Anatomical subdivisions of the frontal lobe in the great apes

Major components of the frontal lobe like the dorsal, mesial and orbital sectors are associated with distinct cognitive attributes. It is here hypothesized that some species-specific adaptations in the behavior of the great apes may be reflected in the relative size of these sectors including the relative size of their gray and white matter. MR scans of the living brain of three bonobos, six chimpanzees, two gorillas and four orangutans were used to estimate the absolute and relative volumes of a) the frontal cortex (FC) and its immediately underlying white matter (WM), and b) the dorsal, mesial and orbital cortices of FC and the white matter underlying each of these cortical regions. Across species the dorsal sector has the lowest ratio of cortex to white matter, followed by the mesial and orbital sectors. The dorsal cortex occupies the largest part of FC (55% to 60%) and the mesial cortex makes up 26%-30% across the great ape species. The orbital cortex forms 15% to 17% of FC in the African apes, but only 10% of FC in the orangutans. Orangutan individual relative values of the orbital cortex do not overlap with individual values of the rest of the apes. The mean ratio of FC to WM is larger in gorillas and orangutans. Individual values overlap across the hominoid species, but most bonobos and chimpanzees have a lower FC to WM ratio than other individual apes. The orbital sector of the frontal lobe is involved in emotional responses to social stimuli and the present results confirm our earlier observations regarding the small size of this part of the brain in orangutans. The present findings also suggest that bonobos and chimpanzees may have increased interconnectivity between closely related areas in the frontal lobe.