Dissociation between the Espinet and perceptual learning effects in flavour aversion conditioning

In three experiments rats were given pre-exposure to two compound flavours, AX and BX, the two compounds being presented for some subjects on alternate trials (the intermixed schedule) and, for others, in separate blocks of trials (the blocked schedule). After aversion conditioning with A (in Experiments 1 and 2), the inhibitory properties of B were tested using both retardation (Experiment 1) and summation tests (Experiment 2). The results failed to support the proposal [Anim. Learn. Behav. 23 (1995) 361] that B should acquire inhibitory properties in the intermixed condition (the "Espinet effect"). Experiment 3 demonstrated that generalisation to BX after conditioning with AX was attenuated by intermixed pre-exposure (a perceptual learning effect). This pattern of results challenges the hypothesis that inhibitory learning during intermixed pre-exposure to AX and BX can account for both the Espinet and the perceptual learning effects.     

Blocked and test-stimulus exposure effects in perceptual learning re-examined

Three experiments re-examined the effects of blocked or alternated exposure to the conditioning and test stimuli and the effect of simple exposure to the test stimulus, on stimulus generalization. In all experiments rats received conditioning where a compound flavor, AX, was paired with LiCl-induced illness. All rats were tested for generalization with another flavor, BX. In Experiment 1, rats that received alternating exposure to the two flavor compounds, AX and BX, prior to conditioning showed less generalization to BX than rats that received no exposure. Exposure to BX or AX alone was also somewhat effective in reducing generalization. In Experiment 2 blocked exposure to AX and BX prior to conditioning was effective in reducing generalization, as was alternated exposure, and extended exposure to BX was more effective than the other procedures. In Experiment 3, exposure to X alone prior to conditioning produced generalization equal to that produced by alternated or blocked exposure and replicated the effect of extended exposure to BX found in the previous experiment. The relevance of the results to the theories proposed by McLaren and Macintosh [Anim. Learn. Behav. 28 (2000) 211] and Hall [Q. J. Exp. Psychol. B 56 (2003) 43] is discussed.     

Order effects after blocked preexposure to two compound flavors

In the first three experiments two groups of rats received prolonged blocked preexposure to AX–BX or to BX–AX. Experiment 1 showed that conditioning of AX after preexposure resulted in less generalization to BX in the AX–BX group than in the BX–AX group. Experiments 2 and 3 constituted, respectively, retardation and summation tests of the inhibitory properties acquired by B after preexposure and conditioning of A. Excitatory conditioning of B was retarded in the AX–BX group (Experiment 2) and only in this group B was able to alleviate the aversion conditioned to a new stimulus (Experiment 3). These results are explained as a consequence of the formation, in the AX–BX group, of inhibitory associations directed from B to A. A fourth experiment provided evidence that the A ↔ X association was preserved until the end of the preexposure phase in this group, which is a requisite for the formation of these inhibitory associations.     

Salience modulation and associative inhibition interaction: Short but not long exposure to similar stimuli protects the salience of the unique elements

In three experiments, rats were given preexposure to two similar flavour compounds, AX and BX. Following preexposure, conditioning trials took place in which AX was paired with an illness-induced unconditioned stimulus. Animals that were given short alternated preexposure to AX and BX, showed higher generalization of conditioned aversion to AX to a new compound, AN, than animals that were given blocked preexposure (short and long) and long alternated preexposure (Experiments 1 and 2); and showed less preference for A when they were given a choice between A and X (Experiment 3). These results have been taken to indicate that the salience of the A element is well preserved after short alternated preexposure, but declines when preexposure goes on for some more trials. The results reported support the notion that perceptual learning is a multi-determined phenomenon that depends on salience modulation processes after relatively short preexposure, and on an associative inhibition mechanism after prolonged preexposure.     

