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.     

Augmentation: Synergistic Conditioning in Taste-Aversion Learning

Recent work in taste-aversion learning has revealed a new phenomenon in classical conditioning. When a preconditioned gustatory cue (taste or odor) is conditioned in compound with a second gustatory cue, conditioning to the second cue is augmented. This enhanced conditioning of the second cue is noteworthy because studies with other forms of classical conditioning have shown blocked conditioning to the second cue. This new phenomenon has been termed augmentation , and it has implications for the study of taste and odor interactions, formal models of learning, and clinical interventions with cancer patients.     

Effects of CS concentration in taste-aversion learning: Problems in assessing aversion strength

The concentration of saccharin (CS) solutions was varied between groups of rats in four taste-aversion learning experiments, using lithium chloride as the aversive agent (US). Saccharin intake was measured on four one-bottle presentations yielding data on initial taste neophobia as well as postconditioning aversion strength. The results indicated that saccharin intake declined progressively with increasing concentrations on the first presentation, and that preconditioning intake to a large extent predicted postconditioning intake. In an attempt to correct postconditioning values for differences of saccharin intake on the preceding trial, a taste suppression ratio (TSR) was computed as the quotient of the amounts of saccharin consumed by individual rats on consecutive presentations. In general, the TSR was found to be too insensitive to detect differences of conditioning strength acquired by different saccharin concentrations. Alternative approaches to the CS-concen-tration problem in taste-aversion learning are discussed.     

Role of the CS-US interval in the US preexposure effect

Theory and research suggest that long delay taste-aversion conditioning should be less affected by proximal US preexposure than short-delay conditioning. The present research tested this hypothesis by administering illness-inducing LiCl shortly before a saccharin-LiCl pairing in which the postsaccharin LiCl was administered either immediately or 20 min. after access to saccharin. The results suggest that US preexposure has a more deleterious effect on conditioning when the illness-inducing US is delayed in conditioning than when it is immediate. These results are to be expected on the basis of classical conditioning principles and question theory and research that suggest more deleterious effects of US preexposure with shorter intervals between the preexposed and conditioning US.     

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.     

The effects of chronic water deprivation on metabolic rate and long-trace taste-aversion conditioning in rats

The effect of chronic water deprivation on metabolic rate and long-trace taste-aversion conditioning was examined in Wistar-derived rats. Subjects were either maintained on a water deprivation regimen or allowed free access to water for a seven-week period prior to conditioning. At conditioning, rats were presented a saccharin CS followed 0-, 45-, 90-, or 180-min later by an i.p. injection of LiCl. Additionally, pseudo-conditioned groups were presented the CS followed immediately by an injection of physiological saline. Heightened oxygen consumption in deprived subjects suggested that chronic water deprivation increased metabolic rate. While no differences in the amount of saccharin intake were observed at conditioning, percent preference for saccharin scores during a 24-h two-bottle water/saccharin test revealed that non-chronically deprived rats supported conditioning at longer CS-US intervals than did chronically water-deprived rats. Results are interpreted in terms of a time-contraction effect stemming from an alteration of an internal metabolic count-down timer.     

Taste-mediated potentiation of noningestional stimuli in rats

Holtzman rats drank saccharin in a distinctive environmental chamber prior to lithium-induced toxicosis. This treatment was administered four times. These animals subsequently drank less water or familiar saline in the chamber than animals which received water in the environment during conditioning. In addition, these environmental stimuli blocked the formation of a lithium-mediated coffee aversion more if they were conditioned in the presence of saccharin than if they were conditioned in the presence of water. Such differential blocking provided further evidence that the presence of a taste during conditioning facilitated aversion learning to the environmental chamber.     

Higher-order conditioning and sensory preconditioning of a taste aversion with an exteroceptive CS1

An exteroceptive stimulus compound (context) was employed as CS₁ in higherorder conditioning (H-OC, Experiments I and II), and sensory preconditioning (SPC, Experiment III) of a saccharin (CS₂) aversion in rats. The results indicated that aversions were established with the H-OC as well as with the SPC procedures. Stimulus generalization and first-order conditioning explanations were ruled out by appropriate controls. A CS₁-extinction period, performed prior to testing, did not affect the H-OC aversion, whereas it reduced the SPC aversion at least partially. These findings imply that interoceptive (taste, nausea) and exteroceptive stimuli (context) are readily associable in rats. Implications of the resemblance between the SPC procedure and long-delay taste-aversion learning are discussed.     

