Posts Tagged 'macaque'

Precuneus and primates

The precuneus displays a remarkable variability in size and shape among adult humans, and it also represents a main difference between human and chimp brain morphology, being larger in our species. It can be argued that precuneus expansion in humans is due to an allometric pattern shared among primates. In this case, a large precuneus is a by-product of a big brain and scaling rules. We have now published a brain shape analysis in non-human primates, suggesting that this seems not the case. The midsagittal brain morphology in non-human primates is probably influenced by cranial architecture more than by brain differences. And, precuneus morphology is apparently not influenced by brain size, with no major differences between monkeys and apes. Therefore, its expansion in humans is likely to be a species-specific character, and not an allometric consequence of a large brain. The exact histological factors involved in this change is still to be investigated, as well as its functional (cognitive) consequences. In general, precuneus morphology is very variable also within other primate species, suggesting a noticeable plasticity. Its areas are crucial for coordination between body and vision (visuospatial integration, visual imaging, simulation, body cognition, autonoesis, etc.), and are influenced by both genetic and environmental factors (i.e., visuospatial training and practice). Its position physically matches those brain districts supposed to have undergone an expansion in the evolution of Homo sapiens, when compared with fossil hominids.

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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.

Deep asymmetries

Stephanie Bogart and colleagues have published an interesting study on sulci asymmetries in chimps and macaques, on NeuroImage. Quantifying cortical depth and surface area, they found consistent  population-level brain asymmetries in chimpanzees but not in macaques. The paper is a good review on many issues related to brain asymmetries and evolution in primates. Asymmetries that, however, are the results of mechanisms and processes which are still poorly known.


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