Microstructural analysis of conditioned and unconditioned responses to maltodextrin

The microstructure of licking responses was analyzed to investigate the interaction between unconditioned responses to maltodextrin and the responses to flavor cues previously associated with maltodextrin. Experiment 1 demonstrated that although the consumption of maltodextrin peaked at intermediate concentrations, the mean lick cluster size showed a positive, monotonic increase with concentration. In Experiment 2, a (conditioned stimulus) CS+ flavor was paired with 16% maltodextrin, whereas a CS- flavor was paired with 2% maltodextrin. During test, consumption of the CS+ was higher than that of the CS- when the flavors were combined with 2% maltodextrin, but not when combined with 16% maltodextrin. In contrast, cluster size was larger with the CS+ than with the CS-, regardless of the concentration of maltodextrin present on test. Previous analyses of licking microstructure indicate that cluster size reflects the palatability of the ingested solution. Thus, the present results indicate that flavor conditioning can change the palatability of the cue flavors. Adding the CS+ flavor to maltodextrin produced results analogous to increasing the concentration of maltodextrin (in terms of both consumption and licking microstructure measures), which is consistent with the idea that after conditioning, responses to the CS+ flavor and to the unconditioned stimulus are mediated via the same representation.     

Latent inhibition of conditioned disgust reactions in rats

The present experiments, using the latent inhibition (LI) paradigm, evaluated the effect of nonreinforced exposure to saccharin on the acquisition of an LiCl-induced saccharin aversion as measured by conditioned disgust reactions in the taste reactivity test and conditioned taste avoidance in a consumption test. When rats were preexposed to saccharin by bottle exposure (Experiments 1 and 3), LI was evidenced only by conditioned taste avoidance (bottle testing), but not by conditioned disgust reactions (intraoral [IO] testing). On the other hand, when rats were preexposed to saccharin by IO infusion (Experiments 2 and 3), LI was evidenced only by conditioned disgust reactions, but not by conditioned taste avoidance. Experiment 4 showed that LI of conditioned disgust reactions does not appear to be affected by a context shift from preexposure to testing phases. These results show that the expression of LI of both conditioned taste avoidance and conditioned disgust reactions depends critically on a common method of flavor exposure during preexposure and testing.     

A learned flavor preference persists despite the extinction of conditioned hedonic reactions to the cue flavors

Learned flavor preferences can be strikingly persistent in the face of behavioral extinction. Harris, Shand, Carroll, and Westbrook (2004) suggested that this persistence may be due to flavor preference conditioning’s producing a long-lasting change in the hedonic response to the conditioned stimulus (CS+) flavor. In the present study, the CS+ flavor was presented in simultaneous compound with 16% sucrose, whereas the CS− flavor was presented with 2% sucrose. During subsequent two- and one-bottle tests, the CS+ and CS− flavors were presented in 2% sucrose. Hedonic reactions during training and test were assessed using an analysis of the microstructure of licking behavior. Conditioning resulted in greater consumption of the CS+ than of the CS− that did not extinguish over repeated two- and one-bottle tests. The mean lick cluster size was higher for the CS+ than for the CS− only on the first cycle of tests. Since lick cluster size can be used as an index of stimulus palatability, the present results indicate that although the hedonic reaction to the CSs did change, this was not maintained across repeated tests. Thus, changes in the hedonic response to the conditioned flavors cannot explain the resistance to the extinction of learned flavor preferences.     

Extinction of a saccharin—lithium association: Assessment by consumption and taste reactivity

Extinction of a conditioned palatability shift preceded extinction of conditioned taste avoidance whether rats were tested using a within-subjects design or a between-subjects design. In each of two experiments, consumption of 0.1% saccharin was paired with either 20 ml/kg of 0.15 M LiCl or equivolume physiological saline on a single trial. In Experiment 1, on each of 10 extinction trials, rats were given a taste reactivity test immediately prior to a consumption test. In Experiment 2, half of the rats were extinguished by taste reactivity testing and half of the rats were extinguished by a consumption test on each of 10 extinction trials. In both experiments, the aversive reactions of gaping and passive dripping were extinguished in a single trial and the suppression of ingestive reactions was extinguished in 2 trials; however, extinction of taste avoidance required 4–5 trials. These results suggest that rats continue to avoid a lithium-paired flavor even when they do not have an aversion to the taste.     

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.     

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.     

