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.

Advertisements

Brainstorm …

As properly remarked in the prequel of the Planet of the Apes, we know everything about our brain, except how it does work. We are aware of such lack of knowledge, at least in theory. In practice, papers are replete of firm sentences and conclusive statements. But we use complex programs and devices, and we should not forget that these tools can only generate models of reality. Models based on algorithms that are trying to represent and simulate only some specific physical or spatial properties. Our brain models are but statistical outputs, not real “brains”. We identify brain activity through indirect blood or metabolic functions, assuming there is a strict correspondence between those signals and our concept of “at work”. A correspondence that is reasonable, but not that strict. Even basic anatomical issues can be blurred after a more detailed scrutiny, mostly when previous knowledge is based on information that has been copy-and-pasted through decades. We are more and more finding strange factors influencing our results. Apparently, the brain undergoes daily variations, and the braincase may suffer seasonal changes. Brain structure and function can be even influenced by head position and posture. These unexpected effects recommend further caution when making too general conclusions from specific and punctual results. Let’s take into account that we still miss much information on gross neuroanatomical components. For example, we still ignore the function of the cerebellum, that has four time the number of neurons of the brain, and we still don’t know all the functions of the glial cells, that may be nine times more numerous than neurons. And, we don’t know how much brain anatomy and functions are the result of genetic programs or environmental influences. In only few weeks, training can easily improve or demote brain complexity. Nothing new under the sun: science is about hypotheses, and hypotheses need to be tested and validated. Our models are tentatively designed with this scope in mind. This summer post is a summary of articles concerning some methodological limitations and some curious result dealing with brain structure and function. And an invitation to interpret results for what they are: evidences supporting or rejecting hypotheses. Remember that those are not neurons: just pixels! Take it easy …

Language and fossils

This week I have published an opinion paper on language and paleoneurology, in a volume of Frontiers in Human Neuroscience dedicated to language, skulls, and brains. I review the fossil evidence on language, suggesting that most of such evidence concerns brain areas that are influenced by cranial structural constraints, or is based on speculations associated with individual bone remains. Thus, strictly speaking, there is no consistent evidence on language evolution when you deal with fossil anatomy. Ralph Holloway already stressed this point before, but it seems that most books and articles introducing this topic simply keep on stating the opposite, following a mantra (usually void of citations) according to which fossils must clearly reveal the cerebral (usually frontal) changes behind language evolution. The lack of scientific evidence in this context does not mean that there is no association between language and brain evolutionary changes in hominids, but just that fossils can provide only a very incomplete (and insufficient) view of this process. Firm statements, scientifically speaking, should be avoided, and relegated to storytelling and science marketing.

The dorsal and medial parietal areas, probably larger in Neanderthals and even more derived in modern humans, are not generally considered when discussing language processes, and most of the debate has been centred on frontal lobe functions. Nonetheless, the parietal areas are crucial for hand coordination and manipulative abilities, both factors that have always been regarded as influential in language evolution. Also, recent evidence suggests that language has an important embodied component: language coding passes through body experience and simulation, something which is profoundly associated with the functions of the deep parietal folds. Therefore, we should consider whether changes in the whole fronto-parietal system may have triggered or facilitated language in the human genus. The paper is open access.

Jebel Irhoud

New fossils and age for Jebel Irhoud. Jean-Jacques Hublin and colleagues have published new specimens, new analyses, and a new chronology pointing at 300 ka. All their results robustly confirm what we already knew on these populations: modern face, primitive braincase. Two major advances of these new findings are i) the morphology of Irhoud 10 (the new skull) is apparently so similar to Irhoud 1 (the old skull found back in the ’60s), suggesting that such phenoptype was common and representative, and not only the result of individual variation, and ii) the age around 300 ka, that suggests an earlier origin for our lineage. The braincase and endocast of the new skull were not analyzed in this study, probably because of some deformation, and there are no photographs of the fossils (in the paper we can only see the virtual reconstruction of the face), so an assessment of its paleoneurological traits is not available yet. But in this article they re-analyze the old specimens (Jebel Irhoud 1 and 2) through shape analysis, confirming a plesiomorph braincase, apparently (Extended Data Figure 4) because of a reduced parietal and frontal size and curvature. Here a 2013 study I coauthored with Osbjorn Pearson on Jebel Irhoud’s endocast, supporting the same conclusion: they were probably modern humans, but without modern brains. If they were our ancestors, something triggered a subsequent change in brain proportions and organization.

Vault and base

This week we have published a study on the integration between the parietal and temporal morphology in the human skull. Modern humans display large and bulging parietal lobes and bones, and large and projecting temporal lobes. It is possible that the latter (the anterior displacement of the temporal lobes) can be a spatial secondary consequence of the former (the enlargement of the parietal district). This survey on adult skulls suggests that this is not the case: parietal bulging and temporal displacement are apparently independent and not related. Nonetheless, when one of these districts undergoes a major variation (bulging in the case of the parietal bone, vertical stretching in the case of the middle cranial fossa), the other undergoes a spatial rotation: shape does not change, but the orientation varies according to the global cranial modification. The enlargement/reduction of the parietal bone have thus a major effect on head orientation, and it is also associated with facial proportions. Hence, it turns out that the general enlargement of the parietal district, a species-specific character of modern human brain and skull, has probably influenced the functional axis of the head, with possible consequences on body organization and posture. These results once more recall the importance of an integrated analysis between brain and braincase and, more generally, of a system-based approach to functional craniology.

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.

Precuneus form and folds

One 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.