Corpus callosum morphology does not depend on hand preference or hemispheric dominance for language

It is traditionally assumed that the corpus callosum has a pivotal role in supporting hemispheric lateralisation, which originated from a series of studies suggesting differences in callosal morphology in relation to handedness. However, recent systematic reviews document that the callosal differences are only inconsistently reported, and it has been speculated that these inconsistencies might arise from focussing on handedness alone, without considering other lateralized functional modules. To address this short-coming, the present pre-registered study was designed to re-examine possible effects on callosal morphology while considering hand preference in interaction with hemispheric dominance for language. It was predicted that only those individuals who write with the hand ipsilateral to their language dominant hemisphere, have an increased need for interhemispheric integration that is reflected in detectable alteration to callosal morphology. That is, individual writing with the left hand (LW) while being left hemispheric dominant for language (LLD) are predicted to have a larger or thicker corpus callosum than individuals in which hand motor and language production are controlled by the same hemisphere. We tested this prediction by comparing the corpus callosum between the three common groups that result when combing the preferred writing hand (LW vs. right writers, RW) and the hemisphere dominant for language processing. For this purpose, language dominance (LLD vs. right dominance, RLD) was determined using a verbal-fluency task in functional magnetic resonance (fMRI) that has been previously validated. The study included N = 220 participants of both sexes, of which 97 were classified as LW/LLD, 73 as RW/LLD, and 50 as LW/RLD. The morphology of the corpus callosum was assessed on T1-weighted structural MR images as midsagittal surface area (subdivided into the three subregions genu, truncus, posterior third) as well as regional thickness (at 100 measuring points). The statistical analyses did not reveal any evidence to support our predictions and our sample size provides sufficient test power to rule out comparatively small effects with reasonable confidence. Thus, the midsagittal corpus callosum appears not substantially affected by the supposed increased requirement for interhemispheric integration in LW/LLD as compared with RW/LLD and LW/RLD individuals.