Super-latent inhibition of conditioned taste preference with a long retention interval

A long delay inserted between conditioning and test phases of a 3-stage Latent Inhibition (LI) procedure produces differential effects on LI depending on the delay context. Thus, enhanced LI has been obtained when the delay is spent in a context that is different from the remaining experimental contexts, but not when it is the same. The present paper examined the effect of delayed testing using a conditioned taste preference procedure. In Experiment 1, three groups received preexposure to either water, or almond, or citrus solutions. Subsequently, the animals were conditioned by pairing the almond and the citrus solutions with sucrose. In a two-bottle preference test conducted 1 day after conditioning, the preexposed flavored solutions were consumed less than the non-preexposed flavors (LI). Experiments 2 and 3 used the same basic procedure, but varied the retention interval duration (1 and 21 days) and the delay context, similar (Experiment 2) and different (Experiment 3) from that of the other stages of the experiments. LI was greater after the long than the short-retention interval when the context was different, thereby demonstrating super-LI. The results were explained by a time-induced context differentiation process.     

There is a time and a place for everything: Bidirectional modulations of latent inhibition by time-induced context differentiation

Latent inhibition (LI) is defined as poorer evidence of learning with a stimulus that previously was presented without consequence, as compared with a novel or previously attended stimulus. The present article reviews the evidence, mostly from three-stage conditioned taste aversion studies (preexposure, conditioning, and test), that LI can be either attenuated or enhanced depending on the length of the retention interval between conditioning and test and where that interval was spent. Time-induced reduction in LI is observed when the interval context is the same as that of the preexposure, conditioning, and test stages. Super-LI is obtained when a long retention interval is spent in a context that is different from that of the other stages. The differential modulations of LI appear to be the result of the strengthening of primacy effects (i.e., first training disproportionately stronger than subsequent training) by long-interval different contexts, thereby producing super-LI, and the reversal of this effect by long-interval same contexts, thereby producing attenuated LI. The bidirectional effects of time/ context modulations on LI, unaccounted for by current learning theories, are explained, in part, by a time-induced context differentiation process. Implications for theories of LI, learning, and memory are discussed.     

Time-induced super-latent inhibition is dependent on the distinctiveness of the retention-interval context from the other experimental contexts

A conditioned taste aversion experiment examined the role of the retention-interval context (between conditioning and test stages) on the modulation of long-delay latent inhibition (LI). A super-LI effect was obtained only when the animals spent the retention interval in a context that was different from that of preexposure, conditioning, and test. Unlike in other super-LI experiments, the context-different conditions were not confounded with the home cage context. Thus, the critical element for producing super-LI is the distinctiveness of the long-delay context relative to the other experimental contexts.     

An empirical analysis of the super-latent inhibition effect

In three conditioned taste aversion experiments, we examined the roles of several variables in producing super-latent inhibition (LI). This effect, greater LI after a long interval than after a short interval between the conditioning and the test stages (De la Casa & Lubow, 2000), was shown to increase with the number of stimulus preexposures (0, 2, or 4; Experiment 1) and with the length of the delay interval (1, 7, 14, or 21 days; Experiment 2). Furthermore, super-LI was obtained when the delay interval was introduced between the conditioning and the test stages (Experiments 1 and 2), but not when it was introduced between the preexposure and the conditioning stages (Experiment 3). The results are discussed in relation to interference explanations of LI.     

Stimulus Preexposure, Comparison, and Changes in the Associability of Common Stimulus Features

In four experiments, rats received preexposure either to both of two compound flavours (AX and BX), or to just one (BX). Experiment 1 demonstrated a perceptual learning effect, showing that, for animals given preexposure to both flavours, an aversion conditioned to AX generalized only poorly to BX. Subsequent experiments assessed the properties of the common feature, X. Experiment 3 showed that the two preexposure treatments did not differ in the extent to which they produced habituation of the neophobia evoked by X. Experiment 2 showed that conditioning to X proceeded more rapidly in subjects given preexposure to both AX and BX than in subjects preexposed to BX alone. In Experiment 4, a similar effect was found when the elements of the compounds were presented serially. It is concluded that the perceptual learning effect of Experiment 1 occurs in spite of the fact that preexposure to two stimuli tends to maintain the associability of their common elements.     

