Taste aversion learning during successive negative contrast

Previous experiments found that acceptance of saccharin by rats was reduced if they had prior experience of sucrose or some other highly palatable solution. This study tested whether such successive negative contrast (SNC) effects involve acquisition of an aversion to the new taste. In three experiments, rats were switched from sucrose exposure in Stage 1 to a less palatable solution containing a new taste in Stage 2. In Experiments 1 and 2, a novel flavor was added to a saccharin solution at the start of Stage 2. In Experiment 1, preference tests revealed a weak aversion to the added vanilla flavor in the Suc-Sacch group, while in Experiment 2 an aversion was found in the Suc-Sacch group to the salty flavor that was used, compared with controls given access either saccharin or water in Stage 1. In Experiment 3, the Suc-Quin group, given quinine solution in Stage 2, displayed a greater aversion to quinine than a Water-Quin control group. These results support the suggestion that taste aversion learning plays a role in the initial suppression of intakes in a qualitative consummatory SNC effect. However, in the light of other evidence, it seems that the unusual persistence of successive negative contrast when rats are switched from sucrose to saccharin is not due to a long-lasting reduction in the value of saccharin.

Switching from sucrose to saccharin: Extended successive negative contrast is not maintained by hedonic changes

Previous experiments found that acceptance of saccharin by rats was reduced if they had prior experience of sucrose or some other highly palatable solution. This reduction in saccharin consumption was particularly extended after a switch from sucrose. On the surface, this seems to correspond to a successive negative contrast (SNC) effect. This term was coined by C. F. Flaherty to describe the situation where consumption of a target solution is reduced by prior experience of a more valuable solution, typically a more concentrated version of the target solution. However, SNC effects are normally transient and assessed relative to a nonshifted control. Here, we confirm that the reduction in consumption seen when shifting from sucrose to saccharin is persistent and is seen relative to the traditional unshifted control. In addition, an analysis of licking microstructure showed that the shift from sucrose to saccharin suppressed the hedonic value of saccharin relative to controls, but this effect was less persistent than consumption suppression. Interestingly, a similar dissociation is observed in extinction of conditioned taste aversion (CTA): suppression of consumption produced by CTA is far more persistent than suppression of hedonic value. The comparison of results across procedures suggests that persistent SNC produced by a qualitative downshift from sucrose to saccharin appears different from quantitative downshifts in the concentration of a single solution, and qualitative downshift effects may involve CTA. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Switching from Sugar- to Artificially-Sweetened Beverages: A 12-Week Trial

BACKGROUND

Consumption of sugar-sweetened beverages (SSBs) forms the primary source of added sugar intake and can increase the risk of metabolic disease. Evidence from studies in humans and rodents also indicates that consumption of SSBs can impair performance on cognitive tests, but that removing SSB access can ameliorate these effects.

METHODS

The present study used an unblinded 3-group parallel design to assess the effects of a 12-week intervention in which young healthy adults (mean age = 22.85, SD = 3.89; mean BMI: 23.2, SD = 3.6) who regularly consumed SSBs were instructed to replace SSB intake with artificially-sweetened beverages (n = 28) or water (n = 25), or (c) to continue SSB intake (n = 27).

RESULTS

No significant group differences were observed in short-term verbal memory on the Logical Memory test or the ratio of waist circumference to height (primary outcomes), nor in secondary measures of effect, impulsivity, adiposity, or glucose tolerance. One notable change was a significant reduction in liking for strong sucrose solutions in participants who switched to water. Switching from SSBs to 'diet' drinks or water had no detectable impact on cognitive or metabolic health over the relatively short time frame studied here. This study was prospectively registered with the Australian New Zealand Clinical Trials Registry (ACTRN12615001004550; Universal Trial Number: U1111-1170-4543).

Unpredictability of access to a high fat/high sugar food can increase rats' intake

When food is readily available, human self-imposed restrictions on consumption of palatable foods can lead to binge eating. Rodent models of human bingeing have produced increased intakes. However, access to highly palatable foods in such models has been largely predictable. The aim of the present study was to examine whether unpredictability of access might increase intakes in an animal model of bingeing, one in which rats had unrestricted access to chow and water throughout. Stage 1 of Experiment 1 gave female rats 2-h access to Oreos on either an unpredictable schedule or daily. In Stage 2 both groups were switched to predictable access on alternate days to test for persistent elevated intakes in the Unpredictable group. Although Oreo consumption did not differ between the two groups in Stage 1, the Unpredictable group ate more Oreos in Stage 2. In Stage 1 of Experiment 2 both groups were given access to Oreos every two days on average. The Predictable group was given alternate day access at a fixed time of day, whereas access days and times could not be predicted by the Unpredictable group. The latter was found to eat more Oreos in Stage 1, but this difference between the groups did not persist in Stage 2. Unpredictability did not appear to impact body weight gain in the study. In conclusion, this study indicates that unpredictability can increase consumption of palatable foods in addition to the increase produced by intermittent access.

