Precuneus

Bruner et al JA2014Paleoneurological studies based on endocranial geometry suggested that a spatial dilation of the deep parietal areas was the major morphological difference between modern and non modern human brains. In our species, the morphogenetic change associated with this parietal bulging was then localized in a very early post-natal period, in a  stage which is absent in chimpanzees or in Neandertals. In the meanwhile the deep parietal areas were demonstrated to have also special cytoarchitectonic elements in modern humans, to be the main functional and structural node of the human brain organization, to be critically involved in major cognitive capacities through the fronto-parietal connections, to be central to the default mode network, and to be essential in human-specific cognitive processes involving imagination and simulation. Such specific parietal modifications have been also tentatively associated with species-specific vulnerability to neurodegeneration in our species. Actually, the early stages of Alzheimer’s disease are associated with metabolic, functional and structural impairments at the deep parietal areas, like the precuneus. These brain districts have been scarcely studied in term of morphology because of their difficult position, multifunctional roles, and blurred anatomical boundaries. Through a MRI shape analysis of adult human brains we have now identified the main character associated with individual brain variation in our species: the geometry of the precuneus. With a negligible effect of brain size or sex, the proportions of the precuneus are the main determinant of the midsagittal brain geometry. The brain morphological variation of the human genus and the brain morphological variation among adult modern humans share the same pattern: parietal bulging. And, at least for modern humans, this pattern is strictly determined by one single character: the longitudinal extension of the precuneal area. Evolutionary and functional evidence both converge toward the neural element which is at the same time the most variable at intra-specific level, strongly influencing our brain form. Many coincidences, which may be the result of the delicate spatial position of the deep parietal areas in the overall brain geometry. Or may there be more than this?

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