Attenuation of the CS-preexposure effect after a retention interval in preweanling rats

The effects of flavor preexposure and retention interval were assessed in 6- and 12-day-old rats. Conditioned aversions to a flavor appeared at both ages. The conditioning of the younger pups was unaffected by conditioned stimulus (CS) preexposure and was not evident after a 10-day retention interval. For the 12-day-old rats, preexposure to either the flavor CS or a different flavor attenuated aversion strength when the rats were tested soon after conditioning. Other 12-day-old rats that were tested 10 days after conditioning also expressed substantial aversions, but with a retention interval of this length, the aversions were equivalent for animals preexposed to the CS and those not preexposed before conditioning. This loss of the CS-preexposure effect over a long interval, which has also been observed in adult rats, identifies the locus of this effect as postacquisition and perhaps at the stage of memory retrieval.     

Specificity of cue to consequence in aversion learning in the rat: Control for US-induced differential orientations

In four experiments, each using a single conditioning trial, rats avoided a light more than a saccharin solution after these stimuli had been paired with footshock, whereas saccharin was avoided more than the light after these stimuli had been paired with lithium injection. The use of a single conditioning trial precludes possible US-induced differential orientations from influencing which stimuli will be associated on the conditioning trial. This cue-consequence specificity effect was obtained even when subjects conditioned with lithium received a non-contingent footshock prior to the test session, and when subjects conditioned with footshock received a noncontingent lithium injection before testing (Experiments 2–4). Weak aversions to the light in rats given a light-lithium pairing and noncontingent footshock and to the saccharin in subjects that received a saccharin-footshock pairing and noncontingent lithium administration were obtained in Experiment 2. However, these weak aversions were not obtained when subjects were given three nonreinforced exposures to the test chamber before the test session (Experiments 3 and 4). These results indicate that US-induced differential orientations do not mediate the cue-consequence effect in aversion learning.     

Variations in the retention of taste aversions: Evidence for retrieval competition

In six experiments, we examined taste and compound taste/taste aversions at different retention intervals. In Experiment 1, saccharin aversions were significantly weaker 1 day after conditioning than 21 days after conditioning. This effect was determined not to be caused by the aftereffects of illness or differential hydration. With the use of a saccharin/denatonium compound, Experiment 2 demonstrated overshadowing of a denatonium aversion at 21- and 1-day retention intervals, Experiment 4 showed a potentiated saccharin aversion only at the 21-day retention interval, and both Experiments 2 and 4 revealed that the aversion of the taste-only controls was stronger at the later retention interval. Experiments 3 and 5 demonstrated that the differences at the two retention intervals were not caused by unconditioned changes in taste preference. Finally, Experiment 6 showed that extinction of the conditioning environment prior to testing results in stronger saccharin aversions than occur in nonextinguished controls. Collectively, these experiments suggest that testing within a 24-h period after conditioning will result in significantly weaker taste aversions. Also, these results support a retrieval-competition explanation that may account for the weakened aversions at the 1-day testing interval of both groups conditioned to single elements and those conditioned to compounds.     

Learned flavor preferences based on calories are independent of initial hedonic value

In three experiments, the learning of flavor preferences due to pairing with calories was examined. In Experiment 1, the relative hedonic values of four isocaloric solutions and saccharin were assessed by offering these substances simultaneously to naive rats. The caloric solutions were then used to condition a flavor preference in separate groups of rats. Although the solutions were reliably different in unconditioned hedonic value, the conditioned flavor preferences were identical. In Experiment 2, we compared solutions of sucrose and saccharin that were equal in unconditioned hedonic value. Only the sucrose conditioned a preference. Finally, in Experiment 3, preferences were found to be sensitive to the number of calories available during conditioning. These results are discussed in terms of a peripheral cholecystokinin (CCK) reflex and the integration of that information along with taste information at the area postrema (AP) and surrounding nuclei. It is proposed that CCK acts centrally to adjust the incentive motivation or hedonic value of flavors.     

A mixed design reveals that glucose moieties facilitate extinction of a conditioned taste aversion in rats

Separate groups of water-deprived rats had four trials with 15-min access to 0.0073 M saccharin, 0.3 M alanine, 0.3 M glucose, 0.1 M maltose, 0.3 M fructose, 0.06 M sucrose, or 0.03 M Polycose. Trials 1–3 were followed by injections of either 0.15 M LiCl (1.33 ml/100 g b.w., i.p.) or saline; Trial 4 (Test) was CS only. Extinction included either 48-h access to water alone or to the appropriate CS, both followed by a 24-h, two-bottle choice of CS and water. This 3-day cycle was repeated five to six times. All rats acquired comparable conditioned taste aversions (CTAs), but extinction rates varied with the test and the taste CS. No CTA extinguished during the two-bottle choices following 2 water days. During one-bottle CS exposure, all CTAs extinguished, but the aversion continued longer in the probe two-bottle tests. Intake of glucose moieties recovered rapidly, often in two cycles; the other CSs took four to six cycles. Thus, CTA extinction varies with the nature of the taste CS.     

