The nature of training in flavor preference learning determines the underlying associative structure.

Pairing a palatable flavor (US) with an initially neutral flavor cue (CS) results in an acquired conditioned preference for the latter. Two main associations have been proposed to explain the acquisition of flavor preferences: Flavor-Flavor and Flavor-Nutrient learning. Although the hedonic reaction triggered by US consumption has also been suggested as a possible additional component underlying acquired flavor preference, this issue has received little attention. Here we explored whether the amount of training to the CS-US compound can favor the formation of a Flavor-Hedonic reaction association using rats as subjects and sucrose as the US. We expected that the more exposure to the CS-US compound, the stronger the S-R type association. Since S-R associations are not sensitive to devaluation procedures, we used a Sensory-Specific Satiety procedure to devalue the US after conditioning and then measured preferences for the CS. On Experiment 1 with a short restrictive training (classic procedure), preference for the CS was decreased after devaluation of the US compared to the control condition. On Experiment 2, with short unrestrictive training, preference for the CS was again weakened. Experiment 3 with a long unrestrictive training, rats expressed preference for the CS regardless of the devaluation procedure. These results suggest that, as with an instrumental paradigm, extensive training in flavor preference learning undermines the US devaluation effect.

Intermixed rapid exposure to similar stimuli reduces the effective salience of their distinctive features.

Intermixed exposure to two similar stimuli, for example, AX and BX, improves subsequent discrimination between them compared to blocked exposure (the intermixed/blocked effect). Salience modulation models, developed mainly from research with nonhuman animals and exposure to widely spaced similar stimuli, explain this effect in terms of increased salience of the unique elements, A and B. Conversely, the results from experiments initially conducted with humans and exposure to close spaced similar stimuli have led to the suggestion that it is the development of well-unitized representations of unique elements that leads to better discrimination, leaving the unique elements with less effective salience. The experiments carried out here aim to replicate the intermixed/blocked effect in rats using an exposure procedure with rapid succession between stimuli and to assess the effective salience of unique elements. In Experiment 1, an aversion to a new flavor, Y, was conditioned and then an external inhibition test with AY was given. In Experiment 2, an aversion to A was conditioned and its extinction was measured on unreinforced trials. In Experiment 3, an aversion to AY was conditioned and the associated aversion to Y was measured. We found after rapid intermixed preexposure a reduction in generalization from the aversive Y element to the compound AY (Experiment 1) as well as a reduction in A's salience (Experiments 2 and 3) compared to the effects of blocked preexposure. The results are discussed in terms of the various mechanisms underlying perceptual learning, which appear to depend on the details of the task. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

The opportunity to compare similar stimuli can reduce the effectiveness of features they hold in common.

In three experiments, rats were given experience of flavored solutions AX and BX, where A and B represent distinctive flavors and X a flavor common to both solutions. In one condition, AX and BX were presented on the same trial separated by a 5-min interval (intermixed preexposure). In another condition, each daily trial consisted of presentations of only AX or only BX (blocked preexposure). The properties acquired by stimulus X were then tested. Experiment 1 showed that after intermixed preexposure X was less able to interfere with a conditioned response established to a different flavor. Experiment 2 showed that X was less effective at overshadowing when trained in compound with another flavor. Simple conditioning, with X as the conditioned stimulus, was not sensitive to the form of preexposure (Experiment 3). These results indicate that the opportunity to compare similar stimuli that is provided by presenting them in close succession can change the properties of features they hold in common, making these features less effective when tested in compound with other stimuli. A loss of effectiveness by such features would contribute to the perceptual learning effect, the enhancement of subsequent discrimination, that is generated by prior exposure to closely spaced similar stimuli. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Perceptual learning after rapidly alternating exposure to taste compounds: Assessment with different indices of generalization.

