Conditioning with taste as the CS in conditioned flavor preference learning

There appear to be unconditioned affective reactions to the four basic tastes: liking for sweet and salt and disliking for sour and bitter. We attempted to modify these reactions by pairing the tastes with calories and with sweeter tastes. Differing sucrose concentrations were ineffective in reversing the preference for salt over citric acid or for saccharin over quinine. We could, however, reverse the preference for salt over citric acid, producing an actual preference for sour over salt, by using sucrose and saccharin as the reinforcers. The initial reactions to tastes could also be modified by reducing the initial difference in affect produced by the tastes through mixing both tastes with sucrose.     

Taste and odor in conditioned flavor preference learning

Almond and peppermint extracts were combined with salt and citric acid as cues in conditioned flavor preference conditioning. In Experiment 1, extracts overshadowed tastes, although tastes and extracts conditioned equally well when presented in isolation. In Experiments 2 and 3, tastes and extracts were conditioned in isolation prior to conditioning of a taste/extract compound. The conditioning history of the tastes and extracts did not affect the overshadowing of taste by extract. The results of Experiment 4 showed that rats could learn to discriminate between a taste and extract presented in isolation vs. the taste/extract compound. Thus, extracts do not interfere with sensing the tastes. We suggest that a taste/extract compound produces a configural stimulus that is more characteristic of the extract than the taste.     

Possible role of confounded taste stimuli in conditioned taste aversions

In many publications, Sjödén, Archer, and their colleagues have studied conditioned taste aversions and found that in addition to taste, the bottle and/or spout containing a solution exerts strong control over the expression of an aversion. For example, Sjödén and Archer claim that this bottle stimulus will support a bottle-illness association even with long delays between the bottle and illness and after only a single conditioning trial. They have interpreted their results as indicating that contextual effects are important in taste-aversion learning. However, a confound in their procedure, stemming from the bottles they used, could explain their results in a simple way. Sjödén and Archer have emphasized that the bottle stimuli they used to distinguish between contexts consisted of one of two different sizes of drinking spouts, the larger of which made a clicking noise when licked. However, the larger spouts were always attached to plastic bottles and the smaller spouts were always attached to glass bottles. If discriminable tastes from the plastic bottles existed, then taste may have been inadvertently manipulated. Support for the likelihood of a plastic taste includes the seminal work on taste-aversion learning that stemmed from the serendipitous use of plastic bottles in the cage that rats were irradiated in and glass bottles in the home cages (see Garcia, McGowan, & Green, 1972). In these early demonstrations, rats learned to avoid the taste of water in the plastic bottles but not the taste of water in the glass bottles.     

Potentiation of taste by another taste during compound aversion learning

Two experiments with rat subjects examined whether a saccharin taste could potentiate the conditioning of an aversion to a salty taste when the two stimuli were presented together prior to lithium-induced illness. In Experiment 1, a 0.1% (w/v) saccharin solution potentiated conditioning of a very dilute (0.03%) NaCl solution, but had no demonstrable effect on two stronger NaCl solutions (0.6% and 1.2%). In Experiment 2, the 0.1% saccharin solution again potentiated the 0.03% NaCl target, but weaker and stronger saccharin concentrations (0.033% and 0.3%) did not. The ability of a taste to potentiate a secondtaste is not consistent with theories that assume that potentiation is unique to compounds composed of tastes and other, functionally different, nontaste cues. Potentiation may occur when the target stimulus is weakly conditionable on its own and when the particular combination of target and potentiator facilitates perceptual integration of the compound.     

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.     

Serial overshadowing of taste aversion learning by stimuli preceding the target taste

Three experiments tested whether events taking place before a rat has access to a target taste, sucrose, can proactively interfere with the acquisition of a sucrose aversion when sucrose is followed by a lithium chloride injection. Using a serial overshadowing procedure with various delays before lithium injection, proactive interference by a taste (Experiments 1 and 3) and by a novel context (Experiment 2) was found following two conditioning sessions, but not after a single conditioning session. Conversely, overshadowing by a taste given after the target was detectable after a single conditioning trial (Experiment 3) and, thus, indicated that retroactive interference involves a process different from that producing proactive interference. A simulation confirmed that the results are consistent with a modified Rescorla and Wagner (1972) interpretation of Revusky??s (1971) concurrent interference theory of delay learning.     

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.     

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.     

The effect of nonnutritive satiation on the learning of a flavor preference by rats

On each day of training in Experiment 1, hungry rats were given one flavored saccharin solution followed by a differently flavored saccharin solution. The rats drank more of the first flavor during training, but preferred the second flavor in a subsequent choice test. In Experiment 2, the two flavored saccharin solutions were provided on alternate days, with one flavor being preceded by nothing and the other flavor by plain saccharin. The rats drank more of the flavor preceded by nothing during training, but preferred the other flavor in a subsequent choice test. These results suggest that a state of nonnutritive satiation can reinforce a flavor preference.     

