In vivo derivation of averaged normalized cortical myelin maps in the NCANDA adolescent and HCP young adult cohorts revealed higher values in the older than younger group particularly in the striate, motor, occipito-parietal cortices. These regions were marked by high T1-weighted / T2-weighted MRI ratios, indicating heavy myelination. These regions are generally considered unimodal, in that they subserve sensory or motor functions and tend to complete development early. By contrast, multimodal cortex subserves integrative, complex cognitive and emotional functions, which develop later and with continued interaction with the environment (Purger et al. 2016; Snaidero and Simons 2017). Multimodal cortex generally contains lightly myelinated fibers that underlie greater intracortical circuit complexity than unimodal circuitry (Glasser et al. 2014).