Impact of brief or extended extinction of a taste aversion on inhibitory associations: Evidence from summation,retardation, and preference tests

In five conditioned taste aversion experiments with rats, summation, retardation, and preference tests were used to assess the effects of extinguishing a conditioned saccharin aversion for three or nine trials. In Experiment 1, a summation test showed that saccharin aversion extinguished over nine trials reduced the aversion to a merely conditioned flavor (vinegar), whereas three saccharin extinction trials did not subsequently influence the vinegar aversion. Experiment 2 clarified that result, with unpaired controls equated on flavor exposure prior to testing; the results with those controls suggested that the flavor extinguished for nine trials produced generalization decrement during testing. In Experiment 3, the saccharin aversion reconditioned slowly after nine extinction trials, but not after three. Those results suggested the development of latent inhibition after more than three extinction trials. Preference tests comparing saccharin consumption with a concurrently available fluid (water in Experiment 4, saline in Experiment 5) showed that the preference for saccharin was greater after nine extinction trials than after three. However, saccharin preference after nine extinction trials was not greater, as compared with that for either latent inhibition controls (Experiments 4 and 5) or a control given equated exposures to saccharin and trained to drink saline at a high rate prior to testing (Experiment 5). Concerns about whether conditioned inhibition has been demonstrated in any flavor aversion procedure are discussed. Our findings help explain both successes and failures in demonstrating postextinction conditioned response recovery effects reported in the conditioned taste aversion literature, and they can be explained using a memory interference account.     

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.     

Neophobias and conditioned taste aversions in rats following exposure to novel flavors

Rats were successively exposed to three solutions with distinctively different flavors and then tested for both neophobia and propensity to form conditioned taste aversions to a fourth distinctively flavored solution. All permutations between the four solutions (salty, bitter, sweet, and sour) were examined. The prior exposures resulted in attenuation of neophobia to novel salty and sour solutions, but not to equally novel bitter or sweet solutions. These effects were found to depend upon thediversity of the prior ingestive events rather than upon either a single specific flavor experience or a summation of the reductions in generalized neophobia accrued by each substance separately; both of the latter findings are inconsistent with stimulus generalization being responsible for the observed attenuation of neophobia to salty and sour solutions following exposure to diverse different solutions. A further test of generalization between the salty and bitter solutions, consisting of associating one flavor with poison and extinguishing the avoidance response in half the animals prior to testing for generalization of conditioned taste aversion to the other flavor, also proved negative. Although these effects of exposure to flavors distinctly different from the test solution may be dependent upon solution concentrations, further research found that the same pretest exposures and same test concentrations failed to inhibit formation of conditioned taste aversions. A demonstration of “latent inhibition” attested to the sensitivity of our procedure to potential interference with acquisition of conditioned taste aversions. The results are considered in light of the relationship between neophobia and conditioned tasted aversions, the differential biological relevancy of specific tastes, and abstraction as a cognitive capability of rats. The possibility is raised that the defense against toxins is not the primary function of neophobia.     

Taste potentiates color-sickness associations in pigeons and quail

The present experiments assessed cue utilization in pigeons and quail on similar tests of poison-based aversion learning. In Experiments 1 and 2, three groups of pigeons were given colored water, flavored water, or colored flavored water prior to induction of sickness; these experiments differed only as to the specific colors and flavors used as stimuli. In both experiments, the birds trained with flavored water exhibited reliable taste aversions when tested with uncolored flavored water. Similar degrees of aversion were observed whether the flavored water had been colored or uncolored during training, suggesting that the color cue had little or no effect on the conditioning of the flavor cue. In contrast, the flavor cue had a pronounced effect on the conditioning of the color cue. When tested with unflavored colored water, the birds trained with colored flavored water exhibited significantly stronger color aversions than those trained with unflavored colored water. That is, the flavor cue enhanced or potentiated the conditioning of the color cue. In a third experiment, quail were trained in the same way as the pigeons with virtually the same result. The pattern of cue utilization observed in the present experiments with pigeons and quail differs markedly from that proposed by Wilcoxon, Dragoin, and Kral (1971) for quail. However, a reexamination of the results obtained by Wilcoxon et al. suggested that they are susceptible to an alternative interpretation consistent with the present results.     

Inhibitory associations between neutral stimuli in flavor-aversion conditioning

In Experiments 1 and 2, rats were exposed to two compound flavors, AX and BX, containing one flavor in common (X). Following this exposure phase, an aversion was conditioned to A in the experimental group by pairing its consumption with an injection of lithium, while a control group drank A without being poisoned. The effect of this treatment was to establish B as a conditioned inhibitor. In Experiment 1, experimental animals were slower than controls to condition an aversion to B when its consumption was paired with lithium (a retardation test of conditioned inhibition). In Experiment 2, B alleviated the suppression of intake of another flavor previously paired with lithium (a summation test). Experiments 3 and 4 established that these effects depended upon prolonged prior exposure to AX and BX.     

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.     

