After stimulation, we observed a preferential increase in local theta-gamma PAC, paralleling the improvement in behavioral accuracy in older adults. This was evidenced by a number of findings following active HD-tACS. First, we identified a cluster of left temporal electrodes showing significantly increased PAC (pcorrected < 0.01; Fig. 3a) between 7–9 Hz phase and 26–34 Hz amplitude frequencies in the memory relative to non-memory control block. Computing PAC in source space produced a similar result in left temporal cortex. Second, the stimulation x block interaction nearest the 8-Hz phase 30-Hz amplitude center frequencies from left temporal electrodes was significant (pcorrected < 0.01; Fig. 3b). Third, older subjects now exhibited significantly nonuniform theta sorted gamma amplitude values (z = −2.041, p = 0.024; Fig. 3c). Fourth, theta phase was predictive of gamma amplitude over left temporal electrodes, rather than the opposite (t41 = 2.235, p = 0.031, dz = 0.345; Fig. 3d). Fifth, after stimulation had restructured theta-gamma PAC in older adults, the strength of this coupling was now significantly predictive of individual working memory accuracy (r41 = 0.571, p <
