Archive for the 'Skull' Category

Brains and eyes

After our first survey on the morphological relationships between eyes and brains, here a comprehensive second study on this same topic. We have analyzed data from computed tomography (orbits and endocranial space) and magnetic resonance (eyes and brain), investigating modern humans, apes, and fossils. Soft tissue variation mainly deals with the distance between eyes and temporal lobes. Cranial variation mainly concerns the orientation of the orbits, probably influenced by parietal morphology and variation of the head functional axis. Phylogenetic differences are generally associated with the distance between orbits and braincase, with fossil humans showing an intermediate position between modern humans and apes. Here a Skull Box post with more details.

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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.

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.

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.

Bones and vessels

Eisova et al 2016The vascular traces left on the bones are remnants of physiological processes associated with blood flow and functions. Craniovascular traits can be used in archaeology, paleontology, and forensic science to deal with normal and pathological variations of the circulatory system, bridging interests between evolutionary and medical fields. Current information on these characters is, at best, scarce. After our recent work on diploic channels, this week we publish another morphometric study on the vascular traces, and specifically on their relationships with parietal bone size and thickness. We provide a quantitative description of the lumen size in adult modern humans for the middle meningeal and diploic vessels, as calculated from cranial anatomy after computed tomography, for different orders of branches. Vessel size and cranial thickness can be proportional if sharing growth factors, or inversely proportional if competing through structural constraints. However, we do not find any clear relationship between vascular size, cranial size, and cranial thickness. This result suggests that bone and vessel morphogenesis are probably influenced by independent factors, at least when dealing with differences among adult individuals.

Diploic veins

Rangel de Lazaro et al 2015Diploic channels run within the vault bones, and are therefore protected from external agents. This condition makes them an interesting topic in paleontology, archaeology, and forensics. At the same time, such secluded position has hampered detailed studies on their morphology, variations, and functions. In 1999 Hershkovitz and colleagues published a first pioneering survey on these “elusive” anatomical elements. This week we publish a segmentation procedure to visualize these channels after computed tomography, applying this method to modern humans and Neandertals. The diploic network displays a marked individual variability. It is frequently connected with the meningeal system at the pteric area, and with the emissary and venous systems at the occipital area. As for the meningeal arteries, also the diploic vasculature is apparently more complex in modern humans than in other hominids, mostly at the parietal area. Taking into account the large size of the parietal lobes and bones of our species, it is likely that such vascular development can be associated with metabolic and thermal functions. Beyond the large diploic channels, this vascular system counts with a widespread network of microvessels, which should be carefully investigated in the future.

Eyes and brains

Brain and Eye - PaleoneurologyThis week, with a team coordinated by Michael Masters (Montana Tech), we have published a correlation analysis to evaluate the relationships between eye, orbit, and brain, in adult modern humans. As already evidenced in other studies in anthropology and primatology, the correlation between eye size and orbit size is very modest. Therefore, the orbit is really a poor predictor of the eye morphology, at both evolutionary and species-specific level. There is also a minor size correlation between the eye and the occipital cortical areas, probably because of their shared visual functions. However, there is also a similar (and even higher) correlation between eye and frontal lobe. In this case there is a structural issue: the frontal lobes lie just above the orbits, generating a spatial interaction between facial and neurocranial elements. Within hominids, this spatial proximity between prefrontal cortex and eyes is generally observed only in modern humans and Neandertals. These two taxa, possibly because of such vertical constraint, enlarged their frontal lobes mainly laterally. These correlations between soft and hard tissues, when dealing with inter-specific trends, can be useful to make inferences on brain proportions based on osteological evidence, providing an heuristic tool for indirect paleoneurology.

So, back to modern human evolution, the situation of the eye was pretty difficult: large eye (due to brain size increase), small orbit (due to facial reduction), upper constraints (the frontal lobes right on the orbital roof), posterior constraints (larger and closer temporal lobes). And, in industrial Countries we can also add more fat between eye and bone. Hard times for the eyeballs, forced to minor deformations blurring images on the retinal screen: myopia. Luckily for us crossing the 40s, the brain stops growing, but the face does not: it grows bigger, giving more space to the eye, which can enjoy a more comfortable environment year by year.


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