Conditioned flavor preferences based on hunger level during original flavor exposure

In five experiments, rats’ preference for a flavor was greater if the flavor had previously been consumed under low rather than high deprivation. This preference was conditioned in as few as three flavor-deprivation pairings (Experiment 1), and persisted through 28 test days, half under each deprivation level (Experiment 2). Rats never preferred the flavor associated with high deprivation even when calories were increased by giving 40 ml of 8% sucrose or when caloric density was increased to the equivalent of 20% sucrose. The preference for the low-deprivation flavor was greater when saccharin solutions were used rather than sucrose solutions, but the preference did emerge when sucrose solutions were used as testing proceeded and when a lower concentration of sucrose was used. We suggest that these preferences may be a result of flavor-taste associations rather than associations between flavors and postingestive consequences, and that the taste of the solutions under low deprivation is preferred to the taste under high deprivation.     

Effects of food deprivation on learning and expression of flavor preferences conditioned by saccharin or sucrose

In four experiments, food deprivation was varied during conditioning and testing of conditioning of flavor preferences by sweeteners. Conditioned preferences for a flavor associated with a more concentrated solution were enhanced by increased deprivation in training whether sucrose or saccharin was used when rats consumed solutions freely during training. When consumption of solutions was controlled and higher deprivation levels were used, preference for the higher concentration of sucrose was still enhanced by increased deprivation in training, but this did not occur with saccharin. We suggest that deprivation may enhance the reinforcing value of sweetness only when calories increase along with sweetness. We also suggest that deprivation can enhance flavor preference learning by increasing consumption and thereby increasing exposure to the flavored solutions.     

Impact of brief or extended extinction of a taste aversion on inhibitory associations: Evidence from summation,retardation, and preference tests

In five conditioned taste aversion experiments with rats, summation, retardation, and preference tests were used to assess the effects of extinguishing a conditioned saccharin aversion for three or nine trials. In Experiment 1, a summation test showed that saccharin aversion extinguished over nine trials reduced the aversion to a merely conditioned flavor (vinegar), whereas three saccharin extinction trials did not subsequently influence the vinegar aversion. Experiment 2 clarified that result, with unpaired controls equated on flavor exposure prior to testing; the results with those controls suggested that the flavor extinguished for nine trials produced generalization decrement during testing. In Experiment 3, the saccharin aversion reconditioned slowly after nine extinction trials, but not after three. Those results suggested the development of latent inhibition after more than three extinction trials. Preference tests comparing saccharin consumption with a concurrently available fluid (water in Experiment 4, saline in Experiment 5) showed that the preference for saccharin was greater after nine extinction trials than after three. However, saccharin preference after nine extinction trials was not greater, as compared with that for either latent inhibition controls (Experiments 4 and 5) or a control given equated exposures to saccharin and trained to drink saline at a high rate prior to testing (Experiment 5). Concerns about whether conditioned inhibition has been demonstrated in any flavor aversion procedure are discussed. Our findings help explain both successes and failures in demonstrating postextinction conditioned response recovery effects reported in the conditioned taste aversion literature, and they can be explained using a memory interference account.     

The role of oral and postingestional cues in the conditioning of taste preferences based on differing caloric density and caloric outcome in weanling and mature rats

Experiments 1, 2, and 3 demonstrated that 23-day-old rat pups can acquire a conditioned taste preference for a flavor paired with either a more calorically dense diet or a diet that provides more calories than another diet of equal caloric density. Experiment 4 showed that 42–49-day-old rats also can acquire conditioned taste preferences based on differing caloric density and differing caloric outcome. Experiments 5 and 6 demonstrated that adult rats form conditioned taste preferences based only on caloric density. Although these experiments indicate that young rats are apparently capable of more diverse learning than adult rats, the question of what US underlies conditioned taste preferences based on diets of differing caloric density remains unresolved.     

The effect of nonnutritive satiation on the learning of a flavor preference by rats

On each day of training in Experiment 1, hungry rats were given one flavored saccharin solution followed by a differently flavored saccharin solution. The rats drank more of the first flavor during training, but preferred the second flavor in a subsequent choice test. In Experiment 2, the two flavored saccharin solutions were provided on alternate days, with one flavor being preceded by nothing and the other flavor by plain saccharin. The rats drank more of the flavor preceded by nothing during training, but preferred the other flavor in a subsequent choice test. These results suggest that a state of nonnutritive satiation can reinforce a flavor preference.     

