Positive and negative patterning after CS preexposure in flavor aversion conditioning

Three experiments examined rats’ ability to discriminate a compound conditioned stimulus (CS) from the individual elements of that compound in a flavor aversion conditioning paradigm. In Experiment 1, presentations of a compound of sucrose and saline solutions were followed by lithium chloride injections, but presentations of those elements individually were nonrein-forced (positive patterning). Conversely, in Experiments 2 and 3, presentations of the individual elements were followed by lithium chloride injection, but compound presentations were non-reinforced (negative patterning). The discriminations were acquired in all three experiments. In addition, all three experiments investigated the effects of preexposure of the discriminative stimuli on subsequent acquisition of the patterned discriminations. In positive patterning, preexposure had no measurable effect on the acquisition of responding (suppression) to the reinforced compound stimulus, but slowed the loss of suppression to the nonreinforced elements. In negative patterning, preexposure slowed the acquisition of suppression to the reinforced elements but had little effect on the loss of suppression to the nonreinforced compound.     

Effects of elements or compound preexposure on conditioned taste aversion as a function of retention interval

The effects of element or compound preexposure and retention interval were examined in three experiments with the taste-aversion paradigm. In Experiment 1, preexposure to the elements of a compound flavor produced less latent inhibition to the compound than did preexposure to the compound itself when a 1-day preexposure-conditioning interval was used. However, preexposing the elements or the compound resulted in equivalent latent inhibition effects when a 21-day retention interval was used. In Experiment 2, a similar pattern of results was observed when the conditioning-test interval was manipulated. Experiment 3 explored the effect of element or compound preexposure when preexposure and test were carried out in different contexts. Attenuated latent inhibition following preexposure to the elements was found when preexposure and test were carried out in the same context. In contrast, preexposure to the elements resulted in as much latent inhibition as did preexposure to the compound when the context was switched from preexposure to testing. The implications of these findings for a retrieval-oriented view of latent inhibition are discussed.     

Taste + odor interactions in compound aversion conditioning

In three experiments with rats, taste + odor interactions in compound aversion conditioning were investigated. In Experiment 1, two odors (0.02% almond and 0.02% orange) were compared on single-element odor aversions, taste (denatonium) potentiated odor aversions, and potentiated odor aversions following taste extinction. Although no odor differences were seen following single-element conditioning, both types of potentiated orange odor aversions were stronger than their almond odor counterparts. These data show that odors of similar conditionability are differentially potentiated by the same taste. To determine whether these differences were due to unique perceptual representations, the effects of elemental extinction or compound extinction on aversions to the compound were investigated in Experiments 2 and 3. In Experiment 2, orange odor extinction weakened responding to the compound significantly more than taste extinction did. In contrast, almond odor extinction and taste extinction produced similar decrements in responding to the compound in Experiment 3. These results suggest that the perceptual representation of these specific taste + odor compounds are different, and they are discussed in regard to configural and within-compound association accounts of potentiation.     

US preexposure: Effects on flavor aversions produced by pairing a poisoned partner with ingestion

If an unpoisoned rat drinks a flavored solution and then is exposed to another rat suffering from lithium poisoning, the unpoisoned rat will later exhibit a flavor aversion. This is called the poisoned partner effect (PPE). The present experiments showed that (1) the US preexposure effect applies to the PPE since, if the unpoisoned rat is preexposed to poisoned partners, the PPE is attenuated, (2) the magnitude of the PPE increases with the dose of poison, and (3) a poisoned partner accustomed to poisoning produces a slightly weaker PPE than a partner naive to poisoning. Two earlier findings were confirmed: (1) The PPE occurs because the poisoned partner is an aversive US, not because a learned aversion is transferred from one rat to another; and (2) the PPE is weaker than an aversion produced by poisoning itself.     

