title: The human brain representation of odor identification.
abstract: Odor identification (OI) tests are increasingly used clinically as biomarkers for Alzheimer's disease and schizophrenia. The aim of this study was to directly compare the neuronal correlates to identified odors vs. nonidentified odors. Seventeen females with normal olfactory function underwent a functional magnetic resonance imaging (fMRI) experiment with postscanning assessment of spontaneous uncued OI. An event-related analysis was performed to compare within-subject activity to spontaneously identified vs. nonidentified odors at the whole brain level, and in anatomic and functional regions of interest (ROIs) in the medial temporal lobe (MTL). Parameter estimate values and blood oxygenated level-dependent (BOLD) signal curves for correctly identified and nonidentified odors were derived from functional ROIs in hippocampus, entorhinal, piriform, and orbitofrontal cortices. Number of activated voxels and max parameter estimate values were obtained from anatomic ROIs in the hippocampus and the entorhinal cortex. At the whole brain level the correct OI gave rise to increased activity in the left entorhinal cortex and secondary olfactory structures, including the orbitofrontal cortex. Increased activation was also observed in fusiform, primary visual, and auditory cortices, inferior frontal plus inferior temporal gyri. The anatomic MTL ROI analysis showed increased activation in the left entorhinal cortex, right hippocampus, and posterior parahippocampal gyri in correct OI. In the entorhinal cortex and hippocampus the BOLD signal increased specifically in response to identified odors and decreased for nonidentified odors. In orbitofrontal and piriform cortices both identified and nonidentified odors gave rise to an increased BOLD signal, but the response to identified odors was significantly greater than that for nonidentified odors. These results support a specific role for entorhinal cortex and hippocampus in OI, whereas piriform and orbitofrontal cortices are active in both smelling and OI. Moreover, episodic as well as semantic memory systems appeared to support OI.
authors.affiliation: Department of Circulation and Medical Imaging, Norwegian University of Science and Technology, Trondheim, Norway.
authors.forename: Grete
authors.initials: G
authors.lastname: Kjelvik
authors.forename: Hallvard R
authors.initials: HR
authors.lastname: Evensmoen
authors.forename: Veronika
authors.initials: V
authors.lastname: Brezova
authors.forename: Asta K
authors.initials: AK
authors.lastname: Håberg
mesh_terms: D000328:Adult; D002540:Cerebral Cortex / Q000502:physiology*; D005260:Female; D006801:Humans; D009415:Nerve Net / Q000502:physiology*; D009812:Odorants / Q000032:analysis*; D021641:Recognition, Psychology / Q000502:physiology*; D012903:Smell / Q000502:physiology*; D013647:Task Performance and Analysis; D055815:Young Adult