Measuring natural hemispheric asymmetry in the brain: quantifying the Yakovlevian torque phenomenon

Abstract: Background: Just by a cursory examination, the two hemispheres of the human brain may appear as mirror images of one another. Yet, the left and right sides exhibit profound differences in anatomy. One of the most obvious expressions of hemispheric asymmetry is the counterclockwise rotation of the brain known as “Yakovlevian torque”. Objective: To measure natural hemispheric asymmetry in the brain by studying the involvement of individual regions in the Yakovlevian torque. Method: Asymmetry was studied on T1-weighted whole-brain atlases of 285 healthy study participants, each labeling 83 anatomical structures. Three techniques were employed: visual scoring, ordinary volumetric asymmetry, and an advanced registration-based technique proposed by Martinez-Torteya et al. [2019]. Results: Regionally specific differences between the two hemispheres were ev ident in all investigated regions, with particularly large asymmetry indices found for the temporal horn of the lateral ventricle, the pre subgenual frontal cortex, and the lateral orbital gyrus. Asymmetry indices obtained from the registration-based measure were higher than the volume measures for 78% of the region pairs. The visual scoring corresponded well with these results but was possibly confounded by differences between the data sets. Conclusion: This study illustrates the distribution of structural asymmetries in the healthy human brain. Automatic quantification of the brain torque was proven more challenging than anticipated, as the investigation did not lead to a suitable method. Moreover, volumetric assessment of asymmetry should be complemented with an index that is also sen sitive to shape