Effect of extended training on generalization of latent inhibition: an instance of perceptual learning

Four experiments examined generalization of latent inhibition (LI) as a function of the length of preexposure in a conditioned taste aversion procedure with rats. Experiment 1 showed that one or four nonreinforced presentations of a flavor compound (BX) retarded subsequent conditioning to another compound (AX). However, after eight presentations of BX, conditioning to AX occurred at the same rate as with no preexposure. These results indicate that generalization of LI decreased as the length of preexposure to BX increased. Experiment 2 replicated this effect of reducing generalization, as well as demonstrating that LI actually increased as the length of preexposure to AX increased. Experiment 3 extended the generality of the effect to a procedure in which both BX and AX were preexposed. Experiment 4 demonstrated a similar reducing-generalization effect when generalization of LI from BX to X was assessed. All of these data are consistent with the notion that prolonged preexposure to BX enhances its discriminability. Different learning mechanisms that might be responsible for this perceptual learning effect are discussed.     

Perceptual learning transfer in an appetitive Pavlovian task

In two experiments, rats were given intermixed or blocked preexposure to two similar compound stimuli, AX and BX. Following preexposure, conditioning trials took place in which AX (Experiment 1) or a novel compound stimulus NX (Experiment 2) was paired with a food-unconditioned stimulus in an appetitive Pavlovian preparation. Animals that were given alternated preexposure showed lower generalization from AX to BX (Experiment 1) and from NX to a new compound, ZX (Experiment 2), than animals that were given blocked preexposure, a perceptual learning and a perceptual learning transfer effect, respectively.     

Changes in stimulus salience as a result of stimulus preexposure: Evidence from aversive and appetitive testing procedures

In two experiments, rats received preexposure to three compound flavor stimuli, AX, BX, and CX, where X represents a saline solution. AX and BX were presented in alternation; CX, on a separate block of trials. The value of X was then modified, being devalued by aversive conditioning in Experiment 1, and rendered valuable by the induction of a state of salt need in Experiment 2. When given a choice between BX and CX, the rats consumed more of BX than of CX in Experiment 1, and more of CX than of BX in Experiment 2, suggesting that B and C differed in their ability to modulate the response governed by the X element. It was suggested that blocked preexposure to CX reduces the salience of the C stimulus but that the salience of B is maintained by preexposure in which BX is alternated with AX. The implications of this result for the phenomenon of perceptual learning are discussed.     

The role of inhibitory associations in perceptual learning

Preexposure to two compound flavors (AX and BX) typically enhances their discriminability: An aversion conditioned to AX will generalize less to BX, especially if the preexposure regime has involved alternated presentations of AX and BX rather than presenting all AX trials before BX trials (or vice versa). One possible explanation of this finding is that alternating preexposure establishes inhibitory associations between the two unique features A and B, thus counteracting the generalization produced by excitatory associations between X and A and between X and B, which might result in either the retrieval of B on a conditioning trial to AX, or the retrieval of A on a test trial to BX. Three experiments on flavor aversion conditioning in rats tested these predictions. Experiment 1 suggested that the more important of these excitatory associations was that which allowed X to retrieve A on the test trial to BX. Experiment 2 suggested that the more important inhibitory association was that which allowed B to inhibit the representation of A on this test trial. Experiment 3 provided direct evidence of the role of this inhibitory B⊣A association.     

The role of comparison in perceptual learning: effects of concurrent exposure to similar stimuli on the perceptual effectiveness of their unique features

In three experiments, rats were given concurrent exposure to a compound flavor (AX) and to one of the elements of the compound (X). The perceptual effectiveness of A was then assessed by a test involving generalization of a conditioned aversion. Comparison was made with a preexposure procedure in which the compound and the common element were presented on separate trials, either in alternation or in separate blocks of trials. The effectiveness of the unique cue was less after blocked preexposure than after either of the other procedures; concurrent preexposure did not produce a greater effect than did alternating preexposure. These results challenge the suggestion that concurrent preexposure engages a special comparison process that will facilitate this form of perceptual learning.     

