Also of interest was whether the association between reduced energy and externalizing disorders was mediated by a reduction in the phase-locking of EEG activity. To simplify this analysis, we carried out these mediation analyses using the model in Figure 1 at the parietal site (which produced stronger effects than the frontal site) for the three composite disorders. We first examined results for delta and found that path c’ was not significantly smaller than path c and no indirect effect (path ab, Figure 1B) was detected, thus failing to provide evidence that for delta, the EVK effect was mediated by PLF. As illustrated in Figure 3, mediation effects were observed for theta. The total effect of thetaEVK (path c) became non-significant (path c’) while the effect of thetaPLF (path b) remained substantial in models predicting Any-SUD (p = .04) and Any-EXT (p = .05). A mediating role of thetaEVK by thetaPLF (path ab) was significant for these two models, suggesting that a reduction in P3-related theta phase-locking at the parietal site may partially explain the total effect of reduced theta energy on externalizing diagnoses, particularly those involving SUDs.
