Spherical Transformation and Registration

The white surface is homeomorphically transformed to a sphere (Fischl, Sereno, Tootell, et al. 1999), thus keeping a one-to-one mapping between vertices of the native geometry and the sphere. Various strategies are available to place these surfaces in register with a common reference and allow intersubject comparisons, including the method used by FreeSurfer (Fischl, Sereno, Tootell, et al. 1999), spherical demons (SD) (Yeo et al. 2010), multimodal surface matching (MSM) (Robinson et al. 2014), among others. Methods that are not diffeomorphic by design but in practice produce invertible and smooth warps can, in principle, be used in registration for areal analyses. In the present analyses, FreeSurfer was used (a complementary comparison with SD is shown in Supplementary Material S2). The measurements of interest obtained from native geometry or in native space, such as area and thickness, are stored separately and are not affected by the spherical transformation or registration.