Animal Learning & Behavior1986. 14 (1). 6-14

Conditioned inhibition in taste aversion learning:Testing methodology and empirical status

ANDREW R. DELAMATER, JOHN M. KRUSE, STUART MARLIN,and VINCENT M. LoLORDO

Dalhousie University. Halifax. Nova Scotia, Canada

Several assays were used in assessing conditioned inhibition within a taste aversion procedure.Following Pavlovian conditioned inhibition training, in which one taste was followed by an in­jection of LiCI on A+ trials, but was followed by access to a second flavored solution on AX­trials, retardation-of-acquisition and summation tests failed to indicate that the X stimulus (NaCl)had become inhibitory. Nor was the X stimulus consistently preferred to water or dilute quininein two-bottle tests, contrary to an earlier report (Best, 1975).

Most assessments of Pavlovian conditioned inhibitionhave used either the retardation-of-acquisition test or thesummation test (Rescorla, 1969). In the retardation test,the putative inhibitor is paired with the unconditionedstimulus (UCS) and the rate of acquisition of an excita­tory conditioned response (CR) is compared with the rateof acquisition in a group that has previously been ad­ministered some control treatment. In the summation test,the putative inhibitor is presented in compound with aknown excitor, and the response to this compound is com­pared with the response to the excitor alone. Rescorla(1969) asserted that when the organism is less responsiveto the compound than to the excitor alone in the summa­tion test, and the putative inhibitor is relatively slow tobecome an excitatory CS in the retardation-of-acquisitiontest, then there is good evidence for conditioned inhibition.

Although relatively few studies of inhibition have in­cluded both test procedures, many experiments using onetest or the other have provided evidence for inhibition indiverse conditioning preparations, including rabbit eye­blink conditioning (Marchant, Mis, & Moore, 1972), au­toshaping in pigeons (Rescorla, 1982; Wasserman, Frank­lin, & Hearst, 1974), and the conditioned emotionalresponse and related procedures in rats and dogs (Ham­mond, 1967; Rescorla & LoLordo, 1965). This researchhas been guided by the notion (Konorski, 1948, 1967;Rescorla & Wagner, 1972) that conditioned inhibition isin some sense the opposite of conditioned excitation, forexample, that it involves the formation of an inhibitoryassociation between CS and UCS centers (Konorski,1948). Furthermore, these discussions of inhibition haveencouraged the belief that conditioned inhibition will bemanifested only in the presence of excitation.

This research was supported by an operating grant from the NaturalSciences and Engineering Research Council of Canada to V.M.L. Duringthe course of this research 1.K. and A.D. were supported by I. W. Kil­lam postdoctoral and predoctoral fellowships, respectively. The authors'mailing address is: Department of Psychology, Dalhousie University,Halifax, NS, Canada B3H 411.

One exception to the standard assays of inhibition hasbeen provided by Best (1975) and Batson and Best (1981),using the conditioned taste aversion paradigm. Theyreported that a taste that had signaled the absence of ill­ness was more acceptable in a two-bottle test of prefer­ence than was the same taste in control conditions. In thecontext of prior work on the problem of conditioned in­hibition, an assertion that a taste predicting the non­occurrence of illness will be preferred to water providesa novel approach to the detection of conditioned inhibi­tion. From such demonstrations one could conclude thatconditioned inhibitors can exert an effect upon behavioreven in the absence of an excitatory context (see Rescorla,1976, 1979). Additionally, these procedures seem to offera new test for conditioned inhibition which is neither aretardation-of-acquisition nor a summation test.

Because the goal of the present research was to com­pare the outcome of the test used by Best (1975) with theoutcome of summation and retardation tests for inhibi­tion, using a preparation very similar to that of Best(1975), a brief description of that study is appropriate atthis point. A Pavlovian conditioned inhibition procedurewas used. The taste of saccharin was rendered excitatoryby pairing it with apomorphine injections in the ex­perimental group; the control groups did not learn an aver­sion to saccharin. The inhibition trial, of which there wasonly one, consisted of 2 min of access to saccharin fol­lowed immediately by 10 min of access to a saline solu­tion but no illness. Apparently the nonoccurrence of theapomorphine injection on the day that the saline solutionwas consumed rendered the salt taste inhibitory. In a sub­sequent two-bottle test, this saline solution was preferredto water in the experimental group, but not in either ofthe control groups. In other experiments, Best showed thatcasein hydrolysate also could serve as the CS-, and thata latent inhibition manipulation prevented his procedurefrom rendering the CS- inhibitory. This final demonstra­tion distinguished the associatively based preference forthe CS- from the increased acceptance of a flavor gener-

Copyright 1986 Psychonomic Society, Inc. 6

ated by learned safety or latent inhibition procedures(Kalat & Rozin, 1973; Rozin & Kalat, 1971).

