New Model Reveals Underlying Similarity in Mammalian Brain Shapes

A novel method for characterizing the cerebral cortex's shape has been devised by researchers, who also present evidence that cortices in all mammalian species share a common, fractal design.

The study's editors characterize it as a useful foundation for comprehending the brain cortex as a fractal shape. The study was published in the journal eLife. According to them, there is strong evidence supporting a universal model of mammalian cerebral cortex folding.

With additional investigation and validation, the method may be able to shed light on how different congenital and degenerative neuropathic disorders emerge.

The cerebral cortex, which is the brain's outermost layer, is in charge of intricate processes including perception, cognition, and decision-making. Gyrification, often referred to as cerebral cortex folding, is the process by which the surface of the brain acquires ridges (gyri) and grooves (sulci).

With this folding, the brain's surface area grows, supporting more neurons and sophisticated information processing. The cortex exhibits a great deal of variation in size and shape both between and within species.

We set out to find a way to define the shape of the cortex, and express what is unique about the complex shapes and folds that comprise each cortex. One can look at an image of a cerebral cortex, and recognize what it is. But how can we tell apart your cortex from mine? Or how can we distinguish a giraffe’s cerebral cortex from that of a marmoset? This requires a more expressive way to describe the shape of the cortex.”

Yujiang Wang, Study Lead Author, Future Leaders Fellow, Computational Neurology, Neuroscience & Psychiatry lab, School of Computing, Newcastle University

Wang and associates started by defining two fundamental ideas. First of all, they understood that cortices are thin sheets of grey matter that are intricately folded around white matter; the size and thickness of these sheets accurately dictate the degree of folding that occurs. As a result, cortices cannot simply take any folded shape.

Scientists refer to this idea as universal scaling. Subsequently, they came up with a method to “melt” the cerebral cortex, meaning that folds smaller than a given threshold may be removed, leaving the remaining folds to be studied separately.

This demonstrated the second principle, which states that cortices are made up of folds of different sizes, with the little folds having a self-similarity to their larger folds. This is similar to the phenomenon known as fractal scaling, in which elaborate patterns appear at ever-smaller scales within a complicated geometric design.

The scientists next examined the cerebral cortex of eleven distinct primate species, including humans, chimpanzees, and marmosets, using these concepts of universal scaling and self-similarity. This showed that all of the species' cortices follow a common scaling law and have a similar fractal shape, despite their obvious visual differences.

As a result, the most complex brain ever studied that of a human begins to resemble that of a chimpanzee when the team's “melting” method is applied to remove the smallest folds. The cortex of a chimpanzee will “melt” to resemble that of a rhesus monkey, and so forth.

These results imply that there is just one method for a cerebral cortex to fold, independent of species. What makes them so obviously different on an MRI scan, then? Their sizes appear to differ; some are much smoother than others, such as the marmoset cortex, and some are extremely folded, similar to the human cortex

The key here is to precisely define what we mean by ‘resemble’. One can imagine a shape that looks like a human cortex, but, as you zoom in, you find within each fold there are infinitely smaller folds. Such a shape cannot exist in nature, but it can be defined mathematically as a fractal shape, as we have done here. What we have shown is that all cortices of the species we have studied resemble this fractal shape for a certain range of fold sizes.”

Bruno Mota, Professor and Study Senior Author, metaBIO Lab, Instituto de Física, Universidade Federal do Rio de Janeiro
 

Consequently, Mota continues, each species has a distinct range of fold sizes for which the likeness holds, which accounts for a major portion of the variances in cortical morphologies seen among these species. This range is wider in chimpanzees, whose brains are more folded, and narrower in marmosets, which have smoother cortexes.

Future research will focus on examining more specialized cortical regions, as the authors point out that their study was restricted to descriptions of full cerebral hemispheres. They will also look into how the fractal form of the cortex is impacted by neurodegenerative illnesses like Alzheimer's.

This could potentially make it possible to identify more precise biomarkers for a variety of neurological disorders and diseases and provide insight into how they progress.

Our results suggest a universal blueprint for mammalian brain shape and a common set of mechanisms governing cortical folding. We hope that our framework for expressing and analyzing cortical shape can become a powerful tool to characterize and compare cortices of different species and individuals, across development and aging, and health and disease.”

Bruno Mota, Professor and Study Senior Author, metaBIO Lab, Instituto de Física, Universidade Federal do Rio de Janeiro