Concomitant heart rate and corneoretinal potential conditioning in the rabbit (Oryctolagus cuniculus): effects of caudate lesions

Rabbits received either bilateral, unilateral, or sham caudate lesions and were subjected to Pavlovian conditioning training. Corneoretinal potential (CRP), electromyographic (EMG), and heart rate (HR) CRs were assessed. Lesions which destroyed the anteromedial two thirds of the head of the caudate nucleus greatly impaired the acquisition of the CRP response during both simple and differential Pavlovian conditioning. However, the magnitude of the HR CR and the HR discrimination were unaffected by these lesions. Measures of free field activity, electromyographic activity, and CRP thresholds to shock revealed no evidence of a motor or sensory deficit in the lesioned animals. These data thus demonstrate a deficit in a specific learned somatomotor response but no impairment in the autonomic changes which usually accompany somatomotor conditioning.     

Blood Pressure and Heart Rate Changes Accompanying Classical Eyeblink Conditioning In The Rabbit (Oryctolagus Cuniculus)

In two experiments employing 38 rabbits differential classical conditioning of heart rate, blood pressure, and corneoretinal potential (CRP) response were examined using l-sec and 4-sec interstimulus intervals ISI respectively. The conditioned response consisted of HK decelerations and DP depressor responses early in conditioning. However, many, but not all, animals revealed pressor responses and HR accelerations after the CRP discrimination was acquired. Significant correlations were also obtained between BP pressor responses, HR accelerations, and the frequency of CRP CRs. These results were discussed within the context of the orienting and defense reflexes.     

Nucleus basalis lesions attenuate aquisition,but not retention,of Pavlovian heart rate conditioning and have no effect on eyeblink conditioning

Summary Rabbits with lesions of the anterior nucleus basalis of Meynert (nBM) were compared with animals with sham lesions or unoperated control animals on a classical conditioning task in which heart rate (HR) and eyeblink (EB) conditioned responses (CRs) were as sessed. The nBM lesions impaired the magnitude of the decelerative HR CR, but had no effect on the EB CR. A second experiment, in which animals were lesioned af ter acquisition was complete, showed that anterior nBM lesions had no effect on retention of either the HR or EB CR. These data suggest that the anterior nBM may participate in the early stages of information processing in which stimuli are evaluated for their significance based on their association with a reinforcer. However, the ante rior nBM is apparently not involved in the selection of a somatomotor response to deal effectively with such changing stimulus contingencies.     

Differential and reversal Pavlovian conditioning in rabbits with mediodorsal thalamic lesions: assessment of heart rate and eyeblink responses

Rabbits received ibotenic acid lesions of the mediodorsal nucleus of the thalamus (MD), or sham lesions. These animals were compared on acquisition and reversal of a Pavlovian conditioned discrimination task in which tones were the conditioned stimuli and paraorbital electric shock was the unconditioned stimulus. Eyeblink and heart rate conditioned responses were assessed. Lesions of MD impaired the reversal, but not original acquisition, of the eyeblink discrimination. Heart rate discrimination was somewhat impaired during both acquisition and reversal. There were no differences between lesion and sham animals on control measures assessing general activity, somatomotor shock thresholds, or heart rate unconditioned responses.     

A Comparison of Two Model Systems of Associative Learning: Heart Rate and Eyeblink Conditioning in the Rabbit

In four classical conditioning experiments heart period and eyeblink responses were assessed concomitantly. The conditioned stimuli (CSs) were tones and the unconditioned stimulus (UCS) was a brief paraorbital electric shock. Using a 0.5-s duration CS, bradycardiac conditioned responses that consisted of a 4–5 ms change from pre-CS baseline occurred within 10–20 CS-UCS presentations. However, eyeblink conditioned responses began to occur only after 100–150 CS–UCS presentations. A 1.0-s CS duration resulted in bradycardiac conditioned responses of 15–30 ms change from pre-CS baseline, which again reached asymptote within 10–20 trials. Using a 4-s CS duration, in a differential classical conditioning paradigm, heart period discrimination between a reinforced CS+ and a nonreinforced CS? occurred within 10 trials; asymptotic performance of the heart period conditioned response occurred within 15 CS-UCS presentations and consisted of a bradycardiac response of 40–50 ms for the 12th interbeat interval following tone onset. These data thus indicate that these two model systems of mammalian learning (based on heart period and eyeblink responses) show quite different acquisition functions. It is also significant that heart rate slowing always accompanied the eyeblink conditioned responses, even though increases in general electromyographic activity as well as eyeblink conditioned responses were simultaneously observed during CS presentation.     

