Longitudinal analysis for upwards of three visits for each of 185 NCANDA adolescents (aged 12-21 years at baseline) confirmed the cross-sectional findings with respect to regions showing highly significant age effects on the normalized myelin content, namely, areas 4, 23c and p24' (Figure 7). As noted in Table 5, significant linear age effects were detected bilaterally for all three regions (P < 0.05 for all regions, where P < 0.001 for area 4 [R], 23c [R] and p24' [L]). A significant deceleration of the normalized myelin content increase was revealed for area 4 [R] (P = 0.004, βage2 = −0.00017) with the quadratic age effect (i.e., with respect to βage2) being negative while the gain with respect to the linear age effect (i.e., with respect to βage) was significantly greater than that of the entire cortex. The two regions with significant acceleration of aging effects (23c [R]: P = 0.001, βage2 = 0.00013; p24' [L]: P = 0.022, βage2 = 0.00011) had significantly smaller gains in normalized myelin content.
