Olfactory function is affected by demographic, cognitive, and genetic factors. In the present thesis, three empirical studies investigated individual differences in olfactory ability. Study I explored demographic and cognitive correlates in common olfactory tasks; odor detection, odor discrimination, and odor identification. The results indicated that old age influenced performance negatively in all tasks, and that semantic memory proficiency and executive functioning were related to odor discrimination and odor identification performance. No cognitive influence was observed for measurements of olfactory threshold. Using population-based data, Study II investigated a potential influence of the ApoE gene on olfactory identification after controlling for health status, semantic memory, and preclinical and clinical dementia. The main finding was that the ApoE- ɛ4 allele interacted with age, such that older ɛ4-carriers had an impaired odor identification performance relative to older non-carriers. Importantly, the negative ApoE- ɛ4 effect on olfactory proficiency was independent of clinical dementia conversion within five years. Study III investigated the effects of the BDNF val66met polymorphism on olfactory change over a five-year interval, in a community dwelling sample of young and old age cohorts. The results showed that age-related decline in olfactory identification was influenced by the BDNF val66met. In middle-aged subjects, no effect of BDNF val66met was observed although older val homozygote carriers showed a selectively larger olfactory decline than the older met carriers. Overall, results suggest that the relative influence of demographic and cognitive factors vary across different olfactory tasks and that two genes (ApoE and BDNF) impact age-related deficits in odor identification. Potential theoretical and practical implications of the findings are discussed as well as potential limitations of association studies in genomics research.

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The capacity to form olfactory images has received less attention than the formation of visual and auditory images. The evidence in favor of such ability is also inconsistent. This thesis explored some of the characteristics of olfactory imagery through three empirical studies. Study I investigated the effects of blocking spontaneous sniffing during olfactory imagery. The results indicated that the prevention of spontaneous sniffing reduced olfactory but not visual imagery capacity. Study II studied the relation between olfactory awareness (as indexed by olfactory dreams, olfactory imagery, and olfactory interest) and olfactory functions (i.e., odor threshold, episodic odor memory, and odor identification). The main findings were that compared to low, high olfactory awareness was associated with better episodic odor memory and identification, but not with higher olfactory sensitivity. Study III investigated the neural correlates of odor evoked autobiographical memories (OEAMs) as (a) a function of cue modality (i.e., odors and their verbal referents), and (b) a function of memory remoteness. The results from Study III showed that OEAMs activated regions generally associated with autobiographical memory. In addition, verbally cued OEAMs were associated with activity linked to olfactory imagery. Odor cues activated the limbic and temporal polar regions more than verbal cues; a result that may explain the phenomenological differences found between the cued memories. Moreover, OEAMs from the first decade of life were associated with higher activity in the secondary olfactory cortex, whereas memories from young adulthood were related to areas linked to semantic memory processing. Taken together these studies favor the notion of a human capacity to form olfactory images.