Structural hemispheric asymmetries in the human precentral gyrus hand representation.

The superior region of the precentral gyrus (preCG) is known to be actively involved with hand function and has been proposed as a possible neural correlate of handedness. To test this hypothesis, we used a combined voxel-based morphometric (VBM) asymmetry analysis of structural MRI, along with diffusion MRI (dMRI) tractography to investigate laterality indices of corticomotor white matter (WM) pathways, based on measures of fractional anisotropy (FA). The relationship between measures of motor performance and FA laterality indices was also investigated. In a cohort of 14 right-handed healthy participants, the VBM asymmetry analysis revealed an area within the preCG associated with hand representation. The tractography analysis revealed that this region possessed a number of major WM intrahemispheric connections to the brain stem, thalamus, cerebellum, postcentral, caudal middle and superior frontal, and superior and inferior parietal corticomotor regions. Within the corticospinal tracts, we found FA was significantly higher in the left hemisphere compared with the right. Furthermore, significant correlations were found between FA asymmetry measures projecting from this region, namely corticospinal tracts and those connecting the postcentral gyri, with grip strength and finger-tapping performance, respectively. A number of the motor pathways projecting from this region also exhibited leftward asymmetry of FA distributions. The findings from this study highlight the role of the left motor cortex in skilled motor performance and provide a framework for the study of the relationship between handedness and preCG hand representation in larger normative populations.