LAUSR

Hemispheric asymmetries can be seen as one of the evolutionary adaptations that allowed the human brain to muster more complex cognitive processes than other primates. In this vein, the study published by Cheng et al. [1] presents a pivotal investigation of both the regional and connectional asymmetries within the inferior parietal lobule (IPL) in human, chimpanzee, and macaque. By investigating 4 sub-divisions of the IPL across the three species, Cheng and colleagues showed that the macroanatomical and connectional architecture of the IPL became more asymmetric throughout the primate lineage. While macaques show little to no structural asymmetries, chimpanzees display a more asymmetric architecture but with both leftward and rightward asymmetries in various connections. In contrast, the human IPL displayed the highest number of asymmetries among the three species with a clear tendency towards more lateralization. This evolutionary trend towards a more lateralized organization of the IPL may have accompanied an improved command of tool-use, stronger forelimb asymmetries, and the increasing complexity of communicative behavior. The IPL is a part of the primate association cortex that plays an important role in language and tool-use along with a multitude of different functions [2]. Given its functional diversity and heterogeneity, a comprehensive analysis of the IPL’s macroanatomy and connectivity requires investigation on a sub-division level. However, translating a set of sub-regions from one species to another poses a central challenge for comparative research. There are various strategies to solve the issue of inter-species comparison, such as establishing a common feature space between species based on structure and/or functional measures [3], or one can investigate homologous regions and/or features in each species. To accomplish this task, Cheng et al. implemented the latter by using a connectivity-based parcellation approach, which creates sub-divisions based on diffusion-weighted probabilistic tractography. This approach can give several solutions that vary in the number of sub-divisions. The authors choose a 4-cluster solution with divisions that follow a rostral-to-caudal (anterior-posterior) organization in macaques, chimpanzees, and humans. This solution maximizes the similarity across species and is consistent with previously reported anatomical and cytoarchitecture parcellations. Utilizing the IPL subdivisions derived by connectivitybased parcellation, Cheng et al. conducted a thorough exploration of the structural asymmetries within the IPL across the three species. The investigation was centered around the gray matter (GM) volume, probabilistic white matter (WM) connections, and the cortical surface vertices of the WM connections. Regarding the different structural measures, the old-world monkeys (macaques) did not present any asymmetries while the great apes (chimpanzees and humans) showed similar and divergent asymmetrical organization. In great apes, rostral subdivisions were leftward asymmetric and caudal subdivisions were rightward asymmetric. This indicates a switch from a symmetrical to an asymmetrical organization in the IPL in a common ancestor of the great apes and old-world monkeys. Asymmetric WM connection from the IPL sub-divisions & Sam Vickery [email protected]