocampus produced no significant deficits in the CR when a loud tone was used as the CS. These results demonstrate that the hippocampus is essential for the DEC when the delay task was rendered more difficult.

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.

Classical eyeblink conditioning using electrical stimulation of caudal mPFC as conditioned stimulus is dependent on cerebellar interpositus nucleus in guinea pigs

AIM

To determine whether electrical stimulation of caudal medial prefrontal cortex (mPFC) as conditioned stimulus (CS) paired with airpuff unconditioned stimulus (US) was sufficient for establishing eyeblink conditioning in guinea pigs, and whether it was dependent on cerebellar interpositus nucleus.

METHODS

Thirty adult guinea pigs were divided into 3 conditioned groups, and trained on the delay eyeblink conditioning, short-trace eyeblink conditioning, and long-trace eyeblink conditioning paradigms, respectively, in which electrical stimulation of the right caudal mPFC was used as CS and paired with corneal airpuff US. A pseudo conditioned group of another 10 adult guinea pigs was given unpaired caudal mPFC electrical stimulation and the US. Muscimol (1 μg in 1 μL saline) and saline (1 μL) were infused into the cerebellar interpositus nucleus of the animals through the infusion cannula on d 11 and 12, respectively.

RESULTS

The 3 eyeblink conditioning paradigms have been successfully established in guinea pigs. The animals acquired the delay and short-trace conditioned responses more rapidly than long-trace conditioned responses. Muscimol infusion into the cerebellar interpositus nucleus markedly impaired the expression of the 3 eyeblink conditioned responses.

CONCLUSION

Electrical stimulation of caudal mPFC is effective CS for establishing eyeblink conditioning in guinea pigs, and it is dependent on the cerebellar interpositus nucleus.

Evaluation of bidirectional interstimulus interval (ISI) shift in auditory delay eye-blink conditioning in healthy humans

Delay eye-blink conditioning is an associative learning task that can be utilized to probe the functional integrity of the cerebellum and related neural circuits. Typically, a single interstimulus interval (ISI) is utilized, and the amplitude of the conditioned response (CR) is the primary dependent variable. To study the timing of the CR, an ISI shift can be introduced (e.g., shifting the ISI from 350 to 850 ms). In each phase, a conditioned stimulus (e.g., a 400- or 900-ms tone) coterminates with a 50-ms corneal air puff unconditioned stimulus. The ability of a subject to adjust the CR to the changing ISI constitutes a critical timing shift. The feasibility of this procedure was examined in healthy human participants (N = 58) using a bidirectional ISI shift procedure while cortical event-related brain potentials were measured. CR acquisition was faster and the responses better timed when a short ISI was used. After the ISI shift, additional training was necessary to allow asymptotic responding at the new ISI. Interestingly, auditory event-related potentials to the CR were not associated with conditioning measures at either ISI.

Reevaluating the role of the medial prefrontal cortex in delay eyeblink conditioning

It has been proposed that the medial prefrontal cortex (mPFC) is not necessary for delay eyeblink conditioning (DEC). Here, we investigated the involvement of the mPFC in DEC with a soft or loud tone as the conditioned stimulus (CS) by using electrolytic lesions or muscimol inactivation of guinea pig mPFC. Interestingly, when a soft tone was used as a CS, electrolytic lesions of the mPFC significantly retarded acquisition of the conditioned response (CR), and muscimol infusions into mPFC distinctly inhibited the acquisition and expression of CR, but had no significant effect on consolidation of well-learned CR. In contrast, both electrolytic lesions and muscimol inactivation of mPFC produced no significant deficits in the CR when a loud tone was used as the CS, or in the unconditioned response (UR) when a soft or loud tone was used as the CS. These results demonstrate that the mPFC is essential for the DEC with the soft tone CS but not for the DEC with the loud tone CS.