Preconditioning exposure to the unconditined stimulus affects the acquisition of a conditioned emotional response

Four experiments examined the UCS preexposure phenomenon using conditioned suppression of food-reinforced responding as a measure of excitatory conditioning, and electric shock as a UCS. In Experiment 1, groups of rats were preexposed to unsignaled 0.8-mA electric shocks for 0, 1, 3, 5, or 10 days, and then conditioned with a 0.8-mA electric shock. Preexposure to electric shock 1 day prior to conditioning enhanced the acquisition of a CER, whereas preexposure to electric shock for 3, 5, or 10 days prior to conditioning attenuated the acquisition of a CER as a direct function of the number of days of preexposure. In Experiments 2 and 2A, groups of rats were preexposed to unsignaled electric shocks of 0.3, 0.5, 0.8, or 1.3 mA for 10 days, and then conditioned with a 0.8-mA electric shocl. All groups preexposed to electric shock acquired the CER at a slower rate than a group not preexposed to electric shock. The greatest attenuation of CER conditioning occurred when the same intensity electric shock was used during both the preexposure and conditioning phases. In Experiment 3, groups of rats were preexposed to signaled electric shocks of either 0.5, 0.8, or 1.3 mA, and then conditioned with a 0.8-mA electric shock. All groups preexposed to electric shock acquired the CER at a slower rate than a group not preexposed to electric shock. As in Experiments 2 and 2A, the greatest attenuation of CER conditioning occurred when the same intensity electric shock was used during both the preexposure and conditioning phases. In Experiment 4, groups of rats were preexposed to series of 0.5, 0.8, or 1.3-mA electric shocks which they could escape by performing a chain-pull response. Rats in each of these groups had yoked partners which received the same number, intensity, and temporal pattern of electric shocks, but could not perform a response to escape shock. All groups were then conditioned with a 0.8-mA electric shock. Rats preexposed to escapable electric shocks showed equal or greater attenuation of CER conditioning than rats which could not escape shock during the preexposure phase. These results are discussed in terms of nonassociative and associative explanations of the UCS preexposure phenomenon.     

Superlatent inhibition and spontaneous recovery: differential effects of pre- and postconditioning CS-alone presentations after long delays in different contexts

In two pairs of three-stage conditioned taste aversion experiments, we examined the effects of delay interval (1 or 21 days) between the second and third stages, and of context in which the animals spent the delay (same as or different from the context of the other stages) on latent inhibition (LI) and spontaneous recovery following extinction. In the LI experiments (Experiments 1A and 1B), the first stage comprised nonreinforced presentations to saccharin or to water. In the second stage, rats were conditioned by saccharin paired with LiCl. In the extinction experiments (Experiments 2A and 2B), the order of the stages was reversed. For all experiments, Stage 3, the test stage, consisted of three presentations of saccharin alone. There was a super-LI effect in the saccharin-preexposed group that spent the 21-day delay in the different context (Experiment 1A). When the delay was spent in the same context, there was no difference in the amount of LI between the short- and long-delay groups (Experiment 1B). Conversely, there was a spontaneous recovery effect in the long-delay/same-context group (Experiment 2B), but not in the long-delay/different-context group (Experiment 2A). The pattern of results, incompatible with current explanations of delay-induced changes in memory performance, was interpreted in terms of an interaction between the delay conditions (same or different delay context), which modulate the extinction of previously acquired context-CS-nothing associations (during CS-alone presentations), and primacy effects.     

