Potentiation by a novel flavour of conditioned place aversions based on both toxicosis and shock

Three experiments examined how a flavour modified the development of aversions to the place in which it was presented and paired with a reinforcer. Experiment 1 confirmed the cue-to-consequence effect: rats made sick after exposure to a flavour in a novel place (Skinner boxes) displayed stronger aversions than shocked animals when they were presented with the flavour in a second, familiar place; rats shocked after exposure to water acquired stronger place aversions than animals made sick. Experiment 2 confirmed the potentiation effect: rats made sick after exposure to the flavour developed stronger place aversions than those made sick after ingestion of water. This experiment also revealed that rats shocked after exposure to the flavour likewise acquired stronger place aversions than the animals shocked after ingestion of water. Experiment 3 replicated this flavour potentiation of place aversions based on shock. The results are not consistent with Garcia, Lasiter, Bermudez-Rottoni, & Deems' (1985) account of how a flavour interacts with an aversive reinforcer to modify what is learned about its associates in a compound.     

Representation-mediated extinction of conditioned flavor aversions

Conditioned flavor aversions were extinguished by presenting without consequence auditory stimuli that had been previously paired with the aversive flavor. In Experiment 1, rats that received tone-sucrose pairings, then sucrose-lithium chloride (LiCl) pairings, and finally repeated tone-alone presentations showed greater sucrose consumption in subsequent testing than rats that received similar sucrose-LiCl pairings and tone-alone presentations but no initial tone-sucrose pairings. Experiment 2 demonstrated the stimulus specificity of the mediated extinction observed in Experiment 1. In Experiment 3, rats that received first-order light-food and second-order tone-light pairings prior to sucrose-LiCl pairings did not show greater subsequent sucrose consumption when extinction of the second-order tone intervened. These results suggest that conditioned stimulus (CS)-evoked representations of events can substitute for those events themselves in the extinction of previously established associations.     

Acquisition of representation-mediated conditioned food aversions

Food aversions were established in rats by administering lithium chloride (LiCl) immediately after the presentation of an exteroceptive conditioned stimulus (CS) which previously had been paired with a food substance. In Experiment 1, rats which received first tone-food and then tone-LiCl pairings showed less food consumption in subsequent testing than rats which received only tone-food or tone-LiCl pairings. Experiments 2 and 3 demonstrated the stimulus specificity of aversions established in this manner. Rats which first received pairings of light and tone CSs with two different food substances and then received pairings of one of those CSs with LiCl showed greater aversion to the food previously associated with the LiCl-paired CS than to the other food substance. Experiment 3 also showed that specific aversions were not acquired if rats received CS-shock rather than CS-LiCl pairings. These results suggest that CS-evoked representations of events can substitute for those events themselves in the formation of new associations.     

Limitations on representation-mediated potentiation of flavour or odour aversions

Odour aversion learning is often potentiated in the presence of flavour stimuli. Establishment of an aversion to an odour is greater when an odour?+?flavour compound is paired with illness than when the odour alone is paired with illness. Holland () showed that under some circumstances auditory or olfactory stimuli previously paired with flavours may also potentiate odour aversion learning. The present experiments examined limitations on this representation-mediated potentiation of aversion learning. The results indicated that conditioned stimuli (CSs) that activate representations of potentiating cues are themselves immune to potentiation by other CS-activated representations, but remain susceptible to potentiation by their real stimulus associates.     

Limitations on representation-mediated potentiation of flavour or odour aversions

Odour aversion learning is often potentiated in the presence of flavour stimuli. Establishment of an aversion to an odour is greater when an odour + flavour compound is paired with illness than when the odour alone is paired with illness. Holland (1983) showed that under some circumstances auditory or olfactory stimuli previously paired with flavours may also potentiate odour aversion learning. The present experiments examined limitations on this representation-mediated potentiation of aversion learning. The results indicated that conditioned stimuli (CSs) that activate representations of potentiating cues are themselves immune to potentiation by other CS-activated representations, but remain susceptible to potentiation by their real stimulus associates.     

Anorexia and conditioned taste aversions in the rat

The present study examined whether or not certain anorexic agents produced flavor aversions. Rats showed comparable anorexia when injected with lithium chloride, ammonium sulfate, arginine·HCl, and d-glucose. Only lithium chloride produced a flavor aversion to either a novel liquid or food. Only arginine was found to interfere with the formation or recall of an association. The effects of the other chemicals were discussed in terms of the relationship between anorexia and induced sickness.     