Strain-dependent sexual dimorphism in the extinction of conditioned taste aversion in mice

Sucrose aversions were induced by lithium chloride toxicosis in 9-week-old male and female mice of the C57BL/6J and DBA/2J strains. Under conditions of ad-lib access to fluids which required a long interval between the sucrose and toxicosis, the conditioned aversions were relatively weak when compared with the sucrose consumption of saline-injected controls. The aversions extinguished rapidly within all groups. Contrary to earlier reports in rats, no sexual dimorphism in extinction rate was observed in this paradigm. When a fluid-deprivation schedule was implemented in the conditioning procedure to shorten the interval between taste experience and toxicosis, strong aversions were noted; yet relatively fast extinction under ad-lib conditions occurred in all groups except male DBA/2J mice. No sexual dimorphism was observed in mice of the C57BL/6J genotype, even when two pairings between sucrose and toxicosis were administered.     

Effects of context exposure during conditioning on conditioned taste aversions

Rats were used in a conditioned taste aversion procedure in order to examine the effects of context exposure duration during the conditioning sessions on conditioned responding. One flavor was paired with lithium chloride during a long session in one context, whereas another flavor was conditioned during a short session in another context. Testing occurred in the home cage. The results showed that conditioning during short sessions produced strong conditioned taste aversions. Conditioning during long sessions produced strong conditioned taste aversions when the conditioned-stimulus-unconditionedstimulus (CS-US) pairing occurred at the end of the lengthy session. Other results showed that context-US associations were formed during the short duration sessions and that these associations supported conditioned responding to the CS trained in that context. The results are discussed with respect to the different influences that contextual cues can exert on conditioned responding.     

One-trial simultaneous and backward fear conditioning as reflected in conditioned suppression of licking in rats

In three experiments, groups of albino rats received one strictly simultaneous pairing of a 4-sec auditory conditioned stimulus (CS) and a 4-sec 1-mA shock unconditioned stimulus (US). Other groups received a backward pairing, in which the US began before the CS, or a forward pairing, in which the CS began before the US. Control groups received only the US or received both the CS and the US but widely separated in time. Later, the CS was presented while the rats licked a drinking tube for water, and CS-elicited suppression of licking was taken as an index of the Pavlovian conditioned response (CR). It was found that groups receiving a single forward or a single simultaneous pairing suppressed more than groups that had received a backward pairing; and the backward groups, in turn, suppressed more than the control groups. It appears, then, that excitatory fear conditioning, as reflected in conditioned suppression of licking in rats, can be produced in a single trial by both backward and simultaneous conditioning procedures.     

Partial reinforcement and latent inhibition effects on stimulus–outcome associations in flavor preference conditioning

Two experiments with thirsty rats explored the harmful effects of non-reinforced exposures to a flavor cue in the control by sensory-specific flavor–sucrose associations in a conditioned flavor preference paradigm. Experiment 1 demonstrated that rats learned to prefer a flavor cue that was consistently paired with sucrose over one that was paired with sucrose the same number of times but was also presented without sucrose on other occasions. However, rats for which sucrose was devalued following the conditioning phase preferred the partially reinforced flavor cue over the consistently reinforced flavor, suggesting that non-reinforcement weakened the ability of that flavor cue to evoke a specific representation of sucrose during the preference test. Experiment 2 demonstrated comparable effects of non-reinforcement in a latent inhibition procedure, although relatively more non-reinforced pre exposures to the flavor, in conjunction with fewer flavor–sucrose pairings, were required to see the effect. Together, the results suggest, as is often found with more traditional learning paradigms, that non-reinforcement of a flavor cue has deleterious effects on preference learning and/or performance.     

Reacquisition of learned taste aversions

In four experiments, it was found that rats subjected to extinction of conditioned taste aversions readily reacquired these aversions when subjected to conditioning once again. Two of the present procedures were very similar to those used by Danguir and Nicolaides in unsuccessful attempts to obtain reconditioning of extinguished taste aversions.     

