Excitotoxic lesions of the hippocampus leave sensory preconditioning intact: implications for models of hippocampal function

Learning about contextual cues is markedly disrupted in rats with hippocampal lesions. One analysis of this disruption is that it reflects a general failure to form associations between the elements of complex events. A straightforward prediction of this analysis is that sensory preconditioning will be disrupted by hippocampal lesions. This prediction was assessed by presenting rats with flavored solutions composed of 2 elements (A and X) before X was paired with an injection of the emetic, lithium chloride. A subsequent test revealed that rats were less willing to consume Solution A than they were to consume a control solution, B. This was true of rats with sham lesions and those with excitotoxic lesions of hippocampus. These findings fail to support the proposition that the hippocampus-dependent deficit in contextual conditioning is due to a general disruption to the process of associating the elements of complex events.

Involvement of the entorhinal cortex in a process of attentional modulation: evidence from a novel variant of an IDS/EDS procedure

Novel behavioral assays were used to assess the role of the entorhinal cortex in modulating attention to components of stimulus compounds. In Stage 1, rats received discrimination training with compounds constructed from 3 dimensions (auditory, visual, and tactile); in each compound the combination of components from 2 dimensions (e.g., auditory and visual) were relevant to the solution of the discrimination, and the remaining dimension (e.g., tactile) was irrelevant. In Stage 2, rats received a different discrimination in which the relevant dimensions were either congruent (auditory and visual) or incongruent (auditory and tactile) with those that were relevant in Stage 1. Sham-operated rats acquired the congruent discrimination more rapidly than the incongruent discrimination--a finding indicative of a process of attentional modulation--whereas rats with excitotoxic lesions of the entorhinal cortex acquired both discriminations equally readily.

Loss of the thalamic nuclei for "head direction" impairs performance on spatial memory tasks in rats

This study sought to characterize the effects of removing the nuclei of primary importance in relaying the thalamic head direction signal to the hippocampal formation (the anterior dorsal [AD] and lateral dorsal [LD] nuclei) on the performance of a variety of spatial and nonspatial tasks. The results indicate that combined excitotoxic lesions of the AD and LD nuclei produce marked deficits on a variety of spatial tasks. These tasks included T-maze alternation and the ability to locate a hidden platform set at a fixed distance and fixed direction from a beacon in a Morris water maze. Although object recognition appeared unaffected, marked impairments were found in the ability to detect when an object was placed in a novel position (object-in-place memory).

Transfer between contextual conditional discriminations: an examination of how stimulus conjuctions are represented

In 2 experiments, when rats were placed in 1 pair of contexts, A and B, 2 relationships were in force (X --> food and Y --> no food), and when they were placed in another pair of contexts, C and D, the complementary relationships were operative (Y --> food and X --> no food). In Experiment 1, rats then received a 2nd discrimination that was either contextually congruent (in A and B, Y --> food and X --> no food; in C and D, X --> food and Y --> no food) or contextually incongruent (in A and D, Y --> food and X --> no food; in C and B, X --> food and Y --> no food) with the 1st discrimination. In Experiment 2, the 1st discrimination, involving X and Y, was interleaved with a 2nd discrimination, involving V and W, that was again either congruent (in A and B, V --> food and W --> no food) or incongruent (in A and D, V --> food and W --> no food) with the 1st discrimination. The congruent discriminations were acquired more readily than were the incongruent discriminations.

A novel contextual dimension for use with an operant chamber: from simple to hierarchical forms of learning

A novel procedure is described in which the floor temperatures (warm and cool) in an operant chamber are used as contextual cues in 2 experiments with rats. Experiment 1 demonstrated that rats learn the relationship between these thermal contexts and auditory stimuli that have been paired with them. Experiment 2 showed that thermal contexts can serve a conditional function that (a) reflects the operation of a mechanism that is common to conventional, visual contexts and (b) is bound to these contexts' ability to retrieve the nature of the relationship between the auditory stimuli and food.

