Summary: A study confirms the role played by the corpus callosum in the lateralization of language.
A study from the HSE Center for Language and Brain confirmed the role of the corpus callosum in language lateralization, the distribution of language processing functions between the hemispheres of the brain.
The authors proposed an innovative linguistic task for their study subjects and applied advanced neuroimaging methods to the collected data.
An article on their findings was published in PLOS ONE.
The functional asymmetry between the two cerebral hemispheres in the execution of higher level cognitive functions is a major characteristic of the human brain. For example, the left hemisphere plays a major role in language processing in most people. However, between 10% and 15% of the human population also use the right hemisphere to varying degrees for the same task.
Traditionally, the lateralization of language to the right hemisphere was explained by laterality, as it is mainly found in left-handed and ambidextrous people (also using both hands). But recent research has demonstrated a genetic difference in how language is processed by left-handed and ambidextrous people.
In addition to this, some right-handed people also involve their right hemisphere in language functions.
These findings have prompted scientists to consider alternative explanations, particularly by examining brain anatomy to discover why language functions may shift to the right hemisphere.
Researchers from the HSE Center for Language and Brain have speculated that language lateralization may have something to do with the anatomy of the corpus callosum, the largest commissural tract in the human brain connecting the two cerebral hemispheres.
The researchers asked 50 study participants to complete a sentence completion task. Subjects had to read aloud a visually presented Russian sentence and complete it with an appropriate last word (eg, “Teper ministr podpisyvaet vazhnoe…” — “Now the minister signs an important…”).
At the same time, participants’ brain activity was recorded by functional magnetic resonance imaging (fMRI). In addition, corpus callosum volume was measured in each subject.
A comparison between fMRI data and measurements of the corpus callosum revealed that the larger the volume of the latter, the less lateralization of language function towards the right hemisphere was observed.
It can be said that in the processing of language, the brain tends to use the resources of the left hemisphere efficiently and to suppress, by means of the corpus callosum, any further involvement of the right hemisphere. The larger a person’s corpus callosum, the less their right hemisphere is involved in language processing (and vice versa).
This finding is consistent with the inhibitory model suggesting that the corpus callosum inhibits the action of one hemisphere while the other is engaged in cognitive tasks.
“The study’s innovative design and use of advanced neuroimaging made this conclusion possible.
“Brain lateralization in language processing is generally difficult to measure accurately because typical speech tasks used in previous studies (e.g., naming pictures, selecting words beginning with a certain letter, or listening to speech) tend to cause activation only in certain parts of the brain responsible for language functions but not others.
“Instead, we have developed a single speech task for fMRI – sentence completion – that reliably activates all language areas of the brain,” says Olga Dragoy, director of the HSE Center for Language and Brain.
It is important to add that the authors reconstructed the volume and properties of the corpus callosum from the MRI data using an advanced tractography technique: constrained spherical deconvolution (CSD).
This is better suited than traditional diffusion tensor imaging for modeling crossed fibers in the smallest unit of volume, the voxel (3D pixel), and is therefore more reliable.
About this language and news in neuroscience research
Author: Anastasia Lobanova
Contact: Anastasia Lobanova – HSE
Picture: Image is in public domain
Original research: Free access.
“Greater volumes of a callosal subregion terminating in posterior language-related areas predict a higher degree of language lateralization: a tractography study” by Victor Karpychev et al. PLOS ONE
Larger volumes of a callosal subregion terminating in posterior language-related areas predict a higher degree of language lateralization: a tractography study
Language lateralization is the most intriguing feature of functional asymmetry for cognitive functions.
Nowadays, the ontogenetic determinants of this trait are largely unknown, but efforts are being made to find its anatomical correlates. In particular, an interhemispheric connection of white matter – the corpus callosum – has been proposed as such.
In the present study, we sought to find the association between the degree of language lateralization and callosal subregion metrics.
We applied a sentence completion fMRI task to measure the degree of language lateralization in a laterality-balanced group of healthy participants.
We obtained the volumes and microstructural properties of callosal subregions with two tractography techniques, diffusion tensor imaging (DTI) and constrained spherical deconvolution (CSD).
Analysis of DTI-based metrics revealed no significant association with language lateralization. In contrast, CSD-based analysis revealed that volumes of a callosal subregion ending in the central posterior language-related areas predict a higher degree of language lateralization.
This finding supports the specific inhibitory pattern implemented through callosal fibers projecting into posterior central language-related areas in the degree of language lateralization, with no relevant contribution from other callosal subregions.