#6: A tract’s position in the hierarchy tracks real variation across people.

In a large developmental sample, tracts that span more of the hierarchy showed greater developmental changes and stronger links to executive function.

#5: Cortical biology shapes tract function.

Tracts staying within a hierarchical level connect regions with similar biological properties. Tracts crossing the hierarchy link biologically heterogeneous areas and support broader functions—acting as anatomical bridges across cortical environments.

#4: Tracts vary in cognitive diversity.

Some are specialists (optic radiation, fornix), while others are generalists supporting many functions (hi IFOF and ILF 👋). And importantly, tracts that span more of the cortical hierarchy are more cognitively diverse.

#3: Tracts have unique cognitive profiles.

We mapped each tract’s cognitive profile, confirming classic roles (arcuate ➡️ language; CST ➡️ movement) and revealing new ones, including empathy for the right arcuate.

#2: The layout of white matter tracts aligns with the layout of cognitive functions.

Multivariate analyses showed that tracts and cognitive functions align along a shared spatial axis. Dorsolateral-connecting tracts ➡️ action functions. Ventromedial-connecting tracts ➡️ affective functions.

#1: Tracts differ a lot in how much of the cortical hierarchy they span.

Some stick to nearby, similarly ranked regions, while others stretch across long distances to link distant points on the S-A axis. So a tract’s spatial span is tightly related to how much of the hierarchy it crosses.

To do this, we used population-level maps linking 52 WM tracts to the cortical surface (bit.ly/4aNI4DJ). This mapping allowed us to place tracts within the cortical hierarchy and characterize their associated cognitive functions and neurobiological properties–leading to SIX exciting findings!