Corticospinal tract asymmetries at the level of the internal capsule: is there an association with handedness?

The functional integrity of the corticospinal tract (CST) is of crucial importance for fine and independent finger movement. Therefore, it has been frequently speculated that interhemispheric asymmetries in this tract might be associated with handedness. The objective of the present study was to examine this proposed relationship, by studying the effects of handedness on macro- and microstructural properties of the CST. For this purpose, 30 consistently left and 30 consistently right-handed participants of either sex were examined applying a combination of anatomical T1-weighted and diffusion-tensor (DTI) MR imaging. In an automated procedure and based on white matter probability maps as well as on DTI fibre direction information, the left and right hemispheric CST was segmented in the individual brain. Focussing the analysis on the posterior limb of the internal capsule, the size as well as two DTI parameters (mean diffusion and fractional anisotropy) was determined. Although the statistical analysis revealed substantial CST asymmetries regarding size, mean diffusion, and anisotropy, neither of these metrics showed a significant hemisphere-by-handedness interaction. From this it was concluded that handedness might be related to motorcortical asymmetries which seem not to be directly reflected in the corticospinal fibre tract characteristics.