The importance of contextual elements in taste-aversion learning

The effect of exteroceptive contextual cues, presented during the conditioned stimulus (CS) and unconditioned stimulus (US) intervals, upon the later development of conditioned saccharin aversions was studied in two experiments. It was found that the presence of the contextual cues during the CS + US and CS intervals resulted in a greater degree of saccharin aversion; the presence of these cues during the US interval was relatively less important. These findings are discussed in terms of conditioning models and retrieval models derived from animal learning and human information-processing data.     

The CS-preexposure effect in conditioned taste-aversion learning in golden hamsters

In Experiment 1, golden hamsters were injected with either 0.9% saline or the nausea-inducing agent, lithium chloride (LiCL), immediately after consuming a flavored diet that was either novel or familiar. The LiCl-induced aversion was strong in hamsters for which the flavored diet was novel, but no significant aversion was observed in hamsters that were familiar with the flavored diet. In Experiment 2, the strength of the LiCl-induced aversion was related inversely to the amount of conditioned-stimulus (CS) preexposure and directly to the duration of the preexposure-conditioning interval. Thus, although some previous researchers have suggested that hamsters may not demonstrate the CS-preexposure effect in a conditioned taste-aversion paradigm, they clearly did so under the conditions of the present experiments, and moreover, the characteristics of the CS-preexposure effect in hamsters were generally similar to those observed in rats.     

Forgetting, preconditioning CS familiarization and taste aversion learning: An animal experiment with implications for alcoholism treatment

The rapid taste aversion acquisition, which typically occurs in many species when ingestion of a novel flavor precedes gastrointestinal distress, is retarded by preconditioning familiarity with the CS flavor. This CS familiarity effect (CSFE) might contraindicate taste aversion approaches to alcoholism treatment since alcoholics are quite accustomed to the tastes of alcoholic beverages. However, many alcoholics do develop strong nausea—induced alcohol aversions under appropriate conditioning parameters. Additionally, the CSFE is attenuated in rats by repeated conditioning trials including discrimination training. The present animal experiment was conducted to determine if the CSFE could additionally be weakened by process of forgetting, i.e. by preconditioning withdrawal of a familiar flavor analogous to an alcoholic's ‘drying out’ before psychotherapeutic intervention. Using saccharin as the CS flavor and cyclophosphamide as the conditioning agent, Sprague-Dawley derived rats acquired no aversions when conditioning was attempted immediately after flavor familiarization. However, significant and equivalent saccharin aversions were observed when conditioning was delayed for either 20 or 100 days after familiarization. These findings imply that the efficiency and cost effectiveness of taste aversion approaches to alcoholism treatment might be enhanced by a pretreatment period of abstinence from alcohol ingestion.     

Learned safety and the CS-US delay gradient in taste-aversion learning

Decrements in taste-aversion learning produced by extensions of the interval between the CS flavor and poisoning (the US) may be attributed to decay of the CS trace during long CS-US intervals. Kalat and Rozin (1973) alternatively proposed that such decrements occur because during extended CS-US intervals subjects learn that the CS is safe. They sought to show that trace decay is not responsible for the CS-US delay gradient by demonstrating that learning is disrupted even if the CS is reintroduced during extended CS-US intervals. The present experiments show that such a second CS presentation during conditioning may (1) enhance subsequent intakes of the CS solution whether or not subjects are poisoned, and/or (2) facilitate aversion learning, the facilitation being greater the closer the second CS exposure is to poisoning. These results question the adequacy of previous evidence for the contribution of learned safety to the CS-US delay gradient and suggest that some other process is also involved.     

