Enhancement of food aversion by exposure to a poisoned conspecific in Norway rats (Rattus norvegicus)

Two experiments were conducted to determine whether exposure to a poisoned conspecific enhances prior food aversion in rats. In Experiment 1, subjects were serially exposed to two foods, cocoa‐flavored and cinnamon‐flavored ones, and were then poisoned 1 h later. On the next day, they were exposed to a poisoned conspecific that had eaten a cocoa‐flavored food. On the subsequent choice test, subjects had an enhanced aversion to cocoa‐flavored food. The result was replicated in Experiment 2, in which a cinnamon‐flavored food was assigned as a target. The results are discussed in relation to previous findings.     

Food choice in rats (Rattus norvegicus): the effect of exposure to a poisoned conspecific

Three experiments were conducted to examine the effects of exposure to a poisoned conspecific on subsequent food aversion in rats. In Experiment 1A, rats that had been aversively conditioned to a cocoa-flavored food were exposed to a poisoned conspecific that had eaten the same food. On the subsequent choice test, the animals increased their aversion to that food. These results were reconfirmed in Experiment 1B, in which a cinnamon-flavored food was used as the stimulus. In Experiment 2, subjects were first exposed to a poisoned conspecific and then conditioned to the food which the conspecific had eaten. On the test, they showed no sign of increased aversion to that food.     

Long-delay taste aversion learning in an unpoisoned rat: Exposure to a poisoned rat as the unconditioned stimulus

M. J. Lavin, B. Freise, and S. Coombes (Behavioral and Neural Biology, 1980, 28, 15–33) have shown that if two rats consume a flavored solution and one is poisoned, the unpoisoned partner will also exhibit a flavor aversion during a later preference test. Experiments 1 and 2 showed that a sufficient condition for obtaining this aversion is that the poisoned partner be present with the unpoisoned rat after it has consumed the flavored solution. It is not necessary that the poisoned partner be present when the flavored solution is consumed or indeed have had any exposure to the flavored solution. Experiment 3 showed that the unpoisoned partner can exhibit a flavor aversion when there is a temporal gap of as long as 6 hr between consumption of the flavored solution and exposure to the poisoned rat.     

Retention of Conditioned Inhibition Produced by Extinction

Rats in a flavor aversion procedure were used to examine the extent to which an extinguished CS can pass a retardation and summation test for conditioned inhibition after either a short or a long retention interval. Rats were given a single pairing of a flavored solution with LiCl, and then received a large number of CS-alone extinction trials. Following extinction, some subjects received a 3-week retention interval following extinction while others did not. Extinction produced evidence of conditioned inhibition on both tests when a short retention interval occurred following extinction. However, conditioned inhibition was attenuated by the long retention interval when assessed with a retardation test, but no effect of the long retention interval was observed when inhibition was evaluated with a summation test.     

Effects of prior exposure on conditioned taste aversion in the rat: androgen- and estrogen-dependent events

The effects of preexposure and gonadal hormone manipulation on the extinction of a conditioned taste aversion were investigated. In Experiment 1, male rats were given one prior exposure to sucrose at some selected time (Days 4, 2, or 1) before a second exposure (Day 0) to sucrose and an LiCl injection. Controls received no prior exposure to sucrose but experienced only the sucrose + LiCl condition (Day 0). under the single exposure condition (Day 0) castrated animals extinguished the aversion faster than either testosterone-treated castrated rats or sham-operated rats. Sham-operated and castrated rats that had received preexposure treatment (Days 4, 2, or 1) were not distinguishable on a within-groups comparison of behavior but differed from their respective controls by exhibiting faster rates of extinction. However, in the testosterone-treated group only the Day 4 preexposure group exhibited a faster extinction rate. In Experiment 2, estradiol, dihydrotestosterone, and testosterone were studied by using only a Day 1 preexposure condition. The testosterone-treated group maintained the aversion for the longest period of time, followed by dihydrotestosterone-treated, sham, castrated, and estradiol-treated groups. It appears that estradiol augments the castration effect whereas dihydrotestosterone attenuates the effect. In Experiment 3, estradiol was administered alone or in combination with two different doses of dihydrotestosterone, and a Day 1 preexposure condition was used. The findings indicate that the outcome of behavior is dependent on the ratio of estradiol to dihydrotestosterone, with variations in this ratio resulting in fast (estrogen effect) to slow (androgen effect) rates of extinction.     

