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 with age, 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 years, from the Dallas Lifespan Brain Study who underwent diffusion-weighted imaging to measure white matter diffusivity, as well as fMRI to measure functional selectivity to viewing photographs from different categories (e.g., faces, houses). In general, decreased ILF anisotropy significantly predicted both focal and broad functional dedifferentiation. Specifically, there was a localized effect of structure on function, such that decreased 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. On the other hand, the whole ILF predicted less selective response across broader ventral visual cortex for viewing animate (e.g., human faces, animals) versus inanimate (e.g., houses, chairs) images. This structure-function relationship became weaker with age and was no longer significant after the age of 70 years. These findings indicate that decreased white matter anisotropy is associated with maladaptive differences in proximal brain function and is an important variable to consider when interpreting age differences in functional selectivity.
Highlights
For models in which mean fractional anisotropy (FA) in inferior longitudinal fasciculus (ILF) was found as a significant predictor, additional regressions were conducted using other indicators of white matter diffusivity in order to determine which diffusion metrics accounted for the effect of FA
Decreased white matter anisotropy in ILF corresponded to less selective activation for animacy across the ventral visual cortex, independent of the effect of age
White matter anisotropy within a smaller and more localized portion of the ILF did predict functional selectivity in fusiform face area. These results suggest that whole ILF white matter may be a stronger predictor of broad differences in selectivity across the entire ventral visual cortex, whereas more focal regions of ILF may be a stronger predictor of dedifferentiation of specialized face processing
Summary
Ventral visual cortex encompasses regions of occipito-temporal cortex that are uniquely specialized for the identification and recognition of visual stimuli (Grill-Spector & Malach, 2004). Researchers have proposed that categorical representations in ventral visual cortex are multifaceted and exist at both a focal and broad level with smaller, specialized regions, like faceselective fusiform associated with narrow category distinctions (i.e., “face” vs “house”) that are nested within larger functional regions involved in superordinate distinctions (i.e., “animate” vs “inanimate”; Grill-Spector & Weiner, 2014) With advanced age, these highly specialized and unique representations associated with processing different visual stimuli become less distinct or “dedifferentiated” (Park et al, 2004; Grady et al, 1994; see Koen & Rugg, 2019 for a review). We considered that younger adults would vary in ILF integrity and that an effect of decreased anisotropy in ILF would be maintained, even after age was controlled
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