Effects of preexposure flavor concentration on conditioned aversion and neophobia

In Experiment 1, 128 experimentally naive, water-deprived rats (Rattus norvegicus) received pretraining access to either 0.25 or 1.5% saccharin, distilled water, or 2.0% saline, followed either by a pairing of 0.25 or 1.5% saccharin with an intraperitoneal injection of 0.15 M lithium chloride (LiCl) or by a pairing of distilled water with LiCl. Preexposure to either saccharin concentration reliably reduced conditioned aversion effects to 0.25% saccharin, relative to that for preexposure to distilled water or saline. But only preexposure to 1.5% saccharin reduced aversion effects to that concentration. In Experiment 2, 48 naive, water-deprived rats received preexposure procedures as in Experiment 1. Afterwards, the rats were tested for neophobia to 0.25 or 1.5% saccharin. Neophobia was reliably greater to the 1.5% concentration. However, preexposure to either saccharin concentration obliterated evidence for neophobia to saccharin, relative to that following preexposure to distilled water or saline.     

Cortical substrates of taste aversion learning: dorsal prepiriform (insular) lesions disrupt taste aversion learning

The functional relation between restricted damage to ventral primary somatosensory neocortex and the ability of rats to acquire conditioned taste aversion (CTA( was examined by a combination of behavioral and neurohistological techniques. Lesions confined exclusively to the established gustatory neocortex (GN) did not disrupt CTA acquisition, nor did lesions confined to suprarhinal cortical areas ventral to the GN. Lesions that encroached on dorsal prepiriform and insular cortices produced CTA acquisition deficits and damaged a large proportion of efferent projections to the prefrontal and precentral neocortex. In a second experiment, lesions of dorsal prepiriform and insular cortices did not modify taste preference-aversion threshold to any of the four taste modalities. It is concluded tha ventral somatosensory neocortical fields, including the established GN, do not mediate CTA acquisition and that rhinal cortices ventral and posterior to the GN are preferentially involved in associative learning for tastes and illness.     

Taste/tactile cue discriminations in taste-aversion learning foIIowing depletion of noradrenaIine

Four experiments were performed to investigate the effect of noradrenaline (NA) depletion, following systemic DSP4 treatment, upon a tastehactile discrimination in taste-aversion learning. In Experiments 1 and 2 , noisy bottle (A) + lithium chloride pairings were alternated with saccharin (B) + saline pairings, and vice versa, during Phase I conditioning. The particular order of reinforcement presentation in each case was then reversed, so that a noisy bottle (A) + saline pairing was now altered with a saccharin (B) + lithium chloride pairing, etc., during Phase II (reversal) conditioning. In Experiments 3 and 4 , saccharin in noisy bottle (AB) + lithium chloride pairings alternated with either noisy bottle (A) + saline or saccharin (B) + saline pairings, and vice versa, during Phase I conditioning; the order of reinforcement presentation was then reversed, as above. None of the four experiments performed offered any evidence of impairments of the discrimination task as a result of NA depletion. These results are discussed in the context of associative preparedness and of discrimination learning in operant tasks and recent findings on compound conditioning, following the loss of NA.     

Conditioned and latent inhibition in taste-aversion learning: clarifying the role of learned safety.

Experiments 1-3 investigated the applicability of the classical conditioning concept of conditioned inhibition to taste-aversion learning. Rats made ill after drinking saccharin and subsequently administered a "safe" exposure to saline (or casein hydrolysate) evidenced an enhanced preference for the safe fluid (relative to either a third, slightly aversive, solution or to water) when compared to controls in which saccharin was not previously poisoned. Such active condition inhibition was significantly reduced in Experiment 4 when two safe exposures to saline preceded saccharin-illness pairings. These results indicate that conditioned inhibition can be established in a taste-aversion procedure and that a latent inhibition manipulation reduces the ability of a taste to become a signal for safety. Implications of these findings for the learned safety theory of taste-aversion learning and the relevance to bait-shyness of principles established within the classical conditioning paradigm are considered.     

