Selective hippocampal lesions disrupt a novel cue effect but fail to eliminate blocking in rabbit eyeblink conditioning

The classical conditioning task of blocking involves the adding of a novel but redundant stimulus to a previously trained stimulus. Both blocking and novelty detection are thought to involve the hippocampus. Previously, Solomon (1977) found that nonselective aspiration lesions of the hippocampal region eliminated blocking in rabbit eyeblink conditioning. We tested the effects of selective ibotenic acid lesions of the hippocampus on blocking, as well as on novelty detection, when training is switched from a tone conditioned stimulus (CS) to a compound tone-light CS in eyeblink conditioning. Selective hippocampal lesions did not eliminate blocking but did lead to a facilitation of conditioned response (CR) acquisition to the tone and to the light, but not to the tone-light compound. Selective hippocampal lesions disrupted a CR decrement observed in sham surgical controls when transferred from tone training to tone-light training. It appears that although selective hippocampal lesions do not eliminate blocking in eyeblink conditioning, they do disrupt novelty detection and may facilitate learning to a previously blocked cue.     

Comparison of Single Unit Responses to Tone, Light, and Compound Conditioned Stimuli during Rabbit Classical Eyeblink Conditioning

Unit recordings and lesion studies have implicated the cerebellum as an essential site for the acquisition and maintenance of the conditioned eyeblink response. The current study looked at the neural characteristics of conditioned stimulus (CS) processing in the interpositus nucleus of the cerebellum after training New Zealand white rabbits (Oryctolagus cuniculus) in one of two conditioning paradigms: (a) compound conditioning (CMP), a compound CS consisting of light and tone paired with an air puff unconditioned stimulus (US); or (b) stimulus compounding (ALT), alternating blocks of tone CS and light CS trials paired with the air puff US. Single unit responses were recorded during five sessions after the animals had reached an asymptotic level of responding. Animals were tested for behavioral and neural responses to CS alone trials that included tone alone, light alone, and compound tone-light trials. For the CMP group, the compound CS elicited 80 to 90% conditioned eyeblink responses (CRs), whereas the individual tone and light CSs elicited only 40 to 50% CRs. For the ALT group, all three CSs (tone, light, and compound) elicited very high levels of responding of at least 80% CRs. For the CMP group, there were roughly equal numbers of cells responding to all of the CSs. This includes cells that responded exclusively to one, and only one, of the three stimuli and also those cells that responded to combinations of two or more. Cells from the ALT group were far more likely to respond exclusively to only one of the CSs. Both the behavioral and physiological results suggest that the compound tone-light stimulus was processed as a distinct stimulus, separate from the component tone and light. These results are discussed in the context of multisensory processing.     

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 (0.5 μL, 10 mM) 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.     

Latent inhibition and stimulus generalization of the classically conditioned nictitating membrane response in rabbits (Oryctolagus cuniculus) following dorsal hippocampal ablation.

Rabbits received 0 to 450 exposures of a tone conditioned stimulus (CS) prior to classical defensive conditioning of the nicitating membrane response based on an infraorbital eye shock unconditioned stimulus. Tone preexposure resulted in retarded conditioning in normal rabbits. This latent inhibition effect was not present in animals with bilateral dorsal hippocampectomy produced by aspiration. Control animals with bilateral neocortical and callosal aspiration lesions demonstrated a latent inhibition effect similar to that shown by normal nonoperated animals. The failure of CS preexposure to retard conditioning in hippocampal rabbits was not due to differences in threshold of the conditioned response to the CS or to differences in response mechanisms as determined by tests of habituation and dishabituation of the unconditioned response. A subsequent experiment employed combined-cue summation tests to confirm the fact that preexposure did not endow the tone with conditioned as well as latent inhibitiory properties. Finally, tests of stimulus generalization along the auditory frequency dimension indicated flatter relative gradients for hippocampals than for nonoperated controls, with cortical controls in between. These findings were discussed in terms of Douglas' model of hippocampal function.     

