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 a new group of older adults participated in one pre-experiment screening day and five experimental days. The design was double blind, sham-controlled, within-subjects, with stimulation order randomized across subjects. Each subject received: (i) sham stimulation to replicate the age-related working memory deficits observed at baseline in Experiment 1, (ii) frontotemporal inphase theta-tuned stimulation (Fig. 1a) to replicate the working memory benefit of stimulation relative to sham in Experiment 1, (iii) unifocal frontal theta-tuned stimulation (Fig. 5a) and (iv) unifocal temporal theta-tuned stimulation (Fig. 5b) on separate days to test the anatomical specificity of the multifocal frontotemporal montage, and (v) frontotemporal inphase 8-Hz nontuned stimulation (Fig. 1a) to test the frequency specificity and need for personalized intervention. Experiment 3 consisted of one screening day and two experimental test days involving sham and frontotemporal antiphase theta-tuned HD-tACS (counterbalanced across subjects), in which the behavior of younger adults was recorded “online” (i.e., during the 10-block delivery period of sham or active stimulation). Experiment