Perceptual learning in flavour aversion conditioning

In each of four experiments, rats drank a solution of saline or of lemon and saline shortly before receiving an injection of lithium chloride, and the generalization of the resulting aversion to sucrose or to lemon and sucrose was measured. There was little generalization from saline alone to sucrose alone, and prior exposure to the two solutions had no effect on their discriminability. An aversion conditioned to lemon-saline, however, did generalize to lemon-sucrose, and the extent of this generalization was substantially reduced by prior exposure to the two compound solutions. This perceptual learning effect was partly, but not entirely, attributable to the latent inhibition of the common element, lemon, produced by exposure to the two compounds: animals pre-exposed to lemon alone discriminated between lemon-saline and lemon-sucrose better than animals pre-exposed to saline and sucrose alone; but exposure to the three elements in isolation was not as effective as exposure to the two compound solutions in enhancing their discriminability. The final experiment established that one critical feature of compound pre-exposure is that it involves experience of saline and sucrose in the presence of the same common element. According to an associative theory of perceptual learning, this would result in the establishment of inhibitory associations between saline and sucrose, thus reducing generalization between the two compound solutions.     

Discriminative taste aversion learning: a learning task for older chickens

The study of learning and memory using the chicken model has relied on three learning paradigms, passive avoidance learning, imprinting and the pebble floor task. Passive avoidance learning and imprinting have been used predominantly in very young chickens and cannot be used to access learning and memory in older chickens. We have established a new behavioural learning paradigm, Discriminative Taste Aversion Learning (DTAL), that can be used with both young and older animals. The task requires chickens to discriminate between food crumbs dyed either red or yellow with one colour being associated with the aversive tasting substance, methylanthranilate. Learning can be tested at various times after the training session by presenting chickens with the coloured food crumbs without an aversive taste. Both chickens tested at 5 and 15 days post-hatch learned to avoid the aversive crumbs. Furthermore, the protein synthesis inhibitor anisomycin (30 mM; 10 microl per hemisphere) injected into the intermediate medial hyperstriatum ventrale 15 min pre-training or 45 min post-training blocked long-term memory for the DTAL task when tested 24 h later. Memory for the task was unaffected by anisomycin injection 120 min post-training or in control animals injected with saline at similar times. The timing of the cellular processes of protein synthesis needed for consolidation of the DTAL appears to be similar to those described for the other behavioural paradigms in young chickens.     

Salience and the effects of CS preexposure on aversion conditioning

Rats (n = 84) received preexposure to distilled water or to one of two differently salient flavors, 5.0% casein or 10.0% sucrose, casein being the more salient. Each preexposure group then received aversion conditioning to a 5.0% casein or a 10.0% sucrose CS. Aversion effects were reliably more enduring to casein than to sucrose. Relative to water-preexposed groups, preexposure to casein attenuated aversion effects to the casein CS reliably less than preexposure to sucrose attenuated aversion effects to the sucrose CS. During preexposure, neophobia was reliably greater to casein than to sucrose, suggesting that the demonstration of salience in taste aversion learning may be based on the inherent aversive properties of novelty.     

Context-dependent latent inhibition in taste aversion learning

The effects of taste stimulus preexposure, either in the presence or in the absence of a specific contextual cue consisting of a specific noise-producing bottle, upon the conditioning and testing of conditioned taste aversions to the taste (saccharin) plus context (noisy-bottle) compound stimulus were investigated. Four groups of rats were given preexposure trials (latent inhibition) to either: (1) novel saccharin in novel noisy bottles, (2) novel saccharin in familiar silent bottles, (3) familiar water in novel noisy bottles, (4) familiar water in familiar silent bottles, in six trials. During conditioning, saccharin was presented in the noisy bottles followed by lithium chloride for all the groups. At testing, saccharin was presented in the noisy bottles for both one-bottle and two-bottle tests of aversion. It was indicated that the conditioning decrement produced after both saccharin and noisy bottle preexposure was overwhelmingly greater than any produced after preexposure to the elements. These results, discussed in relation to current theories of latent inhibition and perceptual learning, further underline the overwhelming significance of exteroceptive contextual elements in conditioned taste aversions.     