An examination of the effectiveness of inflation and deflation treatments in detecting within-compound learning of a taste aversion

Two conditioned taste aversion experiments with rats assessed the relative effectiveness in providing evidence of within-compound learning of different procedures that involve the initial compound presentation of two stimuli, A and X, with the unconditioned stimulus (i.e., AX+). In Experiment 1, following a single AX+ trial, groups A+ and B+ received an additional conditioning trial (i.e., inflation treatment) with A and B, respectively, whereas group A- received an extinction trial (i.e., deflation treatment) with A. The results showed a reduction in the aversion elicited by the target stimulus, X, in group A- relative to both groups A+ and B+, which did not differ. Experiment 2 further investigated the failure of group A+ to increase the aversion to X relative to control group B+ by pairing A or B with either the same unconditioned stimulus that was previously paired with AX (groups A+ and B+) or with a stronger unconditioned stimulus (groups A* and B*). The results showed increased aversion to X in group A* relative to group B*, but not in group A+ relative to group B+. These results are interpreted as indicative of extinction of the within-compound association during the treatment with A, which could likely impair the detection of within-compound learning following an inflation, but not a deflation treatment.     

The taste of alcohol for rats as revealed by aversion generalization tests

In six experiments, naive rats were trained to avoid alcoholby pairing its presentation with lithium chloride-induced illness.Rats were then tested for aversion generalization by presentingvarious test solutions. Rats trained to avoid either 3, 6 or9% (v/v) alcohol generalized the aversion to a sucrose + quininehydrochloride solution. The five remaining binary combinationsof sucrose, sodium chloride, hydrochloric acid and quinine hydrochloridefailed to produce significant generalization in trained rats(Experiment 1). In further experiments, rats trained to avoid6% alcohol showed significant aversion generalization to a varietyof sucrose + other ‘bitter’ solutions (Experiment2) and sucrose + acid solutions (Experiment 3). Varying theconcentration of hydrochloric acid in a sucrose + acid mixtureproduced small but uniform degrees of aversion generalization(Experiment 4). Rats trained to avoid 6% alcohol did not generalizethe aversion to sucrose alone, regardless of concentration (Experiment5). Finally, in Experiment 6, rats trained to avoid 6% alcoholsuppressed consumption over a range of alcohol concentrations.These results confirm that, for rats, the taste of alcohol hasa complex set of characteristics; sweet taste in combinationwith other tastes appears to be the most similar as it is tothese solutions that rats with alcohol aversions show the mostgeneralized avoidance.     

The influence of comparison between similar stimuli on the effectiveness of their unique features

Four groups of rats received stimulus pre-exposure under conditions intended to produce different opportunities for stimulus comparison to occur. Groups AX/BX-L and AX/BX-S received alternating presentations of two compound flavors (AX and BX); the interval between these presentations was long (24 h) for group AX/BX-L, and short (5 min) for group AX/BX-S. Groups AX-L and AX-S matched groups AX/BX-L and AX/BX-S in their pre-exposure conditions except that they received presentations of water rather than presentations of BX. The effective salience of one of the unique stimulus features (A) was then assessed by using this flavor as a conditioned stimulus in a flavor-aversion procedure. It was found that aversion to A was learned about more readily after pre-exposure to AX and BX than after pre-exposure just to AX. However, there was no indication that the rate of conditioning to A was affected by the temporal interval between the presentations of AX and BX. These findings challenge the notion that stimulus comparison engages a process responsible for an increase in the salience of the unique stimulus features, but can be accommodated by the salience modulation mechanism proposed by Hall [Hall, G., 2003. Learned changes in the sensitivity of stimulus representations: associative and nonassociative mechanisms. Q. J. Exp. Psychol. 56, 43-55].     

Within-compound associations are not sufficient to produce taste-mediated odor potentiation

Rats were used in two flavor-aversion experiments to determine if within-compound associations could be detected with a taste+odor compound that would not support taste-mediated odor potentation. In Experiment 1, following taste+odor compound conditioning, postconditioning taste extinction significantly weakened the odor aversion. In Experiment 2, following taste+odor compound conditioning, postconditioning taste inflation significantly strengthened the odor aversion. There was no evidence that taste potentiated the odor aversion in either Experiment 1 or 2. Thus, the results demonstrate that the presence of within-compound associations is not sufficient to produce taste-mediated odor potentiation. We offer a mediated conditioning explanation to account for the results of these two experiments.     