A novel contextual dimension for use with an operant chamber: from simple to hierarchical forms of learning

A novel procedure is described in which the floor temperatures (warm and cool) in an operant chamber are used as contextual cues in 2 experiments with rats. Experiment 1 demonstrated that rats learn the relationship between these thermal contexts and auditory stimuli that have been paired with them. Experiment 2 showed that thermal contexts can serve a conditional function that (a) reflects the operation of a mechanism that is common to conventional, visual contexts and (b) is bound to these contexts' ability to retrieve the nature of the relationship between the auditory stimuli and food.

Associative components of recognition memory

Recent results indicate that visual recognition memory (as assessed by habituation and dishabituation of the orienting response) is influenced by associative knowledge, and that this influence is mediated by the hippocampus. A standard, associative model of learning has been recently reported to provide a parsimonious explanation for these results.

Associative priming in Pavlovian conditioning

Any occasion on which an animal is placed in an experimental setting or context and receives pairings of one event with another provides the opportunity for a variety of associative structures to be acquired. These structures range from simple associations, which allow the presentation of one event to activate or prime a memory of the other, to hierarchical associations, which allow a simple association to be primed by some other event (e.g. the context in which the simple association was acquired). Experiments with rats that reveal priming effects consistent with both of these putative associative structures are reviewed.

Associative modulation of the orienting response: distinct effects revealed by hippocampal lesions

The ability of auditory stimuli to modulate rats' tendency to orient to visual targets was assessed. In Experiment 1, trials where an auditory stimulus (A) signaled one visual array (X) were intermixed with unsignaled presentations of a second array (Y). Comparison of the orienting responses (ORs) to X and Y revealed that A produced a transient (unconditioned) and an emerging (conditioned) disruptive influence on the OR to X. In Experiments 2 and 3, trials where A signaled X were intermixed with others where another auditory stimulus (B) signaled Y. Stimulus A's ability to modulate the OR to X was then assessed by presenting A prior to test arrays containing both X and Y. Control rats were more likely to orient to Y than X (Experiments 2 and 3) and rats with excitotoxic lesions of the hippocampus were more likely to orient to X than Y (Experiment 3). These results show that auditory stimuli exert distinct modulatory influences on the OR to visual stimuli with which they are associated.

Acquired relational equivalence: implications for the nature of associative structures

In 2 experiments, rats received discrimination training in which separate presentations of A and B signaled a common pair of relationships or associations (X-->food and Y-->no food), whereas presentations of C and D signaled a different pair of relationships (X-->no food and Y->food). To assess the nature of the associative structures acquired during this training, rats then received 2 types of reevaluation procedure: In Experiment 1, A was paired with shock and C was not. In Experiment 2, the relationships that A and B had previously signaled (X-->food and Y-->no food) were paired with shock, whereas those that C and D had signaled (Y-->food and X-->no food) were not. After both types of reevaluation treatment, rats showed greater generalized conditioned suppression in the presence of B than D. These results indicate that A, B, C, and D come to evoke memories of the relationship or associations that they have signaled.

Acquired relational equivalence: implications for the nature of associative structures

In 2 experiments, rats received discrimination training in which separate presentations of A and B signaled a common pair of relationships or associations (X-->food and Y-->no food), whereas presentations of C and D signaled a different pair of relationships (X-->no food and Y->food). To assess the nature of the associative structures acquired during this training, rats then received 2 types of reevaluation procedure: In Experiment 1, A was paired with shock and C was not. In Experiment 2, the relationships that A and B had previously signaled (X-->food and Y-->no food) were paired with shock, whereas those that C and D had signaled (Y-->food and X-->no food) were not. After both types of reevaluation treatment, rats showed greater generalized conditioned suppression in the presence of B than D. These results indicate that A, B, C, and D come to evoke memories of the relationship or associations that they have signaled.

