Neuronal substrates of simple associative learning: classical conditioning

Synaptic Mechanisms of Delay Eyeblink Classical Conditioning: AMPAR Trafficking and Gene Regulation in an In Vitro Model

An in vitro model of delay eyeblink classical conditioning was developed to investigate synaptic plasticity mechanisms underlying acquisition of associative learning. This was achieved by replacing real stimuli, such as an airpuff and tone, with patterned stimulation of the cranial nerves using an isolated brainstem preparation from turtle. Here, our primary findings regarding cellular and molecular mechanisms for learning acquisition using this unique approach are reviewed. The neural correlate of the in vitro eyeblink response is a replica of the actual behavior, and features of conditioned responses (CRs) resemble those observed in behavioral studies. Importantly, it was shown that acquisition of CRs did not require the intact cerebellum, but the appropriate timing did. Studies of synaptic mechanisms indicate that conditioning involves two stages of AMPA receptor (AMPAR) trafficking. Initially, GluA1-containing AMPARs are targeted to synapses followed later by replacement by GluA4 subunits that support CR expression. This two-stage process is regulated by specific signal transduction cascades involving PKA and PKC and is guided by distinct protein chaperones. The expression of the brain-derived neurotrophic factor (BDNF) protein is central to AMPAR trafficking and conditioning. BDNF gene expression is regulated by coordinated epigenetic mechanisms involving DNA methylation/demethylation and chromatin modifications that control access of promoters to transcription factors. Finally, a hypothesis is proposed that learning genes like BDNF are poised by dual chromatin features that allow rapid activation or repression in response to environmental stimuli. These in vitro studies have advanced our understanding of the cellular and molecular mechanisms that underlie associative learning.

The role of the cerebellum in cognition and emotion: personal reflections since 1982 on the dysmetria of thought hypothesis, and its historical evolution from theory to therapy

The cognitive neuroscience of the cerebellum is now an established multidisciplinary field of investigation. This essay traces the historical evolution of this line of inquiry from an emerging field to its current status, with personal reflections over almost three decades on this journey of discovery. It pays tribute to early investigators who recognized the wider role of the cerebellum beyond motor control, traces the origins of new terms and concepts including the dysmetria of thought theory, the universal cerebellar transform, and the cerebellar cognitive affective syndrome, and places these developments within the broader context of the scientific efforts of a growing community of cerebellar cognitive neuroscientists. This account considers the converging evidence from theoretical, anatomical, physiological, clinical, and functional neuroimaging approaches that have resulted in the transition from recognizing the cerebellar incorporation into the distributed neural circuits subserving cognition and emotion, to a hopeful new era of treatment of neurocognitive and neuropsychiatric manifestations of cerebellar diseases, and to cerebellar-based interventions for psychiatric disorders.

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.

Response linearity determined by recruitment strategy in detailed model of nictitating membrane control

Many models of eyeblink conditioning assume that there is a simple linear relationship between the firing patterns of neurons in the interpositus nucleus and the time course of the conditioned response (CR). However, the complexities of muscle behaviour and plant dynamics call this assumption into question. We investigated the issue by implementing the most detailed model available of the rabbit nictitating membrane response (Bartha and Thompson in Biol Cybern 68:135-143, 1992a and in Biol Cybern 68:145-154, 1992b), in which each motor unit of the retractor bulbi muscle is represented by a Hill-type model, driven by a non-linear activation mechanism designed to reproduce the isometric force measurements of Lennerstrand (J Physiol 236:43-55, 1974). Globe retraction and NM extension are modelled as linked second order systems. We derived versions of the model that used a consistent set of SI units, were based on a physically realisable version of calcium kinetics, and used simulated muscle cross-bridges to produce force. All versions showed similar non-linear responses to two basic control strategies. (1) Rate-coding with no recruitment gave a sigmoidal relation between control signal and amplitude of CR, reflecting the measured relation between isometric muscle force and stimulation frequency. (2) Recruitment of similar strength motor units with no rate coding gave a sublinear relation between control signal and amplitude of CR, reflecting the increase in muscle stiffness produced by recruitment. However, the system response could be linearised by either a suitable combination of rate-coding and recruitment, or by simple recruitment of motor units in order of (exponentially) increasing strength. These plausible control strategies, either alone or in combination, would in effect present the cerebellum with the simplified virtual plant that is assumed in many models of eyeblink conditioning. Future work is therefore needed to determine the extent to which motor neuron firing is in fact linearly related to the nictitating membrane response.

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.