A systematic review and meta-analysis of animal models of binge eating - Part 1: Definitions and food/drink intake outcomes

Binge eating involves consuming excessive amounts of food within a discrete period of time and is associated with significant impairments in binge-eating disorder and bulimia nervosa. While research on clinical binge eating has provided valuable aetiological insights, animal models allow for closer examination of environmental, biological, and developmental risk factors. Numerous animal models of binge eating exist and differ widely in operational definitions of bingeing, animal characteristics and methodological parameters. The current review aimed to synthesise the available published evidence on these models. A systematic review of binge definitions in 170 articles found most studies displayed good face validity. Meta-analyses on 150 articles confirmed that the amount of food or drink consumed by animals under binge conditions was larger than that of non-binge conditions across many protocols. The meta-regression revealed species, strain, and sex moderated binge effect size, with the largest effect observed in studies with female animals and mice. Risk of bias assessment identified that improved reporting of allocation, baseline characteristics and outcome assessment is required in future studies.

Sex differences in recovery from cognitive and metabolic impairments induced by supplementary sucrose in rats

Consumption of beverages containing around 10% sucrose contributes to worldwide obesity. Studies using rats can increase understanding of the consequences of such consumption. The present experiment aimed to compare male and female rats, first, in terms of cognitive and metabolic impairments produced by excessive intakes of 10% sucrose solution (Stage 1:8 weeks) and, second, with regard to recovery once access to sucrose ceased (Stage 2:4 weeks). All animals had unrestricted access to chow and water throughout. The primary cognitive outcome was performance on a place recognition task. The primary metabolic outcome was retroperitoneal fat pad mass/kg bodyweight at cull, with body weight and glucose tolerance as secondary outcomes. In a 3 × 2 between-subject factorial design the first factor was whether rats had: (1) unlimited access to a 10% sucrose solution and water throughout both stages (Suc-Suc); (2) were switched from sucrose in the 8-week Stage 1 to water only in the 4-week Stage 2 (Suc-Water); or (3) had no access to sucrose in either stage (Water-Water). The second factor was sex. A major metabolic outcome was that of persistent adiposity in both males and females in the Suc-Water condition. As for place recognition, females in the Suc-Suc condition showed greater long-term resistance than males to the impact of excessive sucrose on spatial memory impairment. Overall, few sex differences were found in secondary metabolic outcomes.

Pre-Exposure, But Not Overshadowing, Inhibits Nocebo Hyperalgesia

Nocebo hyperalgesia is a pervasive problem that significantly adds to the burden of pain. Conditioning is a key mechanism of nocebo hyperalgesia and recent evidence indicates that, once established, nocebo hyperalgesia is resistant to extinction. This means that preventive strategies are critical. We therefore tested whether two novel strategies - overshadowing (Experiment 1) and pre-exposure (Experiment 2) - could inhibit conditioned nocebo hyperalgesia. Overshadowing involves introducing additional cues during conditioning that should compete with and overshadow learning about the target nocebo cue. Pre-exposure involves pre-exposing the target nocebo cue in the absence of pain, which should diminish its ability to become associated with pain later. In both studies, healthy volunteers (N = 141) received exposure to a series of electrocutaneous pain stimuli with and without a sham electrode 'activated', which they were led to believe was a genuine hyperalgesic treatment. Nocebo conditioning was achieved by pairing sham activation with high pain prior to testing at equivalent pain intensity. In both studies, standard nocebo conditioning led to clear nocebo hyperalgesia relative to natural history controls. In Experiment 1, there was no evidence that overshadowing attenuated nocebo hyperalgesia. Importantly, however, Experiment 2 found that pre-exposure successfully attenuated nocebo hyperalgesia with post hoc analysis suggesting that this effect was dose-dependent. These findings provide novel evidence that pre-exposure, but not overshadowing, could be a cheap and effective way for mitigating the substantial harm caused by conditioned nocebo hyperalgesia in clinical settings. PERSPECTIVE: Nocebo hyperalgesia causes substantial patient burden with few preventive options available. Our study found novel evidence that pre-exposing treatment cues without pain, but not overshadowing them with other cues, has the capacity to inhibit conditioned nocebo hyperalgesia. Pre-exposure may therefore be an effective preventive strategy to combat nocebo hyperalgesia.

