Archive for the 'Brain morphology' Category

Monkeys’ parietals

Parietal lobes are specialized in primates, and particularly in humans. Nonetheless, the information on their anatomical variation is still scanty. In non-human primates, parietal cortex is investigated only in few species (generally, macaques) and mostly at the histological level. Now we have published a morphometric analysis on the parietal lobes of 11 cercopithecid genera. The study was performed on endocasts, as to broaden the conclusions to the fossil record too. Parietal differences among the main subfamilies have been described before, even in fossils, but without a detailed quantitative analysis. The main shape changes separate genera with large occipital lobes and small parietal lobes (cercopiths) from species with large parietal lobes and small occipital lobes (colobuses and baboons). Allometry is apparently not involved in this feature, and size increase is only associated with taller endocasts (probably due to cranial – not cerebral – factors). These different parietal-occipital proportions are supposed to be related to distinct cognitive organization, hypothetically influenced by diet and locomotion. It would be hence interesting to test the effect of different parieto-occipital ratios on specific behaviors and cognitive capacities. More body or more vision? Different views of the world …

Networks

Network analysis is nowadays employed in so many fields, ranging from economics to engineering, to investigate any kind of system in terms of its elements and their relationships. Sociology is probably the discipline that has mainly exploited and improved this approach, and all the complex methods and statistics behind it. In neuroscience, it is mainly used to investigate neuronal connections. The perspective supplied by network analysis is incredibly powerful because it allows localizing roles and constraints within very complicated systems. Some elements can be crucial as hubs of local influences, some others can be important as bridges between distant components. Some elements can be very sensitive to local changes, some others can be more independent from neighboring variations. Systems can be complicated because of the many elements and relationships (hundreds or thousands or millions of nodes) or because of the nature of their relationships (many variables influencing the relationships). Or both. In any case, network analysis is an amazing tool to step into the organization of functional or structural systems. When applied to anatomical elements, we can talk of Anatomical Network Analysis. In its basic form, nodes are the anatomical elements, and relationships can be just their physical contact. The network, therefore, represents a topological model, in which the position and neighboring properties of each component are the evolutionary result of a selection balancing the architecture of the system, generating at the same time limitations and constraints. In evolutionary neuroanatomy and paleoneurology, too often each macromorphological change in a specific cortical region is directly (and speculatively) interpreted as a functional (cognitive) adaptation. However, brain geometry is also influenced by skull constraints, and each cortical region is also sensitive to spatial variations of its neighboring cortical elements. Therefore, some anatomical changes may be due to intrinsic changes (for example, cell proliferation or growth), while others may be due to secondary extrinsic factors (for example, pressures and strains of the neighboring environment). In a first attempt to apply network analysis to brain macroanatomy, one year ago we analyzed the cortical organization as described by the traditional Brodmann map, finding an interesting correspondence with the general subdivision in “lobes” and cranial fossae, and further integration of the parieto-occipital block. We have now published a more detailed study, on the regions generally considered in evolutionary neuroanatomy and paleoneurology, as to characterize their architectural roles and relevance in terms of overall topology. The posterior cortex is more integrated than the frontal regions, with the precentral gyrus acting as a spatial hinge between these two districts. Topological complexity matches, in this case, sulcal complexity. The temporal cortex is particularly sensitive to distinct cortical influences. We also considered a preliminary model including the main endocranial (bone) elements, evidencing the role of the anterior fossa and frontal bone in influencing the prefrontal cortical morphology. This first targeted analysis represents an invitation to go beyond simplistic approaches to brain morphological changes in paleoneurology and evolutionary neuroanatomy. Yet, it is just a kickoff. More steps are on the way.

Shaping cortical evolution

Happy 2019 to everybody! To begin with this new year, here a new review on human paleoneurology, published in Journal of Comparative Neurology. Some conceptual and methodological issues in functional craniology, digital anatomy and computed morphometrics are introduced and discussed. The case-study on parietal evolution is also briefly summarized, with special attention to connectivity. Nonetheless, more specifically, the review points to theoretical and practical limitations of the field. Living species can provide information on the product of evolution, while fossils are necessary to provide information on the process. In the former case (extant species) we can rely on more comprehensive biological analyses, but results concern the final result of the process, not the process itself. In the latter case (extinct species) we can investigate directly the process, but samples are generally not representative neither at biological nor at statistical level. This dual framework is often not properly acknowledged, confounding taxonomy (the product) with phylogeny (the process). When samples and information are analyzed without these cautions in mind, conclusions can generate misleading hybrid perspectives. From the one hand, living species (monkeys and apes in anthropology and evolutionary neuroscience) are still frequenlty misinterpreted as primitive human ancestors. At the same time, scattered and descriptive information on individual and fragmented fossils are generalized to propose broad and inclusive theories. Both aspects are, scientifically speaking, crucial weaknesses, generating instability and unreliability within the field.

Another issue concerns the Homo-centric perspective that still contaminates evolutionary neuroanatomy and evolutionary anthropology. Apart from generating a deformed evolutionary scenario, anthropocentric views demote attention towards the other primates. Apes are generally used to “shed light on human evolution”. But living apes are not ancestral to humans. They could be bad models to understand our evolution, as we humans are probably bad models to understand their own one. They have their own specialized traits, which merit attention. In fact, apes are themselves an exceptional zoological case study. Anthropology is interesting, but apeology is interesting too. In cognitive terms, for example, apes could have capacities that we have never evolved. Finally, it can be also worth nothing that, charmed in searching for “what makes us humans”, we are neglecting “what makes us primates”. Because these latter features are associated with instincts, emotions, and cognitive constraints, they seriously deserve attention. Mostly when recognizing that they often deal with our social aspects, and with their consequences.