There seem to be no other studies in which a taste wasused as an inhibitor in the conditioned taste aversion(CTA) paradigm. One additional study, which used odorstimuli (Taukulis & Revusky, 1975), mayor may not bedirectly comparable to studies that used only tastes (cf.Bouton & Whiting, 1982; Rusiniak, Hankins, Garcia, &Brett, 1979). Because there has been a history of debateover the degree to which CTA learning parallels otherPavlovian paradigms (e.g., Domjan, 1980; Garcia &Koelling, 1966; Kalat & Rozin, 1972; Palmerino, Rusi­niak, & Garcia, 1980; Rozin & Kalat, 1971), this reportprovides further evidence concerning the empirical sta­tus of Pavlovian conditioned inhibition in the CTAparadigm.

EXPERIMENT 1

The training procedure used here, like Best's, is a var­iant on the common A+, AX- procedure, which gener­ally endows the X stimulus with conditioned inhibitoryproperties. The most common way of administering thistype of training entails simultaneous presentation of thetwo stimuli on compound trials. However, in the case oftaste aversion learning, our pilot work (see also Rescorla& Cunningham, 1978) suggests that mixing the twoflavors tends to result in within-compound associationsand, therefore, excitation rather than inhibition. Conse­quently, we followed Best and presented the two cues,A and X, sequentially, with X following A.

Four different assessment procedures were used to de­tect conditioned inhibition. One was the retardation testrecommended by Rescorla (1969), in which the putativeinhibitor was paired with illness several times. Anotherwas a modified summation test: A new taste was pairedwith illness and then subjects were offered a choice be­tween this taste and a mixture of this taste and the inhibi­tor. The other two tests were preference tests, in whichthe inhibitor served as an alternative to either water ordilute quinine in a two-bottle test.

MethodSubjects. Seventy-two male albino rats of the Sprague-Dawley

strain were obtained from Charles River Canada, Ltd., to serveas subjects. The subjects of Replication 1 were experimentallynaive,whereas the 36 rats used in Replication 2 had previously servedin a conditionedsuppression study; the latter were assigned to groupsin the present study orthogonally to their previous experience. Allsubjects were individually caged and had continuous access to foodand water until 9 days prior to the start of the experiment, whenliquid intake was restricted to 10 min/day. Conditioning manipu­lations took place during the early part of the dark phase of the12-h light-dark cycle.

Procedure. The experiment was conducted in two sequential repli­cations. The procedures used in the two replications were identi­cal. There were 36 subjects in each replication, assigned randomlyto six groups (three pairs, inhibition and control), with one excep­tion, as noted below. The subjects were given 10 min of accessto water each day for 8 days prior to the start of the experiment

INHIBITION IN CTA 7

as well as on all "recovery" days during the course of the experi­ment. There were three conditioning cycles, each 7 days long andeach including one excitatory conditioning trial, two inhibitory con­ditioning trials, and intervening recovery days. Conditioned inhi­bition treatments and discriminative conditioning control treatmentsdiffered from each other only on inhibitory conditioning days.

Each cycle started with an excitatory trial: Instead of the usualwater, a .1 % (w/v) solution of sodium saccharin was made avail­able to the subjects for 10 min, and immediately after this drink­ing session each subject was injected intraperitoneally (ip) with1.8 mEq of a .15-M lithium chloride (LiCI) solution. Injections andalmost-daily weighings occurred in a room adjacent to the colony.On injection days, each subject was carried into this room, injected,and returned immediately to its home cage. Two water recoverydays followed an excitatory conditioning (injection) day. These,in turn, were followed by an inhibitory conditioning trial, a recov­ery day, another inhibitory trial, and another recovery day.

On inhibitory conditioning days, members of the inhibitory con­ditioning groups were given 2 min of access to the .1 % saccharinsolution, followed immediately by 10 min ofaccess to a 0.9% (w/v)solution of sodium chloride (NaCI). Members of the control groupswere given 10 min of access to the NaCI solution with no exposureto the saccharin solution. No injections occurred on these inhibi­tory conditioning days.

