Figure 2.

(a) Mapping Jacobian matrices to the GM and WM surface. The Jacobian matrices are computed from the deformation fields used to spatially normalize all the images. The changes on the cortical gray matter (GM) and the underlying white matter (WM) are transferred onto the boundary surface between the two tissue labels, creating a surface map of the changes on each tissue compartment. (b) 2D schematic representation of the hypothesized thickness changes that occur in each tissue compartment as cortical folding progresses. The starting point illustrated is smooth cortex in which the layers are parallel. The next line drawing shows the transformation to emerging gyri. The thickness of the GM is relatively stable at the gyral crests, and may be deformed positively or negatively along the fundus and pit of a sulcus, which is shown by the nonparallel boundaries. The underlying WM near the cortical boundary (thin gray line) does not have analogous thickness changes (in contrast to surface area change). Rather two processes are simultaneously occurring: cortical folding and regression of the subplate. Consequently, WM appears compressed at sulcal pits and maintains depth at gyri. [Color figure can be viewed at http://wileyonlinelibrary.com]