Odor memory induces brain activation as measured by functional MRI

Purpose: Our goal was to use functional MRI (fMRI) to measure brain activat ion in response to imagination of odors in humans. Method: fMR brain scans were obtained in 21 normal subjects (9 men, 12 wome n) using multislice FLASH MRI in response to imagination of odors of banana and peppermint and to the actual smells of the corresponding odors of amyl acetate and menthone, respectively, in three coronal sections selected fro m anterior to posterior temporal brain regions. Similar studies were obtain ed in two patients with hyposmia using FLASH MRI and in one patient with hy posmia using echo planar imaging, both before and after theophylline treatm ent, which returned smell function to or toward normal in each patient. Act ivation 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 normal subjects and in each patient after imagination of each vapor. In normal subjects, brain activation in response to imagination of odors was significantly less than that in response to the actual smell of these odors, and activation followi ng imagination of banana odor was significantly greater in men than in wome n, as was previously reported for the actual smell of the odor of amyl acet ate. However, in relative terms, albeit at an absolute lower brain activati on level, the ratio of brain activation by imagination of banana to activat ion by actual amyl acetate odor was about twice as high in women as in men. Before treatment, in patients with hyposmia, brain activation in response to odor imagination was greater than after presentation of the actual odor itself. After treatment, in patients with hyposmia in whom smell acuity ret urned to or reward normal, brain activation in response to odor imagination was not significantly different quantitatively from that before treatment, however, brain activation in response to the actual odor was significantly greater than that in response to imagination of the corresponding odor. Br ain regions activated by both odor imagination and actual corresponding odo r were similar and consistent with regions previously described as respondi ng to odors. Conclusion: These studies indicate that (a) odors can be imagined and simil ar brain regions are activated by both imagined and corresponding actual od ors; (b) imagination of odors elicits quantitatively less brain activation than do actual smells of corresponding odors in normal subjects; (c) absolu te brain activation in men by odor imagination is greater than in women for some odors, but on a relative basis, the ratio for odor imagination to act ual smell in women is twice that in men; (d) odor imagination, once the odo r has been experienced, is present. recallable, and capable of inducing a r elatively constant degree of brain activation even in the absence of the ab ility to recognize an actual corresponding odor.