Acquired flavour preferences: Contextual control of adaptation-level effects

This experiment investigated the role of context in the expression of conditioned flavour preferences. Rats were trained on a mixture of almond and sucrose and were then given intermixed exposures to almond in one context (Context A) and to sucrose in a second context (Context S). Finally, choice tests were given in both contexts, with one group given almond-versus-water tests and the other almond + sucrose-versus-sucrose tests. Preference for almond over water was greater in Context A than in Context S. Conversely, preference for almond + sucrose over sucrose was greater in Context S than in Context A. These results suggest that the perceived sweetness of a flavour depends on the context in which it is presented and confirm that expression of a flavour preference depends on the type of test employed.     

Expression of flavor preference depends on type of test and on recent drinking history

The persistence of a conditioned flavor preference was examined in 3 experiments. All contained an initial acquisition phase in which half the rats were given almond odor mixed with sucrose (AS) in some sessions and water (W) only in other sessions, whereas the other half (controls) were given explicitly unpaired exposures to almond (A) and sucrose (S) in separate sessions. Subsequent 2-bottle choice tests revealed a persistent preference for A, despite extinction exposure to A or S, but this depended on the choice offered on test: Exposure to A did not extinguish the preference for A over W but did reduce the preference for AS over S; conversely, exposure to S did not extinguish the preference for AS over S but did reduce the preference for A over W. These results indicate that flavor preferences can be resistant to extinction procedures but suggest that the expression of such preferences in choice tests depends on an adaptation-level process.     

Contrast effects in flavour preference learning

In four experiments the role of contrast effects in producing learned flavour preferences was examined. The experiments showed that contrast effects are pervasive in flavour preference learning, producing results that are often paradoxical from a traditional reinforcement point of view. In Experiment 1, rats preferred a reinforced flavour over a nonreinforced flavour more if the reinforcer was 1% sucrose than if the reinforcer was 8% sucrose. Because the cue flavour was dissolved in 8% sucrose, this represents an anticipatory positive contrast effect. In Experiment 2, the relationship between cue and consequence was shown to be important in flavour preference learning, as expected if contrast effects are involved. In Experiment 3, rats preferred a flavour that was reinforced 4 times, and they preferred this flavour more than one reinforced 8 times, presumably because the greater the expectancy of the consequence, the greater the anticipatory negative contrast. In Experiment 4, rats consumed less of 0.15% saccharin if they received 32% sucrose randomly 90 min either before or after the 0.15% saccharin than if they received only 0.15% saccharin, a simultaneous negative contrast effect.     

Extinction of Within-event Learning Is Contextually Controlled and Subject to Renewal

Five experiments examined within-event learning in rats by inducing an appetite for one of the elements (salt) of a compound stimulus and assessing preference for the other element (almond). Almond preference was conditional upon (1) the almond flavour having been presented in compound with the salt, and (2) the assessment being conducted when the rats were out of sodium balance (Experiment 1). Presentations of the compound in one environment (A) and of the salt and almond elements in a second environment (B) resulted in greater almond preference when rats were tested in A than in B (Experiment 2). Almond preference was reduced when separate presentations of the compound and almond (Experiment 3) or of the compound and salt (Experiment 4) occurred in the same environments but not when these presentations occurred in different environments. Rats exposed to the compound in A and then extinguished to the elements in either A or B showed a reduced almond preference when tested in the extinction environment, but not when tested in the other environment (Experiment 5). Thus, extinction of within-event learning is context-specific and subject to renewal. The results were interpreted in terms of an associative model whereby separate presentations of the elements result in a symmetrical inhibitory link which is contextually gated (Bouton, 1993).     

Contextual factors in neophobia and its habituation: The role of absolute and relative novelty

In four experiments we investigated the role of contextual cues in the habituation of neophobia in rats. Experiment 1 showed that the consumption of a novel flavour increased across a series of presentations in one context (A) but fell when the flavour was subsequently presented in a second, novel, context (B). In Experiments 2 and 3, subjects again received exposure to a flavour in context A, but also were familiarized with the test context, B. These subjects consumed the flavour with equal readiness, whether it was presented in Context A or in Context B at test. Experiment 4 replicated the results of Experiment 1 and also showed that the consumption of a novel flavour was not influenced by whether it was presented in a novel or a familiar context. Several mechanisms by which the novelty or familiarity of the context might interact with the novelty or familiarity of the flavour were discussed.     

