Collapsing across all tract endpoints, we found that superficial tract development aligns with the S-A axis: tract ends near lower-order sensorimotor cortices mature earlier, while those near higher-order association cortices mature later. 9/n

Across all datasets, tracts with large ΔS-A rank (e.g., IFOF, ILF) show significantly greater ΔAge between endpoints than tracts with small ΔS-A rank (e.g., callosal tracts) 🤯 This confirms our suspicions—WM development is closely tied to cortical organization! 8/n

IFOF’s frontal & occipital endpoints sit at opposite poles of the cortical hierarchy defined by the sensorimotor-association (S-A) axis. So, do tracts with larger S-A rank differences (ΔS-A) between endpoints also show greater differences in maturation age (ΔAge)? 7/n

What about tracts linking heterotopic cortices, like the IFOF? The answer—tantalizingly—is NO! The occipital end of IFOF matures by mid-adolescence, but the frontal end does not maturity by the oldest ages in our datasets. Could this relate to cortical hierarchy? 6/n

We next focused on superficial tract regions. After creating a novel workflow to map tracts to their cortical endpoints, we asked—do tracts linking homotopic cortices (e.g., callosal motor) show similar developmental patterns at both ends? The answer: yes! 5/n

Across nearly all tract and datasets, we first identified a deep-to-superficial axis of maturation along long-range tracts. Superficial tract regions closer to the cortical surface exhibit greater age-related change than deep tract regions in the studied age window. 4/n

We think of white matter as the brain's highways—uniform conduits between cortices. But what if we study development along tracts? Turns out the journey matters as much as the cortical destination! We're thrilled to share “Two Axes of White Matter Development” www.biorxiv.org/content/10.1...
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