Potentiation of taste and extract stimuli in conditioned flavor preference learning

In these experiments, we investigated the nature of potentiation in the conditioned flavor preference paradigm. Almond and banana extracts, which have strong odor components, were combined with salt and saccharin (liked tastes; Experiment 1) or quinine and citric acid (disliked tastes; Experiment 2) in a flavor preference procedure that mixed these solutions with a caloric reinforcer (polycose). The results showed that liked tastes potentiated preference conditioning to extracts (Experiment 1), whereas extracts potentiated preference conditioning to disliked tastes (Experiment 2). In both experiments, the presumably less liked stimulus (i.e., the extract in Experiment 1 and the disliked taste in Experiment 2) was the potentiated cue.     

A learned flavor preference persists despite the extinction of conditioned hedonic reactions to the cue flavors

Learned flavor preferences can be strikingly persistent in the face of behavioral extinction. Harris, Shand, Carroll, and Westbrook (2004) suggested that this persistence may be due to flavor preference conditioning’s producing a long-lasting change in the hedonic response to the conditioned stimulus (CS+) flavor. In the present study, the CS+ flavor was presented in simultaneous compound with 16% sucrose, whereas the CS− flavor was presented with 2% sucrose. During subsequent two- and one-bottle tests, the CS+ and CS− flavors were presented in 2% sucrose. Hedonic reactions during training and test were assessed using an analysis of the microstructure of licking behavior. Conditioning resulted in greater consumption of the CS+ than of the CS− that did not extinguish over repeated two- and one-bottle tests. The mean lick cluster size was higher for the CS+ than for the CS− only on the first cycle of tests. Since lick cluster size can be used as an index of stimulus palatability, the present results indicate that although the hedonic reaction to the CSs did change, this was not maintained across repeated tests. Thus, changes in the hedonic response to the conditioned flavors cannot explain the resistance to the extinction of learned flavor preferences.     

Posttraining flavor exposure in hungry rats after simultaneous conditioning with a nutrient converts the CS into a conditioned inhibitor

The present study investigated the decrement in nutrient-based conditioned flavor preference found in hungry rats exposed to a flavor following simultaneous flavor–sucrose conditioning while thirsty. Although a significant decrease in preference was found in the experimental group in each experiment, there was no evidence of either spontaneous recovery (Experiment 1) or reinstatement (Experiment 2). In addition, posttraining flavor exposure weakened the original flavor–sucrose association (Experiment 3). These results suggested that the flavor–US association might have been impaired after posttraining flavor exposure. Two further experiments assessed whether the flavor acquired the properties of a net inhibitor, using the retardation and summation tests for conditioned inhibition. Experiment 4 revealed that the flavor suffered retardation when retraining was conducted after the exposure phase. In Experiment 5, the target flavor decreased the preference shown for a different flavor previously paired simultaneously with sucrose when both were presented forming an unreinforced compound in the summation tests. None of these effects was found in a control group, which had received serial flavor → nutrient presentations during training. Together, these results suggest that a flavor simultaneously paired with sucrose acquires the properties of a net inhibitor when it is subsequently presented outside the compound to hungry animals.     

Partial reinforcement and latent inhibition effects on stimulus–outcome associations in flavor preference conditioning

Two experiments with thirsty rats explored the harmful effects of non-reinforced exposures to a flavor cue in the control by sensory-specific flavor–sucrose associations in a conditioned flavor preference paradigm. Experiment 1 demonstrated that rats learned to prefer a flavor cue that was consistently paired with sucrose over one that was paired with sucrose the same number of times but was also presented without sucrose on other occasions. However, rats for which sucrose was devalued following the conditioning phase preferred the partially reinforced flavor cue over the consistently reinforced flavor, suggesting that non-reinforcement weakened the ability of that flavor cue to evoke a specific representation of sucrose during the preference test. Experiment 2 demonstrated comparable effects of non-reinforcement in a latent inhibition procedure, although relatively more non-reinforced pre exposures to the flavor, in conjunction with fewer flavor–sucrose pairings, were required to see the effect. Together, the results suggest, as is often found with more traditional learning paradigms, that non-reinforcement of a flavor cue has deleterious effects on preference learning and/or performance.     

