CS–US interval determines the transition from overshadowing to potentiation with flavor compounds

The present series of five flavor aversion experiments with rat subjects examined compound conditioning at varying CS-US intervals. Using a taste-taste design, Experiments 1A and 1B demonstrated overshadowing at a 0-min CS-US interval and potentiation at a 120-min CS-US interval, and these effects occurred with both tastes of the compound. Experiment 2 showed that the aversion to a single element is reduced when the CS-US interval is increased to 120 min, but the aversion for a compound taste is not. Experiments 3A and 3B explored odor + taste compound conditioning; the results demonstrated odor potentiation across the trace interval and a transition from taste overshadowing to taste potentiation. Collectively, the data show that the change from overshadowing to potentiation was not due to changes in the aversions produced by compound conditioning but, instead, was due to a more rapid loss of conditionability across a trace interval prior to the US in single-element conditioning. These experiments suggest that following compound conditioning, the aversion to each element represents generalization decrement from the configured compound, but the designation of overshadowing or potentiation actually depends on the status of conditioning in the single-element control.     

A contingency analysis of taste aversion conditioning

The acquisition of conditioned taste aversion was assessed relative to five control procedures. That is, forward conditioning using multiple-trial, brief-duration taste CSs and weak USs over a 30-min CS-US delay was compared to backward, CS alone, US alone, sham CS and sham US, and random control procedures. The outcome supported an associative conditioning interpretation of the learned aversion. While there were no differences between the various control procedures, all were different from the forward conditioning group. The argument was made that some of the distinctive associative and nonassociative phenomena attributed to taste aversion conditioning (but not seen in the present study) may in part be due to the duration and intensity of both the CS and US events.     

Transfer across CS-US intervals and sensory modalities in classical conditioning of the rabbit

Two experiments investigated transfer of the rabbit’s conditioned nictitating membrane response (NMR) from shorter to longer CS-US intervals in conjunction with a change in CS modality, for example, light to tone. In Experiment 1, three experimental groups received initial training with a 400-msec CS-US interval, which produced substantial CR acquisition, and three control groups received initial training with a 2,800-msec CS-US interval, which produced minimal CR acquisition. Subsequently, the experimental and control groups received training with an 800-, 1,800-, or 2,800-msec CS-US trace interval. At the same time, the modality of the CS was changed from tone to light (or vice versa). Experiment 2 contained three groups that received initial exposure to a 400-msec CS-US interval, a 2,800-msec CS-US interval, or just the experimental chambers. Subsequently, all three groups received training with an 800-msec CS-US interval in a different CS modality. The results of both experiments revealed substantial positive transfer across CS modalities from the 400-msec CS-US interval to the 800-msec CS-US interval. There was also significant transfer to the 1,800-msec but not the 2,800-msec CS-US interval. The transfer did not appear immediately on test presentations of the second CS. Rather, the transfer appeared as an enhancement in the rate of CR acquisition after reinforced training with the second CS had commenced. The results are discussed with respect to mechanisms of transfer and facilitation of trace conditioning.     

Long-term retention of a conditioned taste aversion in preweanling and adult rats

The retention and extinction of a conditioned taste aversion after either short (6-day) or long (60-day) intervals was investigated in preweanling (18-day-old) and adult rats. Taste-only and illness-only control conditions were employed, as were variations in the concentration of the US (holding LiCl amount constant). Results indicated that after the short retention interval, retention of the taste aversion was equivalent for both ages. After the long interval, however, the 18-day-old rats exhibited significantly weaker taste aversion than their adult counterparts— infantile amnesia. Manipulation of US concentration had no effect on the magnitude of the taste aversions for either age or retention interval. The results are discussed in terms of their implications for infantile amnesia and general laws of learning.     

