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.     

Distractor effect on the latent inhibition of conditioned flavor aversion in rats

Using a conditioned flavor aversion procedure with rats as subjects, the effect of the addition of a distractor stimulus on the magnitude of the latent inhibition effect was examined. Experiment 1 showed that latent inhibition to vinegar was attenuated by the addition of sucrose during preexposure. On the other hand, sucrose added during conditioning to vinegar did not attenuate latent inhibition. It was also found that the degree of latent inhibition to the vinegar-sucrose compound solution was less when vinegar alone was preexposed (i.e., when sucrose was added only during conditioning) than when the compound solution was preexposed (i.e., when sucrose was added both during preexposure and during conditioning). Experiment 2 gave similar results but with sucrose assigned as the target flavor and vinegar as the distractor. These findings are in full agreement with the generalization decrement account of latent inhibition.     

Lithium-mediated disruptions of latent inhibition: Overshadowing by the unconditioned stimulus in flavor conditioning

In a series of experiments, the ability of a single lithium preexposure to disrupt CS effectiveness was assessed using a latent inhibition procedure. Lithium preexposure administered proximal (90 min) to a saccharin familiarization trial reduced latent inhibition whereas a similar administration more distal (360 min) to flavor familiarization failed to do so. Additional experiments demonstrated that this socalled “US overshadowing” effect was not attributable to sensitization (Experiment 2), excitatory backward conditioning (Experiment 3), or state dependency (Experiment 4). The implications of US overshadowing for proximal US-preexposure effects are discussed.     

The Influence of Retention Interval on the US Preexposure Effect: Changes in Contextual Blocking over Time

A number of studies manipulating the length of the interval between conditioning and testing indicate spontaneous recovery from overshadowing, suggesting that certain instances of overshadowing represent a deficit in memory retrieval rather than a failure of animals to form an association between the overshadowed stimulus and the US. The present series of experiments examined the influence of lengthening the retention interval on blocking, another stimulus selection phenomenon that is typically interpreted as an acquisition deficit. The results indicated that when subjects were tested shortly (3 days) after training conditioning to a taste blocked subsequent conditioning to an odor conditioned in compound with that taste (Experiment 1), whereas prior conditioning to an odor did not block subsequent conditioning to a taste conditioned in compound with that odor (Experiment 2). This pattern of results was essentially unchanged when testing occurred at a longer (21-day) retention interval. However, there was evidence of a US preexposure effect in Experiment 2 when subjects in the US ONLY control condition were tested at the 3-day retention interval, but not when testing occurred 21 days after conditioning. Experiments 3 and 4 examined whether this loss of the US preexposure effect over time might actually represent a change in the degree of contextual blocking as the retention interval is lengthened. Exposure to the conditioning context either during the interval between Phase 1 and Phase 2 of conditioning (Experiment 3) or prior to Phase 1 of conditioning (Experiment 4) alleviated this US preexposure effect suggesting that the loss of the US preexposure effect as the retention interval is lengthened observed in Experiment 2 is due to changes in the degree of blocking by contextual stimuli over time. The results are discussed in terms of differential susceptibility of forgetting of two functional roles played by a contextual stimuli in the current situation-context as a CS and context as a retrieval cue for other CS-US associations.     

Preexposure to the CS alone,US alone,or CS and US uncorrelated: Latent inhibition,blocking by context or learned irrelevance?

Three experiments on conditioned suppression in rats examined the extent to which preexposure to the CS, the US, or uncorrelated presentations of both interfere with future conditioning. Experiment 1 suggested that the interference caused by preexposure to the US alone may result from blocking by contextual cues: signaling the US during preexposure attenuated the interference. Experiment 2 demonstrated that signaling the US by another stimulus did not attenuate the interference caused by exposure to uncorrelated presentations of CS and US. Experiment 3 replicated the results of Experiments 1 and 2 and directly compared the magnitude of these deficits. The results of these experiments imply that the effects of exposure to uncorrelated presentations of CS and US are not reducible to the sum of the effects of exposure to CS or US alone.     