Taste preferences as a function of food deprivation during original taste exposure

It has been reported previously that rats prefer a flavor they consumed under high deprivation to a flavor they consumed under low deprivation (Revusky, 1967). Here it was found that this preference occurs only if nutritive solutions are used and the flavors are given preceding and following eating. If flavors are given separately from the daily feeding, rats prefer the flavor given under low deprivation, whether or not a nutritive solution is used (Experiment 3). If flavors are given before and after the daily feeding, rats prefer the flavor they had under high deprivation (before feeding) more if sucrose solutions are used than if saccharin solutions are used and more on a high-deprivation test than on a low-deprivation test (Experiments 1 and 2). It was concluded that the “incentive value” of consumption is not necessarily higher under high deprivation than under low deprivation. The preference for the low-deprivation flavor obtained here may reflect a greater proportional rewarding effect of consumption under low deprivation or may reflect an aversion to the flavor consumed under high deprivation. Perhaps a small taste of flavor under high deprivation initiates responses of digestion that are unsatisfied and thus aversive, and the more so the higher the deprivation level.     

Foraging on the radial-arm maze: Effects of altering the reward at a target location

Thirsty rats were trained to collect small water rewards from the end of each arm of an eight-arm radial maze. During these training trials and subsequent testing trials, the subjects were allowed to choose a maximum of eight arms. “Preference” for a target maze location was studied by noting when, in the sequence of eight choices, the target was selected. During testing, when one maze location was consistently devoid of water, rats decreased their preference for this arm over trials (Experiment 1). Similarly, rats that learned a saccharin-lithium association demonstrated lower preferences for a maze location that consistently held the conditioned saccharin solution. This was true for animals that received saccharin-lithium conditioning on the maze (Experiment 3A) and for animals conditioned to saccharin in a separate context (Experiment 3B). An increase in preference for a target maze location consistently containing a sweet chocolate milk solution was observed in animals that were water- and food-deprived (Experiment 2). These studies demonstrate that animals will modify their responses toward (preferences for) maze locations that predictably contain an altered reward.     

Conditioning with taste as the CS in conditioned flavor preference learning

There appear to be unconditioned affective reactions to the four basic tastes: liking for sweet and salt and disliking for sour and bitter. We attempted to modify these reactions by pairing the tastes with calories and with sweeter tastes. Differing sucrose concentrations were ineffective in reversing the preference for salt over citric acid or for saccharin over quinine. We could, however, reverse the preference for salt over citric acid, producing an actual preference for sour over salt, by using sucrose and saccharin as the reinforcers. The initial reactions to tastes could also be modified by reducing the initial difference in affect produced by the tastes through mixing both tastes with sucrose.     

Flavor preferences conditioned with starch in rats

Rats were trained to associate artificial cherry or grape flavors with 1% starch suspensions. Conditioning was assessed by offering the rats a choice of the cherry versus grape flavors without starch. Conditioned preferences were moderately strong and persistent; 3 days of conditioning produced a preference that did not fully extinguish within 18 days. Food deprivation substantially increased intake of 1% starch suspension, However, the degree of preference conditioned was not influenced by the availability of food during the conditioning period; the rats that had been food deprived during training acquired as strong a preference as did those fed freely during training. The degree of preference obtained was similar in the rats given reinforcing and nonreinforcing fluids simultaneously or sequentially. Starch conditioned a slightly stronger preference than did the same concentration of glucose, even though these substances contain the same amount of calories. A statistically significant, but weak, preference was conditioned by 0.5%, but not by 0.25%, starch. It is proposed that the flavor of starch, independent of its calories, is reinforcing to rats.     

Preferences and aversions for stimuli paired with ethanol in hungry rats

Two experiments explored the reinforcing effect of ethanol on conditioned location and flavor preferences in hungry rats. In Experiment 1, rats were administered ethanol (.5, 1.0, or 2.0 g/kg, ig) prior to confinement in one side of a shuttlebox with access to a flavored solution. On control trials, H₂O was administered prior to confinement to the opposite side with a different flavored solution. Location choice tests revealed an overall aversion for the ethanol-associated side that was largest at the 2.0-g/kg dose. Flavor choice tests revealed an aversion for the ethanol-associated flavor at the 2.0-g/kg dose, no reliable difference at the 1.0-g/kg dose, and, of particular interest, a preference at the .5-g/kg dose. The results of Experiment 2 suggest that caloric restoration served as the reinforcing mechanism for the conditioned flavor preference. An isocaloric glucose solution conditioned a flavor preference of the same magnitude as that obtained with ethanol. Moreover, when ethanol provided no caloric advantage, the associated flavor was less preferred than a flavor associated with an isocaloric glucose solution.     

