Odor Identification Skills in Adults with ASD Are Associated with Anatomical Abnormalities in the Tractography of Fronto-Occipital Fasciculi

Background: Individuals diagnosed ASD exhibit abnormal chemosensory experiences from childhood into adulthood. Sensory function is likely linked to social communication function (Bennetto et al., 2007) and the manifestation of anxiety (South & Rodgers, 2017). However, there few studies of olfaction in adults with ASD and to date there are no published neuroimaging studies of olfactory processing in ASD.

Objectives: We investigated white matter tractography to determine point-wise quantifications of abnormal anatomical connections in olfactory areas in adults with ASD. We additionally examined correlations of tractography results alongside olfactory identification skills.

Methods: Thirty-four adults with ASD (13F) and 39 neurotypical controls (NT group, 19F) participated in the study. Tracts within inferior fronto-temporal areas, chosen a priori, were defined using automated fiber-tract quantification. High fractional anisotropy (FA) values indicate greater fiber density while mean diffusivity (MD) describes the rotationally-invariant magnitude of water diffusion within brain tissue. The odor identification task required participants to smell 16 odorous pens one at a time after having visualized four images representing the target odor and three distractors, then choosing the correct odor from the four possibilities.

Results: Bayesian models showed that individuals in the ASD group were nearly 3 times more likely to show reduced odor identification skills than NT controls. The tract profiles of infero fronto-occipital and longitudinal fasciculi show abnormalities in ASD especially in rostral sections of the tracts. Odor identification performance was significantly associated with FA values in frontal sections of tract profiles. Odor identification skills in ASD are binomially distributed, in line with other sensory-related experiences in ASD (Bogdashina, 2016).

Conclusions: Taken together, our findings describe for the first time the relationships between odor identification skills and their neural underpinnings in adults with ASD. Results suggest that olfactory perception and neuroimaging can provide non-invasive tools for ASD subgroup characterization. Further exploration of relationships between olfactory processing, social communication skills, and anxiety symptoms is warranted.