Lateralization of brain activation to imagination and smell of odors using functional magnetic resonance imaging (fMRI): left hemispheric localization of pleasant and right hemispheric localization of unpleasant odors.

PURPOSE: Our goal was to use functional MRI (fMRI) of brain to reveal activation in each cerebral hemisphere in response to imagination and smell of odors.

METHOD: FMRI brain scans were obtained in 24 normal subjects using multislice fast low angle shot (FLASH) MRI in response to imagination of banana and peppermint odors and in response to smell of corresponding odors of amyl acetate and menthone, respectively, and of pyridine. Three coronal sections selected from anterior to posterior brain regions were used. Similar studies were obtained in two patients with hyposmia using FLASH MRI and in one patient with hyposmia using echo planar imaging (EPI) both before and after theophylline treatment that returned smell function to or toward normal in each patient and in two patients with birhinal phantosmia (persistent foul odor) and global phantogeusia (persistent foul taste) with FLASH and EPI fMRI before and after treatment with neuroleptic drugs that inhibited their phantosmia and phantogeusia. Activation images were derived using correlation analysis. Ratios of hemispheric areas of brain activation to total hemispheric brain areas were calculated for FLASH fMRI, and numerical counts of pixel clusters in each hemisphere were made for EPI studies. Total pixel cluster counts in localized regions of each hemispheric section were also obtained.

RESULTS: In normal subjects, activation generally occurred in left (L) > right (R) brain hemisphere in response to banana and peppermint odor imagination and to smell of corresponding odors of amyl acetate and menthone. Whereas there were no overall hemispheric differences for pyridine odor, activation in men was R > L hemisphere. Although absolute activation in both L and R hemispheres in response to banana odor imagination and amyl acetate smell was men > women, the ratio of L to R activation was women > men. In hyposmic patients studied by FLASH fMRI, activation to banana odor imagination and amyl acetate smell was L > R hemisphere both before and after theophylline treatment. In the hyposmic patient studied with EPI before theophylline treatment, activation to banana and peppermint odor imagination and to amyl acetate, menthone, and pyridine smell was R > L hemisphere; after theophylline treatment restored normal smell function, activation shifted completely with banana and peppermint odor imagination and amyl acetate and menthone smell to L > R hemisphere, consistent with responses in normal subjects. However, this shift also occurred for pyridine smell, which is opposite to responses in normal control subjects. In patients with phantosmia and phantogeusia, activation to phantosmia and phantogeusia before treatment was R > L hemisphere; after treatment inhibited phantosmia and phantogeusia, activation shifted with a slight L > R hemispheric lateralization. Localization of all lateralized responses indicated that anterior frontal and temporal cortices were brain regions most involved with imagination and smell of odors and with phantosmia and phantogeusia presence.

CONCLUSION: Imagination and smell of odors perceived as pleasant generally activated the dominant or L > R brain hemisphere. Smell of odors perceived as unpleasant and unpleasant phantosmia and phantogeusia generally activated the contralateral or R > L brain hemisphere. With remission of phantosmia and phantogeusia, hemispheric activation was not only inhibited, but also there was a slight shift to L > R hemispheric predominance. Predominant L > R hemispheric differences in brain activation in normal subjects occurred in the order amyl acetate > menthone > pyridine, consistent with the hypothesis that pleasant odors are more appreciated in L hemisphere and unpleasant odors more in R hemisphere. Anterior frontal and temporal cortex regions previously found activated by imagination and smell of odors and phantosmia and phantogeusia perception accounted for most hemispheric differences.