Latent learning and latent inhibition in maze discriminations

Rats were trained on an elevated maze where the rewarded alternative was defined either in terms of intra-maze or in terms of extra-maze cues. Pre-exposure to these cues produced a small latent or perceptual learning effect, i.e. facilitated subsequent learning of both problems, by comparison with animals pre-exposed to the maze with no cues present. Experiment 2 examined whether the effect of pre-exposure on intra-maze discrimination learning varied with the nature of the intra-maze cues. When positive and negative arms were further differentiated by painting the walls white and black, a marginal perceptual learning effect was turned into significant latent inhibition, i.e. a retardation of subsequent learning. Pre-exposure thus reliably facilitated extra-maze discrimnation learning, and its beneficial effects on intra-maze discrimination could be reversed by reducing the overlap between the intra-maze cues. Perceptual learning may therefore depend on requiring animals to discriminate between stimuli containing many common elements.     

Perceptual learning in maze discriminations

In Experiment 1, rats were trained on a discrimination between rubber- and sandpaper-covered arms of a maze after one group had been pre-exposed to these intra-maze cues. Pre-exposure facilitated subsequent discrimination learning, unless the discrimination was made easier by adding further discriminative stimuli, when it now significantly retarded learning. In Experiment 2, rats were trained on an extra-maze spatial discrimination, again after one group, but not another, had been pre-exposed to the extra-maze landmarks. Here too, pre-exposure facilitated subsequent discrimination learning, unless the discrimination was made substantially easier by arranging that the two arms between which rats had to choose were always separated by 135°. The results of both experiments can be explained by supposing that perceptual learning depends on the presence of features common to S+ and S-.     

Effects of pre-exposure to the same or different pattern of extra-maze cues on subsequent extra-maze discrimination

In two experiments, rats learned a spatial discrimination between maze arms defined by their relationship to a variety of extra-maze cues. Prior exposure to the actual arms between which animals were required to discriminate tended to retard subsequent learning (by comparison with a control group either given no pre-exposure to the extra-maze cues or exposed only to arms pointing in the opposite direction), whereas prior exposure to arms intermediate between those used in discrimination training tended to facilitate subsequent learning. These results are consistent with the suggestion that pre-exposure will facilitate discrimination learning when it reduces the associability of features or elements common to the stimuli between which animals are required to discriminate, more than it reduces the associability of the features or elements unique to each.     

Blocking and overshadowing between intra-maze and extra-maze cues: A test of the independence of locale and guidance learning

Rats were trained on an elevated maze where the rewarded alternative was defined either in terms of intra-maze cues (rubber or sandpaper flooring on rewarded and unrewarded arms, regardless of their position) or in terms of extra-maze cues (the correct arm always pointed toward a particular corner of the room, and was sometimes covered with rubber and sometimes with sandpaper), or where both sets of cues were simultaneously relevant. In Experiment 1 rats pretrained with either intra-maze or extra-maze cues alone relevant learned less about the other set of cues than non-pretrained control groups, when, in a second phase of the experiment, both sets of cues were simultaneously relevant. Experiment 2 confirmed that intra-maze cues could block extra-maze cues, and ruled out one alternative explanation of the results of Experiment 1. Experiment 3 showed that extra-maze cues overshadowed intra-maze cues, but that there was no reciprocal overshadowing of extra-maze by intra-maze cues. This was despite the fact that animals learned the intra-maze discrimination significantly faster than the extra-maze discrimination. Experiment 3 also suggested that rats did not solve the extra-maze discrimination by learning to approach or avoid specific extra-maze cues, but rather by locating the correct arm by reference to the entire set of extra-maze cues. The results suggest that locale or place learning and cue or guidance learning, in O'Keefe and Nadel's (1978) terminology, interact with one another in much the same way as does learning about any pair of stimuli in a Pavlovian conditioning experiment.     

Reciprocal overshadowing between intra-maze and extra-maze cues

In three experiments, rats learned a maze discrimination where the location of food was defined either by reference to extra-maze cues alone, or by both extra- and intra-maze cues. Experiment 1 confirmed earlier results in showing that the presence of intra-maze cues failed to overshadow learning about extra-maze cues, in spite of the former's apparently greater salience. Experiment 2, however, suggested that this result was an artefactual consequence of differences between groups in the proportion of reinforced and unreinforced trials during the course of discriminative training. In Experiment 3, the discrimination was taught by a series of reinforced and unreinforced placement trials, and a significant overshadowing effect was observed. Intra-maze and extra-maze cues seem to compete for association with reinforcement in exactly the same way as any other cues.     

