LIMBIC FRONTAL-CORTEX IN HOMINOIDS - A COMPARATIVE-STUDY OF AREA 13

The limbic frontal cortex forms part of the neural substrate responsib le for emotional reactions to social stimuli. Area 13 is one of the co rtical areas long known to be part of the posterior orbitofrontal cort ex in several monkey species, such as the macaque. Its presence nevert heless in the human brain has been unclear, and the cortex of the fron tal lobe of the great and lesser apes remains largely unknown. In this study area 13 was identified in human, chimpanzee, bonobo, gorilla, o rangutan, and gibbon brains, and cortical maps were generated on the b asis of its cytoarchitecture. Imaging techniques were used to characte rize and quantify the microstructural organization of the area, and st ereological tools were applied for estimates of the volume of area 13 in all species. Area 13 is conservative in its structure, and features such as size of cortical layers, density of neurons, and space availa ble for connections are similar across hominoids with only subtle diff erences present. In contrast to the homogeneity found in its organizat ion, variation is present in the relative size of this cortical area ( as a percentage of total brain volume). The human and the bonobo inclu de a complex orbitofrontal cortex and a relatively smaller area 13. On the contrary the orangutan stands out by having a shorter orbitofront al region and a more expanded area 13. Differences in the organization and size of individual cortical areas involved in emotional reactions and social behavior can be related to behavioral specializations of e ach hominoid and to the evolution of emotions in hominids. (C) 1998 Wi ley-Liss, Inc.