Effects of postconditioning inflation on odor + taste compound conditioning

The within-compound association approach has been proposed as an account of synergistic conditioning in flavor aversion learning. One prediction from the within-compound association approach is that following taste + odor compound conditioning, postconditioning inflation of one element of the compound should increase responding to the second element. In four experiments with rats, the AX+/A+ design was used to determine whether postconditioning inflation of A would increase responding to X. In Experiments 1 and 3, responding to X was significantly stronger after AX+/A+ conditioning, as compared with AX+ conditioning. In Experiments 2 and 4, the specificity of the inflation effect was demonstrated, because AX+/A+ conditioning produced a stronger aversion to X than did AX+/B+ conditioning. Furthermore, it appears that the taste + odor association is symmetrical because inflation of the taste aversion increased responding to the odor (Experiments 1 and 2) and inflation of the odor aversion increased responding to the taste (Experiments 3 and 4).     

Generalization of the CS- preexposure effect transfers to taste- aversion learning

Rats repeatedly exposed to a maple, vanilla, banana, or water solution were subsequently poisoned with lithium chloride after drinking the maple solution. Subjects preexposed to water showed the strongest aversions to maple, followed, in descending order, by rats preexposed to banana, vanilla, and maple. These results suggest that the effects of prepoisoning experience with a novel flavor generalized to the maple CS and interfered with the development of the maple aversion. The technique reported here is potentially applicable to study of gustatory discrimination processes.     

The role of amount consumed in flavor preexposure effects and neophobia

Rats received either a small or a large amount of a novel saccharin solution prior to a conditioning episode in which the saccharin flavor was paired with lithium-induced toxicosis. When the time between the preexposure and the conditioning episode was 3.5 h, both the small and large amounts of saccharin decremented conditioning. However, when the time between the preexposure and the conditioning episode was 23.6 h, only the consumption of a large amount of saccharin decremented conditioning. In a second experiment, rats received either a short or a long exposure to the novel saccharin solution and were then given a choice test between the saccharin and water. Rats given the choice test immediately following their preexposure to saccharin avoided consuming it. If they received the choice test 4 h later, they consumed more of the saccharin irrespective of the length of the preexposure. In contrast, when they were given a choice test 24 h after the preexposure, only rats that had received a long preexposure displayed a significant preference for the saccharin. The present results demonstrate that the flavor preexposure effect and the attenuation of neophobia are both determined by the time between preexposure to the novel flavor and the conditioning episode or choice test and the amount of preexposure to the novel flavor. These findings are discussed in relation to the information-processing model developed by Wagner (1976, 1978) and are used to provide an account, in terms of this model, of the delay-gradient seen in flavor-aversion learning.     

A context-specific detrimental effect of UCS preexposure on place conditioning with morphine

The effect of morphine preexposure on place conditioning with morphine was investigated. In the first experiment, five injections of 10 mg/kg morphine were administered to rats prior to place conditioning or taste-aversion training with morphine. Although this number of preexposures retarded taste-aversion learning, there was no effect on place conditioning. In subsequent experiments we investigated the role of context blocking in UCS preexposure in place conditioning. In one experiment, preexposure to five morphine injections prior to place conditioning resulted in a reduced place preference, compared with preexposure and place conditioning in different contexts. However, the overall detrimental effect of morphine preexposure was questionable, because the rats that were preexposed were only marginally different from those that were not preexposed. In a final experiment we examined the effect of a context change from preexposure to place conditioning with 15 preexposures and demonstrated a detrimental effect of preexposure on place conditioning that was context specific. These results support a role of classical conditioning in place-preference conditioning with morphine.     

Higher order conditioning of a taste aversion

Two groups of rats each received 5 drops of NaCl solution from a dropper placed directly inside the mouth. The experimental group was then injected with lithium chloride to establish a conditioned taste aversion. The control group was injected 24 h later. After a recovery day the above procedures were repeated. On the next day both groups received 5 drops of a saccharin solution followed immediately by 5 drops of the NaCl solution. Subsequent preference tests established that the experimental group had learned an aversion to the saccharin solution as a result of its pairing with the NaCl solution which had previously been associated with poisoning. These results demonstrate that higher order conditioning of a taste aversion can be established using tastes as both the first-order stimulus and the second-order stimulus.     

