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Odorant‐induced brain activation as a function of normal aging and Alzheimer’s disease
Author(s) -
Motter Jeffrey N.,
Steffener Jason,
Tabert Matthias H.,
Devanand Davangere P.
Publication year - 2020
Publication title -
alzheimer's and dementia
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 6.713
H-Index - 118
eISSN - 1552-5279
pISSN - 1552-5260
DOI - 10.1002/alz.039116
Subject(s) - orbitofrontal cortex , olfactory system , entorhinal cortex , neuroscience , olfaction , psychology , odor , disease , olfactory memory , dementia , alzheimer's disease , sensory system , medicine , audiology , olfactory bulb , central nervous system , hippocampus , cognition , prefrontal cortex
Background Olfactory dysfunction consistently occurs in patients with Alzheimer’s disease (AD), beyond the mild and gradual decline in olfactory ability found in normal aging. This dysfunction begins early in the disease course, typically before clinical diagnosis, and progresses with disease severity. While odor identification and detection deficits clearly differentiate AD from controls, there remains uncertainty as to whether these are determined by olfactory threshold. The primary purpose of the current fMRI study was to examine the neural correlates of olfactory processing in healthy young and old adults and compare them with AD patients. Method Health young adults (n=10), healthy old adults (n=10), and AD patients (n=12) participated in psychophysical olfactory testing and an fMRI odorant detection paradigm. We analyzed the interplay between age and disease‐related psychophysical olfactory declines and odorant‐induced brain activation. This tested whether odor threshold deficits reflect disease progression and explain group differences in detection, identification and brain activity. Result AD patients had decreased odor detection task‐related signal in all regions of the primary olfactory cortex, with activity in the entorhinal cortex best differentiating the groups. Within the secondary olfactory regions, AD patients showed decreased activity in orbitofrontal cortex compared to healthy age matched individuals. Moderated‐mediation analyses on neuro‐psychophysical relationships found that increased brain activation in the entorhinal cortex moderated the negative effect of disease‐related threshold decline on olfactory detection. Conclusion Even in the face of declining olfactory thresholds, older adults compensated for this effect with increased brain activation in a primary olfactory brain region. This was the case for odor detection but not odor identification. fMRI activation induced by an olfactory identification task may eventually be useful in improving early detection of AD and may, eventually, facilitate early treatment interventions in subjects at risk for AD.