Neural circuitry and plasticity mechanisms underlying delay eyeblink conditioning

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

Multiple sites of extinction for a single learned response

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

A trigeminal conditioned stimulus yields fast acquisition of cerebellum-dependent conditioned eyeblinks

Classical conditioning of the eyeblink response in the rabbit is a form of motor learning whereby the animal learns to respond to an initially irrelevant conditioned stimulus (CS). It is thought that acquired conditioned responses (CRs) are adaptive because they protect the eye in anticipation of potentially harmful events. This protective mechanism is surprisingly inefficient because the acquisition of CRs requires extensive training - a condition that is unlikely to occur in nature. We hypothesized that the rate of conditioning in rabbits could depend on CS modality and that stimulating mystacial vibrissae as the CS could produce CR acquisition faster than the traditional auditory or visual stimulation. We tested this hypothesis by conditioning naïve rabbits in the delay paradigm using a weak airpuff CS (vCS) directed to the ipsilateral mystacial vibrissae. We found that the trigeminal vCS yields significantly faster CR acquisition. We next examined if vCS-evoked CRs are dependent on the intermediate cerebellum in the same fashion as CRs evoked by the traditional auditory CS. We found that vibrissal CRs could be abolished by inactivating the cerebellar interposed nuclei (IN) with muscimol. In addition, injections of picrotoxin in the IN shortened the onset latency of vibrissal CRs. These findings suggest that the tone and vCS-evoked CRs share similar cerebellar dependency.

Associative plasticity in the medial auditory thalamus and cerebellar interpositus nucleus during eyeblink conditioning

Eyeblink conditioning, a type of associative motor learning, requires the cerebellum. The medial auditory thalamus is a necessary source of stimulus input to the cerebellum during auditory eyeblink conditioning. Nothing is currently known about interactions between the thalamus and cerebellum during associative learning. In the current study, neuronal activity was recorded in the cerebellar interpositus nucleus and medial auditory thalamus simultaneously from multiple tetrodes during auditory eyeblink conditioning to examine the relative timing of learning-related plasticity within these interconnected areas. Learning-related changes in neuronal activity correlated with the eyeblink conditioned response were evident in the cerebellum before the medial auditory thalamus over the course of training and within conditioning trials, suggesting that thalamic plasticity may be driven by cerebellar feedback. Short-latency plasticity developed in the thalamus during the first conditioning session and may reflect attention to the conditioned stimulus. Extinction training resulted in a decrease in learning-related activity in both structures and an increase in inhibition within the cerebellum. A feedback projection from the cerebellar nuclei to the medial auditory thalamus was identified, which may play a role in learning by facilitating stimulus input to the cerebellum via the thalamo-pontine projection.

Ventral lateral geniculate input to the medial pons is necessary for visual eyeblink conditioning in rats

The conditioned stimulus (CS) pathway that is necessary for visual delay eyeblink conditioning was investigated in the current study. Rats were initially given eyeblink conditioning with stimulation of the ventral nucleus of the lateral geniculate (LGNv) as the CS followed by conditioning with light and tone CSs in separate training phases. Muscimol was infused into the medial pontine nuclei (MPN) after each training phase to examine conditioned response (CR) retention to each CS. The spread of muscimol infusions targeting the MPN was examined with fluorescent muscimol. Muscimol infusions into the MPN resulted in a severe impairment in retention of CRs with the LGNv stimulation and light CSs. A less severe impairment was observed with the tone CS. The results suggest that CS information from the LGNv and light CSs is relayed to the cerebellum through the MPN. Retrograde tracing with fluoro-gold (FG) showed that the LGNv and nucleus of the optic tract have ipsilateral projections to the MPN. Unilateral inputs to the MPN from the LGNv and nucleus of the optic tract may be part of the visual CS pathway that is necessary for visual eyeblink conditioning.

Medial auditory thalamic input to the lateral pontine nuclei is necessary for auditory eyeblink conditioning

Auditory and visual conditioned stimulus (CS) pathways for eyeblink conditioning were investigated with reversible inactivation of the medial (MPN) or lateral (LPN) pontine nuclei. In Experiment 1, Long-Evans rats were given three phases of eyeblink conditioning. Phase 1 consisted of three training sessions with electrical stimulation of the medial auditory thalamic nuclei (MATN) paired with a periorbital shock unconditioned stimulus (US). An additional session was given with a muscimol (0.5muL, 10mM) or saline infusion targeting the LPN followed by a recovery session with no infusions. The same training and testing sequence was then repeated with either a tone or light CS in phases 2 and 3 (counterbalanced). Experiment 2 consisted of the same training as Experiment 1 except that muscimol or saline was infused in the MPN during the retention tests. Muscimol infusions targeting the LPN severely impaired retention of eyeblink conditioned responses (CRs) to the MATN stimulation and tone CSs but only partially reduced CR percentage to the light CS. Muscimol infusions that targeted the MPN had a larger effect on CR retention to the light CS relative to MATN stimulation or tone CSs. The results provide evidence that the auditory CS pathway necessary for delay eyeblink conditioning includes the MATN-LPN projection and the visual CS pathway includes the MPN.

