Comparisons of dorsal and ventral hippocampus cornu ammonis region 1 pyramidal neuron activity during trace eye-blink conditioning in the rabbit

Previous studies demonstrating a critical role of the hippocampus during trace eye-blink conditioning have focused primarily upon the dorsal portion of the structure. However, evidence suggests that a functional differentiation exists along the septotemporal axis of the hippocampus. In the present study, the activity of 2588 single cornu ammonis region 1 pyramidal neurons of the dorsal hippocampus and ventral hippocampus were recorded during trace and pseudo-eye-blink conditioning of the rabbit. Learning-related increases in dorsal hippocampus neuron firing rates were observed immediately prior to behavioral criterion, and increased over the course of training. Activation of dorsal hippocampus neurons during trace conditioning was also greater than that of ventral hippocampus neurons, including during the trace interval, in well-trained animals. An unexpected difference in the patterns of learning-related activity between hemispheres was also observed. Neurons of the dorsal hippocampus ipsilateral and contralateral to the trained eye, exhibiting significant increases in firing rate [rate increasing neurons], demonstrated the greatest magnitude of activation early and late in training, respectively. Rate increasing neurons of the dorsal hippocampus also exhibited a greater diversity of response profiles, with 69% of dorsal hippocampus rate increasing neurons exhibiting significant increases in firing rate during the conditioned stimulus and/or trace intervals, compared with only 8% of ventral hippocampus rate increasing neurons (the remainder of which were significantly responsive during only the unconditioned stimulus and/or post-unconditioned stimulus intervals). Only modest learning-related activation of ventral hippocampus neurons was observed, reflected as an increase in conditioning stimulus-elicited rate increasing neuron response magnitudes over the course of training. No differences in firing rate between dorsal hippocampus and ventral hippocampus neurons during a 1-day pre-training habituation session were observed. Thus, dorsal hippocampus activation is more robust, suggesting a more substantial role for these neurons in the processing of temporal information during trace eye-blink conditioning.     

Activity profiles of single neurons in caudal anterior cingulate cortex during trace eyeblink conditioning in the rabbit

Acquisition of trace eyeblink conditioning involves the association of a conditioned stimulus (CS) with an unconditioned stimulus (US) separated by a stimulus-free trace interval. This form of conditioning is dependent upon the hippocampus and the caudal anterior cingulate cortex (AC), in addition to brain stem and cerebellar circuitry. Hippocampal involvement in trace eyeblink conditioning has been studied extensively, but the involvement of caudal AC is less well understood. In the present study, we compared neuronal responses from rabbits given either paired (trace conditioning) or unpaired (pseudoconditioning) presentations of the CS and US. Presentation of the CS elicited significant increases in neuronal activity at the onset of both trace conditioning and pseudoconditioning. A robust CS-elicited neuronal response persisted throughout the first 2 days of trace conditioning, declining gradually across subsequent training sessions. In contrast, the magnitude of the CS-elicited excitatory response during pseudoconditioning began to decline within the first 10 trials. Neurons exhibiting excitatory responses to the CS during trace conditioning also exhibited excitatory responses to the US that were significantly greater in magnitude than US-elicited responses during pseudoconditioning. CS-elicited decreases in neuronal activity became more robust over the course of trace conditioning compared to pseudoconditioning. Reductions in activity during the CS interval consistently preceded excitation in both training groups, suggesting that the CS-elicited decreases in neuronal activity may serve to increase the signal-to-noise ratio of the excitatory response to the tone. Taken together, these data suggest that the caudal AC is involved early in trace eyeblink conditioning and that maintenance of the CS-elicited excitatory response may serve to signal the salience of the tone.     

Single neurons in the dentate gyrus and CA1 of the hippocampus exhibit inverse patterns of encoding during trace fear conditioning

Trace fear conditioning is a hippocampus-dependent learning task that requires the association of an auditory conditioned stimulus (CS) and a shock unconditioned stimulus (US) that are separated by a 20-s trace interval. Single-neuron activity was recorded simultaneously from the dentate gyrus (DG) and CA1 of rats during unpaired pseudoconditioning and subsequent trace fear conditioning. Single neurons in DG showed a progressive increase in learning-related activity to the CS and US across trace fear conditioning. Single neurons in CA1 showed an early increase in responding to the CS, which developed into a decrease in firing later in trace conditioning. Correlation analyses showed that DG and CA1 units exhibit inverse patterns of responding to the CS during trace fear conditioning.     