Context dependency of conditioned aversions to familiar and novel fluids

Using a context discrimination procedure and rats as the subjects, the formation of context-dependent aversions to novel and familiar fluids was investigated. Experiment 1 revealed that context dependency could be established to a novel fluid (saccharin) after three cycles of context discrimination training and that the acquired context dependency was revealed also to a second familiar fluid (water) presented in the following test. Experiment 2 showed that the formation of the context-dependent aversion and its transfer to a second fluid was not affected by whether fluid presented during discrimination was novel (saccharin) or familiar (water). Experiment 3 demonstrated that when the same water was given both in the two training contexts and in the home cages of the subjects during discrimination, the context-dependent aversion formed was specific to it. These findings can be explained in terms of a simple summation effect of fluid-nausea and context-nausea associations.     

Extinction of conditioned taste aversions: Effects of concentration and overshadowing

Extinction of conditioned taste aversions was examined as a function of taste concentration and of the presence of an additional taste. The results of Experiment 1 were consistent with previous evidence in that a conditioned aversion to high concentration saline was more persistent in extinction than an aversion to a low concentration. However, when floor effects were avoided the rate of extinction was faster for the higher (1%) concentration than for 0.2% saline (Experiment 2), a result consistent with accounts of extinction in other preparations. Three further experiments examined extinction of a conditioned sucrose aversion. The addition of 1% saline, but not of 0.2% saline, to sucrose during extinction produced overshadowing ("protection from extinction"; Experiment 3). Such overshadowing by saline was detected after two, but not after a single extinction trial (Experiment 4). This last finding suggests that under the conditions of the present experiments sweet and salty tastes function as elemental stimuli competing for loss of associative strength. No overshadowing was found when almond (an aqueous odour) was used in place of saline as the added stimulus, even when high concentrations of almond were used that produced observable neophobia (Experiments 5A and 5B).     

A comparison of two methods of measuring conditioned taste aversions

Two methods of measuring conditioned taste aversions were compared. Following adaptation to a daily 10-min water drinking schedule, rats were made ill with an injection of cyclophosphamide after presentation of a novel-tasting fluid. Three days after this treatment, Ss were tested by (1) measuring the amount of the novel fluid consumed on the continuing 10-min access schedule or (2) measuring their preference between plain water and the novel fluid. When the drinking behavior of the conditioned animals was compared with that of appropriate controls, the preference method proved to be the more sensitive.     

Learning about cues in their absence: Evidence from flavour preferences and aversions

A simple behavioural task may involve the presentation of two or more stimuli. Any learning that takes place in such a situation may be analysed in terms of the formation of an association between the central representations of those stimuli. Presumably performance based on this learning can occur because presentation of one stimulus will then activate the representations of other stimuli that were previously presented with it. To examine the role that these representations play in learning in and of themselves requires that the stimuli themselves are absent. A review of a number of flavour preference and aversion studies indicates that an associatively activated stimulus representation can support learning that is both similar to and the opposite of that maintained by the presentation of the stimulus itself. Which occurs is dependent upon the nature of the reinforcer and the temporal relationships between the training events. Although this pattern of results appears suggestive of separate learning rules, a reanalysis raises the possibility of an explanation in terms of a single associative system.     

Temporal context and conditioned reinforcement value

The effectiveness of a stimulus as a conditioned reinforcer depends on the temporal context of reinforcement, that is, the overall rate of reinforcement in the situation. The dominant view has been that context determines the learned value of a stimulus directly, according to delay-reduction theory. By contrast, the contextual choice model (CCM) maintains that value is independent of context and incorporates the effects of context on choice in the framework of the matching law. The authors report 2 experiments with pigeons as subjects that use transfer tests to assess the value of stimuli in the concurrent-chains procedure. Results strongly support the assumption of CCM that pigeons learn the temporal relations between events independently of context but that context modulates the expression of that learning as choice.     

Time of day-dependent latent inhibition of conditioned taste aversions in rats

We have determined that the temporal context of drinking can modulate latent inhibition of learned saline aversions in Wistar rats by changing the time of day of drinking of the preexposure and conditioning phases. Latent inhibition was absent in the group preexposed and conditioned to saline at different times of the day, but not in the group that was preexposed and conditioned at the same time of day. The results confirm a previous report that the time of day can modulate taste aversion learning independently of other environmental cues. It is proposed that the features and duration of the habituation training to the temporal contexts used may be critical for time-dependent latent inhibition to appear.     

