A comparison of learned aversions to gustatory and exteroceptive cues in rats

The experiment provides a direct comparison of the ability of subjects (rats) to associate gustatory and exteroceptive stimuli with illness. Previous experiments which have made similar comparisons between gustatory and exteroceptive cues have suffered from certain methodological problems involving stimulus control and compounding. The present experiment utilized a between-subjects design wherein half of the subjects had an auditory cue associated with poisoning and half had a taste cue. In both cases, the other cue was present, but was not predictive of poisoning, The auditory cue, like the taste cue, occurred only during drinking. This comparison was made in both an immediate and a delayed poisoning situation. The experiment found that while subjects were able to quickly associate a taste cue with illness, they were unable to form a similar association between poisoning and the exteroceptive stimulus. Results also showed that subjects will fail to acquire a taste aversion to a novel and salient gustatory cue when that cue is followed by illness only 50% of the time. This latter effect was more pronounced in the delayed poisoning situation.     

CS-US temporal relations in blocking

In four trace-conditioning experiments with rats, the influence on the blocking of differences between the blocking cue-unconditioned stimulus (US) and the blocked cue-US trace intervals was explored. Experiment 1 demonstrated blocking despite the blocked cue’s having a shorter trace interval than the blocking cue in both elemental (Phase 1) and compound (Phase 2) training. In Experiment 2, blocking was attenuated when the blocked cue had a longer trace interval than did the blocking cue in both elemental and compound training. In Experiments 3 and 4, the trace intervals of the two cues during compound training were matched (i.e., unlike in Experiments 1 and 2, neither had temporal priority). Blocking was attenuated when the blocking cue trace interval in the elemental phase was shorter (Experiment 3) or longer (Experiment 4) than the compound cue trace during compound training. The findings indicate that subjects encode interstimulus intervals, and they further suggest that cue competition is greatest when the competing cues have the same temporal information as the US.     

Effects of cuing in an “irrelevant” context

Normally, retention of an avoidance response by a rat is impaired when the test context is novel or does not correspond to the training context. Experiment 1 demonstrates that such an impairment of test performance can be alleviated if a rat receives a cuing treatment or reminder of training in the novel test context prior to testing. Experiment 2 indicates that when rats receive avoidance training in one context and then receive a reminder of training in a novel context, they perform more poorly when tested in the training context than do animals that receive no reminder. This finding is discussed in relation to current theories of contextual influence over retention performance.     

Latent inhibition of the conditioning context: Further evidence of contextual potentiation of retrieval in the absence of appreciable context-US associations

Previous research in our laboratory has found superior performance when classically conditioned responses are observed in the training context as opposed to outside it, even when direct context-US associations have been minimized by either the choice of conditioning parameters or extinction to the context. The present experiment used latent inhibition of the conditioning context as an alternative method of examining contextual cue effects in the absence of appreciable direct context-US associations. Water-deprived rats received tone-footshock pairings in one of two distinctly different apparatuses, but all were tested in a common apparatus. Animals conditioned in the test enclosure displayed more lick suppression than those conditioned outside the test enclosure. Other animals tested without the tone present also exhibited more suppression if conditioning had occurred in the test context rather than outside it, indicating that direct associations between the conditioning context and shock had been formed. However, when formation of direct associations to the conditioning context was attenuated in additional animals through extensive preexposure to the context prior to conditioning, more suppression to the tone was still seen when conditioning had occurred in the test context rather than outside it. These results add support to the position that the training context augments recall even when direct associations between the context and the US are attenuated. The phenomenon is discussed in terms of facilitated retrieval of nominal CS-US associations, configural retrieval cues, and conditional discriminations.     

Retrieval of memory in the pigeon by context manipulations

In Experiment 1, 12 pigeons were given eight sessions of VI single stimulus training with a color in a particular context followed by eight sessions of similar training with a line angle in another context. On the next day, half of the subjects were tested for wavelength and angularity generalization in each of the two contexts, a procedure that was thus consistent with training for one dimension and inconsistent for the other. The subjects made significantly more responses to each training stimulus under the consistent context condition, but there was no difference in absolute or relative generalization slopes. In Experiment 2, 12 pigeons were trained as in Experiment 1, but during generalization testing they were exposed to both contexts sequentially. Under the consistent context condition, the subjects responded more to the two training stimuli and yielded sharper absolute and relative wavelength generalization gradients: Under the inconsistent context condition, responding to the training wavelength was substantially disrupted. Thus, under appropriate testing conditions, contextual control over both the amount and the selectivity of responding can be demonstrated.     