Distractor effects upon habituation of complex stimuli

In two experiments, rats were given serial forward (the target followed by the distractor) or backward (the distractor followed by the target) exposure to two compound flavor stimuli that could be either similar (Salt-X/AX) or dissimilar (Salt-X/AY, Experiment 1; Salt/AX, Experiment 2). Following pre-exposure, the Salt element was presented in a compound with a novel flavor, N. The salience or effectiveness of the Salt element was then assessed by presenting the new flavor, N, under a state of salt appetite. Experiments 1 and 2 revealed that the order of presentation modulated the habituation of the Salt element only when the distractor was similar to the target: the Salt element was more salient after forward than backward pre-exposure. In the groups Dissimilar the order of pre-exposure was irrelevant; however, when the Salt element was presented in compound with a second element (Salt-X, Experiment 1), its salience was preserved, whereas when it was presented alone (Salt, Experiment 2) its salience was significantly reduced. These results, which are discussed in terms of Wagner (1981) theory of habituation, inform about the way in which stimuli presented closely in time are processed.     

The sweet-bitter taste of alcohol: aversion generalization to various sweet-quinine mixtures in the rat

Rats were trained to avoid a 5% alcohol solution and then testedwith either sweet + quinine hydrochloride solutions (Experiment1) or sweet + hydrochloric acid solutions (Experiment 2). Thesweet stimuli used were sucrose, glucose, fructose and saccharin.Significant aversion generalization was found only in Experiment1 where trained rats generalized to all four test stimuli, thussuggesting that alcohol has a sweet taste (in combination withbitter) not specific to one sweetener. No significant aversiongeneralization was noted in Experiment 2 when sweet + hydrochloricacid solutions were tested. In Experiment 3, rats were trainedto avoid 6% alcohol and tested with sucrose + quinine hydrochloridemixtures with varying concentrations of each component. In general,rats showed generalization of the alcohol aversions across thevarious concentrations of sucrose and quinine hydrochloridetested.     

Generalization of specific taste aversions to alcohol in the rat

Two experiments were conducted to examine the taste qualitiesof alcohol in the rat. In Experiment 1, rats were trained toavoid sucrose, quinine or a sucrose + quinine mixture. In Experiment2, rats were trained to avoid sucrose, hydrochloric acid ora sucrose + hydrochloric acid mixture. In both experiments ratswere then tested for generalization to 3, 6 and 9% (v/v) alcoholsolutions. Following the alcohol tests rats were tested withthe taste solutions used during training. Results showed thatrats trained to avoid sucrose (Experiments 1 and 2), quinineor the sucrose + quinine mixture generalized that aversion tothe 6% alcohol solution. No generalization was found to the3% alcohol solution. No generalization was displayed by ratstrained to avoid either the hydrochloric acid solution or thesucrose + hydrochloric acid solution. Following the alcoholtests, all trained rats exhibited strong aversions to the solutionthey were trained to avoid. In addition, rats trained to avoida single solution generalized that aversion to the mixture andrats trained to avoid a mixture generalized to the single components.     

Effects of an extinguished CS on competition with another CS

Three experiments were conducted using a conditioned taste aversion procedure with rats to examine the effect of nonreinforced presentations of a conditioned stimulus (CS) on its ability to compete with a target stimulus for manifest conditioned responding. Two CSs (A and B) were presented in a serial compound and then paired with the unconditioned stimulus. CS A was first paired with the US and then presented without the US (i.e., extinction) prior to reinforced presentation of the AB compound. Experiment 1 showed that A was poor at competing with B for conditioned responding when given conditioning and extinction prior to reinforcement of AB relative to a group that received both A and B for the first time during compound conditioning. That is, an extinguished A stimulus allowed greater manifest acquisition to B. Experiment 2 found that extinction treatment produced a poor CR to the pretrained and extinguished CS itself following compound conditioning. Experiment 3 found that interposing a retention interval after extinction of A and prior to compound conditioning enhanced A's ability to compete with B. The results of these experiments are discussed with regard to different theories of extinction and associative competition.     