Latent inhibition disruption by MK-801 in a conditioned taste-aversion paradigm

N-Methyl-D-aspartate (NMDA) receptors appear to be involved in CS processing and memory consolidation. The present paper analyzed the effect of the non-competitive NMDA receptor antagonist Dizocilpine maleate (MK-801) on Latent Inhibition (LI)-retarded learning of a CS-US association after to-be-CS preexposures at time of testing, using Wistar rats as experimental subjects. If NMDA receptors are involved in CS processing, MK-801 administration should affect LI. In fact, previous experiments revealed that a 2.0mg/kg MK-801 dose, administered 20 h before preexposure and conditioning, abolished LI in a conditioned taste-aversion paradigm. In the present paper, MK-801 (0.2 mg/kg) was either injected after preexposure, after conditioning, or after both preexposure and conditioning stages. LI was abolished when MK-801 was injected after preexposure, but not when it was injected after conditioning. These results support the role of NMDA receptors in CS processing and memory consolidation.     

Odor as a conditioned inhibitor: Applicability of the Rescorla-Wagner model to feeding behavior

Hooded rats were given conditioned inhibition training in which the taste of saccharin alone was always followed by induced illness, but the taste of saccharin plus the odor of amyl acetate was not. In a series of three subsequent tests—summation, enhancement of conditioning, and retardation—it was demonstrated that the odor had acquired active inhibitory properties. The results paralleled those obtained with more traditionally studied stimuli and techniques and hence were found to be readily predictable from a recent model of conditioning set forth by Rescorla and Wagner (1972).     

Effects of Cerebroventricle Administration of Acidic Fibroblast Growth Factor on Conditioned Taste Aversion Learning in Rats

Wistar strain rats were given acidic fibroblast growth factor (aFGF) or denatured aFGF into their cerebroventricle before taste aversion conditioning for saccharin solution. Animals administrated with aFGF showed significantly lower aversion threshold for saccharin at the 1st day and preference ratios for saccharin vs distilled water at the 4th, 6th, and 7th day after the conditioning than those administered with denatured aFGF. These results suggests that aFGF in the cerebrospinal fluid facilitates acquisition of the conditioned taste aversion learning.     

Conditioned Inhibition Produced by Extinction of a Conditioned Stimulus

Four experiments used a conditioned taste aversion procedure to examine the potential for CS-alone extinction treatment to produce a conditioned stimulus that possesses inhibitory properties. In Experiment 1, saccharin was paired with LiCl, and then saccharin was presented alone for several trials to produce extensive behavioral extinction. Animals receiving this treatment were retarded in reacquiring conditioned responding to saccharin relative to control subjects receiving conditioning to the flavor for the first time. In Experiment 2, the extinguished saccharin stimulus was shown to decrease conditioned responding to a known excitor when the two stimuli were presented in compound as a summation test. Experiments 3A and 3B replicated the findings of Experiments 1 and 2 while providing evidence that the effects were not due to the differential effects of neophobia during testing. These three experiments revealed that an extinguished conditioned excitor passes retardation and summation tests for conditioned inhibition. Experiment 4 found that extinction of a known excitor was slowed when the excitor was extinguished in compound with a previously extinguished conditioned stimulus. That is, an extinguished CS provided protection from extinction to another CS, a finding also consistent with the view that extinction produces conditioned inhibition.     

Neuronal representation of conditioned taste in the basolateral amygdala of rats

Animals develop robust learning and long lasting taste aversion memory once they experience a new taste that is followed by visceral discomfort. A large body of literature has supported the hypothesis that basolateral amygdala (BLA) plays a critical role in the acquisition and extinction of such conditioned taste aversions (CTA). Despite the evidence that BLA is crucially engaged during CTA training, it is unclear how BLA neural activity represents the conditioned tastes. Here, we incorporated a modified behavioral paradigm suitable for single unit study, one which utilizes a sequence of pulsed saccharin and water infusion via intraoral cannulae. After conditioning, we investigated BLA unit activity while animals experience the conditioned taste (saccharin). Behavioral tests of taste reactivity confirmed that the utilized training procedure produced reliable acquisition and expression of the aversion throughout test sessions. When neural activity was compared between saccharin and water trials, half of the recorded BLA units (77/149) showed differential activity according to the types of solution. 76% of those cells (29/38) in the conditioned group showed suppressed activity, while only 44% of taste reactive cells (17/39) in controls showed suppressed activity during saccharin trials (relative to water trials). In addition, the overall excitability of BLA units was increased as shown by altered characteristics of burst activity after conditioning. The changes in BLA activity as a consequence of CTA were maintained throughout test sessions, consistent with the behavioral study. The current study suggests that the neuronal activity evoked by a sweet taste is altered as a consequence of CTA learning, and that the overall change might be related to the learning induced negative affect.     