Effects of Early Hippocampal Lesions on Trace, Delay, and Long-Delay Eyeblink Conditioning in Developing Rats

The effects of bilateral hippocampal aspiration lesions on later acquisition of eyeblink conditioning were examined in developing Long-Evans rat pups. Lesions on postnatal day (PND) 10 were followed by evaluation of trace eyeblink conditioning (Experiment 1) and delay eyeblink conditioning (Experiment 2) on PND 25. Pairings of a tone conditioned stimulus (CS) and periocular shock unconditioned stimulus (US, 100 ms) were presented in one of three conditioning paradigms: trace (380 ms CS, 500 ms trace interval, 880 ms interstimulus interval [ISI]), standard delay (380 ms CS, 280 ms ISI), or long delay (980 ms CS, 880 ms ISI). The results of two experiments indicated that hippocampal lesions impaired trace eyeblink conditioning more than either type of delay conditioning. In light of our previous work on the ontogeny of trace, delay, and long-delay eyeblink conditioning (Ivkovich, Paczkowski, & Stanton, 2000) showing that trace and long-delay eyeblink conditioning had similar ontogenetic profiles, the current data suggest that during ontogeny hippocampal maturation may be more important for the short-term memory component than for the long-ISI component of trace eyeblink conditioning. The late development of conditioning over long ISIs may depend on a separate process such as protracted development of cerebellar cortex.     

Selective entorhinal and nonselective cortical-hippocampal region lesions,but not selective hippocampal lesions,disrupt learned irrelevance in rabbit eyeblink conditioning

Prior experiments, as well as computational models, have implicated the hippocampal region in mediating the influence of nonreinforced stimulus preexposure on subsequent learning. Learned irrelevance (LIRR) is a preexposure task in which uncorrelated preexposures to the conditioned stimulus (CS) and the unconditioned stimulus (US) produce a retardation of subsequent CS-US conditioning. In the work presented here, we report the results of tests of LIRR in eyeblink conditioning in rabbits with sham lesions, nonselective cortical-hippocampal region lesions, selective hippocampal lesions, and selective entorhinal lesions. Sham-lesioned rabbits that had been preexposed to the CS and the US exhibited slower acquisition of conditioned response, as compared with context-preexposed controls. Nonselective cotical-hippocampal region lesions disrupted LIRR, whereas selective hippocampal lesions had no detrimental effect on LIRR. Selective entorhinal lesions disrupted LIRR. These findings fit other recent empirical findings and theoretical predictions that some classical conditioning tasks previously thought to depend on the hippocampus depend, rather, on the entorhinal cortex.     

Modulation of a discrete Pavlovian conditioned reflex by a putative emotive Pavlovian conditioned stimulus

Three experiments showed the modulation of a rabbit eyeblink conditioned response (CR) to a Pavlovian conditioned stimulus (CS) by 30-s stimuli (A & B) that had been differentially paired with paraorbital shock. The CS (Y) was a 1,050-ms cue that had been paired with paraorbital shock outside A or B. In testing, the amplitude of CRs was greater when Y was presented within A than within B. Differential modulation occurred whether shock in A had been preceded by another 1,050-ms cue, X(AX+,BX-;Experiment 1) or not (A+B-;Experiment 2). Experiment 3 compared the technique of Experiment 1 (AX+) with that of Experiment 2 (A+) and found the latter to be advantageous for facilitation of CRs to Y by A. These data are consistent with the predictions of a model of Pavlovian conditioning (AESOP, Wagner & Brandon, 1989) that distinguishes between emotive and sensory conditioning as did Konorski (1967).     

Classical conditioning of the nictitating membrane response of the lemon shark (Negaprion brevirostris)

An experimental group of lemon sharks received 100 daily presentations of light flash as the conditioned stimulus (CS) and electric shock as the unconditioned stimulus (US) in a classical conditioning situation. The conditioned responses (CRs) and unconditioned responses (URs) under observation consisted of extensions of the nictitating membrane. Separate control groups received either (a) no CS or US, (b) CS-alone, or (c) completely random presentations of CS and US. Few CRs occurred in the experimental group at the outset of conditioning, but the percentage of CRs during the second half of the first acquisition session exceeded 95%. Conditioning stabilized above 95% CRs during Acquisition Sessions 3 through 7. These responses could not be attributed to pseudoconditioning, sensitization, or other nonassociative factors. When the experimental group was subsequently given six CS-alone sessions, the course of extinction was gradual. Most results seemed similar to those previously obtained during classical conditioning of the nictitating membrane in rabbits.     