Persistence of conditioned flavor preferences is not due to inadvertent food reinforcement

Almond preferences were produced by giving rats a mixture of almond and sucrose (Experiments 1-4) or saccharin (Experiment 4). A subsequent extinction procedure consisted of either repeated 2-bottle almond versus water tests (Experiment 1) or repeated exposure to almond alone (Experiments 2-4). The main independent variable was whether access to food following a session was given immediately, 30 min later, or 120 min later. No effect of extinction was found in any experiment. An important finding was that varying the delay until food access had no detectable effect. It was concluded that inadvertent flavor-food associations do not maintain preference for the flavor under extinction conditions.     

Super-latent inhibition of conditioned taste preference with a long retention interval

A long delay inserted between conditioning and test phases of a 3-stage Latent Inhibition (LI) procedure produces differential effects on LI depending on the delay context. Thus, enhanced LI has been obtained when the delay is spent in a context that is different from the remaining experimental contexts, but not when it is the same. The present paper examined the effect of delayed testing using a conditioned taste preference procedure. In Experiment 1, three groups received preexposure to either water, or almond, or citrus solutions. Subsequently, the animals were conditioned by pairing the almond and the citrus solutions with sucrose. In a two-bottle preference test conducted 1 day after conditioning, the preexposed flavored solutions were consumed less than the non-preexposed flavors (LI). Experiments 2 and 3 used the same basic procedure, but varied the retention interval duration (1 and 21 days) and the delay context, similar (Experiment 2) and different (Experiment 3) from that of the other stages of the experiments. LI was greater after the long than the short-retention interval when the context was different, thereby demonstrating super-LI. The results were explained by a time-induced context differentiation process.     

Context specificity of latent inhibition in taste aversion learning

In three experiments rats were given injections of LiCl after consuming distinctively flavoured water. The rats developed an aversion to the flavour and in all experiments the magnitude of the aversion was found to be reduced in subjects that had received pre-exposure to the flavour without aversive consequences. Experiment 1 demonstrated this pre-exposure effect to be a case of latent inhibition. The remaining experiments investigated the effects of pre-exposing the flavour in a context different from that used for conditioning. It was found (Experiment 2) diat latent inhibition transferred perfectly when the context change consisted of a move from one home cage to another. Context specificity of latent inhibition was found (Experiment 3) only when the subjects were given daily sessions in die experimental contexts, these being cages different from the home cage.     

Assessment of the relative importance of S+ and S− in rats using differential training on intercurrent discriminations

The present studies assessed the degree of stimulus control exerted by S+ and S? without confoundings of stimulus novelty and stimulus ambiguity. In Experiment 1, rats were trained on two intercurrent simultaneous discrimination problems with nine times more training given on one than the other. Then the animals were given transfer tests with re-paired stimuli. The results showed that S? exerts greater stimulus control than S+ in a two-choice simultaneous discrimination. Experiment 2 provided a test of the possibility that the relative degree of control by S? varies with different amounts of training. Three groups were trained on two intercurrent simultaneous discrimination problems; each group was given 7, 11, or 15 times more training on one problem than the other. Then transfer tests were given with re-paired stimuli. Again the results showed that S? exerts greater stimulus control than S+ in a two-choice simultaneous discrimination.     

Contextual control of appetitive conditioning: Influence of a contextual stimulus generated by a partial reinforcement procedure

Three experiments with rat subjects examined the effects of a context switch after conditioning treatments in which the conditioned stimulus (CS) was either paired with food on every presentation (continuous reinforcement) or on some of its presentations (partial reinforcement). In each experiment, a target CS was given one of these treatments in Context A, and another CS was given a treatment during sessions that were intermixed in Context B. Final tests of the target CS in Context A and Context B often revealed no loss of responding with the switch to B. However, a loss was observed when partial reinforcement had been associated with Context A and continuous reinforcement had been associated with Context B. Those conditions caused equal decrements in responding to partially reinforced and continuously reinforced targets. The results suggest that under the present conditions partial reinforcement can generate a contextual stimulus that becomes associated with the physical context and controls responding to the CS.     

Caffeine deprivation state modulates expression of acquired liking for caffeine-paired flavours

Previous studies found that caffeine consumers acquired a liking for the flavour of novel caffeinated drinks when these drinks were consumed repeatedly in a caffeine-deprived, but not nondeprived, state. Expression of this acquired liking appeared acutely sensitive to current caffeine deprivation state, but the use of between-subjects designs confounded interpretation of those studies. The present study evaluated these findings further using a within-subject design, with one flavour paired with caffeine (CS + ) and the second with the absence of caffeine (CS–). During four CS + and four CS– training days, 32 moderate caffeine consumers alternatively consumed a novel flavoured drink a CS + paired with caffeine and a CS– flavour paired with placebo. Participants evaluated both drinks before and after training in two motivational states: caffeine deprived and nondeprived. As predicted, pleasantness ratings for the caffeine-paired flavour increased overall. However, this acquired liking was only significant when tested in a caffeine-deprived state. These data are consistent with a conditioned-flavour preference model and imply that expression of acquired liking for a novel caffeinated flavour depends on the need for the effects of caffeine at the time when the drink is evaluated.     