Second-order conditioning detects unexpressed morphine-induced salt aversion

Morphine failed to condition a salt taste aversion at a dose (15 mg/kg) sufficient to produce a robust aversion to a saccharin taste. Indeed, three different concentrations of salt (1%, 1.5%, and 2%) paired with the same morphine dose yielded no direct evidence for conditioned aversion. Yet, when a novel saccharin taste was paired in compound with the previously conditioned salt conditioned stimulus, we found evidence for a conditioning to the saccharin cue alone in three separate experiments. Control groups eliminated alternative accounts such as neophobia and differential exposure to morphine. Combined, these findings indicate that morphine conditioned a salt aversion. Although this aversion was not directly expressed, a second-order conditioning procedure was able to provide a more sensitive index of conditioning.     

Taste avoidance and taste aversion: Evidence for two different processes

The terms conditioned taste avoidance and conditioned taste aversion are often used interchangeably in the literature; however, considerable evidence indicates that they may represent different processes. Conditioned taste avoidance is measured by the amount that a rat consumes in a consumption test that includes both appetitive phases and consummatory phases of responding. However, conditioned taste aversion is more directly assessed with the taste reactivity test, which includes only the consummatory phase of responding. Rats display a conditioned taste aversion as conditioned rejection reactions (gapes, chin rubs, and paw treads) during an intraoral infusion of a nausea-paired flavored solution. Treatments that produce nausea are not necessary for the establishment of taste avoidance, but they are necessary for the establishment of taste aversion. Furthermore, treatments that alleviate nausea modulate neither the establishment nor the expression of taste avoidance, but they interfere with both the establishment and the expression of taste aversion. Considerable evidence exists indicating that these two measures are independent of one another. Taste avoidance may be motivated by conditioned fear rather than conditioned nausea, but taste aversion (as reflected by rejection reactions) may be motivated by conditioned nausea.     

Extinction of the overshadowing CS after overshadowing in conditioned taste aversion

Using a conditioned taste aversion preparation overshadowing of flavor-illness association was produced through the presentation of a second flavor during the interval between the first flavor and illness. The modulatory effects of extinguishing the association between the second (over-shadowing) flavor and illness on conditioned responding to the target flavor was investigated. In Experiment 1, we found that, following one-trial overshadowing, extinction of the overshadowing flavor had no effect on conditioned responding to the target flavor. In Experiment 2, we found a similar absence of an effect of extinction of the overshadowing stimulus in a multitrial over-shadowing paradigm. Experiment 3 confirmed the results of Experiments 1 and 2 using conditioning parameters that were designed to weaken the association between the overshadowed flavor and illness. In Experiments 4 and 5, we used simultaneous presentation of the flavors during conditioning and obtained a weakened aversion to the overshadowed flavor when the overshadowing CS was extinguished. These findings are inconsistent with previous observations in conditioned fear preparations that suggest that extinction of the association between the overshadowing stimulus and the unconditioned stimulus attenuates overshadowing. Possible reasons for the discrepant results are discussed.     

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.     

Potentiation of taste and extract stimuli in conditioned flavor preference learning

In these experiments, we investigated the nature of potentiation in the conditioned flavor preference paradigm. Almond and banana extracts, which have strong odor components, were combined with salt and saccharin (liked tastes; Experiment 1) or quinine and citric acid (disliked tastes; Experiment 2) in a flavor preference procedure that mixed these solutions with a caloric reinforcer (polycose). The results showed that liked tastes potentiated preference conditioning to extracts (Experiment 1), whereas extracts potentiated preference conditioning to disliked tastes (Experiment 2). In both experiments, the presumably less liked stimulus (i.e., the extract in Experiment 1 and the disliked taste in Experiment 2) was the potentiated cue.     

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.     

Generalized conditioned flavor aversions: Effects of toxicosis training with one flavor on the preference for different novel flavors

In two experiments, experimental rats were trained to have strong aversions to one of 10 novel-flavored solutions through contingent lithium injections; control rats were injected with lithium in the absence of prior consumption. Each animal was then tested with each of the 10 solutions, and the preferences of the experimental rats and the control rats were compared. A generalization gradient of the aversion to each conditioned aversive flavor was obtained for each test flavor.     

An extinction cue reduces spontaneous recovery of a conditioned taste aversion

The effect of an auditory cue presented during extinction on spontaneous recovery of a conditioned taste aversion was investigated in three experiments. Experiment 1 demonstrated that the presence of the cue during extinction did not influence saccharin consumption during that phase, and that an aversion to saccharin in the absence of the cue was stronger at 18 days than at 1 day after extinction, representing spontaneous recovery rather than a renewal effect. Experiment 2 showed that a cue presented during extinction and testing reduced spontaneous recovery. Experiment 3 replicated that effect and showed that it depended on the cue’s correlation with extinction and not on an unconditioned effect; cues that had been presented during or prior to conditioning did not reduce spontaneous recovery when presented during testing. The cue’s potential to reduce spontaneous recovery through conditioned inhibition or configural cue learning is discussed, as is the possibility that the cue retrieves a saccharin extinction memory in a manner consistent with Bouton’s (1993) account of spontaneous recovery.