Assessing the Early Lateralization of White Matter in the Infant Language Network

Neural language development involves the maturation of both frontal and temporal language centers and their white matter connections. Leftward asymmetry of white matter tracts has been seen at 5 years of age, and the maintenance of laterality into adulthood likely supports mature language functioning and cortical lateralization. However, it is not known if this laterality is present in infancy or how it relates to early language acquisition. We examined longitudinal changes in white matter microstructure and macrostructure in language (arcuate fasciculus [AF], uncinate fasciculus [UF]) and motor (corticospinal tract [CST]) white matter pathways in typically developing infants. We hypothesized that left hemisphere language tracts would demonstrate more rapid maturation in infancy compared to their right hemisphere counterparts, supporting an early left hemisphere bias for language in the left hemisphere, and we hypothesized that nonmotor tracts would demonstrate concurrent bilateral maturation. We characterized the development of hemispheric asymmetry in the bilateral AF, UF, and CST in 114 typically developing infants from 0 to 24 months of age using data from the HCP Baby Connectome Project. We measured longitudinal changes in fractional anisotropy (FA), mean diffusivity (MD), radial diffusivity (RD), axial diffusivity (AD), probabilistic streamlines, and tract volume. We used linear mixed-effects modeling to estimate the developmental trajectories in micro- and macrostructure in the left and right hemisphere tracts. We additionally reconstructed these tracts in a cohort of healthy adults from the 100 Unrelated Subjects Cohort of the Human Connectome Project. We successfully reconstructed these tracts in the adult brain and demonstrated broad left-lateralization, replicating prior findings. For infants, all tracts demonstrated rapid age-related changes in microstructure, but there were no age-related increases in tract volume or number of streamlines. There were no main effects of sex in any measure. In contrast to adults, while we did see a difference between hemispheres in the number of streamlines in the UF, which was greater in the right hemisphere, we did not find other differences or any asymmetries in rates of maturation between left and right hemisphere tracts. Our methods are capable of identifying laterality differences between left and right hemisphere white matter tracts in adults. However, the picture was quite different in infants. We found that both the left and right AF and UF demonstrated rapid microstructural maturation over the first 2 years of life. However, left lateralization of these tracts was not present in infancy. This may indicate that strong laterality develops as more language skills are acquired or perhaps not until strong cortical lateralization emerges in childhood. Future studies should add to this work by including other language tracts and including data from infancy through childhood, when functional language lateralization begins to emerge and core language acquisition is complete.