Retardation in Human Discrimination Learning as a Consequence of Pre-exposure: Latent Inhibition or Negative Priming?

A series of experiments with human subjects, using black-and-white chequerboard patterns, demonstrated that non-reinforced pre-exposure could impair performance in a subsequent learning task. Subjects were invited to take part in a scenario similar to that of a computer simulated card game. Their task was to turn over a series of cards by mouse-clicking on a pack of cards lying face-down, and then to classify these cards intoone of twocategories. In a subsequent task, subjects were asked todiscriminate betweenpairs ofchequerboards, some of which had previously appeared in the initialcategorizationphase: either directly ('fronts') or incidentally ('backs') involved in categorization. In Experiment 1, for those stimuli used as the backs ofthe cards (thatis,those visible ontopof the packof cards), there was asignificant impairment in performance relative to non-pre-exposed control stimuli. Although the impairment appeared to be specific to the stimuli pre-exposed, when the pre-exposed 'backs' were minimally distorted in the discrimination task of Experiment 2, performance was still significantly impaired relative to non-pre-exposed control stimuli. The results of Experiment 2 donot support the interpretation that retardation in learning following masked pre-exposure in human experiments is comparable to latent inhibition following simple preexposure in other animals. Whilst the impairment in performance appears to be similar to that of latent inhibition, the results may, instead, be better understood in terms of the inhibitory processes involved in negative priming. If this is so, then serious doubt is cast on whether latent inhibition has ever been reliably demonstrated in adult humans.     

Latent inhibition, context specificity, and context familiarity

In two appetitive licking experiments, thirsty rats were pre-exposed to two stimuli prior to conditioning sessions in which one or both of these stimuli were paired with water. Consistent with other results, latent inhibition was disrupted when conditioning took place in a context different from that in which stimulus pre-exposure had occurred. But in animals given prior exposure to the context of stimulus pre-exposure before the start of stimulus pre-exposure, substantial and equivalent latent inhibition was evident whether or not there was a change of context between stimulus pre-exposure and conditioning. These results are discussed in terms of current theories of latent inhibition.     

Contextual conditional discriminations

Three experiments used a discriminated operant procedure to study conditional discrimination learning in rats. The first experiment showed that rats were capable of learning a biconditional discrimination in which two contexts served as conditional cues signalling the reinforcement contingencies associated with two discriminative stimuli. The discrimination was learned equally well when one discriminative stimulus signalled food, the other its absence, and when one stimulus signalled food, the other extinction plus mild footshock.

In Experiment 2 it was shown that prior training on such a conditional discrimination enhanced the subsequent context specificity of simple conditioning relative to control groups of animals for whom the prior training had not been conditional. Experiment 3 showed that a reversal of the significance of one pair of discriminative stimuli produced no spontaneous reversal in performance to a second, target, pair.

The pattern of results is best accounted for by an analysis of contextual conditional discrimination learning in terms of stimulus configurations and offers no support for the notion that rats may learn a general conditional rule or set.     

A search for context-stimulus associations in latent inhibition

Six experiments investigated the effects of pre-exposure to a tone on the subsequent acquisition of conditioned suppression by rats. In Experiments 1-3 the response suppressed was drinking; in Experiments 4-6 it was food-rewarded lever pressing. Repeated exposure to the tone resulted in latent inhibition, i.e., a retardation in the acquisition of suppression. The size of the latent inhibition effect was reduced when a different context was used for conditioning from that used for pre-exposure (Experiment 2). When the context remained the same throughout, a phase of exposure to the context alone, interposed between pre-exposure and conditioning, had no influence on the size of the latent inhibition effect ultimately observed. This last result casts doubt upon Wagner's (1976, 1979) theory of the role of contextual factors in latent inhibition, and alternative accounts are considered.     

