Modelling brain-skull interface

Precise computational modelling of the brain-skull interface is necessary for the prediction, prevention and treatment of acquired brain-injuries. The brain-skull interface comprises complex layers including the osseous cranial tissues, meninges, sub-arachnoidal space and tissues, cerebrospinal fluid (CSF), pia mater and the gray and white cerebral matter. While the tissue properties of the brain-skull interface are known, there is no consensus on how these layers interact during head impact.  To generate computational models of the brain-skull interface with greater accuracy, knowing the boundary conditions or constraints is necessary. Previous experimental studies have relied on modelling the deformation of the brain-skull interface using neural density targets (NDTs) implanted into the cadaver brain, collecting information on tissue displacement during front and rear impact in motor vehicle crash-tests.

Wang et al. (2018) utilized computational bio-mechanics and finite element analysis (FEA), placing nodes in the 3D model in close approximation to the position of the experimental NDTs. Four hypotheses of the brain-skull interface were modeled, each approach placing different boundary conditions to model deformation during simulated head impact. All analyses were validated against previous experimental studies. Results showed that how the brain-skull interface was modeled appreciably affected the results. The 3D model showing the closest agreement with the experimental data, included all tissues of the brain-skull interface, allowed for displacement without separation of the skull and brain tissues, and strongly corresponded with known neuroanatomy. This 3D model indicated that non-linear stress-strain associations between brain and skull tissues best matched experimental results. Further, this 3D model could be closely predicted using an Ogden Hyperviscoelastic Constitutive model which did not over- or under-estimate deformations during head impact. The risks of over- and under-estimating head impact during motor vehicle accidents has implications for vehicle construction and prevention of serious brain trauma during accidents. Ultimately, a better understanding of the interaction between layers of the brain-skull interface can produce more accurate predictions of the likely impact during motor vehicle accidents and prevent violent head injury. Extrapolation of this research into paleoneurology could allow investigations into the structural interaction between the brain and braincase, testing if the resistence of brain-skull tissues during deformation evolved in human species as primary adaptations or secondary adjustments such as allometric responses.

 

Alannah Pearson

Advertisements

Woolly mammoth brain

Endocranial casts are usually the only resource for studying brain gross anatomy of an extinct species. However, sometimes, a frozen mummy can add information not only on the cortical features but also on the internal structures of the brain. Some years ago, Anastasia Kharlamova and Paul Manger published a study on the mummified brain of a Pleistocene woolly mammoth. This Yuka woolly mammoth specimen, dated approximately to 38,000 years ago, was determined to be an adolescent female aged between 6 and 9 years. Its mummified brain is unique in the state of preservation, allowing access to its external and internal morphology through the use of CT imaging techniques. Furthermore, it gave the opportunity to compare mammoths with extant African elephants’ brain, in order to characterize Elephantidae brain evolution, by determining whether elephant-specific features were already present in this specimen. The authors first compared the brains of the two species in terms of overall organization, concluding the Yuka mammoth displays the typical structure of the Elephantidae family. Direct volume comparisons were possible only for the structures which borders were clearly visible on the CT scans. The brain volume of the Yuka mammoth shrank during mummification, due to dehydration, and occupies only about 55% of the endocranial volume. Therefore, the calculated structure masses had to be corrected for such shrinkage. Moreover, due to differences in tissue fat composition, this shrinkage was heterogeneous, differing between hemispheres and between these and the cerebellum, which showed the least shrinkage. Based on subdural volume and on regression equations using extant mammals, the brain mass was determined to range between 4,230 and 4,340g. Given the average brain mass of an adult female elephant is only 300 to 400 g heavier, and that this specimen is immature, the values appear to be close to what would be expected for an adolescent woolly mammoth female. The size of the corpus callosum was also similar to that of female African and Asian elephants suggesting that, like elephants, mammoths also displayed sexual dimorphism in this structure. The comparable size of the amygdala suggests a similar organization of the limbic system, and the similarity in size and organization of the cerebellum point to a similar role in control of the trunk. This further indicates the Elephantidae family holds the largest cerebellum of all mammals, and that the cerebellar sensorimotor integration and learning movements of the trunk is a feature of this family. As the Elephantidae brain structure seems to be evolutionarily conservative, it can be assumed that the woolly mammoth could have achieved the same cognitive capacities as the extant elephants. However, further predictions of behavior and specializations would need a more detailed histological examination, which was not possible in the Yuka specimen. Nonetheless, this study provided an exceptional glimpse into the brain of an extinct species, and helped extending the understanding of the Elephantidae family.

