Augmentation of taste conditioning by a preconditioned odor.

Five experiments explored facilitated taste-aversion conditioning (odor-mediated taste augmentation), using rats that experienced odor (A) and taste (X) in an A+/AX+ design. Augmentation occurred when the stimuli were presented simultaneously during AX+ conditioning, and significantly weaker conditioning occurred after a sequential presentation (Experiment 1). Experiments 2 and 3 demonstrated that augmented conditioning decreased if the odor aversion was reduced through preexposure or extinction following A+ conditioning. A second-order conditioning explanation was not supported by the results of Experiment 4. Experiment 5 showed that extinction of the odor aversion after AX+ conditioning did not alter the strength of the augmented taste aversion. Odor-mediated taste augmentation is similar to potentiation, in which odor and taste cues operate in a synergistic, not competitive, manner.     

Noradrenergic control of odor recognition in a nonassociative olfactory learning task in the mouse

The present study examined the influence of pharmacological modulations of the locus coeruleus noradrenergic system on odor recognition in the mouse. Mice exposed to a nonrewarded olfactory stimulation (training) were able to memorize this odor and to discriminate it from a new odor in a recall test performed 15 min later. At longer delays (30 or 60 min), the familiar odor was no longer retained, and both stimuli were perceived as new ones. Following a post-training injection of the alpha(2)-adrenoceptor antagonist dexefaroxan, the familiar odor was still remembered 30 min after training. In contrast, both the alpha(2)-adrenoceptor agonist UK 14304 and the noradrenergic neurotoxin DSP-4 prevented the recognition of the familiar odor 15 min after the first exposure. Noradrenaline release in the olfactory bulb, assessed by measurement of the extracellular noradrenaline metabolite normetanephrine, was increased by 62% following dexefaroxan injection, and was decreased by 38%-44% after treatment with UK 14304 and DSP-4. Performance of mice in the recall test was reduced by a post-training injection of the beta-adrenoceptor antagonist propranolol or the alpha(1)-antagonist prazosin, thus implicating a role for beta- and alpha(1)-adrenoceptors in the facilitating effects of noradrenaline on short-term olfactory recognition in this model.     

Flavor-illness aversions: potentiation of odor by taste is disrupted by application of novocaine into amygdala

Taste and odor have different properties in toxiphobic conditioning. When each is used alone, taste becomes aversive when followed by immediate or delayed poison, while odor becomes aversive only if followed by immediate poison. However, if odor and taste are presented as a compound and followed by delayed poison, then odor does become aversive when tested alone. It is as if taste has potentiated the odor signal. Several experiments assessed the role of the amygdala in this potentiation effect by anesthetizing the amygdala with 10% novocaine. Novocaine applied 30 min before presentation (Pre-CS) of an odor-taste compound disrupted the potentiated odor aversion but not the taste aversion. In contrast, novocaine applied 1 min after the compound odor-taste or 1 min prior to LiCl poison did not dissociate odor and taste aversions; both odor and taste aversions were facilitated. Novocaine applied 30 min before an odor alone also disrupted an odor aversion induced by immediate LiCl. But identical treatment did not disrupt odor avoidance conditioned by immediate foot-shock, suggesting that amygdala anesthesia does not simply produce anosmia. Pre-CS novocaine treatment also disrupted flavor neophobia prior to conditioning. The results suggest that novocaine applied to the amygdala disrupts the integration of odor with taste and illness during toxiphobic conditioning.     

