Posts Tagged 'frontal lobes'

Frontal surfaces

beaudet-and-bruner-2017

More surfaces. This week we have published a surface comparison of the frontal endocranial morphology in OH9, Buia, and Bodo. The methods are the same applied previously by Amélie Beaudet and colleagues. Despite the importance generally assigned to the frontal cortex in our species, paleoneurology has not managed to reveal clear and patent changes in its gross form. Endocasts can only supply information on the general external appearance of the cortical anatomy, so we should expect they cannot be used to trace many aspects  associated with evolutionary variations. Also, the bad habits to defend firm statements based on single (and often reconstructed and fragmented) individuals unpleasantly crashes against the basic scientific principle of hypothesis testing, something that needs quantification, large samples and statistics. In this paper we compare these three specimens with the general scope of discussing some issues about frontal lobe evolution and paleoneurology. When compared with a modern human endocast, the younger fossils (Buia and Bodo) display flatter dorsal-lateral areas, while the older one (OH9) show a more extensive flattening of the whole dorsal surface. They all fit within a general trend observed in humans and hominoids: the more the eyes go below the frontal cortex, the more the frontal lobe bulges. So it seems reasonable to think that the curvature of the frontal lobes is but a structural consequence of the spatial relationships between face and braincase. In paleoneurology, we should exclude structural changes (cranial constraints and secondary consequences) if we want to localize functional ones, or if we want to reveal specific adaptations and primary evolutionary variations. Surface analysis is one more tool to go in that direction.

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.

Frontal matters

Saccopastore 1 (EBruner)A re-evaluation of brain volumetric data, adjusting for scaling and phylogeny, adds further evidence against the hypothesis of morphological changes in the frontal lobes for our species. Despite a century of firm claims on the patent role of the frontal lobes in our evolution, there are now many different indications suggesting that those statements were probably excessive and not well demonstrated. It seems that there is no clear specific change in the general morphology of the frontal lobes in Homo sapiens, and even the correspondence between anatomy and functions has lost strength. We must take into consideration the possibility that differences may be subtle but important. A minor shift from the general tendency may be irrelevant for the statistical thresholds but important in biological terms (for example, this can be the case for white matter proportions). There may be also changes which have not been detected yet, as well as changes that are not evident from gross morphometrics. Furthermore, even if volumetric changes in our frontal lobes are those expected for our large brain size, the increase in terms of absolute size is patent, and this may be a relevant difference anyway. Throughout this debate, it is interesting to note how the paleoneurological information is generally ignored. Despite the many inferences on the evolutionary changes in the brain human form, there is no mention of the notable advances published on the evolution of brain geometry in our species. This is even more imprudent when considering that anthropology is currently employing very complete and powerful morphometric tools, while in neuroscience most data still refers only to general size measures. However, even using just basic morphometric variables, we know that modern humans and Neandertals experienced at least a change in the proportions of the frontal areas. Excluding the fossil evidence from the debate does not seem to be a good idea, at least when dealing with evolutionary studies.

Brain and eye

Paleoanthropologists are now fairly convinced about the importance of integration in biology and evolution. It is a rare pleasure to see such perspectives successfully applied to every-day life problems. Michael Masters, with a very well documented study on human evolution and functional craniology, suggests that myopia (the primary source of reduced vision throughout the world) may be the consequence of our recent anatomical evolution. Large brains placing the frontal lobes on the orbital roof and constraining the orbital space, while at the same time facial reduction provides further structural limitations. That is, in our species the orbit cannot acknowledge properly the morphological requirements of the eye. Brain and eye compete for space, and the advantages associated with the former are paid with the problems associated with the latter. The consequent packing deforms the eye, leading to vision problems. Allometric and brachycephalic proportions make the situation even a bit more difficult in women and East Asian populations. This hypothesis is an excellent example of interchange between evolutionary biology and medicine. Until now, myopia has been mostly studied considering the eye like an isolated unit. Masters has now provided a very effective example of how induction and deduction can be improved mixing fields, in this case integrating medicine with functional craniology and paleoanthropology.  Interestingly, similar deformations associated with the frontal lobe spatial packing have been also described for some psychiatric disorders.


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