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 elements or compound preexposure on conditioned taste aversion as a function of retention interval

The effects of element or compound preexposure and retention interval were examined in three experiments with the taste-aversion paradigm. In Experiment 1, preexposure to the elements of a compound flavor produced less latent inhibition to the compound than did preexposure to the compound itself when a 1-day preexposure-conditioning interval was used. However, preexposing the elements or the compound resulted in equivalent latent inhibition effects when a 21-day retention interval was used. In Experiment 2, a similar pattern of results was observed when the conditioning-test interval was manipulated. Experiment 3 explored the effect of element or compound preexposure when preexposure and test were carried out in different contexts. Attenuated latent inhibition following preexposure to the elements was found when preexposure and test were carried out in the same context. In contrast, preexposure to the elements resulted in as much latent inhibition as did preexposure to the compound when the context was switched from preexposure to testing. The implications of these findings for a retrieval-oriented view of latent inhibition are discussed.     

Sexually dimorphic extinction of a conditioned taste aversion in rats

The existence of a sexual dimorphism in the extinction of conditioned taste aversions in both fluid-deprived and nondeprived rats was investigated. When nondeprived rats were poisoned with LiCl after consuming a sucrose solution, males extinguished the subsequent conditioned taste aversion more slowly than females. There was no difference in the rate of extinction of a LiCl-induced conditioned taste aversion in fluid-deprived rats. Using marihuana extract distillate as the toxin, it was shown that the dimorphism found in the nondeprived rats is not a toxin-specific effect, and does not appear to be due to the males being made more ill. Possible explanations for the appearance of the dimorphism and its relation to the deprivational state of the organism are discussed.     

Attenuation of the CS-preexposure effect after a retention interval in preweanling rats

The effects of flavor preexposure and retention interval were assessed in 6- and 12-day-old rats. Conditioned aversions to a flavor appeared at both ages. The conditioning of the younger pups was unaffected by conditioned stimulus (CS) preexposure and was not evident after a 10-day retention interval. For the 12-day-old rats, preexposure to either the flavor CS or a different flavor attenuated aversion strength when the rats were tested soon after conditioning. Other 12-day-old rats that were tested 10 days after conditioning also expressed substantial aversions, but with a retention interval of this length, the aversions were equivalent for animals preexposed to the CS and those not preexposed before conditioning. This loss of the CS-preexposure effect over a long interval, which has also been observed in adult rats, identifies the locus of this effect as postacquisition and perhaps at the stage of memory retrieval.     

The effect of drug habituation before and after taste aversion learning in rats

In Experiment I, rats received eight habituation injections of either lithium chloride (LiCl) or sodium chloride (NaCl), then two aversion training trials in which access to saccharin solution was followed by LiCl injections, and finally eight extinction trials with saccharin but no injections. The rats habituated to LiCl showed less aversion to saccharin during training and extinction. In Experiment II, rats received two aversion training trials, then eight habituation trials to either LiCl or NaCl, then eight extinction trials, four more aversion training trials, and eight more extinction trials. The rats habituated to LiCl did not differ during the first extinction period from those habituated to NaCl, but showed less aversion to saccharin during the second training and extinction periods. Consequently, habituation to LiCl reduces the learning of an aversion to saccharin but does not reduce the performance of a previously learned aversion.     

Spontaneous recovery after extinction of a conditioned taste aversion

Four experiments examined whether or not spontaneous recovery could occur after extinction in the conditioned taste-aversion paradigm. After three extinction trials, spontaneous recovery was obtained over an 18-day retention interval (Experiments 1, 2, and 3). The effect was not due to changes in the unconditioned preference for saccharin over the retention interval (Experiment 2) or to an increase in a nonextinguished aversion over time, as indicated by tests with both the original, nonextinguished aversion (Experiment 1) and with a weaker one (Experiment 3). Spontaneous recovery was not obtained when extinction was overtrained (eight trials) and a 49-day retention interval was used (Experiment 4). However, saccharin intake at asymptote reached the level of baseline water intake, and not the highly preferred level shown by never-conditioned controls. Results of all four experiments suggest that extinction does not return an averted taste to the status of an unconditioned one.     

