Immediate and delayed reinforcers for flavor preferences in rats

When rats received, on alternate days, one flavored saccharin solution for 5 min and a differently-flavored saccharin solution for 60 min, they showed no consistent preference between the flavors. On the other hand, when they received one flavor in a concentrated saccharin solution and a different flavor in a dilute one, they preferred the first flavor in tests with saccharin concentration held constant; also, rats learned to prefer a flavor immediately followed by a concentrated saccharin solution to one followed by nothing. They showed no consistent preference, however, between a flavor followed 30 min later by a concentrated saccharin solution and one followed by nothing; but they learned to prefer a flavor followed 30 min later by a dextrose solution to one followed by nothing. In other words, consummatory responding did not reinforce flavor preference, sweet taste did so with immediate but not delayed reinforcement, and nutrition did so even with delayed reinforcement.     

Extinction, spontaneous recovery and renewal of flavor preferences based on taste-taste learning

This paper presents evidence of extinction, spontaneous recovery and renewal in a conditioned preferences paradigm based on taste-taste associations. More specifically, in three experiments rats exposed to a simultaneous compound of citric acid-saccharin solution showed a preference for the citric solution when the preference was measured with a two-bottle test (citric vs. water). Experiment 1 revealed that conditioned preferences were extinguished by repeated presentations of the citric acid solution without the saccharin. In Experiment 2, an additional test trial was introduced 21 days after the extinction treatment. Delayed testing resulted in the recovery of the extinguished preference for the citric, reflecting a spontaneous recovery effect. Experiment 3 revealed a preference renewal effect that was obtained when the extinction was conducted in a context different from that of the remaining experimental stages. The implications of these results are analyzed with attention paid to the mechanisms underlying learning of flavor preferences.     

Reinforcer devaluation in palatability-based learned flavor preferences

Rats exposed to simultaneous compounds of 1 neutral flavor with dilute (2%) sucrose and a 2nd flavor with dilute (2%) maltodextrin subsequently consumed both flavors in preference to a 3rd flavor that was never paired with a palatable taste. Brief training exposure under ad lib food and water minimized the post-ingestive effects of nutrients, emphasizing the contribution of palatability to these preferences. Devaluation of sucrose or maltodextrin by pairing with illness (Experiment 1) or sensory-specific satiety (Experiment 2) selectively reduced the preference for the flavor previously paired with the devalued reinforcer. Such reinforcer-specific devaluation effects suggest that palatability-based learned flavor preferences are under-pinned by a Pavlovian process whereby the cue flavor is associated with the taste of the concurrently consumed palatable reinforcer.     

Interactions between conditioned and unconditioned flavor preferences

Five experiments investigated how rats' conditioned preferences or aversions for aqueous odors paired with sucrose or salt are affected by their unconditioned response to those tastes. Rats preferred an odor paired with 30% sucrose over an odor paired with 5% sucrose when both were presented in 5% sucrose, but they showed no preference or, if thirsty, showed the reverse preference, when the odors were presented in 30% sucrose. These changes in conditioned preference corresponded to changes in the rats' unconditioned preference for the accompanying sucrose solution. Rats' conditioned aversions for odors paired with salt showed a similar dependence on their reaction to the accompanying salt solution. The results were interpreted as showing that conditioned and unconditioned flavor preferences combine additively, as if mediated by the same sensory representation.     

Unconditioned stimulus devaluation effects in nutrient-conditioned flavor preferences

Experiments with different temporal relations between the conditioned stimulus (CS) and the unconditioned stimulus (US) in conditioning assessed whether US devaluation effects can be obtained after nutrient-conditioned flavor preference learning. One flavor (CScarb) was paired with a carbohydrate, Polycose; a 2nd flavor (CSprot) was paired with a protein, casein; and a 3rd flavor (CS-) was presented by itself. Following conditioning, one of the nutrients was devalued through pairings with lithium chloride in the absence of the CS flavors. In a subsequent 2-bottle test, rats preferred CScarb over CSprot; however, this preference was smaller when the carbohydrate was devalued than when the protein was devalued. Results suggest that CS flavors are able to form associations with the sensory features of nutrient USs under a wide variety of circumstances.     

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.     

Teacher report and direct assessment of preferences for identifying reinforcers for young children

Teachers were asked to identify and rank 10 preferred stimuli for 9 toddlers, and a hierarchy of preference for these items was determined via a direct preference assessment. The reinforcing efficacy of the most highly preferred items identified by each method was evaluated concurrently in a reinforcer assessment. The reinforcer assessment showed that all stimuli identified as highly preferred via the direct preference assessment and teacher rankings functioned as reinforcers. The highest ranked stimuli in the direct assessment were more reinforcing than the teachers' top-ranked stimuli for 5 of 9 participants, whereas the teachers' top-ranked stimulus was more reinforcing than the highest ranked stimulus of the direct assessment for only 1 child. A strong positive correlation between rankings generated through the two assessments was identified for only 1 of the 9 participants. Despite poor correspondence between rankings generated through the teacher interview and direct preference assessment, results of the reinforcer assessment suggest that both methods are effective in identifying reinforcers for toddlers.     