The CS-preexposure effect in conditioned taste-aversion learning in golden hamsters

In Experiment 1, golden hamsters were injected with either 0.9% saline or the nausea-inducing agent, lithium chloride (LiCL), immediately after consuming a flavored diet that was either novel or familiar. The LiCl-induced aversion was strong in hamsters for which the flavored diet was novel, but no significant aversion was observed in hamsters that were familiar with the flavored diet. In Experiment 2, the strength of the LiCl-induced aversion was related inversely to the amount of conditioned-stimulus (CS) preexposure and directly to the duration of the preexposure-conditioning interval. Thus, although some previous researchers have suggested that hamsters may not demonstrate the CS-preexposure effect in a conditioned taste-aversion paradigm, they clearly did so under the conditions of the present experiments, and moreover, the characteristics of the CS-preexposure effect in hamsters were generally similar to those observed in rats.     

Non-specific effect of fear conditioning and specific effect of social defeat on social recognition memory performance in female rats

The so called "emotion learning" literature describes the ability of distressing and aversive unconditioned stimuli to classically condition a learned avoidance response. In order to investigate the impact of experience with noxious stimuli in one conditioning context on learning and memory performance in a separate, non-aversively motivated task, juvenile recognition ability was examined in adult female rats exposed previously to one of two environmental stressors. In particular, experimental adult rats were either socially defeated by exposure to an aggressive conspecific rat or fear conditioned using single or multiple pairings with footshock prior to performance of the social recognition task. Experiment 1 established that repeated exposure to a single juvenile resulted in social memory formation reflected in decreased social investigation from the first to the second exposure. Experiment 2 documented that both single and multiple pairings of an environment with footshock produced robust freezing behavior (90-95% suppression of activity). In addition, fear conditioning produced a non-specific 5-60% increase in social investigation time in both single and multiple-pairing fear conditioned groups which confounded the ability of the social recognition measure to assess effects of fear conditioning on learning and memory performance per se. In contrast, Experiment 3 documented that when social recognition memory performance was impaired to 85% of control levels by imposition of a 2 h delay, exposure to a social defeat stressor reinstated optimal social recognition memory performance. These findings suggest that the after effects of fear conditioning include non-specific alteration of social investigation whereas exposure to conspecific aggression enhances subsequent social recognition memory.     

The effect of three procedures for eliminating a conditioned taste aversion in the rat

Although extinction procedures have been the most common techniques used to eliminate conditioned taste aversions, several studies have employed postconditioning exposure to the US instead. To date, a comparison of the effectiveness of these two techniques has not been possible, because there were differences in the conditioning phases of these studies. In the present study, after an aversion to saccharin had been established, rats were administered 1, 5, or 10 extinction trials, 1, 5, or 10 postconditioning exposures to the US, or a period of no treatment. Then, all rats received seven two-bottle extinction tests. Five or ten extinction trials reduced the aversion to saccharin most effectively. Five or ten postconditioning presentations of the US were also effective. However, this effect was not noticeable until the fourth of seven test trials, suggesting an elimination procedure-test trials interaction. Neither the extinction nor the US postexposure procedure completely eliminated the aversion. In addition: (i) a single postconditioning exposure to either the CS or the US was ineffective in attenuating the aversion; and (ii) when no postconditioning treatment was administered, the strength of the aversion was undiminished for 20 days.     

Flavour-odour compound conditioning: Odour-potentiation and flavour-attenuation

Five experiments employed a toxiphobia conditioning paradigm to examine the strengths of odour and flavour aversions when conditioned separately and in compound. When conditioned in compound, odour aversions were stronger than when conditioned separately, i.e., the flavour potentiated the odour (Experiment Ia), but flavour aversions were weaker than when conditioned separately, i.e., the odour attenuated the flavour (Experiment Ib). The duration of exposure to the reinforced compound governed the nature of the interaction between the components: at a brief exposure, the flavour overshadowed the odour; at a long exposure, the flavour potentiated the odour (Experiment II). The remaining experiments examined the mechanism subserving the potentiation effect. Experiment III demonstrated that extinction of the flavour associate of the odour attenuated the odour aversion but further conditioning of the flavour did not strengthen the odour aversion. Experiment IV confirmed this effect of extinction but also found a comparable attenuation of the odour aversion from extinction of a separately conditioned flavour. Experiment V examined the previous failure to influence the strength of the odour aversion by strengthening the flavour aversion. In this experiment, conditioning the flavour associate or a separately conditioned flavour with a more potent US augmented the strength of the odour aversion. The results did not provide support for the idea that the potentiation phenomenon reflects the formation of within-compound associations but did indicate that a potentiated odour aversion could be modulated by manipulations designed to alter the US representation.     

