Persistence of Preference for a Flavor Presented in SimultaneousCompound With Sucrose

Justin A. HarrisUniversity of Sydney

Fiona L. Shand, Louisa Q. Carroll, andR. Frederick Westbrook

University of New South Wales

Rats exposed to a simultaneous compound of a flavor and sucrose subsequently exhibited a preferencefor the flavor over water. This preference persisted across repeated testing even though the flavor waspresented in the absence of sucrose. The preference did, however, extinguish if the rats were hungry whentrained or tested, or if they had been reexposed to sucrose between training and test. Though failing toextinguish the preference, presentation of the flavor outside the compound protected it from the effectsof sucrose devaluation, indicating that these presentations extinguished the within-compound associationbetween the flavor and sucrose. The authors conclude that the hedonic reaction elicited by sucrose imbuesthe flavor with the same hedonic properties, and these properties maintain the preference independentlyof the flavor–sucrose association.

Associative learning is one of the mechanisms used by rats andpeople to solve the problem of selecting from a vast range ofpotentially edible substances what to accept for ingestion and whatto reject (Capaldi, 1996; Rozin & Zellner, 1985). Experimentalevidence for the involvement of learning in food selection wasprovided by the classic studies on cue-to-consequence and longdelay learning in rats (e.g., Garcia, Ervin, & Koelling, 1966;Garcia & Koelling, 1966). These studies demonstrated that adversepostingestive consequences selectively condition aversions to theflavors of recently consumed foods and do so even when there areconsiderable delays between ingestion of the flavor conditionedstimulus (CS) and the adverse effects produced by the gastricunconditioned stimulus (US).

In addition to developing aversions to flavors that signal thepresence of toxins in foods, rats learn about flavors that signal thepresence of nutrients in foods. For example, rats acquire prefer-ences for the flavors of foods that are rich in calories (Capaldi &Myers, 1982; Fedorchak & Bolles, 1987; Mehiel & Bolles, 1984)or contain specific vitamins in which the rat is deficient (Garcia,Ervin, & Yorke, 1967; Rodgers & Rozin, 1966). The developmentof such learned preferences is particularly well illustrated in thestudies conducted by Sclafani and colleagues. In these studies,

intragastric infusion of nutrients (polycose, glucose, or fats) fol-lowed ingestion of one flavored solution (CS�), whereas intra-gastric infusion of water followed ingestion of another (CS�).After this training, rats displayed substantial preferences for CS�over CS�, or for CS� over plain water, even when the nutrientUS was no longer infused after ingestion of CS� (e.g., Azzara &Sclafani, 1998; Lucas, Azzara, & Sclafani, 1997; Perez, Lucas, &Sclafani, 1998; Sclafani, 1991; Sclafani, Nissenbaum, & Vigorito,1987).

Calories and toxins are correlated with foods that taste sweetand bitter, respectively, and this correlation is reflected in the biasrats and people show toward acceptance of sweet substances andrejection of bitter ones. The correlation can be broken experimen-tally by exposing rats to sweet but nonnutritive tastes (e.g., sac-charin) or to bitter but nontoxic ones (e.g., quinine). Althoughwithout intrinsic postingestive consequences, such tastes persis-tently elicit hedonic reactions and function as reinforcers in theirown right, changing subsequent ingestive behavior toward otherassociated flavors (Fanselow & Birk, 1982; J. A. Harris & West-brook, 1998; Holman, 1975).

An unusual property attributed to learned flavor preferences,established either by following ingestion of the flavor with nutri-ents or by combining the flavor with a sweet taste, is their persis-tence when assessed in the absence of their reinforcer (Capaldi,Myers, Campbell, & Sheffer, 1983; Elizalde & Sclafani, 1990;Fedorchak, 1997; Sclafani, 1991). This persistence, or resistance toextinction, can be illustrated by an experiment reported by Mehiel(1991) in which rats acquired a preference for a CS� flavorpresented in simultaneous compound with sucrose over a secondunpaired flavor (CS�). Although the preference declined acrossthe first three preference tests (conducted under extinction), therats maintained a stable preference for the flavor associate ofsucrose for the next 11 tests. A similar pattern has been reported byDrucker, Ackroff, and Sclafani (1994) for rats that had acquired a

Justin A. Harris, School of Psychology, University of Sydney, Sydney,New South Wales, Australia; Fiona L. Shand, Louisa Q. Carroll, and R.Frederick Westbrook, School of Psychology, University of New SouthWales, Sydney, New South Wales, Australia.

This research was supported by Grants A79917204, A79800074, andDP02019523 from the Australian Research Council. We thank BobBoakes, Peter Lovibond, Vin LoLordo, and Gavan McNally for commentsand discussion.

Correspondence concerning this article should be addressed to Justin A.Harris, School of Psychology, University of Sydney, Sydney, New SouthWales 2006, Australia. E-mail: justinh@psych.usyd.edu.au

Journal of Experimental Psychology: Copyright 2004 by the American Psychological AssociationAnimal Behavior Processes2004, Vol. 30, No. 3, 177–189

0097-7403/04/$12.00 DOI: 10.1037/0097-7403.30.3.177

177

preference for a flavor paired with intragastric infusion ofpolycose.

There is an interpretive problem with many of the studiesreporting that conditioned flavor preferences are resistant to ex-tinction (e.g., Drucker et al., 1994; Elizalde & Sclafani, 1990;Fedorchak, 1997; Mehiel, 1991; Sclafani, 1991). The problemarises from the fact that preference was assessed by a choicebetween the flavor that had been paired with the reinforcer (CS�)and a flavor explicitly unpaired with the same reinforcer (CS�).We have observed that rats learn to avoid a flavor that has beenexplicitly unpaired with an attractive reinforcer (J. A. Harris,Gorissen, Bailey, & Westbrook, 2000). Therefore, the above mea-sure of preference confounds the selection of the paired flavor withan avoidance of the unpaired flavor. Furthermore, if preferencewere based on avoidance of the explicitly unpaired flavor, it wouldbe maintained across testing. This is particularly plausible becausethe inhibitory properties of cues are not reduced or extinguished bysimple presentations of these cues in the absence of the reinforcer;rather such extinction requires pairing the cues with the reinforcer(Zimmer-Hart & Rescorla, 1974). Evidence in support of thisargument comes from an experiment conducted by Drucker et al.(1994). First, they showed that rats’ absolute intake of a flavorpaired with intragastric infusion of polycose did decline acrossextinction tests, even though the rats continued to show a clearpreference for that flavor over an unpaired flavor. Second, thepreference did extinguish if the rats were tested for a choicebetween the paired flavor and water, rather than between the pairedflavor and an unpaired flavor. Thus, a learned flavor preferencemay extinguish when the test for that preference is not confoundedby avoidance of a second flavor.