Effects of bilateral hemispherectomy on genetically variable stimulus preferences and imprinting in quail chicks

Bilateral ablation of the telencephalon on the 1st posthatch day prevented learning from imprinting to colored stimuli in Japanese quail chicks C. coturnix japonica, but it spared their artificially selected approach preferences for particular colors and patterns. Post-imprinting decerebration on the 3rd posthatch day erased the imprinted memory, but did not revert the chicks' approach choices to the colors of their genetically distinct initial preferences. The genetically determined and the acquired preferences were both lost in the latter birds, regardless whether they were imprinted to the initially preferred or the initially nonpreferred stimuli before decerebration. Discussion focuses on implications of the data for modeling specific interactions between and the interrelated neural representation of genetic and environmental influences in early behavioral development.

5B4-CAM expression parallels neurite outgrowth and synaptogenesis in the developing rat brain

In order to study whether 5B4-CAM expression parallels neurite outgrowth and synaptogenesis, a monoclonal antibody, 5B4, was used, which recognizes both fetal (185-250 kD) and adult (140 kD, 180 kD) forms of the neural cell adhesion molecule (N-CAM), to identify and localize the antigen in rat tissue during developmental ages P1 through P31 and in adults between P60 and 2 years of age. A ubiquitous pattern of intense immunolabelling was detected during the earliest stages of development. 5B4-CAM expression paralleled process outgrowth and the early stages of synaptogenesis in the cerebral cortex, hippocampal formation, and cerebellum. In the adult, immunoreactivity was generally less intense, but the cerebral cortex and hippocampal and cerebellar molecular layers, all areas implicated in learning-associated plasticity, retained substantial immunoreactivity. The inner one-third of the dentate gyrus molecular layer, an area implicated in axonal sprouting and reactive synaptogenesis, was particularly intensely labelled. Evidence from this work suggests that 5B4-CAM expression may be useful in monitoring neurite outgrowth and the early stages of synapse formation during development and possibly axonal sprouting and reactive synaptogenesis in the adult.

Intracortical auditory evoked potentials during classical aversive conditioning in cats

Auditory evoked potentials (EPs) were recorded simultaneously from six different depths of the auditory cortex of freely moving cats. In a classical aversive learning paradigm the change of the pitch of regularly given acoustic stimuli served as warning signal. With conditioning the amplitude of a middle latency negative component increased and its latency decreased. Based on laminar analysis of EP profiles we propose that the observed EP changes were generated within the upper cortical layers and reflected a temporary aurousal reaction induced by the signal as a result of discrimination learning. For the description of this effect the term "learned aversive arousal" is suggested.

Hypoplasia of cerebellar vermal lobules VI and VII in autism

Autism is a neurologic disorder that severely impairs social, language, and cognitive development. Whether autism involves maldevelopment of neuroanatomical structures is not known. The size of the cerebellar vermis in patients with autism was measured on magnetic resonance scans and compared with its size in controls. The neocerebellar vermal lobules VI and VII were found to be significantly smaller in the patients. This appeared to be a result of developmental hypoplasia rather than shrinkage or deterioration after full development had been achieved. In contrast, the adjacent vermal lobules I to V, which are ontogenetically, developmentally, and anatomically distinct from lobules VI and VII, were found to be of normal size. Maldevelopment of the vermal neocerebellum had occurred in both retarded and nonretarded patients with autism. This localized maldevelopment may serve as a temporal marker to identify the events that damage the brain in autism, as well as other neural structures that may be concomitantly damaged. Our findings suggest that in patients with autism, neocerebellar abnormality may directly impair cognitive functions that some investigators have attributed to the neocerebellum; may indirectly affect, through its connections to the brain stem, hypothalamus, and thalamus, the development and functioning of one or more systems involved in cognitive, sensory, autonomic, and motor activities; or may occur concomitantly with damage to other neural sites whose dysfunction directly underlies the cognitive deficits in autism.

