Comparing 48 candidate tracts, microstructural differences of three fornix-related fiber tracts were highly predictable based on gray matter differences within the DMN (Fig. 2), explaining up to 24% of population variance in this major hippocampus output pathway of the limbic system. These associations persisted across different diffusion parameters (SI Appendix, Fig. S2). Gray matter volume differences in DMN subregions were predictive of microstructure in a specific subset of anatomical tracts. The prediction accuracy for the common FA measure of white matter integrity ranged from R2 = 0.00 to 0.24 across all tracts, with mean performance of 0.04 ± 0.04 (±SD) in unseen individuals (out-of-sample cross-validation). Across the atlas tracts, pattern detection performance was high for the fornix (Fig. 2 A and B) at an explained variance of 24% (R2 = 0.24 ±0.03 SD across cross-validation splits), fornix fibers in the bilateral cres and stria terminalis (right: R2 = 0.09 ± 0.01; left: R2 = 0.08 ± 0.02), anterior corona radiata of the thalamus (left: R2 = 0.10 ± 0.01; right: R2 = 0.09 ± 0.01), posterior limb of the internal
