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Is the superior temporal sulcus involved in the reinforced configural processing of a binary odor mixture?. 35. annual meeting - association for chemoreception sciences (AChemS)

Supported by grants from the Burgundy Regional council and EU-ERDF, European Dijon-Dresden Laboratory (LEA 549); Odors we perceive from our environment arise from the processing of mixtures of odorants. Some mixtures can lead to configural or elemental perception depending, in part, on experience. However, the neural bases of such influences are still unknown. In the present study, we examined the neurophysiological correlates of the configural and elemental processing of a binary odor mixture (AB). This AB mixture has previously been shown to blend into one percept, but also to produce a more or less configural perception depending on pre-exposure to either the mixture itself or to the single components of the mixture. Therefore, subjects were pre-exposed twice either to AB (group Gmix, n=12) or to its single components (A & B; Gcomp, n=14) in order to favor configural processing in the Gmix group or analytical processing in the Gcomp group. During fMRI, subjects were stimulated with AB, A and B, following a block design, using a computer-controlled olfactometer. At the end of the session, subjects rated intensity, pleasantness, and complexity for each stimulus. Psychophysical data revealed no significant difference in pleasantness but a small difference in intensity (B slightly less intense). Concerning complexity, Gmix subjects perceived the mixture as simple as the components whereas Gcomp subjects perceived the mixture as more complex, revealing experience-induced analytical processing of the mixture for this group. For fMRI data recorded when AB was the stimulus, the contrast Gcomp minus Gmix did not yield any significant differences in activation. However, the contrast Gmix minus Gcomp indicated an increased activation in five brain regions, among which the right superior frontal gyrus, already found to be specifically activated in a PET study on mixtures, and the hippocampus known to be involved in holistic processing of complex visual scenes. These preliminary results shed some light on cerebral mechanisms implied in the processing of complex odors.

International audience

International audience; Perfumers and flavorists are familiar with odor blending phenomenon and often report that a minimum number of odorants has to be mixed for a good odor blend to arise. Previous studies in humans and animals suggested that a configural process could be involved in the perceptual analysis of mixtures of odorants. It has been proposed that the perceptual blending phenomenon corresponds to a configural perception of odorant mixtures. In the present study, we investigated the influence of chemical complexity on the configural perception of odorant mixtures. Six mixtures including 2 to 6 odorants as well as each unmixed odorant were assessed for their odor quality by a panel of 55 healthy untrained subjects (Ss). Two verbal description tasks were performed: Free Description and Choice between Attributes. For each mixture, a target odor name was expected; i.e. a name for odor quality emerging from each mixture through the perceptual blending phenomenon. A typicality rating task was also performed. In this task, Ss had to rate, on a linear scale, how typical was the stimulus in relation to the target odor name. Analysis of typicality ratings as well as comparison of the frequency of quotation of the target names, in both verbal tasks, between the mixtures and their components, revealed that perceptual blending did occur in 2 of the tested mixtures. Indeed the typicality was found to be significantly higher for mixtures as compared to their components and mixtures were more often described with the expected target name as compared to their components. Moreover it appeared that perceptual blending was more pronounced in the mixtures containing the higher number of odorants. Taken together, these findings confirm that mixtures of odorants can be perceived in a configural way by humans and that such a configural process may explain why odorant mixtures can elicit novel odor percepts. Additionally, we propose that a configural process would be more likely engaged when more than 4 odorants are mixed which is in line with the limit in the capacity of humans to identify odorants in a mixture (Laing & Francis, 1989).

Diplôme : Dr. d'Université; Dans le monde de la parfumerie et de l'aromatique, il existe un terme particulier définissant la formation d'une nouvelle qualité odorante à travers le mélange et la fusion de plusieurs odeurs : l'accord. Ce terme, largement employée parmi ces experts, n'avait jamais donné lieu à des études scientifiques approfondies.L' objectif de cette thèse a été de démontrer la réalité de l'accord en olfaction et de montrer qu'il repose sur des interactions perceptives influençant la qualité de l'odeur. Nous avons en particulier étudié les déterminants physico-chimiques, perceptifs et cognitifs des accords aromatiques. Dans un premier temps, nous avons démontré que la perception de ces accords ne provient pas d'interactions physico-chimiques au sein de la phase des mélanges odorants. Dans un second temps, nous avons montré que la perception de ces accords n'est pas "réservée" aux experts mais que des néophytes sans expériences particulières des odeurs perçoivent aussi la qualité émergente du mélange. Par ailleurs, la complexité chimique de ses mélanges contribue à la formation et à l'affinage de ces accords. Dans un troisième temps, nous avons montré que la perception de ces accords est tributaire d'un fragile équilibre entre les proportions de chacun des composés du mélange. Enfin, des aspects cognitifs tels que les apprentissages olfactifs perceptifs, le type d'attention porté (processus synthétique versus analytique) et l'expertise peuvent moduler fortement la perception de ces accords aromatiques. Au final l'ensemble des résultats de cette thèse démontre, sur le plan perceptif chez l'homme, qu'une nouvelle note odorante spécifique peut apparaitre dans un mélange de composés odorant comme cela a pu être suggéré sur la base de données neurophysiologiques récentes dans le domaine de l'olfaction.