Latent inhibition and conditioning after preexposure to the training context

Experiment 1, using the conditioned suppression technique with rats, showed that the retardation of learning produced by prior exposure to a stimulus (latent inhibition) was more marked in subjects given an initial phase of preexposure to the training context. This effect was confirmed and extended in Experiment 2 in which an appetitive conditioning procedure was used. Experiments 3 and 4, again using conditioned suppression, found no effect of preexposure to the context on the acquisition of suppression when training was given with a novel stimulus, either immediately after preexposure or after a delay; but context preexposure was again found to be effective when exposure to the to-be-conditioned stimulus was given in the delay interval between context preexposure and conditioning. The implications of these findings for accounts of the role of contextual factors in latent inhibition are discussed.     

Outcome pre- and postexposure effects: retention interval interacts with primacy and recency

Effects of outcome-alone pretraining and posttraining exposure were investigated in conditioned suppression experiments conducted within a sensory preconditioning preparation with rats. Experiment 1 found that interference by outcome postexposure was stronger than that by outcome preexposure, suggesting a recency effect. Experiment 2 found that after a long retention interval, outcome preexposure produced more interference than outcome postexposure, suggesting a shift from recency to primacy with increasing retention interval. Experiment 3 showed that presentation of a priming stimulus that had been embedded within the earlier phase of treatment also caused a shift from recency to primacy. These results suggest that, at least in a sensory preconditioning paradigm, retrievability of outcome-alone exposure memory is an important determinant of any outcome-alone exposure effect.     

Some determinants of latent inhibition in human predictive learning

The present experiments assessed the effects of different manipulations between cue preexposure and cue-outcome pairings on latent inhibition (LI) in a predictive learning task with human participants. To facilitate LI, preexposure and acquisition with the target cues took place while participants performed a secondary task. Presentation of neither the target cues nor the target outcome was anticipated based on the instructions. Experiment 1 demonstrated the LI effect in the new experimental preparation. Experiment 2 analyzed the impact on LI of different activities that participants performed during the interval between preexposure and acquisition. Experiment 3 assessed LI as a function of changes in the secondary task cues made between preexposure and acquisition, namely presenting novel cues and reversing the cue-outcome contingencies. All of the manipulations in Experiments 2 and 3 resulted in a decrease in LI. The attenuation of LI by these manipulations challenges most current theories of learning and is best accommodated by Conditioned Attention Theory (Lubow, Weiner, & Schnur, 1981).     

The effects of preexposure-test and conditioning-test intervals on the magnitude of latent inhibition

Using the conditioned taste aversion paradigm, two experiments were conducted to examine the effects of the interval between preexposure and test and that between conditioning and test on the magnitude of latent inhibition. Experiment 1 revealed that the degree of latent inhibition was attenuated when rats were given a 21‐day interval between preexposure and test. It was also found that this attenuation was more marked in subjects which were given conditioning immediately after preexposure than those which were conditioned shortly before the test. Retention interval between preexposure and test was reduced to 12 days in Experiment 2, and exactly the same pattern of results as those found in Experiment 1 was obtained. These findings suggest that the memory of conditioning as well as that of preexposure decreases its retrievability after a long retention interval, although the former is more retainable than the latter.     

The influence of flavor preexposure and test interval on conditioned taste aversions in the rat

The effects of flavor preexposure and test interval on conditioned taste aversions were examined in four experiments. In the first three experiments, prior experience with a flavor different from that used as a conditioned-stimulus (CS) produced attenuated aversions when testing occurred after a 1-day interval but not after a 21-day interval. Preexposure to the same stimulus used as a CS produced attenuated aversions at both 1- and 21-day intervals. In Experiment 4, a delay interval between flavor preexposure and conditioning eliminated the attenuating effect of preexposure, but only when different stimuli were used for preexposure and conditioning. These data could not be easily accounted for by contemporary interpretations of preexposure as an event that interferes with subsequent acquisition of a conditioned aversion. An alternative retrieval interference hypothesis was outlined.     