Sucrose-based flavor preferences in rats: Factors affecting detection of extinction

Rats that have consumed a novel target flavor added to a sucrose solution will develop a preference for that flavor. Such preferences appear to persist over the course of many presentations of the flavor alone when animals are not food-deprived. However, previous research indicates that an extinction effect (a reduction in preference) can be obtained when training or testing is carried out in animals that are hungry. In a series of experiments that produced flavor preferences in hungry rats by adding the flavor to a sucrose solution, three (Experiments 1, 2A, 2B) established that the concentration of sucrose and the nature of the flavor influenced the results but failed to detect extinction. Two-bottle choice tests showed some loss of preference but this occurred both in subjects given the extinction treatment (flavor-only presentations) and in control subjects given just water. A loss of preference in rats given an extinction treatment as opposed to controls given only water was, however, found in Experiments 3 and 4. These experiments differed from Experiments 1 and 2 in that the extinction stage involved the presentation of two bottles containing the flavor, thus matching the two-bottle procedure used in the test phase. These results confirm that experiencing a flavor alone can result in extinction of a conditioned flavor preference in hungry rats but indicate that the effect is highly context-specific, requiring the conditions of the test to match those of the extinction procedure. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Comparable metabolic effects of isocaloric sucrose and glucose solutions in rats

Much of the global increase in sugar intake is attributable to rising consumption of sugar-sweetened beverages (SSBs). Because people compensate poorly for liquid calories, SSB consumption increases total energy intake, raising the risk of harmful metabolic effects in addition to possible effects of sugars per se. Glucose and fructose, the constituent sugars in sucrose, can exert distinct effects on metabolism and also differ in their satiating properties, suggesting that compensation for the calories in these sugars may also vary. In light of claims that the fructose within sucrose is particularly harmful, the present study compared the effects of giving rats access to either a sucrose or an isoenergetic glucose solution. Adult male rats were fed standard chow and water supplemented with 95 ml of 10% glucose (Glucose group; n = 10), 9% sucrose solution (Sucrose group; n = 10) or water only (Control group; n = 10) daily for 7 weeks. Sugar-fed groups had higher total energy intakes than the Control group, but the extent of this incomplete compensation did not vary between Sucrose and Glucose groups. In a short-term compensation test, sugar groups were less sensitive to the effects of a sweet pre-meal, with no differences between the Glucose and Sucrose groups. Relative to water, both sugars reduced insulin sensitivity after 4 weeks on the diets and elevated fat mass at 7 weeks. Results suggest that sucrose and glucose induce comparable metabolic impairments and alter the homeostatic regulation of food intake even under conditions where daily access is capped.

Sugar consumption and behavioural inhibition in the rat

The metabolic effects of sugary drinks have been extensively studied, whereas the effects on psychological processes have received relatively limited attention. Several studies have found that high-sugar diets can produce impaired performance by rats on tests assessing spatial learning and memory. In contrast, despite claims that weakened inhibitory control underlies many sugar-induced deficits, evidence supporting this proposal has been limited. The aim of the present study was to assess the impact of high-sugar diets on response inhibition, as measured by rats' performance on a differential reinforcement of low rates schedule (DRL) in Experiments 1 and 2 and on a Pavlovian discrimination reversal task in Experiment 3. In all three experiments a 30-day diet stage, in which Sugar groups were given unrestricted access to 10% sucrose solution and Control groups had access to water only, was followed by behavioural tests. In Experiment 1 the Sugar group performed poorly on a spatial memory task, but no difference was detected between the performances of the two groups in the DRL test. In Experiment 2 longer DRL training was given and post-diet performance was assessed both before and after access to sugar was withdrawn. Null results were obtained under both conditions. In Experiment 3 rats' performance on a discrimination learned prior to the diet intervention was not affected by the high-sugar diet, but neither was performance once the discrimination was reversed. The implications of these results for understanding of sugar-induced psychological deficits are discussed.

Reduced acceptance of saccharin solutions by rats previously consuming more highly palatable solutions

Rats first given 24-h access to 10% sucrose for 4 or 12 days (Stage 1) were then switched to a saccharin solution for a 12-day Stage 2. The initial result of this switch was that these Sucrose groups drank less saccharin than Water groups that had been given only water to drink in Stage 1. This difference was maintained throughout Stage 2 by the females that served in Experiments 1 and 4 and by the males that served in Experiment 3. Experiment 1 also found that access to 10% glucose in Stage 1 produced an essentially identical decrease in subsequent saccharin acceptance as that produced by giving 10% sucrose in Stage 1. The impact on subsequent acceptance of saccharin was also tested in rats given two types of maltodextrin solution. The first type of maltodextrin (Myopure brand) was used with the males in Experiment 2; this failed to find any difference between the Maltodextrin and the Water group. However, when a second type of maltodextrin (SolCarb brand) was given to males in Stage 1 of Experiment 3, the results for this group were similar to those from a group given sucrose in Stage 1. The final experiment confirmed that prior exposure to maltodextrin solutions can reduce saccharin acceptance by female rats. Overall, the results suggest that acceptance of saccharin is sensitive to a contrast effect, in that it is reduced by prior exposure to a solution that is more palatable but not necessarily sweet.