The influence of a distractor during compound preexposure on latent inhibition

Nonreinforced exposure to a nontarget stimulus that was followed by nonreinforced exposure to a target/nontarget simultaneous compound stimulus resulted in enhanced latent inhibition of the target. Conditioning was slower after this treatment than after nonreinforced exposure to the target stimulus alone (Experiment 1). However, a salient auditory stimulus presented immediately after the compound in the second phase reduced levels of latent inhibition, relative to the enhanced latent inhibition produced when no such extracompound stimulus was presented (Experiments 2 and 3). This effect was not noted if the salient auditory cue was presented 10 sec after the termination of the compound stimulus (Experiment 4). In Experiment 5, there was no disruption of simple latent inhibition produced by a salient stimulus. These results are consistent with enhanced latent inhibition’s being produced by the formation of within-compound associations, which are disrupted by the salient extracompound stimuli.     

A contingency analysis of taste aversion conditioning

The acquisition of conditioned taste aversion was assessed relative to five control procedures. That is, forward conditioning using multiple-trial, brief-duration taste CSs and weak USs over a 30-min CS-US delay was compared to backward, CS alone, US alone, sham CS and sham US, and random control procedures. The outcome supported an associative conditioning interpretation of the learned aversion. While there were no differences between the various control procedures, all were different from the forward conditioning group. The argument was made that some of the distinctive associative and nonassociative phenomena attributed to taste aversion conditioning (but not seen in the present study) may in part be due to the duration and intensity of both the CS and US events.     

The effects of stimulus preexposure on taste-mediated environmental conditioning: Potentiation and overshadowing

On four occasions, Holtzman rats drank saccharin in a distinctive environment prior to lithium-induced toxicosis. Preconditioning exposure to saccharin either in the home cage or in the distinctive environment interfered significantly with the establishment of an environmental aversion. Animals preexposed to the experimental environment, however, showed environmental aversions substantially stronger than those in animals preexposed to saccharin and only slightly higher than those with no preexposure to either the taste or the environment. Subsequent saccharin tests revealed significantly stronger aversions in the group that received environmental preexposure than in any of the other groups. This pattern of outcomes demonstrates taste-mediated potentiation of novel and familiar environmental stimuli as well as overshadowing of the taste by novel environmental stimuli. Furthermore, it indicates that previous demonstrations of taste-mediated environmental potentiation involve facilitated conditioning of the environmental stimuli and decremented conditioning of the taste stimuli.     

Effects of pretraining on conditioning-enhanced neophobia: Evidence for separable mechanisms of neophobia and aversion conditioning

In two experiments, rats (n = 228) received pretraining access to a distinctive novel flavor (saline) followed by aversion conditioning to a different novel conditioned stimulus (CS) (saccharin). Then the rats were tested for aversion to the CS (saccharin) or for conditioning-enhanced neophobia to a third novel flavor (casein hydrolysate). Pretraining access to a distinctive novel flavor that differed from the CS reliably reduced the magnitude of conditioning-enhanced neophobia to casein, but did not reliably affect conditioned aversion effects to the CS. Pretraining access to the CS reduced aversion effects to the CS and reduced postconditioning neophobia to casein to the performance level shown by ingestion-toxin controls. Results were consistent with the view (Braveman & Jarvis, 1978) that conditioned aversion and neophobia may be independent phenomena with separable underlying mechanisms.     

Nonreinforced flavor exposure attenuates the effects of conditioned taste aversion on both flavor consumption and cue palatability

Nonreinforced exposure to a cue tends to attenuate subsequent conditioning with that cue—an effect referred to as latent inhibition (LI). In the two experiments reported here, we examined LI effects in the context of conditioned taste aversion by examining both the amount of consumption and the microstructure of the consummatory behavior (in terms of the mean size of lick clusters). The latter measure can be taken to reflect affective responses to, or the palatability of, the solution being consumed. In both experiments, exposure to a to-be-conditioned flavor prior to pairing the flavor with nausea produced by lithium chloride attenuated both the reduction in consumption and the reduction in lick cluster sizes typically produced by taste aversion learning. In addition, we observed a tendency (especially in the lick cluster measure) for nonreinforced exposure to reduce neophobic responses to the test flavors. Taken together, these results reinforce the suggestion from previous experiments using taste reactivity methods that LI attenuates the effects of taste aversion on both consumption and cue palatability. The present results also support the suggestion that the failure in previous studies to see concurrent LI effects on consumption and palatability was due to a context specificity produced by the oral taste infusion methods required for taste reactivity analyses. Finally, the fact that the pattern of extinction of conditioned changes in consumption and in lick cluster sizes was not affected by preexposure to the cue flavors suggests that LI influenced the quantity but not the quality of conditioned taste aversion.     