Imprinting, learning and development: from behaviour to brain and back

Neural and behavioural analyses have shown that the formation of filial preferences in young, precocial birds involves at least two separate processes. One process is an emerging predisposition to approach stimuli with the characteristics of the natural mother. The other (learning) process of filial imprinting results in chicks preferentially-approaching a stimulus to which they have been exposed and involves forming links between the components of the exposed stimulus. The neural substrate for the predisposition is different from that underlying imprinting, and different regions of the chick brain are involved in distinct aspects of learning about imprinting stimuli.

Hippocampal lesions disrupt an associative mismatch process

Novel assays were used to assess inter alia whether the hippocampus is involved in detecting novelty per se or in an associative mismatch process. During training, rats received two audiovisual sequences (tone-left constant light and click-left flashing light). In both sham-operated control rats and those with excitotoxic hippocampal lesions, novel visual targets provoked an orienting response that habituated during training. Moreover, like sham-operated rats, rats with hippocampal lesions acquired associations between the elements of two audiovisual sequences. However, subsequent test trials in which the auditory stimuli preceding the visual targets were switched (click-left constant light and tone-left flashing light) provoked renewed orienting to the visual targets in sham-operated rats but not in hippocampal rats. These results support the view that hippocampal damage results in a failure to detect (or act on) mismatches that are generated when an auditory stimulus associatively evokes the memory of one visual stimulus and a different (familiar) visual stimulus is present in the environment.

Combining CSs associated with the same or different USs

In three experiments, hungry rats received appetitive training with four stimuli, A, B, X, and Y. In each Experiment, A and B were paired with one unconditioned stimulus (US; e.g. food pellets) whereas X and Y were paired with a second US (e.g. sucrose). Subsequently, rats responded more vigorously to combinations of stimuli associated with different USs (A-Y & X-B) than to combinations of stimuli associated with the same US (A-B & X-Y; Experiments 1, 2, & 3). This effect was observed when the stimuli were presented simultaneously and during the second elements of serial compounds (Experiments 2 & 3). Moreover, combining CSs associated with different USs resulted in a more marked CR than combining CSs that had each been paired with both US1 and US2 (Experiment 3). These results suggest that the sensory properties of appetitive reinforcers have an important influence on performance.

Dissociable effects of selective lesions to hippocampal subsystems on exploratory behavior, contextual learning, and spatial learning

Rats received excitotoxic lesions of different subsystems within the hippocampal system--either the hippocampus proper (cornu ammonis and dentate gyrus; hippocampal lesions) or the entorhinal cortex-subicular region (entorhinal lesions). Subsequently, their activity in an operant chamber was monitored both before and after footshock had been delivered (Experiment 1). Rats with hippocampal lesions showed enhanced activity before the delivery of footshock and reduced freezing after the delivery of shock. Rats with entorhinal lesions showed control levels of activity before the delivery of footshock and control levels of freezing after the delivery of footshock. Both types of lesion impaired spatial learning in a water maze (Experiment 2). These results suggest that the deficits arising from damage to the hippocampal system in contextual and spatial learning have different origins.

Dissociable effects of selective lesions to hippocampal subsystems on exploratory behavior, contextual learning, and spatial learning

Rats received excitotoxic lesions of different subsystems within the hippocampal system--either the hippocampus proper (cornu ammonis and dentate gyrus; hippocampal lesions) or the entorhinal cortex-subicular region (entorhinal lesions). Subsequently, their activity in an operant chamber was monitored both before and after footshock had been delivered (Experiment 1). Rats with hippocampal lesions showed enhanced activity before the delivery of footshock and reduced freezing after the delivery of shock. Rats with entorhinal lesions showed control levels of activity before the delivery of footshock and control levels of freezing after the delivery of footshock. Both types of lesion impaired spatial learning in a water maze (Experiment 2). These results suggest that the deficits arising from damage to the hippocampal system in contextual and spatial learning have different origins.