Developmental changes in eyeblink conditioning and simple spike activity in the cerebellar cortex

The activity of neurons in the cerebellum exhibits learning-related changes during eyeblink conditioning in adult mammals. The induction and preservation of learning-related changes in cerebellar neuronal activity in developing rats may be affected by the level of maturity in cerebellar feedback to its brainstem afferents, including the inferior olive. Developmental changes in cerebellar plasticity were examined by recording the activity of Purkinje cells in eye regions of cerebellar cortical lobule HVI (lobulus simplex) in infant rats during eyeblink conditioning. The percentage and amplitude of eyeblink conditioned responses increased as a function of age. Analyses of Purkinje cell simple spike activity revealed developmental increases in the number of units that exhibited stimulus-evoked and learning-related changes in activity. Moreover, the magnitude of these changes exhibited a substantial age-related increase. The results support the view that the emergence of learning-specific cerebellar plasticity and the ontogeny of eyeblink conditioning are influenced by developmental changes in the synaptic interactions within brainstem-cerebellum circuits.     

Cortical involvement in acquisition and extinction of trace eyeblink conditioning

Previous studies have implicated 2 cortical regions interconnected with the hippocampal formation, the retrosplenial cortex (RSC) and the medial prefrontal cortex (mPFC), as loci important for the acquisition of hippocampally dependent trace eyeblink conditioning. These loci have also been proposed to serve as long-term storage sites of task critical information. This study used lesions made prior to training to investigate the roles of the RSC, as well as the caudal and rostral subdivisions of the mPFC, in the acquisition and subsequent extinction of trace eyeblink conditioning in the rabbit. The caudal mPFC and rostral mPFC were shown to be critical for acquisition and extinction of the conditioned reflex, respectively. The data indicate that the RSC is not critical for acquisition or extinction of the trace conditioned reflex.     

Hippocampal response patterns during discriminative eyeblink/jaw movement conditioning in the rabbit

Rabbits were given concurrent training in eyeblink (EB) and jaw movement (JM) conditioning in which 1 tone predicted an airpuff and another tone predicted water. After 10 days of discrimination training, the animals were given 10 days of reversal training. In the discrimination phase, acquisition of the 2 conditioned responses was not significantly different; however JM discrimination errors were much more frequent than were EB errors. In the reversal phase, correct performance on EB trials increased gradually, as was expected, whereas there was immediate behavioral reversal on JM trials. Differences in size and topography of dorsal CA1 multiple-unit responses reflected the ability of the hippocampus to discriminate between stimuli in trained animals, corresponding to the performance of the behavioral discrimination. During JM trials, the rhythmicity of the neural response was further modulated by the type of the prior trial, suggesting the coding of sequential events by the hippocampus. Thus, hippocampal conditioned activity can rapidly change its magnitude and pattern depending on the specific trial type during a concurrent EB/JM discrimination task and its reversal. (PsycINFO Database Record (c) 2008 APA, all rights reserved).     

Theta-contingent trial presentation accelerates learning rate and enhances hippocampal plasticity during trace eyeblink conditioning

Hippocampal theta activity has been established as a key predictor of acquisition rate in rabbit (Orcytolagus cuniculus) classical conditioning. The current study used an online brain--computer interface to administer conditioning trials only in the explicit presence or absence of spontaneous theta activity in the hippocampus-dependent task of trace conditioning. The findings indicate that animals given theta-contingent training learned significantly faster than those given nontheta-contingent training. In parallel with the behavioral results, the theta-triggered group, and not the nontheta-triggered group, exhibited profound increases in hippocampal conditioned unit responses early in training. The results not only suggest that theta-contingent training has a dramatic facilitory effect on trace conditioning but also implicate theta activity in enhancing the plasticity of hippocampal neurons.     

