Posts Tagged 'sulcal patterns'

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.


Subparietal morphology

Pedro-Pereira and Bruner 2016In this last years we have been studying the morphology, surface and position of the precuneus in adult humans and chimps. This week we publish a survey on its coronal anatomy: lateral extension and sulcal pattern. The aim of this article is to provide a quantitative description of its parasagittal variation in terms of morphometrics and folding schemes. The subparietal sulcus is larger on the right side, and possibly larger in males. The size of the subparietal sulcus is not associated with the sulcal scheme, which is very variable even between hemispheres of the same individual. The height of the precuneus influences the outer cortical profile, but the morphology and width of the subparietal sulcus have no apparent effect on the external brain geometry. The precuneus in general influences the upper cortical shape, with scarce or no influence on the lateral outline of the upper parietal lobules. Therefore, shape changes in this lateral areas are more likely to be associated with changes of the intraparietal fold. Correlations between inner and outer morphology are useful to evaluate whether changes in deep anatomical elements can be indirectly evidenced in paleoneurology, through the analysis of the outer (endocranial) surface.


Gomez-Robles et al 2015Brain evolution involves changes in size and morphology, but also changes in the capacity to be changed. Plasticity refers to the range of phenotypic variation allowed within a given genetic structure. Environment is a major factor influencing the phenotypic expression, and we humans have a special additional environmental component called “culture”. The ecological, cultural, and social niches, shape each others, with dynamics which are far from being understood. Aida Gómez-Robles and colleagues have now published a morphological analysis of human and chimp brain, taking into consideration heritability. In both species cranial capacity is highly heritable, more for humans than for chimps. Also the general dimensions of the main brain areas show in both species an apparent genetic component. The situation is different when dealing with sulcal morphology, which are still heritable for chimps but not that much for humans. This means that brain morphology in chimps has a stricter genetic program, while humans are more sensitive to non-genetic factors and individual responses. Environmental influences are supposed to be the key, mostly when considering the altricial condition and heterochronic changes associated with the human brain growth and development. In a recent review on the evolution of visuospatial integration with Atsushi Iriki we focused on the necessity to understand to what extent brain changes associated with human evolution are due to genetic, epigenetic or environmental factors. We pointed to the sensitivity of the brain to be “trained” through feedbacks between biology and culture as a crucial variable targeted by selection. This new study stresses further the possibility that selection can act on the capacity to change, more than on the change itself.


Fan et al 2015 (Curr Biol)The skull has represented, since ever, a crystal ball to investigate history and geography of past and present human populations. Considering the reciprocal influences between brain and braincase, we can wonder whether the brain can also provide traces of that long run. It looks like it does, according to a recent study which evidences a correlation between cortical patterns and genetic ancestry. It seems not a matter of size or surface area, but of cortical organization and sulcal geometry. If confirmed, these results are extremely interesting. Taking into account these differences among human groups, the authors of the study cast some doubts on the possibility to obtain robust information from fossil species, questioning the relationships between brain shape changes and specific volumetric variations of the brain districts. I must confess I can’t really see an antagonistic relationship between these results and the paleoneurological data. Intra-specific and inter-specific variations do not necessarily undergo the same rules and patterns. Most importantly, paleoneurological evidence is also aimed at considering specific changes of surface and volume proportions, beyond sulcal appearance. And, as recently described for the precuneus, larger size of a brain element may generally mean larger cortex of that element. Brain and braincase share a lot of morphogenetic mechanisms, but of course they are also influenced by independent factors. Their boundaries may vary according to different rules, but the intimate relationships between their respective surfaces allow at least a gross quantification of spatial organization, volumetric changes, and relative proportions among brain areas. We know that morphological changes as shown on endocasts are only a part of the story, and we know they are not always associated with neural (or even cognitive) changes. But reductionists approaches should be avoided in any fields, including genetics or neuroimaging, not only in paleoneurology. Conversely, a genetic signal on the sulcal pattern may promote further interest in brain shape variation. Last but not least, the study seems to support, rather than contradict, the information available from bones: it turns out that the conclusions of the analysis match the results of the Howells‘ craniometric studies. Good.

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