Effects of context exposure during conditioning on conditioned taste aversions

Rats were used in a conditioned taste aversion procedure in order to examine the effects of context exposure duration during the conditioning sessions on conditioned responding. One flavor was paired with lithium chloride during a long session in one context, whereas another flavor was conditioned during a short session in another context. Testing occurred in the home cage. The results showed that conditioning during short sessions produced strong conditioned taste aversions. Conditioning during long sessions produced strong conditioned taste aversions when the conditioned-stimulus-unconditionedstimulus (CS-US) pairing occurred at the end of the lengthy session. Other results showed that context-US associations were formed during the short duration sessions and that these associations supported conditioned responding to the CS trained in that context. The results are discussed with respect to the different influences that contextual cues can exert on conditioned responding.     

Variations in the retention of taste aversions: Evidence for retrieval competition

In six experiments, we examined taste and compound taste/taste aversions at different retention intervals. In Experiment 1, saccharin aversions were significantly weaker 1 day after conditioning than 21 days after conditioning. This effect was determined not to be caused by the aftereffects of illness or differential hydration. With the use of a saccharin/denatonium compound, Experiment 2 demonstrated overshadowing of a denatonium aversion at 21- and 1-day retention intervals, Experiment 4 showed a potentiated saccharin aversion only at the 21-day retention interval, and both Experiments 2 and 4 revealed that the aversion of the taste-only controls was stronger at the later retention interval. Experiments 3 and 5 demonstrated that the differences at the two retention intervals were not caused by unconditioned changes in taste preference. Finally, Experiment 6 showed that extinction of the conditioning environment prior to testing results in stronger saccharin aversions than occur in nonextinguished controls. Collectively, these experiments suggest that testing within a 24-h period after conditioning will result in significantly weaker taste aversions. Also, these results support a retrieval-competition explanation that may account for the weakened aversions at the 1-day testing interval of both groups conditioned to single elements and those conditioned to compounds.     

Ontogenetic differences in conditioning to context and CS as a function of context saliency and CS-US interval

A series of experiments examined ontogenetically the effects of context saliency and CS-US interval on the learning of CS-US and context-US associations. Although adult rats generally showed an inverse relationship between CS and context learning that depended upon CS-US interval and context saliency, the opposite relationship held for preweanlings. Consistent with many contemporary learning theories, adult rats expressed CS-context competition in learning about the US. Preweanlings, on the other hand, exhibited potentiated responding to either the context or the CS as a result of strong learning about the other element. These results suggest that adult and infant rats process information in a qualitatively different manner.     

Changes in the retrievability of associations to elements of the compound CS determine the expression of overshadowing

In two experiments, rats were presented with a taste conditioned stimulus (CS) alone, an odor CS alone, or an odor-taste compound followed by lithium chloride injection. When tested 1 day following conditioning, there was evidence that the odor cue overshadowed conditioning to the taste; however, there was no indication of overshadowing following a longer (21-day) retention interval, despite undiminished strength of the aversion in animals conditioned with only the single element (taste). The overshadowing observed at the 1-day retention interval was not reciprocal. Rats conditioned with the odor CS alone or with the compound CS expressed odor aversions of comparable strength—that is, no overshadowing. However, in contrast to the taste aversion, overshadowing of conditioning to the odor by taste was evident following a 21-day retention interval. Rather than reflecting a failure of the overshadowed stimulus to acquire associative strength, these data suggest that overshadowing may be expressed, or not expressed, as a result of changes in the relative retrievability of learned associations over time.     

Taste-potentiated color aversions in pigeons: Examination of CS preexposure and subsequent effects on postconditioning extinction of the taste aversion

In the present study, we investigated the mechanisms of potentiation using pigeons as subjects. Half the groups were given unreinforced preexposures to the CS on 2 consecutive days before aversion training. After training, the taste aversion was extinguished in some of the groups conditioned with a colored, tasty compound. Postconditioning extinction of the taste aversion was found to attenuate potentiated color aversions. This was the case for the subjects that both were and were not preexposed to the CS. These results lend support to the summation theory of potentiation (Durlach & Rescorla, 1980) and weaken the claim that CS preexposures are necessary for obtaining such support. Alternative accounts of the data are discussed.     