Following conditioning, two groups, water inhibition (WI) andwater control (WC), first received a IO-min two-bottle practice testwith both bottles full of water to habituate them to the two-bottletesting procedure. On the following day, these subjects receiveda 10-min test of preference between water and the salt solution;the positions of the bottles were switched midway through this test.

Two additional groups, quinine inhibition (QI) and quinine con­trol (Qc), received one exposure to a .005% solution of quininesulfate; this was followed, after a water day, by two separate two­bottle tests of preference between the quinine sulfate solution andthe NaCI solution. The position of the bottles was switched mid­way through each IO-min test. Two further groups, retardation in­hibitory (RI) and retardation control (RC), were administered aretardation test: Groups RI and RC were administered three trialsconsisting of IO-min access to the NaCI solution followed immedi­ately by an injection of LiCI (I. 8 mEq). Two water recovery daysintervened between successive conditioning trials.

Finally, the subjects of Groups WI and WC were administereda summation test. They were given IO-min access to a .1 % (v/v)vanilla (Club House, London, Canada) solution followed immedi­ately by an injection ofLiCI. Following 2 recovery days, they weregiven two separate two-bottle tests of preference between vanillaand a mixture of vanilla and NaC!.

ResultsAcquisition of an aversion to saccharin was substantial

after three trials in all groups (trials, F = 5lO.9). Thenonreinforced presentations of saccharin on inhibitiondays apparently attenuated the aversion, as the membersof the inhibition groups drank slightly more saccharin onTrials 2 and 3 than did the control group members [groupsX trials, F(lO,120) = 2.15, p=.025]. However, therewas no overall effect of groups [F(5,60) = 1.91,p > .lO].

Subjects in Replication 2 consumed less saccharin thanthose in Replication 1 [F(1,60) = lO.67, p=.OO2], andthey also acquired an aversion more quickly and/or com­pletely [replication x trials, F(2,120) 17.59,P < .001]. On the last conditioning trial, subjects ofReplication 1 drank a mean of 2.9 ml of saccharin,whereas subjects of Replication 2 drank only 0.4 ml, The

8 DELAMATER, KRUSE, MARLIN, AND LoLORDO

Figure 1. Percentage of NaCI consumed (NaCI/(NaCI + water»when NaCI was presented in a two-bottle test with water as the al­ternative.

three-way interaction was not significant (F < 1). Withineach replication, differences among the inhibition groupsand among the control groups were negligible.

The patterns of saccharin consumption during the 2-mininhibition trials were also analyzed. As was the case onexcitatory conditioning trials, subjects of Replication 2consumed less saccharin during Trials 2 and 3 than didsubjects of Replication 1. On Trial 3, group means (n=6)for consumption of saccharin ranged from 1.5 to 0 mI.

Consumption of the saline solution in Replication 1averaged 16.8 mI on Trial 1 and 22.4 mI on Trial 6. Con­sumption of saline in Replication 2 started at 17.4 mI onTrial 1 and rose to 23.9 mI on Trial 6. Consumption in­creased similarly in all groups.

Following conditioning, one pair of groups was initiallyadministered a two-bottle test of preference between sa­line and water. As inspection of Figure 1 shows, the in­hibition group drank a higher percentage of the putativeinhibitor than did the control group. An ANOVA (groupsx replications) indicated a significant effect of groups[F(I,20) = 4.99,p= .035] but no other significant effectsor interactions (Fs < 1).

The results of two-bottle tests of preference betweenthe quinine solution and an NaCI solution administeredto another pair of groups appear in Figure 2, which sug­gests that on one test the inhibition group showed a greaterpreference for the NaCI solution than did the controlgroup. An ANOV A revealed that there was a significanteffect of replications [F(l,20) = 7.15, p= .014], indicat­ing that subjects drank a significantly higher percentageof NaCI in the second replication than in the first, but thisdid not interact with either groups or tests at conventional

. levels of significance (Fs < 1). There was a significantgroups X tests interaction [F(I,20) = 6.35, p=.OI9].Further tests indicated that the two groups, combined over

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replications, differed from each other only on the secondtest [F(l,20) = 5.46; F < 1 for first test). No other maineffects or interactions were statistically significant.

