The relationship between olfaction and cognitive function in the elderly
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The Journal of Physiological Sciences Open Access
SHORT COMMUNICATION
The relationship between olfaction and cognitive function in the elderly Sae Uchida1* , Chiho Shimada2, Naoko Sakuma3, Fusako Kagitani1, Akiko Kan4 and Shuichi Awata3
Abstract This study investigated the relationship between olfaction and cognitive function in 12 elderly people (age: 80.9 ± 1.6) living in the community. Olfactory function was assessed by the identification threshold for rose odor. Four cognitive measures consisting general cognitive ability assessed by Mini-Mental State Examination (MMSE), its sub-domains, and attentional ability assessed by drawing a line to connect the numbers consecutively (trail-making test part A; TMT-A), were assessed. Subjects with a higher olfactory threshold (≥ 5) declined more in the performance speed of TMT-A (73% ± 7%, p = 0.05) compared with those subjects with a lower threshold (≤ 4) (averaged value was set at 100%). Other cognitive statuses assessed by MMSE tended to decline in subjects with higher thresholds. Because attentional function relates to the basal forebrain cholinergic system, our results suggest that olfactory impairment links to the decline in cognitive function, particularly of attention-relating cholinergic function. Keywords: Odor identification threshold, Olfactory function, Cognitive function, Attention, Elderly people Introduction The decline in olfactory function is one of the earliest symptoms of Alzheimer’s disease [1]. Difficulty in identifying odors predicts the subsequent transition from mild cognitive impairment to Alzheimer’s disease, and also from normal cognition to mild cognitive impairment [2, 3]. However, in these studies, they did not evaluate cognitive dysfunction, based on the levels of olfactory impairment. To establish the olfactory deficits as an early preclinical indicator for Alzheimer’s disease, on the basis of the level of olfactory impairment, decline in cognitive function should be evaluated, in the elderly people living in the community, using appropriate assessments. The rose odor is one of the odorant items that has been reported to become harder to identify with the cognitive decline [4, 5]. Attentional ability, which relates to the basal forebrain cholinergic system [6, 7], is the first non-memory
domain to be affected in Alzheimer’s disease [8]. Basic animal research has revealed that the activation of basal forebrain cholinergic neurons produces an increase in extracellular acetylcholine release in the neocortex, hippocampus, and olfactory bulb [9–15]. On the basis of these background studies, in early Alzheimer’s disease, we speculated that impairments of both the olfactory identification ability and the cognitive functions, especially attention, may arise from basal forebrain cholinergic deficits. This study aimed to investigate the relationship between olfactory identification ability and cognitive functions, including attention, in elderly people living in the community. In this study, the identification threshold for the rose odor was as
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