Stimulus preexposure speeds or slows subsequent acquisition of associative learning depending on learning test procedures and response measure

Prior exposure to a conditioned stimulus (CS) typically results in latent inhibition—slower acquisition of associative learning about that stimulus in subsequent training. Here, we found that CS preexposure had different effects on the appetitive conditioning of rats with a sucrose unconditioned stimulus (US) depending on training test procedures, the similarity of preexposure and training procedures, and the choice of response measure. Preexposure to a visual or an auditory stimulus produced facilitation of acquisition of food-cup-directed responding when both of those cues were (separately) paired with sucrose delivery in the training test (Experiments 1 and 3). By contrast, the same preexposure procedure resulted in latent inhibition of food-cup learning if the second stimulus in the test phase was of the same modality as the preexposed stimulus (Experiment 2). In Experiment 3, latent inhibition was enhanced if both phases included a single CS or both phases included both auditory and visual CSs, compared to treatments in which only one CS was presented in one phase but two CSs were presented in the other phase. In Experiment 4, preexposure of an auditory cue slowed subsequent learning about it if the context was salient but enhanced learning if the context was of weaker salience. Finally, a measure of general activity revealed latent inhibition after preexposure in all conditions in all 4 experiments. We discuss the results within several classes of latent inhibition theories, none of which provides a comprehensive account.     

Disruption of latent inhibition by interpolation of task-irrelevant stimulation between preexposure and conditioning

Latent inhibition refers to attenuated responding to a conditioned stimulus (CS) that was repeatedly presented without reinforcement prior to the CS-unconditioned stimulus (US) pairings. Using water-deprived rats as subjects, we observed that interpolating task-irrelevant stimulation between the preexposure and conditioning phases of a latent inhibition procedure attenuated latent inhibition (Experiments 1A, 1B, and 2). Apparently, interpolated stimulation segments the preexposure and conditioning treatments into two separate experiences, much in the same way that a change of context would. Consistent with this view, the interpolated stimulation did not disrupt latent inhibition if it was also presented during both preexposure and conditioning (Experiment 3). We view these results as analogous to those of Escobar, Arcediano, and Miller (2003), who suggested that the difficulty in observing latent inhibition in human adults is related to the segmentation between preexposure and conditioning caused by the usual interpolation of instructions in preparations with humans.     

Context sensitivity of conditioned suppression following preexposure to the conditioned stimulus

Conditioned suppression in rats is often unaffected when the context (or set of background stimuli) is changed following conditioning. This suggests that responding to the conditioned stimulus (CS) can be relatively insensitive to the context in which the CS is presented. In two experiments, we examined whether sensitivity to contextual stimuli is affected by preexposure to the CS. In Experiment 1, when the CS was novel at the outset of conditioning, conditioned suppression was not affected when the context was changed following conditioning. However, when the CS had been preexposed, responding was weaker when extinction occurred outside of the conditioning context. In Experiment 2, responding was again sensitive to the test context, regardless of whether preexposure occurred in the conditioning context or in an alternate context. These results suggest that the extent to which responding is sensitive to context can depend on the conditioning history of the CS.     

Control of pigeons’ keypecking by the left-right arrangement of stimuli

Control of pigeons’ keypecking by conditionalities in the spatial arrangement of two element stimuli (designated A and B) was investigated. In Experiment 1, reinforcement for keypecking was made conditional upon the left-right location of A and B: Reinforcement was available when A was on the left and B was on the right (AB), but not on BA, AA, or BB trials. The pigeons successfully discriminated the rewarded AB configuration, but only after a stage in which a particular element in a particular location (e.g., A on left) primarily controlled pecking. Experiments 2 and 3 systematically replicated these findings and included controls to discount discrimination of the AB compound on the basis of the temporal order (e.g., A followed by B) rather than the spatial configuration of the elements. During a generalization test in Experiment 4, the elements were presented singly either in the left (AX, BX) or right (XA, XB) positions. As would be expected had the animals learned “A on the left, B on the right is rewarded,” responding on AX trials exceeded that on XA trials, and responding on XB trials exceeded that on BX trials.     

Retrieval practice after multiple context changes,but not long retention intervals,reduces the impact of a final context change on instrumental behavior

Recent evidence from this laboratory suggests that a context switch after operant learning consistently results in a decrement in responding. One way to reduce this decrement is to train the response in multiple contexts. One interpretation of this result, rooted in stimulus sampling theory, is that conditioning of a greater number of common stimulus elements arising from more contexts causes better generalization to new contexts. An alternative explanation is that each change of context causes more effortful retrieval, and practice involving effortful retrieval results in learning that is better able to transfer to new situations. The current experiments were designed to differentiate between these two explanations for the first time in an animal learning and memory task. Experiment 1 demonstrated that the detrimental impact of a context change on an instrumental nose-poking response can be reduced by training the response in multiple contexts. Experiment 2 then found that a training procedure which inserted extended retention intervals between successive training sessions did not reduce the detrimental impact of a final context change. This occurred even though the inserted retention intervals had a detrimental impact on responding (and, thus, presumably retrieval) similar to the effect that context switches had in Experiment 1. Together, the results suggest that effortful retrieval practice may not be sufficient to reduce the negative impact of a context change on instrumental behavior. A common elements explanation which supposes that physical and temporal contextual cues do not overlap may account for the findings more readily.     