Exposure to two similar stimuli (AX and BX; e.g., two tastes) reduces the extent to which a conditioned response later established to BX generalizes to AX. This example of perceptual learning is more evident when AX and BX are exposed in an alternating manner (AX, BX, AX, BX,…) than when AX and BX occur in separate blocks (e.g., AX, AX,…BX, BX,…). We examined in male rats (N = 126) the impact of rapid alternation to AX and BX on generalization of a taste aversion from BX to AX. Experiment 1 showed that such alternating presentations (with 5-min intervals between AX and BX) reduced generalization relative to blocked exposure; but only as assessed by consumption levels and not by lick cluster size (an index of hedonic reactions). Experiment 1 also showed that the nature of exposure did not affect how A influenced performance to a novel conditioned taste, Y. Experiment 2 replicated the pattern of results involving the different influences of rapidly alternating and blocked exposure on generalization from BX to AX, and showed that this effect was only evident when rats received access to water during the 5-min intervals between AX and BX. These results reinforce parallels between perceptual learning effects in rats and humans, both at empirical and theoretical levels. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Habituation as an underlying mechanism for Sensory Specific Satiety: An assessment using flavor consumption and preference in rats.

Sensory specific satiety refers to a decline in the hedonic value of the sensory properties of a particular food as it is consumed. This phenomenon is characterized by a decrement in responding as a consequence of repeated exposure, is stimulus specific, and recovers after time. All these characteristics are shared with the habituation phenomenon and for this reason, habituation has been proposed as the underlying mechanism that explains this eating regulatory system. However, several studies conducted with human models have yielded mixed results. Using rats as experimental subjects, the present study tested the following three characteristics of habituation within a Sensory Specific Satiety (SSS) framework: spontaneous recovery, dishabituation and the distractor effect. Experiment 1 demonstrated the basic effect of SSS and its spontaneous recovery over time. In Experiment 2 we found that the presentation of a dishabituator after a pre-feeding procedure had no impact on the SSS effect. Finally, in Experiment 3 the presence of a distractor during a pre-feeding procedure did not alter the expression of SSS. These results challenge the idea that SSS constitutes a typical case of habituation, at least with the procedure used here.

Motivational factors controlling flavor preference learning and performance: Effects of preexposure with nutritive and nonnutritive sweeteners.

In three experiments thirsty rats were given exposure to a sweet solution (saccharin in some experiments, sucrose in others) prior to consuming a compound of the sweet substance and almond flavoring. Preference for that flavor, in a choice test of almond vs. water, was then assessed. In some cases the rats were hungry, in others they were not. When the sweetener used was saccharin, preexposure reduced the magnitude of the preference obtained on test in both hungry and nonhungry rats. When the sweetener was sucrose, preexposure had this effect only when the rats were hungry. The effects produced after preexposure to saccharin are interpreted as being the result of habituation to its sensory features that reduces the ability of these features to engage in subsequent learning. These effects will occur whether the animal is hungry or not. The results for sucrose are interpreted in terms of the fact that it possesses both sensory and nutritional properties, the role of the latter being dependent on the motivational state of subject. It is suggested that the sensory features of sucrose do not undergo habituation, but that an effect of preexposure can be obtained in hungry rats when the source of the learned preference will depend on learning about the nutritive consequences of the sucrose.

Contextual control of the retardation of flavour aversion learning by preexposure to the unconditioned stimulus: Acquisition or retrieval deficit?

Two experiments, using rats as the subjects, and flavour aversion learning with an injection of lithium chloride (LiCl) as the unconditioned stimulus (US), examined the effects of a context shift between phases of the procedure on the retardation of learning produced by preexposure to the US. Experiment 1 showed that the US-preexposure effect (the reduction in the size of the conditioned aversion) was not attenuated when the animals were given both preexposure to the US and the conditioning procedure in a novel context but received the test phase in a different context (the home cages). Experiment 2 showed that, after degrading the injection cues-illness association by interpolating saline injections between LiCl preexposures, the US-preexposure effect was attenuated when there was a context shift between preexposure and conditioning, but that the context shift was without effect when it occurred between conditioning and test. These results are consistent with the proposal that US preexposure obtained in this procedure has its effect by interfering with the formation of the target association; they provide no support for the suggestion that the effect depends on interference at the test stage.

Prenatal dietary choline supplementation modulates long-term memory development in rat offspring.