Midbrain reticular stimulation produces patterns of metabolic activation and suppression in the cerebellum and vestibular nuclei: a 2-deoxyglucose study

Autoradiographic 2-deoxyglucose (2-DG) procedures were used to map the activity in the cerebellum and vestibular nuclei during electrical stimulation of the midbrain reticular formation (MRF) in unrestrained rats. The major finding was a large increase in 2-DG uptake observed in the flocculus of MRF-stimulated rats. The peak of labeling in the flocculus was greater than any other peak of labeling measured in the cerebellum of MRF-stimulated or control rats. Structures showing significant decreases in 2-DG uptake included the 3 deep cerebellar nuclei and the 3 vestibular nuclei. The most pronounced suppressive effects of MRF stimulation were on the medial and lateral vestibular nuclei. The changes in metabolic activity revealed by 2-DG provide a first anatomical demonstration of: the activating effects of MRF stimulation on the flocculus; and the suppressive effects of MRF stimulation on deep cerebellar and vestibular nuclei. The observed patterns of metabolic activation and suppression were correlated with the known electrophysiological properties of the structures affected by MRF stimulation. The findings are consistent with specific effects of MRF stimulation on floccular-vestibular-visual interactions that may be disruptive to learning functions such as adaptability of the vestibulo-ocular reflex. The effects of MRF stimulation on the deep cerebellar nuclei are also consistent with a potential disruption of somatomotor learning-related activities in these nuclei. The results support the existence of MRF mechanisms for the modulation of integrative sensory-motor functions in the cerebellum.

Classical conditioning in 3-, 30-, and 45-month-old rabbits: behavioral learning and hippocampal unit activity

Evidence has accumulated to suggest that the hippocampus is impaired by aging processes. Hippocampal multiple unit activity was examined in 3-, 30-, and 45-month-old rabbits during classical conditioning in the trace paradigm. Animals were trained daily with 126 paired trials of tone and airpuff. The 3-month-olds learned to a criterion of 8/9 CR's in a mean of 3.2 days, while the 30-month-olds took a mean of 9.4 days, and the 45-month-olds took a mean of 11.75 days. Neural unit activity in the hippocampus modeled the behavioral eyelid response but took much longer to develop in the older animals. Results suggest that rabbits as young as 2 1/2 years old show behavioral and correlated neural response deficits.

The use of avoidance training in studies of modulation of memory storage

Studies of the modulatory effects on memory of many treatments have relied in large part on the use of inhibitory (passive) avoidance training procedures. Recent critiques have questioned the validity of data obtained with the inhibitory avoidance task. This paper addresses these comments and describes many of the advantages of using the procedure in studies of processes which regulate the neurobiological mechanisms which store new information.

Retraining of functional gait through the reduction of upper extremity weight-bearing in chronic cerebellar ataxia

Patients with chronic ataxia resulting from traumatic central nervous system (CNS) damage frequently attain a gait pattern dependent upon upper extremity weight-bearing (UEWB). Traditional attempts to train these patients may perpetuate UEWB dependency, by not allowing optimal facilitation of balance and associated movement control required for independent ambulation. In the present study an ambulation retraining approach designed to facilitate coordination and balance while minimizing UEWB was used. This approach can allow the patient to train outside of the clinic; only about 5% of the actual therapy hours involved a therapist.

The role of hippocampus in memory: a hypothesis

The role of the hippocampus in memory storage in the mammalian brain is examined. The intrinsic anatomical organization of the hippocampus is such that a multidimensional mapping of other brain regions is represented. Emerging knowledge of the cortico-limbic-subcortical anatomy suggests that the hippocampal representations preserve the topological features of the targets and possess reciprocal connectivity. Long-Term Potentiation (LTP) is a prominent physiological characteristic of hippocampal synapses and is a promising candidate mnemonic device. The hypothesis is advanced that the pattern, or index, of specific neocortical (and other) areas activated by an experiential event is represented, or indexed, in the hippocampus by means of LTP. This hypothesis, termed the Memory Indexing Theory, suggests that experiential events are initially stored in an index of neocortical locations maintained in hippocampus. Subsequently, other regions, notably neocortex itself, permanently encode these experiential events and the interrelationships between them.

Adaptive plasticity in the spinal stretch reflex: an accessible substrate of memory?

The study of the substrates of memory in higher vertebrates is one of the major problems of neurobiology. A simple and technically accessible experimental model is needed. Recent studies have demonstrated long-term adaptive plasticity, a form of memory, in the spinal stretch reflex (SSR). The SSR is due largely to a two-neuron monosynaptic arc, the simplest, best-defined, and most accessible pathway in the primate central nervous system (CNS). Monkeys can slowly change SSR amplitude without a change in initial muscle length or alpha motoneuron tone, when reward is made contingent on amplitude. Change occurs over weeks and months and persists for long periods. It is relatively specific to the agonist muscle and affects movement. The salient features of SSR adaptive plasticity, combined with clinical and laboratory evidence indicating spinal cord capacity for intrinsic change, suggest that SSR change eventually involves persistent segmental alteration. If this is the case, SSR plasticity should be a powerful model for studying the neuronal and synaptic substrates of memory in a primate.