In recent years, objective whole-brain techniques (voxel-based morphometry, VBM) have become available that allow segmentation of brain structures into grey matter, white matter and cerebrospinal fluid. In the present study we used them to investigate the correlation between individual grey matter thickness and olfactory function. Forty-six subjects (27 women, 19 men) underwent extensive olfactory testing including odor identification, detection thresholds, intensity discrimination and quality discrimination. The behavioral results and subjects’ anatomical MRI scans were analyzed using two MNI in-house programs (CIVET, Surfstat).
A global analysis demonstrated that subjects with better overall performance had a significantly thicker cortex in the right dorsal postcentral gyrus and the right insula. Moreover, a region of interest analysis revealed a correlation of olfactory function with grey matter thickness in the right medial orbitofrontal cortex.
Of the individual olfactory tasks, odor identification was correlated with gray matter thickness in the right anterior transverse temporal gyrus, adjacent to the right insula, where quality discrimination showed an association with cortical thickness. Quality discrimination was also associated with grey matter thickness in the right intraparietal sulcus and pre- and postcentral areas bilaterally, considered being responsible for voluntary respiration and sniffing. In addition, in the right entorhinal cortex, occipital cortex, and insula the correlation between olfactory function and cortical thickness was different for women and men, possibly showing a biological underpinning for sex differences in olfactory performance.
These results indicate that performance on individual olfactory tests is reflected in brain anatomy, in both classical chemosensory areas as well as in areas usually considered to be dedicated to other (sensory) tasks.