Stimulus comparison in perceptual learning: roles of sensory preconditioning and latent inhibition

Rats were given exposure to a compound flavour (AX) and to one element of that compound (X). Two drinking tubes were made available to the rat on each exposure trial. For group concurrent (CNC) one tube contained AX and the other X. For groups alternating (ALT) and blocked (BLK), however, both tubes contained the same flavour (AX or X). Group ALT received AX and X on alternate trials; group BLK received AX in the first block of trials and X on the second, or vice versa. After an aversion had been established to X the groups were tested with AX. It was found that group ALT showed less generalization from X to AX than did group BLK. This difference was not accompanied by a parallel difference in the level of conditioning to X. However, group CNC showed both stronger conditioning to X and greater generalization from X to AX than groups ALT and BLK. Implications for the role of stimulus comparison in the perceptual learning effect are discussed.     

Haloperidol treatment at pre-exposure phase reduces the disturbance of latent inhibition in rats with neonatal ventral hippocampus lesions

Animals with neonatal ventral hippocampal lesions develop during or after adolescence abnormal behaviors related to schizophrenia such as anxiety and latent inhibition disruption. The aim of this study was to test whether haloperidol injection prior to pre-exposure session in the latent inhibition test would facilitate latent inhibition.     

Familiarity interferes with filial imprinting

The present study was performed to investigate whether and how pre-exposure to an object affects subsequent filial imprinting to that object. In Experiment 1 junglefowl chicks (Gallus gallus spadiceus) were first exposed to either a red object alone (control group), or a red and a yellow object simultaneously (experimental group; phase 1). Subsequently, all chicks were exposed to the yellow object in the presence of a black and blue one (phase 2). At the end of phase 1, most experimental chicks had developed a preference for the red object over the yellow one. At the end of phase 2, preferences of experimental chicks were shifted away from the yellow object towards the novel black and blue object, relative to preferences of control chicks. This shows that pre-exposure may interfere with imprinting. Experiment 2 revealed that when control chicks were tested with the yellow object at the end of phase 1, filial responses were as strong as in experimental chicks. This shows that the yellow object had not acquired control over filial behaviour during phase 1, and also that the relatively impaired imprinting on that object in phase 2 was not due to reduced generalization from the red object. One possible explanation why pre-exposure may interfere with imprinting is that familiarity alters the level of attention attracted by an object, a mechanism suggested to underlie ‘latent inhibition’ in conditioning.     

Input coding in animal and human associative learning

Two appetitive conditioning experiments with rats investigated whether the degree of generalization between a compound and its component parts is fixed or variable. Both experiments used a two-stage transfer design. In Stage 1, the elemental groups learned that a compound and its component parts signaled the same outcome (i.e. C+, D+, CD+), whereas the configural groups learned that a compound and its component parts signaled different outcomes (i.e. C+, D+, CD-, where '+' is pellets and '-' is no pellets). In Stage 2, the rats were tested for reductions in generalization. Experiment 1 found no evidence that past configural learning reduced generalization when a new set of alike-treated A and B elements were presented in compound for the first time. Experiment 2 found no evidence that past configural learning reduced generalization when the stimuli of Stage 1 were presented in a new C-, D-, CD+ relation. In contrast to findings with humans, these results suggest that past experience plays a minor role in how stimuli are encoded in animal conditioning.     

