Dear Rudolph Pienaar, could you please tell me if the Folding index and the sulc are in a way correlated? thank you
Hi David -
This question does not really have a simple yes or no answer, mostly because each measures different features. These features are both derived from the topology of the surface, but are not directly related.
The 'sulc' measurement is the distance that a specific vertex is away from a hypothetical "mid-surface" that exists between the gyri and the sulci. This surface is chosen so that the "mean" of all the displacements is zero. The 'sulc' gives a indication then of linear *distance* and displacements: how "deep" and how "high" are the folds, but doesn't say much about how "sharply" folded they are, or in what manner are they folded (like cylinders, like spheres, etc).
The FI is a *curvature* (not distance) measure. It uses the k1 and k2 principal curvatures at each vertex to calculate a value according to (per Van Essen/Drury 1997):
FI_i = fabs(k1) * (fabs(k1) - fabs(k2));
Conceptually the FI tries to give a measure of how (relatively) tightly or sharply folded a gyrus is in a cylindrical sense. The more cylindrical a gyrus is, the more the FI increases along its length. For regions with spherical or saddle folding, k1 = k2 and the FI is zero. So it tells us more about how much like cylinders the folds are, but tell us nothing about how "deep" the folds extend.
Essentially, the 'sulc' and the FI are related of course by the topology of the surface but they each measure conceptually different things. It is not possible to determine 'sulc' from the FI or vice versa.