Characteristics of the lithium-mediated proximal US-preexposure effect in flavor-aversion conditioning

Taste-aversion learning in rats is disrupted if the subjects are exposed to the unconditioned stimulus (US) shortly before the conditioning trial but not if this single US preexposure treatment occurs 1 day or more before conditioning. Several characteristics of this proximal US-preexposure phenomenon were explored. Experiment 1 showed that the time course of the interference with conditioning is directly related to the preexposure drug dose. Experiment 2 demonstrated that the interference effect is evident even if the test for aversion learning is conducted following a drug injection, thereby minimizing stimulus generalization decrement for the preexposed subjects. Finally, Experiment 3 showed that disruption of the contingent relationship between tastes and drug effects is probably not responsible for the proximal US-preexposure phenomenon because the interference with conditioning occurs regardless of whether or not the preexposure drug treatment is paired with a novel flavor. These findings, together with previous research, demonstrate the remarkably robust character of the proximal US-preexposure phenomenon.     

Inhibitory associations between neutral stimuli: A comparative approach

In Experiments 1A, 1B, and 1C, nonhuman subjects, rats, received long alternated exposures to two compound flavors, AX and BX, that shared one flavor in common, X. Following this, conditioning of an aversion to A was sufficient to establish B as a conditioned inhibitor of the aversive unconditioned stimulus, passing both summation and retardation tests. Two additional experiments (Experiments 2 and 3) expanded the generality of these results to humans, using similar designs but an auditory discrimination learning task. A set of notes sequentially presented served as cues and fictitious composers served as outcomes. Both summation and retardation effects were found (Experiments 2 and 3, respectively). Experiment 4 then sought to clarify the mechanism underlying these effects. The results are discussed within several theoretical frameworks, most centrally the McLaren, Kaye, and Mackintosh (1989) theory of perceptual learning.     

Contextual effects in latent inhibition with an appetitive conditioning procedure

In three experiments, rats received presentations of a diffuse 30-sec stimulus (a light) and of food, and their tendency to enter the food tray was monitored. Experiment 1 showed that when the light was made to signal the delivery of food, the response of entering the food tray increased in frequency during the stimulus. The acquisition of this conditioned response to the light was retarded in subjects that had received preexposure to the stimulus. In Experiments 2 and 3, subjects received preexposure to the stimulus, some in the same context as that subsequently used for stimulus-food pairings and some in a different context. Those experiencing the change of context acquired the response more readily. It is argued that these results demonstrate a latent inhibition effect that is attenuated by contextual change.     

Context specificity of sensory preconditioning: Implications for processes of within-event learning

A variant of the taste aversion procedure for sensory preconditioning, as used by Rescorla and Cunningham (1978), was employed in a study of the context dependency of within-event learning. In two experiments, rats received Phase 1 exposure to a simultaneous flavor compound, AX; flavor X was paired with illness during Phase 2, and any tendency for the resulting aversion to be elicited by A was measured. It was found that subjects were less likely to shun flavor A as a consequence of this training if the Phase 1 and test episodes were conducted in distinctively different contexts. This effect was evident both when the change of context occurred just before the test with flavor A (Experiment 1) and when it occurred before Phase 2 (Experiment 2). These results were taken to imply that, as is often found with serial associations, the retrieval of within-event associations is subject to contextual control. The implications of these findings for the interpretation of perceptual learning effects are discussed.     

Flavor aversions and deprivation state

In the first four experiments, it was found that aversions to saccharin solution produced by contingent poisoning were similar regardless of whether the rats had been trained under the test deprivation or under a different deprivation; the two deprivation states used were thirst and satiety. In Experiment 5, rats were poisoned after drinking grape juice while hungry or poisoned after drinking milk while thirsty, but they were not poisoned after grape-thirst or milk-hunger combinations. In abstract terms, poisoning occurred after AX and BY stimulus combinations, but did not occur after AY and BX combinations. There was some learning under these discrimination conditions.     