Loss of latent inhibition in conditioned taste aversion following exposure to a novel flavour before test

Killcross, Kiernan, Dwyer, andWestbrook (1998b) observed that latent inhibition(LI) of contextual fear was attenuated if animals received post-conditioning exposure to a novel context similar to the pre-exposure context. Six experiments used a conditioned taste aversion (CTA) procedure to examine this effect. Experiments 1A-1C demonstrated that LI of CTA was attenuated by a similar post-conditioning manipulation, establishing the generality of previous findings. Experiment 2A manipulated the taste elements to which animals were exposed after conditioning, revealing that exposure to a common element X, present at pre-exposure and conditioning, was not responsible for loss of LI. Experiment 2B manipulated test solution and showed that loss of LI depended on the presence of the full pre-exposed cue AX at test. These two results are contrary to predictions derived from the Dickinson-Burke (Dickinson & Burke, 1996) theory of retrospective revaluation or comparator theory (Miller & Matzel, 1988), and they support recent findings suggesting that retrospective effects may occur by several mechanisms. Experiment 3 showed that a novel element B had to be present during post-conditioning exposure for an attenuation of LI to be observed. Implications for the loss of LI following aretention interval between conditioning and test and retrieval-failure theories of LI are discussed.     

Delay-induced attenuation of latent inhibition with a conditioned emotional response depends on CS–US strength

Latent Inhibition (LI) attenuation when a long delay is introduced between acquisition and test phases has been repeatedly observed using aversive conditioning procedures (e.g., Aguado, Symonds, & Hall, 1994). This effect has been used as evidence to support those theories that consider LI to be the result of a retrieval failure. We designed three experiments intended to control for a possible effect of incubation of fear as a possible source of delay-induced attenuated LI. Specifically, we examined the effects of a retention interval between conditioning and testing stages on LI using a 3-stage conditioned emotional response procedure (preexposure, conditioning, and testing). Experiment 1 showed that the LI effect was completely abolished in the delayed testing condition. Experiment 2 evaluated whether a process of fear incubation, developed during the retention interval but obscured by a ceiling effect, produced the attenuation of LI. To this end, we reduced magnitude of conditioning by decreasing US intensity and number of acquisition trials. Experiment 3 directly assessed the relationship between CS–US strength and fear incubation. Experiments 2 and 3 revealed that the apparent reduction of LI after the delay was, at least partially, the result of an incubation effect that is a function of the strength of the CS–US association. The results are discussed with respect to their implications for the different theories of LI.     

Palatability shifts in taste and flavour preference conditioning

Changes in palatability of tastes and flavours as a result of flavour preference conditioning were examined. In Experiment 1, when tastes were paired with glucose in a reverse-order differential conditioning paradigm, rats acquired conditioned preferences for CS + and displayed more hedonic responses to CS + than to CS - in a postconditioning taste reactivity test. In Experiment 2, rats that received oral infusions of flavours as CSs during a reverse-order conditioning procedure expressed both palatability shifts and conditioned preferences for CS + . Rats that received a forward conditioning procedure acquired a preference for CS + , but the palatability of CS + was unchanged. In Experiment 3, hungry rats drank mixtures of a flavour CS and a calorific or sweet tasting reinforcer in a long-exposure conditioning paradigm. When tested hungry, rats preferred CS + whether they had acquired flavour-calorie or flavour-taste associations. However, CS + became more palatable only for rats that acquired flavour-calorie associations. These results suggest that acquisition of flavour preferences, as measured by 2-bottle tests, may not always be accompanied by enhanced palatability.     

Flavour aversion produced by running and attenuated by prior exposure to wheels

In two experiments hungry rats were given access to running wheels. When given the novel flavour, almond, prior to novel access to the wheels, a conditioned aversion to almond was revealed by a subsequent two-bottle test. No such aversion was found in rats with previous experience of wheel running, whether this prior running occurred in the absence of any novel flavour, as in Experiment 1, or following access to saccharin, as in Experiment 2. These results suggest that the failure of rats with prior experience of the running wheels to develop a flavour aversion (unconditioned stimulus, US, preexposure effect) is unlikely to be due to associative blocking. Instead it seems that increasing exposure to a wheel produces habituation of its nausea-inducing properties.     

