Figure 3. CST analysis using tractography.

(A) Illustration of crossed (blue) and uncrossed (red) CSTs. Images show the ROI used to reconstruct the CST at the base of the pontine nuclei (i), the anterior pyramid in the upper medulla (ii), and the crossed lateral (yellow) funiculus of the upper cervical cord (iii). For example, the crossed CST from the right primary motor cortex (M1) to the left upper cervical cord was reconstructed excluding fibers reaching the left medial and the right lateral and medial funiculus (red). (B) Tractography of the CST (same color coding as in A) superimposed on the individual FA color map (sagittal views on the left, coronal views in the middle, zoom of the tracts inferior to the decussation on the right) of a control subject and an NTN1 patient (NTN1-1.9). The CST laterality coefficient, expressed as (NF crossed – NF uncrossed)/(NF crossed + NF uncrossed), was positive for the control (0.93, indicating more connections in the crossed CST) and negative for the NTN1 patient (–0.94, indicating more connections in the uncrossed CST). (C) The CST laterality coefficients of 20 control subjects were compared with that of the NTN1-1.9 patient (Crawford-Howell t test, **P = 0.001). (D) Mean FA along the crossed CST. The mean tract from the upper brainstem to the funiculus of the upper spinal cord is represented in yellow and is superimposed on the coronal view of the FA map of the control subject. Note that the z axis displays the anatomical correspondence between the coronal view (left) and the graph of the mean FA values (right), indicating that the pyramidal decussation occurs between 17<z<25. Mean FA values increased for the NTN1-1.9 patient (diamonds) compared with values for the control subject (squares) at the level of the pyramidal decussation. Error bars represent standard errors.