Why does our brain look like a giant walnut?

A recent study published in the journal Nature Physics and presented on Harvard’s website reveals the secrets of the brain’s folds and creases, or cerebral convolutions. The researchers examined how exactly our brain takes on this very unique form. How are the human brain’s many folds created? Is it genetics or mechanics?

The specific valleys and peaks of the human brain are only present in a handful of animal species, such as primates, dolphins, elephants, and pigs. On average, skull volume varies between 1,100 and 1,700 cm3. But if we smooth out the brain, it would cover a surface of 1 to 2 m2. Gyrification (the degree of folding) is an important brain characteristic because it allows a large cortex to be stored in a small space.

Several hypotheses have been proposed so far to explain the appearance of the brain’s creases and folds. Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences, in collaboration with Finnish and French scientists have succeeded in showing that brain creasing results from the mechanical compression of the organ in the skull. This compression is also quite useful since the creases bring neurons closer together, promoting shorter and faster connections.

To validate their hypothesis, the research team collaborated with neuroanatomists and radiologists in France to build a 3D model of a human fetus brain using MRI images. The observations showed that the brain of a human fetus is smooth throughout the first twenty weeks of development; it is from the twentieth week of gestation that folding begins. It continues until the child is about 18 months old. The scientists then sought to replicate cortical growth to understand the phenomenon. They applied a layer of elastomer gel to the surface of their 3D model, then immersed it in a solvent. After a few minutes, the researchers observed the layer of gel had swelled, which created mechanical compression forces and caused the formation of folds similar in size and shape to those of a real brain.

The study’s authors also created mathematical simulations of the brain and showed that, following a normal development path, the convolutions also appeared. The brain’s geometry thus serves to guide the folds in certain directions.

This discovery allows us to better understand the development of certain diseases and affect the diagnosis and treatment of a number of neurological disorders. According to JY Chung, one of the study's co-authors: “Brains are not exactly the same from one human to another, but we should all have the same major folds in order to be healthy. Our research shows that if a part of the brain does not grow properly, or if the global geometry is disrupted, we may not have the major folds in the right place, which may cause dysfunction in the brain.”
Source: Tuomas Tallinen, Jun Young Chung, François Rousseau, Nadine Girard, Julien Lefèvre and Lakshminarayanan Mahadevan. On the growth and form of cortical convolutions. Nature physics, Feburary 2016.


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