The retardation-of-acquisition test administered toanother pair of groups yielded rather equivocal evidencefor conditioned inhibition. Figure 3 suggests that the in­hibition group was retarded relative to the control grouponly in the second replication. An ANOVA indicated anonsignificant main effect of groups [F(I,20) = 1.37],a nonsignificant groups x trials interaction [F(2,40) =1.92], but a significant groups X replication x trials in­teraction [F(2,40) = 8.61, p= .001]. Further analyses ofthis interaction (Kirk, 1%8) revealed that in Replication 2,on Trial 2, the inhibition group consumed significantlymore saline than the control group [F(l,40) = 18.96).This indicates that in Replication 2 acquisition of an aver­sion was slower, but eventually as complete, in the inhi­bition group as in the control group. The inhibition andcontrol groups did not differ from each other at any otherpoint in testing, and the only other significant effect indi­cated by the analysis was that of trials [F(2,40) = 257.4],indicating that pairing the saline solution with LiCI in­jections resulted in reduced intake of the saline solution.

The subjects that were initially administered a water­saline preference test were subsequently administered twotests of preference between vanilla and a vanilla-NaClmixture. Figure 4 suggests that the inhibition group drank

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INHIBITION IN CTA 9

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a higher percentage of the vanilla-NaCl mixture than didthe control group (means = 47.8 and 30.8, respectively),but an ANDVA (groups X replications x trials) indicatedthat the effect of groups was nonsignificant [F(I,20) =2.91, p= .10], as were the interactions involving groups.The replication X trial interaction was significant [F(1 ,20)= 6.71, p= .016], but this is uninformative with respectto conditioned inhibition. No other effects or interactionswere significant (ps > .20).

seemed to be appropriate, the discriminative condition­ing procedure was compared with another control treat­ment in Experiment 3.

The NaCl-water two-bottle test provided the moststraightforward evidence for conditioned inhibition, interms of statistical reliability, inasmuch as the effect ofthe experimental manipulation was observed when bothreplications were combined. This finding confirms those

50

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Figure 4. Percentage of vanilla-NaCI mixture consumed when themixture was presented in a two-bottle test with a previously poi­soned vanilla solution as the alternative.

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DiscussionThese results provide some evidence of conditioned in­

hibition. Some of the tests for conditioned inhibition sug­gested that the saline solution had become inhibitory,whereas others did not. Even in the positive tests, thedifferences between the inhibition and control groups werenot very large. It might be argued that, in fact, the dis­criminative control procedure produced weak. conditionedinhibition in the control groups, and that comparison ofthe experimental and control groups led to underestima­tion of the amount of inhibition in the former. However,pilot work immediately preceding the present experimentshad suggested that this control procedure generated thesame acceptability value for the saline solution as did aprocedure in which the saccharin was not paired with ill­ness but did precede the saline solution on inhibitory trials.Although some experimenters have found that the dis­criminative conditioning procedure occasionally gener­ates some inhibition (e.g., Rescorla, 1982), other studieshave used this procedure as a baseline against which theeffects of inhibitory procedures have been compared (e.g.,Rescorla & Holland, 1977). Although, in the present in­stance, the discriminative conditioning control procedure

10 DELAMATER, KRUSE, MARLIN, AND LoLORDO

of Best (1975) and Batson and Best (1981). It is interest­ing that this test should provide the strongest evidence ofinhibition, since the theoretical understanding of inhibi­tors has been that they in some sense counteract the ten­dencies evoked by conditioned excitors, rather than ac­quiring control over behavior of their own (see Rescorla,1969). In the present test there was no obvious excita­tory stimulus present.

The observation that the preference for NaCI over qui­nine was only about 60% is consistent with the idea thatNaCI acquired aversive properties as a result of its un­conditional effects upon the organism. Devenport (1973)has demonstrated that the postingestive consequences ofisotonic saline can result in a mild aversion to the tasteof the saline solution. Even the preference for saline whenpitted against water (Figure 1) was not very high; in theinhibition group, saline intake was approximately 48 %.This salt preference score is less than that obtained byDevenport (1973) and Wong (1977), but it is consistentwith unconditioned salt preference values obtained in thislaboratory with Charles River rats (Kruse & LoLordo,1986; also see Wiener & Deutsch, 1967).

