Grammar acquisition in preschool children is related to white matter maturation of the dorsal language network

In preschool years, children take important steps in grammar acquisition, which are essential to learning their native language. A central aspect is the acquisition of the morpho-syntactic rule system, which forms an intersection between words and sentences. In adults, rule-based linguistic processes are supported by the dorsal fiber pathway to BA44, the arcuate fascicle. This pathway matures late in development, raising the question of whether it already supports grammar processes in the early preschool years, or whether early grammar acquisition is supported by different, earlier-maturing fiber pathways. In a large sample of 3- to 5-year-old children (N = 120), we examined the association between maturation of the language network and children's morpho-syntactic abilities. This revealed consistent differences between 3-year-olds and 4- to 5-year-olds. The 4- to 5-year-olds, but not 3-year-olds, showed a relation of morpho-syntax with both the dorsal pathway to BA6, supporting phonological processes, and the dorsal pathway to BA44, supporting syntactic processes in adults. Additionally, although less consistent, a relation with the ventral pathway, supporting semantic processes, was found in the 4- to 5-year-olds. Our results suggest that maturation of the language network enhances the acquisition of morpho-syntactic rules in the preschool period from 4 years of age.