HD-tACS exerted a preferential influence on maintenance-related frontotemporal theta phase synchronization in older adults. As illustrated in Figure 4a, after stimulation, a significant cluster (pcorrected < 0.01) in prefrontal cortex showed increased PLV with left temporal cortex during memory relative to control blocks, whereas the same subjects in the sham condition generated no significant clusters of temporal-seeded PLV in the theta band. This resulted in a significant difference in memory-specific theta PLV between active and sham conditions for older adults (Fig. 4a). Further, the expected information flow direction between prefrontal and temporal regions was effectively reinstated (t41 = −2.592, p = 0.013, dz = 0.400). Repeating the seeded PLV analysis for the gamma band yielded a significant cluster (pcorrected < 0.01) of memory-related gamma in regions nearest lateral occipital cortex for older adults during both sham and active stimulation conditions (Fig. 4b). This resulted in no significant difference in memory-specific gamma PLV across conditions (Fig. 4b). Both stimulation conditions showed no preferred direction of information flow among occipitotemporal gamma synchronization (ts41 < −1.221, ps > 0.229, dsz > 0.188). The
