Olfactory preference conditioning changes the reward value of reinforced and non-reinforced odors

Effects of food deprivation on conditioned orthonasal olfactory preferences with caloric and non-caloric reinforcers

Three experiments were conducted to investigate Conditioned Olfactory Preferences using orthonasal inhalation, which is a less explored perceptual pathway compared to retronasal inhalation. In these experiments, odors were impregnated onto plastic disks to prevent the subjects from consuming or tasting them. The reinforcers used were a sucrose solution (Caloric groups) and a saccharin solution (Non-Caloric groups). The influence of nutritional deprivation was analyzed, with unrestricted access to food throughout the procedure in Experiment 1, food restriction during the conditioning phase in Experiment 2, and limited access to food during the test phase in Experiment 3. The results revealed conditioned preferences using both sucrose and saccharin as reinforcers. Furthermore, dietary restriction reduced the conditioned preference induced by saccharin, but not the preference induced by sucrose. These findings are discussed in light of the potential differences between orthonasal and retronasal presentation of odors during conditioning.

Odor-taste pairings lead to the acquisition of negative hedonic qualities by the odor in aversion learning

Three experiments examined the affective responses conditioned to an odorous stimulus in the taste-mediated odor aversion learning paradigm. Experiment 1 analyzed the microstructure of licking behavior during voluntary consumption. Before conditioning, water-deprived rats had access to a bottle containing either a tasteless odor (0.01% amyl acetate) diluted in water or mixed with 0.05% saccharin. Next, the rats were injected with either LiCl or saline immediately after drinking saccharin. At test, they received the odor and taste solutions on separate days. Lick cluster size was used as a direct measure of the hedonic response to the odor cue. Rats receiving odor-taste pairings prior to the saccharin devaluation showed both lower consumption and lick cluster size, reflecting a reduced hedonic evaluation of the odor. Experiments 2a and 2b used the orofacial reactivity method. After pretraining in the drinking boxes with the odor alone or mixed with saccharin, the rats were intraorally infused with saccharin before injection with LiCl or saline. At test, they were infused in separate sessions with the odor and taste and their orofacial reactions video recorded. There were increased aversive orofacial responses to the odor in rats that had prior odor-taste experience, a result indicating a negative hedonic evaluation of the odor. These results provide evidence of conditioned changes in affective value of odor cues through taste-mediated learning and are consistent with the idea that odor-taste pairings lead to the acquisition of taste qualities by the odor.

Experience Informs Consummatory Choices for Congruent and Incongruent Odor-Taste Mixtures in Rats

Experience is an essential factor informing food choice. Eating food generates enduring odor-taste associations that link an odor with a taste's quality and hedonic value (pleasantness/unpleasantness) and creates the perception of a congruent odor-taste combination. Previous human psychophysical experiments demonstrate that experience with odor-taste mixtures shapes perceptual judgments related to the intensity, familiarity, and pleasantness of chemosensory stimuli. However, how these perceptual judgments inform consummatory choice is less clear. Using rats as a model system and a 2-bottle brief-access task, we investigated how experience with palatable and unpalatable odor-taste mixtures influences consummatory choice related to odor-taste congruence and stimulus familiarity. We found that the association between an odor and a taste, not the odor's identity or its congruence with a taste, informs consummatory choice for odor-taste mixtures. Furthermore, we showed that the association between an odor and a taste, not odor neophobia, informs consummatory choice for odors dissolved in water. Our results provide further evidence that the association between an odor and a taste, after odor-taste mixture experience, is a fundamental feature guiding consummatory choice.