The effects of language conditioning on various aspects of anxiety

Staats' three-function learning theory provides the basis for investigating the effects of emotionally-relevant self-verbalizations (SV) on the physiological, subjective-affective and behavioural aspects of anxiety. Using aversive electric UCSs and slides of snakes (CS), anxiety was classically conditioned in 88 volunteer Ss. In 20 subsequent language-conditioning trials (without aversive electric UCSs), the same snakes slides were paired with UCS verbalizations having either positive or negative connotative meanings. Half of the Ss were exposed to a living snake prior to language conditioning. The results show a complete extinction of the conditioned anxiety response in groups with positive SV whereas negative SV impeded extinction; the latter effect could only be found in groups without exposure to snakes prior to language conditioning. In general, the affective evaluation of snakes improved in groups with positive SV and deteriorated in groups with negative SV. However, these effects were more pronounced in groups without exposure to snakes. Although the results indicate that Ss in all groups with positive SV exhibited more approach behaviours than Ss with negative SV, this trend was not statistically significant. The possible relevance of the results for a language-conditioning approach to anxiety reduction is briefly discussed.     

Characterization of a dose-response curve for nicotine-induced conditioned taste aversion in rats: relationship to elevation of plasma beta-endorphin concentration

In the first experiment a conditioned taste aversion paradigm was used to characterize a dose-response curve for the aversive properties of nicotine in male Sprague-Dawley rats. Doses of nicotine ranging from 0.01 to 0.46 mg/kg, 2.0 ml of 0.47 M lithium chloride, or saline were injected, ip, 10 min after exposure to a novel saccharin solution. Amount of saccharin consumed in a two-bottle test was assessed 72 h later. Nicotine doses of 0.046 mg/kg and above produced a significant degree of conditioned taste aversion. In a second experiment, four groups of 10 rats each were injected with saline, 0.022 mg/kg nicotine, 0.46 mg/kg nicotine, or 2.0 ml 0.47 of M LiCl. Doses of 0.46 mg/kg nicotine and 0.47 M LiCl elevated plasma beta-endorphin concentrations significantly above saline control values. The 0.022 mg/kg dose, the highest dose that did not produce conditioned taste aversion in Experiment 1, did not significantly increase plasma beta-endorphin concentrations. This finding suggests that doses of nicotine that produce conditioned taste aversion also promote the release of pituitary stress hormones. Taken together these data suggest that some of the pharmacological and behavioral effects attributed to nicotine, including the release of endogenous neuromodulators, may be dose-dependent concomitants of the aversive effects of nicotine in nicotine-naive animals.     

Signalling and incentive processes in instrumental reinforcer devaluation

We have previously reported that conditioning an aversion to the reinforcer using an isotonic lithium chloride (LiCl) solution following instrumental training reduces performance in a subsequent extinction test only if animals are re-exposed to the reinforcer prior to the test. Rescorla (1992), in contrast, reported an immediate devaluation effect using a hypertonic LiCl solution that did not depend upon re-exposure. In two experiments we examined the effect of using a hypertonic LiCl solution to condition the aversion to the reinforcer on subsequent instrumental performance in extinction, with and without re-exposure. In Experiment 1 thirsty rats were trained to press a lever for a sucrose solution before being injected with 0.6 M LiCl either immediately or after a delay. Half of the immediate and delay groups were then re-exposed to the sucrose in the absence of the lever, with the remainder being exposed to water. Contrary to the previously reported effects of isotonic LiCl, a hypertonic solution induced a reinforcer devaluation effect in all the immediately poisoned animals, which did not depend upon re-exposure to the reinforcer. In Experiment 2 the possibility that this devaluation effect was induced by the discomfort associated with the hypertonicity of the solution was assessed by replicating Experiment 1 but, in addition, using two immediately poisoned groups given the LiCl injection under anaesthesia. In the absence of anaesthesia, the devaluation effect observed without re-exposure to the reinforcer in Experiment 1 was replicated. When the injection was given under anaesthesia, however, a reinforcer devaluation effect was observed only in animals that were re-exposed to the reinforcer prior to the extinction test. These results were interpreted as evidence that a reinforcer devaluation effect induced by pairing the reinforcer with illness depends upon a process of incentive learning, whereas a devaluation effect mediated by learning a signalling relationship between the reinforcer and somatic discomfort does not.     