On the basis of these considerations, our purpose in the presentexperiments was to provide a further investigation of the develop-ment and extinction of flavor preferences in rats. To avoid theconfound that complicates interpretation of most previous studieson the extinction of flavor preferences, we have measured prefer-ence by testing rats with a choice between the flavor and water.

Experiments 1A and 1B

These experiments had two aims. The first was to confirm thatrats acquire a preference for either an aqueous odor (almond,Experiment 1A) or a taste (salt, Experiment 1B) presented insimultaneous compound with sucrose. The second aim was toassess the persistence of this preference across testing when theodor or taste was pitted against water. In both experiments, threegroups of rats were trained for 4 days: One group (AS/N) wasexposed to a solution composed of almond (A) and sucrose (S)alternating with exposures to a salt solution (N), a second group(NS/A) was exposed to a salt–sucrose compound alternating withexposures to almond, and the third group (AN/S) was exposed toan almond–salt compound alternating with exposures to sucrose.Thus, all groups had an equivalent history of exposure to almond,sucrose, and salt, but differed with respect to the relations amongthese elements. After completion of training, the three groups weretested repeatedly with almond versus water (Experiment 1A) orsalt versus water (Experiment 1B).

Method

Subjects. Twenty-four male albino Wistar rats (Rattus norvegicus)were used in each experiment. They had been used in a shock-conditioningexperiment, but their allocation to the three conditions here was orthogonalto their previous treatments. They weighed 350–450 g and were obtainedfrom the colony of Specific Pathogen-Free rats maintained by the Com-bined Universities Laboratory Animal Services (Sydney, New SouthWales, Australia). They were housed in groups of eight in plastic boxes (65cm in length � 40 cm in width � 32 cm in depth) with food and watercontinuously available unless otherwise stated. The boxes were kept in anair-conditioned colony room under natural lighting. All experimental pro-cedures occurred between 9 a.m. and 11 a.m. and between 4 p.m. and6 p.m. Each rat was handled 3–5 min each day across 3 days before thestart of the experiment. The experimental procedures followed the ethicalguidelines established by the American Psychological Association andwere approved by the Animal Care and Ethics Committee of the Universityof New South Wales.

Apparatus. Eight plastic buckets with lids, each measuring 38 cm inheight � 20 cm in diameter, were used. The outside of each bucket wasfitted with two clasps, 6 cm apart, each of which held an inverted,calibrated, plastic cylinder. Each cylinder was fitted with a metal spoutwhose nozzle contained a ball bearing. The spouts protruded 2 cm into thebucket, at a height of 2 cm above the floor. Buckets were wiped clean withtap water after each rat was removed. All solutions were made up using tapwater. These consisted of a 4% wt/vol sucrose solution (Ajax Chemicals,Sydney, New South Wales, Australia), a 1% vol/vol almond solution(Aeroplane, Sydney, New South Wales, Australia), and a 1.16% wt/volsolution of sodium chloride (NaCl, Ajax Chemicals, Sydney, New SouthWales, Australia). The compound solutions were composed of 4% sucroseand 1% almond, 1.16% salt and 1% almond, or 4% sucrose and 1.16% salt.The buckets were located in an air-conditioned room in the laboratoryunder natural lighting.

Procedure. The water bottles were removed from the home boxes24 hr before the start of each experiment. The only fluid obtained by therats was that drunk in the plastic buckets where they received fluid for10 min each morning and afternoon. All fluids were presented in thetwo cylinders attached to each bucket, and the positions of the cylinderswere interchanged 5 min into each session. On any one training session,the same fluid was presented in both bottles. On each of Days 1– 4, ratsin group AS/N received alternating presentations of the almond–sucrosecompound and salt, those in group AN/S were presented with analmond–salt compound alternating with sucrose, whereas rats in groupNS/A received alternating presentations of the salt–sucrose compoundand almond. The solution presented in the morning versus afternoonwas reversed across days (i.e., the rats received the compound in themorning and element in the afternoon on odd days but received theelement in the morning and compound in the afternoon on even days).On each of Days 5–12, rats were tested each morning and afternoon. InExperiment 1A, the test consisted of presenting almond flavored waterin one of the cylinders and water in the other. In Experiment 1B, the testconsisted of presenting the salt solution in one cylinder and water in theother. Each test was 10 min in duration and the position of the cylinderswas interchanged after 5 min.

Statistical analysis. The test data for each rat were converted intopreference ratios, calculated as the intake of the target solution over thetotal amount consumed. These ratios were analyzed using a multivariate,repeated-measures analysis of variance (O’Brien & Kaiser, 1985). In theseand in the remaining experiments, significance was set at .05 and theexperiment-wise error rate was controlled using the Bonferroni inequalityprocedure (R. J. Harris, 1994).

178 HARRIS, SHAND, CARROLL, AND WESTBROOK

Results

Experiment 1A. The total amount of fluid consumed across thetrainings days (Days 1–4) was similar for the three groups (n � 8).The mean intakes were 14.0 ml (SEM � 0.97) for the almond–sucrose compound and 9.0 ml (SEM � 1.08) for the salt (groupAS/N), 11.0 ml (SEM � 0.98) for the almond–salt compound and13.6 (SEM � 0.77) for the sucrose (group AN/S), and 10.0 ml(SEM � 0.88) for the sucrose–salt compound and 12.0 ml (SEM �0.96) for the almond (group NS/A). However, the data of majorinterest are the test intakes of almond versus water. The top left

panel of Figure 1 shows the mean intakes of almond and water forrats in each group on the first and last days of testing (averagedacross the morning and afternoon sessions). The ratios of theintake of almond over total intake for each of the 7 test days areshown in the bottom left panel of Figure 1. It is clear that ratsexposed to almond in simultaneous compound with sucrose ex-hibited a persistent preference for the almond in comparison withthe preferences shown by those in the remaining groups. Thestatistical analysis confirmed that rats in group AS/N exhibited asignificantly greater preference for the almond than did those in

Figure 1. Results of Experiments (Exp) 1A and 1B. Top: Mean intakes of the flavor (F) and water (W) for eachgroup on the first and last days of testing in both experiments (flavor was almond in Experiment 1A and salt inExperiment 1B). Bottom: Average preference for flavor (as ratio of flavor intake over total intake) for each groupon each of the 7 test days in both experiments. Prior to testing, rats had been given four exposures to a compoundof almond (A) with sucrose (S) alternating with exposures to salt (N), to a compound of A with N alternatingwith S, or to a compound of N with S alternating with A.