Timing of interfering events in one-trial serial overshadowing of a taste aversion

This set of experiments examined the question of when a stimulus would be most effective in overshadowing the acquisition of long-delay taste aversion learning. In Experiment 1 rats drank sucrose, the target solution, followed by a hydrochloric acid (HCl) solution before lithium injection some time later; HCl was presented either early or late in the interval. The late condition produced greater overshadowing than the early condition. The importance of the HCl-injection interval was confirmed by Experiment 2, in which the sucrose-injection interval was varied. Experiment 3 found that even placement in a different context – an event that normally produces little overshadowing of a CTA – produced one-trial overshadowing of a sucrose aversion as long as the context was novel and exposure to it occurred immediately before lithium injection. No current theoretical account of one-trial overshadowing predicts that a late event produces more overshadowing than an early event. This result can, however, be accommodated within a modified version of the Rescorla-Wagner model.     

Attentional changes in blocking are not a consequence of lateral inhibition

In three human causal learning experiments, we examined attentional modulation in the blocking task, in which participants typically learn little about a novel cue B when it is paired with a previously trained, predictive cue A. Evidence indicates that this blocking training led to a decrement in attention to the blocked cue B. The present experiments addressed whether this decrease in attention to the blocked cue could be better explained as being due to lateral inhibition from the pretrained cue A to the blocked cue B, or as a cue-specific property that is not conditional on the presence or absence of other stimuli. Strong effects of learned predictiveness were observed on participants’ causal judgments (Experiment 1) and choice behavior (Experiments 2 and 3). However, no evidence for lateral inhibitory processes emerged in any of the experiments, despite explicit attempts to maximize experimental sensitivity to this effect. The results are discussed in the context of formal models of the operation of attentional processes in human and animal learning.     

The effect of subadditive pretraining on blocking: limits on generalization

Recent evidence indicates that prior learning about a set of cues may determine how new cues are processed. If subjects are taught that two reliable predictors of an outcome do not summate when the cues are presented together (i.e., subadditive pretraining), the subjects will tend to show a less profound blocking effect when trained with different cues. Three experiments investigated the conditions necessary for subadditive pretraining to generalize to new cues. Experiment 1 demonstrated that subadditive pretraining is less effective in reducing blocking when it is experienced in a context other than that in which the blocking training is experienced. In Experiment 2, the effectiveness of subadditive pretraining waned with time. Experiment 3 showed that subadditive pretraining is more effective when the temporal characteristics of pretraining cues are similar to those of the cues used in blocking training. These results provide information concerning the conditions under which learning will generalize from one set of cues to another.     

Context specificity of sensory preconditioning: Implications for processes of within-event learning

A variant of the taste aversion procedure for sensory preconditioning, as used by Rescorla and Cunningham (1978), was employed in a study of the context dependency of within-event learning. In two experiments, rats received Phase 1 exposure to a simultaneous flavor compound, AX; flavor X was paired with illness during Phase 2, and any tendency for the resulting aversion to be elicited by A was measured. It was found that subjects were less likely to shun flavor A as a consequence of this training if the Phase 1 and test episodes were conducted in distinctively different contexts. This effect was evident both when the change of context occurred just before the test with flavor A (Experiment 1) and when it occurred before Phase 2 (Experiment 2). These results were taken to imply that, as is often found with serial associations, the retrieval of within-event associations is subject to contextual control. The implications of these findings for the interpretation of perceptual learning effects are discussed.     

Chemosensory-based contextual conditioning inHermissenda crassicornis

In two experiments, the marine molluskHermissenda crassicornis was exposed to context discrimination training. In one context, defined by the presence of a diffuse chemosensory stimulus (shellfish extract A), brief, unsignaled, unconditioned stimuli (USs; high-speed rotation) were presented; in a second context, defined by the presence of shellfish extract B, no USs were presented. Animals were then tested (at both 1.5 and 24 h) by exposing them to small pieces of the shellfish meat used to define the two contexts. The latency to strike at the meat served as an index of the context-US association. In Experiment 1, the latency to strike at the cue associated with rotation was reduced relative to both preconditioning strike latencies and the associatively neutral cue. However, in a two-choice test where the animals could approach the conditioned or neutral stimulus, the animals regularly avoided the stimulus paired with rotation. Moreover, if, following conditioning, the animals were presented with an unsignaled rotation in the conditioned context or the neutral context, the animals exhibited more effective defensive clinging (an unconditioned reflex normally elicited by rotation) in the conditioned context, suggesting that it “prepared” the animal for the aversive US. In total, these results demonstrate thatHermissenda is capable of making associations to diffuse background (contextual) stimuli. Moreover, the results suggest that pairing the chemosensory cue with an aversive US elicits a strike response inHermissenda when the animal is placed in forced contact with the cue and an active avoidance response when the animal can choose between that cue and a neutral cue.     