Potentiation of a conditioned taste aversion in preweanling and adult rats

Preweanling (18 days old) and adult rats were made ill with LiCl either 2 min or 1 hr after tasting controlled amounts of either one of two single flavors (saccharin or NaCl) or a compound mixture of the two. Conditioning was assessed with a single test 4 days later relative to explicitly unpaired control conditions. Generally, potentiation of the aversions to either flavor occurred for animals conditioned to the compound. The potentiation effect was decreased or eliminated by nonreinforced exposure to the alternative flavor of the compound. These effects tended to be stronger for the younger rats. Specifically, adult animals did not express potentiation of the saccharin aversion whereas preweanlings expressed potentiated salt aversions. Nonreinforced exposure to the alternative element eliminated the potentiation effect. Conditions conducive to potentiation are discussed in light of investigators who have not observed this effect in similar studies with compound stimuli.     

A re-examination of one-trial blocking in conditioned suppression

Three experiments on the conditioned suppression of licking in rats examined the amount of conditioning produced by a single conditioning trial to a clicker-light compound and the effect of prior conditioning to the light on the level of conditioning to the clicker. In Experiment I, prior conditioning to the light, far from blocking conditioning to the clicker, actually enhanced it, whether the clicker was presented in a simultaneous compound with the light or in a serial compound preceding the light. Experiment II, however, showed that this potentiation effect could be abolished if a trace interval was inserted between the clicker and light in the serial compound arrangement. Moreover, Experiment III demonstrated a significant blocking effect when a trace interval separated the clicker and light on the single compound trial. These results establish that one-trial blocking of conditioned suppression is possible, and suggest that in some earlier studies blocking may have been masked by higher-order conditioning to the target stimulus.     

Taste aversions conditioned by the aversiveness of insulin and formalin: role of CS specificity

Experimenters in the past have reported that when insulin is used as the unconditioned stimulus (US), rats will learn an aversion to a sodium chloride but not a sucrose solution, whereas with formalin as the US, they will learn an aversion to a sucrose but not a saline solution. The present experiments failed to confirm these findings. Aversions to sucrose were conditioned with insulin and aversions to sodium chloride were conditioned with formalin. The use of a more concentrated sucrose solution in the present study may have been responsible for the successful sucrose-aversion conditioning with insulin. Although the source of the discrepancy in findings concerning aversion conditioning with formalin remains unclear, experiments ruled out numerous possibilities. These experiments also showed that sodium chloride aversion conditioning with formalin is a highly robust phenomenon that occurs with a variety of conditioned stimulus durations and formalin doses, with distributed and massed training, in male and female rats, and even if saline is not the only novel solution presented during conditioning. Furthermore, the aversion can be detected with both single-stimulus and choice test procedures.     

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.     

Effects of endorphin blocking on conditioned SCR in humans

In order to test the hypothesis that low levels of endogenous opioids (endorphins) predispose to strong conditioning effects, female Ss (N = 36) were assigned to a placebo group, a low-dose naltrexone group, or a high-dose naltrexone group and then underwent a classical conditioning procedure. This procedure consisted of an acquisition phase in which all Ss received 5 pairings of a CS+ (neutral picture) and a UCS (100 dB white noise). The CS− (neutral picture) was never followed by a UCS. During extinction, Ss received 4 unreinforced presentations of CS+ and CS−. Throughout the experiment, skin conductance responses (SCRs) to the CSs and UCSs were recorded. Acquisition was successful in that CS+ slides elicited stronger SCRs than CS − slides. However, during acquisition, there was no interaction between drug and differential response (CS+ vs CS−). During extinction, there was no overall remaining effect of conditioning. Again, no evidence was found to suggest that (remaining) effects of conditioning were stronger in the naltrexone treated Ss than in the placebo S's. If anything, the opposite seemed to be true with especially high-dose naltrexone Ss showing relatively weak conditioning effects.     

Within-compound associations between the context and the conditioned stimulus

Three experiments were conducted to test for the presence of associations between contextual cues and the nominal conditioned stimulus (CS) in fear conditioning. Rats were given tone-footshock pairings and were tested for their fear of the nominal CS, the tone, in a different context. Experiments 1 and 2 demonstrated that rats given nonreinforced exposure to the training context following conditioning were less fearful of the CS. Experiment 3 indicated that additional footshock presentations in the training context following conditioning produced greater fear of the CS. In both procedures postconditioning treatments designed to directly alter only the associative strength of the training context yielded parallel changes in the conditioned response to the CS. These data suggest that within-compound associations are formed between the context and the CS during classical conditioning.