Potentiation by a taste of toxicosis-based context conditioning: Effects of varying the test fluid

Rats that drank water in a distinctive environment and were then injected with lithium chloride (water-lithium condition) were compared with those given an added taste on those conditioning sessions (sucrose-lithium condition). In three experiments this taste potentiated a conditioned aversion to the context, as measured by suppression of intake of another solution: either a novel sour taste (Experiments 1 and 2) or a familiar saline solution (Experiment 3). In contrast, this potentiation effect was not detected when subjects were tested with water, whether a high or low dose of lithium was used (Experiment 2). Instead, in Experiments 1 and 2 water-lithium subjects drank less water than did the sucrose-lithium subjects on such tests i.e. an apparent overshadowing effect, which was the opposite outcome to that found previously using almost identical procedures. Intake on recovery sessions in another context suggested that, when water is used as the test fluid, potentiation can be masked by two factors: a context-dependent aversion to water in water-lithium subjects, and a conditioned inhibition effect of water in sucrose-lithium subjects. These may account for previous failures to detect potentiation of context conditioning.     

Conditioned taste aversions: Two-stimulus tests are more sensitive than one-stimulus tests

Weanling and mature rats were presented with saccharin or saline solutions for 1 h on alternate days. Following exposure to saccharin, rats were injected with 0, 21, or 37 mg/kg of cyclophosphamide. Injections had no significant effect on saccharin preference in one-stimulus tests, but had a highly significant effect in two-stimulus tests.     

Developmental flavor experience affects utilization of odor, not taste in toxiphobic conditioning

Feeding experiences were varied in developing rats and the effects upon flavor neophobia and lithium chloride-induced flavor aversions were observed. In Experiment 1, nursing experience of neonate rats was reduced by artificial feeding via intragastric cannula; the rats then were tested with apple juice paired with lithium chloride injection at weaning or maturity. Conditioned aversions were not affected, but neophobia to novel apple juice was attenuated in artificially-reared rats tested at maturity. In Experiment 2, rats received enriched feeding experience after weaning, which consisted of (a) obtaining many complex flavors, a few of which were paired with poisoning, effortlessly in the home cage, or (b) foraging for various foods on an elevated maze. No dramatic effects on neophobia or conditioned taste aversion for saccharin water were apparent. In Experiment 3, rats were given experience after weaning with vanilla-scented water either paired or unpaired with quinine water, and then tested with the odor of almond or that odor compounded with saccharin water for neophobia and lithium-induced aversions. Flavor-experienced rats exhibited more pronounced odor conditioning and more resistance to extinction of the odor aversion after both simple and compound conditioning. In contrast, saccharin taste aversions were relatively unchanged. Apparently, enriched feeding and drinking experience facilitates the utilization of odor more than taste cues.     

Effects of stress on nonassociative learning processes in male and female rats

In this study we assessed habituation and sensitization of the acoustic startle response (ASR) to discern whether intense, inescapable stress affects nonassociative learning differently in male and female rats. Rats were inescapably stressed 2 hours per day over 3 consecutive days. ASR magnitudes were measured at several times post-stress (1, 4, 8, and 15 days after cessation). Females generally showed greater ASR magnitudes (compared to males), but both sexes exhibited short and long-term habituation across the testing days. ASR magnitudes were only affected by stress in male subjects. The effect in males was an increase in short-term sensitization of the ASR on post-stress day-4. The results suggest that stressed males and females react differently to ASR testing, in that stress males appear to develop an exaggerated ASR response over repeated test sessions due to short-term sensitization. The source of the short-term sensitization is discussed with regards to possible stress-induced enhanced contextual learning during ASR testing on post-stress day-1.     

Defensive burying of aversive fluids in rats: The possible role of odor

Defensive burying of flavored fluids paired with lithium chloride injections was examined. Rats showed little inclination to bury conditioned saccharin or salt solutions (Experiments 1 and 2). However, they buried tabasco sauce or dilute milk solutions readily (Experiment 3). It was hypothesized that the amount of olfactory stimulation provided by a solution may determine whether a rat engages in burying. This hypothesis received support in a final experiment which demonstrated that rats readily buried an arbitrarily selected odorous solution which was previously paired with lithium chloride. Thus the present experiments begin the process of identifying the circumstances under which burying does and does not occur.     