Assessment of forebrain-dependent trace eyeblink conditioning in chronic cannabis users

While CB1 knockout mice exhibit striking impairments on a cerebellar-dependent task called delay eyeblink conditioning (dEBC), these animals demonstrate intact forebrain-dependent trace EBC (tEBC). Although heavy human cannabis users also show impaired delay EBC, their performance on tEBC is currently unknown. Therefore, 13 heavy cannabis users and 13 cannabis naive controls completed a tEBC procedure. The cannabis group exhibited similar rates of conditioned responding compared to controls in the acquisition and extinction phase. Consistent with reports of overt attentional abnormalities, the cannabis group exhibited decreased N100 ERP amplitudes to the tone CS that were unrelated to mean levels of conditioning across blocks during the acquisition phase. The lack of a significant effect of heavy cannabis use on tEBC reported here, combined with the previous report of impaired dEBC in such users, mirrors the findings observed in CB1 knockout mice, and suggests that the cannabinoid system differentially mediates forebrain- and cerebellar-dependent learning processes in both humans and animals.     

Impaired delay and trace eyeblink conditioning performance in major depressive disorder

BACKGROUND: Preliminary evidence obtained in our lab has revealed that depressive symptoms impair associative learning, as measured by acquisition of eyeblink classical conditioning (EBCC) tasks. The current study assesses EBCC acquisition in individuals with major depressive disorder (MDD). METHODS: The 17-item Hamilton Rating Scale for Depression (HAM-D(17)) and the 30-item Inventory for Depressive Symptomatology, Self-Report (IDS-SR(30)) were used to quantify severity of depressive symptoms. Participants received 60 trials each in delay 500, trace 500, and trace 1000 conditioning paradigms. A 150-ms, 5-7 psi air puff served as the unconditioned stimulus (US), and an 80-dB, 1-kHz tone as the conditioned stimulus (CS). Mean percent conditioned responses (CRs) served as the primary measure of task acquisition. RESULTS: The MDD group generated significantly fewer CRs on delay 500 and trace 500 tasks, and approached significance on the trace 1000 task compared to healthy controls. Furthermore, presentation of successive trials did not increase CR production in the depressed group, in contrast to progressive increases observed in the control group. LIMITATIONS: The presentation of multiple EBCC tasks precludes some detailed analyses of task-specific performance. Future studies may also benefit from including sufficient numbers of subjects to assess differential characteristics of depression (e.g., length of episode, depressive subtype) and treatment effects. CONCLUSIONS: These data suggest that MDD impairs acquisition of EBCC, providing behavioral support for cerebellar and hippocampal dysfunction in depression. Delineating the neural substrates involved in MDD may aid in future treatment approaches for this pervasive disorder.     

Ontogeny of delay versus trace eyeblink conditioning in the rat

The ontogeny of delay versus trace eyeblink conditioning was examined in 19-, 23-, and 30-day-old rat pups. Pairings of a tone conditioned stimulus (CS) and periocular shock unconditioned stimulus (US; 100-ms) were presented in one of three conditioning paradigms: standard delay [380-ms CS, 280-ms interstimulus interval (ISI)], trace (380-ms CS, 500-ms trace interval), or long-delay (980-ms CS, 880-ms ISI). The results of two experiments indicated that standard delay conditioning emerged between 19 and 23 days of age whereas trace and long-delay eyeblink conditioning emerged more slowly from postnatal Days 19 to 30. Because the acquisition profile for long-delay paralleled that of trace and not standard delay, it appears that the relative deficits in the emergence of trace eyeblink conditioning during development reflect difficulty in forming associations over long ISIs rather than the short-term memory demands of the trace conditioning paradigm.     

Awareness is necessary for differential trace and delay eyeblink conditioning in humans

Squire et al. have proposed that trace and delay eyeblink conditioning procedures engage separate learning systems: a declarative hippocampal/cortical system associated with conscious contingency awareness, and a reflexive sub-cortical system independent of awareness, respectively ( [Clark and Squire, 1998] and [Smith et al., 2005]). The only difference between these two procedures is that the conditioned stimulus (CS) and the unconditioned stimulus (US) overlap in delay conditioning, whereas there is a brief interval (e.g., 1 s) between them in trace conditioning. In two experiments using the same procedure as Clark and Squire's group, we observed differential conditioning only in participants who showed contingency awareness in a post-experimental questionnaire, with both trace and delay procedures. We interpret these results to suggest that, although there may be multiple brain regions involved in learning, these regions are organized as a coordinated system rather than as separate, independent systems.     

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.     

