We show results of linear-mixed modelling of local MT (averaged within 6 mm sphere around peak effects presented in main results Figs. 3a, b and 4a) in left SFG, amPFC, left anterior insula, right IFG, left ACC (anterior midcingulate) and left ventral striatum depicted in columns. We use a design matrix accounting simultaneously for both compulsivity and impulsivity regressors and their respective time/visit interactions (n = 497/288 scans/subjects with both covariates available, 51.7% female). Otherwise, age_mean, visit, sex, socioeconomic status, interactions, and confounds are entered as fixed effects covariates. Rows illustrate model’s trait by time/visit interaction and respective statistics from fixed effect coefficient (t-value, p-value, two-sided, df = 479) suggesting trait-dependent rates of local MT change over study visits for compulsivity (top row) and impulsivity (bottom row). Plots show adjusted longitudinal data and model prediction (including effects of interest intercept, time/visit, trait by time interaction) with model trajectory illustrated in more red to yellow colours for low to higher scores of each trait (z = [−2,1,0,1,2]). Statistics suggest specificity of effects reported in Fig. 3a, b and Fig. 4a for each dimension in all (except one) peak effect region even when accounting for variations of both traits simultaneously. The insula shows attenuated growth as (additive linear) function of both traits when controlling for variations of the other. 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 (time) and data adjusted for effects of no interest (covariates and confounds).