Phonological decoding ability is associated with fiber density of the left arcuate fasciculus longitudinally across reading development.

Numerous studies have linked reading ability to white matter microstructure using diffusion tensor imaging, but findings have been inconsistent and lack specificity. Fiber-specific diffusion-weighted magnetic resonance imaging (dMRI) models offer enhanced precision in measuring specific microstructural features, but they have not yet been applied to examine associations between reading ability and white matter microstructure development as children learn to read. We applied constrained spherical deconvolution (CSD) and fiber-specific modelling to characterize developmental changes in fiber density of key white matter tracts of the reading network, and investigated associations between tract-wise fiber density and children's phonological decoding abilities. Fiber density was measured from ages 2-13 years, and decoding ability (pseudoword reading) was assessed at ages 6 years and older. Higher decoding ability was associated with greater fiber density in the left arcuate fasciculus, and effects remained consistent over time. Follow-up analysis revealed that asymmetry changes in the arcuate fasciculus were moderated by decoding ability: good decoders showed leftward asymmetry from early childhood onward, while poorer decoders shifted toward leftward asymmetry over time. These results suggest that densely organized fibers in the left arcuate fasciculus serve as a foundation for the development of reading skills from the pre-reading stage through fluent reading.