Cornering the fear engram: long-term synaptic changes in the lateral nucleus of the amygdala after fear conditioning

Use-dependent synaptic modifications in the lateral nucleus of the amygdala (LA) have been suggested to be the cellular analog of memory trace after pavlovian fear conditioning. However, whether neurophysiological changes in the LA are produced as a direct consequence of associative learning awaits additional proof. Using microstimulation of the medial geniculate nucleus of the thalamus as the conditioned stimulus (CS), we demonstrated that contingent pairings of the brain-stimulation CS and a footshock unconditioned stimulus lead to enhanced synaptic efficacy in the thalamic input to the LA, supporting the hypothesis that localized synaptic alterations underlie fear memory formation.

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.

Stimulation of the lateral geniculate, superior colliculus, or visual cortex is sufficient for eyeblink conditioning in rats

The role of the cerebellum in eyeblink conditioning is well established. Less work has been done to identify the necessary conditioned stimulus (CS) pathways that project sensory information to the cerebellum. A possible visual CS pathway has been hypothesized that consists of parallel inputs to the pontine nuclei from the lateral geniculate nucleus (LGN), superior colliculus (SC), pretectal nuclei, and visual cortex (VCTX) as reported by Koutalidis and colleagues in an earlier paper. The following experiments examined whether electrical stimulation of neural structures in the putative visual CS pathway can serve as a sufficient CS for eyeblink conditioning in rats. Unilateral stimulation of the ventral LGN (Experiment 1), SC (Experiment 2), or VCTX (Experiment 3) was used as a CS paired with a periorbital shock unconditioned stimulus. Stimulation was delivered to the hemisphere contralateral to the conditioned eye. Rats in all experiments were given five 100-trial sessions of paired or unpaired eyeblink conditioning with the stimulation CS followed by three paired sessions with a light CS. Stimulation of each visual area when paired with the unconditioned stimulus supported acquisition of eyeblink conditioned responses (CRs) and substantial savings when switched to a light CS. The results provide evidence for a unilateral parallel visual CS pathway for eyeblink conditioning that includes the LGN, SC, and VCTX inputs to the pontine nuclei.

Metabolic mapping of rat forebrain and midbrain during delay and trace eyeblink conditioning

While the essential neural circuitry for delay eyeblink conditioning has been largely identified, much of the neural circuitry for trace conditioning has yet to be determined. The major difference between delay and trace conditioning is a time gap between the presentation of the conditioned stimulus (CS) and the unconditioned stimulus (US) during trace conditioning. It is this time gap, which accounts for the additional memory component and may require extra neural structures, including hippocampus and prefrontal cortex. A metabolic marker of energy use, radioactively labeled glucose analog, was used to compare differences in glucose analog uptake between delay, trace, and unpaired experimental groups (rats, Long-Evans), to identify possible new areas of involvement within forebrain and midbrain. Here, we identify increased 2-DG uptake for the delay group compared to the unpaired group in various areas including: the medial geniculate nuclei (MGN), the amygdala, cingulate cortex, auditory cortex, medial dorsal thalamus, and frontal cortices. For the trace group, compared to the unpaired group, there was an increase in 2-DG uptake for the medial orbital frontal cortex and the medial MGN. The trace group also exhibited more increases lateralized to the right hemisphere, opposite to the side of US delivery, in various areas including: CA1, subiculum, presubiculum, perirhinal cortex, ventral and dorsal MGN, and the basolateral and central amygdala. While some of these areas have been identified as important for delay or trace conditioning, some new structures have been identified such as the orbital frontal cortex for both delay and trace groups.