Trace decay and the priming of short-term memory in long-delay taste aversion learning: Disruptive effects of novel exteroceptive stimulation

A variation of Kalat and Rozin's two-presentation paradigm was used to test the hypothesis that the first, as opposed to the second, presentation of a flavor conditioned stimulus (CS) constitutes the functional CS in two-presentation experiments involving moderate interflavor intervals (IFIs), and results in flavor aversions that are a function of the primary, as opposed to the secondary, conditioned stimulus-unconditioned stimulus (CS-US) interval. Contrary to the hypothesis, it was shown in Experiment 1 that holding the primary CS-US interval constant at 4 hr for each of three groups, while decreasing the secondary CS-US interval (i.e., the interval between the second flavor presentation and the illness) from 3.75 hr to 2.5 hr to .5 hr, resulted in the flavor aversion increasing as the secondary CS-US interval decreased. However, the aversion acquired by the group with a 0.5 hr secondary CS-US interval was also found to be significantly weaker than that acquired by a single-presentation 0.5 hr control group. In Experiment 2 it was demonstrated that animals exposed to novel exteroceptive stimulation (NES) immediately prior to a second flavor presentation that preceded the US by 0.5 hr acquired an aversion as strong as that acquired by a 0.5-hr control group. In Experiment 3 it was demonstrated that, in the absence of a second flavor presentation, animals exposed to novel exteroceptive stimulation 0.5 hr prior to the US acquired a weaker flavor aversion than did animals not exposed to novel exteroceptive stimulation during the 4-hr flavor CS-illness US interval. The contrasting effects of novel exteroceptive stimulation observed in Experiments 2 and 3 were replicated in Experiment 4. The results suggest, consistent with the trace-decay hypothesis and Wagner's (1976) general model of stimulus processing, that exposure to novel exteroceptive stimulation disrupts continued processing of the short-term memory (STM) trace of the initial presentation of a flavor CS, and hence minimizes stimulus preexposure effects attributable to the priming of STM.     

The Importance of Context in the US Preexposure Effect in CTA: Novel versus Latently Inhibited Contextual Stimuli

Three experiments were conducted to examine the role of novel contextual stimuli in producing the unconditioned stimulus (US) preexposure effect. Experiment 1 demonstrated that novel contextual stimuli produce a significantly stronger US preexposure effect than familiar or “latently inhibited” contextual stimuli. Moreover, subjects preexposed in the presence of latently inhibited contextual cues failed to show a significant US preexposure effect. Experiments 2 and 3 attempted to provide evidence that the addition of a single novel stimulus to the latently inhibited context would result in a significantly stronger US preexposure effect than when no such novel cue was present. Experiment 3 was able to demonstrate this effect. Results are consistent with the Rescorla–Wagner (1972) model of conditioning.     

Analysis of the US-preexposure effect in flavor acceptance conditioning

In three experiments, rats received exposure to a sucrose solution followed by conditioning with a neutral flavor as the conditioned stimulus (CS) and sucrose as the unconditioned stimulus (US). In Experiments 1 and 2, some rats were given both the preexposure and the conditioning phases in a highly familiar context (the homecage), whereas other animals received both phases in a novel and distinctive context. In both cases the magnitude of the conditioning effect was reduced by preexposure to the US. Experiment 3 directly assessed the possible role of contextual cues by changing the context between the exposure phase and the conditioning phase but found no loss of the US-preexposure effect in these conditions. These results lend no support to the blocking-by-context account of the US-preexposure effect; alternative interpretations are considered.     

Analysis of the learned irrelevance effect in appetitive Pavlovian conditioning

In Experiments 1 and 2 rats received uncorrelated presentations of a light conditioned stimulus (CS) and a food unconditioned stimulus (US) on each day of a preexposure phase. Control subjects received the same number of USs during the first half of preexposure and the same number of CSs during the second. Uncorrelated preexposure retarded inhibitory conditioning. Experiment 3 showed, however, that the different patterns of US preexposure experienced by the two groups could in itself influence the course of subsequent inhibitory conditioning. When this factor was equated by restricting the uncorrelated treatment to the first half of the pre-exposure phase (Experiment 2) or by extending the control treatment throughout the phase (Experiment 4) it was found that uncorrelated preexposure retarded excitatory conditioning, but facilitated inhibitory conditioning. This outcome challenges an interpretation in terms of the concept of learned irrelevance, which predicts that uncorrelated preexposure should retard both forms of conditioning.     

Tolerance to drug-induced (poly I:C) natural killer cell activation: congruence with a Pavlovian conditioning model

Two experiments were conducted to assess the role of conditioning factors on the tolerance of drug-induced natural killer (NK) cell activity. A protocol in which mice were given four weekly injections of the immunostimulatory synthetic polynucleotide (poly I:C) paired with a complex environmental stimulus produced a reliable tolerance effect. The sensitivity of the observed tolerance to known decremental conditioning procedures--extinction (Experiment 1) and preexposure to the conditioning stimulus (latent inhibition, Experiment 2)--was investigated. The results indicated that posttreatment exposure to drug-signaling cues (i.e., extinction) significantly reversed tolerance such that NK cell activity was at a level comparable to that for controls receiving the drug for the first time (Experiment 1). In a similar fashion, pretreatment exposure to the drug-signaling cues (CS preexposure) inhibited the development of tolerance (Experiment 2). These results provide initial evidence that nonpharmacological manipulations of drug-signaling environmental cues can affect tolerance to the immunostimulatory effect of poly I:C, as would be expected with a conditioning analysis of such tolerance.     

