Reproducibility of leftward planum temporale asymmetries in two genetically isolated populations of chimpanzees ( Pan troglodytes )

Abstract

Once considered a hallmark of human uniqueness, brain asymmetry has emerged as a feature shared with several other species, including chimpanzees, one of our closest living relatives. Most notable has been the discovery of asymmetries in homologues of cortical language areas in apes, particularly in the planum temporale (PT), considered a central node of the human language network. Several lines of evidence indicate a role for genetic mechanisms in the emergence of PT asymmetry; however, the genetic determinants of cerebral asymmetries have remained elusive. Studies in humans suggest that there is heritability of brain asymmetries of the PT, but this has not been explored to any extent in chimpanzees. Furthermore, the potential influence of non-genetic factors has raised questions about the reproducibility of earlier observations of PT asymmetry reported in chimpanzees. As such, the present study was aimed at examining both the heritability of phenotypic asymmetries in PT morphology, as well as their reproducibility. Using magnetic resonance imaging, we evaluated morphological asymmetries of PT surface area (mm 2 ) and mean depth (mm) in captive chimpanzees ( n = 291) derived from two genetically isolated populations. Our results confirm that chimpanzees exhibit a significant population-level leftward asymmetry for PT surface area, as well as significant heritability in the surface area and mean depth of the PT. These results conclusively demonstrate the existence of a leftward bias in PT asymmetry in chimpanzees and suggest that genetic mechanisms play a key role in the emergence of anatomical asymmetry in this region.

Asymmetry of cortical functional hierarchy in humans and macaques suggests phylogenetic conservation and adaptation

“…Moreover, by comparing humans, macaques, and chimpanzees, 45 report evolutionary modifications to this tract in humans relative to other primates, possibly derived from auditory pathways 71 . At the same time, other structural studies have also observed leftward asymmetry of language areas in chimpanzees, indicating that asymmetry of language-regions per se may not be a humanspecific feature 46,72 . Fittingly, there are no significant differences of thickness and area asymmetry between humans and chimpanzees in superior temporal lobe 72 .…”

“…As inter-hemispheric asymmetry has been observed consistently in human brain structure and function, there may be important (phylo)genetic factors supporting lateralized human cognition 2,[39][40][41][42][43] . Previous work has reported that brain structure asymmetry is heritable 1,3 , especially in the language areas, and differentiates between humans and non-human primates [44][45][46][47] . At the same time, it has been shown that both humans and apes show asymmetry of brain shape 47 , indicating that asymmetry is not a uniquely human brain feature.…”

How Asymmetries Evolved: Hearts, Brains, and Molecules

“…In humans, the planum temporale overlaps with Wernicke's area, one of the major language areas, and is larger on the left than on the right [53], but the same asymmetry is present in great apes [54][55][56], and in both adult [57] and infant baboons [58]. This asymmetry may therefore date back at least to the common ancestor of humans, great apes and Old World monkeys, 30-40 million years ago, and is not specifically connected to language.…”

Are We Right about the Right TPJ? A Review of Brain Stimulation and Social Cognition in the Right Temporal Parietal Junction

“…While there are debates among the definitions, we defer to a broader (i.e., greater area) inclusionary definition. There are asymmetrical anatomical differences (i.e., hemispheric), one of the most notable is the planum temporale, in which we and others have discovered significant structural lateralized differences [31][32][33][34][35][36][37], which lead to significant functional differences in non-social domains [38][39][40]. For example, lesions of the lTPJ (left TPJ) typically lead to aphasias and verbal language disruption [41], visual-spatial neglect, however, is a common outcome of rTPJ damage [42,43].…”