The effect of dose expectancies on caffeine withdrawal symptoms during tapered dose reduction

BACKGROUND

Negative expectancies can exacerbate withdrawal symptoms via the nocebo effect. As such, information provided about dose reductions during attempts to taper a drug could contribute to withdrawal symptoms and increase the likelihood of relapse. The current study tested whether blinding participants to dose reductions during a supervised caffeine dose taper reduced these nocebo-induced withdrawal symptoms.

METHODS

Three groups of moderate to heavy coffee drinkers had their dose of caffeine reduced (tapered) from 300 mg per day to 0 mg over the course of five days and reported withdrawal symptoms twice daily. Groups were given differing information about how much caffeine they were receiving. An Open Reduction group was given accurate information about dose reductions. A Blind Reduction group was given no dose information whatsoever. A Deceptive Reduction group was misinformed about dose, with instructions suggesting that the dose remained on 300 mg for three days then dropped to 0 mg.

RESULTS

The Open Reduction group reported more pronounced caffeine withdrawal symptoms than the Deceptive Reduction group on the days with the greatest discrepancy between actual dose and informed dose, indicating a nocebo effect of open versus deceptive reductions. In addition, the rate of increase in reported withdrawal symptoms in the Blind Reduction and Deceptive Reduction groups was less than that of the Open Reduction group.

CONCLUSION

These results suggest that awareness of dose reductions during a dose taper can result in a nocebo withdrawal effect, and that removing this awareness can reduce withdrawal. This has important implications for standard supervised dose-tapering practice, where patients are aware of the timing and magnitude of dose reductions.

Variety overcomes the specificity of cue-potentiated feeding in rats

Cue-potentiated feeding (CPF) describes the stimulation of food consumption by cues that have become associated with food. Determining under what Conditions CPF occurs is important to better understand how exposure to food cues contributes to overeating. CPF is typically found to be specific: cues enhance consumption only of the food they have signaled. Further, previous research has focused largely on discrete cues rather than multimodal cues such as a feeding environment. The present experiments paired a "Plus" context with highly palatable food and a "Minus" context with no food or chow in adult female rats. Experiment 1 confirmed that the Plus context enhanced consumption of the paired food (Froot Loops) but not a different food (banana bread). Experiments 2 and 3 tested whether pairing a variety of foods with the Plus context would overcome this specificity. In Experiment 2 the Plus context either contained bland chow (Chow group), 1 (Single group), or 3 palatable foods (Variety group). The test food, Froot Loops, was familiar but never paired with the Plus context. The Variety group exhibited CPF by eating more Froot Loops in the Plus than in the Minus context, while Single and Chow groups ate equivalently in the 2 contexts. Experiment 3 replicated this effect when the Minus context contained chow during training and when a novel food was tested. These findings have important implications for overeating given that modern food environments are typified by variety and that food consumption often occurs outside the home. (PsycINFO Database Record

Learning about cues that prevent an outcome: Conditioned inhibition and differential inhibition in human predictive learning

Summation tests were used to assess whether participants in a human predictive judgement task learned that some cues (foods) prevented the occurrence of the outcome (migraine). In three experiments the preventative effect of a conditioned inhibitor, I, trained in the design, P+, PI–, I–, was stronger than that of a negative control cue, N, that had been presented alone with the same frequency and simply predicted no outcome. This control cue, N, also passed the summation test when compared to a novel control cue. Experiments 2 and 3 examined whether the latter negative control effect was attributable to differential inhibition. Manipulations designed to alter context learning and the magnitude of the negative contingency did not affect the properties of the negative control cue. Thus, these experiments did not support the possibility that a cue given simple negative training acquires differential inhibition. Regardless of the mechanism underlying the negative control effect, the consistent finding that the conditioned inhibition cue, I, reduced prediction of the outcome more than did the negative control cue, N, provides evidence for true conditioned inhibition whose acquisition requires co-occurrence of the preventative cue with a positive training cue.

Second-order conditioning in human predictive judgements when there is little time to think

Associative accounts uniquely predict that second-order conditioning might be observed in human predictive judgements. Such an effect was found for cue X in two experiments in which participants were required to predict the outcomes of a series of training trials that included P + and PX−, but only when training was paced by requiring participants to make a prediction within 3 s on each trial. In Experiment 1 training on P + ended before training was given on PX − . In Experiment 2 trials with P+, PX−, T + and other cues were intermixed. In the unpaced group inhibitory learning was revealed by a summation test, TX versus TM, where M was a control stimulus. These results suggest either that pacing interferes with learning successive associations more than with learning simultaneous associations or that lack of time to think interferes with inferential processes required for this type of inhibitory learning.

