Posts Tagged 'parietal lobes'

Precuneus form and folds

bruner-et-al-aa2017One more paper on the morphology of the precuneus. This time we have analyzed a racially heterogenous sample, confirming that precuneus size is a major source of brain form variation also when a wider genetic variability is taken into account. It is a variation that is apparently independent from sex, race, or hemisphere, although males could have slightly larger proportions than females. A larger precuneus can be associated with additional folds, often in its anterior district, although this association is feeble. Geometric models suggest that the areas involved in this variations are the anterior-dorsal ones, roughly corresponding to area 7a. This area is the largest and more variable of the precuneus, and it includes the medial cortex but also the dorsal external cortex of the upper parietal lobule. It is functionally associated with the integration of somatic and visual information, and with self-centered mental imagery. These results also suggest that upper and lower areas of the precuneus should be considered separately when dealing with functional or evolutionary neuroanatomy. Our former papers on this topic concerned the shape of the precuneus, its cortical surface area, its sulcal patterns and  lateral extension, and the differences between humans and chimpanzees. Apart from the relevance in modern neuroanatomy, these same endocranial regions also display a corresponding spatial enlargement in modern human evolution.

Integrated paleoneurology

Zollikofer et al 2016Together with the recent article on modern vs Neandertal endocranial ontogeny, the team coordinated by Christoph Zollikofer has now published also a large and comprehensive study on endocranial ontogeny in humans and apes. The paper focuses on a specific question: to what extent endocranial differences are due to brain differences, and to what extent they are due to cranial constraints? Definitely, this is a key-paper in paleoneurology. They considered the integration between and within the main cranial districts to evaluate the influence on brain shape of two major cranial effects: spatial packing and facial orientation. Their analyses suggest that endocranial differences between humans and apes, as well as differences among apes, are the result of all those factors, the cerebral and the cranial ones. Therefore, the endocranial form is due to a complex admixture of specific brain differences (already present at birth) and cranial constraints. Comparisons among endocranial ontogenetic patterns of living hominoids, among adult fossil specimens, and among different neuroanatomical aspects of living species, can give different results, suggesting that the relationships between anatomical, morphological, and cytological elements is far from being understood. In my opinion, a limit of many shape analyses in general concerns the use of surface semi-landmarks to analyze brain geometry. Surface landmarks are necessary because of the lack of good anatomical references on the endocasts. Unfortunately, they can’t take into account the contribution of distinct cerebral areas, and as a consequence they consider brain morphology as a single homogeneous surface. The identification of boundaries or distinct and independent elements within this surface might seriously influence the multivariate output. I am particularly interested in the analysis of the parietal districts. When using surface landmarks the analysis of the parietal surface may give different (and sometimes contrasting) results. Hence, we may wonder whether the observed parietal variations are the result of brain differences (cortical expansion/reduction) or of geometry (bulging and flexion). Nonetheless, previous morphological studies based on cortical landmarks suggest that modern humans show an actual (absolute and relative) increase not only of the parietal “surface”, but also and specifically of the parietal “lobe”, when compared with extinct hominids or with living chimps. The localization of anatomical boundaries on endocasts may be difficult, although those results have been replicated on different samples. The identification of anatomical landmarks in living species is, in contrast, definitely more reliable. Therefore, whatever the result of a global surface analysis of the whole endocranium, we should not forget that comparisons of specific areas are suggesting a differential contribution of distinct brain components.

Subparietal morphology

Pedro-Pereira and Bruner 2016In this last years we have been studying the morphology, surface and position of the precuneus in adult humans and chimps. This week we publish a survey on its coronal anatomy: lateral extension and sulcal pattern. The aim of this article is to provide a quantitative description of its parasagittal variation in terms of morphometrics and folding schemes. The subparietal sulcus is larger on the right side, and possibly larger in males. The size of the subparietal sulcus is not associated with the sulcal scheme, which is very variable even between hemispheres of the same individual. The height of the precuneus influences the outer cortical profile, but the morphology and width of the subparietal sulcus have no apparent effect on the external brain geometry. The precuneus in general influences the upper cortical shape, with scarce or no influence on the lateral outline of the upper parietal lobules. Therefore, shape changes in this lateral areas are more likely to be associated with changes of the intraparietal fold. Correlations between inner and outer morphology are useful to evaluate whether changes in deep anatomical elements can be indirectly evidenced in paleoneurology, through the analysis of the outer (endocranial) surface.

Precuneus and chimps

Bruner et al 2016 - Brain Structure and FunctionWe modern humans have larger parietal bones and large parietal lobes when compared with extinct human species and living apes. We also have an ontogenetic parietal bulging stage, a stage which is absent in apes and Neandertals. Interestingly, a main factor of variability in our brain morphology is the size of the precuneus, in terms of proportions and cortical surface area. Now we have compared human and chimp brains, and here it goes again: the main difference is a much larger precuneus in our species. It doesn’t look like an allometric issue, being possibly associated with that bulging stage specific of modern humans, and even absent in large-brained Neandertals. The precuneus is essential in visuospatial integration, coordinating brain, body, and environment, and bridging the somatosensorial experience with simulation and self-awareness. It is a key element to integrate space, time, and  social perception. It is also worth noting that parietal lobes are particularly vascularized in our species, and the precuneus is a high-metabolic and heat-accumulating element. This may be interesting when considering that it suffers early metabolic impairments in Alzheimer’s disease, a pathology particularly associated with our species. The precuneus is also a central hub of the default mode network. Interestingly, at least in adult modern humans the size of the parietal lobes is inversely correlated with the size of the frontal and temporal lobes, introducing some phylogenetic issues on the evolution of the fronto-parietal system.

