The rodent PFC was defined as a combination of the orbital frontal regions, the prelimbic areas, the secondary motor cortex, and area 24 ( 27, 28). The primate PFC was defined as a combination of the medial PFC, the dorsolateral PFC, and the ventrolateral PFC. The PFC and the cingulate were added as intermediate level 3 of segmentation. The frontal lobe was divided into the orbital frontal cortex (gustatory cortex, orbital proisocortex and preiallocortex), the medial PFC (BA 32, 14, 24, 25), the dorsolateral PFC (BA 10, 9, 46, 8, 55, 33, 9/46) and the ventrolateral PFC (BA 47, 45, 44, the precentral opercular area, and the inferior frontal sulcus). Results Developing a Method of Comparison across Atlases. We ultimately revealed that the frontal and parietal lobes were both disproportionately expanded in catarrhini compared to other mammalian species. We then compared all possible combinations of region volume pairs with prediction interval deviation matrices (PIDMs), a method that we developed in order to analyze large datasets based on phylogenetic generalized least squares (PGLS). We identified 20 3D digital brain atlases of 18 mammalian species and adapted each of them into a common anatomical classification to cover the whole brain. The segmentations of these atlases were often based on cytoarchitecture, as well as a variety of magnetic resonance imaging (MRI) techniques, including functional MRI, diffusion tensor imaging, and MRI contrast ( 18– 20). However, three-dimensional (3D) atlases of various species (humans, macaques, mouse lemurs, mice, etc.) have been recently published to digitally reveal histological and functional boundaries ( 14– 17). This presented a challenge, as traditional histological volume quantification in the whole brain is currently impractical. Our approach lay in using homologous segmentation of brain areas to compare brain volumes across mammalian species. Nevertheless, humans remain unique in carrying the most relatively enlarged frontal and parietal lobes in an infraorder exhibiting a disproportionate expansion of these areas. Our findings suggest that the human frontal and parietal lobes are not disproportionately enlarged when compared to other catarrhini. Across all species, expansion of the frontal lobe appeared to be systematically linked to the parietal lobe. Within the catarrhini’s frontal lobe, the prefrontal cortex was the principal driver of frontal expansion. Frontal and parietal slopes were statistically different in catarrhini when compared to other species through bootstrap analysis. Cerebral lobe volumes slopes plotted for catarrhini species were ranked as parietal∼frontal > temporal > occipital, contrasting with the ranking of other mammalian species (occipital > temporal > frontal∼parietal). Our main finding suggests a divergence in primate evolution, orienting the stereotypical mammalian cerebral proportion toward a frontal and parietal lobe expansion in catarrhini (primate parvorder comprising old world monkeys, apes, and humans). Our approach used data-driven analysis based on phylogenetic generalized least squares to evaluate anatomical expansion covering the whole brain. We therefore sought to address this gap by adapting anatomical areas from the digital atlases of 18 mammalian species, to create a common interspecies classification. This question has left the study of over/under-expansion in other structures relatively unexplored. Whether the size of the prefrontal cortex (PFC) in humans is disproportionate when compared to other species is a persistent debate in evolutionary neuroscience.
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