Eyeblink conditioning using cochlear nucleus stimulation as a conditioned stimulus in developing rats

Previous studies demonstrated that the development of auditory conditioned stimulus (CS) input to the cerebellum may be a neural mechanism underlying the ontogenetic emergence of eyeblink conditioning in rats. The current study investigated the role of developmental changes in the projections of the cochlear nucleus (CN) in the ontogeny of eyeblink conditioning using electrical stimulation of the CN as a CS. Rat pups were implanted with a bipolar stimulating electrode in the CN and given six 100-trial training sessions with a 300 ms stimulation train in the CN paired with a 10 ms periorbital shock unconditioned stimulus (US) on postnatal days (P) 17-18 or 24-25. Control groups were given unpaired presentations of the CS and US. Rats in both age groups that received paired training showed significant increases in eyeblink conditioned responses across training relative to the unpaired groups. The rats trained on P24-25, however, showed stronger conditioning relative to the group trained on P17-18. Rats with missed electrodes in the inferior cerebellar peduncle or in the cerebellar cortex did not show conditioning. The findings suggest that developmental changes in the CN projections to the pons, inferior colliculus, or medial auditory thalamus may be a neural mechanism underlying the ontogeny of auditory eyeblink conditioning.

Ontogenetic change in the auditory conditioned stimulus pathway for eyeblink conditioning

Two experiments examined the neural mechanisms underlying the ontogenetic emergence of auditory eyeblink conditioning. Previous studies found that the medial auditory thalamus is necessary for eyeblink conditioning with an auditory conditioned stimulus (CS) in adult rats. In experiment 1, stimulation of the medial auditory thalamus was used as a CS in rat pups trained on postnatal days (P) 17-18, 24-25, or 31-32. All three age groups showed significant acquisition relative to unpaired controls. However, there was an age-related increase in the rate of conditioning. Experiment 2 examined the effect of inactivating the medial auditory thalamus with muscimol on auditory eyeblink conditioning in rats trained on P17-18, 24-25, or 31-32. Rat pups trained on P24-25 and P31-32, but not P17-18, showed a significant reduction in conditioned responses following muscimol infusions. The findings suggest that the thalamic contribution to auditory eyeblink conditioning continues to develop through the first postnatal month.

Medial auditory thalamus inactivation prevents acquisition and retention of eyeblink conditioning

The auditory conditioned stimulus (CS) pathway that is necessary for delay eyeblink conditioning was investigated using reversible inactivation of the medial auditory thalamic nuclei (MATN) consisting of the medial division of the medial geniculate (MGm), suprageniculate (SG), and posterior intralaminar nucleus (PIN). Rats were given saline or muscimol infusions into the MATN contralateral to the trained eye before each of four conditioning sessions with an auditory CS. Rats were then given four additional sessions without infusions to assess savings from the initial training. All rats were then given a retention test with a muscimol infusion followed by a recovery session. Muscimol infusions through cannula placements within 0.5 mm of the MGm prevented acquisition of eyeblink conditioned responses (CRs) and also blocked CR retention. Cannula placements more than 0.5 mm from the MATN did not completely block CR acquisition and had a partial effect on CR retention. The primary and secondary effects of MATN inactivation were examined with 2-deoxy-glucose (2-DG) autoradiography. Differences in 2-DG uptake in the auditory thalamus were consistent with the cannula placements and behavioral results. Differences in 2-DG uptake were found between groups in the ipsilateral auditory cortex, basilar pontine nuclei, and inferior colliculus. Results from this experiment indicate that the MATN contralateral to the trained eye and its projection to the pontine nuclei are necessary for acquisition and retention of eyeblink CRs to an auditory CS.

Inferior colliculus lesions impair eyeblink conditioning in rats

The neural plasticity necessary for acquisition and retention of eyeblink conditioning has been localized to the cerebellum. However, the sources of sensory input to the cerebellum that are necessary for establishing learning-related plasticity have not been identified completely. The inferior colliculus may be a source of sensory input to the cerebellum through its projection to the medial auditory thalamus. The medial auditory thalamus is necessary for eyeblink conditioning in rats and projects to the lateral pontine nuclei, which then project to the cerebellar nuclei and cortex. The current experiment examined the role of the inferior colliculus in auditory eyeblink conditioning. Rats were given bilateral or unilateral (contralateral to the conditioned eye) lesions of the inferior colliculus prior to 10 d of delay eyeblink conditioning with a tone CS. Rats with bilateral or unilateral lesions showed equivalently impaired acquisition. The extent of damage to the contralateral inferior colliculus correlated with several measures of conditioning. The findings indicate that the contralateral inferior colliculus provides auditory input to the cerebellum that is necessary for eyeblink conditioning.