Potentiation and overshadowing in preweanling and adult rats

Four experiments compared the aversion acquired by 18-day-old and 60-day-old rats to a flavor that was either tasted alone or in combination, simultaneously or successively, with another flavor when paired with illness. The purpose was to study temporal variables and theoretical issues pertinent to potentiation and overshadowing while investigating ontogenetic differences in these phenomena. When either the preweanlings or the adults were presented a simultaneous compound flavor (sucrose/coffee) followed by lithium chloride-induced illness, greater sucrose aversions were found than for animals conditioned on sucrose alone--that is, potentiation. Preweanlings demonstrated greater potentiation than did adults, whether the conditioned stimulus-unconditioned stimulus (CS-US) delay was 0 or 1 hr. This potentiation was eliminated by nonreinforced presentations of the alternative CS element. Potentiation was not seen when the two flavors were presented successively as the CS; instead, overshadowing occurred. Tests of configuring by extinction procedures indicated a tendency for these animals to form a configured representation of the simultaneous compound solution. This disposition for configuring tended to be more pronounced for preweanlings than for adults. Ontogenetic differences in response to configuration, CS saliency, and generalization decrement seem consistent with at least one model of potentiation and the ontogenetic differences in potentiation seen in the present experiments.     

Stimulus generalization of conditioned taste aversion in rats

Relatively little information is available regarding the intradimensional stimulus generalization of conditioned taste aversion (CTA). Experiment 1 employed a between-groups generalization test to examine the extent to which conditioned flavor aversion to one sucrose solution generalized to other concentrations of sucrose in adult rats. Evidence of a gradient of aversion was obtained. Because generalization gradients in other tasks have been found to flatten over a retention interval, Experiment 2 investigated the effects of delayed testing (2, 7, or 21 days) upon the slope of the generalization gradient. The generalization gradient flattened at the intervals, suggesting that subjects forgot the specific attributes of the conditioning concentration and avoided generalized stimuli as if they were the original CS. Experiment 3 used a long delay between taste and toxicosis to degrade the associative contingency and found no evidence that the generalization gradients found in the first two experiments could be explained in terms of enhanced neophobia due to poisoning. These findings provide further evidence (cf. A. W. Logue, 1979, Psychological Bulletin, 86, 276-296; M. Domjan, 1980, in J. S. Rosenblatt, R. A. Hinde, C. Beer, & M. Busnel (Eds.), Advances in the study of behavior, Vol. 11, New York: Academic Press) that CTA shares a number of similarities with other learning processes. Further, they illustrate that stimulus forgetting can be detected in a paradigm considered relatively immune to retention loss.     

The addition of saccharin to taste cues affects taste preference conditioning in thirsty rats

Previous failures to condition preferences for the unacceptable taste cues sucrose octaacetate (SOA) and citric acid (CA) using a reverse-order, differential conditioning procedure (Forestell & LoLordo, 2000) may have been the result of low consumption of the taste cues in training or of their relatively low acceptability to rats that are thirsty and hungry. In the present study, rats that were thirsty but not hungry readily consumed the SOA and CA conditioned stimulus solutions in training. In test, stronger preferences were displayed for CS+ if the taste cues had been mixed in water (i.e., for the previously unacceptable taste cues) than if they had been made more acceptable by the addition of saccharin in training and test. In Experiment 2, again with rats that were thirsty but not hungry, stronger preferences for CS+ were observed when taste cues were presented in water in the test than when they were presented in saccharin. Addition of saccharin to the taste cues in test eliminated the preference for CS+.     

Difficulty in evoking odor images: The role of odor naming

Many studies report that people have difficulty in evoking odor images. In this article, we explore whether this results from another commonly observed phenomenon, difficulty in naming odors. In Experiment 1, participants both named and attempted to imagine either odors or their common visual referents. More-difficult-to-name odors were reported as being more difficult to evoke as olfactory images, in comparison with the visual condition. In Experiment 2, participants received training prior to forming odor images and naming the same set of odors. As in Experiment 1, more-difficult-to-name odors were harder to imagine, but participants who had learned the odor names during training were significantly better, by their own report, at imagining many of these stimuli, relative to participants who were either exposed to the odors, exposed to their names, or who received no pretraining. In sum, these experiments suggest that odor naming may account for some of the difficulty reported by participants when attempting to evoke odor images; we discuss an associative basis for this effect.     

