In Experiment 2, we sought to replicate the principal behavioral findings in a new cohort of older adults and examine the frequency and anatomical specificity of HD-tACS by using nontuned stimulation and montages that targeted frontal and temporal regions, separately (see Methods; Figs 1a, 5a, b). There were four main findings (Fig. 6). First, during post-stimulation memory blocks of the sham condition, performance was impaired in the new group of older adults compared to the 20-year-olds from Experiment 1 (RT, t68 = 2.173, p = 0.033, dz = 0.519; accuracy, t68 = 4.186, p < 0.01, dz = 1.001), and these impairments were observed at all nine sequential 4-minute-long time bins over the full duration of the experiment, including stimulation and post-stimulation periods (RT, ts68 > 2.546, ps < 0.014, dsz > 0.609; accuracy, ts68 > 3.332, ps < 0.01, dsz > 0.797), replicating the between-group differences of Experiment 1. Second, frontotemporal theta-tuned stimulation relative to sham preferentially enhanced the accuracy of working memory behavior in older adults independent of RT, and these post-stimulation accuracy gains were large enough to
