Fig. 6. Evidence of selective degeneration of crossing fibres with age in the internal capsule.

(a) Tractography studies in the literature (such as Hosey et al., 2008, as shown) show secondary thalamic radiations crossing the primary corticospinal tract in the internal capsule (red box). The white box demarcates a representative crossing scenario. (b) In the current study, the internal capsule exhibits a positive association of both tissue fractional anisotropy (FAt) and tissue mode of anisotropy (MOt) with age, which implies an increasingly linear tensor shape with age. (b) Bedpostx tractography suggests this increase in FAt and MOt with age may be due to selective degeneration of secondary crossing fibres: Secondary fibres exhibit greater decline with age than primary fibres in these regions. Abbreviations: FAt, tissue fractional anisotropy; MOt, tissue mode of anisotropy.