Effects of expected vs. unexpected proximal US preexposure on taste-aversion learning

Animals were first conditioned to expect lithium treatment following exposure to one taste solution (the CS+) and to expect no drug treatment following exposure to another flavor (the CS?). All subjects then received a saccharin taste-aversion conditioning trial. In Experiment 1, this conditioning trial was preceded 0, 1, 2, 4, or 6 h earlier by exposure to the CS+ flavor for independent groups. The CS+ exposure attenuated saccharin aversion learning if it occurred immediately before the saccharin conditioning trial but not if it occurred 1 h or more before conditioning. In Experiment 2, the saccharin conditioning trial was preceded 3 or 4.5 h earlier by a lithium injection. This proximal US preexposure injection was either unannounced (Li) or preceded by exposure to the CS+ (CS+Li) or the CS? (CS?Li) stimuli. The US preexposure attenuated saccharin aversion learning in all cases. However, the interference effect was less when the preexposure injection was expected (CS+Li) than when it was unexpected (CS?Li). This outcome could not be explained in terms of direct effects of the CS+ and CS? stimuli on the saccharin conditioning trial, and shows that the proximal US preexposure effect is a function of not only the drug dosage and preexposure interval, but also the anticipation of the drug pretreatment.     

Expression of a taste aversion conditioned with an odor-taste compound: Overshadowing is relatively weak in weanlings and decreases over a retention interval in adults

Adult rats were injected with lithium chloride (LiCl) after consumption of a novel flavor (chocolate milk) that either was or was not presented together with a novel ambient odor (banana) as a compound conditioned stimulus (CS). In Experiment 1, the adults’ consumption of the flavor 24 h after conditioning was compared with that of weanling rats given the same conditioning treatment on Postnatal Day 21. The results confirmed previous indications that the reduction in aversion observed for adults conditioned with the compound CS (overshadowing) was weak or nonexistent in weanlings. After a longer retention interval (21 days), there was no evidence of overshadowing in adults despite maintained retention of the basic conditioned aversion. In Experiment 2 this decrease in overshadowing after a long retention interval was replicated with adult animals and extended to a different method of testing. The form of the effect was the same as in Experiment 1: The decrease in overshadowing occurred over the retention interval without loss in retention of the basic taste aversion; the decrease in overshadowing was a consequence of anincrease in the flavor aversion displayed by animals conditioned with the compound CS. The impaired flavor aversion (i.e., the overshadowing) observed shortly after conditioning apparently was due to factors associated with memory retrieval, rather than to reduced attentional or associative strength.     

The effect of number and minimum duration of post-response-prevention escapes on the resistance to extinction of an avoidance response

After one-way avoidance training, rats were exposed, during avoidance response prevention, to light (CS-only) presentations or to light-shock (CS-US) pairings. Subgroups were then given 1, 5, or 10 trials during which they could escape immediately (unrestricted) or after 5 sec (restricted) by means of the previously conditioned avoidance response from a simultaneous light-shock compound. All animals were then exposed to avoidance extinction. The number of unrestricted escapes increased responding for CS-only animals, but had no significant effect on the performance of CS-US animals. Nevertheless, resistance to extinction was considerably less for CS-only animals given 10 unrestricted escapes than for CS-US animals given one unrestricted escape. One restricted escape had no more effect than one unrestricted escape for either response-prevention group. However, 5 restricted escapes elevated responding for CS-only animals to the level of CS-US animals. Extinction responding for CS-US animals increased significantly only after 10 restricted escapes. Since CS-only animals showed no further increase, resistance to extinction once more was greater for CS-US animals. These results, together with the very brief unrestricted escape latencies of CS-only animals, support a greater role for Pavlovian extinction than for response competition in the facilitation of avoidance extinction by CS-only response prevention. The fact that 10 restricted escapes were required to elevate resistance to extinction for CS-US animals over that obtained with one unrestricted escape attests to the effectiveness of Pavlovian conditioning during avoidance response prevention in elevating CS aversiveness to a near ceiling level.     

