2.4. CST and its subfiber parcellation

In terms of anatomy, the origins of CST fibers include four cortical areas, namely, M1, PMC, SMA, and S1. ^{26 , 27 , 35} In this study, the bilateral CSTs and their subfibers (i.e., M1, PMC, SMA, and S1 fibers) were retrieved from a recent study whose authors reconstructed the fine maps of CSTs and their subfibers using data from 50 healthy adults from the Human Connectome Project (http://www.humanconnectomeproject.org/). ³¹ A maximum probability map was created with a population probability of ≥50% for each CST subfiber originating from the M1, PMC, S1, and SMA (Figure 1). Besides, a mask for the whole CST for each side with a population probability of >50% was also generated. Finally, the averaged diffusion metrics within each CST and CST subfiber for each subject were calculated from the unsmoothed normalized diffusion maps for region of interest (ROI)‐wise statistics. These parcellation maps were also used as masks for voxel‐wise statistics.

The maximum probability maps of the CST subfibers. (A) Cortical regions of interest for reconstructing CST subfibers, including M1 (red), PMC (green), S1 (blue), and SMA (yellow) fibers. (B) The CST subfibers (red, M1 fibers; green, PMC fibers; blue, S1 fibers; and yellow, SMA fibers) are reconstructed with 50% probability. CST, corticospinal tract; L, left; M1, primary motor area; PMC, premotor cortex; R, right; S1, primary sensory area; SMA, supplementary motor area.