Figure 1. Schematics of postulated models for brain cortical folding.

(a) Intracortical differential growth model (Richman et al., 1975). Brain cortex is roughly divided into two layers with the outer layer growing faster (indicated by “++”) than the inner layer (“+”). All other underlying tissue is treated as a softer elastic foundation without any growth (“0”). Differential growth results in cortical buckling. (b) Axon tension hypothesis (Van Essen, 1997). Tension (black arrows) in axons that strongly interconnect two cortical regions pulls them closer to each other to form an outward fold. An inward fold forms between two outward folds to separate weakly interconnected cortical regions (grey arrows). (a’) Phased differential growth model. Cortical growth in region 1 (t < t_c) followed by cortical growth in region 2 (t > t_c) produces two folds. The underlying subplate grows to provide stress relaxation in response to the cortical growth. (b’) Actual distributions of axon tension based on present dissection and histology data. Axons are under tension (black arrows), and the majority of them are located circumferentially in the subcortical white matter tract and radially in the subplate or the cores of outward folds (from around P18). No circumferential tension (grey arrows) or axons (grey dotted lines) was detected in the cores (subplate) of outward folds.