Background: Previous studies on preclinical models have shown that giving supplemental choline during the embryonic period improves performance on memory tasks during adulthood. However, the effects of an early intervention on the development of cognitive functions in the immature brain have not been widely studied. In addition, it has been well established that short-term memory in rats emerges at an earlier stage than long-term memory.Objective: The aim of this work was to examine the effect of prenatal dietary choline supplementation on long-term memory development in rats.Methods: In order to assess long-term memory, we used an object-recognition task, which evaluates the ability to recall a previously presented stimulus. Pregnant rats were fed with the diets AIN 76-A standard (1.1 g choline/Kg food) or supplemented (5 g choline/Kg food) between embryonic days (E) 12 and E18. On the first post-natal day (PN 0), male offspring of the rats fed with the supplemented and standard diet were cross-fostered to rat dams fed a standard diet during pregnancy and tested at the age of PN21-22 or PN29-31 applying 24-hour retention tests.Results: The supplemented animals spent less time exploring the familiar object after a 24-hour retention interval, an effect that was observed in both the group tested at PN21-22 days of age and that tested at PN29-31 days. The non-supplemented rats only showed this effect in the group tested at PN29-31 days.Conclusions: These results suggest that prenatal supplementation with choline accelerates the development of long-term memory in rats.

The amount of exposure determines generalization in animal perceptual learning using short inter-stimulus intervals.

While human and animal perceptual learning (PL) had sometimes yielded similar results, there is evidence of some striking discrepancies. It has been proposed that such differences reflect the existence of multiple species-specific mechanisms, especially regarding to humans. However, it is also possible that those discrepancies are caused by procedural differences. One of the most important differences between PL experiments in humans and laboratory animals is the inter-stimulus interval (ISI) used. In the former, short ISI reliably produces PL, while in the latter reducing the ISI leads to paradoxical results. We report two experiments with rodents to prove that the length of exposure is a key element under such conditions. In the first experiment we replicated the paradoxical results already present in the literature using a short exposure. In a follow up experiment, we increased the exposure trials and obtained normal PL in animals using short ISI. Our results support current associative theories of PL and highlight the impact of procedural differences on this phenomenon.

Effect of exposure to similar flavours in sensory specific satiety: Implications for eating behaviour.

Sensory-specific satiety (SSS) refers to a decline in pleasantness of eaten foods in comparison to other non-ingested meals. Although SSS is specific to the eaten food, it can also generalize to other meals that share similar properties to the satiated food. It is possible that this phenomenon could become more specific after extensive experience, as repeated exposure to an assortment of similar food could cause perceptual learning (i.e. an increase in the distinctiveness of each specific item). This in turn would reduce generalization from the consumed food to other similar products, so they will be more readily consumed. To assess this hypothesis, two experiments were carried out using rats as experimental subjects and flavoured solutions as stimuli. In Experiment 1 our main goal was to find the basic SSS effect with two different solutions. As expected, the results showed that rats tended to consume a higher amount of non-sated solutions in comparison to sated ones. Experiment 2 evaluated how repeated exposure to two similar solutions affected generalization of the SSS. Results showed that rats that did not have extensive exposure to the flavoured solutions showed no preference for the non-sated drink. However, rats that had repeated exposure to the flavours showed SSS. The results suggest that easy and continuous access to a high variety of similar unhealthy foods might have long-term effects on food consumption, and highlight a potential mechanism linking obesogenic environments with dietary habits.

Dietary choline supplementation in adult rats improves performance on a test of recognition memory.

In two experiments adult rats (aged at least 6 months at the start of the procedure) received a diet enriched with added choline for a period of 10 weeks; control subjects were maintained on a standard diet during this time. All rats then underwent the spontaneous object recognition (SOR) procedure in which they were exposed to a pair of objects and then tested, after a retention interval, to a display with one object changed. Exploration of the changed object indicates retention and use of information acquired during the exposure phase. All subjects showed retention with a 24-h interval (Experiments 1 and 2) and when retested after a further 24 h (Experiment 1). But when tested for the first time after a 48-h interval (Experiment 2), control subjects showed no evidence of retention, exploring both objects equally, whereas those given the dietary supplement continued to show a preference for the changed object. This supports the conclusion that dietary choline supplementation can enhance performance on a task regarded as a test of declarative memory, and will do so even when the supplementations is given in adulthood.

The role of stimulus comparison in animal perceptual learning: Effects of a distractor placement.