Effect of a retention interval between pre-exposure and conditioning on latent inhibition in humans using a blink conditioning procedure

Latent inhibition, retarded learning after pre-exposure to the to-be-conditioned stimulus, was examined using a blink conditioned procedure in humans. Experiment 1 showed that the procedure is suited to inducing the latent inhibition effect. In Experiment 2, the introduction of a 3-minute interval between pre-exposure and conditioning phases attenuated latent inhibition. These results contribute to identify the mechanisms involved in pre-exposure and subsequent conditioning of a stimulus, which is particularly important if we bear in mind that latent inhibition has been used repeatedly as an instrument to analyze the course of attentional processes in normal and pathological populations.     

Sex differences in aversive memory in rats: Possible role of extinction and reactive emotional factors

Studies usually show better spatial learning in males and stronger emotional memory in females. Spatial memory differences could relate to diverse strategies, while dissimilar stress reactions could cause emotional memory differences. We compared male and female rats in two emotional (classical emotional conditioning and aversive discrimination memory) and two emotionally “neutral” tasks: (1) plus-maze discriminative avoidance, containing two open and two enclosed arms, one of which presenting aversive stimuli (light/noise). No differences were found in learning, retrieving, or basal emotional levels, while only male rats presented extinction of the task; (2) contextual fear conditioning – a cage was paired to mild foot shocks. Upon reexposure, freezing behavior was decreased in females; (3) spontaneous alternation – the animals were expected to alternate among the arms of a four-arm maze. No differences between genders were found and (4) open-field habituation was addressed in an arena which the rats were allowed to explore for 10 min. Habituation was similar between genders. Differences were found only in tasks with strong emotional contexts, where different fear responses and stress effects could be determinant. The lack of extinction of discriminative avoidance by females points out to stronger consolidation and/or impaired extinction of aversive memories.     

Rats acquire spatial learning sets

This experiment was designed to examine the development of a spatial learning set in rats and some of the variables influencing the retention of individual problems. The apparatus was a plus maze. At the beginning of each test, the rat was put on two arms, each in a different place. Food was present in one of the arms, but not in the other. The rat was then given a choice between these two places; the correct response was to return to the place that previously contained food (win-stay, lose-shift, response-reinforcement contingency). Fifty different two-choice spatial discriminations were given, each in a different location. At the end of testing, the mean percentage of correct responding for the first choice between the two places was 83%. Control procedures showed that the discriminative stimuli were distal, extramaze spatial stimuli. Variations of the procedure examined the influence of proactive interference and temporal delay on the memory for each discrimination. These results demonstrate that rats can develop a spatial learning set and provide new information about the characteristics of the memory underlying learning sets.     

Effects of Retention Interval on Latent Inhibition and Perceptual Learning

Repeated, non-reinforced preexposure to a context slowed development of conditioned freezing to that context when it was subsequently paired with footshock (latent inhibition) and enhanced discriminability of that context from a similar context (perceptual learning) whether assessed by a generalization test or by explicit discrimination training. Latent inhibition was eliminated by a delay between conditioning sessions and test (Experiments 1a and 1b) and reduced by a delay between preexposure and conditioning (Experiment 2). However, perceptual learning was unaffected by either of these intervals (Experimnets 1b and 2). These results are discussed in terms their impact on theories that have latent inhibition as a possible mechanism of perceptual learning, and on theories of latent inhibition that consider the retardation of conditioned responding to be the result of an acquisition failure.     

Learning about non-predators and safe places: the forgotten elements of risk assessment

A fundamental prerequisite for prey to avoid being captured is the ability to distinguish dangerous stimuli such as predators and risky habitats from non-dangerous stimuli such as non-predators and safe locations. Most research to date has focused on mechanisms allowing prey to learn to recognize risky stimuli. The paradox of learned predator recognition is that its remarkable efficiency leaves room for potentially costly mistakes if prey inadvertently learn to recognize non-predatory species as dangerous. Here, we pre-exposed embryonic woodfrogs, Rana sylvatica, to the odour of a tiger salamander, Ambystoma tigrinum, without risk reinforcement, and later try to teach the tadpoles to recognize the salamander, a red-bellied newt Cynops pyrrhogaster—a closely related amphibian, or a goldfish, Carassius auratus, as a predator. Tadpoles were then tested for their responses to salamander, newt or fish odour. Pre-exposure to salamander did not affect the ability of tadpoles to learn to recognize goldfish as a predator. However, the embryonic pre-exposure to salamanders inhibited the subsequent learning of salamanders as a potential predator, through a mechanism known as latent inhibition. The embryonic pre-exposure also prevented the learned recognition of novel newts, indicating complete generalization of non-predator recognition. This pattern does not match that of generalization of predator recognition, whereby species learning to recognize a novel predator do respond, but not as strongly, to novel species closely related to the known predator. The current paper discusses the costs of making recognition mistakes within the context of generalization of predators and dangerous habitats versus generalization of non-predators and safe habitats and highlights the asymmetry in which amphibians incorporate information related to safe versus risky cues in their decision-making. Mechanisms such as latent inhibition allow a variety of prey species to collect information about non-threatening stimuli, as early as during their embryonic development, and to use this information later in life to infer the danger level associated with the stimuli.     

