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RESEARCH PRODUCT

Perception of a blending odor mixture: an fMRI study in humans

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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.