Caffeine deprivation state modulates expression of acquired liking for caffeine-paired flavours

Previous studies found that caffeine consumers acquired a liking for the flavour of novel caffeinated drinks when these drinks were consumed repeatedly in a caffeine-deprived, but not nondeprived, state. Expression of this acquired liking appeared acutely sensitive to current caffeine deprivation state, but the use of between-subjects designs confounded interpretation of those studies. The present study evaluated these findings further using a within-subject design, with one flavour paired with caffeine (CS + ) and the second with the absence of caffeine (CS-). During four CS + and four CS- training days, 32 moderate caffeine consumers alternatively consumed a novel flavoured drink a CS + paired with caffeine and a CS- flavour paired with placebo. Participants evaluated both drinks before and after training in two motivational states: caffeine deprived and nondeprived. As predicted, pleasantness ratings for the caffeine-paired flavour increased overall. However, this acquired liking was only significant when tested in a caffeine-deprived state. These data are consistent with a conditioned-flavour preference model and imply that expression of acquired liking for a novel caffeinated flavour depends on the need for the effects of caffeine at the time when the drink is evaluated.     

Motivational state regulates the content of learned flavor preferences

Rats acquired a preference for an aqueous odor (almond) presented in simultaneous compound with sucrose. Separate presentations of saccharin reduced this preference in rats with ad-lib access to food during training or at test, but not in rats that were hungry during both training and test. In contrast, separate presentations of sucrose reduced the preference for the almond irrespective of deprivation state during training and test. We interpret the results to mean that a hungry rat forms odor-taste and odor-calorie associations, and its motivational state on test determines which of these associations controls the preference. In contrast, a rat that is not hungry during training only forms an odor-taste association, and its performance on test is independent of its level of hunger.     

Palatability shifts in taste and flavour preference conditioning

Changes in palatability of tastes and flavours as a result of flavour preference conditioning were examined. In Experiment 1, when tastes were paired with glucose in a reverse-order differential conditioning paradigm, rats acquired conditioned preferences for CS + and displayed more hedonic responses to CS + than to CS - in a postconditioning taste reactivity test. In Experiment 2, rats that received oral infusions of flavours as CSs during a reverse-order conditioning procedure expressed both palatability shifts and conditioned preferences for CS + . Rats that received a forward conditioning procedure acquired a preference for CS + , but the palatability of CS + was unchanged. In Experiment 3, hungry rats drank mixtures of a flavour CS and a calorific or sweet tasting reinforcer in a long-exposure conditioning paradigm. When tested hungry, rats preferred CS + whether they had acquired flavour-calorie or flavour-taste associations. However, CS + became more palatable only for rats that acquired flavour-calorie associations. These results suggest that acquisition of flavour preferences, as measured by 2-bottle tests, may not always be accompanied by enhanced palatability.     

Flavour liking and preference conditioned by caffeine in humans

When caffeine consumers repeatedly experience a novel flavoured drink containing caffeine, the rated pleasantness of the drink flavour increases progressively. These results could be interpreted in terms of the flavour acting as a Pavlovian conditioned stimulus (CS) predicting the consequences of caffeine ingestion. However, all studies of this phenomenon to date have used between-subjects designs, and one criticism of this is that changes in pleasantness might have arisen from nonspecific effects. A more rigorous test is to examine changes in pleasantness for two drinks, a CS+ flavour paired with caffeine and CS- paired with placebo. Accordingly, 20 moderate caffeine consumers consumed both CS+ and CS- drinks in counterbalanced order over eight conditioning trials at breakfast, with hedonic and sensory characteristics evaluated on each trial. As predicted, the rated pleasantness of the CS+ drink increased whereas pleasantness of the CS- drink did not change. Despite this, participants did not have an overall preference for the CS+ flavour posttraining. However, both those who chose the CS+ and those who chose the CS- at the end showed the same direction and rate of change in pleasantness for the two drinks during training, but spurious differences in baseline preference obscured this effect in terms of an overall change in preference. Overall these data suggest that changes in pleasantness of drinks paired with caffeine delivery are best explained in terms of Pavlovian associations between drink flavour and the postingestive effects of caffeine.     

