Limited exposure to language during childhood alters adult brain structure

In a recent study published in the Proceedings of the National Academy of Sciences, researchers assess the impact of restricted language access among children on the organization of the adult brain in specific language regions.

Study: Restricted language access during childhood affects adult brain structure in selective language regions. Image Credit: Prostock-studio / Shutterstock.com

Language development in children

Language is a distinctively human cognitive function that requires a left-lateralized frontotemporal brain network. However, it remains unclear how this extremely efficient and sophisticated language network forms in the brain during early development.

Most of the postnatal brain development is dependent on experience. Environmental learning and experience influence several brain processes, including pruning, synaptogenesis, and myelination.

The slower growth of language-related regions relative to sensory-motor regions suggests a more significant duration of neural plasticity, during which language from the environment might change this brain network. In addition, previous research has demonstrated that the amount of language exposure children receive influences the rate of language development.

About the study

In the present study, researchers explore the impact of early language experiences on the adult human brain by assessing the anatomical characteristics of individuals born deaf with either typical or limited language experience during childhood.

The impacts of three variables were evaluated by employing three cortical measures, including adjusted volume, cortical area, and cortical thickness. These three variables included the age of ASL acquisition (AOA), left or right hemisphere, and somatomotor or language brain areas.

Using gender and age as variables and region of interest (ROI) as random errors within a linear mixed model, the volume of the brain regions was compared after adjusting for age and gender. The left hemisphere (LH) and somatomotor areas served as baseline values.

To further discern the characteristics of the bilateral AOA consequences, altered anatomical features were described for each language-relevant ROI for all deaf signers employing AOA as an independent covariate.

Since the timespan of restricted language experience during childhood influences cortical assessments in adults born deaf, the team also determined whether the visual-motor modality related to the language experience among deaf signers affected the anatomical features of selected language regions of the brain. This was achieved by comparing deaf signers whose guardians or parents signed to them since birth to a cohort of hearing non-signers exposed to speech since birth.

Study findings

The team preselected ROIs within each hemisphere, including 17 in language-specific regions and 19 in somatomotor regions as controls. All enrolled individuals were born deaf; however, their ages at first exposure to ASL varied.

No remarkable effect of AOA on control somatomotor ROIs within the LH was observed. However, considerable interaction was observed between the language areas and AOA. This indicated a negative correlation between adjusted cortical volume and the timespan of restricted language exposure during childhood within language-relevant ROIs in the LH.

The lack of hemispheric interactions with ROI type and AOA suggests that there were no discernible impacts of laterality in the language areas. However, concerning cortical thickness, a remarkable effect of AOA on somatomotor ROIs was observed, in addition to noteworthy interactions between LH language areas and AOA. Thus, AOA appears to have adverse effects in somatomotor and language areas, with a more significant effect on language areas.

No hemispheric interactions with ROI type and AOA were observed, suggesting that AOA’s effects in bilateral language areas were comparable. A similar linear mixed model revealed no AOA effects. Furthermore, when the cortical area was employed as a dependent variable. These findings indicate that restricted availability of language during childhood had a negative association with both cortical thickness and the adjusted cortical volume within language-relevant areas of both hemispheres.

Two language brain areas in the LH, as well as three language brain areas regions in the right hemisphere (RH), including left TE1p, left BA45, right TPOJ1, right A4, and right A5 were associated with a significant AOA effect.

Linear models that considered hearing and deaf groups, along with gender and age as covariates, revealed no remarkable sensory-modality effects linked with newborn language experience across all six brain structural characteristics with substantial negative AOA effects. Furthermore, there was a nonsignificant pattern for higher cortical thickness in the right TPOJ1 of deaf native signers.

No evidence of distinctions regarding infant language experience with deaf participants having sign language experience and hearing participants having spoken language experience was observed.

Conclusions

The study findings showed that restricted language experience in early childhood was related to negative alterations in various anatomical aspects in bilateral language-relevant areas, particularly in left frontotemporal areas that are essential for the development of the linguistic structure. The absence of any effect when comparing deaf native signers and hearing native speakers demonstrated the importance of early language experience on language system development in the brain, regardless of language modality.

Thus, the development of the brain language system was extremely susceptible to language exposure during early life.