Our cortical myelin development hypothesis (shown as graphs in Figure 2) was supported by the cross-sectional trajectories of both the raw and normalized myelin content (see Appendix A.4). The raw myelin content was greater with age (ages 12 to 35) throughout the cortex; however, individual cortical regions differed with respect to age-dependent myelination trajectories. To highlight this difference, normalization of the raw myelin scores removed the growth pattern of the entire cortex so that a positive residual score (i.e., normalized myelin content) revealed greater myelin content and a negative normalized score encoded less myelin content relative to the myelin content of the entire cortex (Glasser et al. 2014). The resulting age-dependent myelination trajectories showed greater gains in myelin with age in the motor cortex and smaller gains with age in the midcingulate cortex. These development patterns were more extreme (i.e., far larger or far smaller gains) during adolescence than in young adulthood.
