Diffusion neuroimaging of speech acquisition in infants.

How multisystem white-matter maturation supports early speech acquisition in infancy remains unclear. Using longitudinal diffusion MRI from the Baby Connectome Project, we quantified neurite density maturation from birth to 24 months across 33 white-matter tracts spanning motor, auditory, visual, ventral-language, and dorsal-language systems. Rather than being confined to canonical language pathways, maturation was coordinated across systems: dorsal-language association tracts were relatively immature near birth but showed steep postnatal growth, progressively rebalancing with motor and sensory systems. Tract-behavior coupling was likewise distributed across developmental domains, with broadly positive age-adjusted associations for motor outcomes, a sparser, partly motor-like profile for gesture-related phrase comprehension, and predominantly negative associations for lexical speech-related outcomes. Exploratory follow-up analyses further suggested motor and social precursor capacities that may help organize these indirect developmental routes from white-matter maturation to later speech-related outcomes. These findings support a multisystem developmental account in which early speech acquisition emerges not from an isolated language pathway, but from coordinated maturation across multiple systems and the precursor skills that link them.