Renewal of Pavlovian conditioned inhibition

Prior research has demonstrated renewal, which is the ability of contextual cues to modulate excitatory responding to a Pavlovian conditioned stimulus (CS). In the present research, conditioned lick suppression in rats was used to examine similar contextual modulation of Pavlovian conditioned inhibition. After Pavlovian conditioned inhibition training with a CS in one context, subjects were exposed to pairings of the CS with an unconditioned stimulus (US) either in the same or in a second context. Results indicated that, when the CS was paired with the US in the second context, the CS retained its inhibitory control over behavior, provided that testing occurred in the context used for inhibition training. However, when the CS-US pairings occurred in the inhibition training context, the CS subsequently proved to be excitatory regardless of where testing occurred. These observations indicate that conditioned inhibition is subject to renewal.     

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.     

Cross-modality transfer effects in conditioning-enhanced neophobia in chicks (Gallus domesticus): Evidence for the separability of novelty from specific stimulus characteristics

In this study, we investigated the separability of novelty from specific stimulus characteristics (e.g., color or taste quality) in the transfer of aversion effects. Ninety-six chicks (Gallus domesticus) received a novel visual (red water) or taste (3.0% vinegar) CS paired with an injection of lithium chloride or saline. The chicks were then tested for aversion to the CS or for conditioning-enhanced neophobia in response to a different novel visual cue (green water) or taste cue (1.0% saline). Aversions to the CSs were reliable and similar to each other. Reliable evidence of conditioning-enhanced neophobia occurred with respect to each test stimulus, irrespective of the type of CS, but conditioning with the vinegar CS produced reliably greater enhancement of neophobia than did conditioning with red water. For each CS, postconditioning neophobia was more persistent in testing with saline than with green water. The results for postconditioning neophobia suggested that novelty is a general stimulus property that is separable from specific stimulus characteristics.     

A within-event analysis of taste-potentiated odor and contextual aversions

A series of experiments was conducted to examine the phenomenon of potentiation. Experiment 1 demonstrated potentiation of odor aversions by taste when morphine served as the unconditioned stimulus (US). Experiment 2 provided evidence that the observed potentiation was due to a within-event association between odor and taste stimuli, rather than reflecting an enhanced odor-morphine association. In Experiment 3, morphine supported place conditioning to contextual cues and aversive conditioning to a taste cue, but potentiation of place conditioning by a taste cue was not obtained. Apparently the absence of potentiation was due to the dual nature of the morphine US, as potentiation of a contextual aversion by taste was obtained in Experiment 4 when a strictly aversive US (lithium) was used. These data suggest that potentiation depends on (1) an initially weak association between the to-be-potentiated conditioned stimulus (CS) element and the US, and (2) the elicitation of qualitatively similar responses by the individual elements of the CS compound. Collectively, these results support an explanation of potentiation based on within-event learning.     

Some determinants of second-order conditioning

In a Pavlovian conditioning situation, an initially neutral stimulus may be made excitatory by nonreinforced presentations in compound with an established conditioned excitor [i.e., second-order conditioning (SOC)]. The established excitor may be either a punctate cue or the training context. In four conditioned suppression experiments using rats, we investigated whether SOC phenomena parallel other cue interaction effects. In Experiment 1, we found that the response potential of a target stimulus was directly related to the intertrial interval when SOC was mediated by a punctate cue, and inversely related to the intertrial interval when SOC was mediated by the training context. Experiment 2 demonstrated that punctate- and context-mediated SOC are oppositely affected by posttraining context extinction, and Experiments 3 and 4 demonstrated that context- and punctate-mediated SOC are differentially affected by conditioned stimulus (Experiment 3) and unconditioned stimulus (Experiment 4) preexposure treatments. These findings parallel phenomena in conditioned inhibition and cue competition situations.     