Effects of repeated lithium injections on temperature,activity, and flavor conditioning in rats

Two experiments investigated the effects of lithium chloride on activity and rectal temperature in rats. Lithium injections, relative to saline injections in controls, caused decreases in both activity and temperature. Importantly, lithium’s effects did not diminish over the course of eight repeated injections, although preexposure to lithium did attenuate its conditioning effectiveness in a flavor-conditioning trial. The results support the idea that drug tolerance is not of great importance in the lithium-mediated US-preexposure effect in flavor conditioning.     

Effects of CS preexposure on inhibition of delay

Two experiments examined the effects of preexposure to a stimulus on the subsequent acquisition of conditioned suppression by rats. Variations in the level of suppression within conditioning trials were noted so thatinhibition of delay (taken here to mean less suppression at the beginning of a trial than at the end) could be detected. Inhibition of delay was observed both during the acquisition of suppression and (in Experiment 1) when suppression began to wane with continued postasymptotic training. Preexposure to the to-be-conditioned stimulus retarded acquisition of suppression and slowed the appearance of inhibition of delay both in acquisition and (in Experiment 1) in postasymptotic performance. Experiment 2 demonstrated that inhibition of delay was attenuated during conditioning that followed preexposure in which the stimulus was paired with a weak reinforcer. These results provide no support for the suggestion that preexposure to a stimulus retards later conditioning because it allows the subject to acquire information about stimulus duration that in turn fosters the development of inhibition of delay. Rather, they are compatible with the suggestion that preexposure causes the stimulus to lose associability.     

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.     

Extinction and retraining of simultaneous and successive flavor conditioning

In three experiments, rats received pairings of flavor conditioned stimuli with polycose unconditioned stimuli, in either a simultaneous or a sequential relation. Both temporal relations produced excellent conditioned increases in consumption of the flavors. Separate presentation of the flavors resulted in extinction in both cases. However, restoring the pairing of the flavor with polycose resulted in reconditioning only with the sequential, not with the simultaneous, relation.     

The role of injection cues in the production of the morphine preexposure effect in taste aversion learning

The attenuation of an LiCl-induced conditioned taste aversion (CTA) by LiCl preexposure is mediated primarily by associative blocking via injection-related cues. Given that preexposure to morphine attenuates morphine-induced CTAs, it was of interest to determine whether injection cues also mediate this effect. Certain morphine-induced behaviors such as analgesic tolerance are controlled associatively, via injection-related cues. Accordingly, animals in the present experiments were preexposed to morphine (or vehicle) every other day for five total exposures, followed by an extinction phase, in which the subjects were given saline injections (or no treatment) for 8 (Experiment 1) or 16 (Experiment 2) consecutive days. All of the animals then received five CTA trials with morphine (or vehicle). The morphine-preexposed animals in Experiment 1 displayed an attenuation of the morphine CTA that was unaffected by extinction saline injections, suggesting that blocking by injection cues during morphine preexposure does not mediate this effect. All of the morphine-preexposed subjects in Experiment 2 displayed a weakened preexposure effect, an effect inconsistent with a selective extinction of drug-associated stimuli. The attenuating effects of morphine preexposure in aversion learning are most likely controlled by nonassociative mechanisms, like drug tolerance.     

Taste-potentiated color aversions in pigeons: Examination of CS preexposure and subsequent effects on postconditioning extinction of the taste aversion

In the present study, we investigated the mechanisms of potentiation using pigeons as subjects. Half the groups were given unreinforced preexposures to the CS on 2 consecutive days before aversion training. After training, the taste aversion was extinguished in some of the groups conditioned with a colored, tasty compound. Postconditioning extinction of the taste aversion was found to attenuate potentiated color aversions. This was the case for the subjects that both were and were not preexposed to the CS. These results lend support to the summation theory of potentiation (Durlach & Rescorla, 1980) and weaken the claim that CS preexposures are necessary for obtaining such support. Alternative accounts of the data are discussed.