Stimulus comparison and perceptual learning: further evidence and evaluation from an imprinting procedure

Two experiments used chicks to investigate the role of stimulus comparison in perceptual learning. In Experiment 1, chicks received exposure to two views of a jungle fowl, SV (side view) and BV (back view), intermixed within a session (mixed exposure), exposure to SV in one session and BV in a different session (separate exposure), or no exposure to either view. All chicks then received a heat-reinforced discrimination with SV and BV serving as discriminanda. Chicks given mixed exposure acquired the discrimination more readily than did either those given separate exposure or those given no exposure. In Experiment 2, all chicks received mixed exposure to the two stimuli. For one group the interval between presentations of the stimuli was short (short-mixed), for the other group it was long (long-mixed). Subjects in the long-mixed condition acquired the heat-reinforced discrimination more rapidly than those in the short-mixed condition. These results suggest that the intermixed nature of stimulus exposure is an important determinant of the magnitude of perceptual learning effects.

Functionally distinct memories for imprinting stimuli: behavioral and neural dissociations

Precocial chicks exposed to a stimulus subsequently approach that stimulus in preference to other, novel stimuli. Previous investigations of the neural basis of these imprinting preferences suggest that imprinting training results in the formation of two memories. The first memory is formed rapidly and is located in the intermediate and medial part of the hyperstriatum ventrale (IMHV) of the left hemisphere; the formation of the second, in another memory system, S', takes several hours and can be prevented by a lesion placed in the right IMHV soon after training. The results of the present study suggest that the functional characteristics of these memories differ. Although memories in both left IMHV and S' supported imprinting preferences (Experiments 1a and 2a), only memories in S' influenced the acquisition of a heat-reinforced discrimination in which imprinted objects served as discriminanda (Experiments 1b and 2b).

Functionally distinct memories for imprinting stimuli: behavioral and neural dissociations

Precocial chicks exposed to a stimulus subsequently approach that stimulus in preference to other, novel stimuli. Previous investigations of the neural basis of these imprinting preferences suggest that imprinting training results in the formation of two memories. The first memory is formed rapidly and is located in the intermediate and medial part of the hyperstriatum ventrale (IMHV) of the left hemisphere; the formation of the second, in another memory system, S', takes several hours and can be prevented by a lesion placed in the right IMHV soon after training. The results of the present study suggest that the functional characteristics of these memories differ. Although memories in both left IMHV and S' supported imprinting preferences (Experiments 1a and 2a), only memories in S' influenced the acquisition of a heat-reinforced discrimination in which imprinted objects served as discriminanda (Experiments 1b and 2b).

Within-event learning during filial imprinting

Newly hatched domestic chicks (Gallus gallus domesticus) that have received a period of exposure to a visual stimulus (V1) approach V1 in preference to a novel stimulus (V2). The learning process that underlies such changes in chicks' behavior is known as filial imprinting. Under natural conditions, and in laboratory studies, V1 is often accompanied by an auditory stimulus, the maternal call of a hen (A1). Experiments 1a and 1b demonstrated that simultaneous exposure to V1 and A1 potentiated chicks' preference for V1 in a test in which A1 was absent. Subsequent experiments revealed that additional exposure to A1 after (Experiment 2) or before (Experiment 3) exposure to the V1A1 compound reduced the preference for V1. These findings suggest that the potentiation of visual imprinting is a product of within-event learning that is undermined when one element of the compound is presented in isolation.

Within-event learning during filial imprinting

Newly hatched domestic chicks (Gallus gallus domesticus) that have received a period of exposure to a visual stimulus (V1) approach V1 in preference to a novel stimulus (V2). The learning process that underlies such changes in chicks' behavior is known as filial imprinting. Under natural conditions, and in laboratory studies, V1 is often accompanied by an auditory stimulus, the maternal call of a hen (A1). Experiments 1a and 1b demonstrated that simultaneous exposure to V1 and A1 potentiated chicks' preference for V1 in a test in which A1 was absent. Subsequent experiments revealed that additional exposure to A1 after (Experiment 2) or before (Experiment 3) exposure to the V1A1 compound reduced the preference for V1. These findings suggest that the potentiation of visual imprinting is a product of within-event learning that is undermined when one element of the compound is presented in isolation.