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.     

Hippocampal lesions and pavlovian cardiovascular conditioning

New Zealand albino rabbits received either sham, cortical control, or hippocampal lesions and were subjected to differential Pavlovian conditioning in which tones of different frequencies served as conditional stimuli (CSs), and a brief paraorbital electric shock train served as the unconditional stimulus. Heart rate (HR), blood pressure (BP), and electromyographic (EMG) conditional responses (CRs) were recorded. Animals with cortical and hippocampal damage or animals with cortical damage alone revealed attenuated bradycardiac CRs, but HR CRs of the former two groups did not significantly differ. BP changes were minimal; reliable differences were not obtained between CS+ and CS?. However, these changes consisted of small but reliable depressor responses, which were not affected by either cortical or hippocampal lesions. Few EMG CRs were obtained. These data, combined with those of previous experiments, suggest that forebrain structures may modulate higher level processing of stimulus information, perhaps in terms of assessing the biological significance of such stimulation.     

Lesions of the entorhinal cortex impair acquisition of hippocampal-dependent trace conditioning

Rabbits with the electrolytic lesions of bilateral entorhinal cortex (EC) were trained with the hippocampal-dependent trace conditioning of the nictitating membrane response. The multiple-unit activity of the hippocampal CA1 region was recorded during conditioning. The conditioned stimulus was a tone (1 kHz, 85 dB, 200-ms duration), the unconditioned stimulus was a corneal air puff (3 psi, 150-ms duration), and the interstimulus interval was 750 ms. The EC-lesioned animals showed only 30% conditioned response (CR) by the ninth session while the sham-operated animals showed above 80% CR. The lesioned animals did not show learning-related changes in the hippocampal activity. When the training was switched to the 300-ms interstimulus interval trace conditioning, both groups learned above 80% CR. The EC-lesioned animals, however, showed less learning-related activity in the hippocampus than the sham-operated animals. These results suggest that the development of the learning-related activity in the hippocampus depends on the intact EC, and that the EC may provide a possible pathway conveying learning information from the cerebellum or cerebral cortex to the hippocampus during the trace conditioning.     

Fear develops to the conditioned stimulus and to the context during classical eyeblink conditioning in rats

In classical eyeblink conditioning, non-specific emotional responses to the aversive shock unconditioned stimulus (US), which are presumed to coincide with the development of fear, occur early in conditioning and precede the emergence of eyeblink responses. This twoprocess learning model was examined by concurrently measuring fear and eyeblink conditioning in the freely moving rat. Freezing served as an index of fear in animals and was measured during the inter-trial intervals in the training context and during a tone conditioned stimulus (CS) presented in a novel context. Animals that received CS-US pairings exhibited elevated levels of fear to the context and CS early in training that decreased over sessions, while eyeblink conditioned responses (CRs) developed gradually during acquisition and decreased during extinction. Random CS-US presentations produced a similar pattern of fear responses to the context and CS as paired presentations despite low eyeblink CR percentages, indicating that fear responding was decreased independent of high levels of learned eyeblink responding The results of paired training were consistent with two-process models of conditioning that postulate that early emotional responding facilitates subsequent motor learning, but measures from random control animals demonstrate that partial CS-US contingencies produce decrements in fear despite low levels of eyeblink CRs. These findings suggest, a relationship between CS-US contingency and fear levels during eyeblink conditioning, and may serve to clarify further the role that fear conditioning plays in this simple paradigm.     

