Regional differences in cerebral asymmetries of human cortical white matter.

The form of the structural asymmetries across the cerebral hemispheres, that support well-established functional asymmetries, are not well understood. Although, many previous studies have investigated structural differences in areas associated with strong functional asymmetries, such as language processes, regions of the brain with less well established functional laterality have received less attention. The current study aims to address this by exploring global white matter asymmetries of the healthy human brain using diffusion tensor imaging (DTI) and tractography. DTI was conducted on twenty-nine healthy right-handed males, and pathways from the four major lobes were reconstructed using probabilistic tractography. Mean FA, parallel and perpendicular diffusion values were calculated and compared across hemispheres for each pathway generated. Significant asymmetries in the parietal (rightward asymmetry) and occipital (leftward asymmetry) pathways were found in FA measures. However, asymmetric patterns in parallel and/or perpendicular diffusion were observed in all four lobes, even in pathways with symmetrical FA. For instance, significant rightward asymmetry in parallel diffusion was found in the parietal and frontal lobes, whereas significant leftward asymmetry was found in the temporal and occipital lobes. We suggest that these different patterns of diffusion asymmetry reflect differences in microanatomy that support the known patterns of differential functional asymmetry. The different directions of anatomical asymmetry support the notion that there may be a number of different lateralising influences operating in the brain.