Developmental reorganization of functional and structural connectivity in children's language networks is consistent with interactive specialization.

Interactive Specialization hypothesizes that brain regions initially have poorly defined roles and gradually specialize through interactions with the environment and other brain regions during postnatal development. The mechanisms underlying language acquisition are without consensus, split between innate maturation and experience-dependent theories. We analyzed a four-year longitudinal dataset of children aged 5, 7, and 9, which included developmental surveys, psychoeducational tests, fMRI, and diffusion-weighted imaging data. We observed evidence of increased functional integration over time, with region-specific improvements in brain network efficiency and functional activity correlations changing substantially. Notably, the functional coupling between the left and right cerebral white matter regions changed significantly across this age window (z=-2.99,p=0.003), declining from a strong correlation of task-related BOLD activity at age 5 (ρ=0.85) to a substantially weaker correlation by age 9 (ρ=0.21). Within language-associated networks, relay-involved tracts were consistently longer than non-relay tracts (p<0.0001 at each age) and this gap widened between ages 5 and 9, whereas no comparable difference or developmental trend was observed in non-language networks, indicating a network-specific reconfiguration toward a more economical wiring configuration. While findings support the theory of interactive specialization, no direct link was found between daily activities and either language outcomes or network efficiency, and associations with standardized language ability were small and mixed in direction.