 

Sofia Pedro


María

A new PhD student at the paleoneuro lab! María Silva is an archaeologist, and she did a master in human evolution, working on cognitive archaeology and visuospatial integration. Now she will keep on making research in this field, working on visuospatial capacity and experiemental archaeology. She is now fully enrolled in the PhD program at the University of Burgos, Spain. She will integrate concepts in archaeology, cognitive sciences, and psychology, as to investigate issues in brain-body-environment functional relationships. She will work in a joint team with Annapaola, further integrating this group that mixes fossils and behavior. She is also one of the editors of our blog on evolution and prehistory. So, more and more cognition here in this lab!


Cranial vault thickness in South African Australopiths

In a recent paper, Beaudet and colleagues analyze the cranial vault thickness of StW 578, a partial cranium of Australopithecus not yet assigned to a species. The authors explore the utility of cranial vault thickness and of the organization of the diploe and cortical tables as potential diagnostic criteria for hominin species. For that, they also analyze a comparative sample including other South African Late Pliocene-Early Pleistocene fossils, extant humans, and chimpanzee specimens. Fossils include specimens of Australopithecus and Paranthropus recovered from Sterkfontein, Swartkrans, and Makapansgat sites. Based on cranial landmarks, the authors defined homologous parasagittal and coronal sections on the CT scans, preferentially on the right hemisphere, which is better preserved in StW 578. The thickness of the diploe, the thickness of the inner and outer cortical tables, and the total thickness were measured automatically in various points sampled throughout the length of the sections. The proportion of each layer was computed by dividing the thickness by the surface area calculated between two successive points. Specimens that preserved only the left side were used for qualitative comparison. Results emphasize differences between Australopithecus and Paranthropus. The former genus tends to have thicker vaults, with a larger proportion of the diploic layer, while the latter tends to have thinner vaults, with a larger proportion of the inner and outer tables. The distribution of thickness also differs, as StW 578 and other Australopithecus crania from Sterkfontein display disproportionately thicker frontal and posterosuperior parietal regions, while Paranthropus (SK 46) and extant chimpanzees have thickest regions on cranial superstructures (supraorbital and occipital tori). As the authors suggest, thickening of the cranial vault in frontal and parietal regions needs further investigation, as to unveil a possible correlation between bone thickness and brain anatomy. Moreover, as the increase in thickness is associated with an increase in diploe proportions, variation in this layer might indicate physiological (thermoregulation) or biomechanical differences between Australopithecus and Paranthropus. In sum, cranial vault thickness patterns of StW 578 are equivalent to those of other specimens from Sterkfontein (StW 505 and Sts 71). The presence of a Paranthropus-like pattern in two of the three Mangapansgat specimens further indicates the presence of different morphs or species of Australopithecus in this site. This methodology and results provide a fine base for further studies on the taxonomic significance of the cranial vault thickness. The authors suggest beginning by including more Paranthropus specimens, and by evaluating chronological, geographic, and taxonomic variation.

 

Sofia Pedro


A History of Surgery

The Chirurgeon’s Apprentice is a popular blog on the website of medical historian Dr Lindsey Fitzharris who received her doctorate from University of Oxford in medical, technology and science history. Dr Fitzharris discusses the apt naming of the blog with the word ‘chirurgeon‘ the first historical reference to a practitioner of surgery. The website illuminates the often grisly but fascinating historical developments in Medicine and Surgery with focus on the Victorian era and the rapidly developing techniques and methods occurring in all scientific disciplines at this time.  Under the Knife is a well-researched and often darkly humorous video series delivered by Fitzharris where each episode details different aspects of the history of Surgery and Medicine. Dr Lindsey Fitzharris is also the author of a recent book The Butchering Art about the Victorian surgical pioneer Jospeh Lister and the development of antiseptic practices.