Attenuation of latent inhibition after compound conditioning

Using a conditioned suppression procedure with rats, three experiments examined the effects of compound conditioning on the degree of latent inhibition. Experiment 1 suggested that latent inhibition of the preexposed target was not enhanced but rather attenuated when a second stimulus was presented in compound conditioning. Experiment 2 showed that a similar result was obtained when, in subjects given only compound conditioning, the salience of the target was reduced to the level where it was overshadowed by the second stimulus. Experiment 3 proved that addition of the second stimulus only during preexposure, or during both preexposure and conditioning, did not attenuate the latent inhibition to the target. These results are difficult to explain by any model of Pavlovian conditioning which assumes that both latent inhibition and overshadowing effects are a consequence of acquisition deà cit; other possible accounts are discussed.     

Amphetamine, conditioned stimulus, and nondebilitating preshock effects on activity and avoidance: further evidence for interactions between associative and nonassociative changes

A literature survey and preliminary experiments with rats on the consequences of shock preexposure on subsequent activity and escape or avoidance showed the need for further work on the interactions between nondebilitating preshock and various test and treatment factors. The two main experiments used 16 preexposure conditions, namely, presence or absence of unavoidable punishment (36 shocks of 2.5 mA and 5 sec subdivided in three daily sessions), a light CS, a central partition in the shuttle-box, and dl-amphetamine sulfate (1 mg/kg ip 15 min before each session). In both experiments the four factors studied exerted more than additive effects on activity in preexposure sessions, leading to a very high frequency of crossing in the CS-shock-no-partition-drug condition. Upon retesting for activity (Experiment 1) suppression of locomotion by prior shock was less marked in animals preexposed to CS-US pairings in the absence of partition, while proactive amphetamine effects consisted mainly of a progressive increase of activity over successive retest sessions in the groups not preshocked. Upon retesting for light-cued, two-way avoidance acquisition (Experiment 2) the groups preexposed to US only were mostly retarded, while those preexposed to paired CS and US were mostly facilitated. Other changes, including drug pretreatment consequences, were negligible or unsystematic, but in general the data showed that the effects of various preexposure conditions on activity could not account for those on avoidance. Overall, it appears that the interactions between nondebilitating preshock and other test and treatment factors can be further exploited to clarify the respective roles of various associative and nonassociative mechanisms in modulation of activity and adaptive responding in aversive situations.     

Rats' Memory for Serially Presented Novel Flavours: Evidence for Non-spatial Primacy Effects

Four experiments examined the effects of serially presenting a number of novel flavours to rats on their subsequent consumption of those flavours. In Experiments 1-4, rats were orally infused with 0.5 ml of flavour over 30 sec for each of five flavours in the exposure phase of the experiment. In these studies, primacy and recency effects emerged, the size of the primacy effect being related to the length of the retention interval, which varied from zero to twenty-four hours. Thus, both primacy and recency effects can be generated using nonspatial stimuli with rats.     

Latent inhibition experiments with goldfish (Carassius auratus)

Evidence of latent inhibition was sought in a series of experiments with goldfish. In Experiment 1, goldfish were given nonreinforced preexposure to a color that subsequently predicted shock in an activity conditioning situation; their performance did not differ from that of control animals preexposed to a markedly different color. In Experiment 2, a group of goldfish given nonreinforced preexposure to a tone and an unstimulated control group were trained in an appetitive situation, with the tone serving either as a conditioned excitor or as a conditioned inhibitor. Preexposure had no significant effect in the conditioned excitation training, but it reduced the level of responding both to the positive stimulus and to the negative compound in the conditioned inhibition training. In Experiments 3 and 4, classical aversive conditioning was studied in the shuttle box. In Experiment 3, excitatory conditioning to a color was found to be impaired (relative to the performance of nonpreexposed control animals) as much by nonreinforced preexposure to the training color as by nonreinforced preexposure to a markedly different color; substantial variation in amount of preexposure was without significant effect. In the conditioned inhibition training of Experiment 4, animals with nonreinforced preexposure responded less than did unstimulated control animals both to the positive stimulus and to the negative compound. The results for goldfish can be understood on the assumption that the effect of preexposure in these animals is simply to reduce general responsiveness or level of arousal.