The effects of conditioned taste aversions on schedule-induced polydipsia: An analysis of the initiation and postpellet temporal distribution of licking

Animals poisoned following the schedule-induced consumption of saccharin initially continued to drink following spaced pellet deliveries. Neither the initiation of postpellet drinking (i.e., bout, initiation) nor the size and duration of the bouts was effected by the conditioned aversion procedure. With repeated conditioning trials (i.e., repeated pairings of saccharin and LiCl), schedule induced drinking was eventually reduced. The specific components underlying schedule-induced consumption, however, were differentially affected by the aversion training. Specifically, the decrease in schedule-induced drinking was effected primarily by a decrease in licking occurring between 10 and 60 sec after pellet delivery. Bout initiation and licking immediately postpellet (i.e., within the first 10 sec following pellet delivery) were most resistant to suppression and appeared to be responsible for the relative insensitivity of schedule-induced drinking to conditioned taste aversions. The differential effects of taste aversion conditioning on individual components of elicited behavior are discussed.     

Habituation to illness: Effects of prior experience with the US on the formation of learned taste aversions in rats

Experiment 1 investigated the effects of US habituation on the acquisition and extinction of learned taste aversions in rats. Subjects receiving five noncontingent LiCl intubations prior to conditioning failed to develop a conditioned taste aversion, while control subjects experiencing a single saccharin/LiCl pairing displayed a pronounced taste aversion which weakened during subsequent poisonings. Experiment 2 examined whether habituation, defined as a waning of responses to repeated presentation of an illness stimulus, was a possible mechanism for explaining the results of Experiment 1. Subjects showed a decrease in motor activity following an initial LiCl intubation, but less attenuation of activity with successive intubations.     

Conditioned flavor preferences as a function of deprivation level: Preferences or aversions?

Rats tend to prefer flavors previously consumed under low deprivation to flavors previously consumed under high deprivation (Capaldi & Myers, 1982). We attempted to distinguish among possible associative explanations by determining whether this conditioning phenomenon was based upon conditioned preferences, conditioned aversions, or both. We compared preference for flavors presented exclusively under either high or low deprivation with preference for a neutral flavor. In Experiments 1A and 1B the neutral flavor was one that had been randomly paired with both high and low deprivation, whereas in Experiments 2 and 3 the neutral flavors had not been associated with either high or low deprivation. Our results strongly suggest that this conditioning phenomenon is based upon an actual increase in preference for the flavor consumed under low deprivation rather than on any form of aversion conditioning.     

Evaluating conditioning of related and unrelated stimuli using a compound test

Three experiments used a compound test procedure to evaluate whether superior conditioning results from the pairing of stimuli that are related to each other. In each case, a stimulus compound was tested after its component conditioned stimuli (CSs) had been conditioned by the same unconditioned stimuli (USs) arranged such that either related or unrelated CSs and USs were paired. Experiment 1 explored auditory and gustatory stimuli conditioned by LiCl or shock, using rats. Experiments 2 and 3 used second-order conditioning in pigeons to pair stimuli that were similar by virtue either of qualitative features or of shared physical location. In each case, the compound test provided clear evidence that pairing related stimuli produces superior associative learning.     

Flavor preferences conditioned with starch in rats

Rats were trained to associate artificial cherry or grape flavors with 1% starch suspensions. Conditioning was assessed by offering the rats a choice of the cherry versus grape flavors without starch. Conditioned preferences were moderately strong and persistent; 3 days of conditioning produced a preference that did not fully extinguish within 18 days. Food deprivation substantially increased intake of 1% starch suspension, However, the degree of preference conditioned was not influenced by the availability of food during the conditioning period; the rats that had been food deprived during training acquired as strong a preference as did those fed freely during training. The degree of preference obtained was similar in the rats given reinforcing and nonreinforcing fluids simultaneously or sequentially. Starch conditioned a slightly stronger preference than did the same concentration of glucose, even though these substances contain the same amount of calories. A statistically significant, but weak, preference was conditioned by 0.5%, but not by 0.25%, starch. It is proposed that the flavor of starch, independent of its calories, is reinforcing to rats.     

The effect of nonreinforced stimulus exposure on the strength of a conditioned taste aversion as a function of retention interval: Do latent inhibition and extinction involve a shared process?