Assessments of changes in the effective salience of stimulus elements as a result of stimulus preexposure

Rats received exposure to 3 flavor compounds, AX and BX, presented in alternation, and CX, presented on a separate block of trials. The hypothesis that this treatment would leave B effectively more salient than C was tested in 3 ways. Experiment 1 showed that the unconditioned response evoked by B was stronger than that evoked by C. Experiment 2 showed that B was more effective than C when used as a reinforcer in a sensory preconditioning procedure. Experiment 3 showed that B was learned about more readily than C as a conditioned stimulus in flavor aversion conditioning. Alternating preexposure to 2 similar stimuli may protect their distinctive features from the loss of salience normally produced by nonreinforced exposure to a stimulus.     

The easy to hard effect and perceptual learning in flavor aversion conditioning

In experiments on the easy to hard effect, pretraining on an easy discrimination results in better performance on a harder version of the discrimination than pretraining on the hard discrimination itself. In addition, some theories posit that unreinforccd preexposure to the easy discrimination should be as effective as differentially reinforced easy pretraining in producing the easy to hard effect. Two experiments on flavor aversion conditioning in rats demonstrated the basic easy to hard effect. However, in neither experiment was easy preexposure more effective than hard preexposure in enhancing learning of the hard discrimination. Indeed, in Experiment 2, rats preexposed to an easy discrimination learned the hard discrimination significantly more slowly than those preexposed to the hard discrimination itself.     

Latent inhibition experiments with goldfish (Carassius auratus)

Evidence of latent inhibition was sought in a series of experiments with goldfish. In Experiment 1, goldfish were given nonreinforced preexposure to a color that subsequently predicted shock in an activity conditioning situation; their performance did not differ from that of control animals preexposed to a markedly different color. In Experiment 2, a group of goldfish given nonreinforced preexposure to a tone and an unstimulated control group were trained in an appetitive situation, with the tone serving either as a conditioned excitor or as a conditioned inhibitor. Preexposure had no significant effect in the conditioned excitation training, but it reduced the level of responding both to the positive stimulus and to the negative compound in the conditioned inhibition training. In Experiments 3 and 4, classical aversive conditioning was studied in the shuttle box. In Experiment 3, excitatory conditioning to a color was found to be impaired (relative to the performance of nonpreexposed control animals) as much by nonreinforced preexposure to the training color as by nonreinforced preexposure to a markedly different color; substantial variation in amount of preexposure was without significant effect. In the conditioned inhibition training of Experiment 4, animals with nonreinforced preexposure responded less than did unstimulated control animals both to the positive stimulus and to the negative compound. The results for goldfish can be understood on the assumption that the effect of preexposure in these animals is simply to reduce general responsiveness or level of arousal.     

A Comparison of Latent Inhibition and Learned Irrelevance Pre-Exposure Effects in Rabbit and Human Eyeblink Conditioning

The learning of an association between a CS and a US can be retarded by unreinforced presentations of the CS alone (termed latent inhibition or LI) or by un-correlated presentations of the CS and US (termed learned irrelevance or LIRR). In rabbit eyeblink conditioning, there have been some recent failures to replicate LI. LIRR has been hypothesized as producing a stronger retardation effect than LI based on both empirical studies and computational models. In the work presented here, we examined the relative strength of LI and LIRR in eyeblink conditioning in rabbits and humans. In both species, a number of preexposure trials sufficient to produce LIRR failed to produce LI (Experiments 1 & 3). Doubling the number of CS pre-exposures did produce LI in rabbits (Experiment 2), but not in humans (Experiment 4). LI was demonstrated in humans only after manipulations including an increased inter-trial interval or ITI (Experiment 5). Overall, it appears that LIRR is a more easily producible pre-exposure retardation effect than LI for eyeblink conditioning in both rabbits and humans. Several theoretical mechanisms for LI including the conditioned attention theory, stimulus compression, novelty, and the switching theory are discussed as possible explanations for the differences between LIRR and LI. Overall, future work involving testing the neural substrates of pre-exposure effects may benefit from the use of LIRR rather than LI.     

