The research team from HSE University, Lomonosov State University, and Institute of Linguistics, Russian Academy of Sciences, Moscow, Russia, conducted the first tractography study that examined the association between the volumes and microstructural properties of the corpus callosum (CC) subregions and the degree of language lateralization. They demonstrated that the volumes of callosal subregions terminating in the language-related posterior brain regions predict a stronger degree of language lateralization.
About the transcallosal cortico-cortical transmission
From the first observations made in the late 19th century, generations of scientists became interested in the origin and consequences of brain asymmetry. Pioneering studies have pointed to the left hemisphere dominance in language, as well as the right hemisphere dominance in the affective, prosodic and intonational aspects of spoken language and emotional word processing.
Later studies have also documented a dominance of the left hemisphere for language in most human beings. However, about 10–15% of individuals show atypical dominance for language in the right hemisphere or no clear hemispheric dominance. This is more frequently represented in the left-handed and ambidexters than in the right-handed people.
The corpus callosum (CC), as the largest inter-hemispheric commisure connects the cortical zone of the left hemisphere and the right hemisphere. The anterior regions of the CC contain axons from the frontal, premotor and motor cortices, whereas the posterior regions of the CC contain fibers from the somestetic, parietal, occipital and temporal lobes.
Transcallosal cortico-cortical transmission is mainly excitatory, but, the main and longer lasting effect in the contralateral hemisphere is inhibitory, probably because most of the excitatory callosal fibers terminate on pyramidal neurons which then activate inhibitory interneurons. These activated inhibitory cells may then induce a widespread inhibition in homotopic regions of contralateral neurons.
The role of the CC has two major aspects. The first aspect suggests that the CC is associated with functional cortical lateralization at the level of inter-hemispheric inhibition, while the second implies that CC contributes to brain symmetry because the impairment of CC leads to intra-hemispheric isolation. Accordingly, two different models explain how the CC can contribute to language lateralization. The excitatory model suggests the functional activation of both hemispheres through the CC because most of its fibers rely on excitatory glutamate neurotransmitters. The inhibitory model argues for the suppression of the subdominant hemisphere during language tasks by the dominant hemispheres through the inhibitory interneurons.
About the study
The objective of this study was to investigate the relationship between the structural properties of each callosal subregion and the degree of language lateralization in the corresponding cortical area using diffusion tensor imaging (DTI) and constrained spherical deconvolution (CSD). The volumes of the callosal subregions, for the first time, were explored in relation to the degree of language lateralization.
Early tractography studies of language lateralization investigated only the microstructural properties obtained with DTI. However, neither of these studies examined volumes of the callosal subregions. Therefore, the researchers additionally applied the CSD, an advanced tractography technique for modeling the CC crossing fibers, and directly compared the results obtained with DTI and CSD.
Researchers applied a block-designed language task with alternating sentence completion, which activates both anterior and posterior language-related areas, to measure the degree of language lateralization in neurologically healthy individuals. Of the 50 subjects included, 20 were right-handed, another 20 were left-handed, and 10 participants were ambidextrous. During the language task the participants’ brain activity was recorded using functional magnetic resonance imaging (fMRI). In addition, the volumes and microstructural properties of the callosal subregions with two tractography techniques, DTI and CSD were measured in each subject.
The authors noted that over the past decades the efforts have been made to find anatomical correlates for language lateralization in gray and white matter structures. Among gray matter structures, insular asymmetry has been suggested to predict language lateralization. However, it has been shown that asymmetry in areas clearly related to the language, the planum temporale and Broca’s area, does not correlate with the lateralization of the language.
Results showed a significantly larger volume in CSD than in DTI for each of the callosal subregions, which is in line with some earlier work emphasizing that CSD can provide a more complete reconstruction of the crossing callosal fibers. Results also showed that the microstructural properties of callosal fibers did not affect the degree of language lateralization, regardless of the tractography method.
However, the variability of the microstructural properties and the potential functional specialization of the callosal subregions led the researchers to analyze the degree of language lateralization in each of the cortical areas corresponding to the specific callosal subregions. Analysis of DTI-based metrics has not shown any significant association with language lateralization. However, the CSD-based analysis found a significant impact only in the area that included language-related posterior brain regions.
Specifically, the volumes of callosal subregions terminating in the posterior parietal, temporal and occipital lobes predicted a stronger degree of language lateralization. According to the authors, their findings support the specific inhibitory model implemented through the callosal fibers projected into the language-related core posterior areas, with no relevant contribution from other callosal subregions. In addition, the relationship between the volume of CC fibers projecting into the parietal, temporal, occipital lobes and the degree of language lateralization can be explained by the role of the posterior callosal subregion in the comprehension of language.
The authors concluded that this is the first tractography study that investigated the relation between the volumes and microstructural properties of callosal subregions and the degree of language lateralization, using both DTI and CSD. In line with the inhibitory model, greater volumes in the CSD, but not in the DTI, predicted a stronger degree of language lateralization in the language-related posterior brain regions, the posterior parietal, temporal and occipital lobes. According to the results of this study, the influence of callosal fibers on the degree of language lateralization is not equipotential, but rather anatomically specific.
This article was published in Plos ONE. Karpychev V et al. Greater volumes of a callosal sub-region terminating in posterior language-related areas predict a stronger degree of language lateralization: A tractography study. PLoS ONE 2022; 17(12): e0276721. (Open Access). https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0276721