The role of cue additivity in salience in taste aversion conditioning

In toxiphobia conditioning certain cues are more easily associated with malaise (i.e., more salient) than others. The present experiments show that cue additivity can account for previously found differences in salience of certain fluids. The relative salience of saccharin, saline and casein hydrolysate was tested in normal rats and in animals rendered anosmic by treatment of their nasal passages with zinc sulphate. The normally more salient casein was reduced in salience to the same level as saccharin and saline in the anosmic animals, while anosmia did not affect the relative salience of saccharin and saline. A second study tested the cue properties of vanilla, a fluid of very low salience. Anosmic animals could not show a conditioned aversion to vanilla. Normal animals readily learned the aversion but could not show it following a later anosmia treatment. Anosmia did not disrupt a saccharin aversion previously conditioned in normal animals. The data show that some of the “tastes” used in toxiphobia conditioning have olfactory components, and some are purely olfactory. Further, olfactory cues can summate with taste cues to form a more salient stimulus.     

Noradrenaline loss and the disruption of between-CS stimulus generalisation effects in aversion learning

In stimulus blocking the previous conditioning to one stimulus, A, followed by conditioning to the compound stimulus, AB, results in less conditioning to the other stimulus, B. Three experiments were performed to study the effect of the prior pairing of a stimulus A (noisy bottle) with lithium chloride followed by stimulus AB (saccharin plus noisy bottle) and lithium chloride pairings upon the strength of the aversion to the target stimuls B (saccharin) in NA-depleted and control rats. The results obtained were not in keeping with a "blocking" explanation since enhancement, rather than blocking, of the saccharin aversion was obtained, and confirm several recent demonstrations of the "anti-blocking" effect from investigations using the conditioned suppression procedure. DSP4 treatment attenuated the aversion-enhancement ("anti-blocking") effects in all three experiments. The possibility that DSP4-induced noradrenaline depletions may cause some disruption of between-CS associations leading to a lesser higher order conditioning or stimulus generalisation is discussed and the present findings do add further evidence to the conclusion that noradrenaline is intimately involved in attentional processes.     

Cortical spreading depression and conditioned taste aversion: an attempt to resolve a controversy

The claim (Winn, Todd, & Elias, Behav. Biol. 19, 55-63 (1977) ) that cortical spreading depression (CSD) can serve as US in the conditioned taste-aversion (CTA) paradigm was experimentally examined. Rats given 15-min access to novel 0.1% sodium saccharin (CS) followed by ip NaCl and bilateral or unilateral CSD (US) displayed similar saccharin preference (54%) as the sham-CSD-treated controls in a multiple-bottle retention test. Rats receiving ip LiCl (0.15 M, 2% body weight) and sham CSD as the US showed marked saccharin aversion. It is concluded that CSD does not elicit CTA and that some claims to the contrary can perhaps be ascribed to CSD-induced disruption of attenuation of neophobia.     

Exteroceptive context in tasteaversion conditioning and extinction: odour, cage, and bottle stimuli

Five experiments investigated the extent to which the exteroceptive context, present on a saccharin aversion conditioning trial with rats, controlled the resulting aversion on one-bottle extinction tests and subsequent preference tests. The presence or absence of the specific odour which had been present on the conditioning trial was found not to influence saccharin intake on extinction tests, whereas the presence of the particular compartment in which, and the bottle from which, the saccharin had been consumed greatly suppressed saccharin intake as compared to the absence of these elements. Preference tests, performed in the respective conditioning contexts, showed extinction to be specific to the compartment + bottle context: groups that had extinguished their saccharin aversion in a context different from the conditioning context, retained their aversion in the conditioning context. No such specificity was found for the odour context. However, in the absence of the taste stimulus during the extinction phase, the odour that had been present on the conditioning trial did control the amount of water consumed, whereas the compartment+bottle context did not. Moreover, on preference tests, groups that had consumed water during extinction in the presence of the odour context, evidenced a lesser saccharin aversion than groups not exposed to the odour. The results are interpreted as demonstrating that rats learn about taste, odour, cage and bottle stimuli on a taste-aversion conditioning trial, and that taste and bottle stimuli seem to be the most salient.     

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 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.     

Discriminative properties of morphine that modulate associations between tastes and lithium chloride

Wistar rats learned to withhold consumption of a target solution when morphine preceded presentation of the target solution and lithium chloride (LiCl) and to consume the same target solution when saline preceded the presentation of the solution. After this serial feature discrimination training, morphine did not block the formation of a Pavlovian association between saccharin and LiCl but did suppress consumption of familiar tap water. After Pavlovian conditioning, morphine blocked the formation of an association between saccharin and LiCl but did not suppress consumption of a familiar tap water solution. The roles of morphine and saline can be interchanged. It appears that the morphine discriminative stimulus is calling up a representation of neither the conditioned stimulus nor the unconditional stimulus alone, but rather a modified representation of some aspect of their association.