Overshadowing of a context aversion by a novel incentive in operant conditioning

Two experiments examined the processes underlying the suppression of instrumental behaviours by lithium in rats, as reported by Meachum (1988 and this issue). Experiment 1 examined whether presenting a novel sucrose solution prior to lithium chloride administration would overshadow aversion learning to either the stimuli of the operant chamber or to familiar food pellets. After lever pressing had been established, and in the absence of responding, animals received free deliveries of a novel sucrose solution, familiar food pellets, or both, or they were exposed to only the cues of the operant chamber, prior to lithium injections. Lever pressing for food pellets was then assessed. It was found that the animals receiving the novel sucrose, either alone or with the familiar food pellets, pressed more for pellets than either the group receiving only food pellets or the group exposed to only the context. In addition, there was no appreciable difference in the response rates between the context-only group and the group that received the familiar food pellets. These outcomes were interpreted in terms of the novel sucrose overshadowing aversion learning to the context. Experiment 2 investigated whether in fact aversive contextual conditioning could be obtained using the present parameters. This was accomplished by directly manipulating the contexts. In this experiment animals were trained to lever press in two distinctive contexts. Subsequently, one context was paired with the novel sucrose, and the second was experienced in the absence of reinforcement prior to toxicosis. During a subsequent non-reinforced test it was found that responding in the context paired with the novel sucrose was considerably higher than responding in the context that was experienced alone. These findings stand in contrast to the taste-mediated contextual potentiation observed when a consumatory response is used to assess aversive contextual conditioning.     

Failure of serial taste-taste compound presentations to produce overshadowing of extinction of conditioned taste aversion

Two experiments were conducted to study overshadowing of extinction in a conditioned taste aversion preparation. In both experiments, aversive conditioning with sucrose was followed by extinction treatment with either sucrose alone or in compound with another taste, citric acid. Experiment 1 employed a simultaneous compound extinction treatment and found results indicative of overshadowing of extinction. By contrast, Experiment 2, in which extinction treatment involved serial compound presentations, failed to obtain evidence of overshadowing of extinction. The results of Experiment 2 indicate that the serial presentation of two tastes was processed as equivalent to the separate presentation of the tastes. The results are discussed in relation to: (1) Convergent evidence from research on latent inhibition, (2) competing theories of learning and, (3) their possible adaptive value in food-selection learning.     

All we need is a cue to remember: the effect of an extinction cue on renewal

In the present study, it was investigated by employing a human fear conditioning paradigm whether an extinction retrieval cue can attenuate renewal of conditioned responding after an extinction treatment procedure, and if so, what the precise role of such an extinction cue comprises. It was hypothesized that such a cue can attenuate renewal and would function as a safety signal capable of directly inhibiting the expectancy of an aversive outcome and conditioned skin conductance responding to a conditioned stimulus. The results demonstrated that the extinguished expectancy of an aversive outcome was renewed when the CS was presented outside the extinction context and that an extinction cue attenuated this effect. This extinction cue, however, only transferred its inhibitory properties to other, non-extinguished stimuli when there was no contextual switch. This safety signal function was not observed after a switch in context. Possible functions of the extinction cue and its application in extinction-based exposure treatments are discussed.     

Social blockade of taste-aversion learning in Norway rats (Rattus norvegicus): is it a social phenomenon?