Alannah Pearson


Seasonal brain changes

As we already discussed, small mammal species with short life spans and high metabolisms which experience seasonal fluctuation of resources tend to undergo seasonal changes in skull size and morphology. More recently, Lázaro et al. examined how this seasonal variation affected brain size and organization in the common shrew (Sorex araneus). They collected specimens in Southern Germany, covering all seasons and the whole lifespan of the shrew, which is about 18 months. The sample was divided into three age groups: summer juvenile, winter subadult, and spring-summer adult (sexually mature). Right hemispheres were used to investigate the volumes of the different brain regions, and the left hemispheres for examining neuron morphology. They confirmed the patterns of seasonal volume variation, and observed there were differences between brain regions and between sexes. The overall volume decrease from summer to winter was more pronounced in females, while spring regrowth was similar for both sexes, thus resulting in adult sexual dimorphism with females having smaller brain volumes. Regarding the brain regions, most significant changes were observed in the hypothalamus and the thalamus, both in the winter decrease and spring regrowth. Neocortex and striatum (mostly caudoputamen) volumes decreased in winter but did not regrow in spring. Cerebellar volumes were smaller in females during winter, but reached the same volumes as males during spring regrowth. According to the authors, as the volume reduction from juveniles to subadults occurs before winter, it is more likely genetically encoded than a direct result of temperature or resources fluctuation. Furthermore, the independent variation of the different brain regions suggests a mosaic adaptation of each structure to the cognitive requirements and energetic limitations of each season. Other explanations for the different patterns of variation between the different regions might be associated with differences in energetic demands and in the potential for plasticity between brain structures. However, the authors could not find correlations between seasonal volume changes and functional demands, developmental timing, or metabolic consumption of the different brain regions. They conclude the variation in each brain structure might be influenced by functional adaptations and plasticity to different degrees. The authors also registered the variation in neuron size and morphology in three regions. The caudoputamen showed a decline in dendritic length and volume, in soma size, and in spine number and density, from juvenile to adult. The somatosensory cortex displayed only decline in soma size from summer to winter and in spine density from winter to adult. In the anterior cingulate cortex there was a reduction in soma size from juvenile to adult but in dendrite volume only from juvenile to subadult. These results cannot explain adequately the observed volumetric changes in the respective regions, and other factors that might affect volume, such as the space between cells and neuron density, should be considered in further studies. Moreover, changes in axonal innervations and myelin, and in the density of microvessels should be considered as these can also influence energetic costs.

Sofia Pedro


Chimpanzee Sulci

Studying the evolution of brain form requires paleoneruologists to rely on casts from the cranial cavity from fossil species. Due to the lack of soft-tissue preservation in fossils, descriptions of macroanatomy and cytoarchitecture  are taken from comparative non-human primates to serve as hypothetical models of early hominin brain form. Using extant non-human primates as models for fossil species ignores the separation of lineages, any specific adaptations and lineage-specific evolution since divergence. Furthermore, extant species risk being relegated as ‘living fossils’ with the issue worsened by the absence of identifiable fossils for either Pan or Gorilla. The untenable assumption is that extant chimpanzee anatomy should resembles  the original form prior to the PanHomo split. Nonetheless, comparison among living hominoids is still mandatory to investigate the evolutionary radiation of this taxon.

Previous published descriptions of chimpanzee sulcal patterns occur in classic literature but were based on only a few post-mortem dissections. Recently, Falk and colleagues aimed to increase knowledge of chimpanzee sulcal variation by describing sulcal patterns present in in-vivo Magnetic Resonance Imaging (MRI) from eight chimpanzees. Results suggested that, contrary to previous opinion, two sulci do occur in both chimpanzees and humans. To elaborate, these two sulci are the middle-frontal sulcus located in the frontal lobe, and lunate sulcus located between the parietal and occipital lobes.

No quantitative analyses were conducted in this study, but Falk et al. (2018) provide detailed descriptions of the variation between individuals, highlighting why descriptions based on only one or two individuals cannot be used to reliably describe the brain anatomy of a species. The authors argue the presence of the middle-frontal sulcus and lunate sulcus in chimpanzees invalidates previous claims that these sulci represent derived states found only in the human lineage. Further quantitative analyses with much larger samples, including both extant and fossil species will aid in a better understanding of the brain anatomy of humans and other great ape species.