For a long time we have been looking for subtle differences between human and ape brain. This one looks not that subtle. Any functional or histological change behind this expansion, at inter-specific and intra-specific level, is still to be investigated. But most of all, it remains to be established the nature of such morphological variations. Genetic factors and selective processes cannot be excluded, but these areas are also particularly sensitive to environmental influences, including training and cultural effects.

Fronto-parietal highways

Caminiti et al 2015Roberto Caminiti and his colleagues have just published a very large and detailed review on the fronto-parietal network, comparing functional anatomy, histology, and connectivity in humans and macaques. The organization of the fronto-parietal system is similar in these two taxa, suggesting a shared conservative structure rooted in a long evolutionary history. However, there are also discrete differences, most of all at the intraparietal sulcus and in the precuneus. Because such differences were apparently put forward on a shared background, they support the hypothesis of Fred Coolidge about exaptation of the parietal lobes, as reuse of primitive characters to achieve new functions. Visuo-spatial integration and the eye-hand system are of course central in this perspective, but those parietal elements are also involved in different kind of processes ranging from consciousness to numerosity. As usually, we are supposing that macaques show a primitive organization, and humans a derived one. As recently discussed, such assumption is very general and it has no logic or experimental support, and caution is recommended in this sense. In fact, both macaques and humans could display derived characters, evolved independently. The review carefully considers also the human paleoneurological evidence, supplying a very complete image which effectively synthesizes at once more than ten years of works published by myself and by Simon Neubauer and Philipp Gunz.

Brain and braincase

Bruner et al  - JAnat 2015Parietal lobes are a main source of morphological variation within humans and within hominids. This month we have published a study on the relationships between bones and lobes, to evaluate how and how much such variation in the cerebral areas can influence the variation of the corresponding cranial points. There is a size correlation  between parietal bones and parietal lobes, but it is small. There is a lot of individual variation, most of all in the precuneus. Changes of the brain proportions do not seem to influence the spatial relationships of the bones. Therefore, when the boundaries of the parietal lobes change, the boundaries of the parietal bone do not. It is like the brain “slides” under the bones, without strict constraints due for example to the connective meningeal interface. So, the larger the parietal lobe, the more it approaches the frontal bone. Such lack of marked correspondence between bones and lobes suggest cautions when using cranial landmarks to estimate brain boundaries, like in neurosurgery or in paleoneurology. Now, two hypotheses can be put forward, taking into consideration that the growth of the parietal area in our species is characterized by a very early post-natal stage. First hypothesis: such lack of correspondence can be present since the beginning, and hence that early parietal bulging will separate the limits of the parietal lobe and bone. Second hypothesis: during that early stage parietal lobe and bone grow together (the latter in response to the former), but later stages of spatial changes in the anterior districts (frontal bone and lobe) alter their original correspondence. This study deals with modern humans, and it will be interesting to consider the same spatial relationships in other primates. Nonetheless, at least for Homo sapiens, we can say that between parietal lobes and parietal bones there is a good geometrical correspondence (overall curvature), a modest size correlation (length), and a variable spatial relationship (boundaries).

Visuospatial evolution

Descartes - Meditations metaphysiques 1641Visuospatial integration is essential in handling, tooling, simulation, and many specific tasks which are supposed to be crucial for human evolution. However, it may be even more important for theories on extended cognition, taking into account the relevance in coordinating the relationships among brain, body, and environment. This is something directly associated with concepts like embodiment, material engagement, and brain-artefact interface. And this is pretty intriguing when considering that the upper and medial parietal areas, which are major functional nodes of visuospatial integration, show a remarkable enlargement only in Homo sapiens. Together with Atsushi Iriki (Riken Brain Institute), we have now published a review trying to interlace all these issues: Extending mind, visuospatial integration, and the evolution of the parietal lobes in the human genus. We have tried to integrate topics in neurobiology, paleoneurology, cognitive archaeology, and comparative primatology, to understand why and how visuospatial integration may have been important, in our genus and in our species, for enhancing material engagement and embodying capacities. This article will be part of an issue of Quaternary International dedicated to the importance of “Material dimensions of cognition”. At the same time, the Journal of Anthropological Sciences is now publishing a second forum on the “three hands” of the Neandertals. The hypothesis of a mismatch between visuospatial functions and cultural complexity in this human species is further discussed with comments by Leee Overmann, Enza Spinapolice, Joseba Rios Garaizar, Ariane Burke, Carlos Lorenzo, and Duilio Garofoli. All the papers of the forum are free to download.


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