Effects of post-treatment retention interval and context on neophobia and conditioned taste aversion

We have repeatedly observed that a delay between acquisition and test, and the nature of the context in which the delay is spent, modulates latent inhibition (LI) of conditioned taste aversion (CTA; e.g. [Anim. Learn. Behav. 28 (2000) 389; Anim. Learn. Behav. 30 (2002) 112]). The present paper analysed the effects of delayed testing and treatment context after flavor exposure on the recovery of neophobia (Experiment 1) and on extinction after simple conditioning (Experiment 2). Two experiments were conducted with the same factorial design (2x2: 1 day versus 21 days of delay between first and second stage, and home versus experimental cages as place of experimental treatment). There were independent effects of both variables on habituation of neophobia and conditioning strength as measured on extinction trials. The long delay produced a reduction of neophobia (Experiment 1) and an increase in conditioning (Experiment 2). In addition, more of the flavored solution was consumed when the experimental treatment was conducted in the home cage than in the experimental cage (Experiment 1), and there was stronger conditioning when the delay period took place in the experimental cages than in the home cages (Experiment 2). The implications of these results for LI, as well as their relevance for experiments that use the CTA paradigm, are discussed.     

Summation of latent inhibition and overshadowing in a generalized bait shyness paradigm of rats

This paper reports a single experiment using four groups of rats. A group of thirsty rats showed aversion to sodium chloride (NaCl) solution in testing after drinking lithium chloride (LiCl) solution, demonstrating generalized bait shyness due to the common "salty" taste. The other two groups of rats were treated identically except that the rats were either exposed to NaCl solution prior to drinking the LiCl solution or allowed to drink LiCl-sucrose solution instead of the LiCl solution. The weaker salt aversion observed in these groups indicates latent inhibition and overshadowing, respectively. The rats in the fourth group were preexposed to NaCl and allowed to drink the LiCl-sucrose solution. The weakest salt aversion in this group suggests summation of latent inhibition and overshadowing effects.     

Anxiolytic effects of lavender oil inhalation on open-field behaviour in rats

To establish a valid animal model of the effects of olfactory stimuli on anxiety, a series of experiments was conducted using rats in an open-field test. Throughout, effects of lavender oil were compared with the effects of chlordiazepoxide (CDP), as a reference anxiolytic with well-known effects on open-field behaviour. Rats were exposed to lavender oil (0.1-1.0 ml) for 30 min (Experiment 1) or 1h (Experiment 2) prior to open-field test and in the open field or injected with CDP (10 mg/kg i.p.). CDP had predicted effects on behaviour, and the higher doses of lavender oil had some effects on behaviour similar to those of CDP. In Experiment 3, various combinations of pre-exposure times and amounts of lavender oil were used. With sufficient exposure time and quantity of lavender the same effects were obtained as in Experiment 2. Experiment 4 demonstrated that these behavioural effects of lavender could be obtained following pre-exposure, even if no oil was present in the open-field test. In Experiments 2-4, lavender oil increased immobility. Together, these experiments suggest that lavender oil does have anxiolytic effects in the open field, but that a sedative effect can also occur at the highest doses.     

Symmetrical effect of pre-exposure between alcohol and morphine on conditioned taste aversion

It has previously been reported that pre-exposure to a psychoactive drug can block the conditioned taste aversion associated with that drug. This study was an attempt to investigate alcohol-morphine interactions using this pre-exposure paradigm. After two weeks of adaptation to a schedule of daily 30-minute access to water, rats were pre-exposed to morphine, ethanol, or the respective vehicle control every second day for three days before (Days 1, 3, 5) and after the first conditioning day (Days 8, 10, 12). On conditioning days (Days 7, 14), animals were first presented with a saccharin solution for 30 minutes following which animals that were pre-exposed to morphine were injected with ethanol while those pre-exposed to ethanol were administered with morphine. Saccharin was again presented on three more occasions (Days 21, 28, 35) without drug injection. Using the percent change in saccharin consumed from the first presentation as a measure of aversion, it was found that pre-exposure to morphine blocked ethanol conditioned taste aversion. Similarly, animals pre-exposed to ethanol showed less aversion to the saccharin paired with morphine. This is the first demonstration of a symmetrical relationship between alcohol and the opiates.