questionnaire 69 and visual analog scale 70, which included questions regarding attention, concentration, mood, vision, headache, fatigue, and skin sensations under the stimulating electrodes. Scores on these ratings did not significantly differ by stimulation condition (Experiment 1, Fs1, 41 < 0.322, ps > 0.574; Experiment 2, Fs4, 108 < 0.446, ps > 0.669; Experiment 3, Fs1, 17 < 0.693, ps > 0.417; Experiment 4, Fs1, 13 < 1.168, ps > 0.299). In addition, all subjects were asked at the end of the experiment whether they could guess the presence of stimulation and were at chance levels (Experiment 1, 52.4% hit rate; Experiment 2, 19.0% hit rate; Experiment 3, 55.6% hit rate; Experiment 4, 55.0% hit rate). Finally, to avoid expectancy and demand characteristic effects, in Experiment 1 all younger adults were treated the same as the older adults in the sham condition, that is, HD-tACS electrodes were placed, and the sham stimulation was used to induce the same itching and tingling experienced by older adults.
