For each region with significant age effects, the average regional normalized myelin content was tallied over vertices in that region whose βage value agreed with the significant finding, e.g., βage of a vertex and region indicated a positive age effect. The resulting myelin score was then correlated with the Grooved Pegboard Test scores (time in seconds needed to complete the test) (Lafayette-Instrument 2002) separately for the dominant and non-dominant hand of the 194 non-ambidextrous adolescents (right handed=180; left handed=14). To identify significant correlations (P-value < 0.05), a linear regression model removed the confounding factors of age, sex, svol, site, ethnicity, and handedness from the regional myelin and performance scores. The Pearson correlation ‘r’ between the corresponding residual scores was calculated for each region and hand. The P-value of ‘r’ was computed by Permutation testing (Boca et al. 2014) of the BRAVO toolbox V.2.0 (https://sites.google.com/site/bravotoolbox; iterations: 5000; one-sided t-test).