Situational relevance: Context as a factor in serial overshadowing of taste aversion learning

In a serial overshadowing procedure, a target stimulus, A, is followed after an interval by a potentially interfering stimulus, B, and this is then followed by an unconditioned stimulus (US). An untested proposal from over four decades ago was that the degree to which B overshadows conditioning of A depends on whether or not the two events take place in the same context. To test this, two experiments used a 1-trial long-delay conditioned taste aversion (CTA) procedure: sucrose served as the target taste (A) and dilute hydrochloric acid (HCl) as the overshadowing taste (B), with lithium chloride injection providing the US. In Experiment 1, these tastes were novel: weaker overshadowing by HCl of an aversion to sucrose was found when the two tastes were presented in different contexts. Experiment 2 tested whether the effect of pre-exposure to HCl, thereby rendering it less effective in overshadowing a sucrose aversion, was also context dependent. In the conditioning session, rats again received either context-same or context-different presentations of sucrose and HCl. However, for some rats, HCl was pre-exposed in the same context to which it was later presented during conditioning (Consistent), while others were pre-exposed to HCl in a different context to the one in which it was presented during conditioning (Inconsistent). The Inconsistent group produced greater overshadowing than the Consistent group and thus confirmed that the latent inhibition effect was also context dependent. This study confirms the concept of situational relevance.

Sweetening yoghurt with glucose, but not with saccharin, promotes weight gain and increased fat pad mass in rats

The claim that non-nutritive sweeteners accelerate body weight gain by disrupting sweet-calorie associations was tested in two experiments using rats. The experiments were modelled on a key study from a series of experiments reporting greater body weight gain in rats fed yoghurt sweetened with saccharin than with glucose (Swithers & Davidson, 2008). Both of the current experiments likewise compared groups fed saccharin- or glucose-sweetened yoghurt in addition to chow and water, while Experiment 1 included a third group (Control) given unsweetened yoghurt. In Experiment 1, but not in Experiment 2, rats were initially exposed to both saccharin- and glucose-sweetened yoghurts to assess their relative palatability. We also tested whether the provision of an energy-dense sweet biscuit would augment any effects of saccharin on food intake and weight gain, as seemingly predicted by Swithers and Davidson (2008). In Experiment 1 there were no differences in body weight gain or fat pad mass between the Saccharin and Control group, whereas the Glucose group was the heaviest by the final 5 weeks and at cull had the largest fat pads. Greater acceptance of saccharin predicted more weight gain over the whole experiment. Consistent with past reports, fasting blood glucose and insulin measures did not differ between the Saccharin and Control groups, but suggested some impairment of insulin sensitivity in the Glucose group. Experiment 2 found similar effects of glucose on fat mass, but not on body weight gain. In summary, adding saccharin had no detectable effects on body-weight regulation, whereas the effects of glucose on fat pad mass were consistent with previous studies reporting more harmful effects of sugars compared to non-nutritive sweeteners.

Placebo caffeine reduces withdrawal in abstinent coffee drinkers

BACKGROUND

Expectancies have been shown to play a role in the withdrawal syndrome of many drugs of addiction; however, no studies have examined the effects of expectancies across a broad range of caffeine withdrawal symptoms, including craving.

AIMS

The purpose of the current study was to use caffeine as a model to test the effect of expectancy on withdrawal symptoms, specifically whether the belief that one has ingested caffeine is sufficient to reduce caffeine withdrawal symptoms and cravings in abstinent coffee drinkers.

METHODS

We had 24-h abstinent regular coffee drinkers complete the Caffeine Withdrawal Symptom Questionnaire (CWSQ) before and after receiving decaffeinated coffee. One-half of the participants were led to believe the coffee was regular caffeinated coffee (the 'Told Caffeine' condition) and one-half were told that it was decaffeinated (the 'Told Decaf' condition).

RESULTS

Participants in the Told Caffeine condition reported a significantly greater reduction in the factors of cravings, fatigue, lack of alertness and flu-like feelings of the CWSQ, than those in the Told Decaf condition.

CONCLUSIONS

Our results indicated that the belief that one has consumed caffeine can affect caffeine withdrawal symptoms, especially cravings, even when no caffeine was consumed.

Mediated overshadowing and potentiation of long-delay taste aversion learning: Two versus six cue-taste pairings

Mediated overshadowing occurs when an evoked representation of one stimulus interferes with the formation of an association between two other stimuli. This study tested whether such an effect can be found in long-delay taste aversion learning. The general methodology was to pair a cue with a sour taste (hydrochloric acid [HCl]) and then introduce the cue during the delay between the target taste, sucrose, and injection with lithium chloride (LiCl). Either 2 or 6 cue-HCl pairings were given. In Experiment 1, introduction of the cue, an almond flavor, produced overshadowing of the sucrose aversion in the group given 2 cue-HCl pairings (Paired-2), relative to an unpaired control, but potentiation of the sucrose aversion in the group given 6 cue-HCl pairings (Paired-6). This confirms that few pairings can be better than many in determining whether representation-mediated effects occur (Holland, 1990). A possible explanation for the Paired-6 results is that almond evoked an aversive response rather than memory of the sour HCl and that this added to the aversion produced by the sucrose-lithium pairing. Experiment 2 obtained similar results when a context was used as the cue intended to evoke an HCl representation.

