Experiment 1, phase-amplitude coupling results.a, Results show a significantly increased phase-amplitude coupling (PAC) cluster in left temporal electrodes (top) and voxels (bottom) for theta (7–9 Hz) phase and gamma (26–34 Hz) amplitude frequencies on memory compared to control blocks in younger adults at baseline and older adults after frontotemporal inphase theta HD-tACS (pcorrected < 0.01, two-sided cluster permutation test, n = 42). No other clusters were identified. b, Comodulograms show group x block and stimulation x block interactions for all combinations of low-frequency phase (2–16 Hz) and high-frequency amplitude (18–120 Hz) from left temporal electrodes showing significant PAC. Crosshairs indicate values where t > −2.6 or −2.4, respectively (pcorrected < 0.01, two-sided permutation test, n = 42). c, Box plots of gamma (26–34 Hz) amplitude binned by 8-Hz phase shown across groups and conditions (n = 42). d, Cross-frequency directionality between 6.5–10.5-Hz phase and 26–34-Hz amplitude from left temporal electrodes for each group and condition (center, mean; shaded region, SEM; n = 42). e, Individual subject Pearson correlations (two-tailed) between accuracy and theta-gamma PAC from left temporal electrodes for each group and condition (n = 42). Box-plot center, median; box limits, lower and upper quartiles; whiskers, lower and upper extreme values; points, outliers.
