Individualized structure-function coupling reveals behavioral signatures in the adolescent brain

Adolescence is a critical period of brain maturation, yet how functional dynamics relate to white-matter microstructure at the individual level remains poorly understood. We developed a machine learning workflow to predict fractional anisotropy (FA) from distributed resting-state functional connectivity (FC) in two large adolescent cohorts: NCANDA (n = 814, 12-22 years, longitudinal) and HCP-Development (n = 472, 12-22 years, cross-sectional). Whole-brain FA could be modestly predicted from FC ( $r$ = 0.16 in NCANDA; $r$ = 0.27 in HCP-D). The accuracy of predicting regional FA significantly varied across 27 white-matter regions, with the highest structure-function coupling detected in fiber tracts subserving unimodal cortical regions. These regional accuracy scores were reproducible between datasets ( $r$ = 0.95). Region-specific analyses also revealed tract-clustered FC predictors, highlighting distinct large-scale functional circuits underlying regional microstructural integrity. We then defined a "structure-function gap" as the residual between predicted and observed FA in each white-matter region. These gap measures were significantly associated with a broad constellation of cognitive and behavioral measures, particularly involving memory, impulsivity, and executive function. Notably, significant behavioral coupling emerged in Corticospinal and Cingulum pathways. Together, these findings establish individualized structure-function coupling as a reproducible, anatomically specific, and behaviorally informative marker in youth, offering a new framework to link distributed FC patterns to white-matter development and behavioral variability.