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.     

Context excitation and modulation of conditioned sexual behavior

Three experiments were conducted to investigate direct and modulatory influences of context in the conditioned sexual behavior of male Japanese quail. A preference test procedure was used to assess the acquisition of contextual excitation. In Experiment 1, following direct context-unconditioned stimulus (US) pairings, male quail shifted their contextual preference from an initially preferred context to one in which they received copulatory opportunity with a female quail (US). Unpaired control group subjects did not demonstrate this shift in preference. This place preference procedure was used in Experiments 2 and 3 to assess contextual excitation when context was trained in the presence of a discrete conditioned stimulus (CS). Experiment 2 provided evidence that context can modulate responding to a discrete CS. In Experiment 3, we varied the spatial contiguity between the context and the US. Some subjects received the US directly in the training context, whereas other subjects received the US in an alternate context. Contextual excitation was evident only in subjects that received the former. Thus, there is a dissociation between the modulatory and excitatory properties of context in sexual conditioning that may depend on the context-US spatial contiguity.     

Interaction between conflicting memories in the rat: Contextual pretest cuing reverses control of behavior by testing context

Two experiments were run in order to investigate the influence of external contextual cues on the interaction between two conflicting memories. Rats were trained, in the same apparatus, on a passive-avoidance task and, 10 min later, on an active-avoidance task, then submitted to a 24-h delayed test (without reinforcement). When contextual cues remained unchanged throughout these three phases, the animals exhibited proactive interference, as shown by longer response latencies than those of control animals that had learned only the active-avoidance task (Experiment 1). When training contexts were made slightly different from one task to the other (by the presence of a continuous tone during either the first or the second task), the animals behaved at subsequent testing in accordance with the response contingencies present in the context exactly similar to the test context; this control of behavior by testing context was demonstrated both when the tone was absent (Experiments 1 and 2) and present (Experiment 2) at testing. A cuing procedure—a 90-sec exposure to the tone in the experimental room 5 min prior to testing—led the animals to behave in accordance with the response previously acquired in the presence of the tone (Experiments 1 and 2), exactly in the same way as animals tested in the presence of the tone (Experiment 2). The same cuing treatment was ineffective when administered 1 h before testing (Experiment 2). These results are interpreted in terms of a dual function of contextual cues at the time of retrieval: the general contextual information present during testing or during pretest cuing is assumed to induce concurrent reactivation of both memories. Consequently, the experimentally manipulated contextual cue (the tone) would have a discriminative function, leading the animals to choose between the two equally available representations.     

The effects of varying the interreinforcement interval on appetitive contextual conditioning

Appetitive contextual conditioning in rats and ringdoves was investigated in six experiments. In Experiment 1, differential contextual training produced greater anticipatory activity in rats in the presence of a context paired with food than it did in rats in the presence of a different context in which food was never presented. Furthermore, the rats showed a preference for the context associated with food when they were given a simultaneous choice test between contexts. In Experiment 2, rats were more active in and preferred a context associated with a variable-time 30-sec (VT30) schedule as opposed to a VT180 schedule. Experiment 3 was a between-subjects replication of the previous experiment. As expected, rats exhibited significantly more anticipatory activity in a context in which food had been presented on a VT30 schedule than they did in a context in which food had been presented on a VT180 schedule. Experiment 4 showed that anticipatory activity was a reflection of context-US associations in ringdoves, and in Experiments 5 and 6, ringdoves also exhibited an inverse relationship between the. amount of anticipatory activity and the length of the interreinforcement interval (IRI). These results reveal a relation between ERI and contextual conditioning opposite from that obtained in studies of aversive conditioning.     

Context specificity of conditioning in flavor-aversion learning: Extinction and blocking tests

In three experiments using rats as subjects, we investigated the degree to which a conditioned flavor aversion transfers from one context to another. Experiment 1, using a one-trial conditioning procedure, found no effect of a change of context on a conditioned aversion. Experiment 2 employed a multitrial procedure and demonstrated that a conditioned aversion was extinguished more rapidly after a change of context. Experiment 3 showed that context change decreased the effectiveness with which a conditioned flavor could block acquisition of an aversion by a second flavor. It is argued that these data cannot be explained in associative terms, and that they constitute evidence of conditionality in a simple aversive conditioning procedure.