Chimpanzee brain anatomical network analysis

In the past year we have published a series of three studies that introduced anatomical network analysis to the brain, specifically in humans (Homo sapiens). Our aim was to highlight pivotal elements and general phenotypic patterns in the morphological organization of the human brain by investigating its intrinsic and extrinsic spatial constraints through topology—that is, patterns of physical interaction. We started with the human brain itself and found that the parahippocampal gyrus of the limbic lobe (i.e., the deep medial foil to the temporal lobe) is of great structural relevance in cerebral architecture. Then we proceeded to investigate community detection as a proxy for modularity in human brain morphology and detected the simultaneous occurrence of two modular patterns which mirror the organization of the surrounding braincase: a vertical division in line with the different ontogenetic processes associated with cranial base and vault, respectively, and a longitudinal gradient consistent with the distinct morphogenetic environments of the three cranial fossae. Lastly, to actually test whether our findings could be contextualized in light of the braincase, as we suspected, we included the skull into the model. Besides matching our expectations in regard to the anatomical system’s modularity, this study suggested that the sphenoid bone and parahippocampal gyrus are the elements of the highest structural relevance, especially due to their role as an interface between soft and hard tissues of the head.

Now we have extended our analysis to a new species, the chimpanzee (Pan troglodytes), and have compared its results to our prior research (Schuurman & Bruner, 2024). Chimpanzees and humans share morphologically complex inferior-medial cerebral regions and a topological organization that corresponds to the spatial arrangement of the braincase. These mutual topological characteristics are interesting because they can probably be traced back 7-10 million years ago, to the Pan–Homo Last Common Ancestor. However, some crucial differences are found between the chimpanzee and human brains as well, namely, in the structural relevance and roles of their respective cerebral components. Most notably, in chimpanzees, the cerebellum is embedded in a more intricate topological context than the parahippocampal gyrus, even compared with humans. The structural relevance of the cerebellum is likely to stem from the lack of expansion and reorganization in the chimpanzee temporal lobes. This could have caused the forebrain to envelop the midbrain and hindbrain more notably than in humans, leading to more spatial interactions between the cerebellum and the other brain regions. We hope this information will help to interpret macroanatomical changes in the brains of fossil hominids.

Tim Schuurman

Human craniocerebral network analysis

We have published a new paper that expands upon our anatomical network analysis of the human brain from the last few years by adding further context to the model: the skull (Schuurman & Bruner, 2024a). The morphology of the human brain is spatially constrained by numerous intrinsic and extrinsic physical interactions, which help identifying the source of morphological variability. By modeling physical interactions among brain and skull structural components, we highlight pivotal elements, as well as general phenotypic patterns, in the craniocerebral topological organization.

Previous studies yielded useful, yet disconnected insights: an anatomical network analysis of Brodmann’s map pointed to the retrosplenial area as burdened topologically and evidenced a longitudinal modular partition of the brain (Bruner, 2022). Subsequently, a comprehensive analysis that included the brain’s subcortical elements, highlighted the parahippocampal gyrus as structurally relevant and revealed a vertical partition of the brain (Schuurman & Bruner, 2023). Finally, an inquiry on community detection as a proxy for modularity in human brain morphology proposed that the best modular partition was indeed twofold: the simultaneous presence of a longitudinal and a vertical community pattern (Schuurman & Bruner, 2024b). An effort was made to contextualize these results through the literature, arguing that our findings were likely to stem from the topology of the skull. Yet, these suspicions remained untested. Here, we directly incorporate the skull into the model to assess whether our prior justification was sensible.

The results suggest that the sphenoid bone and parahippocampal gyrus are embedded in a severely intricate topological region, subjected to strong spatial constraints that are likely to make these elements highly influential in the evolution of brain morphology. This is partly due to their role as an interface between hard and soft tissues of the head. Additionally, the ethmoid bone acts as a local hub, integrating the entire facial block, while the parietal bone is constrained by its many physical interactions with the brain. Regarding the system’s modularity, the craniocerebral complex is still marked by two complementary community patterns: a vertical and a longitudinal gradient. Indeed, the former reflects the distinct morphogenetic organization of the cranial base and vault, while the latter corresponds to that of the anterior, middle, and posterior cranial fossae. Altogether, this information is fundamental to understanding the ontogeny and phylogeny of brain morphology, especially concerning where the brain molds the skull, and vice versa.

Tim Schuurman