Research on perceptual learning shows that the way stimuli are presented leads to different outcomes. The intermixed/blocked (I/B) effect is one of these outcomes, and different mechanisms have been proposed to explain it. In human research, it seems that comparison between stimuli is important, and the placement of a distractor between the pre-exposed stimuli interferes with the effect. Results from animal research are usually interpreted in different terms because the type of procedure normally used in animal perceptual learning does not favour comparison. In our experiments, we explore the possibility that a distractor placed between the to-be-discriminated stimuli may interfere with the perceptual learning process in rats. In Experiment 1, two flavoured solutions are presented in an I/B fashion, with a short time lapse between them to favour comparison, showing the typical I/B effect. In Experiment 2, we introduced a distractor in between the solutions, abolishing this effect. Experiment 3 further replicates this by comparing two intermixed groups with or without distractor. The results replicate the findings from human research, suggesting that comparison also plays an important role in animal perceptual learning.

Flattening of a generalization gradient following a retention interval: Evidence for differential forgetting of stimulus features.

In two experiments, rats received exposure to a compound consisting of a solution of salt plus a distinctive flavor (A), followed by an injection of furo-doca to induce a salt need. Experiment 1, established that this procedure successfully generated a preference for flavor A in a subsequent choice test between A and water. Experiment 2 used this within-event learning effect to investigate generalization, testing the rats with both A and a novel flavor (B). For different groups the interval between the training phase and the test phase was varied. Subjects tested immediately after training showed a steep generalization gradient (i.e., a strong preference for A, and a weak preference for B). Subjects given a 14-day retention interval showed a flattened gradient, a reduced level of preference for A and an enhanced preference for B. These results are interpreted in terms of changes in stimulus representations over the retention interval that act to reduce the effectiveness of the distinctive features of stimuli (the features that are necessary to ensure discrimination between them).

The role of instructions in perceptual learning using complex visual stimuli.

Although modeled on procedures used with nonhuman animals, some recent studies of perceptual learning in humans, using complex visual stimuli, differ in that they usually instruct participants to look for differences between the to-be-discriminated stimuli. This could encourage the use of mechanisms not available to animal subjects. To investigate the role of instructions, in 2 experiments, participants were given preexposure to checkerboards that were similar except for the presence of a small distinctive feature on each. For participants instructed to look for differences, performance on a same-different test was enhanced by preexposure in which the critical stimuli were presented on alternate trials-the usual perceptual learning effect. No such effect was found in 2 other preexposure conditions: when participants were told only to look at the stimuli and not explicitly told to look for differences; and when participants were instructed on an alternative task requiring attention to the stimuli. These results indicate a role for a learning process reinforced by success in finding stimulus differences; they challenge previous interpretations of results from studies using complex visual stimuli in the study of perceptual learning. (PsycINFO Database Record

The effect of additional exposure to the unique features in a perceptual learning task can be attributed to a location bias.

It has been suggested that human perceptual learning could be explained in terms of a better memory encoding of the unique features during intermixed exposure. However, it is possible that a location bias could play a relevant role in explaining previous results of perceptual learning studies using complex visual stimuli. If this were the case, the only relevant feature would be the location, rather than the content, of the unique features. To further explore this possibility, we attempted to replicate the results of Lavis, Kadib, Mitchell, and Hall (2011, Experiment 2), which showed that additional exposure to the unique elements resulted in better discrimination than simple intermixed exposure. We manipulated the location of the unique elements during the additional exposure. In one experiment, they were located in the same position as that when presented together with the common element. In another experiment, the unique elements were located in the center of the screen, regardless of where they were located together with the common element. Our results showed that additional exposure only improved discrimination when the unique elements were presented in the same position as when they were presented together with the common element. The results reported here do not provide support for the explanation of the effects of additional exposure of the unique elements in terms of a better memory encoding and instead suggest an explanation in terms of location bias.

Effects of dietary choline availability on latent inhibition of flavor aversion learning.

OBJECTIVE: It has been previously reported that dietary choline supplementation might affect latent inhibition (LI) using a conditioned suppression procedure in rats. We have assessed the effect of dietary choline on LI of flavor aversion learning. METHOD: Adult male Wistar rats received a choline supplemented (5 g/kg), deficient (0 g/kg), or standard (1.1 g/kg) diet for 3 months. After this supplementation period, all rats went through a conditioned taste aversion (CTA) procedure, half of them being pre-exposed to the conditioned stimulus before the conditioning. RESULTS: The results indicated that choline deficiency prevents LI of conditioned flavor aversion to cider vinegar (3%) induced by a LiCl (0.15 M; 2% body weight) intraperitoneal injection, while choline supplementation enhances CTA leading to slower extinction. DISCUSSION: The role of the brain systems modulating attentional processes is discussed.

