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Are inter-hemispheric shape asymmetries of white matter tracts related to language lateralization?

Poster A2 in Poster Session A, Thursday, October 6, 10:15 am - 12:00 pm EDT, Millennium Hall

Ieva Andrulyte¹, Christophe de Bezenac¹, Simon S. Keller¹; ¹University of Liverpool

Introduction: Interhemispheric anatomical asymmetries are thought to be related to language lateralization. White matter (WM) tracts provide the basis for neural communication between cortical regions important for language. There have been some studies that have examined whether interhemispheric asymmetries of fundamental diffusion scalar metrics of WM tracts known to be important for language are consistent with the side of language lateralization in healthy and brain damaged populations, with inconsistent results. In the present study, we examined whether asymmetric morphometric features of WM tracts are related to the side of language lateralization in a large cohort of healthy individuals. Methods: We studied 1040 healthy young (22-35 years) adult subjects from the Human Connectome Project (HCP), an open-access data initiative, which offers high quality anatomical and functional neuroimaging in the healthy human brain using magnetic resonance imaging (MRI), behavioural, and demographic data. Participants underwent both functional and diffusion magnetic resonance imaging. The language comprehension task used in Human Connectome Project was designed by Binder and colleagues (2011), which consists of two runs each consisting of four blocks of a story task alternating with four blocks of a math task, with a matched length and difficulty. A laterality index (LI) was calculated to determine hemispheric language lateralization for each subject’s fMRI. The language tracts, such as a superior longitudinal fasciculus and corpus callosum were reconstructed in MNI space using q-space diffeomorphic reconstruction to obtain the spin distribution function (Yeh et al, 2010). Then, shape measures of bundles (length, span, curl, elongation, diameter, volume, and surface area) were extracted. The motivation for doing tract geometry analyses came from the recent study (Yeh, 2020) that showed geometrical laterality differences in the arcuate fasciculus. An asymmetry index (AI) for all shape descriptors was calculated to determine structural asymmetries of all WM tracts. A linear regression analysis was performed to assess the relationship between functional LI and structural AIs. Results: Based on the laterality index of the fMRI language task, 978 participants were classed as left-hemisphere dominant (93%), 60 as bilateral (6%), and the remaining 11 as right-hemisphere dominant (1%). fMRI analysis revealed asymmetric activation in core language areas, including the inferior frontal gyrus, superior temporal gyrus, angular gyrus, posterior cingulate cortex, and lateral prefrontal cortex. No evidence was found for associations between LI and the AI of each shape metric for all white matter tracts analysed. Discussion: Our results indicate that interhemispheric shape asymmetries of white matter fibre tracts are not related to the lateralization of language comprehension functions. On the one hand, this is consistent with other studies that indicate no relationship between language lateralization and interhemispheric asymmetries of white matter (Ocklenburg et al, 2016) and grey matter (Keller et al, 2010) structure, suggesting that the lateralization of language functions may not have a gross morphological basis.

Topic Areas: Language Development/Acquisition, Computational Approaches