Transfer of conditioned inhibition across different aversive reinforcers in the rat

Three experiments using a conditioned suppression procedure in rats assessed transfer of conditioned inhibition across different aversive reinforcers. Experiment 1, using shock and car horn blast as reinforcers, showed that a noise negatively correlated with 0.7-mA shock between sessions attenuated suppression of lever pressing elicited by the signals for 0.7-mA shock and 110-db horn blast. Similarly, when the noise signaled the omission of horn blast, it attenuated suppression elicited by the signals for horn blast and shock. Experiment 2, using the same procedure, replicated transfer of conditioned inhibition from shock to horn found in Experiment 1, and indicated that such effects should be attributed to associative factors. Experiment 3, using the conditioned inhibition paradigm and different chambers during training and testing, replicated transfer from horn to shock and indicated that transfer cannot be attributed to multiple-background-reinforcer associations established during inhibitory training. The results of these experiments have implications for theories of conditioned inhibition.     

Visual orientation preferences in the rat

An experiment was performed which demonstrated a preference in hooded rats for vertical rather than horizontal striatums. It was demonstrated that experience in discriminating between horizontal and vertical striations facilitates subsequent discrimination between squares and circles, regardless of which type of striation was positive in initial training. This finding has implications for theories of shape recognition, which are discussed.     

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.     

Context influence on the perception of figures as conditional upon perceptual organization strategies

Two experiments tested the effect of context on figure perception. Subjects were shown rapid sequences of three figures: a prime, a whole, and a part. They were asked to decide if the third figure was a part of the second (Experiment 1) or if the second and third figures were the same or different with respect to a particular angle (Experiment 2). The prime served to establish a context for stimuli that followed to be compared. Priming had influence on the part—whole comparison in Experiment 1, but not on the local comparison in Experiment 2. The results of Experiment 1 were interpreted as evidence for a role of prior information in perceptual organization. Experiment 2 showed that the task must require an integrative perceptual organization strategy for the priming effects to occur.     

Modification of slot-machine preferences through the use of a conditional discrimination paradigm

The present experiment investigated the impact of contextually trained discriminations on gambling behavior. Nine recreational slot-machine players were initially exposed to concurrently available computerized slot machines that were each programmed on random-ratio schedules of reinforcement and differed only in color. All participants distributed responding equally across the two slot machines. A conditional discrimination procedure was then used to teach the contextual cues representing the arbitrary relations of "greater than" and "less than." Following contextual cue training, participants were reexposed to the concurrent slot-machine task. After training of the contextual cues, a higher proportion of responses were made to the slot machine that shared formal properties (i.e., color) with the contextual cue representing "greater than."     

The influence of flavor preexposure and test interval on conditioned taste aversions in the rat

The effects of flavor preexposure and test interval on conditioned taste aversions were examined in four experiments. In the first three experiments, prior experience with a flavor different from that used as a conditioned-stimulus (CS) produced attenuated aversions when testing occurred after a 1-day interval but not after a 21-day interval. Preexposure to the same stimulus used as a CS produced attenuated aversions at both 1- and 21-day intervals. In Experiment 4, a delay interval between flavor preexposure and conditioning eliminated the attenuating effect of preexposure, but only when different stimuli were used for preexposure and conditioning. These data could not be easily accounted for by contemporary interpretations of preexposure as an event that interferes with subsequent acquisition of a conditioned aversion. An alternative retrieval interference hypothesis was outlined.     

Roles of dopamine D1 and D2 receptors in the acquisition and expression of fat-conditioned flavor preferences in rats

