Fig. 1.

Left: examples of two geometrically different topological defects that are topologically equivalent: a handle that bridges a sulcus, and a hole in the bank of a gyrus. Right: a graphical example of the difficulty of using surface deformation techniques to model the cortical surface. Typically we want a smooth surface, but much of the cortical surface is buried deep inside folds forcing surfaces to pass through regions (indicated by the blue arrow) where the evolving surface has to bunch up to get enough surface area inside the fold to model the surface. Another problem is finding energy terms that will draw the surface into the deep fissure, and away from the narrow opening, which also means pulling it away from the true cortical surface to traverse the sulcal opening and arrive at the boundary on the other side.