Reflex facilitation during eyeblink conditioning and subsequent interpositus nucleus inactivation in the rabbit (Oryctolagus cuniculus)

In eyeblink conditioning in the rabbit (Oryctolagus cuniculus), not only is a conditioned response (CR) acquired, but also the original reflex is modified as a function of training. In Experiment 1, by comparing unconditioned responses in unpaired and paired groups, 3 types of reflex facilitation were distinguished. One type was linked to exposure to the unconditioned stimuli (USs) and/or experimental setting. The 2nd type was related to the formation of the memory trace for conditioned eyeblink. The 3rd type was linked to the conditioned stimulus immediately preceding the US in the paired group. In Experiment 2, reversible inactivation of the interpositus nucleus (IPN) abolished the CR and reduced the CR-related reflex facilitation, indicating that the latter depends on the plasticity of the IPN.     

Connections of the caudal anterior cingulate cortex in rabbit: neural circuitry participating in the acquisition of trace eyeblink conditioning

The caudal anterior cingulate cortex (cAC) is an essential component of the circuitry involved in acquisition of forebrain-dependent trace eyeblink conditioning. Lesions of the cAC prevent trace eyeblink conditioning [Weible AP, McEchron MD, Disterhoft JF (2000) Cortical involvement in acquisition and extinction of trace eyeblink conditioning. Behav Neurosci 114(6):1058-1067]. The patterns of activation of cAC neurons recorded in vivo suggest an attentional role for this structure early in training [Weible AP, Weiss C, Disterhoft JF (2003) Activity profiles of single neurons in caudal anterior cingulate cortex during trace eyeblink conditioning in the rabbit. J Neurophysiol 90(2):599-612]. The goal of the present study was to identify connections of the portion of the rabbit cAC previously demonstrated to be involved in trace eyeblink conditioning, using the neuronal tract tracer wheat germ agglutinin conjugated to horseradish peroxidase, to better understand how the cAC contributes to the process of associative learning. Reciprocal connections with the claustrum provide a route for the transfer of sensory information between the cAC and neocortical and allocortical regions also involved in learning. Connections with components of the basal forebrain cholinergic system are described, with relevance to the proposed attentional role of the cAC. Reciprocal and unidirectional connections were in evidence in multiple thalamic regions, including the medial dorsal nucleus, which have been implicated in a variety of conditioning paradigms. Anterograde connections with the caudate and lateral pontine nuclei provide access to forebrain motor and brainstem sensory circuitry, respectively. The relevance of these connections to acquisition of the trace conditioned reflex is discussed.     

Neuronal activity in rabbit neostriatum during classical eyelid conditioning

Extracellular multiple- and single-unit recordings were made from the neostriatum of rabbits during classical eyelid conditioning. Neostriatal neurons processed information regarding the conditioned auditory stimulus (CS) and conditioned eyelid response (CR) as well as the unconditioned stimulus/response (US/UR). These data are consistent with previous reports that neostriatal neurons respond to movement and movement-related sensory stimuli. In most cases, neostriatal neurons increased activity to the US during the early phase of training, but to the CR as training progressed. A close temporal correlation was found between neuronal activity and CR onset with unit discharges typically preceding CR onset by 10–50 ms. The activity of some multiple and single units was monitored after injection of haloperidol, a neuroleptic and dopamine antagonist known to disrupt neostriatal function. Interestingly, haloperidol caused a greater disruption of CRs at low-intensity than at high-intensity CSs, but conditioning-related neuronal activity was disrupted equally at both intensities. These data are discussed in terms of a possible role for the neostriatum in eyelid conditioning.     

The ontogeny of human learning in delay, long-delay, and trace eyeblink conditioning

The ontogeny of associative learning in delay (750-ms conditional stimulus [CS], 650-ms interstimulus interval [ISI]), long-delay (1,350-ms CS, 1,250-ms ISI), and trace (750-ms CS, 500-ms trace interval, 1,250-ms ISI) eyeblink conditioning was examined in 5-month-old human infants and adults. Infants and adults showed different acquisition rates but reached equivalent asymptotes of conditional responses (CRs) in standard delay conditioning. In long-delay and trace conditions, infants exhibited less robust conditioning than adults and minimal ability to appropriately time CRs. During infancy, the ISI, rather than the conditioning procedure, predicted rate and effectiveness of CRs. These findings suggest that higher order cognitive abilities begin emerging early in development. Across ontogeny, however, there are changes in the limits and parameters that support associative learning.     

