Gray matter asymmetries in chimpanzees as revealed by voxel-based morphometry.

Determination of whether nonhuman primates exhibit neuroanatomical asymmetries would inform our understanding of the evolution of traits in humans that show functional hemispheric dominance, including language and handedness. Here we report the first evidence of population-level asymmetries in the chimpanzee neocortex using voxel-based morphometry (VBM). MRI scans of the brain were collected in a sample of 31 chimpanzees including 9 males and 22 females, and the resulting images were segmented into gray matter, white matter and CSF. Gray matter images were then co-registered to a template and these normally oriented volumes were flipped on the left-right axis to create mirror volumes. In total, significant asymmetries were found in 13 regions including several that have been described previously in great apes using traditional region-of-interest approaches. The results from this VBM analysis support previous reports of hemispheric lateralization in chimpanzees and reinforce the view that asymmetries in the central nervous system are not uniquely human.