The observation that only the second NaCl-quinine testresulted in evidence for a NaCI preference may be dueto some general effect of familiarity with the two-bottletesting procedure. In any event, the results of this test weresimilar to the results of the water-NaCI test. In both cases,the inhibition group exhibited a stronger preference forthe taste that had predicted the nonoccurrence of illnessthan did the control group.

The retardation-of-acquisition and vanilla versus NaCI­vanilla tests are more like the traditional tests for condi­tioned inhibition. The evidence yielded by these tests mustbe taken as equivocal, since the inhibition group wasretarded in acquisition in only one of the replications.Moreover, our summation test, which pitted vanillaagainst a vanilla-NaCI mixture, provided no evidence forconditioned inhibition. Perhaps responses to saline werealtered by prior experience with the saline-water test.

In summary, several conventional assays failed toproduce strong evidence for conditioned inhibition. Incontrast, results of the test in which saline and waterserved as alternatives most strongly indicated that salinehad become inhibitory.

EXPERIMENT 2

This experiment was originally intended to be a repli­cation and extension of Experiment 1. The saline-watertest for conditioned inhibition seemed to provide the moststable results in Experiment 1; consequently, it waschosen as the assay for Experiment 2. Zimmer-Hart andRescorla (1974) reported that repeated presentation of con­ditioned inhibitors in isolation following training did notreduce their inhibitory effect in subsequent tests. In thepresent case, their effect was sought in the context of CTAlearning. Confirmation of the findings of Zimmer-Hartand Rescorla (1974) would strengthen our conviction that

the target taste had truly been rendered inhibitory. Afterthe first test for inhibition was conducted, the remaininganimals in this experiment were subjected to a modifiedsummation test for conditioned inhibition, to be describedbelow.

MethodSubjects. Fifty-two experimentally naive male albino rats, ob­

tained and housed as in Experiment I, served as subjects. They wereassigned randomly to groups of 13.

Procedure. The subjects were divided among four groups; threeinhibitory conditioning groups and one control group. The train­ing procedures were the same as in the first experiment. That is,there were three training cycles, each consisting of an excitatorytrial and two inhibitory conditioning trials and all followed by waterrecovery days. As before, an excitatory trial consisted of lO-rninaccess to saccharin followed by an ip injectionof 1.8 mEq ofO.15-MLiCl. The inhibitory trials consisted of 2 min of saccharin followedby 10 min of NaCI for the inhibitory groups, and 10 min of NaCI(not preceded by saccharin) for the control group.

One of the inhibitory groups and the control group were testedwith a two-bottle preference test, as in Experiment 1. They werefirst given a two-bottle test with water in both bottles, to habituatethem to the procedure. They were then given two tests (separatedby recovery days) of preference between NaCI and water.

Given the results of the two-bottle tests administered to the firstpair of groups in this experiment, the remaining subjects were notused for the originally intended purpose. They had all had the sameinhibitory training, and for this test they were split into two groupsand administered another type of test for the existence of condi­tioned inhibition. This test can be considered a variant of the sum­mation test (Rescorla, 1969), except that the elements, excitatoryand inhibitory, were presented sequentially, rather than simultane­ously. Half of the subjects were administered 2 min of access tothe saline solution followed immediately by 10 min of access tothe saccharin solution. The remainder of the subjects were ad­ministered 2 min of access to water, followed immediately by10 min of access to saccharin. There were three such trials al­together, conducted on separate days and followedby recovery days.For all subjects, a day of 10 min of access to saccharin intervenedbetween Trials 2 and 3 of this test, in an attempt to attenuate a pos­sible floor effect resulting from strong saccharin aversions.

ResultsAs in the first experiment, the acquisition of an aver­

sion to saccharin proceeded slightly more slowly in theinhibition groups than in the control group. On the last1O-min training trial, rats in the control group drank amean of 0.4 ml of saccharin solution, whereas the meansfor the three inhibition groups were 1.1, 1.6, and 1.8 ml.The animals drank approximately 22.5 ml of the salinesolution on the last inhibition trial.

The two groups given the two-bottle test with NaCI andwater as the alternatives drank approximately the sameprecentage of NaCI, as Figure 1 shows. An ANOVA in­dicated that these values did not differ significantly fromeach other [F(1 ,24) = .16] and that none of the other ef­fects or interactions were significant (ps > .30).

