Cortical myelin undergoes growth from pre-adolescence through young adulthood (Grydeland et al. 2013; Miller et al. 2012; Tomassy et al. 2016). Trajectories of the normalized myelin content (i.e., the myelin content normalized to the average Conte69 template) varied in the gain detected over several regions during adolescence. As displayed in Figure 3, the normalized myelin growth was spatially more homogenous across the entire cortex among young adults than adolescents, which may reflect the culmination of local intracortical circuitry development. Specific to adolescents, myelin developmental trajectories were relatively symmetrical by cerebral hemisphere and similar in male and female youth. Cortical myelin growth either had smaller gains (relative to the overall gain in myelin density of the cortex), as occurred in the mid and posterior cingulate cortices ([Vogt] areas p24' and 23c) supporting cognitive processes (Vogt et al. 1995), or greater gains, as occurred in the primary motor cortex ([Brodmann] area 4) supporting primary motor function (Nieuwenhuys 2013). Longitudinal data in the adolescents verified these cross-sectional slope patterns. Heterochronicity in cortical structural (Giedd et al. 2015; Raznahan et al. 2012; Shaw et