Two experiments examined the effects of nonreinforced flavor exposure on the strength of a conditioned taste aversion. Rats were conditioned by pairing maple flavor with LiC1. Prior to or subsequent to this pairing, some animals received nonreinforced exposure to either maple or saccharin. In separate subjects, preference for maple was tested 1 or 21 days after the last training episode. In the first experiment, the nonreinforced stimulus exposure occurred before conditioning (latent inhibition, or LI, procedure); in the second experiment, the nonreinforced exposure occurred after conditioning (extinction, or EXT, training). In both experiments, nonreinforced exposure to maple or saccharin reduced the magnitude of a conditioned maple aversion when testing occurred soon after conditioning. When testing was delayed, however, the attenuation due to nonreinforced saccharin exposure dissipated, both with the LI procedure and with EXT. In contrast, the nonreinforced exposure to maple was found to attenuate conditioned reactions at both short and long retention intervals. The release from generalized LI and spontaneous recovery following generalized EXT training are discussed in terms of retrieval processing. The possibility that the same mechanism may underlie LI and EXT is considered.     

The influence of the information value provided by prior-cuing treatment on the reactivation of memory in preweanling rats

In two experiments, the influence of exposure to a CS? on the acquisition and retention of a conditioned odor aversion was examined. Preweanling rats were given exposure to the CS? either prior to (CS?/CS+) or following (CS + /CS?) the pairing of a second odor (the CS+) with footshock. The results of Experiment 1 indicated that subjects in both of the treatment conditions acquired aversions of comparable strength to the odor paired with footshock and that retention of the odor aversion was not affected by order of stimulus presentation during conditioning. Experiment 2 indicated, however, that the effectiveness of pretest exposure to various elements of the conditioning episode in reactivation of the memory for conditioningwas dependent on the order of stimulus presentation during conditioning. This differential effectiveness of the various reactivation treatments is discussed in terms of their relationship to the associative “status” of the stimuli present during conditioning and in terms of the information provided to the animal by the reactivation treatment.     

Appetitive-aversion interactions: Facilitation of aversive conditioning by prior appetitive training in the rat

In Experiment 1, four groups of rats received conditioned suppression training in which a tone was reinforced with shock. If the tone had been previously paired with response-independent food, aversive conditioning was slightly facilitated by comparison to control groups preexposed either to the tone randomly associated with food or to the tone and food unpaired. However, by comparison to a control which was not preexposed to the tone, animals receiving prior pairings of the tone and food showed retarded aversive conditioning. Experiment 2 replicated the facilitation in aversive conditioning after the tone had been paired with food relative to the random control condition and demonstrated that this difference occurred even if the tone and background stimuli continued to be associated with response-independent food during aversive conditioning. This result suggests that pairing a stimulus with an appetitive reinforcer reduces the retardation of aversive conditioning produced by stimulus preexposure.     

Release from latent inhibition with delayed testing

A conditioned emotional response procedure was used to study the interactive effects of stimulus preexposure and retention interval in rats. In Experiment 1, the subjects were conditioned by presenting a light CS paired with mild footshock as the US. Half of the subjects were given nonreinforced preexposure to the CS, and the others were not. Separate preexposed and nonpreexposed groups were then tested 1,7, or 21 days after conditioning. Suppression of ongoing activity was used to assess the degree of conditioned fear. Latent inhibition was found at the 1-day retention interval; the preexposed subjects displayed less conditioned fear than did the nonpreexposed subjects. In contrast, equally strong conditioned fear was expressed by the preexposed and the nonpreexposed groups tested after the 7- and the 21-day retention intervals. These results indicate a release from latent inhibition similar to that obtained with conditioned taste aversions (Kraemer & Roberts, 1984). The results of Experiment 2 suggest that retention-interval-induced increases in sensitization, pseudoconditioning, or neophobia cannot account for the release from latent inhibition effect obtained in Experiment 1. The implications of these findings for a retrievaloriented view of latent inhibition are discussed.     

Conditioned flavor preferences based on hunger level during original flavor exposure

In five experiments, rats’ preference for a flavor was greater if the flavor had previously been consumed under low rather than high deprivation. This preference was conditioned in as few as three flavor-deprivation pairings (Experiment 1), and persisted through 28 test days, half under each deprivation level (Experiment 2). Rats never preferred the flavor associated with high deprivation even when calories were increased by giving 40 ml of 8% sucrose or when caloric density was increased to the equivalent of 20% sucrose. The preference for the low-deprivation flavor was greater when saccharin solutions were used rather than sucrose solutions, but the preference did emerge when sucrose solutions were used as testing proceeded and when a lower concentration of sucrose was used. We suggest that these preferences may be a result of flavor-taste associations rather than associations between flavors and postingestive consequences, and that the taste of the solutions under low deprivation is preferred to the taste under high deprivation.