Abstract
The humans’ brain asymmetry is observed in the early stages of life and known to change further with age. The developmental trajectory of such an asymmetry has been observed for language, as one of the most lateralized cognitive functions. However, it remains unclear how these age-related changes in structural asymmetry are related to changes in language performance. We collected longitudinal structural magnetic resonance imaging data of children from 5 to 6 years to investigate structural asymmetry development and its linkage to the improvement of language comprehension abilities. Our results showed substantial changes of language performance across time, which were associated with changes of cortical thickness asymmetry in the triangular part of the inferior frontal gyrus (IFG), constituting a portion of Broca’s area. This suggests that language improvement is influenced by larger cortical thinning in the left triangular IFG compared to the right. This asymmetry in children’s brain at age 5 and 6 years was further associated with the language performance at 7 years. To our knowledge, this is the first longitudinal study to demonstrate that children’s improvement in sentence comprehension seems to depend on structural asymmetry changes in the IFG, further highlighting its crucial role in language acquisition.
Highlights
The human brain is anatomically and functionally asymmetric
We examined the association between language performance and cortical thickness asymmetry separately for each time point, i.e., when they were 5 and 6 years old, we did not find any significant correlation for neither time point
We found that the language performance in 7-year-old children was negatively correlated with the cortical thickness asymmetry in the triangular part of inferior frontal gyrus (IFG) at age 5 years (r = -0.40, p = 0.005, Bonferroni corrected, see Fig. 3B and Table S5 in the supplements) and at age 6 years (r = -0.42, p = 0.003, Bonferroni corrected, see Fig. 3C and Table S5 in the supplements), while controlling for covariates of no interest, suggesting that children with superior language performance at age 7 showed lower cortical thickness in the left compared to the right triangular part of the IFG already at ages 5 and 6
Summary
The human brain is anatomically and functionally asymmetric. This so-called brain lateralization has been observed for various cognitive functions (Toga and Thompson, 2003), such as language, favoring the left hemisphere in the healthy adult brain. The lateralization of language-related regions has been revealed in the brain’s cortical thickness, supposed to be derived from different thinning rates across hemispheres during development (Shaw et al, 2009; Zhou et al, 2013). That language abilities are related to the brain’s asymmetry becomes especially apparent when looking at brain asymmetry of individuals with language difficulties, such as developmental dyslexia (Altarelli et al, 2014; Bishop, 2013; Qi et al, 2016), assumed to rely on a phonological processing deficit. Because atypical brain asymmetry is already present in individuals with a familial risk of dyslexia during their early childhood (Vanderauwera et al, 2018), it could be suggested that atypical brain asymmetry is one of the causes of this developmental disorder
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