Extinction and latent inhibition of within-event learning are context specific

Exposure to a solution composed of an odor (almond) and a taste (salt) produced a context-independent preference when rats were subsequently tested with almond under a salt appetite. Postcompound exposure to either the almond or the salt alone reduced almond preferences but only when rats were tested in the extinction context. Exposure to either the almond or the salt in 1 context in advance of exposure to the compound in a different context also reduced preferences but only when the rats were tested in the context in which the element had been pre-exposed. These results show that extinction and latent inhibition of within-event learning are context specific.     

Peak shift in honey bee olfactory learning

If animals are trained with two similar stimuli such that one is rewarding (S+) and one punishing (S?), then following training animals show a greatest preference not for the S+, but for a novel stimulus that is slightly more different from the S? than the S+ is. This peak shift phenomenon has been widely reported for vertebrates and has recently been demonstrated for bumblebees and honey bees. To explore the nature of peak shift in invertebrates further, here we examined the properties of peak shift in honey bees trained in a free-flight olfactory learning assay. Hexanal and heptanol were mixed in different ratios to create a continuum of odour stimuli. Bees were trained to artificial flowers such that one odour mixture was rewarded with 2 molar sucrose (S+), and one punished with distasteful quinine (S?). After training, bees were given a non-rewarded preference test with five different mixtures of hexanal and heptanol. Following training bees’ maximal preference was for an odour mixture slightly more distinct from the S? than the trained S+. This effect was not seen if bees were initially trained with two distinct odours, replicating the classic features of peak shift reported for vertebrates. We propose a conceptual model of how peak shift might occur in honey bees. We argue that peak shift does not require any higher level of processing than the known olfactory learning circuitry of the bee brain and suggest that peak shift is a very general feature of discrimination learning.     

Flavour aversion produced by running and attenuated by prior exposure to wheels

In two experiments hungry rats were given access to running wheels. When given the novel flavour, almond, prior to novel access to the wheels, a conditioned aversion to almond was revealed by a subsequent two-bottle test. No such aversion was found in rats with previous experience of wheel running, whether this prior running occurred in the absence of any novel flavour, as in Experiment 1, or following access to saccharin, as in Experiment 2. These results suggest that the failure of rats with prior experience of the running wheels to develop a flavour aversion (unconditioned stimulus, US, preexposure effect) is unlikely to be due to associative blocking. Instead it seems that increasing exposure to a wheel produces habituation of its nausea-inducing properties.     

Perceptual learning enhances retrospective revaluation of conditioned flavor preferences in rats

Forward and backward blocking of taste preference learning was compared in rats. In the forward condition, thirsty rats were exposed to a flavor (A) in sucrose solution (+) or in water (-), after which they were exposed to A in compound with another flavor (B) in sucrose solution (i.e., AB+). In the backward condition, these phases were reversed. Consumption of B alone was assessed when rats were food deprived. In the forward condition, rats given A+ consumed less B than rats given A-, providing evidence of forward blocking, whereas in the backward condition, rats given A+ drank more of B than those given A-. Subsequent experiments found that alternating but not blocked preexposure to A and B, when given prior to training, produced blocking of B whether A+ was given before or after AB+, suggesting that prior failures to observe backward blocking reflect failures of discrimination.     

Emergent Simple Discrimination via Transfer from Differentially Reinforced S+ Stimuli: A Further Test of the Stimulus-Response Interaction Model

This study examined emergent form discrimination through contiguity of the forms (B stimuli) with colours (A stimuli) associated with different magnitudes of reinforcement. In Experiment 1, children were trained on two colour discriminations. In both, the colour stimuli had the same form: A1B1/A2B1 and A3B1/A2B1. Responding to A1/B1 was followed by three units of reinforcement, to A3B1 by one unit of reinforcement, and to A2B1 by no reinforcement. There followed five tests in which stimuli were presented without programmed consequences. These assessed: (1) the maintenance of accurate performance on the training tasks; (2) S+ preference (A1B1/A3B1); (3) stimulus generalization (A1B2/A2B3, A3B4/A2B5); (4) S+ preference again (A1B6/A3B6); and (5) transfer from the S+ colours to the forms (A4B2/A4B4). On the final test, seven of the eight subjects selected A4B2. Experiment 2 demonstrated that a preference for A4B2 also occurred when the S+ preference tests were omitted. This preference was also found when the total amounts of reinforcement associated with the S+ colours (A1 and A3) were the same (Experiment 3). These findings: (a) indicate that children can acquire new discriminations under non-reinforced conditions through contiguity with differentially reinforced S+ stimuli; (b) support the Stimulus-Response Interaction model (Smeets, 1993) for specifying transfer.