Little Foot

The endocast of the australopith StW 573 is pretty complete, and now Amélie Beaudet and colleagues have published a very detailed and comprehensive anatomical analysis of its features. For many paleoneurological traits we still miss a reliable knowledge on intra- and inter-specific variation but, according to what we can currently see in Australopithecus, Paranthropus and chimpanzees, StW 573 does not display derived sulcal patterns in the frontal and parietal regions. Its overall endocranial form resembles the morphology of some Paranthropus specimens, although in this case there are still some issues on deformation and possible taphonomic effects (specially at the frontal bone). The study supplies a careful description of the vascular patterns, in particular for the middle meningeal artery. In humans, only our species has generally a complex vascular network, while vessels are more scarce and less connected in extinct human taxa. Nonetheless, these same vessels (or, at least, their analogous networks) are more developed in apes. Therefore, australopiths are a key group to understand what happened with these traits, and to assess the polarity of these features in the evolution of distinct hominoid branches.

Globularity genes

Today, Philipp Gunz and colleagues have published a real milestone for paleoneurology: a comprehensive analysis integrating brain anatomy and paleogenetics to identify the genes involved in brain form differences between modern humans and Neanderthals. They compute an individual globularization index for a very large modern human sample size, and then look for the effect of supposedly introgressed Neanderthal genes. They found correlations between our individual brain globularity and genes involved in neurogenesis and myelination, most of all in putamen and cerebellum. Interestingly, they don’t find morphometric signals for parietal changes, even if there is evidence of actual parietal cortical differences among humans, between modern and extinct humans, and most of all between humans and apes. Furthermore, putamen and cerebellum are seriously involved in motor circuitry (including tool use?), something which is crucially coordinated by the parietal cortex, at physical (body) and virtual (visual imaging) level. As usually, caution is required when such complex methods are employed (in this case, the many assumptions in shape analysis, the many assumptions in brain imaging, and the many assumptions in paleogenetics). These results should be probably intended more to support hypotheses than to supply conclusive answers. Although these results point to individual brain shape differences among modern humans associated with neurogenesis and myelination, the study does not provide specific comments about possible functional or cognitive aspects, naming only some very general behavioral issues. Some relevant cogntive effects are, indeed, expected. The issue is definitely thorny (Neanderthal introgressed genes into our own species associated with consequences in individual brain form and development!), but should have probably deserved a more courageous interpretation. After all, also in science one must take into account that old and wise adage: if you don’t like the answer, don’t ask the question. In the supplementary information there is an amazing comparison (S1) between CT endocasts and MRI brains. This supplementary analysis is, in my opinion, a real jewel for this field, and I really hope that more future papers will be dedicated to what is here a single figure. Here an article from the New York Times.

Human brain variation

One year ago Croxson and colleagues published a survey on human and macaques brain variation, a paper which has been issued this month in Cerebral Cortex. They considered variation in white and grey matter, comparing inter and intra-specific patterns, and discussing similarities between the evolutionary and individual degree of variability. This study evidences the importance of variation as a source of evolutionary possibilities and constraints. The survey was based on only 10-20 individuals and, despite any statistical reassurance, we have to recognize that this is an unusual sample size for a study targeted to describe and quantify intra-specific diversity. Furthermore, in these kinds of analyses one has constantly the sensation that phylogenetic differences (macaque-human) are still interpreted as evolutionary differences (ancestral-descendant), which is definitely an inappropriate perspective when dealing with extant species. Also, the fact that we keep on using the term “monkey” when referring to one single species of hundreds of living, independent and diverse ones, denotes a still-alive linear approach to the evolutionary schemes (the old fashion progression monkey -> ape -> human). This paper was then commented by Aida Gómez-Robles, who discussed the pros and cons of this study. Some months later, Reardon and colleagues published a similar analysis, but on a huge sample. In her review, Aida Gómez-Robles pointed to endocasts as a potential source of additional information on intra-specific brain variation. Definitely a good point, and a valiant position to be presented in a mainstream journal on cognition. Endocasts and macroanatomy are issues which are often neglected in neuroscience. Nonetheless, two aspects must be taken into account. First, macroanatomy and morphology still hide many issues which suffer a dramatic lack of information, and that can reveal unexpected suprises. This is also true taking into account traditional neuroanatomy and, for example, in our last survey on human brain variation (on 265 individuals) the precuneus still stands as a major source of gross morphological human diversity. Second, although endocasts can’t provide a comprehensive information on brain biology, they can remarkably help to increase the sample size when dealing with primates and especially hominoids, because of the many collections available as dry or digital skulls. A recent study on the degree of endocranial metric variation in apes, humans, and hominids can be found here.

Newborn folding

Amazing study this one on neonate cortical folding! They analyzed almost 600 newborn brains with an automated method based on surface and curvature and, according to the results, the adult folding scheme is already expressed after birth. This means that most folding mechanisms act before birth. Taking into account that the brain then undergoes a dramatic increase in size, we can probably say that size-related effects may be important but indeed not determinant, for the final sulcal scheme. There could be allometric influences in brain folding but, if a neonate already displays the adult cortical pattern, this means that other early non-allometric factors are involved. Here a post on this study. And, a recent review by Van Essen and colleagues on brain development and evolution, addressing the issue of cortical folding.


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