The role of stimulus comparison in perceptual learning: an investigation with the domestic chick

In two experiments an imprinting procedure was used to familiarize chicks with two stimuli, A and B, that subsequently served as the discriminanda in a simultaneous discrimination. On the first day of each experiment, subjects either received presentations of A and B that were intermixed within a session (mixed exposure) or presentations of A in one session and of B in another (separate exposure). For half of the subjects in each of the exposure conditions, A and B differed in both colour and form; for the remainder A and B differed in form alone. On the second day of the experiments, the chicks were placed into a cool test apparatus and given training in which approaching A was rewarded by the delivery of a stream of warm air, but approaching B was not. Acquisition of this discrimination was more rapid when A and B differed in two respects than when they differed in form alone. When A and B differed in both colour and form, the heat-reinforced discrimination was acquired more rapidly after separate exposure than after mixed exposure; but when A and B differed in form alone, discrimination learning was more rapid following mixed exposure than separate exposure. The latter finding, that the opportunity to compare stimuli differing in only one dimension facilitates subsequent discrimination learning, is consistent with earlier suggestions (Gibson, 1969) regarding the conditions that promote perceptual learning.

Measurement of the spectral absorption coefficient in the ocean with an isotropic source

Closed-form equations that describe the vector irradiance from an isotropic source embedded in the oceanare rigorously derived from the steady-state radiative transfer equation. The equations are exact for a homogeneous medium and are believed to be an excellent approximation along the vertical axis for a plane-parallel ocean. The equations are solved for the absorption coefficient as a function of distance from the source. For clear ocean water, it is shown that vector irradiance measurements alone provide sufficient information for an accurate calculation of the absorption coefficient. Measurements in Pacificwater of the vector irradiance from an isotropic source are presented, and the absorption coefficient is computed. The estimated value of the absorption coefficient from a linear least-squares fit to the data has a standard error of ~ 1%.

Selective hippocampal lesions abolish the contextual specificity of latent inhibition and conditioning

The contextual specificity of the conditioned response (CR) and latent inhibition (LI) was examined in rats with selective hippocampal lesions. Acquisition of the CR to a novel conditioned stimulus (CS) was equally rapid in control and hippocampal rats (Experiments 1 and 2), and CS preexposure disrupted acquisition, (i.e., produced LI) to an equal extent in both groups (Experiment 2). In control subjects, however, the CR established in one context transferred incompletely to a second context (Experiment 1), and LI was attenuated when CS preexposure and conditioning occurred in different contexts (Experiment 3). This context specificity of the CR and LI was not apparent in hippocampal rats--the CR and LI transferred readily from one context to another. In addition, hippocampal rats were impaired in a spatial learning task (Experiment 2) but were unimpaired in learning a Pavlovian contextual discrimination (Experiment 3). These results suggest that a common contextual retrieval process underlies the contextual dependence of the CR and of LI and that this process is mediated by the hippocampus.

Selective hippocampal lesions abolish the contextual specificity of latent inhibition and conditioning

The contextual specificity of the conditioned response (CR) and latent inhibition (LI) was examined in rats with selective hippocampal lesions. Acquisition of the CR to a novel conditioned stimulus (CS) was equally rapid in control and hippocampal rats (Experiments 1 and 2), and CS preexposure disrupted acquisition, (i.e., produced LI) to an equal extent in both groups (Experiment 2). In control subjects, however, the CR established in one context transferred incompletely to a second context (Experiment 1), and LI was attenuated when CS preexposure and conditioning occurred in different contexts (Experiment 3). This context specificity of the CR and LI was not apparent in hippocampal rats--the CR and LI transferred readily from one context to another. In addition, hippocampal rats were impaired in a spatial learning task (Experiment 2) but were unimpaired in learning a Pavlovian contextual discrimination (Experiment 3). These results suggest that a common contextual retrieval process underlies the contextual dependence of the CR and of LI and that this process is mediated by the hippocampus.