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groups AN/S and NS/A, F(1, 21) � 60.91, MSE � 0.05, p � .01,and that the almond preferences in these two latter groups did notdiffer (F � 1). There was a significant linear trend, F(1, 21) �25.8, MSE � 0.01, p � .01, which, from inspection, indicates thatpreference ratios for the almond increased across days of test.There were no significant Trend � Group interactions (F � 1),which confirms that the differences between the preferences of ratsin group AS/N versus those of rats in groups AN/S and NA/S werejust as great at the end as they had been at the start of testing.

Experiment 1B. The total amount of fluid consumed across thetrainings days (Days 1–4) was similar for the three groups (n � 8).The mean intakes were 15.0 ml (SEM � 0.63) for the salt–sucrosecompound and 12.0 ml (SEM � 0.68) for the almond (groupNS/A), 16.0 ml (SEM � 0.83) for the almond–sucrose compoundand 8.4 ml (SEM � 0.95) for the salt (group AS/N), and 11.0 ml(SEM � 0.73) for the almond–salt compound and 17.5 ml (SEM �0.73) for the sucrose (group AN/S). The top right panel of Figure 1shows the mean intakes of salt and water for each group on the firstand last test day (averaged across the morning and afternoonsessions). The ratios of salt intake to total intakes for each test dayare shown in the bottom right panel of Figure 1. The statisticalanalysis of the ratios confirmed what is clear from inspection ofthese panels. Rats in group NS/A exhibited significantly greatersalt preferences than those in the other two groups, F(1, 21) �35.46, MSE � 0.30, p � .01, and rats in group AN/S showedsignificantly stronger salt preferences than those in group AS/N,F(1, 21) � 4.50, MSE � 0.30, p � .05. There was no evidence fora linear trend or interaction between trend and groups (Fs � 1).

Discussion

Rats that had consumed an almond–sucrose solution subse-quently showed a clear preference for the almond when it waspitted against water (Experiment 1A). Similarly, rats that con-sumed a salt–sucrose solution subsequently showed a clearpreference for salt over water (Experiment 1B). In both cases,the development of the preference was contingent on the al-mond or the salt being presented in simultaneous compoundwith sucrose rather than on the familiarity resulting from theirrepeated presentations or on their presentation in a compound.Further, the preference for the almond associate or the saltassociate of sucrose persisted across 7 days of twice-dailytesting, even though on each of these 14 tests the almond or saltwas unaccompanied by the sucrose that had been critical for theacquisition of the preference.

Experiment 1B also showed that rats in group AS/N developedan avoidance of salt, presumably because it had been explicitlyunpaired with sucrose. Given this, we might have expected toobserve a similar avoidance of salt in group AN/S, because saltwas also explicitly unpaired with sucrose for these rats. We at-tribute the failure to see such an avoidance of salt to overshadow-ing by almond because salt and almond were presented in com-pound. We might also have expected to observe an avoidance ofalmond after it was explicitly unpaired with sucrose in Experiment1A. Although we did not see this effect, we did observe such anavoidance of almond in a previous study (J. A. Harris et al., 2000).These findings provide empirical grounds for the suggestion thatinferences about conditioned flavor preference and their resistance

to extinction based on a comparison of the relative intakes of apaired (CS�) and an unpaired (CS�) flavor may well have con-founded preference for CS� with avoidance of the CS�. Althoughflawed by this confound, the inference concerning the persistenceof learned flavor preferences has been confirmed by the presentdemonstrations of such preferences when the associate of sucrosewas pitted against water.

Experiments 2A and 2B

Experiments 1A and 1B demonstrated that exposing rats to acompound containing a flavor and sucrose establishes a preferencefor the flavor that persists across repeated exposure to the flavor inthe absence of sucrose. Recently, we have shown that the prefer-ence rats acquire for a flavor associated with sucrose has twodistinct components: an association between the flavor and thecalories provided by the sucrose, and an association between theflavor and the sweet taste of the sucrose (J. A. Harris et al., 2000).We showed that the differential contributions of these two asso-ciations can be separated by manipulating the rats’ hunger stateduring training and test. Specifically, if rats were trained or testedwhile food deprived, their preference was based, at least in part, ona flavor–calorie association. If, on the other hand, they weretrained and tested while sated for food, then their preference wasbased predominantly on an association between the flavor andsweet taste.

In Experiments 1A and 1B, the rats had continuous access tofood in their home boxes. Hence their preference should havebeen based on a flavor–taste association. However, they mayalso have been hungry because their access to water was re-stricted to twice-daily drinking bouts across the experiment.Feeding and drinking are normally coupled in rats, and as such,the rats could have suffered a latent hunger. Therefore, thepreference could also have been based on a flavor– calorieassociation (cf. J. A. Harris et al., 2000). To investigate whetherthe two types of preference differ in how readily they can beextinguished by exposure to the odor or sucrose, in the presentexperiments we specifically manipulated the rats’ hunger dur-ing training (Experiment 2A) or test (Experiment 2B). Thus, inExperiment 2A, the rats were made either hungry (by explicitfood deprivation) or sated (being neither food nor water de-prived) during training but were tested with ad-lib access tofood (but water deprived). In Experiment 2B, the rats werewater deprived but had ad-lib access to food during training,and they were either hungry (food and water deprived) or sated(water deprived but not food deprived) during test.

Method

Subjects and apparatus. In both experiments, 16 experimentally naiverats (350–425 g) of the same stock and sex, and obtained from the samesource, were used. They were kept under the conditions described previ-ously. The apparatus was the set of plastic buckets described in Experi-ments 1A and 1B.

Procedure. Rats were allocated to two weight-matched groups (n � 8).On Days 1 and 2, they received twice-daily (a.m. and p.m.) exposures tothe almond–sucrose compound. From Day 3, they were then repeatedlytested with almond versus water twice daily in the manner describedpreviously.

180 HARRIS, SHAND, CARROLL, AND WESTBROOK

In Experiment 2A, one group (trained hungry) was placed on a fooddeprivation schedule in which food was made available for 2 hr eachevening, starting from the evening before Day 1 of the experiment. Theother group (trained sated) had ad-lib access to food. Rats in both groupswere maintained with ad-lib access to water to ensure that there was nolatent hunger that might otherwise have been induced by water deprivation.On the evening of Day 2 (prior to testing), ad-lib food was returned to thetrained hungry group, and the water bottles were removed from bothgroups.

