FUNCTIONAL MRI OF HUMAN OLFACTION

Purpose: Our goal was to use functional MRI (fMRI) to measure brain ac tivation in response to olfactory stimuli. Method: fMRI brain scans we re obtained in 17 normal subjects (9 men, 8 women) using multislice FL ASH MRI in response to three olfactory stimuli (pyridine, menthone, am yl acetate) in three coronal brain sections selected from anterior to posterior temporal brain regions. Activation images were derived using correlation analysis, and ratios of areas of brain activated to total brain areas were calculated. Results: Activation was present in each section in all subjects. Subjective estimation of vapor intensity foll owed relative vapor pressure of stimuli presented (pyridine > amyl ace tate > menthone) and were similar for both men and women. However, bra in activation did not follow subjective responsiveness order but rathe r pyridine > menthone > amyl acetate, a pattern demonstrated by both m en and women. Brain activation in women was consistently lower than in men for all vapors in all brain sections, although regions of activat ion did not differ. Activation occurred in regions previously recogniz ed as associated with olfactory stimulation, including orbitofrontal a nd entorhinal cortex; however, extensive regions within frontal cortex including cingulate gyrus were also activated. Brain regions activate d to odors considered pleasant or unpleasant did not differ. Conclusio n: The techniques used in this study demonstrated that brain activatio n to olfactory stimuli could be measured quantitatively such that diff erences between groups of subjects (in this case men and women) could be compared. Although localization of brain activation was not the maj or thrust of this study, activation to olfactory stimuli was found not only in brain regions previously associated with processing of olfact ory information but also in several other areas of frontal cortex, in cingulate gyrus, and in several components of the limbic system. This is the first study in which activation in human brain parenchyma of no rmal humans to olfactory stimuli has been quantitated by fMRI.