In this report, MRIs of 226 healthy adolescents from the NCANDA study and 686 young adults from the HCP study are repurposed to characterize the developmental trajectory of regional myelin density from adolescence to adulthood. Accordingly, the estimate of myelin contents across these two large, publicly-available datasets were harmonized so that myelin scores of the HCP data set were similar to those provided by the original study. We predicted that heavy myelination occurs in cortical regions with relatively direct, predetermined circuitry supporting unimodal sensory or motor functions and shows a steep developmental slope until young adulthood when further myelination decelerates (for example, area 4). By contrast, light myelination occurs in regions with highly plastic circuitry supporting complex functions (for example, areas 23/24) and follows a temporally lagging developmental trajectory. For the NCANDA cohort, functional ramifications of the regionally different myelin contents were tested with a speeded, eye-hand coordination test. Further, longitudinal NCANDA data were used to validate the cross-sectionally observed developmental trajectories. To do so, our approach harmonized the computations of myelin content across visits by matching the intensities of