In Experiment 2B, both groups were put on a water deprivation schedule(as for rats in Experiments 1A and 1B) on the evening before Day 1. Onthe evening of Day 2 (prior to commencement of testing on Day 3), onegroup (tested hungry) was placed on a food deprivation schedule, whichconsisted of free access to their regular food for 1 hr between 5 p.m. and6 p.m. and no access at other times. Food was continuously available in thehome box for the other group (sated).

Results

Experiment 2A. The amount of the almond–sucrose compoundthat was consumed increased across the four training trials, but thetwo groups drank similar total amounts of the solution (Ms � 40.6and 36.8 mls, SEMs � 1.7 and 4.0, for the hungry and satedgroups, respectively; t � 1). The mean intakes of almond andwater (averaged across the morning and afternoon sessions) on thefirst and last days of test for both groups are shown in the upperleft panel of Figure 2. The preference ratios for both groups oneach of the 7 days of testing are shown in the lower left panel ofthe figure. The group trained sated showed a strong preference foralmond that remained high across all tests. By contrast, the grouptrained hungry showed a preference on the first test day but thisrapidly decreased across subsequent tests. The statistical analysis

Figure 2. Results of Experiments (Exp) 2A and 2B. Top: Mean intakes of almond (A) and water (W) for eachgroup on the first and last days of testing. Bottom: Average preference for almond (as ratio of almond intake overtotal intake) for each group on each of the 7 test days. Prior to testing, rats had been given four training exposuresto a compound of almond with sucrose. In Experiment 2A, the rats were either food deprived (Hungry) or hadad-lib access to food (Sated) during training; in Experiment 2B, the rats were hungry or sated during testing.

181PERSISTENCE OF ODOR PREFERENCES

of the ratios revealed that the two groups differed significantlyoverall, F(1, 14) � 9.35, MSE � 0.19, p � .01. There was nosignificant linear trend overall (F � 1), nor was there an interac-tion between the groups and linear trend, F(1, 14) � 3.60, MSE �0.02, p � .08. However, there was a significant quadratic trend,F(1, 14) � 4.86, MSE � 0.01, p � .05, and a significant interac-tion between group and quadratic trend, F(1, 14) � 14.74, MSE �0.01, p �.01. The significant quadratic trend and interaction bearsout the observation that the preference ratio decreased very rapidlyacross the initial tests for the group trained hungry but not thegroup trained sated.

Inspection of the intakes of water and almond on test raises thepossibility that the change in almond preference among hungryrats may have been due to increased intake of water rather than adecrease in almond consumption. Total volume consumed on testdid increase in both groups of rats across days, presumably as therats adjusted to the change in their feeding or drinking schedulebetween training and test. The important point here is that therelative proportions of each solution consumed changed in thehungry group but not in the sated group. That is, although thehungry rats began drinking more on successive tests, they alsobecame less discriminating in directing their intake preferentiallyto the almond over the water.

Experiment 2B. The mean intakes of almond and water (av-eraged across the morning and afternoon sessions) on the first andlast days of test for both groups are shown in the upper right panelof Figure 2. The preference ratios for both groups on each of the7 days of testing are shown in the lower right panel of the figure.Rats tested while sated for food showed a strong preference foralmond across all tests, whereas the rats tested while hungryshowed an initial preference that rapidly decreased across tests.The statistical analysis of the ratios revealed that the two groupsdid not differ significantly overall, F(1, 14) � 2.97, MSE � 0.19,p � .11, but there was a significant overall linear trend, F(1, 14) �4.88, MSE � 0.02, p � .04, and a significant interaction betweenthe groups and linear trend, F(1, 14) � 5.00, MSE � 0.02, p � .04.The significant interaction confirms that the preference ratiosdecreased (extinguished) across tests for the group tested hungrybut not the group tested sated.

Discussion

These experiments have shown that the preferences acquiredand displayed by hungry rats differ from those of sated rats in theirsensitivity to extinction by repeated exposure to the flavor withoutsucrose. If rats were trained and tested while maintained on ad-libaccess to food, their preference for the flavor associated withsucrose was resistant to extinction. However, if they were fooddeprived during training or test, their preference for the flavorshowed evidence of extinction across days of testing.

Food deprivation should change the salience of calories to therat and thus influence what the rat selects to consume as well aswhat it learns about substances it consumes (J. A. Harris et al.,2000). When food deprived rats in Experiment 2A drank thealmond–sucrose solution, their hunger would have biased them tolearn about the association between the flavor and the caloriesprovided by the sucrose. This association then exerted control overtheir flavor preference when they were subsequently tested with a

modest level of hunger (induced by water deprivation). In contrast,rats in Experiment 2A that were neither food nor water deprived,and thus completely sated for food, would have failed to learn theflavor–calorie association, and thus their preference on test wascontrolled exclusively by the flavor–taste association. When rats inExperiment 2B consumed the almond–sucrose compound whilewater deprived but not food deprived, their modest level of hungerwould have ensured they learned about both the flavor–calorieassociation as well as the odor–taste association. When these ratswere finally tested, the content of their preference would have beencontrolled by their level of hunger—explicitly food deprived ratswould have shown a preference based predominantly on theflavor–calorie association, whereas non-food-deprived rats wouldhave based their preference on the flavor–taste association (J. A.Harris et al., 2000).

The present findings identify a correspondence between thedifferential contribution of flavor–calorie versus flavor–taste as-sociations to the preference with a differential sensitivity of thepreference to the absence of sucrose. That is, because rats trainedor tested while food deprived showed some loss of their prefer-ence, whereas rats trained and tested without food deprivationfailed to show any such loss, this implies that the flavor–calorieassociation does extinguish whereas the flavor–taste associationdoes not. This conclusion is consistent with the results of the studyreported by Drucker et al. (1994). These researchers trained ratswith a flavor paired with intragastric infusion of polycose. Thepreference these rats subsequently showed for the flavor can onlyhave been due to the association between the flavor and calories.Thus, the fact that the preference did extinguish across repeatedtest trials with the flavor alone is consistent with our own conclu-sions that conditioned preferences based on flavor–calorie associ-ations do extinguish.

