Archive for the 'Endocasts' Category

Surfin’ endocasts

Endocasts and brains are difficult to analyze through traditional anatomical landmarks, because of the smooth morphology, blurred boundaries, and a noticeable individual variation. Currently, semilandmarks and surface analyses are good alternatives. Nonetheless, these two methods analyze the geometry of an “object”, ignoring its anatomical nature. If such geometrical modelling is interpreted too strictly, it may generate speculations and even incorrect conclusions. Numerical transformations behind spatial and geometrical models can be very complex and entangled, and the long chains of algorithms cannot be disentangled in any research article (the same occurs in any other field, like molecular biology, where long chains of reactions and engineering processes can’t be resumed in detail in every single paper and, necessarily, we must blindly rely on their proper functioning). In those many numerical steps, we must be aware that there may be incorrect passages, or simply algebraic assumptions that are not consistent with the real biological and evolutionary processes. More importantly, the brain is formed by so many independent elements, histological components, and cortical areas, and a pooled geometrical analysis can generate hybrid results. Anatomical landmarks are still necessary to mark boundaries and proportions, as to evaluate the real contribution of each element. Of course, anatomical landmarks are difficult to assess, they require experience, and they require inferences: as in every experimental paradigm, as in every field of science. Shape analyses deals with models, not with real anatomical entities. And models only take into considerations some specific properties of those anatomical systems, following algebraic rules that, right or wrong, represent conventional and operational assumptions. Here an opinion paper on all these issues.

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At Holloway’s

Ralph Holloway is at the Columbia University (New York) since 1964. More than half century dedicated to paleoneurology, brain evolution, fossils and endocasts. Some weeks ago I was visiting his laboratory, an amazing place, full of books, experience, and history. And collections. Endocasts are everywhere, witnessing at once the evolution of the human brain and the evolution of the moulding techniques. So I took the opportunity to ask Ralph some quick comments for this blog …

Why do we still need physical casts? (but do we?)

I think for the most part, we can do very well with virtual endocasts (as long as these don’t get hacked…), although these can never provide the same haptic experience as a true cast, even if it is a good 3D print. At the moment, I am working on LES1, Homo naledi, and while I have a 3D print from scan data of the endocast surface, and good images provided by Heather Garvin, I am making an endocast from a 3D print of the cranial portion, as I need the best resolution I can get of the occipital portion in particular. Even micro-CT scanning doesn’t always provide the subtle variations on the endocast surface that are critical for correctly identifying convolutional details. The downside of course is possible damage to original specimens, and lack of sharing with colleagues at other institutions unless they visit the lab where these are made. Furthermore, the accuracy of virtual endocasts depends on the software, the researcher’s experience, expertise, and whether the algorithms used to correct for distortion, etc, are accurate.

What is the main current challenge in paleoneurology?

The major challenge is to synthesize the overall size data (ECV’s) with whatever sulcal and gyral information (e.g., lunate sulcus, fronto-orbital sulcus, Broca’s cap regions, etc) is available with morphometric analyses for each specimen with temporal and archaeological evidence, so that actual hypotheses can be generated than can be tested within (or even beyond) the paleoneurological community. This requires researchers fully cognizant of anatomical details, and both nonhuman primate and human neuroscience. Needless to say, but many more hominin and hominid endocasts need to be found and studied if paleoneurology is to become a better science.

Advices to those who begin working in this field …

Know your neuro- and cranial anatomies! Stay humble, lose your hubris, and keep in mind how rare endocasts are, and how imperfect these usually are, and how difficult, if not impossible it is to really know what the brain was like when the hominin was alive, and realize that you will probably never see an endocast that fully captures all the convolution details that were part of that once throbbing brain. A lack of hubris will be essential for good science, and don’t dismiss earlier works in paleoneurology simply because these are not modern or based on the last decade of morphometric advances. Staying up to date, or being current with the findings coming out of neuroscience will be particularly difficult.

Australopithecus africanus

Amélie Beaudet and coauthors have published a shape analysis of the endocranial surface of two specimens included in the hypodigm of Australopithecus africanus, namely Sts 5 and Sts 60. They used a deformation-based approach, as to quantify the global endocranial surface variations. They compared the endocasts of the two specimens with modern humans (Homo sapiens) and chimps (Pan troglodytes and Pan paniscus). The endocranial geometry of the two piths is similar to the shape displayed by the apes. The incomplete Sts 60 lies right in between the two chimpanzees, while Sts 5 is slightly displaced toward the human form. The main difference between humans and the two australopiths can be localized in the parietal surface (larger in our species). Nonetheless, piths may present some minor shape difference on the frontal (orbital) cortex, when compared with chimps. Back in the ’20s Raymond Dart stated that, if extant African apes show a primitive scheme, a displacement of the lunate sulcus in Australopithecus might suggest a relative enlargement of the parietal cortex in these early hominids. Unfortunately, there are still disagreements on this point, because of the uncertainties associated with the identification of sulcal landmarks on fossil endocasts. Also frontal changes have been already proposed for other australopithecines. However, in this case the spatial interaction between frontal cortex and the facial system (orbits and ethmomaxillary block) may hamper a straight separation of the effects due to actual brain changes and those due to cranial constraints.