Negative priming in associative learning: evidence from a serial-conditioning procedure

Three experiments investigated the suggestion that a predicted or primed stimulus commands less processing and consequently elicits a weaker conditioned response (CR) than a stimulus that is not primed. In each experiment rats received initial training in which the presentation of each of 2 serial compounds, A-X and B-Y, was followed by the delivery of food. Subsequently, X's capacity to elicit the CR, approaching the site of food delivery, was assessed when X was preceded by Stimulus A (i.e., primed) or was presented after Stimulus B. Stimulus X elicited a more vigorous response when it was presented after B than when it followed A. These results show that the ability of one event to elicit its CR is reduced if its presentation has been predicted by some other event. This negative priming effect supports one aspect of Wagner's (1981) model of Pavlovian conditioning.

Negative priming in associative learning: evidence from a serial-conditioning procedure

Three experiments investigated the suggestion that a predicted or primed stimulus commands less processing and consequently elicits a weaker conditioned response (CR) than a stimulus that is not primed. In each experiment rats received initial training in which the presentation of each of 2 serial compounds, A-X and B-Y, was followed by the delivery of food. Subsequently, X's capacity to elicit the CR, approaching the site of food delivery, was assessed when X was preceded by Stimulus A (i.e., primed) or was presented after Stimulus B. Stimulus X elicited a more vigorous response when it was presented after B than when it followed A. These results show that the ability of one event to elicit its CR is reduced if its presentation has been predicted by some other event. This negative priming effect supports one aspect of Wagner's (1981) model of Pavlovian conditioning.

Contextual factors in neophobia and its habituation: the role of absolute and relative novelty

In four experiments we investigated the role of contextual cues in the habituation of neophobia in rats. Experiment 1 showed that the consumption of a novel flavour increased across a series of presentations in one context (A) but fell when the flavour was subsequently presented in a second, novel, context (B). In Experiments 2 and 3, subjects again received exposure to a flavour in context A, but also were familiarized with the test context, B. These subjects consumed the flavour with equal readiness, whether it was presented in Context A or in Context B at test. Experiment 4 replicated the results of Experiment 1 and also showed that the consumption of a novel flavour was not influenced by whether it was presented in a novel or a familiar context. Several mechanisms by which the novelty or familiarity of the context might interact with the novelty or familiarity of the flavour were discussed.

Conditioning and contextual retrieval in hippocampal rats

The role of the hippocampus in associative learning was investigated in 3 experiments with rats as subjects. Hippocampal rats were impaired in the acquisition of conditioned responding both when food was signaled by the insertion of a lever (Experiment 1) and when the presentation of auditory or visual events served as the conditioned stimuli (Experiment 2). Experiment 2 also evaluated the suggestion that deficits in the acquisition of conditioned responding reflect the failure of hippocampal subjects to use contextual cues to retrieve associative information. This experiment showed that hippocampal rats were impaired in learning that a given stimulus was reinforced in Context A but nonreinforced in Context B. Experiment 3 demonstrated that hippocampal rats were unimpaired in learning a simple Pavlovian contextual discrimination. This pattern of results suggests that the hippocampus is involved in a higher order contextual retrieval process.

Acquired equivalence and distinctiveness of cues using a sensory-preconditioning procedure

In Experiment 1 rats were presented with two flavours, A and B, that were accompanied either by a third common flavour, X, or by two different flavours, X and Y, respectively. An aversion was then established to A, and the extent to which this aversion generalized to B was measured. Stimulus generalization was found to be more marked when A and B had both been presented with the same flavour, X, during pre-exposure than when A and B had been presented with X and Y. In Experiment 2 three target flavours, A, B, and C, were initially presented. In one pre-exposure condition presentations of A and B were accompanied by X, and C was presented alone; in a second condition A and B were presented in isolation, and C was accompanied by X. After an aversion had been established to A, half of the animals in each of the pre-exposure conditions were tested with B, and the remainder were tested with C. In both of the pre-exposure conditions the generalized aversion was more substantial to B than to C. An analysis of these results in terms of the mechanisms that have been supposed to underlie sensory preconditioning is presented.