Neural activity in the medial geniculate nucleus during auditory trace conditioning

 In classical trace conditioning the acquisition of a conditioned response (CR) is possible even though an interval (the trace interval) elapses between the conditioned stimulus (CS) and unconditioned stimulus (US). This implies that some neural representation of the CS (the stimulus trace) is able to support association between the two stimuli. The medial geniculate nucleus (MGN), particularly the medial division (mMGN), has been identified as one site in the auditory pathway where associative related changes in neural activity occur. If neurons in the MGN are involved in such a sensory trace and in acquisition of a CR, then it is expected that activity following an acoustic CS should be related to both stimulus and response. This study examined the extracellular activity of single units in the MGN during differential auditory trace conditioning of the rabbit nictitating membrane response (NMR). Two 150-ms tones (600 Hz and 1200 Hz) served as CS+ and CS–, and the US was periorbital electrostimulation. Changes in activity during the stimulus and trace interval were largest in the medial and dorsal MGN divisions on CS+ trials and on trials in which a CR was made. Examination of probe stimuli of short (50 ms) and long (600 ms) duration suggested that both CR latency and activity changes in the trace interval were related to stimulus duration and time-locked to stimulus offset. Comparisons of neural activity on the basis of fast or slow CR responses revealed different patterns of response – activity on fast CR trials was generally greater and tended to occur earlier. These results suggest that MGN neurons are involved in the maintenance of a sensory memory trace and possibly play a part in CR generation and timing. Received: 2 June 1995 / Accepted: 30 August 1996     

A developmental comparison of trace and delay eyeblink conditioning in rats using matching interstimulus intervals

The effects of conditioning paradigm and interstimulus interval (ISI) were evaluated in this study of the development of associative learning in rats. The acquisition of classical eyeblink conditioning (EBC) was examined in two paradigms (trace vs. delay), with three different ISIs (short, medium, or long) at two ages (postnatal Days 21-23 or 29-31). These data provide the first parametric analysis of ISI in developing animals trained with trace EBC procedures. Further, by comparing trace and delay EBC, it was determined that when ISI is held constant, acquisition is similar for both conditioning paradigms regardless of developmental age. This suggests a similar ontogeny of associative processes for delay and trace EBC that relies on common neural substrates. However, conditioned response timing (onset and peak latencies) was affected by age and paradigm, suggesting that different neural mechanisms may play a role in timing delay versus trace conditioned responses.     

Procedural memory system supports single cue trace eyeblink conditioning in medial temporal lobe amnesia

A number of studies investigating trace eyeblink conditioning have found impaired, but not eliminated, acquisition of conditioned responses (CRs) in both animals and humans with hippocampal removal or damage. The underlying mechanism of this residual learning is unclear. The present study investigated whether the impaired level of learning is the product of residual hippocampal function or whether it is mediated by another memory system that has been shown to function normally in delay eyeblink conditioning. Performance of bilateral medial temporal lobe amnesic patients who had a prior history of participating in eyeblink conditioning studies was compared to a control group with a similar training history and to an untrained control group in a series of single cue trace conditioning tasks with 500 ms, 250 ms, and 0 ms trace intervals. Overall, patients acquired CRs to a level similar to the untrained controls, but were significantly impaired compared to the trained controls. The pattern of acquisition suggests that amnesic patients may be relying on the expression of previously acquired, likely cerebellar based, procedural memory representations in trace conditioning.     

Mediodorsal thalamic lesions and Pavlovian conditioning of heart rate and eyeblink responses in the rabbit

Rabbits received ibotenic acid lesions of the mediodorsal nucleus of the thalamus (MD) or sham lesions. These animals were compared on 4 sessions of Pavlovian eyeblink and heart rate conditioning, in which a tone was the conditioned stimulus and a paraorbital electrical shock was the unconditioned stimulus. Lesions of MD retarded acquisition of the eyeblink conditioned response and abolished the late-occurring tachycardiac component of the heart rate conditioned response. The data are compatible with previous experiments suggesting that MD participates in the sympathetic control associated with somatomotor learning.