Disruption of rabbit (Oryctolagus cuniculus) nictitating membrane conditioning by posttrial electrical stimulation of hippocampus

Two experiments were conducted involving classical conditioning of the nictitating membrane response of rabbits with posttrial electrical brain stimulation. Hippocampal posttrial stimulation (PTS) presented immediately after each CS-US paired trial was found to cause massive disruption of conditioning by comparison with performance of either a cortical PTS group, an operated but unstimulated group, or an unoperated control group. The major effect of PTS was to postpone CR emergence. Once CRs began to occur, conditioning seemed to proceed normally regardless of the presence or absence of stimulation. Hippocampal PTS initiated after conditioning had no effect on the elicitation of conditioned responses. The results indicated that hippocampal PTS disrupted consolidation but did not affect either registration of stimuli or execution of unconditioned or conditioned responses. It was concluded that classical conditioning of the rabbit nictitating membrane response is an excellent model neural system for the investigation of neurophysiological aspects of memory.     

Cholesterol modifies classical conditioning of the rabbit (Oryctolagus cuniculus) nictitating membrane response

Cholesterol plays an important role in synapse formation, receptor function, and synaptic plasticity, and animal studies show that modifying cholesterol may improve learning and memory. Other data show that feeding animals cholesterol can induce beta amyloid accumulation. Rabbits (Oryctolagus cuniculus) fed 2% cholesterol for 8 weeks were given trace conditioning of the nictitating membrane response using a 100-ms tone, a 700-ms trace, and periorbital electrical stimulation or airpuff. Rabbits fed cholesterol showed significant facilitation of trace conditioning to airpuff and conditioning-specific reflex modification to periorbital electrical stimulation and airpuff. The cholesterol-fed rabbits had beta amyloid accumulation in the cortex, but little in the hippocampus. The data suggest cholesterol had facilitative effects that outweighed potential amnesic effects of cortical beta amyloid.     

Age-related changes in Pavlovian conditioning: central nervous system correlates

Two groups of New Zealand Albino rabbits, average age 6 and 40 months, were subjected to Pavlovian conditioning in which eyeblink (EB) and heart rate (HR) responses were assessed. At the end of conditioning biogenic amine content was assessed in several brain areas; dopamine receptor binding was assessed in caudate nucleus. Older animals revealed an impairment in EB conditioning relative to the young animals; the conditioned decelerative HR response was increased in magnitude in the old animals. Significant deficits in cortical norepinephrine and serotonin were observed in the old subjects, but in both old and young animals the magnitude of the bradycardia was positively correlated with forebrain norepinephrine and inversely correlated with serotonin concentration. Receptor binding experiments revealed that 3H-spiroperidol receptor density was decreased in caudate nuclei of old animals; however, receptor affinity was unaffected by age.     

Prefrontal control of trace eyeblink conditioning in rabbits (Oryctolagus cuniculus) II: effects of type of unconditioned stimulus (airpuff vs. periorbital shock) and unconditioned stimulus intensity

Previous research has implicated the medial prefrontal cortex (mPFC) in the control of classically conditioned autonomic and somatomotor responses. In eyeblink (EB) classical conditioning prefrontal involvement appears to be limited to paradigms that are more difficult to learn, in that acquisition is slower. These include trace conditioning and discrimination/reversal. Some of this research suggests that the participation of mPFC in classical EB conditioning is related to the intensity or type of unconditioned stimulus (US) employed. In the present two experiments we thus studied the effects of manipulation of periorbital shock intensity as the US in Experiment 1 and in Experiment 2 the intensity of a corneal airpuff as the US on Pavlovian trace EB conditioning. The results indicate that there are optimal intensities of both airpuff and periorbital shock as the US in the demonstration of mPFC control of trace classical EB conditioning.     

