Figure 2.
Steps used to calculate sulcal depth. The white and pial surfaces were averaged to create an intermediate surface that ran halfway through the cortical ribbon (the result is show in the top left). Multiple dilation and erosion operations were then applied to this surface to obtain a model of the cerebral hull (top middle). The geodesic distance from the hull to each point on the intermediate surface was measured (in mm) to produce a depth map of the entire surface [top right; see Van Essen, 2005, for more details]. Inclusive borders were delineated along the gyral crowns abutting the cingulate, paracingulate, and superior rostral sulci (CS, PCS, and SRS, respectively; bottom left). Beginning at the hull (i.e., 0 mm) and progressing deeper in increments of 0.5 mm, a threshold was set on the depth map of each individual to create a mask that comprised only buried cortex (i.e., cortex on gyral crowns was excluded; bottom middle). The thresholded mask was then applied to the inclusive sulcal traces to yield regions comprising only cortex buried in each sulcus (bottom right). In this regard, our depth measures are similar in principle to those described by Rettmann et al. [2006].