Taste quality and extinction of a conditioned taste aversion in rats

Rats (Rattus norvegicus) that received a taste cue (saccharin, saline, quinine, or sucrose) paired with a lithium chloride (LiCl) injection displayed a robust decrease in consumption of that taste, relative to controls that had the taste unpaired with LiCl. Consumption of the paired taste increased with each nonreinforced presentation (i.e., extinction). After asymptotic extinction, rats that had had a 0.1% saccharin cue paired with LiCl consumed less of the saccharin solution than did controls. A similar data pattern was observed with a 10% sucrose solution. These results are consistent with the view that some aspect of the excitatory CS-US association remains after extinction. On the other hand, rats that had a bitter (0.005% or 0.001% quinine) or salty (1% or 0.5% saline) solution paired with LiCl drank similar amounts of the fluid as controls after asymptotic extinction treatment. Together, these experiments suggest that a taste that is either sweet or preferred is required in order to demonstrate the chronic decrease in fluid consumption after extinction treatment. The data suggest that the conditioning experience prevents the later development of a preference for the sweet taste, rather than there being a retained aversion that suppresses fluid consumption.     

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 effects of taste extinction on ingestional potentiation in weanling rats

Four experiments investigated taste potentiation in weanling rats. In Experiment 1, the animals that drank a conditioning compound of denatonium and saccharin consumed significantly less on the test than controls that drank only saccharin during conditioning. This enhanced saccharin aversion was decremented by postconditioning extinction to denatonium in Experiment 2, and no generalization of saccharin aversions to the denatonium was observed in Experiment 3. Extinction of either saccharin or denatonium aversions after compound conditioning was shown in Experiment 4 to result in substantial decrements in aversions to the compound. The relationship of these outcomes to a multiple-association account of potentiation and to the role of discrimination processes in ingestional learning is discussed.     

Potentiation of taste by another taste during compound aversion learning

Two experiments with rat subjects examined whether a saccharin taste could potentiate the conditioning of an aversion to a salty taste when the two stimuli were presented together prior to lithium-induced illness. In Experiment 1, a 0.1% (w/v) saccharin solution potentiated conditioning of a very dilute (0.03%) NaCl solution, but had no demonstrable effect on two stronger NaCl solutions (0.6% and 1.2%). In Experiment 2, the 0.1% saccharin solution again potentiated the 0.03% NaCl target, but weaker and stronger saccharin concentrations (0.033% and 0.3%) did not. The ability of a taste to potentiate a secondtaste is not consistent with theories that assume that potentiation is unique to compounds composed of tastes and other, functionally different, nontaste cues. Potentiation may occur when the target stimulus is weakly conditionable on its own and when the particular combination of target and potentiator facilitates perceptual integration of the compound.     

Higher order conditioning of a taste aversion

Two groups of rats each received 5 drops of NaCl solution from a dropper placed directly inside the mouth. The experimental group was then injected with lithium chloride to establish a conditioned taste aversion. The control group was injected 24 h later. After a recovery day the above procedures were repeated. On the next day both groups received 5 drops of a saccharin solution followed immediately by 5 drops of the NaCl solution. Subsequent preference tests established that the experimental group had learned an aversion to the saccharin solution as a result of its pairing with the NaCl solution which had previously been associated with poisoning. These results demonstrate that higher order conditioning of a taste aversion can be established using tastes as both the first-order stimulus and the second-order stimulus.     

Slow reacquisition of a conditioned taste aversion

Rats were used to examine the extent to which extinction of an acquired conditioned taste aversion retards subsequent reacquisition. A saccharin-flavored solution (sac) was paired with LiCl and then followed by CS-alone extinction trials with this flavor. A control group received a different flavor, decaffeinated-coffee (coff), during initial conditioning and extinction. Sac was then paired with LiCl for all rats during a second conditioning phase. Reacquisition of the aversion to sac was retarded relative to the acquisition of an aversion to sac by the control group. A similar experiment with fewer extinction trials, but still with complete loss of the initial aversion, did not obtain slow reacquisition. The results are discussed with respect to an interference view of extinction and the slow-reacquisition effect.     

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.     