Fear Develops to the Conditioned Stimulus and to the Context during Classical Eyeblink Conditioning in Rats

In classical eyeblink conditioning, non-specific emotional responses to the aversive shock unconditioned stimulus (US), which are presumed to coincide with the development of fear, occur early in conditioning and precede the emergence of eyeblink responses. This two-process learning model was examined by concurrently measuring fear and eyeblink conditioning in the freely moving rat. Freezing served as an index of fear in animals and was measured during the inter-trial intervals in the training context and during a tone conditioned stimulus (CS) presented in a novel context. Animals that received CS-US pairings exhibited elevated levels of fear to the context and CS early in training that decreased over sessions, while eyeblink conditioned responses (CRs) developed gradually during acquisition and decreased during extinction. Random CS-US presentations produced a similar pattern of fear responses to the context and CS as paired presentations despite low eyeblink CR percentages, indicating that fear responding was decreased independent of high levels of learned eyeblink responding. The results of paired training were consistent with two-process models of conditioning that postulate that early emotional responding facilitates subsequent motor learning, but measures from random control animals demonstrate that partial CS-US contingencies produce decrements in fear despite low levels of eyeblink CRs. These findings suggest a relationship between CS-US contingency and fear levels during eyeblink conditioning, and may serve to clarify further the role that fear conditioning plays in this simple paradigm.     

The Influence of Retention Interval on the US Preexposure Effect: Changes in Contextual Blocking over Time

A number of studies manipulating the length of the interval between conditioning and testing indicate spontaneous recovery from overshadowing, suggesting that certain instances of overshadowing represent a deficit in memory retrieval rather than a failure of animals to form an association between the overshadowed stimulus and the US. The present series of experiments examined the influence of lengthening the retention interval on blocking, another stimulus selection phenomenon that is typically interpreted as an acquisition deficit. The results indicated that when subjects were tested shortly (3 days) after training conditioning to a taste blocked subsequent conditioning to an odor conditioned in compound with that taste (Experiment 1), whereas prior conditioning to an odor did not block subsequent conditioning to a taste conditioned in compound with that odor (Experiment 2). This pattern of results was essentially unchanged when testing occurred at a longer (21-day) retention interval. However, there was evidence of a US preexposure effect in Experiment 2 when subjects in the US ONLY control condition were tested at the 3-day retention interval, but not when testing occurred 21 days after conditioning. Experiments 3 and 4 examined whether this loss of the US preexposure effect over time might actually represent a change in the degree of contextual blocking as the retention interval is lengthened. Exposure to the conditioning context either during the interval between Phase 1 and Phase 2 of conditioning (Experiment 3) or prior to Phase 1 of conditioning (Experiment 4) alleviated this US preexposure effect suggesting that the loss of the US preexposure effect as the retention interval is lengthened observed in Experiment 2 is due to changes in the degree of blocking by contextual stimuli over time. The results are discussed in terms of differential susceptibility of forgetting of two functional roles played by a contextual stimuli in the current situation-context as a CS and context as a retrieval cue for other CS-US associations.     

Posttraining prefrontal lesions impair jaw movement conditioning performance,but have no effect on accompanying heart rate changes

This experiment examined the role of the medial prefrontal cortex (mPFC) in regulating learned autonomic and somatomotor responses in rabbits using appetitive Pavlovian conditioning. Interstimulus interval (ISI) duration [i.e., the time between the onset of the conditioned stimulus (CS) and unconditioned stimulus (US)] was manipulated in order to determine whether ISI duration was related to the heart rate (HR) responses obtained during conditioning. Two groups received either a 1- or a 4-s ISI, with a tone as the CS and an intraoral pulse of water as the US. Another two groups received explicitly unpaired presentations of either the 1- or 4-s tone CS and water US. Few conditioned jaw movement (JM) or HR conditioned responses (CRs) were observed in the unpaired conditions. Significant JM conditioning was, however, elicited by the paired conditions, especially to the 4-s ISI. Consistent CS-evoked HR accelerations were observed in both ISI conditions. After five sessions of training, the mPFC was lesioned in half the animals. A separate group of paired animals received sham lesions. After surgical recovery, all animals received 3 days of postoperative training. During the first postoperative training session, JM CRs significantly declined in both groups with mPFC lesions in comparison to the groups with sham lesions. The mPFC lesions, however, did not affect the CS-evoked cardiac accelerations, which again occurred during postoperative training.     