The effects of relative intensity of cue and marker on marked trace conditioning in pigeons

Pigeons were trained on trace conditioning procedures (autoshaping) in which a keylight conditioned stimulus (CS) was presented for 5 sec and followed after a 10-sec temporal gap by food. In Experiment 1, acquisition of conditioned responding to the CS was facilitated when it was immediately followed by a brief stimulus, but only when the brief stimulus was of greater intensity than the trace CS. In Experiment 2, this facilitation effect was found when a more intense brief stimulus followed the trace CS immediately, but not when it was delayed. In both experiments, responding to the target CS was compared within-subjects to that to a control CS, which eliminated factors such as second-order conditioning as an explanation for the facilitation effects. Together, the results of Experiments 1 and 2 suggest that a brief stimulus may mark immediately preceding events in memory and thus facilitate their association with delayed reinforcement only if the brief stimulus is relatively more intense, and thus more salient, than the target events. In Experiment 3, we found substantial generalization of this facilitation effect from the marked CS to the control CS and conclude that the results of Experiments 1 and 2 may underestimate the magnitude of these marking effects.     

Latent inhibition and conditioning after preexposure to the training context

Experiment 1, using the conditioned suppression technique with rats, showed that the retardation of learning produced by prior exposure to a stimulus (latent inhibition) was more marked in subjects given an initial phase of preexposure to the training context. This effect was confirmed and extended in Experiment 2 in which an appetitive conditioning procedure was used. Experiments 3 and 4, again using conditioned suppression, found no effect of preexposure to the context on the acquisition of suppression when training was given with a novel stimulus, either immediately after preexposure or after a delay; but context preexposure was again found to be effective when exposure to the to-be-conditioned stimulus was given in the delay interval between context preexposure and conditioning. The implications of these findings for accounts of the role of contextual factors in latent inhibition are discussed.     

Contextual Control of Latent Inhibition by the Reinforcer

Three experiments examined the contextual control of latent inhibition (LI) by the unconditioned stimulus (US) using a within-subjects conditioned suppression procedure with rats. The effect of reducing the context change produced by the introduction of the shock US was investigated by presenting this US during preexposure to the conditioned stimulus (CS). Although limited CS preexposure in the absence of the US had no impact on subsequent conditioning, preexposure in the presence of the shock retarded both excitatory and inhibitory conditioning. We conclude that the introduction of the US during the conditioning phase of a normal LI experiment can produce a contextual change that reduces the observed magnitude of LI.     

The influence of word shading and word length on eye movements during reading

In three experiments rats were given short or long preexposure (4 or 10 sessions) to two compound flavours, AX and BX, according to an intermixed or a blocked schedule. Following preexposure, aversion conditioning trials were given with AX as the conditioned stimulus (CS). In Experiments 1 and 2, retardation and summation tests were then carried out to assess the inhibitory properties of B (an Espinet procedure). In Experiment 3, test trials evaluated generalization from AX to BX (the standard perceptual learning procedure). The results showed that B performed as an inhibitor of the unconditioned stimulus (US; an Espinet effect) only after long intermixed preexposure, whereas a reliable perceptual learning effect was observed both after short and after long preeexposure. The observation that B had no detectable inhibitory properties after short preexposure casts doubt on the suggestion that inhibitory learning is responsible for perceptual learning after brief exposure to AX and BX.     

Latent inhibition: A trace conditioning phenomenon?

An informal model of latent inhibition (LI), the trace hypothesis, is described. This hypothesis holds that preexposure (PE) to the to-be-conditioned stimulus alters the salience (associability) of the CS such that only the onset and initial segments, but not the later segments, are capable of supporting conditioning when paired with a US. Thus, LI is effectively a trace conditioning phenomenon. Four experiments used rats and a one-trial fear-conditioning task to test predictions which stem from this hypothesis. Experiment 1 showed conditioning performance decreased as trace intervals increased between 0 and 12 s, thus demonstrating the sensitivity of this task to trace intervals proposed by the trace hypothesis to produce LI. Consistent with the trace hypothesis, LI was found to be an increasing function of the CS duration (Experiment 2), the number of PE trials (Experiment 3), and the US onset during a 15-s CS (Experiment 4). These data are somewhat problematic for several explanations of LI, but, in general, the trace hypothesis extends extant explanations by focusing on changes in salience within a given presentation of the CS.     

The Espinet and the perceptual learning effects in flavour aversion conditioning: do they depend on a common inhibitory mechanism?

In three experiments rats were given short or long preexposure (4 or 10 sessions) to two compound flavours, AX and BX, according to an intermixed or a blocked schedule. Following preexposure, aversion conditioning trials were given with AX as the conditioned stimulus (CS). In Experiments 1 and 2, retardation and summation tests were then carried out to assess the inhibitory properties of B (an Espinet procedure). In Experiment 3, test trials evaluated generalization from AX to BX (the standard perceptual learning procedure). The results showed that B performed as an inhibitor of the unconditioned stimulus (US; an Espinet effect) only after long intermixed preexposure, whereas a reliable perceptual learning effect was observed both after short and after long preeexposure. The observation that B had no detectable inhibitory properties after short preexposure casts doubt on the suggestion that inhibitory learning is responsible for perceptual learning after brief exposure to AX and BX.