A metric survey on the sagittal and coronal morphology of the precuneus in adult humans.

The size, cortical surface area, length and height of the precuneus vary greatly in adult humans. The precuneus is also proportionally much larger in humans than in other primates, and its dorsal region is larger in our species compared with extinct hominids. Its longitudinal and vertical proportions can depend on a real cortical expansion, or on a deformation of the whole brain form due to cranial constraints. In this study, we analyse the correlation between its sagittal and coronal morphology to study the variation of its proportions along the three dimensions. Shape analysis was performed on magnetic resonance scans of 50 adult humans, through both traditional and geometric morphometrics. Results suggest that the degrees of longitudinal, vertical and lateral extension are independent, pointing at different factors involved, and concerning different cortical areas or specific connectivity pathways. This absence of correlation suggests that the individual and evolutionary morphological variations are probably due to actual brain changes, and not to general volume deformation constrained by cranial architecture. These results are discussed in the wider frame of homology and phylogenetic differences, and provide indications for further studies and more targeted surveys.