In the sequential summation test, rats appeared to con­sume slightly more saccharin after consuming NaCI thanafter consuming water (Figure 5), but the statistical anal­ysis indicated that the effect was not reliable. The onlysignificant effect in the sequential tests was that of trials[F(2,48) = 7.3, p=.OO2], indicating that the subjects

INHIBITION IN CTA 11

4

EXPERIMENT 3

of-acquisition tests. In Experiment 1 there was the pos­sibility of greater generalization decrement from nonrein­forced conditioning trials to retardation-of-acquisition testtrials in the conditioned inhibition group than in thedifferential conditioning control. Such differential gener­alization decrement would have opposed the demonstra­tion of conditioned inhibition. To control for this possi­bility, the differential conditioning control was replacedby a control group which received the same saccharin­saline sequence as the experimental group on nonrein­forced trials, but received unpaired, rather than paired,presentations of saccharin and LiCl. This control group,which was used by Best (1975), was also added for ratswhich were to receive the summation test.

In Experiment 1, the summation test, which pitted avanilla excitor against a mixture of vanilla and saline, wasadministered to rats that had already been given a test ofpreference for saline versus water. To control for the pos­sibility that the inhibitory effect of the saline extinguishedas a result of the first test, and thus was not revealed inthe summation test, in the present experiment the salineversus water test was omitted and replaced by a two-bottletest of water versus water to familiarize the rats with thetwo-bottle test procedure.

The vanilla used as the new excitatory flavor in the sum­mation test of Experiment 1 might have had an aversiveodor as well as an aversive taste. If this was the case, thenthe odor might have kept the rats away from the spoutsso that they failed to sample the saline enough to forma preference. To control for this possibility, a "pure"taste, HCI, was substituted for vanilla.

MethodSubjects. Sixty experimentally naive male albino rats, obtained

and housed as in Experiment 1, served as subjects. They were as­signed randomly to groups of 12.

Procedure. The subjects were divided among three groups dur­ing the conditioning phase of the experiment. The conditioned in­hibition (N=24) and differential conditioning control (N= 12) groupswere treated exactly like their counterparts of the first two experi­ments. The unpaired control (N=24) group received lO-min ac­cess to saccharin on one occasion during each conditioning cycleand 2-min access to saccharin followed by lO-min access to salineon two occasions during each cycle, just as the conditioned inhibi­tion group did. However, the rats in the unpaired control groupreceived LiCI injections following water consumption, 24 h afterexposure to saccharin.

Twelve of the rats in the conditioned inhibition and unpaired con­trol groups were given a retardation-of-acquisition test for inhibi­tion. The procedure of Experiment 1 was repeated, except that therewere four, rather than three, pairings of saline and LiC!.

The remaining 12 rats in each of the conditioned inhibition andunpaired control groups, along with the 12 rats in the differentialconditioning control group, received a summation test for inhibi­tion. Following the conditioning phase, these rats received a sin­gle pairing of 10-min access to a 1.5 % normal solution of HCI withan ip injection of 1.8 mEq of O.15-M LiC!. Following 2 water­recovery days, the rats received lO-min two-bottle tests with waterin both bottles on 2 successive days. The positions of the two bot­tles were switched 5 min into the tests. This procedure was designedto adapt the rats to the two-bottle testing procedure. Then the ratsreceived two summation tests pitting HCI against a mixture of HCIand saline; these tests were separated by a water-recovery day. Thestarting position for the HCI and saline mixture was counterbalanced

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drank increasing amounts of saccharin over trials. Themain effect of groups was nonsignificant (F < 1), as wasthe interaction between groups and trials (F < 1). Dur­ing the immediately prior 2-min exposure to saline orwater, the consumption of saline by the inhibitory condi­tioning group was slightly greater than the consumptionof water by the control group (mean saline consumed =4.86 ml for the inhibition group, and mean water con­sumed = 3.88 ml for the control group). Such a differ­ence could tend to mask an inhibitory conditioning effectof saccharin intake if enhanced fluid consumption duringthis 2-min period were to alleviate thirst motivation.However, there was also a slight positive correlation be­tween the amount of fluid consumed during this periodand the amount of saccharin consumed during the subse­quent lO-min exposure (r= .42), indicating that no suchmasking had occurred. In the lO-min exposure to saccha­rin between Trials 2 and 3 of this phase, the two groupsconsumed equal amounts of saccharin (F < 1).

