Posts Tagged 'parietal lobes'

Modern human brain shape

In a very comprehensive (and elegant!) article Simon Neubauer and colleagues have now analyzed brain shape variation along the modern human lineage. Since the description of the skull and endocast of Jebel Irhoud, it was clear that modern human brain form could have evolved after modern human origin. So, at that time (150,000-300,000 years ago) we had modern humans without modern brains. If Jebel Irhoud was Homo sapiens, then “early modern humans” lacked our characteristic globular brain shape, which is due to parietal lobe bulging and cerebellar form. Then, some later “archaic modern humans” seem to display a sort of intermediate morphology. Only recently (30,000-100,000 years ago) modern humans have evolved modern brains, at least in terms of general proportions and gross appearance. Of course, it’s difficult to say whether this transition was gradual or more abrupt. This article of the Max Planck team follows a previous one on the same specimens, and provides a very detailed analysis of many fossils that describe the evolution of our own species. Although the fossil record is not continuous because of the many chronological gaps, results suggest that a gradual change was likely. They also emphasize that a full-globularity can be found at the same time in which we find the archaeological evidence of behavioural modernity (arts, symbols, complex tools …). I remarked this same point many years ago, but the statement was not much appreciated because of the many uncertainties on the cultural “modern revolution” (more or less gradual, more or less discontinuous). Whatever the process behind, the appearance of a modern brain form (largely influenced by parietal districts associated with visuospatial functions, body cognition and visual imagery) matches the appearance of a modern behaviour (largely based on visual cognition and visuospatial managements, ranging from simulation and imaging to body-tool integration). Maybe it is but a coincidence, but nonetheless … they match.

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Precuneus and primates

The precuneus displays a remarkable variability in size and shape among adult humans, and it also represents a main difference between human and chimp brain morphology, being larger in our species. It can be argued that precuneus expansion in humans is due to an allometric pattern shared among primates. In this case, a large precuneus is a by-product of a big brain and scaling rules. We have now published a brain shape analysis in non-human primates, suggesting that this seems not the case. The midsagittal brain morphology in non-human primates is probably influenced by cranial architecture more than by brain differences. And, precuneus morphology is apparently not influenced by brain size, with no major differences between monkeys and apes. Therefore, its expansion in humans is likely to be a species-specific character, and not an allometric consequence of a large brain. The exact histological factors involved in this change is still to be investigated, as well as its functional (cognitive) consequences. In general, precuneus morphology is very variable also within other primate species, suggesting a noticeable plasticity. Its areas are crucial for coordination between body and vision (visuospatial integration, visual imaging, simulation, body cognition, autonoesis, etc.), and are influenced by both genetic and environmental factors (i.e., visuospatial training and practice). Its position physically matches those brain districts supposed to have undergone an expansion in the evolution of Homo sapiens, when compared with fossil hominids.

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.


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