Fibre-specific laterality of white matter in left and right language dominant people

Despite the typical symmetrical appearance of the human brain, several functional and structural asymmetries have been reported. Language is the most commonly described lateralised cognitive function, relying relatively more on the left hemisphere in over 90% of the population. This is in line with white matter studies which have revealed leftwards lateralisation of the arcuate fasciculus, a white matter tract that connects important language-related regions. Most research to date examining the structure-function relationship of language lateralisation only included people showing a left language hemisphere dominance. As such, the reported correlations do not allow for inferences of relationships between the directions of functional and structural lateralisation of language. In this work, we applied a state-of-the-art "fixel-based" analysis approach, which allows to statistically analyse white matter micro- and macrostructure on a fibre-specific level. To study lateralisation using this framework, we defined a bespoke fibre-specific laterality index which allowed us to examine whole-brain white matter asymmetries in samples of participants with left and right language dominance (LLD and RLD respectively). Both groups showed similar extensive and intricate patterns of significant white matter lateralisation. Few group differences were found between both groups, with a similar leftwards lateralisation of the arcuate fasciculus, regardless of functional language lateralisation of the participants. A significant group difference of lateralisation was detected in the forceps minor, with a leftwards lateralisation in LLD and rightwards lateralisation for the RLD group. In conclusion, we showed that fixel-based analysis of fibre-specific lateralisation indices is an effective approach to study white matter asymmetries. Our results suggest that the lateralisation of language functioning and the arcuate fasciculus are driven by independent biases. The exact relationship between forceps minor asymmetry and language dominance could be an interesting subject of future studies.