What the rat’s nose tells the rat’s mouth: Long delay aversion conditioning with aqueous odors and potentiation of taste by odors

In Experiment 1, olfactory bulbectomized and control rats were trained using operant conditioning to determine the taste threshold of aqueous amyl acetate. Concentrations below gustatory threshold were used in Experiments 2–5 to compare the effectiveness of odors with various concentrations of saccharin as cues for illness. The results showed the following: (1) The effectiveness of odor and taste was directly related to concentration; (2) the strength of an aversion to a concentration of taste could be matched by an appropriate concentration of an odor; (3) odor was as effective as taste with CS-US delays of 4 h; and (4) an effective odor potentiated an aversion to an otherwise ineffective taste. The results challenge the privileged role accorded tastes in food aversion learning and the manner in which tastes are held to interact with odors according to the sensory-and-gate channeling analysis of potentiation (Rusiniak, Hankins, Garcia, & Brett, 1979).     

General transfer across sensory modalities survives reductions in the original conditioned reflex in the rabbit

Two experiments demonstrated that transfer of training between CSs from different sensory modalities survived substantial reductions in responding to the first CS. In both experiments, animals received three stages of training. Stage 1 entailed CS-US training with a CS from one modality (e.g., light), and Stage 3 entailed CS-US training with a CS from another modality (e.g., tone). The experiments differed in treatment during Stage 2. In Experiment 1, animals either remained in their home cages or received unreinforced exposures to the first CS, which extinguished the original CR. In Experiment 2, the animals received either continued CS-US training or exposure to the CS and US but at a long interval (2,800 msec), which eliminated the original CR. As the baseline for detection of transfer effects, each experimental group had a control group that received Stage 1 training with a 2,800-msec CS-US interval, which produced minimal CR acquisition. The results of both experiments revealed substantial positive transfer across CS modalities regardless of the treatment during Stage 2. The transfer did not appear immediately on test presentations of the second CS in Stage 3. Rather, the transfer appeared as an enhancement in the rate of CR acquisition after reinforced training with the second CS had commenced. The results are discussed with respect to stimulus generalization, neutralization of background stimuli, and learning processes superordinate to specific associations.     

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.     

Second-order conditioning of the rabbit’s nictitating membrane response as a function of the CS2-CS1 and CS1-US intervals

Three experiments investigated conditioning of the rabbit’s nictitating membrane response (NMR) in a second-order conditioning procedure which intermixed first-order trials (CS1-US) and second-order trials (CS2-CS1) from the outset of training. Experiment 1 provided a controlled demonstration that substantial levels of second-order conditioning can be obtained with the NMR preparation. Experiment 2 showed that the level of CR acquisition to CS2 was an inverse function of the CS2-CS1 interval over the values of 400, 800, and 2,400 msec. Experiment 3 found that CR acquisition to CS2 and CS1 in second-order conditioning varied in a parallel fashion across CS1-US intervals. Similarly, Experiment 3A found that the level of CR acquisition to the two components of a serial compound (CSA-CSB-US) varied in a parallel fashion as a function of the CSB-US interval. The results of the CS2-CS1 and CS1-US interval manipulations were all predictable from the known CS-US interval effects in NMR conditioning with a single CS. The present results are discussed with regard to their implications for accounts of serial compound conditioning and second-order conditioning.     

