(a) Impulsivity was found to be associated with a reduced growth of MT in subcortical white matter areas, especially adjacent to striatum (internal capsule), anterior core white matter and close to pyramidal tracts. We show projections of Z-maps from one-sided Wald-test of a negative impulsivity by time/visit interaction (p < .05, voxelwise FDR, n = 497/288 scans/subjects and test applies to panels a-b, 51.7% female). (b) More extensive presentation of main results in Fig. 4c. Subjects with higher impulsivity show reduced myelin-sensitive MT especially in anterior medial and lateral (IFG, MFG), insular and parieto-temporal areas. We present voxel-based analysis Z-maps (p < .05, voxelwise FDR) projected on cortical white matter and striatum, directed testing for negative main effect of impulsivity. SPM results are shown in Supplementary Table 5. We account for effects of visit/time, age_mean, sex, socioeconomic status, interactions and confounds. (c) Post-hoc specificity analysis within regions observed in main compulsivity analysis using two separate compulsivity sub-scores PI-WSUR and OCI-R and linear mixed effects modelling of local MT (averaged within 6 mm sphere around peak effects presented in main results Fig. 3a) in left SFG (left column) and ACC (right column). We explore effects of scores and their respective time/visit interactions with otherwise age_mean, time/visit, sex, socioeconomic status, interactions, and confounds entered as fixed-effects covariates (n = 413/217 scans/subjects). Rows illustrate model’s trait by time/visit interaction and respective statistics of fixed effects coefficients (t-value, p-value, two-sided, df = 400) suggesting sub-score-dependent rates of local MT change over study visits for PI-WSUR (top row) and OCI-R (bottom row). Plots show adjusted longitudinal data and model prediction (including effects of interest intercept, time/visit, score by time interaction) with model trajectory illustrated in more red to yellow colours for low to higher values of each sub-score (z = [−2,1,0,1,2]). Statistics suggest that OCI-R score difference are predictive of local MT changes in those areas while PI-WSUR shows similar, but less strong tendencies. Y-axis: MT; x-axis: time of scan in years relative to each subject’s mean age over all visits. All plots show effects of interest and MT data adjusted for effects of no interest (covariates and confounds).