Precuneus proportions and cortical folding: A morphometric evaluation on a racially diverse human sample.

Recent analyses have suggested that the size and proportions of the precuneus are remarkably variable among adult humans, representing a major source of geometrical difference in midsagittal brain morphology. The same area also represents the main midsagittal brain difference between humans and chimpanzees, being more expanded in our species. Enlargement of the upper parietal surface is a specific feature of Homo sapiens, when compared with other fossil hominids, suggesting the involvement of these cortical areas in recent modern human evolution. Here, we provide a survey on midsagittal brain morphology by investigating whether precuneus size represents the largest component of variance within a larger and racially diverse sample of 265 adult humans. Additionally, we investigate the relationship between precuneus shape variation and folding patterns. Precuneus proportions are confirmed to be a major source of human brain variation even when racial variability is considered. Larger precuneus size is associated with additional precuneal gyri, generally in its anterior district. Spatial variation is most pronounced in the dorsal areas, with no apparent differences between hemispheres, between sexes, or among different racial groups. These dorsal areas integrate somatic and visual information together with the lateral elements of the parietal cortex, representing a crucial node for self-centered mental imagery. The histological basis and functional significance of this intra-specific variation in the upper precuneus remains to be evaluated.