White matter microsctructure predicts focal and broad functional dedifferentiation of visual processing in normal aging

Abstract Ventral visual cortex exhibits highly organized and selective patterns of functional activity associated with visual processing. However, this specialization decreases in normal aging, with functional responses to different visual stimuli becoming more similar, a phenomenon termed “dedifferentiation”. The current study tested the hypothesis that age-related degradation of the inferior longitudinal fasciculus (ILF), a white matter pathway involved in visual perception, could account for dedifferentiation of both localized and distributed brain activity in ventral visual cortex. Participants included 281 adults, ages 20-89, from the Dallas Lifespan Brain Study who underwent diffusion weighted imaging to measure white matter diffusivity and functional magnetic resonance imaging to measure functional activity selective to viewing photographs from different image categories (e.g., faces and houses). In general, decreased ILF anisotropy significantly predicted functional dedifferentiation in both face-selective fusiform gyrus and across the whole ventral visual cortex, beyond the effect of age. Specifically, there was a local effect of structure on function, such that anisotropy in a smaller mid-fusiform region of ILF predicted less selective (i.e., more dedifferentiated) response to viewing faces in a proximal face-responsive region of fusiform, whereas the whole ILF predicted less selective response across ventral visual cortex for viewing animate (e.g., human faces and animals) versus inanimate (e.g., houses and chairs) images. These results suggest decreased white matter anisotropy is associated with maladaptive differences in proximal brain function, and may be an important variable to consider when interpreting age-differences in functional activity.