Reply to Pellón and Killeen's (2015) commentary on Boakes, Patterson, Kendig, and Harris (2015)

This reply notes the importance of data from variable-time schedules for resolving the issue of whether competition from magazine approach behavior is a major factor responsible for the concentration of schedule-induced drinking early in interpellet intervals. Pellón and Killeen propose that this temporal distribution of schedule-induced drinking reflects the effectiveness of adventitious reinforcement of this behavior over a delay, as detailed in their model. However, there is no evidence to support either of their key assumptions: (a) that schedule-induced behaviors are maintained by adventitious reinforcement ("are operants") and (b) that behaviors vary in their sensitivity to a delay before the subsequent occurrence of a reinforcer.

Metabolic Effects of Access to Sucrose Drink in Female Rats and Transmission of Some Effects to Their Offspring

The aims of this study were, first, to examine the metabolic consequences for female rats of having unrestricted access to 10% sucrose solution and, second, to test for effects of this dietary intervention on their offspring. In Stage 1 females were mated following a 4-week period in which one group was given the sucrose in addition to their normal chow and a control group was given chow and water only. Sucrose was removed at parturition and the pups monitored until weaning. Despite the development of glucose intolerance in sucrose-fed mothers, no effects were detected on litter size or pup weights. In Stage 2 voluntary activity of offspring was assessed over postnatal days (PND) 51-60 and their glucose tolerance measured at PND89-94. Again no effect of maternal diet was detected. Only male offspring were used in Stage 3, which began when they were 13 weeks old. Four groups were given 10% sucrose solution for 48 days in a 2 x 2 design, in which one factor was maternal diet and the other was whether they were given 2-h access to an activity wheel on alternate days. Higher fasting glucose levels were found in offspring of sugar-fed mothers. Exercise increased insulin sensitivity in these rats but not in offspring of control mothers. Behavioural measures of memory in Stage 3 did not reveal any effects of maternal diet or exercise. Overall, this study suggested that, while providing 10% sucrose solution ad-libitum was sufficient to impair maternal metabolism, the impact of this dietary manipulation on offspring may be revealed only when the offspring's diet is similarly manipulated.

Proximal, but not distal, pre-exposure reduces serial overshadowing in one-trial taste aversion learning

This experiment tested whether pre-exposing a taste would reduce its ability to overshadow conditioning to a target taste and whether this effect would depend on the delay between pre-exposure and conditioning. Two groups of rats were pre-exposed to an interfering taste (HCl) either a week before conditioning (Group Distal) or the day preceding conditioning (Group Proximal). In the single conditioning trial, rats were given the target taste (sucrose) and 65min later were injected with lithium. The groups differed as to what they were given to drink 50min after sucrose: The Distal, Proximal and Novel groups were given HCl, while the Control group was given water. Pre-exposure to HCl reduced overshadowing of the sucrose aversion by HCl in Group Proximal but not in Group Distal. Possible explanations for the latter result include extinction of the context-HCl association and loss of context control over an HCl-no outcome association.

Temporal distributions of schedule-induced licks, magazine entries, and lever presses on fixed- and variable-time schedules

In this article, schedule-induced drinking (SID) refers to increased drinking by hungry rats exposed to intermittent delivery of food pellets. Two major accounts of SID differ in their explanation of why such drinking tends be concentrated soon after pellet delivery. Temporal discrimination theories propose that drinking is a form of displacement activity that occurs when a pellet is least likely. Adventitious reinforcement theories propose that drinking is displaced to early in an interpellet interval (IPI) by magazine-directed behavior that occurs toward the end of an IPI. The main aim of this study was to examine the latter response-competition account by recording distributions of both licking and magazine entries as SID developed when pellets were delivered to different groups either on a fixed-time (FT 30 s) or on a variable-time schedule (VT 30 s), as in Experiment 1. Although VT 30-s schedules produced essentially flat distributions of magazine entries, licking still tended to be concentrated early in an IPI. Furthermore, there was no indication (Experiments 1 and 2) that magazine entry distributions developed ahead of licking distributions. Experiment 3 examined distributions of lever presses instead of licks: Initially high rates of lever pressing declined both with response-independent schedules (FT and VT) and when a minimal response-dependency was introduced (recycling conjunctive schedule), yet this response also tended to be most frequent soon after pellet delivery. Overall, the data were generally consistent with temporal conditioning theories.

Taste Aversion Learning Despite Long Delays: How Best Explained?