Reversal of long-delay conditioned taste aversion learning in rats by sex hormone manipulation

Conditioned taste aversion (CTA) learning is an adaptive, robust, well-established learning and memory paradigm. Strong taste, aversions develop to the conditioned stimulus (CS=saccharin) despite long delays between exposure to the CS and unconditioned stimulus (US=LiCl). Rats display a sexually dimorphic pattern of long-delay CTA learning (Foy et al., 1996). The present study examines whether this sex difference is a result of activational or organizational hormone action, because here we implanted gonadectomized rats with their normal hormone replacements, or with opposing hormones prior to testing in a 4-hr delayed CTA learning task. We found that gonadally intact male rats displayed a more robust CTA response than intact female rats. Gonadectomy essentially eliminated this sex difference; gonadectomized males and gonadectomized females displayed similar CTA responses. In gonadectomized rats, when their normal sex hormones were replaced with implanted hormone pellets, the sex difference in CTA learning was reinstated. In contrast, when gonadectomized rats were implanted with opposing hormones, the sex difference was reversed. Gonadectomized female rats implanted with 5α-DHT pellets (metabolite of testosterone) displayed a stronger CTA response compared to gonadectomized males implanted with 17β-estradiol pellets. Regardless of the original developmental hormonal environment, our study suggests that an activational manipulation of circulating hormones serves to significantly influence long-delay CTA learning in rats.     

Paradoxical olfactory function in combat Veterans: The role of PTSD and odor factors

Stress- and trauma-related disorders, including posttraumatic stress disorder (PTSD), are characterized by an increased sensitivity to threat cues. Given that threat detection is a critical function of olfaction and that combat trauma is commonly associated with burning odors, we sought a better understanding of general olfactory function as well as response to specific trauma-related (i.e. burning) odors in combat-related PTSD. Trauma-exposed combat Veterans with (n = 22) and without (n = 25) PTSD were assessed for general and specific odor sensitivities using a variety of tools. Both groups had similar general odor detection thresholds. However, the combat Veterans with PTSD, compared to combat Veterans with comparable trauma exposure but without PTSD, had increased ratings of odor intensity, negative valence, and odor-triggered PTSD symptoms, along with a blunted heart rate in response to burning rubber odor. These findings are discussed within the context of healthy versus pathological changes in olfactory processing that occur over time after psychological trauma.     

Conditioned nausea in rats: Assessment by conditioned disgust reactions, rather than conditioned taste avoidance

The terms conditioned taste avoidance and conditioned taste aversion are often used interchangeably in the literature; however, considerable evidence indicates that they may represent different processes. Conditioned taste avoidance is measured by the amount that a rat drinks in a consumption test that includes both appetitive phases and consummatory phases of responding. However, conditioned taste aversion is more directly assessed using the taste reactivity (TR) test that includes only the consummatory phase of responding. Rats display a conditioned taste aversion as conditioned disgust reactions (gapes, chin rubs, and paw treads) during an intraoral infusion of a nausea-paired flavored solution. Only treatments that produce nausea produce conditioned disgust reactions, but even rewarding drugs produce conditioned taste avoidance. Furthermore, treatments that alleviate nausea prevent the establishment and the expression of conditioned disgust reactions, but they do not necessarily modify conditioned taste avoidance. Considerable evidence exists indicating that these two measures can be independent of one another. The potential of a compound to produce conditioned disgust reactions is a reflection of its nausea-inducing properties. Taste avoidance may be motivated by conditioned fear rather than conditioned nausea, but conditioned disgust is motivated by conditioned nausea. (PsycINFO Database Record (c) 2008 APA, all rights reserved).