Flavor avoidance learning based on missing calories but not on palatability reduction

Avoidance of a target flavor can be produced by providing rats with a highly nutritious solution of 20?% maltodextrin (20?%Malto) in some sessions and a 3?% maltodextrin (3?%Malto) solution containing the target flavor in intermixed sessions. Since 20?%Malto is both more nutritious and more palatable than 3?%Malto, flavor avoidance could arise because the flavor signals either a reduction in calories or reduced palatability, or both. Pilot testing established that rats strongly preferred 3?%Malto plus 0.1?% saccharin to both unflavored 3?%Malto and unflavored 20?%Malto. The two main experiments tested whether the palatability difference, which the pilot data had suggested was larger than the difference between 20?%Malto and 3?%Malto, could produce flavor avoidance. In both experiments, one group of rats were given 3?%Malto plus 0.1?% saccharin on some days, intermixed with other days on which this group was given 3?%Malto plus the target flavor, almond. Neither when trained and tested under conditions of food deprivation (Experiment 1) nor when trained and tested sated (Experiment 2) did palatability reduction produce almond avoidance. In contrast, calorie reduction produced almond avoidance under both conditions. These results suggest that flavor avoidance can be produced by intermixed training involving solutions that differ in nutritious value and palatability, but not when they differ only in palatability.     

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

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

Variable processing of flavors in rat STM

Hooded Lister rats exhibited less neophobia towards (i.e., drank more of) a novel fluid (3% lemon or 5% sucrose) on a 10-min test if given a 6-min exposure to that fluid 6 h earlier. Presentation of a distractor (1.26% coffee) immediately after preexposure to the test solution enhanced neophobia habituation to lemon (Experiment 1), but disrupted habituation to sucrose (Experiment 3). This bidirectional distractor effect was not due to distractor-induced change in the hedonic value of the preexposed test flavor (Experiment 4). Evidence was obtained (Experiment 5) indicating that the rat perceives lemon to be more similar to coffee than is sucrose. It is suggested that when test flavor and distractor are dissimilar, processing of the distractor denies the preexposed test flavor sufficient processing in STM to allow encoding of information about that flavor in LTM. Consequently, the rat responds to a subsequent presentation of the test flavor as it would to a novel stimulus. When test flavor and distractor are similar, however, the distractor elicits less processing in STM (cf. Wagner, 1976) and is therefore less able to disrupt STM processing of the preexposed test flavor. The resultant loss of neophobia to the test flavor resulting from encoding of information about that flavor in LTM may then be augmented by generalization of attenuated neophobia to the distractor. Consistent with this analysis, coffee was shown to suffer more proactive interference when preceded by lemon than when preceded by sucrose (Experiment 6).     

Spontaneous recovery after extinction of a conditioned taste aversion

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

Latent inhibition of conditioned disgust reactions in rats

The present experiments, using the latent inhibition (LI) paradigm, evaluated the effect of nonreinforced exposure to saccharin on the acquisition of an LiCl-induced saccharin aversion as measured by conditioned disgust reactions in the taste reactivity test and conditioned taste avoidance in a consumption test. When rats were preexposed to saccharin by bottle exposure (Experiments 1 and 3), LI was evidenced only by conditioned taste avoidance (bottle testing), but not by conditioned disgust reactions (intraoral [IO] testing). On the other hand, when rats were preexposed to saccharin by IO infusion (Experiments 2 and 3), LI was evidenced only by conditioned disgust reactions, but not by conditioned taste avoidance. Experiment 4 showed that LI of conditioned disgust reactions does not appear to be affected by a context shift from preexposure to testing phases. These results show that the expression of LI of both conditioned taste avoidance and conditioned disgust reactions depends critically on a common method of flavor exposure during preexposure and testing.     

Food deprivation and conditioned flavor preferences based on sweetened and unsweetened foods

In four experiments, rats’ preferences for flavors consumed under high deprivation versus low deprivation were measured. In Experiment 1, rats preferred flavors received in unsweetened food under high deprivation to flavors received in unsweetened food under low deprivation. This preference did not vary with amount of food used to deliver the flavors (1-g vs. 16-g wet mash). Sweetening the food (0.10% saccharin) eliminated this preference when 16 g of mash was received, but not when 1 g of mash was received (Experiments 2 and 3). Sweetening the mash even more (0.15% saccharin) eliminated the preference when 1 g of mash was received, as well as when 20 g of mash was received. We suggested that the reinforcing value of sweetness is reduced by increasing deprivation level.