Planum temporale grey matter asymmetries in chimpanzees (Pan troglodytes), vervet (Chlorocebus aethiops sabaeus), rhesus (Macaca mulatta) and bonnet (Macaca radiata) monkeys

Abstract

Brain asymmetries, particularly asymmetries within regions associated with language, have been suggested as a key difference between humans and our nearest ancestors. These regions include the planum temporale (PT) - the bank of tissue that lies posterior to Heschl’s gyrus and encompasses Wernicke’s area, an important brain region involved in language and speech in the human brain. In the human brain, both the surface area and grey matter volume of the PT is larger in the left compared to right hemisphere, particularly among right-handed individuals. Here we compared the grey matter volume and asymmetry of the PT in chimpanzees and three other species of nonhuman primate in two Genera including vervet monkeys (Chlorocebus aethiops sabaeus), rhesus macaques (Macaca mulatta) and bonnet macaques (Macaca radiata). We show that the three monkey species do not show population-level asymmetries in this region whereas the chimpanzees do, suggesting that the evolutionary brain development that gave rise to PT asymmetry occurred after our split with the monkey species, but before our split with the chimpanzees.

Sex differences in the relationship between planum temporale asymmetry and corpus callosum morphology in chimpanzees (Pan troglodytes): A combined MRI and DTI analysis

“…The evidence of robust leftward bias in the PT suggests that this particular dimension of brain asymmetry has a strong evolutionary foundation in humans and chimpanzees (Hopkins, 2013). Previous studies suggest that population-level asymmetries in the surface area of the PT are not present in more distantly related Old World monkey primates (Gannon et al, 2008; Heilbronner and Holloway, 1988, 1989; Lyn et al, 2011) and therefore this may be recently evolved trait.…”

Asymmetry of the Brain: Development and Implications

“…An asymmetry in the PT has been observed in chimpanzees, macaques, and vervet monkeys, although in these last two species, sidedness is not consistent at the population level. (103). Rodent brains are also asymmetrical at various structural levels, although here again orientation of these asymmetries is subject to variation depending on age, gender, species, and environmental factors (24,46,84,150).…”

On the other hand: including left-handers in cognitive neuroscience and neurogenetics

“…Moreover, chimpanzees exhibit population-level asymmetries of some brain structures that are homologous to human brain structures that show asymmetry, including the planum temporale (a part of the temporal cerebral cortex that overlaps with Wernicke's classically defined language region) [28][29][30] . The marked population-level bias in human handedness compared with that in chimpanzee handedness together with a modest association between left-handedness and atypical language dominance in humans (see below) have contributed to the hypothesis that language had its evolutionary origins in manual gesture 31,32 .…”