For the regions, whose average myelin content significantly correlated with the scores of the dominant hand, the regression on the original regional and performance scores was repeated with respect to all confounding factors but age. For each hand, a Multiple Mediation Model (Figure 1) examined the effect of age (independent variable) on the pegboard test scores (dependent variable) and identified which of two regional residual scores (intermediate variable) had a greater indirect effect, i.e., accounted for greater variance. The P-value of an effect was inferred from repeating the previous permutation test with respect to the normalized βN value, which was the β value of the effect divided by the β value of the direct (unmediated) effect between age and the pegboard score.
