"The Relation Between Language Function Lateralization and the Organization of White Matter pathways in the brain "

The mechanisms of language lateralization are still not fully understood today. It is known that multiple factors may be related to the distribution of language between the left and the right hemispheres of the brain. The current study aimed to examine three of them – the relation between individual handedness, familial sinistrality and tractography metrics of the corpus callosum (CC), on the one hand, and language lateralization, on the other hand. I hypothesized that the larger CC volume is, the greater left-hemisphere lateralization is. This hypothesis was developed and partially confirmed by Josse and colleagues (2009); however, they used the indirect metric of CC (measurement of midsagital СС surface using anatomical T1-weighted images). I was first who reconstructed the CC using tractography data and correlated its metrics with the language lateralization. In the present study, I correlated language lateralization indices (LI) based on fMRI data with the fractional anisotropy, mead diffusivity and the volume of CC. Fifty neurologically healthy Russian speakers with different hand preference took part in the study. I collected fMRI and DTI data, as well as information about individual handedness and familial sinistrality (FS). The participants grouped into six categories: 10 right-handers FS+, 10 right-handers FS-, 10 left-handers FS+, 10 left-handers FS-, 4 ambidexters FS+ and 6 ambidexters FS-. One of the results is that the involvement of the right hemisphere in language processing increased with the decrease of right-handedness scores obtained with the Edinburgh inventory (Oldfield, 1971), from right-handers to ambidexters and left-handers. Also, the individual variability was shown. But the main and new finding is that language lateralization is related to the volume of CC. Specifically, people with greater right-hand preference and people with larger CC had greater left-hemisphere lateralization of language-related brain activation.