Taste aversion learning (aka conditioned taste aversions or CTA) can occur even when there is delay of some hours between experience of the taste and the subsequent onset of illness. This property of CTA is quite distinct from other forms of associative learning, where typically no association between two events is acquired if they are separated by more than a minute. This paper provides an overview of a series of recent experiments based on the assumption that long-delay CTA is possible only when no potentially overshadowing – or ‘concurrently interfering’ (Revusky, 1971) – events occur during the delay. The general method is one in which in a single conditioning session the rats are first given 8% sucrose, providing the sweet target taste, and 65 min later are injected with lithium chloride. What vary across experiments are the potentially interfering events occurring during the 65-min delay period. When the interfering event is a second, and quite different, taste, namely sour-tasting hydrochloric acid solution (HCl), this produces 1-trial overshadowing of the sucrose aversion, to a degree that is greater when HCl is given late in the delay period, greater when HCl is given in the same context as sucrose and greater when HCl has not been pre-exposed. Other intervening events can also overshadow sucrose aversion learning. These include placement into a novel context, as long as this occurs immediately before injection, and even stimuli that evoke memories of food-related experiences. These results can be accounted for by adding to the Rescorla-Wagner model (Rescorla & Wagner, 1972) the assumption that sickness is comprised of a succession of mini-bouts and the assumption that context-event associations (Wagner, 1981) are important in long-delay CTA.

Temporal contiguity in associative learning: Interference and decay from an historical perspective

The greater the separation in time between 2 events, A followed by B, the less likely they are to become associated. The dominant explanation of this temporal contiguity effect has been trace decay: During the interval between A and B, the trace left by A becomes too weak by the time B occurs for an association to be formed between them. Pavlov adopted this idea in the context of classical conditioning and Hull used it to account for the deleterious effect of delaying reinforcement on the acquisition of instrumental responses. By 1960 various studies supported the conclusion that animals could not learn to associate 2 events separated by more than around 45 s. Research on human skill acquisition with delayed feedback and later studies using causal or predictive judgment tasks indicated that explicit cognitive processing is generally involved when humans associate events separated by more than a few seconds. The discovery of long-delay taste aversion learning prompted Revusky's (1971) alternative analysis of contiguity effects in terms of interference: The greater the separation between A and B, the more likely that extraneous events compete for association with A and B. Although the analysis of overshadowing provided by associative learning theories provides a context for this account, none of these theories provide a satisfactory account of evidence on temporal contiguity from a wide range of animal studies. Alternative timing theories are arguably also unsatisfactory.

Low-volume exercise can prevent sucrose-induced weight gain but has limited impact on metabolic measures in rats

PURPOSE

Rats given sugar-sweetened drinks can develop glucose intolerance, insulin resistance and dyslipidaemia. The aim of this study was to investigate whether such metabolic disruptions and also possible weight gain induced by chronic sucrose consumption could be attenuated by low-volume exercise.

METHODS

Using a 2 × 2 factorial design, rats were given free access for 57 days to either a 10% sucrose solution (Suc and SucEx) or water only (Con and ConEx), while exercise rats (SucEx and ConEx) received 20-min treadmill training every 3 days. Caloric intake and body weight were measured throughout this dietary intervention. Oral glucose tolerance tests were performed on days 29 and 54. Plasma insulin, triglycerides and leptin were also measured, together with post-mortem measures of retroperitoneal fat pads and liver triglycerides.

RESULTS

In groups given sucrose, exercise reduced calorie consumption, reduced weight gain and decreased leptin relative to non-exercised controls. Exercise was found to improve glucose tolerance and insulin action at day 29, but not day 54.

CONCLUSIONS

Low-volume exercise can be effective in preventing weight gain in sucrose-fed rats, probably via reduction of subcutaneous fat, but prevention of the glucose intolerance and dyslipidaemia produced by sucrose consumption may be transient.

Warning about side effects can increase their occurrence: an experimental model using placebo treatment for sleep difficulty

Patients in clinical practice and participants in clinical trials are warned about side effects that may result from their treatment. Such warnings could lead to placebo-induced side effects if they create an expectation of these effects. We used an experimental model to test this possibility. Undergraduates reporting sleep difficulty received placebo treatment disguised as a hypnotic for one week and were warned about either one or four bogus side effects. Placebo treatment significantly improved sleep difficulty relative to a no treatment control group, as indicated by self-report and by objective outcomes. At the end of the treatment week participants who had been warned about a single side effect showed better recall of this effect than those warned about four side effects. Most importantly, participants tended to report experiencing a side effect they had been warned about, with a trend towards a larger effect in participants warned about one side effect. This evidence for placebo-induced side effects may need to be considered when interpreting data on side effects from clinical trials.

Blocking of acquisition of a taste aversion by a context experienced prior to the taste

This experiment tested the proposal that events taking place before a rat has access to a taste can proactively interfere with acquisition of an aversion to the taste when this has been followed by lithium chloride injection. Rats were initially given context discrimination whereby placement in one distinctive context (target) was followed by lithium injection, while placement in a second context (safe) was followed by saline injection. In the subsequent 1-trial taste conditioning session, rats were first placed in either their target context (Blocking group), their safe context (Control-Safe group) or a neutral context (Control-Neutral group), then given access to sucrose and 30 min later were injected with lithium. Subsequent tests of sucrose intakes revealed a blocking effect. These results indicate that proactive interference with taste aversion learning by a context can occur that is unlikely to be based on generalization decrement.

