Figure 2.

Schematic illustrations of the sulcal contours defining the frontal operculum in the human (left) and chimpanzee (right) brain (not to scale). The top row indicates the typical sulcal configurations, and the bottom row indicates the variability of the sulcal contours based on the data reported by Keller et al. (2007b) in humans and Sherwood et al. (2003) in chimpanzees. Arrows on the bottom row indicate the “deviation” from the typical configuration: single arrow, presence of a new sulcus (ds/ipcs); double arrow, discontinuity of a sulcus (ifs); triple arrow, presence of connection of rami (ar/hr); dashed arrow, presence of connection of sulci (ipcs/ifs). The shaded area indicates the frontal operculum in both species. The grainy shaded area in the bottom right illustration indicates the possible extension of the operculum when an additional segment of the inferior precentral sulcus is present. ar, Anterior ascending ramus of the Sylvian fissure; cs, central sulcus; ds, diagonal sulcus; fos, fronto-orbital sulcus; ifs, inferior frontal sulcus; ipcs, inferior precentral sulcus.