Alannah Pearson


Pulling faces

Two different papers have been published this month on the evolution of the supraorbital anatomy in humans. The first article is on Neanderthal facial morphology, and it was coordinated by Stephen Wroe, of the FEAR lab. Here a comment on the Daily Mail. The second article, by Ricardo Miguel Godinho and coauthors, links supraorbital morphology and social dynamics, and it was commented in a News and Views by Markus Bastir.


Fossil Primate Brains

Primates are unique among mammals for having a brain much larger than expected for body size. An important  aim in paleoneurology is  understanding how cerebral structures reorganized to accomodate primate cerebral expansion. The brain comprises only soft-tissue and does not fossilize  so paleoneurologists rely on endocasts, either physical or digital molds of the cranial cavity, to estimate the macro-anatomy of the brain. Continuing computational advances and powerful imaging techniques have allowed the generation of increasingly higher-resolution digital endocasts. Gonzales et al. (2015) generated a high-resolution endocast of the 15 Myr-old fossil cercopithecine Victoriapithecus macinnesi using micro-CT scans. By using computational methods, taphonomic distortion was corrected and a new endocranial volume (ECV)  of 35.6 cm3 reported for Victoriapithecus which is much smaller than the previous value 54 cm3. This new, smaller ECV places  Victoriapithecus within the range of extant strepsirrhines but outside the range expected of extant and fossil cercopithecoids including the 32 Myr-old fossil species Aegyptopithecus zeuxis which had an ECV within the expected range for fossil cercopithecoids.

Despite Victoriapithecus exhibiting a very small ECV and falling below the range for extant cercopithecoids, the fossil does exhibit the ‘frog-shaped’ sulcal pattern shared only among cercopithecines. This sulcal pattern suggests Victoriapithecus is a cercopithecine, the ‘frog-shaped’ sulcal pattern is such a diagnostic trait that it is not shared by the leaf-eating colobines but only present in cercopithecines. The olfactory bulbs in Victoriapithecus are unusually large relative to the small ECV. Large olfactory bulbs are present in extant strepsirrhines and the fossil catarrhine Aegyptopithecus zeuxis but reduced in all extant and fossil cercopithecoids and hominoids. The presence of small olfactory bulbs in the 18 Myr-old hominoid Proconsul versus the large bulbs in  Victoriapithecus suggested olfactory bulb reduction may have evolved independently in both cercopithecoids and hominoids.

Harrington et al. (2016) compared digital endocasts generated from micro-CT of three adapiform fossil primates including the 48 Myr-old Notharctus tenebrosus, 47 Myr-old Smilodectes gracilis and 45 Myr-old Adapis parisiensis. Results of endocranial volume (ECV) were consistent with other studies revealing an ECV of 7.6 cm3 for Notharctus, an ECV of 8.3 cm3 for Smilodectes while Adapis had an ECV of 8.8 cm3. The sulcal morphology of these adapiforms was also consistent with previous studies showing the defining feature of the primate brain, the Sylvian sulcus, is species-specific in these adapiforms. The Sylvian sulcus is well-defined in Adapis, occurs only as a shallow depression in Notharctus but is entirely lacking in Smilodectes. The absence of the Sylvian sulcus in Smilodectes is not understood but as it is absent in other mammals, this may represent a retained ancestral trait from before the divergence of primates from other mammals.

The cerebral organization of Notharctus and Smilodectes showed both possessed larger temporal and occipital lobes relative to brain size with smaller olfactory bulbs and frontal lobes. This trend might indicate cerebral reorganization favoring larger visual-auditory structures located in the temporal-occipital regions of the brain versus smaller visual-olfactory structures in the frontal region. The olfactory bulbs of these adapiforms were small and blunt relative to endocranial volume and predicted body mass but uniquely, Adapis parisiensis had the largest olfactory bulbs, placing it within the range of extant strepsirrhines. These studies reveal how little is understood about primate paleoneurology and the evolutionary trends of different primate lineages with implications for the human fossil record.

Alannah Pearson


Galway

Visiting Peter Dockery and the amazing facilities of the brand new Human Biology Building at the National University of Ireland, Galway