Posts Tagged 'shape analysis'

Surfaces

beaudet-et-al-jhe2016Amélie Beaudet and colleagues have published a comprehensive and detailed paleoneurological study on South African fossil cercopithecoids. The paper supplies three main advances. First, it provides key information on primate paleoneurology, in particular on Plio-Pleistocene monkeys, belonging to the genera Theropithecus, Parapapio, and Cercopithecoides. Paleoneurology is often more focused on humans and hominoids than on monkeys, and therefore this article is particularly welcome. Furthermore, the study is based on a surface-based method, that compares the rough geometry of the object. Surface analyses can represent an additional and interesting alternative for computing endocast comparisons. There are many complex techniques currently available in shape analysis, and we should always carefully consider that their results depend upon their specific criteria and constraints. Morphometric outputs are “ordered representations” of a given sample variation according to specific numerical and logical assumptions. Consequently, methods are crucial in determining the comparative framework. Different methods, different criteria. For example, surface analysis is not constrained by anatomical correspondence, but it is only sensitive to geometrical correspondence. Hence, the approach misses the information on anatomical boundaries between different elements and areas, distributing variation all through a homogeneous and undifferentiated object.This can be an advantage when taking into consideration form alone, or a disadvantage if one want to investigate the contribution of specific anatomical components. Finally, this study presents a semi-automatic approach for sulcal detection, that is a geometry-based method for the identification of surface relieves, curvature lines, and topographical variations. This approach may seriously represent a major advance in paleoneurology. Nonetheless, it should be taken into account that we still ignore many mechanisms behind cortical folding, and that folding patterns could be the result of passive biomechanical constraints with uncertain phylogenetic or functional meaning.

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Ontogenetic dilemma

Ponce de Leon et al 2016

Marcia Ponce de León and colleagues have published a comprehensive shape analysis on modern human and Neandertal early ontogenetic endocranial changes, as Philipp Gunz and his team did back in 2010. Interestingly, results are different. The previous study from the Max Planck Institute concluded that only modern humans have a species-specific postnatal stage in which the braincase bulges (globularization stage). In contrast, this new analysis, coordinated by Christoph Zollikofer, suggests that after birth Neandertals and modern humans share a similar pattern of endocranial shape change. In this case, any endocranial difference between these two species must occur before birth. The discrepancy between the two studies may be due to differences in the samples (which, recognizing the good samples used in these analyses, would reveal a problematic instability of most paleoanthropological studies) or to differences in the reconstructions of the specimens (which, recognizing the good experience of both teams, would reveal a problematic instability of most paleoanthropological studies). Nonetheless, we must also take into account that both articles rely on very complex statistical and algebraic passages, and methodological biases should not be ruled out. After all, also paleontology deals with the same limits of any science: we do not work with skulls or brains, but with models made of variables and parameters. Models that work well in some cases, and do a worse job in some others, depending on the questions involved. In this new study, the fact that endocranial shape differences between Neandertals and modern humans are prenatal is used to state that there are no cognitive differences between the two species. Of course, cognition is more than shape, so the relationship between the timing of these changes (before or after birth) and the statement on cognition is not particularly straight. Inferences on cognition should be made on multiple evidence, dealing with something that goes well beyond a surface analysis.

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?

Pan & Pan

Although geometric morphometrics is currently the most promising method to analyze endocasts, there are alternatives. Durrleman and colleagues propose an approach based on deformations between surfaces. This method can help with non-linearity of the ontogenetic processes, lack of morphological references, or continuity of the anatomical tissues. The approach is definitely more complex and less intuitive than geometric morphometrics. This may mean sometimes more analytical power, sometimes more analytical bias.  The case-study is the endocranial ontogeny in chimps and bonobos: some shared patterns, but interesting differences too.

Corpus callosum and allometry

Size and shape variations of the human corpus callosum have always been controversial. Limits in samples and techniques have not helped in this sense. This month we have published a paper on midsagittal shape variation of the corpus callosum. Taking into account the results of this analysis, it seems that the shape differences between males and females are largely allometric. That is, they are secondary consequences of size variation, and not sex-related characters. Differences between groups are, however, very small, mostly because of the large personal differences among individuals. This is probably also due to a limited morphological integration within the corpus callosum, which is influenced by different and independent factors along its outline. It remains to be understood when shape variation is due to intrinsic components of the corpus callosum, and when it is due to indirect influences of external structures.


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