The insular taste cortex contributes to odor qualiting coding
Despite distinct peripheral and central pathways, stimulation of both the olfactory and the gustatory systems may give rise to the sensation of sweetness. Whether there is a common central mechanism producing sweet quality sensations or two discrete mechanisms associated independently with gustatory and olfactory stimuli is currently unknown. Here authors used fMRI to determine whether odor sweetness is represented in the piriform olfactory cortex, which is thought to code odor quality, or in the insular taste cortex, which is thought to code taste quality. Fifteen participants sampled two concentrations of a pure sweet taste (sucrose), two sweet food odors (chocolate and strawberry), and two sweet floral odors (lilac and rose). Replicating prior work we found that olfactory stimulation activated the piriform, orbitofrontal and insular cortices. Of these regions, only the insula also responded to sweet taste. More importantly, the magnitude of the response to the food odors, but not to the non-food odors, in this region of insula was positively correlated with odor sweetness rating. These findings demonstrate that insular taste cortex contributes to odor quality coding by representing the taste-like aspects of food odors. Since the effect was specific to the food odors, and only food odors are experienced with taste, authors suggest this common central mechanism develops as a function of experiencing flavors.
(A) Coronal and sagittal sections of the area of insula where neural response to the food odors shows a positive correlation with the sweetness ratings of those odors. The main images display the unmasked regression maps while the insets labeled 1 depict the masked regressions (i.e., using the “sweet odors and sweet taste overlap” inclusive mask). The insets labeled 2 depict the analysis with the pleasantness and familiarity ratings as covariates. (B) Shows neural response (in parameter estimate) in the insula (at the maximally responding voxel at −45 −3 9), plotted against sweetness ratings for the food (red squares) and floral odors (yellow diamonds). (C) Illustrates the magnitude of the correlation (averaged across all voxels ± standard deviation) in the insula for food odors (in red squares) and non-food odor (yellow diamonds) in comparison to the responses in other areas for food and non-food odors vs. odorless. Note that although there appears to be a significant difference between food and floral odors in the left OFC, this effect this effect does not survive authors' criterion for significance in SPM and is therefore not discussed.