Sugars and fats elicit innate and learned flavor preferences with the latter mediated by flavor-flavor (orosensory) and flavor-nutrient (post-ingestive) processes. Systemic dopamine (DA) D1 (SCH23390: SCH) and D2 (raclopride: RAC), but not opioid antagonists blocked the acquisition and expression of flavor-flavor preferences conditioned by sugars. In addition, systemic D1, but not D2 or opioid antagonists blocked the acquisition of flavor-nutrient preferences conditioned by intragastric (IG) sugar infusions. Given that DA antagonists reduce fat intake, the present study examined whether systemic D1 or D2 antagonists altered the acquisition and/or expression of conditioned flavor preferences (CFP) produced by pairing one novel flavor (CS+, e.g., cherry) with a 3.5% corn oil (CO: fat) solution relative to another flavor (CS-, e.g., grape) paired with a 0.9% CO solution. In an expression study, food-restricted rats were trained to drink either flavored 3.5% or 0.9% CO solutions on alternate days. Subsequent two-bottle tests with the CS+ and CS- flavors mixed in 0.9% CO solutions occurred 0.5h after systemic administration of vehicle (VEH), SCH (50-800 nmol/kg) or RAC (50-800 nmol/kg). The rats displayed a robust CS+ preference following VEH treatment (87-88%) the expression of which was attenuated by treatment with moderate doses of RAC, and to a lesser degree, SCH. In an acquisition study, six groups of rats received VEH, SCH (25, 50, 200 nmol/kg) or RAC (50, 200 nmol/kg) 0.5 h prior to 1-bottle training trials with CS+ flavored 3.5% and CS- flavored 0.9% (CS-) CO solutions. A seventh Limited VEH group was trained with its training intakes limited to that of the SCH and RAC groups. Subsequent two-bottle tests were conducted with the CS+ and CS- flavors presented in 0.9% CO without injections. Significant and persistent CS+ preferences were observed in VEH (75-82%), Limited VEH (70-88%), SCH25 (75-84%), SCH50 (64-87%), SCH200 (78-91%) and RAC200 (74-91%) groups. In contrast, the group trained with RAC50 displayed a significant initial CS+ preference (76%) which declined over testing to 61%. These data indicate limited DA D1 and D2 receptor signaling involvement in the expression and acquisition of a fat-CFP relative to previous robust effects for sugar-CFP.     

Two-component arterial blood pressure conditional response in rat

The objective of these experiments was to quantify the pattern of change in arterial blood pressure (BP) during a discriminative aversive classical conditioning paradigm in rat using a new “high resolution” computer analysis. Sprague-Dawley rats (n=5) were restrained in a soft, conical cloth pouch and conditioned using a 6 sec. pulsed tone (CS+) followed by a 0.5 sec. tail shock; a steady tone, never followed by shock, served as a CS-. BP peaked at 16.4±6.5 mm Hg (mean±SD) above control at 1.5±0.1 sec. after onset of CS+. This “first component” (“C₁”) also occurred during CS- (12.1±3.8 mm Hg), although the magnitudes of the two were significantly (p<0.05) different. Another group of rats (n=8) was treated identically except the tones were 15 seconds long. The conditional BP response consisted of two components. C₁ was reminiscent of that seen using the short tone: for CS+ a peak of 13.6±5.6 mm Hg at 1.5 sec. or, for CS-, of 10.0±4.3 at 1.3 sec. (p<0.05). In CS+ trials BP peaked again (“C₂,” 7.4±2.5 mm Hg) at 8.3±1.2 sec. There was no statistically significant C₂ for CS- trials, clearly demonstrating discrimination between tones. The unconditional BP response in both groups consisted of two large, closely spaced peaks in BP. Respiration was recorded in 3 additional rats. After shock delivery these subjects often showed a sudden shift between (1) a regular respiratory pattern with moderate chest excursion and (2) apneic episodes interspersed with single, deep breaths. This latter pattern was associated with large, low frequency fluctuations in BP. Continued development of the rat conditioning paradigm is especially warranted because of the ability to record sympathetic nerve activity in intact, awake subjects and the large number of readily available genetic strains, which model human pathological states.     

Social transmission of flavor preferences in two species of hamsters (Mesocricetus auratus and Phodopus campbelli)

Golden hamsters (Mesocricetus auratus) and dwarf hamsters (Phodopus campbelli) interacted with a conspecific demonstrator that had recently consumed a flavored food. When given a choice between their demonstrator's flavor and another flavor, the dwarf hamsters preferred the flavor their demonstrator had eaten. Golden hamsters did not prefer their demonstrators' diets when the demonstrators were unrelated adults or littermates, but they did when the demonstrator was their mother. Videotaping the interactions between demonstrators and observers revealed that adult golden hamsters did not investigate foods hoarded by their demonstrators whereas dwarf hamsters did. These results are interpreted in terms of the stimuli that activate feeding behavior systems in these 2 hamster species.     

Short-term and long-term effects of reinforcers on choice

The relation between molar and molecular aspects of time allocation was studied in pigeons on concurrent variable-time variable-time schedules of reinforcement. Fifteen-minute reinforcer-free periods were inserted in the middle of every third session. Generalized molar matching of time ratios to reinforcer ratios was observed during concurrent reinforcement. Contrary to melioration theory, preference was unchanged during the reinforcer-free periods as well as in extinction. In addition to this long-term effect of reinforcement, short-term effects were observed: Reinforcers increased the duration of the stays during which they were delivered but had little consistent effect either on the immediately following stay in the same schedule or on the immediately following stay in the alternative schedule. Thus, an orderly effect of reinforcer delivery on molecular aspects of time allocation was observed, but because of its short-term nature, this effect cannot account for the matching observed at the molar level.