Effects of food deprivation on learning and expression of flavor preferences conditioned by saccharin or sucrose

In four experiments, food deprivation was varied during conditioning and testing of conditioning of flavor preferences by sweeteners. Conditioned preferences for a flavor associated with a more concentrated solution were enhanced by increased deprivation in training whether sucrose or saccharin was used when rats consumed solutions freely during training. When consumption of solutions was controlled and higher deprivation levels were used, preference for the higher concentration of sucrose was still enhanced by increased deprivation in training, but this did not occur with saccharin. We suggest that deprivation may enhance the reinforcing value of sweetness only when calories increase along with sweetness. We also suggest that deprivation can enhance flavor preference learning by increasing consumption and thereby increasing exposure to the flavored solutions.     

Circadian-temporal context and latent inhibition of conditioned taste aversion: Effect of restriction in the intake of the conditioned taste stimulus

Latent inhibition of conditioned taste aversion (CTA) is sensitive to changes in the temporal context. A change in the time of day of conditioning with respect to the time of day of the preexposure can disrupt the latent inhibition. This contextual change in the time of day may reveal a temporal specificity of latent inhibition. The optimum procedure to induce this temporal specificity is not well established. For example, it has been shown that a long period of habituation to temporal contexts is one factor that can determine the effect. However, the experimental conditions on the conditioning day that facilitate this phenomenon are unknown. The aim of this study is to elucidate whether a restriction in the intake of the conditioned taste stimulus affects the temporal specificity of latent inhibition. Two main groups of Wistar rats were tested in a latent inhibition of CTA paradigm, in which the temporal specificity of this phenomenon was analyzed by a change in the time of day of conditioning. The intake of the taste stimulus was restricted in the conditioning day in one of the groups, but this restriction was not applied in the other group. The results indicated temporal specificity of latent inhibition only in the group without restriction, but not in the group with limitation in the intake of the taste stimulus during conditioning. These findings can help to elucidate the characteristics of the procedure to induce temporal specificity of latent inhibition.     

A history of morphine-induced taste aversion learning fails to affect morphine-induced place preference conditioning in rats

Drugs of abuse have both rewarding and aversive effects, as indexed by the fact that they support place preferences and taste aversions, respectively. In the present study, we explored whether having a history with the aversive effects of morphine (via taste aversion conditioning) impacted the subsequent rewarding effects of morphine, as measured in the place preference design. In Experiment 1, rats were exposed to a taste aversion procedure in which saccharin was followed by morphine. Place preference conditioning was then initiated in which animals were injected with morphine and placed on one side of a two-chambered apparatus. Animals with a taste aversion history acquired place preferences to the same degree as controls without such a history, suggesting that morphine’s affective properties condition multiple effects, dependent on the specific stimuli present during conditioning. To determine whether these results were a reflection of processes operating in traditional associative conditioning, in a modified blocking procedure, place preference conditioning was attempted in the presence of a taste previously associated with morphine (Exp. 2). Under these conditions, animals still acquired morphine-induced place preferences comparable to those of animals without a morphine or conditioning history. These results are consistent with the position that drugs of abuse have multiple stimulus effects (positive and negative) that are differentially associated with specific stimuli (environmental and taste) that drive different behavioral responses (approach and avoidance).     

Slow reacquisition of a conditioned taste aversion

Rats were used to examine the extent to which extinction of an acquired conditioned taste aversion retards subsequent reacquisition. A saccharin-flavored solution (sac) was paired with LiCl and then followed by CS-alone extinction trials with this flavor. A control group received a different flavor, decaffeinated-coffee (coff), during initial conditioning and extinction. Sac was then paired with LiCl for all rats during a second conditioning phase. Reacquisition of the aversion to sac was retarded relative to the acquisition of an aversion to sac by the control group. A similar experiment with fewer extinction trials, but still with complete loss of the initial aversion, did not obtain slow reacquisition. The results are discussed with respect to an interference view of extinction and the slow-reacquisition effect.     

Cross-modality contrast: Exteroceptive context habituation enhances taste neophobia and conditioned taste aversions

The relationship between absolute and relative stimulus novelty was examined within the context of the conditioned taste aversion paradigm in which the relative novelty of the conditioned interoceptive stimulus was manipulated by differential exteroceptive context habituation. Rats received similar isolation histories but either 5 or 30 days of habituation to the test environment prior to treatment. One group was administered lithium chloride following saccharin consumption, a second group was administered isotonic saline following saccharin consumption, and a third group was administered saline after water consumption. The animals habituated for 30 days exhibited greater conditioned avoidance and greater neophobic avoidance of saccharin than did animals habituated for only 5 days. The results are interpreted in terms of a cross-modality stimulus contrast effect which implicates context habituation as an important parameter of both taste neophobia and taste aversion learning.     