Operant conditioning and discrimination of alpha: some methodological limitations inherent in response-discrimination experiments

Studies on the operant conditioning of central nervous system activity have produced results interpreted as demonstrating that responses, certain properties of responses, or response-produced stimuli can function as discriminative stimuli. It is assumed that the feedback stimulus in biofeedback makes the subject aware of the internal response and that by becoming aware of the response, the subject can acquire voluntary control over it. In this context, awareness is operationally defined as the ability to use the response as a discriminative stimulus. Since direct evidence for the assumed relationship between control and discrimination is lacking, an attempt was made to test the hypothesis that discrimination of a response automatically leads to control over that response. The discriminative stimuli were the presence and absence of occipital alpha electroencephalograph (EEG) activity. Data from two experiments are reported. The first study, employing naive subjects, was designed to answer the following questions: (a) Since pilot data indicated that subjects seemed to match their responses to the more probable type of trial, would increases in the probability of a correct response result when the probabilities of alpha and nonalpha trials were held near .50? (b) If correct responding does increase, would performance of these subjects in an alpha feedback task be enhanced relative to that of subjects not previously given discrimination training? and (c) If subjects could not learn the discrimination task, would feedback training enhance their performance in a subsequent discrimination task? Results from this study indicate that holding the probabilities of alpha and nonalpha discrimination trials near .50 results in an absence of learning curves, but leaves open the possibility that sophisticated subjects are capable of discriminating alpha and nonalpha activity. The second study deals with two questions: (a) Can sophisticated subjects learn to discriminate occipital alpha activity from nonalpha activity? and (b) Does the procedure of providing subjects with salient stimuli, contingent on the presence and absence of alpha activity, establish stimulus control of the presence and absence of alpha activity? Results indicate that it is not possible to conclude that subjects can learn to discriminate alpha and nonalpha activity. However, learning to increase percent-time nonalpha or decrease percent-time alpha with respect to baseline levels by means of EEG-contingent stimulation provides subjects with the ability to suppress percent-time alpha in the absence of feedback. Information gained in both studies through subject interviews indicates that subjects most often acquired their control of alpha activity during feedback by a specific strategy and then used the strategy during the stimulus-control tests.     

Spatial learning set acquisition in rats

Development of a learning set reflects the ability to abstract from a given problem the rules and procedures that can be generalized to novel stimuli. This ability has been demonstrated readily in rats only with tasks employing olfactory stimuli, suggesting a modality-specific capability. In the present experiment, rats were tested on a series of three go/no-go spatial discrimination tasks in order to evaluate their ability to develop a learning set in a modality other than olfaction. Each discrimination task was presented in a different room providing unique extra-maze cues. All rats reached the criterion performance of 18 correct responses in 20 consecutive trials on all three discriminations and showed a learning set. Some rats performed the third discrimination perfectly after only a single error. These results indicate that the ability to develop a learning set is not specific to the olfactory modality but reflects a more general cognitive capacity.     

Relational Learning in Pigeons?

Two experiments investigated whether discrimination learning and transposition by pigeons were facilitated by the opportunity to compare rectangles differing in luminance or stars differing in number of vertices. In Experiment 1, one group was trained with stimuli from the same dimension appearing simultaneously on each trial, but for a second group such stimuli appeared on separate trials. The opportunity to compare stimuli from the same dimension on a single trial facilitated the learning of the luminance discrimination but not of the stars discrimination. Such comparison also resulted in greater luminance but not greater stars transposition. Using a different training procedure, Experiment 2 confirmed that the opportunity for comparison facilitated a luminance discrimination. The results for the star discrimination are entirely consistent with 'absolute' theories of discrimination learning; but the results for the luminance discrimination suggest some kind of 'relational' learning. Given the difference between the star and luminance discriminations, a low-level, sensory theory of relational learning seems most consistent with the data.     