Experiment 3

Experiments 1A and 1B demonstrated that the acquisition offlavor preference is contingent on the presentation of that flavorwith sucrose, but that once established, the preference persistseven in the absence of the sucrose. Experiments 2A and 2Bprovided evidence that repeated nonreinforced presentations of theflavor does extinguish the preference in hungry, but not sated, rats.The results of our previous research (J. A. Harris et al., 2000),suggest that the preference among hungry rats is based on anassociation between the flavor and calories provided by sucrose,whereas the preference among sated rats is based on an associationbetween the flavor and the sweet taste of sucrose. In turn, thissuggests that the flavor–sucrose association among sated rats sur-vives repeated exposures to the flavor without sucrose. Experiment3 used sensory preconditioning of a conditioned taste aversion(CTA) to examine this possibility. Rats were initially trained withthe almond–sucrose compound (while on ad-lib food). The designthen consisted of a 2 � 2 factorial. Half the rats received exposuresto the almond while the remaining rats were exposed to water(Factor 1). The sucrose was then paired with lithium chloride(LiCl) for half of the rats in each of these conditions but not for theremaining rats (Factor 2). Finally, all rats were tested with almondversus water. We hypothesized that the rats given paired exposuresto sucrose and LiCl should acquire an aversion to the sucrose. As

182 HARRIS, SHAND, CARROLL, AND WESTBROOK

a result, if the almond–sucrose association is intact, these ratsshould show some aversion to almond, and so reject the almond ontest in comparison to the preferences expected among rats notaverted to the sucrose. This was certainly expected for the nonex-tinguished rats exposed to water prior to the pairing of sucrosewith LiCl (Rescorla, 1980, 1982; Rescorla & Cunningham, 1978).The question of interest, however, is whether rats exposed toalmond prior to the development of a sucrose aversion will alsoreject the almond on test. If the repeated exposures to almondextinguish the odor–sucrose association, then these rats should notshow any aversion to the almond.

Method

Subjects and apparatus. Thirty-two experimentally naive rats (300–350 g) of the same stock and sex, and from the same source, were used.They were kept under the conditions described previously. The apparatuswas that used in Experiments 1A and 1B.

Procedure. Rats were fluid deprived and allocated to four weight-matched groups (n � 8). On Days 1–4, they were placed in the drinkingboxes for 10 min each morning and afternoon. On one of these occasionsa compound solution of 1% almond essence and 4% sucrose was presentedin both drinking cylinders; on the other occasion water was presented inboth cylinders. The order of these presentations was reversed betweendays. On Days 5–14, two groups (extinction) were tested for 10 min eachmorning and afternoon. During these sessions, one cylinder contained asolution of 1% almond and the other contained water. The two other groups(no extinction) were given equivalent drinking experience but with water inboth cylinders. On Days 15 and 18, all groups were placed into the drinkingchambers for 10 min with a 4% sucrose solution available in both cylin-ders. For rats in groups extinction CTA and no extinction CTA, each ofthese sucrose exposures was followed immediately by intraperitoneal (i.p.)injection of 10.0 ml/kg of 0.15M LiCl (BDH Laboratory Supplies, Poole,England). For rats in groups extinction no CTA and no extinction no CTA,an equal volume of saline was injected. All groups received counterbal-anced injections 6 hr later—the CTA groups were injected with saline,whereas the no CTA groups received LiCl. These injections took place inthe colony room. On Days 16–20, all groups were retrained with water inthe chambers. Finally, the rats were tested on Days 21–23. This consistedof a single 10-min exposure on each of these days to almond in one bottleand water in the other.

Results

The mean intakes of almond and water on the initial and finalextinction tests by each of the extinction groups are presented inthe top left panel of Figure 3. The ratios of almond intake over totalintake for each of the 10 days of testing in these two groups areshown in the top right panel of the same figure. It is clear that,apart from isolated and inexplicable low preferences on Day 1 forgroup extinction no CTA and on Day 6 for group extinction CTA,both groups showed equivalent preferences for the almond thatendured across 10 days of testing (20 tests in all). The analysisconfirmed that there was no overall linear trend across tests norany differences between the two groups (Fs � 1).

Both of the CTA groups acquired aversions to the sucrose afterits pairing with the effects of LiCl. Thus, on Day 18, when thesucrose was presented for the second pairing with LiCl, rats in thetwo CTA groups consumed much less sucrose (4.8 ml and 5.0 mlfor groups extinction CTA and no extinction CTA, respectively)than did the two no CTA groups (17.1 ml and 16.9 ml for groups

extinction no CTA and no extinction no CTA, respectively). Thisdifference was highly significant, F(1, 28) � 217.7, MSE � 5.40,p � .01, but there were no differences between the two CTAgroups (F � 1) indicating that these groups acquired the tasteaversion at the same rate.

The lower panels of Figure 3 show the mean intakes of almondand water (left), and the preference ratios based on these intakes(right), for each of the four groups across the tests performed afterdevaluation of the sucrose. From the figure it is clear that rats ingroup no extinction CTA avoided the almond on test, whereas theother three groups showed similar preferences for the almond. Thestatistical analysis confirmed a main effect for the two CTA groupsversus the two no CTA groups, F(1, 28) � 14.33, MSE � 0.04,p � .01, and a significant main effect for the two extinction groupsversus the two no extinction groups, F(1, 28) � 7.41, MSE � 0.04,p � .01. There was also a (just) significant interaction betweenthese factors, F(1, 28) � 4.21, MSE � 0.04, p � .05. A trendanalysis across tests confirmed that all groups showed equivalentlystable preferences across the three tests (Fs � 1). Post hoc analysesidentified that the interaction between extinction and CTA factorswas due to the fact that group no extinction CTA showed lowerpreferences than group extinction CTA, F(3, 28) � 10.13, MSE �0.05, p � .01, whereas the latter group was not different fromeither group extinction no CTA, F(3, 28) � 1.63, MSE � 0.04,p � .22, or group no extinction no CTA, F(3, 28) � 1.16, MSE �0.02, p � .30, which were not different from each other (F � 1).

Discussion

This experiment confirmed that exposure to an almond–sucrosecompound resulted in an almond preference that persisted acrossthe 20 tests in spite of the absence of the sucrose. It also replicatedresults from previous investigations of within-event learning (e.g.,Rescorla & Cunningham, 1978). Specifically, devaluation of thesucrose element of an almond–sucrose compound reduced thepreference shown by rats toward the almond (group no extinctionCTA vs. group no extinction no CTA). Finally, it confirmed thatthis effect of sucrose devaluation is itself reduced if the almond ispresented outside the compound after training but before sucrosedevaluation (group extinction CTA). In other words, exposure tothe almond outside the almond–sucrose compound weakened theassociation formed between the almond and sucrose. The impli-cation of this conclusion, therefore, is that the persistent preferenceexhibited toward the almond when presented outside the com-pound was not carried by the association between the almond andthe taste of sucrose.

