Dissociated neural representations induced by complex and simple odorant molecules

- Odorant molecules are characterized by a large range of structural complexity.
- How odorant complexity influences verbal processing of odors remains unclear.
- Structurally complex odorants evoked more verbal labels than simple odorants.
- Complex odorants induced also activation in the anterior cingulate cortex (ACC).
- ACC activity could stem from conflict between labels evoked by complex odorants.

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.