In Experiment 1, hooded rats (Rattus norvegicus) were exposed to a novel diet in a food dish or on a conspecific; they were allowed to consume the same diet and then were injected with a toxin LiCl. Later both groups ate more of the novel diet than animals that had not been exposed, and the conspecific-exposed group ate more than the dish-exposed group. Reducing aversion learning by exposure on a conspecific is known as social blockade. We examined if this effect is because a conspecific intensifies dietary cues and thereby increases latent inhibition. Experiment 2 failed to show that diet on a conspecific is a more effective conditioned stimulus for taste-aversion learning than diet in a dish, and Experiment 3 showed that diet in a dish is an effective overshadowing stimulus in aversion learning but diet on a conspecific is not. These results suggest that social blockade cannot readily be assimilated to a latent-inhibition model and may be a distinctly social form of learning.     

Blocking of inhibitory conditioning within a serial conditioned stimulus-conditioned inhibitor compound: Maintenance of acquired behavior without an unconditioned stimulus

Well-trained classically conditioned stimuli, presented unreinforced, were protected from extinction when they were followed by a signal of the omission of the reinforcer (conditioned inhibitor Konorskian type) in eight cats. An aversive classical conditioning paradigm with shock as the reinforcer was used. Of several behavioral (leg flexion, vocalization) and organismic arousal (heart rate, respiration rate, respiration amplitude) measures of conditioned responses, the respiration amplitude changes were found to be most informative for the continuous assessment of elicited arousal of low and medium intensity. In all subjects conditioned stimuli presented during extinction in serial compound with the conditioned inhibitor elicited larger responses than did conditioned stimuli presented alone during extinction. The mechanism of protection from extinction in a paradigm in which the elicitor of learned behavior occurs prior to the conditioned inhibitor provides the organism with the mechanism for the maintenance of learned behavior in the absence of a reinforcer.     

Alcohol seeking by rats: Action or habit?

In two experiments, we examined the relative susceptibility to outcome devaluation of lever pressing by rats for either a 10% ethanol solution or food pellets. The rats were trained to press different levers for these two reinforcers using a sucrose-substitution procedure. An aversion was then conditioned from either the ethanol solution or the food pellets by pairing consumption with illness induced by lithium chloride. When instrumental performance was subsequently tested in extinction, the rats pressed less on the pellet lever if the pellets, rather than the ethanol, had been devalued by aversion conditioning. By contrast, performance on the ethanol lever was unaffected by whether the ethanol or pellets were devalued. Moreover, noncontingent presentations of the devalued reinforcer had no impact on test performance. The differential resistance to outcome devaluation suggests that, in contrast to food seeking, alcohol seeking is a stimulus-response habit rather than a goal-directed action mediated by a representation of the action-outcome contingency.     

Instrumental responding following reinforcer devaluation

In two experiments, hungry rats were given instrumental lever-press training for an appetitive reinforcer and, in addition, were exposed to another type of food which was not contingent on lever pressing. In the first experiment, exposure to each type of food was on separate days, whereas in the second experiment rats were exposed to each type of food in strict alternation within each session. Subsequently, a food aversion was conditioned to the reinforcer for the experimental group and to the non-contingent food for the control group. In both experiments, animals with an aversion to the reinforcer responded less in an extinction test than animals with an aversion to the non-contingent food. Subsequent reacquisition tests confirmed that the aversion to the non-contingent food in the control group was of comparable strength with that to the reinforcer in the experimental group. The results were discussed in terms of whether the reinforcer is encoded in the associative structure set up by exposure to an instrumental contingency.     

The CS-Preexposure Effect in Conditioned Taste-Aversion Learning in Golden Hamsters

In Experiment 1, golden hamsters were injected with either 0.9% saline or the nausea-inducing agent, lithium chloride (LiCL), immediately after consuming a flavored diet that was either novel or familiar. The LiCl-induced aversion was strong in hamsters for which the flavored diet was novel, but no significant aversion was observed in hamsters that were familiar with the flavored diet. In Experiment 2, the strength of the LiCl-induced aversion was related inversely to the amount of conditioned-stimulus (CS) preexposure and directly to the duration of the preexposure-conditioning interval. Thus, although some previous researchers have suggested that hamsters may not demonstrate the CS-preexposure effect in a conditioned taste-aversion paradigm, they clearly did so under the conditions of the present experiments, and moreover, the characteristics of the CS-preexposure effect in hamsters were generally similar to those observed in rats.