Experiment 4

Experiment 3 demonstrated that the preference accruing to aflavor associated with sucrose is maintained across assessments ofthis preference even though the flavor–sucrose association itselfwas weakened. The present experiment also investigated whetherextinguishing the flavor–sucrose association affects the acquiredflavor preference, but here the association was extinguished byrepeated exposure to the sucrose instead of the flavor. The logic ofthis experiment derives from previous demonstrations that, whenrats are presented with a flavor and taste in a simultaneous com-

183PERSISTENCE OF ODOR PREFERENCES

pound, the learned association between them can be extinguishedby presentation of either element outside the compound (Rescorla,1982; Rescorla & Cunningham, 1978; Westbrook et al., 1995).Experiment 4 followed the same design as Experiment 3 exceptthat the extinction groups were exposed to sucrose, instead of thealmond, in order to extinguish the flavor–sucrose association. Ifexposure to sucrose weakens the association, then subsequentdevaluation of the sucrose should leave the almond preferenceunaffected.

Method

Subjects and apparatus. Thirty-two experimentally naive rats (340–380 g) of the same stock and sex, and from the same source, were used.They were kept under the conditions described previously. The apparatuswas that described in Experiments 1A and 1B.

Procedure. Rats were fluid deprived and allocated to four weight-matched groups (n � 8). They received a single drinking session (acrosstraining and testing) each day. Accordingly, rats received 20 min ofsupplementary access to water at 6 p.m. each day. Once per day for Days1–4, they were placed in the drinking chambers where a compoundsolution of 1% almond and 4% sucrose was presented in both drinkingcylinders. On Days 5–14, the rats were placed in the drinking chambers for10 min, during which time both cylinders contained either 4% sucrose(extinguished groups) or water (not extinguished groups). On Days 15, 18,and 21, all groups were placed into the drinking chambers for 10 min witha 4% sucrose solution available from both cylinders. For two of the groups(CTA), these sucrose exposures were followed by i.p. injections of 10.0ml/kg of 0.15M LiCl. The injection occurred immediately after the drink-ing session for rats in the extinguished CTA group, whereas the injectionwas delayed by 90 min for the not extinguished CTA group. Theseintervals were selected on the basis of the results of pilot studies, which

Figure 3. Results of Experiment 3. Top left: Average intakes of almond (A) and water (W) on the first and lastextinction exposures to almond for both extinction (Ext) groups. Top right: Average preference for almond (asratio of almond intake over total intake) on each of the 10 extinction days for the two extinction groups. Bottomleft: Average intakes of almond and water for all four groups averaged across the three tests conducted afterconditioning of a taste aversion to sucrose (CTA). Bottom right: Average preference ratio for almond for eachgroup on each of the three tests conducted after sucrose devaluation.

184 HARRIS, SHAND, CARROLL, AND WESTBROOK

indicated that they produced similar levels of aversions to the reexposedversus nonreexposed sucrose. The other two groups (no CTA) were in-jected with an equal volume of saline at the corresponding times. Allgroups received counterbalanced injections 6 hr later in the colony room—the CTA groups were injected with saline, whereas the no CTA groupsreceived LiCl. On Days 16–19, all groups were placed into the drinkingchambers for 10 min and given water. Rats were tested on Day 20 withalmond pitted against water. As in Experiment 3, a further 2 days of testingwere scheduled (Days 21 and 22), but this was extended by another 3 days(making a total of six tests) to confirm the pattern of results emergingacross the first three tests.

Results

The mean intakes of sucrose for each group across sucrose–LiCL exposures are presented in the upper panels of Figure 4. Thetwo no CTA groups consistently consumed large amounts ofsucrose. The two CTA groups initially consumed as much as theno CTA groups (before the first LiCl injection), but their intakesdropped sharply across successive conditioning episodes. Forpresent purposes, it is important that the intakes were similar forthe two CTA groups, and, if anything, the extinguished groupshowed a more rapid development of taste aversion than the notextinguished group. The statistical analysis of the intakes acrossthese three tests showed that the two CTA groups were signifi-cantly different from the two no CTA groups both in terms ofoverall intake, F(1, 28) � 383.9, MSE � 5.11, p � .01, and inlinear trend across days, F(1, 28) � 196.4, MSE � 7.21, p � .01.The two CTA groups did not differ in overall intake, F(1, 28) �3.67, MSE � 5.11, p � .07, but did differ in linear trend acrossdays, F(1, 28) � 11.72, MSE � 7.21, p � .01, which reflects themore profound suppression on the final test in the extinguishedgroup.

The mean intakes of almond and water on the first and final daysof testing are shown in the center panels of Figure 4. The notextinguished CTA group showed the lowest intake of almond andhighest of water, whereas the extinguished CTA group showed thehighest intake of almond and the lowest of water. The two no CTAgroups initially showed high intakes of almond and low intakes ofwater, but across successive tests, their relative intakes of almondand water converged. These differences were confirmed by thepreference ratios across each of the six tests, shown in the lowerpanels of Figure 4. On the first of these tests, rats in the notextinguished CTA group showed a much lower preference than theother three groups, indicating that sucrose devaluation resulted inthe rejection of almond. By contrast, the extinguished CTA groupshowed high preferences (as strong as for the no CTA groups),indicating that the reexposure to sucrose had prevented subsequentsucrose devaluation from imbuing the almond with aversive prop-erties. Across successive tests, the initially high almond preferenceamong the two no CTA groups declined, indicating extinction, butthere was no change in preference among either of the CTAgroups.

The statistical analysis of the ratios showed there was no sig-nificant difference between the CTA and no CTA groups (F � 1).However, there was a significant difference between the extin-guished and not extinguished groups, F(1, 28) � 20.11, MSE �0.05, p � .01, and a significant interaction between these factors,F(1, 28) � 13.63, MSE � 0.05, p � .01. There was a significant

Figure 4. Results of Experiment 4. Top: Mean consumption of sucroseby extinguished and not extinguished groups across each of threesessions in which an aversion was conditioned to sucrose (CTA) or noaversion was conditioned (No CTA). Middle: Mean intakes of almond(A) and water (W) on test by extinguished and not extinguished ratssubsequent to devaluation of sucrose (CTA) or no devaluation (NoCTA). Bottom: Average preference ratio for almond on test for eachgroup.

185PERSISTENCE OF ODOR PREFERENCES

overall linear trend across tests, F(1, 28) � 9.44, MSE � 0.03, p �.01, but this did not interact with any of the between-groupdifferences, Fs(1, 28) � 2.71, MSE � 0.03, p � .11. Separate posthoc analyses were conducted on the ratios from the first (Day 24)and last (Day 30) of testing (for these, the F critical[3, 28] � 8.9).They confirmed what is clear from the figure: On the first test day,the extinguished CTA group did not differ from the two no CTAgroups, F � 2.47, but these three groups did differ from the notextinguished CTA group, F � 28.84; by contrast, on the last testday, the not extinguished CTA group did not differ from the twono CTA groups (F � 1), and the difference between these threegroups and the extinguished CTA group fell just short of statisticalsignificance (F � 8.41).

