In Experiment 1, younger adults visited the laboratory on two test days (one pre-experiment screening day and one experimental day). The experimental day consisted of recording EEG and behavior during and after the administration of sham HD-tACS. In Experiment 1, older adults visited the laboratory on three separate test days (one pre-experiment screening day and two experimental days). On the experimental days, subjects had their EEG and behavior recorded during and after the administration of frontotemporal inphase theta-tuned and sham HD-tACS (Fig. 1a). The two-day design for older adults was double blind, sham-controlled, and within-subjects, with stimulation order counterbalanced across subjects. HD-tACS was administered over the first 10 blocks (or ~25 minutes) of the task (Fig. 1b). After stimulation, all subjects continued to perform the task for another 20 blocks (or ~50 minutes). Behavioral data were analyzed during and after stimulation, whereas EEG data were analyzed on post-stimulation blocks only to avoid potential confounds associated with tACS 52, 53. Experiment 2 was similar to Experiment 1, but only behavior was collected, and the experiment consisted of six days in which