Serial overshadowing of taste aversion learning by stimuli preceding the target taste

Three experiments tested whether events taking place before a rat has access to a target taste, sucrose, can proactively interfere with the acquisition of a sucrose aversion when sucrose is followed by a lithium chloride injection. Using a serial overshadowing procedure with various delays before lithium injection, proactive interference by a taste (Experiments 1 and 3) and by a novel context (Experiment 2) was found following two conditioning sessions, but not after a single conditioning session. Conversely, overshadowing by a taste given after the target was detectable after a single conditioning trial (Experiment 3) and, thus, indicated that retroactive interference involves a process different from that producing proactive interference. A simulation confirmed that the results are consistent with a modified Rescorla and Wagner (1972) interpretation of Revusky??s (1971) concurrent interference theory of delay learning.     

A mixed design reveals that glucose moieties facilitate extinction of a conditioned taste aversion in rats

Separate groups of water-deprived rats had four trials with 15-min access to 0.0073 M saccharin, 0.3 M alanine, 0.3 M glucose, 0.1 M maltose, 0.3 M fructose, 0.06 M sucrose, or 0.03 M Polycose. Trials 1–3 were followed by injections of either 0.15 M LiCl (1.33 ml/100 g b.w., i.p.) or saline; Trial 4 (Test) was CS only. Extinction included either 48-h access to water alone or to the appropriate CS, both followed by a 24-h, two-bottle choice of CS and water. This 3-day cycle was repeated five to six times. All rats acquired comparable conditioned taste aversions (CTAs), but extinction rates varied with the test and the taste CS. No CTA extinguished during the two-bottle choices following 2 water days. During one-bottle CS exposure, all CTAs extinguished, but the aversion continued longer in the probe two-bottle tests. Intake of glucose moieties recovered rapidly, often in two cycles; the other CSs took four to six cycles. Thus, CTA extinction varies with the nature of the taste CS.     

The effect of nonreinforced stimulus exposure on the strength of a conditioned taste aversion as a function of retention interval: Do latent inhibition and extinction involve a shared process?

Two experiments examined the effects of nonreinforced flavor exposure on the strength of a conditioned taste aversion. Rats were conditioned by pairing maple flavor with LiC1. Prior to or subsequent to this pairing, some animals received nonreinforced exposure to either maple or saccharin. In separate subjects, preference for maple was tested 1 or 21 days after the last training episode. In the first experiment, the nonreinforced stimulus exposure occurred before conditioning (latent inhibition, or LI, procedure); in the second experiment, the nonreinforced exposure occurred after conditioning (extinction, or EXT, training). In both experiments, nonreinforced exposure to maple or saccharin reduced the magnitude of a conditioned maple aversion when testing occurred soon after conditioning. When testing was delayed, however, the attenuation due to nonreinforced saccharin exposure dissipated, both with the LI procedure and with EXT. In contrast, the nonreinforced exposure to maple was found to attenuate conditioned reactions at both short and long retention intervals. The release from generalized LI and spontaneous recovery following generalized EXT training are discussed in terms of retrieval processing. The possibility that the same mechanism may underlie LI and EXT is considered.     

Conditioned inhibition of rotation-induced taste aversion

In three experiments, we investigated the existence of conditioned inhibition of body rotation-induced taste aversion. Rats were given conditioned inhibition training in which the taste of saccharin was always followed by rotations, but the taste of vanilla was not. Flavor-preference tests, retardation-of-acquisition tests, and summation tests of inhibition indicated that the vanilla stimulus had acquired conditioned inhibitory properties. These findings could not be interpreted as functions of either initial solution preferences or simple nonassociative effects of flavor preexposure. They lend support to a theory that views learning mechanisms as being central to the phenomenon of motion sickness, and suggest that inhibitors might be effectively employed to ameliorate its symptoms.     

Novelty of contextual cues in taste aversion learning

The purpose of this experiment was to test the theory of Lubow, Rifkin, and Alek (1976) concerning the effects of stimulus preexposure on later learning. This hypothesis predicts that conditioning will occur faster when either the stimulus or the testing environment is novel relative to the other than when the stimulus and the environment are equally novel or equally familiar. The theory was tested in a taste aversion conditioning paradigm in which groups of rats were presented with either the familiar (preexposed) solution or the novel nonpreexposed solution, in either the familiar or the novel environment. Conditioning was affected by the novelty of both the stimulus and the environment, with novel stimuli enhancing learning and novel environments retarding it. However, no interaction between stimulus and environmental novelty was evident, and thus Lubow’s hypothesis was not confirmed.