Gyrification from constrained cortical expansion

TL;DR

This paper demonstrates that brain surface folding (gyrification) can be explained by a mechanical instability caused by cortical expansion constrained by white matter, supported by physical models and simulations.

Contribution

It introduces a simple mechanical model showing how cortical expansion leads to gyrification, explaining brain folding patterns through physical and numerical simulations.

Findings

Gyrification results from a mechanical instability due to cortical expansion.

Patterns depend on cortical expansion and thickness relative to brain size.

Model explains variations and anomalies in brain folding.

Abstract

The exterior of the mammalian brain - the cerebral cortex - has a conserved layered structure whose thickness varies little across species. However, selection pressures over evolutionary time scales have led to cortices that have a large surface area to volume ratio in some organisms, with the result that the brain is strongly convoluted into sulci and gyri. Here we show that the gyrification can arise as a nonlinear consequence of a simple mechanical instability driven by tangential expansion of the gray matter constrained by the white matter. A physical mimic of the process using a layered swelling gel captures the essence of the mechanism, and numerical simulations of the brain treated as a soft solid lead to the formation of cusped sulci and smooth gyri similar to those in the brain. The resulting gyrification patterns are a function of relative cortical expansion and relative thickness (compared with brain size), and are consistent with observations of a wide range of brains, ranging from smooth to highly convoluted. Furthermore, this dependence on two simple geometric parameters that characterize the brain also allows us to qualitatively explain how variations in these parameters lead to anatomical anomalies in such situations as polymicrogyria, pachygyria, and lissencephalia.