Divergent Connectional Asymmetries of the Inferior Parietal Lobule Shape Hemispheric Specialization in Humans, Chimpanzees, and Macaque Monkeys

The inferior parietal lobule (IPL) is one of the most expanded and structurally and functionally asymmetric regions in the human cerebral cortex. Whether the structural and connectional asymmetries of IPL subdivisions differ across primate species and whether this relates to functional asymmetries remain unclear. We identified IPL subregions that exhibited symmetric positive allometric scaling across macaque monkeys, chimpanzees, and humans. Patterns of IPL subregions asymmetry were similar in chimpanzees and humans, whereas no IPL asymmetries were evident in macaques. Among the comparative sample of primates, humans showed the most widespread asymmetric connections in the frontal, parietal, and temporal cortices, constituting leftward asymmetric networks that may provide an anatomical basis for language and tool use. Unique human asymmetric connectivity between the IPL and the primary motor cortex may be related to handedness. These findings suggest that structural and connectional asymmetries may underlie hemispheric specialization of the human brain.