Odour conditioning of positive affective states: Rats can learn to associate an odour with being tickled

Most associative learning tests in rodents use negative stimuli, such as electric shocks. We investigated if young rats can learn to associate the presence of an odour with the experience of being tickled (i.e. using an experimenter’s hand to mimic rough-and-tumble play), shown to elicit 50 kHz ultrasonic vocalisations (USVs), which are indicative of positive affect. Male, pair-housed Wistar rats (N = 24) were all exposed to two neutral odours (A and B) presented in a perforated container on alternate days in a test arena. Following 60s of exposure, the rats were either tickled on days when odour A (n = 8) or odour B (n = 8) was present, or never tickled (n = 8). When tickled, rats produced significantly more 50 kHz USVs compared to the days when not being tickled, and compared to control rats. The level of anticipatory 50 kHz USVs in the 60s prior to tickling did not differ significantly between the tickled and control rats. As a retrieval test following the odour conditioning, rats were exposed successively in the same arena to three odours: an unknown neutral odour, extract of fox faeces, and either odours A or B. Compared to controls, 50 kHz USVs of tickled rats increased when exposed to the odour they had previously experienced when tickled, indicating that these rats had learned to associate the odour with the positive experience of being tickled. In a test with free access for 5 min to both arms of a T-maze, each containing one of the odours, rats tickled with odour A spent more time in the arm with this odour. This work is the first to test in a fully balanced design whether rats can learn to associate an odour with tickling, and indicates that positive odour conditioning has potential to be used as an alternative to negative conditioning tests.

Retronasal Odor Perception Requires Taste Cortex, but Orthonasal Does Not

Smells can arise from a source external to the body and stimulate the olfactory epithelium upon inhalation through the nares (orthonasal olfaction). Alternatively, smells may arise from inside the mouth during consumption, stimulating the epithelium upon exhalation (retronasal olfaction). Both ortho- and retronasal olfaction produce highly salient percepts, but the two percepts have very different behavioral implications. Here, we use optogenetic manipulation in the context of a flavor preference learning paradigm to investigate differences in the neural circuits that process information in these two submodalities of olfaction. Our findings support a view in which retronasal, but not orthonasal, odors share processing circuitry commonly associated with taste. First, our behavioral results reveal that retronasal odors induce rapid preference learning and have a potentiating effect on orthonasal preference learning. Second, we demonstrate that inactivation of the insular gustatory cortex selectively impairs expression of retronasal preferences. Thus, orally sourced (retronasal) olfactory input is processed by a brain region responsible for taste processing, whereas externally sourced (orthonasal) olfactory input is not.

Activity in the rat olfactory cortex is correlated with behavioral response to odor: a microPET study

How olfactory cortical areas interpret odor maps evoked in the olfactory bulb and translate odor information into behavioral responses is still largely unknown. Indeed, rat olfactory cortices encompass an extensive network located in the ventral part of the brain, thus complicating the use of invasive functional methods. In vivo imaging techniques that were previously developed for brain activation studies in humans have been adapted for use in rodents and facilitate the non-invasive mapping of the whole brain. In this study, we report an initial series of experiments designed to demonstrate that microPET is a powerful tool to investigate the neural processes underlying odor-induced behavioral response in a large-scale olfactory neuronal network. After the intravenous injection of [¹⁸F]Fluorodeoxyglucose ([¹⁸F]FDG), awake rats were placed in a ventilated Plexiglas cage for 50 min, where odorants were delivered every 3 min for a 10-s duration in a random order. Individual behavioral responses to odor were classified into categories ranging from 1 (head movements associated with a short sniffing period in response to a few stimulations) to 4 (a strong reaction, including rearing, exploring and sustained sniffing activity, to several stimulations). After [¹⁸F]FDG uptake, rats were anesthetized to perform a PET scan. This experimental session was repeated 2 weeks later using the same animals without odor stimulation to assess the baseline level of activation in each individual. Two voxel-based statistical analyses (SPM 8) were performed: (1) a two-sample paired t test analysis contrasting baseline versus odor scan and (2) a correlation analysis between voxel FDG activity and behavioral score. As expected, the contrast analysis between baseline and odor session revealed activations in various olfactory cortical areas. Significant increases in glucose metabolism were also observed in other sensory cortical areas involved in whisker movement and in several modules of the cerebellum involved in motor and sensory function. Correlation analysis provided new insight into these results. [¹⁸F]FDG uptake was correlated with behavioral response in a large part of the anterior piriform cortex and in some lobules of the cerebellum, in agreement with the previous data showing that both piriform cortex and cerebellar activity in humans can be driven by sniffing activity, which was closely related to the high behavioral scores observed in our experiment. The present data demonstrate that microPET imaging offers an original perspective for rat behavioral neuroimaging.