Location preference and flavor aversion reinforced by amphetamine in rats

To associate the identical drug state with both a location and a flavor, rats were given intraperitoneal amphetamine injections and then confined for 20 min in one side of a shuttlebox with access to a flavored solution; on control trials without injections, they were confined for 20 min in the opposite side with a different flavor. In the first experiment, the rats were placed in the shuttlebox immediately after injections; in the second experiment, they were placed in the shuttlebox 20 min after injections. Subsequent free-choice tests in both experiments revealed an increased choice of the side of the shuttlebox associated with amphetamine but also an aversion to the flavored solution associated with the drug.     

Flavor-flavor associations induce hedonic shifts in taste preference

Two experiments allowed rats to drink freely two neutral flavors (almond and vanilla) in simultaneous compound with two hedonically valued flavors (quinine and saccharin). The neutral flavor previously paired with saccharin was subsequently preferred. The neutral flavor that had been paired with quinine was subsequently avoided. Experiment 3 found similar results when the animals were hand-fed a preset amount of the solution. Preference shifts were not obtained when differential amounts of the neutral flavors were consumed in isolation. The data indicate that flavor-flavor associations can shift taste preferences.     

Taste quality and extinction of a conditioned taste aversion in rats

Rats (Rattus norvegicus) that received a taste cue (saccharin, saline, quinine, or sucrose) paired with a lithium chloride (LiCl) injection displayed a robust decrease in consumption of that taste, relative to controls that had the taste unpaired with LiCl. Consumption of the paired taste increased with each nonreinforced presentation (i.e., extinction). After asymptotic extinction, rats that had had a 0.1% saccharin cue paired with LiCl consumed less of the saccharin solution than did controls. A similar data pattern was observed with a 10% sucrose solution. These results are consistent with the view that some aspect of the excitatory CS-US association remains after extinction. On the other hand, rats that had a bitter (0.005% or 0.001% quinine) or salty (1% or 0.5% saline) solution paired with LiCl drank similar amounts of the fluid as controls after asymptotic extinction treatment. Together, these experiments suggest that a taste that is either sweet or preferred is required in order to demonstrate the chronic decrease in fluid consumption after extinction treatment. The data suggest that the conditioning experience prevents the later development of a preference for the sweet taste, rather than there being a retained aversion that suppresses fluid consumption.     

Injected flavor as a CS in the conditioned aversion preparation

Three experiments were conducted to determine the effectiveness of intravenous (IV) flavor injections in the formation of conditioned taste aversions and in the attenuation of neophobia. In Experiment 1, two groups of rats were permitted to drink either a .1% saccharin solution or tap water followed immediately by IV injections of lithium chloride (LiCl), and two more groups were given IV injections of a 2% saccharin solution followed immediately by IV injections of either LiCl or distilled water. Injected flavor did not serve as an effective CS for the conditioning of an aversion to .1% saccharin. The second experiment employed a two-bottle procedure to detect attenuation of neophobia using the injected-flavor technique. It was found that, whether saccharin had been injected intravenously (2%), injected intraperitoneally (2% IP), or orally consumed (.1%), neophobia for .5% saccharin was attenuated equally relative to controls. CS-US intervals were manipulated in the final experiment such that IP injections of 2% saccharin solution were followed 0–480 min later by IP injections of LiCl. In this case, it was shown that injected flavor (2% saccharin) could act as an effective CS if the US was delayed (optimally about 120 min) and when the test solution was .1% saccharin. The delay gradient found in Experiment 3 was interpreted as a generalization gradient where optimum conditioning was displayed at the point where the concentration of saccharin circulating in the animal at the time of illness onset most closely matched the concentration of the test solution.     

Sexually dimorphic extinction of a conditioned taste aversion in rats

The existence of a sexual dimorphism in the extinction of conditioned taste aversions in both fluid-deprived and nondeprived rats was investigated. When nondeprived rats were poisoned with LiCl after consuming a sucrose solution, males extinguished the subsequent conditioned taste aversion more slowly than females. There was no difference in the rate of extinction of a LiCl-induced conditioned taste aversion in fluid-deprived rats. Using marihuana extract distillate as the toxin, it was shown that the dimorphism found in the nondeprived rats is not a toxin-specific effect, and does not appear to be due to the males being made more ill. Possible explanations for the appearance of the dimorphism and its relation to the deprivational state of the organism are discussed.