Discussion

This experiment has shown that repeated exposure to sucrose,like similar exposure to the almond, weakens the almond–sucroseassociation established when rats consume almond and sucrose insimultaneous compound. Sucrose devaluation reduced the prefer-ence for almond among rats exposed to water between the com-pound and the sucrose–LiCl pairings (group not extinguishedCTA) in comparison to the preference shown by control rats notaverted to the sucrose (no CTA groups). In contrast, no suchrejection of almond was observed among rats that had receivedintervening exposures to sucrose prior to the establishment of aconditioned aversion to sucrose (group extinguished CTA). Thus,exposure to either sucrose or almond outside the compound extin-guished the almond–sucrose association (e.g., Rescorla & Cun-ningham, 1978).

In spite of this extinction of the almond-sucrose association, ratsthat received interpolated exposures to the sucrose (group extinc-tion CTA) continued to show a strong preference for almond. Thisconfirms the evidence in Experiment 3 that the almond preferenceis maintained independently of the almond–sucrose association.

An unexpected finding of this experiment was that almondpreference did appear to extinguish across repeated tests in both noCTA groups. This extinction had not been observed in our Exper-iments 1–3. The main difference between those experiments andthis one is that the rats in this experiment were reexposed tosucrose before test. Thus, it would seem that exposure to sucrose,but not almond, can extinguish the preference. We have sinceconfirmed this finding in several unpublished experiments. How-ever, this effect is not an inevitable consequence of reexposure tosucrose because it was not observed among rats in the extinguishedCTA group, despite the fact that this group received many expo-sures to sucrose. Therefore, it appears that any decrease in almondpreference caused by reexposure to sucrose can itself be reversed(i.e., the preference is reinstated) if the sucrose is devalued bypairing with illness.

General Discussion

When rats are exposed to a compound composed of sucrose andeither an aqueous odor (almond) or another taste (salt), they learnabout the relation between the elements within the compound. Thepresent experiments provided evidence of this learning in twoways. First, in Experiment 1A, rats exposed to an almond–sucrose

compound subsequently exhibited strong preferences for the al-mond, whereas rats exposed to almond in isolation, or in com-pound with another taste (salt), did not. Likewise, in Experiment1B, rats exposed to a salt–sucrose compound subsequently pre-ferred the salt, whereas rats presented with the salt in isolation, orin compound with an aqueous odor (almond), did not. Second,when rats were exposed to an almond–sucrose compound, theirpreference for the almond was reduced by subsequent devaluationof the sucrose, indicating that their response to the almond trackedthe value of the sucrose (Experiments 3 and 4).

A simple explanation of these findings is that the associationformed between the sucrose and the other flavor (e.g., the almond)enabled that flavor to borrow the properties of the sucrose. Ac-cordingly, rats exhibited a preference for the flavor when thesucrose was valuable, and a reduction in this preference when thesucrose was devalued by its association with the effects of LiCl.But this explanation is too simple, because any association be-tween the almond and sucrose should be extinguished by repeatedpresentation of almond alone. Although such presentations didprotect the almond against the impact of the sucrose devaluation,they did not result in any extinction of the almond preference.Therefore, if repeated presentations of almond extinguished thealmond–sucrose association, then this association could not havebeen the basis of the almond preference.

Like almond, presentations of sucrose extinguished the almond–sucrose association, evidenced by the loss of a sensory precondi-tioned aversion following devaluation of sucrose (Experiment 4).However, unlike almond, presentations of sucrose lead to aneventual reduction of the conditioned flavor preference. Theseeffects of sucrose reexposure could be taken as evidence that thealmond–sucrose association is necessary for expression of theflavor preference. But there are two reasons for rejecting thisconclusion. First, it is at odds with the evidence that repeatedpresentation of almond also extinguishes the almond–sucrose as-sociation without reducing the preference. Second, it is under-mined by another finding from Experiment 4—the persistence ofthe flavor preference was reinstated if, between reexposure andtest, the sucrose was devalued by pairing with LiCl. That is, thegroup (extinction CTA) in Experiment 4 for which the almond–sucrose association had been most clearly extinguished was thevery group whose preference remained intact.

One explanation for the dissociation reported here is that theflavor preference and the sensory preconditioned aversion differ intheir sensitivity to changes in the strength of the flavor–sucroseassociation. That is, both the preference and sensory precondi-tioned aversion are mediated by the same flavor–sucrose associa-tion, but a decrease in the strength of that association has moreimpact on the sensory preconditioned response than on the pref-erence. However, this notion is difficult to reconcile with theresults of Experiment 4. In that experiment, the almond prefer-ences exhibited by the two no CTA groups extinguished acrossrepeated testing, presumably as a result of their earlier exposuresto sucrose. One of these groups received only three exposures tosucrose, the same number of exposures given to the not extin-guished CTA group, which showed a change in their response toalmond following sucrose devaluation. Thus, three exposures didnot extinguish the almond–sucrose association as indexed by thesensory preconditioning procedure, but they were effective in

186 HARRIS, SHAND, CARROLL, AND WESTBROOK

causing an eventual reduction of the conditioned preference foralmond. In other words, in this case the preference was moresensitive than the sensory preconditioned response to extinction ofthe almond–sucrose association.

The persistence of the flavor preference despite extinction of thesensory preconditioned response suggests that the two responsesare mediated by different associations. For example, while drink-ing the almond–sucrose compound, rats may have learned twodistinct associations, one between the almond and the taste ofsucrose and a second between the almond and the hedonic reactionresulting from ingestion of the compound. The association be-tween the almond and taste of sucrose can be invoked to explainthe impact of sucrose devaluation on the almond, whereas theassociation between the almond and the hedonic reaction consti-tutes a mechanism by which the preference to almond is expressedin the absence of the sucrose. Although this explains the dissoci-ation between the preference and sensory preconditioning, it doesnot explain why the association between the almond and thehedonic reaction should be resistant to extinction.

