Multidimensional representation of odors in the human olfactory cortex.

What is known as an odor object is an integrated representation constructed from physical features, and perceptual attributes mainly mediated by the olfactory and trigeminal systems. The aim of the present study was to comprehend how this multidimensional representation is organized, by deciphering how similarities in the physical, olfactory and trigeminal perceptual spaces of odors are represented in the human brain. To achieve this aim, we combined psychophysics, functional MRI and multivariate representational similarity analysis. Participants were asked to smell odors diffused by an fMRI-compatible olfactometer and to rate each smell along olfactory dimensions (pleasantness, intensity, familiarity and edibility) and trigeminal dimensions (irritation, coolness, warmth and pain). An event-related design was implemented, presenting different odorants. Results revealed that (i) pairwise odorant similarities in anterior piriform cortex (PC) activity correlated with pairwise odorant similarities in chemical properties (P < 0.005), (ii) similarities in posterior PC activity correlated with similarities in olfactory perceptual properties (P <0.01), and (iii) similarities in amygdala activity correlated with similarities in trigeminal perceptual properties (P < 0.01). These findings provide new evidence that extraction of physical, olfactory and trigeminal features is based on specific fine processing of similarities between odorous stimuli in a distributed manner in the olfactory system. Hum Brain Mapp 37:2161-2172, 2016. © 2016 Wiley Periodicals, Inc.

Dissociated neural representations induced by complex and simple odorant molecules.

An important challenge in olfaction research is to understand how percepts relate to the molecular structure of stimuli. Previous psychophysical studies showed that, whereas structurally simple odorant molecules evoked a more uniform qualitative perception as revealed by the use of a small number of labels to describe their olfactory quality, more complex odorants evoked a larger variety of olfactory qualities, reflecting a more heterogeneous qualitative perception. The present study examined how this influence of odorant molecular complexity on perception is reflected in the human brain. To this end, participants were stimulated with structurally simple and complex odorant molecules and their brain responses were assessed by functional magnetic resonance imaging (fMRI). Low- and high-complexity odorants were judged to have the same intensity, pleasantness and familiarity (p>0.05 in all cases), whereas complex odorants induced more quality labels than simple odorants (p<0.02) as expected. Imaging analysis of complex vs. simple odorants revealed significant activation in dorsal anterior cingulate gyrus, but not in primary olfactory areas. Taken together, these findings suggest dissociated neural representations of uniform and heterogeneous olfactory perception, highlighting for the first time the impact of odorant complexity on activity of the cingulate gyrus.

A portable experimental apparatus for human olfactory fMRI experiments.

Human olfactory perception can be measured using psychophysical tools or more complex odor generating devices systems, namely olfactometers. The present paper is aimed at presenting a new inexpensive, non-voluminous portable olfactometer adapted for human fMRI experiments. The system adjusts odorant stimulus presentation to human nasal respiration and records behavioral responses in the same experimental device. Validation by psychophysical measures and photo-ionization detection showed a linear increase in both odor intensity perception and vapor concentration as a function of odorant concentration. Further validation by brain imaging revealed neural activation in typical olfactory areas. In summary, the system represents a new low-cost, easy-use, easy-maintenance portable olfactometry tool for brain imaging, opening up new possibilities for investigating neural response to odors using event-related fMRI designs.

Molecular complexity determines the number of olfactory notes and the pleasantness of smells.

One major unresolved problem in olfaction research is to relate the percept to the molecular structure of stimuli. The present study examined this issue and showed for the first time a quantitative structure-odor relationship in which the more structurally complex a monomolecular odorant, the more numerous the olfactory notes it evokes. Low-complexity odorants were also rated as more aversive, reflecting the fact that low molecular complexity may serve as a warning cue for the olfactory system. Taken together, these findings suggest that molecular complexity provides a framework to explain the subjective experience of smells.