Figure 3.

Illustration of the three basis vectors defining the gyral coordinate system. The radial axis in red aligns with the normal to the sulcal wall and points along the short axis of the gyrus. The sulcal axis in green aligns with the sulcal depth gradient and tends to point from deep white matter to the gyral crown. The gyral axis in blue is orthogonal to the sulcal axis to define the tangential plane. The arrow on the left panel points to a white matter voxel underlying the gyral crown, which illustrates that even close to the gyral crown the gyral coordinates tend to be defined by the sulcal walls rather than crown (e.g., the radial orientation aligns with the sulcal wall normal). Note that because the gyral coordinates rely fully on the orientation of the WM/GM boundary (white) and the pial surface (cyan), it can be very sensitive to errors in reconstructing these surfaces (e.g., around the “dimple” in the pial surface at the gyral crown in the left panel).