Figure 5. Consumption of saccharin during each to-min one-bottletest when access to NaCI (inhibition group) or water (control group)had preceded it for 2 min.

DiscussionThe subjects in this experiment were trained in exactly

the same manner as those of the first experiment. Fol­lowing this, the saline-water test used in Experiment 1was conducted. In contrast to the results of the first ex­periment, the present results gave no indication that ex­posure to the conditioned inhibition procedure had in­creased the preference for the saline solution.

In light of these negative results, we turned to yetanother attempt to detect conditioned inhibition. The se­quential summation test is similar in logic to more con­ventional summation tests. However, the results of thistest provided no evidence that the saline solution had be­come inhibitory.

This experiment was a further attempt to demonstrateconditioned inhibition using summation and retardation-

12 DELAMATER, KRUSE, MARLIN, AND LoLORDO

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Figure 6. The left panel illustrates percentage of HCI-NaCI mixture consumed when the mixture waspresented in a two-bottle test with a previously poisoned HCI solution as the alternative. The right panelillustrates the total intake from both bottles during this test.

Figure 7. Absolute consumption of NaCI when it was paired withan LiCI injection over four trials in Experiment 2.

DiscussionNeither the summation nor the retardation-of-acquisition

test provided any evidence for conditioned inhibition. In

tion. Although the effect of groups was not significant[F(l,22) = .96], the effect of trials [F(3,66) = 166] andthe group X trial interaction [F(3,66) = 3.18] were sig­nificant.

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ResultsAcquisition of an aversion to saccharin was substantial

after three trials for the conditioned inhibition anddifferential conditioning control groups; the unpaired con­trol group maintained its consumption of saccharin(groups X trials, F = 67.36). As in Experiment 1, mem­bers of the inhibition groups drank slightly more saccha­rin on Trials 2 and 3 than did rats in the differential con­ditioning control groups. Saline consumption increasedacross trials, as in Experiment 1, and again the groupsdid not differ in this respect.

Figure 6 illustrates the total fluid consumed during eachsummation test and the percentage of HC1-NaCl mixtureconsumed during each test. The three groups failed todiffer on either measure during either test [allFs(2,33) < 1]. However, there was an increase in totalconsumption from the first to the second summation test[F(1,33) = 58.3], suggesting that the failure of the groupsto differ on the preference measure during the first testwas not the result of a ceiling effect on consumption.

In the subsequent test of preference for NaCl aloneversus NaCl mixed with HCl, virtually all rats preferredNaCI alone. The effect of flavor was significant [F(1,33)= 177], whereas the effect of groups and the group Xflavor interaction were not significant [Fs(2,33) < 1.13].

Figure 7 illustrates the results of the retardation-of­acquisition test. The unpaired control group acquired anaversion to saline more slowly than did the conditionedinhibition group. Statistical analysis supported this asser-

across subjects. Finally, the rats underwent a treatment designedto show that they could discriminate 1.5% HCI when it was mixedwith the saline. Following a water-recovery day, all rats from thesummation testing procedure received lO-min access to HCI fol­lowed by an injection of a 3-mEq dose of LiCl. After 2 water­recovery days, NaCI was pitted against HCI mixed with NaCI ina two-bottle test.

the case ofthe summation test, the conditioned inhibitiongroup failed to differ from either a differential condition­ing control group, like that of Experiment I, or an un­paired control group, like that used by Best (1975).Moreover, the effects of the two control procedures weresimilar. In the retardation test, the unpaired control group,which should have suffered as much generalization decre­ment as the experimental group, acquired a taste aver­sion more slowly than the conditioned inhibition group,a result opposite that expected on the basis of conditionedinhibition.

GENERAL DISCUSSION

The general pattern of results from all the present workon conditioned inhibition in the CTA paradigm suggeststhat this is an elusive phenomenon. Data from several as­says do not converge upon the existence of such aphenomenon. This conclusion contrasts sharply with thatdrawn by Best (1975), who obtained a robust effect inseveral experiments. There are several differences be­tween the present work and Best's. One difference is thatwe used LiCI as the unconditioned stimulus (DCS) andBest used apomorphine. This could be an important differ­ence, since recent evidence suggests that conditioned tasteaversions induced by apomorphine and LiCI are based ondifferent physiological mechanisms (Pratt & Stolerman,1984). However, Batson and Best reported conditionedinhibition in an experiment with LiCI as the DCS.

