Summary: A new study confirms the role of the corpus callosum in determining how language functions are distributed between the brain’s hemispheres.
Source: HSE
Researchers from the HSE Centre for Language and Brain have demonstrated a clear anatomical correlate of language lateralization: the corpus callosum. Their work links the size of specific callosal sub-regions to how much the right hemisphere is recruited for language processing.
To reach this conclusion, the team developed an effective sentence-completion task for functional magnetic resonance imaging (fMRI) and applied advanced tractography to measure callosal anatomy. Their findings are reported in PLoS ONE.
Functional asymmetry between the cerebral hemispheres is a defining feature of human cognition. Most people rely primarily on the left hemisphere for language, yet roughly 10–15% of individuals show varying degrees of right-hemisphere involvement. Historically, this variation has been linked to handedness—right-hemisphere language is more common among left-handed and ambidextrous people—yet recent evidence suggests genetic and developmental factors also play a role. Importantly, some right-handed individuals likewise recruit the right hemisphere for language.
Given this diversity, the HSE team explored whether brain anatomy could explain why language functions sometimes shift away from the dominant left hemisphere. They focused on the corpus callosum, the brain’s largest commissural tract that connects the two hemispheres and mediates interhemispheric communication.
Fifty participants took part in the study. During fMRI scanning, each person read silently or aloud visually presented Russian sentence fragments and produced an appropriate final word (for example: “Teper ministr podpisyvaet vazhnoe…” — “Now the minister is signing an important …”). This sentence-completion design reliably engages multiple language areas across both hemispheres, making it well suited to measure the true degree of lateralization.
Alongside functional imaging, the researchers reconstructed volumes and microstructural properties of callosal sub-regions using diffusion MRI tractography. They compared diffusion tensor imaging (DTI) metrics with results from constrained spherical deconvolution (CSD), a more advanced method better suited to model crossing fibers within a voxel.
The combined analysis revealed a clear pattern: participants with larger volumes in a particular callosal sub-region that terminates in posterior language-related cortex showed reduced recruitment of the right hemisphere for language tasks. In other words, when that callosal region is larger, language processing is more strongly lateralized to the left hemisphere; conversely, smaller volumes in that region are associated with greater right-hemisphere involvement.
This relationship supports an inhibitory model in which callosal fibers suppress activity in one hemisphere while the other carries out a cognitive function. The team did not find meaningful associations between language lateralization and other callosal sub-regions when using DTI metrics, whereas CSD-based tractography identified the specific posterior-projecting callosal subdivision as the relevant anatomical predictor.
“The study’s innovative task design and the application of advanced neuroimaging methods made it possible to measure language lateralization more comprehensively than many previous studies,” says Olga Dragoy, Director of the HSE Centre for Language and Brain. Typical speech tasks used in past research often activate only parts of the language network, whereas the sentence-completion task reliably engages core anterior and posterior language regions across participants.
By combining a robust fMRI paradigm with CSD tractography, the researchers obtained a more precise link between structure and function: the volume of a callosal subdivision connected to posterior language areas predicts the degree to which language remains lateralized to the left hemisphere. This supports a model in which interhemispheric inhibition via callosal fibers contributes to individual differences in language lateralization.
About this language and neuroscience research news
Author: Anastasia Lobanova
Source: HSE
Contact: Anastasia Lobanova – HSE
Image: The image is in the public domain
Original Research: Open access.
“Greater volumes of a callosal sub-region terminating in posterior language-related areas predict a stronger degree of language lateralization: A tractography study” by Victor Karpychev et al. PLOS ONE
Abstract
Greater volumes of a callosal sub-region terminating in posterior language-related areas predict a stronger degree of language lateralization: A tractography study
Language lateralization remains one of the most striking examples of functional asymmetry in human cognition. While the developmental determinants of this trait are still poorly understood, researchers have sought anatomical correlates that might explain individual differences.
This study tested the association between the degree of language lateralization and metrics of callosal sub-regions. Using a sentence-completion fMRI task in a cohort balanced for handedness, the team measured language lateralization and reconstructed callosal sub-regions with two tractography approaches: diffusion tensor imaging (DTI) and constrained spherical deconvolution (CSD).
DTI-based metrics did not show significant associations with language lateralization. In contrast, CSD-based tractography revealed that the volume of a callosal sub-region projecting to core posterior language areas predicts the strength of left-hemisphere dominance for language. These results support a specific inhibitory model in which callosal fibers projecting to posterior language cortex contribute to the degree of language lateralization, while other callosal sub-regions appear less relevant in this context.