Varying temporal contiguity and interference in a human avoidance task

Temporal contiguity and number of interfering events were manipulated in a human avoidance (Martians) task, which required participants to prevent an "invasion" when a particular visual stimulus ("shield") appeared by releasing the space bar before the shield was activated. A particular symbol, 1 of up to 6, functioned as a brief warning signal. The signal-offset to shield-onset (S-S) interval varied between groups, as did the number of additional symbols acting as distractors. In Experiments 1 and 2, speed of learning declined as a linear function of both trace interval and number of distractors. Path analysis showed that the effects of the S-S interval depended primarily on the number of distractors during this interval. Experiment 2 further demonstrated that participants who failed to suppress responding were generally unable to identify which symbol was the signal, suggesting that the presence of distractors disrupted detection of the contingency rather than performance. Overall, the results indicated that learning to associate 2 temporally separated events depends mainly on the amount of interference and little on the time interval between them.

Behavioral contrast and response independent reinforcement

Four pigeons received pre-training that included presentation of the reinforcer independently of behavior and then baseline training on a variable-interval schedule of reinforcement. With the introduction of a multiple schedule, in which the first stimulus was associated with a response contingent and a second stimulus with a response independent, 1-min variable-interval schedule, a reduction in response rate was obtained in the second component, which was not accompanied by a behavioral contrast effect in the first component. A further three pigeons were given the same pre-training and baseline training before the introduction of an otherwise identical multiple schedule, in which no reinforcement occured in the second component. Behavioral contrast was obtained from all three subjects. The results indicated that under conditions of constant reinforcement density a reduction in responding is not a sufficient condition for the occurrence of behavioral contrast.

The bisection of a brightness interval by pigeons

Pigeons were trained on a discrete trials, successive discrimination procedure, in which the stimuli were two luminance values on the center key. Behavior was maintained by 25% reinforcement of correct responses on two side-keys. During occasional test trials the luminance of the center key was maintained at one of a number of values, intermediate to those of the two training stimuli, and a function relating the relative frequency of responses on the two side keys to stimulus intensity was obtained. The intersection of this function with the 50% line provided an estimate of the bisection point. Since no bisection point occurred below the geometric mean of the interval, the results were not consistent with a logarithmic scale of brightness but fitted the general mean theorem with an exponent of 0.24. With continued testing, the performance of individual subjects oscillated in an irregular manner about the mean bisection point. The relative stability of the test behavior and the absence of context effects indicated that the method was suitable as a general procedure for measuring stimulus distances.

Response additivity: effects of superimposed free reinforcement on a variable-interval baseline

Three experiments examined the effects of superimposing free reinforcement (Free VI 30-sec) on behavior maintained by a response dependent mult VI 2-min VI 2-min schedule of reinforcement. Experiment I used pigeons as subjects, key pecking as the response, and colors of response key as the stimuli associated with the multiple-schedule components. When free reinforcement was added during only one component (Differential condition) a large and highly significant increase in response rate developed in this component. Adding free reinforcement during both components (Nondifferential condition) produced smaller and far less-consistent effects. An entirely different pattern of results was obtained in two subsequent experiments, where similar procedures and reinforcement conditions were used with rats as subjects and bar pressing as the response. In both Experiments II and III, response rates decreased to the stimulus associated with added free reinforcement in the Differential condition. These findings are interpreted as the result of interactions between behavior maintained by response-reinforcer contingencies and behavior maintained by stimulus-reinforcer contingencies. As such, they support the main assumption of an autoshaping theory of behavioral contrast, that additivity of responding generated by the two kinds of contingency can occur only in situations favorable to autoshaping.

Observing stimulus sources that signal food or no food

Pigeons were given a choice between observing a stimulus source that was uncorrelated with food or one that was informative. The informative source was either positive, in which a stimulus change signalled food, or negative, in which change signalled no food. If observing is supported by the reduction of uncertainty, the negative as well as the positive source should be preferred to the uncorrelated source. On the other hand, if observing requires support by conditioned reinforcement, the negative source should not be preferred to the uncorrelated source. Two keys served as stimulus sources in a discrete trial procedure. The keys were lighted together, remained on for a variable length of time, and went off together. A key could change color 1 sec before going off. In the uncorrelated source, the change occurred equally often on trials ending with or without food. In the positive information source, the change occurred only on food trials, whereas in the negative information source, it occurred only on no-food trials. All stimulus changes and food delivery were response independent. As measured by orientation and autoshaped pecking, the positive information source was preferred to the uncorrelated source. However, the uncorrelated source was preferred to the negative information source. The latter result does not support the view that observing behavior is reinforced by the reduction of uncertainty. The positive and negative information sources reduced uncertainty equally but only the positive source provided a signal that could act as a conditioned reinforcer by virtue of its relation to food.