Any explanation for why the conditioned preference should beresistant to extinction first requires a description of the mecha-nisms mediating extinction. The most widely accepted view ofextinction holds that the subject must detect the discrepancy be-tween what it experiences under the conditions of extinction andwhat it expected to occur on the basis of prior experience ofreinforcement (for discussion, see Rescorla, 2001). Therefore, onedescription of why the rats’ preferences failed to extinguish is thatthey failed to detect that their hedonic reactions to the flavor alonewas less than the anticipated reaction based on previous experiencewith the flavor–sucrose compound. It is not obvious why thisshould happen, but it appears to be sensitive to reexposure tosucrose. One possible consequence of reexposure to sucrose is aninflation of the hedonic value of the reinforcer, or at least the rat’smemory of that value. The consequence of this would be toincrease the discrepancy between the rat’s expectation of rein-forcement and the level of reinforcement experienced in responseto the flavor alone, thereby enabling extinction. Moreover, if thereinforcer is subsequently devalued (in this case, by pairing it withLiCl), this would again remove the basis on which the rat coulddetect a negative discrepancy between its expectation of reinforce-ment and its experience on test. That is, if the attractiveness of thesucrose is reduced, then the flavor is no longer less attractive thanthe reinforcer.

Unlike the flavor preference shown by rats with ad-lib food, thepreference did extinguish in rats that were food deprived duringtraining or test (Experiments 2A and 2B). The effect of hungercould be viewed in terms of the role played by this motivationalstate in determining what component of the almond–sucrose asso-ciation controlled responding. In an earlier study (J. A. Harris etal., 2000), we showed that preexposing rats to sucrose blocks theacquisition of conditioned preferences for a flavor presented incompound with sucrose. The important extension to this was thatpreexposure to saccharin likewise blocked the acquisition of flavorpreferences among sated rats but not among hungry rats, whereaspreexposure to sucrose blocked preference conditioning in bothcases. Because saccharin is sweet but contains no calories, it couldfunction to block conditioning on the basis of the taste of sucrosebut not the caloric content of sucrose. Thus, we concluded that (a)

the hungry rats based their preferences for almond on its associa-tion with the calories provided by sucrose, whereas (b) the satedrats based their preference on the association between the almondand the sweet taste of sucrose.

Accordingly, in the present experiments, rats trained or testedwhile hungry would have been subjected to a discrepancybetween what was expected of the almond based on its priorassociation with calories and the absence of that outcome acrosstesting. This discrepancy could serve as the basis for the reduc-tions in almond preferences among those rats. However, thepresent results are not entirely consistent with our secondconclusion—that sated rats based their preference on the asso-ciation between almond and the taste of sucrose— because, asnoted previously, the preference among these rats survivedextinction of the almond–sucrose association. We have arguedabove that the persistent preference is maintained by an asso-ciation between the almond and a positive hedonic responseelicited by sucrose. Therefore, one interpretation of the resultsfrom Experiments 2A and 2B is that the conditioned hedonicresponse controls the preference in sated rats more than inhungry rats, the latter basing their preference, at least to someextent, on an expectancy of calories (cf. Myers & Sclafani,2003).

The impact of food deprivation on persistence of the flavorpreference could be taken as evidence for a different explanationof the effect. According to this explanation, repeated nonrein-forced tests with the flavor did extinguish the original associativebasis for the preference (the flavor–sucrose association), but thiswas progressively replaced by a surrogate association between theflavor and calories ingested during free access to food immediatelyfollowing each test. The extinction of the flavor preference amongrats tested hungry (Experiment 2B) could therefore be viewed asrevealing the true effect of presenting the flavor outside the flavor–sucrose compound. A similar line of argument could be couched interms of the hydrating effects of the almond-alone solution acrossrepeated testing under conditions of water deprivation. That is,despite extinction of the almond–sucrose association, the almondpreference could be carried by an association between almond andthe hydrating effects of water. However, three lines of evidenceargue against these explanations. First, in Experiments 1A and 1B,control rats were repeatedly exposed to the flavor when thirsty andwere allowed to feed immediately following these exposures, yetthese rats failed to acquire any flavor preference as a result of theseexperiences. Therefore, any potential association between the fla-vor and hydration or feeding is not able to establish a preference.Second, rats that had been trained while food deprived in Exper-iment 2A also showed extinction of their almond preference eventhough they had free access to food after each test and weremaintained on water deprivation. Similarly, the flavor preferenceextinguished across tests among rats in the two no CTA groups ofExperiment 4, even though these groups were maintained understandard conditions (water deprived but with ad lib access to food).Therefore, the opportunity to acquire an association between theflavor and feeding, or between the flavor and hydration, was notsufficient to maintain the flavor preference. Third, reexposure tosucrose in Experiment 4 successfully extinguished the almond–sucrose association, but this did not have an immediate impact onthe almond preference. This is important because these rats did not

187PERSISTENCE OF ODOR PREFERENCES

have the opportunity to acquire a surrogate association between thealmond and posttest feeding, or between the almond and hydration,while their almond–sucrose association was extinguishing. There-fore, the opportunity to acquire an association between the flavorand feeding, or between the flavor and hydration, was not neces-sary for the flavor preference to survive extinction of the almond–sucrose association.

Rather than differing with respect to the contents of what hadbeen learned, the preferences exhibited by sated and hungry ratsmay simply differ in the magnitude of their hedonic response tothe reinforcer. There is evidence that hunger increases thehedonic response elicited by sucrose (e.g., Berridge, 1991;Capaldi, 1991), suggesting that the value of sucrose is inflated.Therefore, rats that were trained or tested while hungry shouldexperience a stronger expectation of reinforcement. Althoughceiling effects may have prevented its detection on the initialpreference tests, this difference could explain why extinctionproceeded in the hungry rats but not in sated rats. As suggestedearlier, the failure to observe extinction of the preferenceamong sated rats may have been due to their failure to detect thediscrepancy between their anticipated reaction to the almondand that actually experienced. By inflating the value of thereinforcer, hunger would increase the discrepancy between thelevel of reinforcement expected and that experienced, therebyenabling extinction.

In conclusion, we have shown that rats acquire a preference fora flavor paired with sucrose that is extremely enduring, persistingfor as many as 20 exposures to the flavor outside the compound,even though these exposures extinguish the association betweenthe flavor and sucrose. Further, if rats are hungry when trained ortested with the flavor, or they are reexposed to sucrose before testwith the flavor, their preference for the flavor does extinguish. Weinterpret these findings as showing that the preference exhibited bysated rats does not extinguish because these rats fail to detect thediscrepancy between the expected hedonic reaction to the rein-forcer and that elicited by the flavor. Hunger, or reexposure tosucrose, inflates the hedonic value of the sucrose reinforcer, andthus increases the discrepancy, thereby facilitating extinction ofthe preference.

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Received September 2, 2003Revision received December 16, 2003

Accepted January 7, 2004 �

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