Inactivation of the anterior cingulate cortex impairs extinction of rabbit jaw movement conditioning and prevents extinction-related inhibition of hippocampal activity

Although past research has highlighted the involvement of limbic structures such as the anterior cingulate cortex (ACC) and hippocampus in learning, few have addressed the nature of their interaction. The current study of rabbit jaw movement conditioning used a combination of reversible lesions and electrophysiology to examine the involvement of the hippocampus and the ACC during acquisition, performance, and extinction. We found that microinfusions of procaine into the ACC did not significantly alter the rate of behavioral learning or the amplitude of hippocampal conditioned unit responses, but that they disrupted the rhythmic periodicity of conditioned jaw movements. During extinction, whereas controls showed a rapid decline in behavioral CRs and active inhibition of hippocampal unit responses, ACC lesioned rabbits showed a persistence of conditioning-related hippocampal activity and behavioral responding. The results show that the ACC can be important for adaptive suppression of conditioned behavior and suggest a crucial physiological modulation of hippocampus by ACC during extinction.     

Conditioning the unconditioned response: modification of the rabbit's (Oryctolagus cuniculus) unconditioned nictitating membrane response

Conditioning-specific reflex modification (CRM) occurs when classical conditioning modifies responding to an unconditioned stimulus (US) in the absence of a conditioned stimulus (CS). Three experiments monitored rabbit nictitating (Oryctolagus cuniculus) membrane unconditioned responses to 5 intensities and 4 durations of periorbital electrical stimulation before and after CS or US manipulation. CRM occurred after 12 days of CS-US pairings but not following unpaired CS/US presentations or restraint. CRM survived CS-alone and CS/US-unpaired extinction of the conditioned response (CR) but not presentations of the US alone, although CRs remained intact. Thus, CRs could be weakened without eliminating CRM and CRM could be weakened without eliminating CRs. Data indicate CRM is a reliable, associative effect that is more than a generalized CR and may not be explained by habituation, stimulus generalization, contextual conditioning, or bidirectional conditioning.     

Blocking between occasion setters and contextual stimuli

Two appetitive conditioning experiments with rats used a blocking procedure to compare the mechanisms through which contexts and positive occasion setters control responding to conditioned stimuli (CSs) in discrimination learning. In Experiment 1, a light (L) initially set the occasion for food reinforcement of a tone CS (T). Then a compound of L and a novel context signaled reinforcement of T. Previous learning about L blocked contextual control of responding to T. Blocking was not due to simple excitation conditioned to L during discrimination training; comparable excitation to L in a control group did not result in blocking. Conversely, in Experiment 2, initial learning about the context blocked the acquisition of occasion setting to L. Excitation conditioned to the context could not account for the blocking. In general, the results suggest that contexts and occasion setters may control responding to CSs through similar mechanisms.     

Human classical conditioning. The status of the CS

Recent reports of failure to obtain blocking in human galvanic skin response (GSR) conditioning, together with our own equivocal results with eyelid conditioning, have motivated us to re-examine the status of the conditioned stimulus (CS) in human conditioning studies. The issues raised by compound stimuli, by contextual cues and occasion setting stimuli, and by cross-modal transfer are considered in the light of data from our laboratories. These data include observations on the interchangeability of stimulus modalities during acquisition, the use of varying information loads embedded in occasion setting displays, the comparison of alternative blocking designs and the analysis of response topography in relation to stimulus variability. They suggested that an adequate account of the CS in human conditioning studies must recognize that it is dynamically processed and reprocessed both during and after acquisition.     

Temporal specificity in cross-modal transfer of the rabbit nictitating membrane response

Two experiments examine cross-modal transfer of response features specific to the interstimulus interval (ISI) between a conditioned stimulus (CS) and an unconditioned stimulus. Rabbits were given initial training with a stimulus (CSA) in one modality (e.g., tone) at a designated ISI (e.g., 600 ms). Training was then shifted to a new stimulus (CSB) in another modality (e.g., light) at a new ISI (e.g., 400 ms). The timing of early conditioned responses (CRs) to CSB reflected the ISI of CSA. Ultimately, CRs to CSB shifted to a temporal location conforming to the ISI of CSB. When the ISI of CSB was shorter than that of CSA, CRs to CSA also shifted to a locus conforming to the ISI of CSB. The present results confirmed previous findings that training in one CS modality accelerates CR acquisition to a CS in another modality. The findings are compared with the transfer of response patterns in instrumental learning sets and are discussed regarding their implications for theories of cross-modal transfer.