Duan and colleagues developed a new computational method for automatic detection of patterns of cortical folding in large datasets. This method extracts multiple features that characterize the folding patterns, such as sulcal bottoms and gyral crest curvatures. Then, an overall similarity matrix is calculated that contains information on shared patterns and individual variation. Finally, the subjects are clustered on groups that represent a common folding pattern. The authors show their method is more efficient than previous ones in detecting folding patterns and clustering subjects into affinity groups. They validate its reproducibility and reliability in two main samples. They demonstrate the application of their methodology on a large sample of 595 healthy neonate brains, to characterize folding patterns in newborns. Then, they compare their results to a dataset of adult brains from the Human Connectome Project. They focus on four cortical regions, the superior temporal gyrus (STG), the inferior frontal gyrus (IFG), the cingulate cortex, and the precuneus, considering both sex differences and hemispheric asymmetries. Overall, the typical folding patterns of infants were consistent with those of adults, evidencing that cortical folds are largely established from an early age. On the other hand, some differences were also identified. For instance, four folding patterns were recognized in infant STG, while adults have an extra pattern. In contrast, one of the neonates’ IFG pattern is absent in adults. In both samples, there are sex differences in the proportions of some of the folding patterns of STG, IFG, and cingulate cortex. Hemispheric asymmetries were observed in the cingulate and STG, being more significant in the latter, which the authors suggest might reflect language lateralization. Considering the precuneus, their method revealed three main gyral patterns which were not associated with sex differences or hemispheric asymmetries. These gyral patterns appear to be mainly grouped based on the presence of either a gyral structure (patterns 1 and 2, more frequent) or a deep sulcus (pattern 3) in the middle of the precuneus. According to the authors, these groups are similar to the ones described in a previous study by our lab on a sample of adult healthy brains.
We have published one more paper on the morphology of the precuneus, this time featuring a sample of non-human primates, in collaboration with James Rilling and Todd Preuss from the Emory University (Atlanta, USA). Modern humans have a much larger precuneus than chimpanzees both in absolute and relative size. Taking into account the large brain size in our species, we investigated the midsagittal morphology in non-human primates as to test whether precuneus proportions are influenced by allometric factors. We did a geometric morphometric analysis on a total of 42 MRIs from the National chimpanzee brain resource database, including 5 species of apes and 4 species of monkeys. A first analysis, conducted on the species averages, showed that the main pattern of midsagittal variation involves the general shape of the braincase, which might be due to cranial constraints rather than to changes in proportions of specific brain regions. This main shape pattern separates monkeys from apes, as the former display flatter, elongated brains (with capuchins being the flattest), while the latter exhibit rounder brains with frontal bulging (especially orangutans). This morphological variation correlates with brain size, except for gorillas (which brain is large but elongated), and gibbons (which have smaller but round brains). A second analysis was conducted only on chimpanzees and macaques, to compare two species with different brain size. In neither case the proportions of the precuneus displayed major differences between species or size-related changes. However, as in humans, precuneus size is very variable within each species, suggesting a remarkable plasticity. Overall, the results suggest that precuneus expansion in modern humans is a species-specific characteristic of our species, rather than a simple consequence of increase in brain size. Further studies should address the histological and functional processes involved in this morphological change.
This month we have published a study featuring the cortical extension and anatomy of the precuneus, dealing with its metrics as well as with its sulcal pattern. The analysis was based on a MRI sample of 50 adult humans from both sexes. Our previous works concerned the variation, position, and surface, as well as the phylogenetic differences in the midsagittal plane. Instead, in this survey metrics was assessed on the coronal plane, “cutting” the precuneus in its anterior, middle and posterior sections, taking into account its curvature. The lateral (inner) extension of the precuneus was measured along the subparietal sulcus and its height was measured from the subparietal sulcus to the endocranial wall. Then a set of 10 two dimensional landmarks were digitized on the middle section along the outline of the parietal lobe, to analyze the correlation between outer brain profile and inner precuneus dimensions. We found that, on average, the precuneus extends 14 mm laterally and 36 mm vertically. It is wider on the anterior and middle sections, and usually larger on the right hemisphere, possibly due to the length of the fold (surface area) rather than to the thickness of the grey matter. The precuneus height influences the outer brain morphology (vertical stretching), but the subparietal size apparently has no influence on the external outline. Therefore, at least the former trait could be investigated in paleoneurology, by indirect inference on the inner dimensions as evaluated through their external effects. The lateral (parasagittal) surface of the upper parietal lobules seems to be unrelated to the size of the precuneus. Therefore, when a change of these areas is detected (like in Neandertals) the intraparietal sulcus is a better candidate as the cortical element involved in the morphological variations.
The sulcal pattern was analyzed on the average density projection of the 5 most sagittal stacks (5 mm of the cortex), a thickness which displays most of the sulcal features. Three characteristics were taken into consideration: the connections of the subparietal sulcus, the connections of other sulci in the precuneus, and the general sulcal scheme. Some of these features have been analyzed in other surveys, but the consideration of other folds beyond the subparietal one is specific of this study. Roughly half of the precuneus sulci that reach the external surface are not linked to the subparietal sulcus. Contrary to other studies which found higher frequencies of an H-like pattern of the subparietal sulcus, we found a larger proportion of an inverted-T pattern (subparietal sulcus connected with one precuneus sulcus). The differences between studies might be due to random effects of the samples, because of the relevant individual variability of these traits. The left hemisphere displays more sulci reaching the external surface while the right hemisphere displays deeper folding. The anterior area shows more sulcal complexity than the posterior one. There seems to be no relationship between the size of the subparietal sulcus and its folding pattern, and these characteristics might be hence influenced by genetics or folding biomechanics.
Evidence for expansion of the precuneus in human evolution
Emiliano Bruner, Todd M. Preuss, Xu Chen, James K. Rilling
Brain Structure and Function 2016
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“Journal of Anatomy Best Paper Prize 2014” to Bruner E., Rangel de Lázaro G., de la Cuétara JM., Martín-Loeches M., Colom R. & Jacobs HIL. 2014. Midsagittal brain variation and MRI shape analysis of the precuneus in adult individuals, Journal of Anatomy, Volume 224, Issue 4, April 2014, pp 367-376, as the most outstanding article published during 2014! The prize is awarded by the Anatomical Society.