Effects of a flavor-placement reversal test after different modalities of taste aversion learning

Taste aversion learning is induced through two different behavioral procedures: a short-term or concurrent (two-daily flavors) and a long-term or sequential (one-daily flavor) procedure. For the concurrent group of animals, two gustatory/olfactory stimuli are presented separately but at the same time on a daily basis. One is paired with simultaneous intragastric administration of hypertonic NaCl and the other with physiological saline. For the sequential group, the two stimuli are presented on alternate days, one of them followed by intragastric injection of the aversive stimulus and the other by saline, both after a delay of 15 min. The two groups learned the task, but when they were subjected to a flavor-placement reversal test only the sequential group was successful in achieving it. In a second experiment, three groups of animals had to learn concurrent or sequential discrimination tasks (with either simultaneous or delayed administration of the visceral stimulus) using only spatial/proprioceptive cues. The data show that none of the groups learned them under these conditions. The results are discussed in terms of the different modalities of learning. Short-term and long-term taste aversion learning are different in the anatomical structures involved, the number of trials required for acquisition and, as shown in this paper, flexibility.     

Analysis of the perceptual learning effect in flavour aversion learning: Evidence for stimulus differentiation

Rats received exposure to two compound flavours, AX and BX, where A and B were sucrose and saline and X was acid. For group intermixed (I), exposure consisted of alternating trials with AX and BX; group blocked (B) received a block of AX trials and a separate block of BX trials. Experiment 1 showed that generalization to BX after conditioning with AX was less profound in group I than in group B. Separate examination of the elements of the compound showed that the source of this difference lay in the strength acquired by the X element. X acquired less strength in group I than in group B (Experiments 1 and 2), whereas for the A element (Experiments 3 and 4) the reverse pattern was obtained. These results support the proposal that the perceptual learning effect (restricted generalization from AX to BX in group I) depends on a process that enhances the effectiveness of unique stimulus elements (A and B) and reduces that of common elements (such as X).     

Selective associations in one-day-old rats: taste-toxicosis and texture-shock aversion learning

One-day-old rats learned aversions to a novel taste paired with lithium-induced distress and to a tactile stimulus paired with brief electric shocks. However, aversions did not develop when taste was paired with shock or when the tactile stimulus was paired with lithium treatment. The aversions occurred only when lithium treatment immediately followed taste exposure and when shock was concurrent with exposure to the tactile stimulus. These findings indicate that selective associations in aversion learning are mediated by innate mechanisms that govern conditioning in the absence of extensive ontogenetic experience. The present research also shows that selective associations are sufficient for the occurrence of long-delay learning.     

The effect of stimulus preexposure upon the context effect in taste-aversion learning in noradrenaline-depleted rats

The present investigation was designed to study the effect of preexposure to a saccharin plus noisy bottle stimulus compound, or to each element by itself, upon the extinction-dependent "context effects" (due to the presencelabsence of the noisy bottle contextual cue) in taste-aversion learning, following noradrenaline (NA) depletion via a single systemic DSP4 injection. Since contextual manipulations have provided a reliable means of studying selective attentional processes the current theorising on the role of noradrenaline can be effectively accommodated. It was found that, under the No Preexposure and Noisy Bottle Preexposure conditions, the "context effect" during the saccharin preference tests, after reinstatement of the noisy bottles for all groups, was totally absent for the NA-depleted rats; in the Saccharin plus Noisy Bottle Preexposure condition the "context effect" was partially blocked for the NA depleted rats and in the Saccharin Preexposure condition it was at least as strong as in the control condition, These findings may be interpreted as supporting our conclusions, concerning NA-depleted rats derived from several different investigations, i.e. that following NA loss rats fail to attend sufficiently to contextual cues in taste-neophobia and taste-aversion situations. Thus, the attentional deficit may be described as an inability to attend to all aspects of the taste stimulus plus exteroceptive context stimulus compound. Although the selective attentional hypothesis of noradrenaline function requires a good deal of alteration, it remains the most parsimonious account that can be described at present.