Sulcal imprints

The fuzzy geometry of the brain surface shapes the endocranial wall, and endocasts can show traces and imprints of the cortical sulcal patterns. Individual variation is noticeable, and the precise mechanisms behind these folding schemes are not clear at all. Hence, it is not recommended to use this information in a simplistic “phrenological” fashion, as unfortunately it has been done in many evolutionary studies. At the same time, cortical morphology is the direct result of neurons growth and development, and therefore even the pretentious rejection of this information seems unwise. Many authors dismiss any result based on brain gross morphology, simply because it is “just brain form”. This is probably because they ignore the developmental processes behind that forms, and they don’t take into account that when we talk about “brain form” we are implicitly referring to those processes, and not to a crude geometrical appearance. At least, sulcal patterns are useful (and the only available macroscopic) boundaries to detect the absolute or relative extension of some cerebral districts or cortical areas. So, despite all the uncertainties, they are directly providing information on cortical proportions. Proportions means “some areas are larger and some others are smaller”. Size is not always a matter of more or less neurons, but it is however matter of more or less “something”. Whatever it is, it should be functional, and maybe even adaptive some way, associated with some specific histological factor, or with some indirect physiological consequence. This is why the issue is not trivial.

Sulcal imprints are generally more visible on smaller and younger skulls. A recent study investigates the expression of the sulcal traces in macaques. Anterior folds (frontal and temporal lobes) leave more traces than the posterior ones (parietal and occipital). There are no many differences among young ontogenetic stages but then, during aging, the expression of the traces decreases noticeably, and imprints become more blurred. Local anatomical differences in the barrier between brain and skull (meninges, vessels, etc) can have a role in this size-related differences. Nonetheless, probably it is a matter of growth. In earlier ages, the brain generates a constant pressure on the vault bones, shaping the bone surface. But in later ages, when brain growth is concluded, that intimate physical relationship is looser. During aging, the brain even undergoes a shrinkage of about 7-8%, and the contact is further lost. This study is simple and effective, a good paper to approach the topic. Between an uncritical phrenological approach and a snobbish rejection of the evidence, we should consider an intermediate approach, in which we evaluate what kind of information we can obtain from these traits. To do that, we have to investigate their phenotypic factors and their mechanical influences, their structural associations and their variability.

Endocasting

I have found a very useful article published one year ago by Amy Balanoff and colleagues on Journal of Anatomy, a guide on “Best Practice for Digitally Constructing Endocranial Casts”. The paper is a detailed and comprehensive methodological overview on digital endocasting, introducing techniques, parameters, programs, problems, tools, and many suggestions on procedures and operational choices. Although the paper is more focused on birds and dinosaurs, it can be perfectly suited for human paleoneurology as well. The authors have organized the article as a set of replies to essential questions dealing with endocranial cast digital reconstruction. Pretty clear flow charts supply quick solutions for basic technical issues. The paper takes into account technical aspects (machines, physics, programs) as well as biological aspects (bone, skull, brain). Indeed, an extremely useful lecture for those who want to step into digital anatomy and paleoneurology.

Brains and teeth

gomez-robles-et-al-pnas2017In anthropology it is commonly accepted that the evolution of larger brains was associated with the reduction of posterior teeth. Factors ranging from diet to cognitive ability have been used to explain this inverse correlation between cerebral complexity and masticatory structures. Aida Gómez-Robles and colleagues have analyzed brain and teeth changes using a multiple-variance Brownian motion approach, providing evidence against a brain-teeth phylogenetic association. Brain shape was analyzed by using eight linear variables as measured on endocasts. Teeth shape was analyzed through geometric morphometrics. The study found that endocranial proportions and dental geometry are largely characterized by similar rates of variation, which are indicative of a neutral and non-directional pattern of evolution. Brain size and tooth size show different rates of change throughout the phylogenetic tree, and the hypothesis of a reciprocal and inverse correlation is not supported. This seems to suggest independent factors at environmental and/or genetic level. Two characters show faster rates of change in specific lineages, and are probably associated with specific selective and adaptive processes: brain size in early Homo and brain globularity in Homo sapiens. The first result suggests that brain evolution in the genus Homo is strongly based on size increase rather than on changes of specific cortical proportions. However, caution is needed in this sense: the study is based on simple linear metrics such as arcs and chords, and reflects only the external appearance of endocranial anatomy. Despite these limitations, this result is consistent with other kinds of evidence. The second result reflects an exception to this size-only pattern of change: the globular brain shape in modern humans. Parietal lobe variations are again an issue.

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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