Startle reflex facilitation as a function of classical eyeblink conditioning in humans

I investigated whether classical eyeblink conditioning increases the activating effect of the conditional stimulus (CS) on the relevant alpha motoneuron pool in the facial nucleus. The activating effect was measured by the reflex modification method, where startle reflexes, sharing the facial nerve motor pathway with conditional and unconditional eyeblink reflexes, were elicited in the presence of reinforced and nonreinforced CSs (CS+ and CS?). To reduce influences of conditional arousal on startle, a weak airpuff to the eye was used as unconditional stimulus and relatively short stimulus onset asynchronies (SOAs) of 300 and 600 ms were used in three startle tests presented while conditioning was in progress. Two groups (N= 72) received 100% or 0% reinforcement (Groups 100 and 0) of a tone CS+. The CS? was not reinforced. Three startle tests, consisting of acoustic startle probes presented alone and after CS+ and CS? onset, were conducted early, middle, and late in conditioning. Results showed increased startle reflex facilitation after CS+ onset in the late startle test in Group 100, indicating that startle increased as a function of classical conditioning. Although reflex facilitation at the 300-ms SOA in Group 100 is consistent with the hypothesis that CS presentations activated motoneurons in the facial nucleus, the present study cannot itself definitively support this hypothesis. Reflex facilitation at the 600-ms SOA in Group 100 could be influenced by conditional arousal.     

Disruption of maternal behavior in rats with lesions of the septal area

Rats given septal lesions prior to mating did not build nests or nurse their pups. During retrieving tests they carried pups about the cage repeatedly and dropped them in scattered locations. Virgin rats induced to be maternal by housing with foster pups showed similar, but less marked, behavioral deficits. Rats administered septal lesions after parturition became hyperresponsive, ceased all maternal behaviors, and cannibalized their pups. The results indicated that the aberrant maternal behaviors shown by rats with septal lesions did not result from alterations in hormonal status. It was concluded that the lesions disrupted the typical pattern or sequence of maternal activities and that the deficits were probably related to an enhancement of response perseveration.     

Magnitude and timing of nictitating membrane movements during classical conditioning of the rabbit (Oryctolagus cuniculus)

A trial-by-trial, subject-by-subject analysis was conducted to determine whether generation of the conditioned response (CR) occurs on a continuous or all-or-none basis. Three groups of rabbits were trained on different partial reinforcement schedules with the conditioned stimulus presented alone on 10%, 30%, or 50%, respectively, of all trials. Plots of each rabbit's nictitating membrane movements revealed that their magnitude rose in a continuous fashion. Response growth during acquisition followed a sigmoidal curve, and the timing of CR-sized movements was largely stable throughout the experiment. The results are discussed with respect to alternative models of CR generation.     

Conditioned bradycardia in the rabbit: effects of knife cuts and ibotenic acid lesions in the lateral hypothalamus

Summary New Zealand albino rabbits received either parasagittal or coronal knife cuts in the lateral hypothalamus (LH). Other animals received ibotenic acid lesions of LH. Pavlovian conditioning was studied in these animals and compared with that of sham operated, vehicle, or unoperated control animals. Tones served as conditioned stimuli (CSs) and periorbital electric shocks as unconditioned stimuli (USs). Coronal knife cuts that interrupted fibers passing through LH abolished the bradycardia elicited by these contingencies, as well as the cardiac component of the orienting reflex (OR), which also consisted of bradycardia. Parasagittal knife cuts medial to the temporal lobe but lateral to the major nuclei of the hypothalamus also completely abolished conditioned bradycardia, but had no effect on the OR, compared to sham and unoperated control animals. These lesions also had no effect on concomitantly occurring eyeblink conditioning. Ibotenic acid lesions of LH had no effect on conditioned bradycardia but diminished the magnitude of the cardiac OR. Control experiments suggest that the damage produced by these manipulations had no effect on either sensitivity to the CS or US and produced no general motoric difficulties. These data thus suggest that structures rostral to the hypothalamus, probably in the amygdala and/or agranular prefrontal cortex, mediate the bradycardia associated with classical conditioning contigencies, but that hypothalamic mechanisms may mediate the OR.     