Conditioning and contextual retrieval in hippocampal rats

The role of the hippocampus in associative learning was investigated in 3 experiments with rats as subjects. Hippocampal rats were impaired in the acquisition of conditioned responding both when food was signaled by the insertion of a lever (Experiment 1) and when the presentation of auditory or visual events served as the conditioned stimuli (Experiment 2). Experiment 2 also evaluated the suggestion that deficits in the acquisition of conditioned responding reflect the failure of hippocampal subjects to use contextual cues to retrieve associative information. This experiment showed that hippocampal rats were impaired in learning that a given stimulus was reinforced in Context A but nonreinforced in Context B. Experiment 3 demonstrated that hippocampal rats were unimpaired in learning a simple Pavlovian contextual discrimination. This pattern of results suggests that the hippocampus is involved in a higher order contextual retrieval process.

Context-specific conditioning in the conditioned-emotional-response procedure

In 3 experiments, rats were used to investigate the conditions that influence the transfer of a conditioned emotional response from 1 context to another. The subjects experienced training in 2 contexts on each day. In Experiment 1, subjects received a single conditioning trial with a different target stimulus in each of the 2 contexts. Conditioned responding was found to be more vigorous when the target was presented subsequently in the context in which conditioning had taken place than when it was presented in the other context. Experiments 2 and 3 confirmed these results and also showed that neither the unconditioned response evoked by the target stimulus nor the conditioned response acquired after multiple training trials showed evidence of context specificity. Possible reasons for the difference in outcome between single-trial and multitrial conditioning procedures are discussed.

Stimulus generalization as a function of stimulus novelty and familiarity in rats

Three experiments used rats as subjects to investigate the generalization of conditioned responding between stimuli as a function of the subjects' exposure to these cues prior to conditioning. Experiment 1 used a between-subjects design, food as the reinforcer, and measured the tendency of subjects to approach the site of food delivery during the stimuli. Generalization of this response was more marked when the training and test stimuli were equated in terms of their novelty (i.e., when both were novel or both were familiar) than when the stimuli differed in this respect (i.e., when one was novel and the other was familiar). Experiments 2a and 2b used within-subjects designs to confirm the reliability of the results of Experiment 1. Implications of these results for current theories of stimulus representation are discussed.

Attenuation of latent inhibition after compound pre-exposure: associative and perceptual explanations

Three experiments used rats as subjects and the conditioned emotional response (CER) paradigm to examine the effect of pre-exposure to a compound stimulus on the extent of latent inhibition to an element of that compound. In Experiment 1a a group of rats exposed to a compound that comprised a tone and a click exhibited less latent inhibition to the tone than did a group that had received pre-exposure to the tone in isolation. Experiment 1b showed that pre-exposure to the tone/click compound also resulted in an attenuation of latent inhibition to the click relative to a group that was pre-exposed to the click in isolation. Experiment 2 demonstrated that latent inhibition to the tone was left intact following pre-exposure to the tone in compound with a light. This pattern of results seems to be most plausibly explained in terms of the presence and absence of generalization decrement following compound pre-exposure.

Acquired equivalence and distinctiveness of cues

In Experiments 1 and 2, rats received initial training in which two stimuli (A and N) were either followed by the same consequence (food) or by different consequences (food and no food). Subsequently N was paired with electric shock and the generalization of conditioned suppression to A was assessed. Suppression to A was more marked when A and N had both been followed by food than when they had had different outcomes. In Experiment 3, 3 stimuli (A, B, and N) were presented initially. For one group, A and N were paired with food and B was nonreinforced: for a second group, B was paired with food and A and N were nonreinforced. Generalization of suppression was found to be more substantial to A than to B for both groups. These results indicate that the extent to which stimuli are treated as being equivalent is partly determined by their reinforcement histories.