Age of Language Acquisition Affects Topology of Brain White Matter Network: A Diffusion Tensor Magnetic Resonance Imaging Study

Background/Objectives: In bilinguals, experience-based inter-individual differences, including language acquisition age, influence the functional and structural organization of the brain. Although many studies have explored the effects of age of acquisition on non-tonal languages, only a few studies have investigated the relationship between the age of tonal language acquisition and brain networks. Therefore, the aim of the study is to investigate how the age of language acquisition affects the topological organization of structural brain networks in proficient Japanese-Chinese/Chinese-Japanese speakers. Methods: A total of 33 proficient Japanese-Chinese/Chinese-Japanese speakers were recruited and divided into an early group into early and late groups according to their age of Chinese acquisition (AoCA). Sixteen of them constituted the early group, with AoCA at birth, whereas the late group consisted of 17 participants whose AoCA was after age 6. In addition to AoCA, the the age of second language acquisition (AoA-L2) was significantly younger in the early group than in the late group. All participants underwent diffusion MRI scanning. Structural brain networks were reconstructed, and graph-theoretical metrics were computed to compare topological properties between early and late groups. Results: Higher local efficiency and global clustering coefficient were found in the late group. For nodal parameters, the early group, compared to the late group, showed a higher nodal clustering coefficient in the right precuneus and higher betweenness centrality in the right Heschl’s gyrus (HG). Conclusions: Alterations in the right precuneus may be associated with earlier AoA-L2, consistent with its involvement in cognitive control processes required for managing multiple languages. Changes in the right HG could be related to earlier Chinese acquisition, particularly lexical tone learning, given the right HG’s role in pitch processing. This study showed how the linguistic environment, particularly language acquisition age, influences the topological properties of the structural connectivity, shedding light on its far-reaching effects on brain networks responsible for language processing and executive control.