Hand movement fMRI-lateralization and white matter motor tract quantification for handedness groups.

Handedness is the preference for one upper limb side generalized across different sensorimotor skills and strong population-level right-handedness is especially pronounced in humans. The lateralization of functional representations in the primary motor cortex for unilateral upper limb movements depends on task complexity (e.g., simple tapping movements versus sequential finger movements) and brain morphology (e.g., quantification of the pyramidal tract and transcallosal connectivity). Previous studies investigating lateralization differences between right- and left-handed individuals did not consider these two main drivers of lateralization. Therefore, we investigated the functional lateralization of upper limb movements of varying complexity, alongside white matter quantification, in 26 right-handed and 26 left-handed healthy volunteers. We were particularly interested in possible differences in functional magnetic resonance imaging (fMRI) lateralization between the groups, as well as possible associations between white matter tract quantification and fMRI activation. Our study replicated prior findings on the impact of increasing demand on decreasing lateralization of the primary motor cortex (M1). White matter quantification of the corticospinal tract and the callosal fibers originating in M1 were comparable for both groups and were not associated with performance or fMRI-lateralization. For simple movements, fMRI lateralization between handedness groups was comparable, independent of the hand used (dominant or non-dominant). However, for more complex movements of the non-dominant hand, ipsilateral activation was stronger in right-handers. This finding supports the model for a possibly higher need to suppress the dominant hemisphere from executing mirror movements in right-handed participants.