US-preexposure effects in flavor-preference and flavor-aversion learning with nonnutritive USs.

In two experiments, rats received exposure to either a saccharin or quinine solution followed by conditioning with a solution of almond as the conditioned stimulus (CS) and either saccharin or quinine as the unconditioned stimulus (US). In Experiment 1, rats received preexposure and conditioning using saccharin as the US; in Experiment 2 quinine was the US. In both cases the magnitude of the conditioning effect (an enhanced preference for the CS in Experiment 1; a reduced preference in Experiment 2) was reduced by preexposure to the US. The results provided confirmation of the occurrence of the US-preexposure effect in the flavor-preference procedure and demonstrate that the effect can be obtained with nonnutritive USs that lack significant post-oral consequences. The implications of these results for theories of the US-preexposure effect are discussed.

Long-lasting effects of prenatal dietary choline availability on object recognition memory ability in adult rats.

Choline is an essential nutrient required for early development. Previous studies have shown that prenatal choline availability influences adult memory abilities depending on the medial temporal lobe integrity. The relevance of prenatal choline availability on object recognition memory was assessed in adult Wistar rats. Three groups of pregnant Wistar rats were fed from E12 to E18 with choline-deficient (0 g/kg choline chloride), standard (1.1 g/kg choline chloride), or choline-supplemented (5 g/kg choline chloride) diets. The offspring was cross-fostered to rat dams fed a standard diet during pregnancy and tested at the age of 3 months in an object recognition memory task applying retention tests 24 and 48 hours after acquisition. Although no significant differences have been found in the performance of the three groups during the first retention test, the supplemented group exhibited improved memory compared with both the standard and the deficient group in the second retention test, 48 hours after acquisition. In addition, at the second retention test the deficient group did not differ from chance. Taken together, the results support the notion of a long-lasting beneficial effect of prenatal choline supplementation on object recognition memory which is evident when the rats reach adulthood. The results are discussed in terms of their relevance for improving the understanding of the cholinergic involvement in object recognition memory and the implications of the importance of maternal diet for lifelong cognitive abilities.

Chronic dietary choline supplementation modulates attentional change in adult rats.

In two experiments adult rats were maintained on a diet enriched with added choline for 12 weeks prior to behavioral testing; control rats remained on the standard diet during this time. In Experiment 1 all rats received training in the Hall-Pearce negative transfer paradigm in which prior training with a conditioned stimulus (CS) paired with a small reinforcer retards further learning when the size of the reinforcer is increased. This effect, which has been attributed to a loss of associability by the CS, was obtained in control subjects but not in those given the supplement. Experiment 2 investigated the effect of prior nonreinforced exposure of the to-be-CS (latent inhibition). Such exposure retarded subsequent learning in control subjects, but latent inhibition was not obtained in those given the supplement. We conclude that the mechanism that reduces the attention paid to a stimulus that accurately predicts its consequences does not operate effectively after choline supplementation. These results are consistent with a role for the cholinergic system of the basal forebrain in modulation of attention.

Choline dietary supplementation improves LiCl-induced context aversion retention in adult rats.

Previous studies have demonstrated that choline is an essential nutrient during prenatal and early postnatal developmental periods. Thus, the availability of choline during these periods produces some beneficial effects on hippocampal-dependent learning and memory in rats. However, research on the effect of adult choline supplementation on learning and memory abilities is scarce. In the present study, 3-4 month-old male Wistar rats receiving a 7-week choline-supplemented diet (4.5 fold that of a standard diet) and control rats receiving a standard diet were trained in a LiCl-induced contextual aversion task. Short and long-term context aversion retention was assessed by recording the consumption of a flavoured solution in the aversive and safe contexts over two subsequent tests. Statistical analysis showed that the supplemented group exhibited greater intake suppression in the aversive context than in the safe context when two retention tests were applied 3 and 15 days after conditioning. These results suggest that increasing dietary choline availability during adulthood may favour the retention of a context aversion.