Latent inhibition as a performance deficit resulting from CS—context associations

Treatments that attenuate latent inhibition (LI) were examined using conditioned suppression in rats. In Experiment 1, retarded conditioned responding was produced by nonreinforced exposure to the CS prior to the CS-US pairings used to assess retardation (i.e., conventional LI). In Experiment la, retarded conditioned responding was induced by preexposure to pairings of the CS and a weak US prior to retardation-test pairings of the CS with a strong US (i.e., Hall-Pearce [1979] LI). Both types of LI were attenuated by extensive exposure to the training context (i.e., context extinction) following the CS-US pairings of the retardation test. Experiment 2 examined the specificity of the attenuated LI effect observed in Experiment 1. After preexposure to two different CSs in two different contexts, each CS was paired with a US in its respective preexposure context. One of the two contexts was then extinguished. This attenuated LI to a greater degree for the CS that had been trained in the extinguished context. Experiment 3 differentiated the roles in LI of CS-context associations and context-US associations. Following preexposure to the CS in the training context, LI was reduced by further exposure to the CS outside the training context. This observation was interpreted as implicating the CS-context association as a factor in LI. Thus, the results of these experiments suggest that LI is a performance deficit mediated by unusually strong CS-context associations. Implications for Wagner’s (1981) SOP model and Miller and Matzel’s (1988) comparator hypothesis are discussed.     

Massive preexposure and preexposure in multiple contexts attenuate the context specificity of latent inhibition

Latent inhibition, which refers to attenuated responding to a conditioned stimulus (CS) after CS-unconditioned stimulus (CS-US) pairings as a result of CS-alone presentations prior to the pairings, is often attenuated if preexposure and conditioning occur in different contexts (i.e., it is context specific). Here we report two conditioned lick suppression experiments, using rat subjects, that examined whether manipulations known to attenuate the context specificity of extinction could also eliminate the context specificity of latent inhibition. Context specificity of latent inhibition was eliminated when the CS was preexposed in multiple contexts (Experiment 1) and when the CS was massively pre-exposed in the training context alone (Experiment 2). These results and their practical implications are discussed in the framework of contemporary theories of latent inhibition.     

Changes in attention to an irrelevant cue that accompanies a negative patterning [corrected] discrimination

Pigeons were trained in two experiments with negative patterning discriminations that were accompanied by an irrelevant cue. For Experiment 1, the discriminations were of the form AX+ BX+ ABX–, where A and B were relevant, X was irrelevant, and + or – indicate whether or not reinforcement was delivered. The discriminations for Experiment 2 were of the form A+ B+ AX+ BX+ ABX–. A subsequent test phase in both experiments revealed that the associability of A and B, and hence the attention paid to these stimuli, was less than the associability of X. The results are explained with a modified version of a configural theory of associative learning.     

Varieties of perceptual learning

Although most studies of perceptual learning in human participants have concentrated on the changes in perception assumed to be occurring, studies of nonhuman animals necessarily measure discrimination learning and generalization and remain agnostic on the question of whether changes in behavior reflect changes in perception. On the other hand, animal studies do make it easier to draw a distinction between supervised and unsupervised learning. Differential reinforcement will surely teach animals to attend to some features of a stimulus array rather than to others. But it is an open question as to whether such changes in attention underlie the enhanced discrimination seen after unreinforced exposure to such an array. I argue that most instances of unsupervised perceptual learning observed in animals (and at least some in human animals) are better explained by appeal to well-established principles and phenomena of associative learning theory: excitatory and inhibitory associations between stimulus elements, latent inhibition, and habituation.     

Retrieval of memory in the pigeon by context manipulations

In Experiment 1, 12 pigeons were given eight sessions of VI single stimulus training with a color in a particular context followed by eight sessions of similar training with a line angle in another context. On the next day, half of the subjects were tested for wavelength and angularity generalization in each of the two contexts, a procedure that was thus consistent with training for one dimension and inconsistent for the other. The subjects made significantly more responses to each training stimulus under the consistent context condition, but there was no difference in absolute or relative generalization slopes. In Experiment 2, 12 pigeons were trained as in Experiment 1, but during generalization testing they were exposed to both contexts sequentially. Under the consistent context condition, the subjects responded more to the two training stimuli and yielded sharper absolute and relative wavelength generalization gradients: Under the inconsistent context condition, responding to the training wavelength was substantially disrupted. Thus, under appropriate testing conditions, contextual control over both the amount and the selectivity of responding can be demonstrated.