Release from latent inhibition with delayed testing

A conditioned emotional response procedure was used to study the interactive effects of stimulus preexposure and retention interval in rats. In Experiment 1, the subjects were conditioned by presenting a light CS paired with mild footshock as the US. Half of the subjects were given nonreinforced preexposure to the CS, and the others were not. Separate preexposed and nonpreexposed groups were then tested 1,7, or 21 days after conditioning. Suppression of ongoing activity was used to assess the degree of conditioned fear. Latent inhibition was found at the 1-day retention interval; the preexposed subjects displayed less conditioned fear than did the nonpreexposed subjects. In contrast, equally strong conditioned fear was expressed by the preexposed and the nonpreexposed groups tested after the 7- and the 21-day retention intervals. These results indicate a release from latent inhibition similar to that obtained with conditioned taste aversions (Kraemer & Roberts, 1984). The results of Experiment 2 suggest that retention-interval-induced increases in sensitization, pseudoconditioning, or neophobia cannot account for the release from latent inhibition effect obtained in Experiment 1. The implications of these findings for a retrievaloriented view of latent inhibition are discussed.     

Compound-component differentiation as a function of CS-US interval and CS duration in the rabbit’s conditioned nictitating membrane response

The acquisition of the rabbit’s nictitating membrane response to a tone + light compound and its components was examined as a function of the CS-US interval (Experiment 1) and CS duration (Experiment 2). In Experiment 1, responding to the compound attained high levels in all groups, but responding on component test trials declined to low levels as the CS-US interval increased across values of 300, 800, and 1,300 msec. Further disparities between the compound and components appeared when the animals were shifted to a positive patterning schedule. In Experiment 2, disparities between the compound and components increased as the duration of the CS was increased across values of 50, 200, and 800 msec within a fixed CS-US interval of 800 msec. The results are discussed with respect to distributive processes, configuration, and speed-accuracy tradeoffs.     

Extinction of the overshadowing CS after overshadowing in conditioned taste aversion

Using a conditioned taste aversion preparation overshadowing of flavor-illness association was produced through the presentation of a second flavor during the interval between the first flavor and illness. The modulatory effects of extinguishing the association between the second (over-shadowing) flavor and illness on conditioned responding to the target flavor was investigated. In Experiment 1, we found that, following one-trial overshadowing, extinction of the overshadowing flavor had no effect on conditioned responding to the target flavor. In Experiment 2, we found a similar absence of an effect of extinction of the overshadowing stimulus in a multitrial over-shadowing paradigm. Experiment 3 confirmed the results of Experiments 1 and 2 using conditioning parameters that were designed to weaken the association between the overshadowed flavor and illness. In Experiments 4 and 5, we used simultaneous presentation of the flavors during conditioning and obtained a weakened aversion to the overshadowed flavor when the overshadowing CS was extinguished. These findings are inconsistent with previous observations in conditioned fear preparations that suggest that extinction of the association between the overshadowing stimulus and the unconditioned stimulus attenuates overshadowing. Possible reasons for the discrepant results are discussed.     

Effects of proximal unconditioned stimulus preexposure on ingestional aversions learned as a result of taste presentation following drug treatment

Taste aversions were conditioned by exposing subjects to a 1.0% saccharin solution 30 min after an injection of lithium chloride. The aversion learning was disrupted if subjects had also received an additional lithium injection some time earlier (Experiments 1–3). This interference effect of US preexposure was a decreasing function of the preexposure interval, beyond the optimal interval (105 min) for observing the phenomenon (Experiment 1), and was directly related to the dose of the preexposure injection (Experiment 2). No interference with conditioning occurred at short (e.g., 30-min) preexposure intervals (Experiment 1), probably because under these circumstances the preexposure injection itself conditioned a strong aversion (Experiment 4). At moderate (105-min) but not at short (30-min) preexposure intervals, the interference with aversions learned as a result of taste exposure following drug injection was comparable to the interference with learning in a more conventional forward conditioning procedure (Experiments 3 and 4). These findings are similar to previously documented effects of proximal CS- and US-preexposure and are consistent with recent stimulus rehearsal and opponent-process theories.