Dissociated representations of pleasant and unpleasant olfacto-trigeminal mixtures: an FMRI study

Abstract

How the pleasantness of chemosensory stimuli such as odorants or intranasal trigeminal compounds is processed in the human brain has been the focus of considerable recent interest. Yet, so far, only the unimodal form of this hedonic processing has been explored, and not its bimodal form during crossmodal integration of olfactory and trigeminal stimuli. The main purpose of the present study was to investigate this question. To this end, functional magnetic resonance imaging (fMRI) was used in an experiment comparing brain activation related to a pleasant and a relatively unpleasant olfacto-trigeminal mixture, and to their individual components (CO(2) alone, Orange alone, Rose alone). Results revealed first common neural activity patterns in response to both mixtures in a number of regions: notably the superior temporal gyrus and the caudate nucleus. Common activations were also observed in the insula, although the pleasant mixture activated the right insula whereas the unpleasant mixture activated the left insula. However, specific activations were observed in anterior cingulate gyrus and the ventral tegmental area only during the perception of the pleasant mixture. These findings emphasized for the firs time the involvement of the latter structures in processing of pleasantness during crossmodal integration of chemosensory stimuli.

Figure 1. Perceptual ratings and brain activations.

(a) Three-D view of fMRI activation maps showing activation to the pleasant mixture after subtraction of activation to their individual components: brain responses were seen in the superior temporal gyrus, insula and cingulate gyrus. (b) Three-D view of fMRI activation maps showing activation to the unpleasant mixture after subtraction of activation to their individual components: brain responses were seen in the superior temporal gyrus and insula, but not in cingulate gyrus. (c) Differential activation patterns, showing activation to ([pleasant mixture] vs. [unpleasant mixture]). Brain responses were seen in the cingulate gyrus. (d) Contrast estimates at voxel coordinates [−3, 15, 45] in cingulate cortex for each mixture condition. (e) Compound pleasantness ratings. (f) Compound intensity ratings. Bars represent SEM.