Contextual control in discrimination reversal learning

In 3 human predictive learning experiments, the authors examined contextual control of responding in discrimination reversal learning. In Phase 1, a discrimination between 2 stimuli (A+, B-) was trained in Context 1. During Phase 2, participants received discrimination reversal training (A-, B+) in Context 2. Testing occurred in Context 1 and Context 2 (Experiments 1A and 1B) or in Context 1 and Context 3 (Experiment 2). During the test phase, performance in Context 1 and Context 2 reflected the contingencies trained during Phase 1 and Phase 2, respectively. When testing occurred in Context 3, there was no discriminative responding between A and B. In addition, the experiments demonstrated that discriminating stimuli with a consistent reinforcement history were also affected by contextual manipulations. Results indicate that each training context acquires the ability to control performance. Unique-cue and configural approaches account for a major part of the results.     

Acetylcholine release in the hippocampus and striatum during place and response training

These experiments examined the release of acetylcholine in the hippocampus and striatum when rats were trained, within single sessions, on place or response versions of food-rewarded mazes. Microdialysis samples of extra-cellular fluid were collected from the hippocampus and striatum at 5-min increments before, during, and after training. These samples were later analyzed for ACh content using HPLC methods. In Experiment 1, ACh release in both the hippocampus and striatum increased during training on both the place and response tasks. The magnitude of increase of training-related ACh release in the striatum was greater in rats trained on the response task than in rats trained on the place task, while the magnitude of ACh release in the hippocampus was comparable in the two tasks. Experiment 2 tested the possibility that the hippocampus was engaged and participated in learning the response task, as well as the place task, because of the availability of extra-maze cues. Rats were trained on a response version of a maze under either cue-rich or cue-poor conditions. The findings indicate that ACh release in the hippocampus increased similarly under both cue conditions, but declined during training on the cue-poor condition, when spatial processing by the hippocampus would not be suitable for solving the maze. In addition, high baseline levels of ACh release in the hippocampus predicted rapid learning in the cue-rich condition and slow learning in the cue-poor condition. These findings suggest that ACh release in the hippocampus augments response learning when extra-maze cues can be used to solve the maze but impairs response learning when extra-maze cues are not available for use in solving the maze.     

Further evidence for a dissociation between different forms of mnemonic expressions in a mouse model of age-related cognitive decline: effects of tacrine and S 17092, a novel prolyl endopeptidase inhibitor

It has been demonstrated previously on the radial maze that the emergence of an age-related mnemonic impairment is critically dependent on the form which the discrimination problems took. Hence, when the arms were presented one by one (i.e., successive go-no-go discrimination), both adult and aged mice learned to distinguish between positive (baited) and negative (unbaited) arms readily, as evidenced by their increased readiness to enter positive relative to negative arms (i.e., by a differential in arm-entry latencies). A selective impairment in the aged mice was seen when these arms were presented subsequently as pairs, such that the mice were confronted with an explicit choice (i.e., simultaneous 2-choice discrimination). When discriminative performance was measured by the differential run speed between positive and negative arms, aged mice were also impaired. This was particularly pronounced in the 2-choice discrimination condition. We examined the effects of tacrine (3mg/kg, subcutaneously) or S 17092 (10mg/kg, orally) in aged mice on the three behavioral indices of this 2-stage spatial discrimination paradigm. The results indicated that: (1) Tacrine, but not S 17092, enhanced the acquisition of go-no-go discrimination as reflected in arm-entry latencies; (2) both drugs improved choice accuracy in simultaneous discrimination, although the effect of tacrine was less striking and, in particular, far from statistical significance in the very first 2-choice responses; and (3) neither drugs significantly affected run-speed performance. We conclude further that the specific patterns of drug effects on the three indices of discriminative performance might suggest that each index is associated with a distinct form of mnemonic expression relying on separate neural systems.