Amygdala central nucleus lesions: effect on heart rate conditioning in the rabbit

The present study was designed as an initial step in determining the specific anatomical systems of the amygdala which may contribute to the expression of conditioned heart rate responding during aversive Pavlovian conditioning in the New Zealand rabbit. Animals receiving either small or large radio-frequency lesions of the central nucleus of the amygdala demonstrated a significant attenuation of the conditioned bradycardia response to the conditioned stimulus when compared to that demonstrated by control animals. No significant effects of the lesions on baseline heart rate or on the heart rate orienting response to the conditioned stimulus were observed. Central nucleus lesions also produced a significant effect on the unconditioned heart rate response to the unconditioned stimulus manifested in an increased duration and prolonged habituation of the unconditioned response. The results are consistent with anatomical and physiological evidence suggesting the involvement of the central nucleus in the expression of emotional responses including accompanying cardiovascular alterations.     

A re-examination of the effects of septal lesions on leverpress shock escape

Following training on a leverpress shock escape procedure, rats were exposed to bilateral destruction of septal nuclei. Although the force and amount of leverholding were reduced following syrgery, all animals continued to escape shock efficiently. Analysis of video tape recordings of performance suggested that septal damage may have interrupted the “rearing” response, an essential component of the dominant response strategy prior to surgery. Observations did not reveal consistent septal lesion-induced changes in activity level, reactivity, or disinhibition of competing responses.     

Cross-maze avoidance behavior and septal lesions in the rat

Rats with lesions of the septum were compared with control rats on the acquisition of free choice and discriminated active avoidance behaviors in a cross-maze. The rats with septal damage exhibited facilitated acquisition of the avoidance response under both conditions. The groups also differed in the patterning of their avoidance responses. The lesioned rats tended to respond sequentially to each of the four alleys while the predominant response of the control rats was to shuttle back and forth between two of the four alleys.     

Heart rate and somatomotor activity in rats during signalled escape and yoked classical conditioning

In the first of three studies, heart rate, movement, and EMG activity during signalled escape and regular classical conditioning were examined in unrestrained and restrained groups of rats. The direction of the HR CR was accelerative in the unrestrained groups and decelerative in the restrained groups, regardless of the presence or absence of the escape contingency. Both CRs were enhanced by the presence of directionally similar changes in movement to the CS. The decelerations of the restrained groups were correlated with decreases in movement and increases in EMG activity. Increasing the somatomotor activity associated with the US failed to modify the decelerative direction of the HR CRs of restrained rats in the second and third studies. It was suggested that HR CRs in rats may be accompaniments of naturally occurring defense behaviors such as freezing in restrained rats and aggression in unrestrained rats.     

Studies of binaural detection in the rabbit (Oryctolagus cuniculus) with Pavlovian conditioning.

A Pavlovian conditioned eyeblink response in rabbits (Oryctolagus cuniculus) was used to study psychoacoustical phenomena previously demonstrated in human listeners and other animals. This article contains the results of a tone-in-noise detection study to examine 2 psychoacoustical phenomena in rabbit and in human listeners: (a) the binaural masking level difference (BMLD) and (b) differential performance across reproducible noise masker waveforms. The rabbits demonstrated a BMLD comparable in size to other species. Significant differences in performance across reproducible noise masker waveforms were seen in the rabbits. This performance was compared with the performance of human listeners using the same set of waveforms.     

Square-runway avoidance behavior and septal lesions in the rat

The acquisition of a square-runway avoidance response was studied in rats with septal lesions The lesioned rats acquired the square-runway avoidance response significantly faster than the control rats. In a second experiment lesioned and control rats were compared on the acquisition of a regular one-way avoidance response in the same apparatus. This change in procedure facilitated the avoidance responding of the control rats and inhibited avoidance acquisition by the rats with septal lesions.     

Classical conditioning of heart rate in rats using direct vagal stimulation as a US

Classically conditioned heart-rate deceleration was established in rats using a 2-sec stimulation of the intact cervical vagus nerve as the US. Although small in magnitude, changes in the form of the CR over training indicated that there may have been inhibition of delay similar to that obtained using a traditional painful shock US. Independent assessments of motivational potency revealed that the vagal US did not lead to conditioned lever-press suppression in a CER procedure, but did produce reliable escape responding in a shuttle-box preference situation. It was suggested that the mildly aversive motivational effects of vagal stimulation may have stemmed from the activation of a vagally mediated system involved in the regulation of hunger.