Generating conditioned inhibition might be expected torequire a large number of training trials, relative to thenumber required to generate conditioned excitation (e.g. ,Wagner & Rescorla, 1972), and it is certainly the casethat more trials are used for inhibitory conditioning thanfor excitatory conditioning in most preparations.However, empirically, the rate of acquisition of condi­tioned inhibition is not well understood. The present ex­periments involved more trials than Best's experiments,and approximately the same number as were presentedby Batson and Best. The present procedure seems reason­able in the context of our goals-to replicate and extendprevious work.

Another difference between the present manipulationsand those used by Best is that we administered repeatedcycles of excitatory trials and inhibitory trials, rather thanadministering two excitatory trials and then one inhibi­tory trial as he did. However, the present training proce­dures are more similar to common practices (see Mar­chant & Moore, 1974; Rescorla, 1982; Rescorla &LoLordo, 1965), as are those of Batson and Best (1981).

Historically, there has been considerable interest in thedifferences between conditioned taste aversions and otherconditioned responses (see, e.g., Garcia & Koelling,1966; Rozin & Kalat, 1971; Rusiniak et al., 1979). Kalatand Rozin (1972) speculated that CTAs might be formedby some mechanism more "primitive" and less cogni­tive than those underlying the acquisition of other condi­tioned responses; they based their suggestion on their

INHIBITION IN CTA 13

failure to observe blocking in a CTA preparation (but seeGillan & Domjan, 1977; Revusky, 1971). Garcia,Hankins, and Rusiniak (1974) suggested that in acquir­ing a taste aversion, unlike in say the CER procedure,the rat "does not act as if it were acquiring an 'if-then'strategy. It acts as if a hedonic shift, or a change in theincentive value of the flavor were taking place" (p. 831).If the A+, AX-procedure typically renders stimulus Xinhibitory because X is nonreinforced at a time when Aevokes an expectation of the DCS, then the suggestionmade by Garcia and his colleagues would imply that theA+, AX-procedure used in the present experimentsshould not generate conditioned inhibition, because A, ataste, evokes no expectation of illness to be violated inthe presence of X. Why then should Best (1975) and Bat­son and Best (1981) have established conditioned inhibi­tion? In both of those studies, unlike the present ones,the A cue consisted not only of access to a taste but alsoof being placed in a distinctive environment. Perhaps theset of contextual cues evoked an expectation of illness,and thus permitted the AX-trials to establish X as a con­ditioned inhibitor.

Consistent with this suggestion, Best, Dunn, Batson,Meachum, and Nash (1985) reported the finding, in threeexperiments, of an enhanced preference for a flavortrained as an inhibitor in a context that otherwise reliablypreceded illness. On excitatory conditioning trials, the ratswere exposed to the novel context for 10 min (the last5 min of which water was made available in Experi­ments 2 and 3) and then injected with LiCL On inhibi­tory conditioning trials, a vinegar solution was presentedduring the last 5 min of exposure to this context. In con­trast to a procedure that uses a taste excitor, this proce­dure should especially encourage the development of in­hibition not only because the contextual stimuli mightevoke an expectation of the DCS, but also because thepresumed expectation is evoked at a time when the puta­tive inhibitor is simultaneously present (Konorski, 1948;Pavlov, 1927).

Procedures in which a taste is paired with recovery fromthe illness induced by poisoning (e.g., Hasegawa, 1981)or thiamine deficiency (Zahorik, Maier, & Pies, 1974)can be considered formally similar to backward and ces­sation conditioning procedures, which often result in con­ditioned inhibition (see LoLordo & Fairless, 1985). Tastespaired with recovery from illness in these procedures dobecome preferred to familiar tastes (Zahorik et al., 1974),an effect analogous to the inhibitory effect we sought.However, it should be noted that the occurrence of thisso-called "medicine effect" does not imply that the A+,AX- procedure used in the present experiments shouldalso yield inhibitory effects, for the mechanism of themedicine effects and other inhibitory conditioned effectsof backward and cessation conditioning procedures maybe very different from the mechanism of inhibition in theA+, AX- procedure. Presumably, the former involveno violation of expectations. Thus, our results are plau­sible despite the occurrence of medicine effects.

14 DELAMATER, KRUSE, MARLIN, AND LoLORDO

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(Manuscript received August 14, 1985;revision accepted for publication January 27, 1986.)