Folding forces: How brains get into shape

The idea of mechanical factors shaping the cortical features goes back as far as the end of the 19th century [1] and has been recently revived [2, 3] and investigated with computational models [4]. Our current model deals with the impact of various mechanical factors on the formation of cortical convolutions, by simulating gyral collisions. Our finite element model implements a collision detection and repulsion mechanism for intersections of the growing cortex with both the skull and with other portions of the cortex. For a fixed skull size, we observed that if the growing cortex impacts the skull prior to folding onset, no folds form after the impact, and the growth of the cortical region only occurs radially [F1]. If the cortex is already folded at impact, growing gyri may overlap despite repelling mechanisms [F2]. For a variable skull size with similar growth parameters as the cortex [F3], there was no influence on convolution pattern. The presence of a skull had a limiting effect on growth, leading to a deepening of sulci, without much influence on the shape of gyri. We also investigated the potential role of radial fibers in cortical folding, as proposed by Toro and Burnod [4]. Our simulations show similar folding onsets in the presence [F4] or in the absence of radial fibers [F5]. However, the overall smaller brain size rendered by the presence of radial connections and/or skull appears to facilitate the collisions of gyri. [F2-4]. We consider these collisions o