Spatiotemporal white-matter development across early childhood.

Early childhood development is scaffolded by rapid maturation of brain white matter structure, believed to support the emergence of cognitive and socioemotional functions. Previous whole-tract studies have suggested patterns of white matter development occurring along posterior-anterior, deep-superficial, and inferior-superior axes. However, these have largely been cross-sectional and employed nonspecific metrics of white matter organization. Using longitudinal diffusion imaging data from 133 children (4 to 8 years; 76 females), the present work characterizes along-tract patterns of white matter development across association, commissural, and projection bundles using fixel-based analysis. Within long range association bundles, faster age-related changes were observed for segments adjacent to the visual cortices relative to segments located near association regions, supporting a sensorimotor-association axis of brain development. An inferior-superior pattern was found for projection tracts, with faster age-effects observed for segments near the brainstem. Lastly, while several association and commissural bundles exhibited faster maturation within central segments; indicative of a deep-superficial axis, effects were mixed between micro- and macrostructure, underscoring the unique developmental timing of these different fiber properties. Our findings provide evidence that within-tract white matter maturation unfolds along key spatiotemporal axes, and suggests that increased spatial precision can advance our understanding of early childhood brain development.