Another potential contributor to decreased GM volume and cortical thickness is the ongoing myelination of small axons at the interior cortical border. This would lead to voxels changing classification from GM to WM along this border, resulting in cortical thinning as assessed by MR volumetrics, without necessarily entailing changes in synaptic density (Sowell et al. 2001). At present it is thought both factors are likely involved, but a definitive answer awaits studies such as imaging of nonhuman primates with post-mortem validation.
