We took several measures to ensure that information about the experiments would not lead to biasing of results, using previously established methods 21, 58, 63–68. First, all experiments were within-subjects in design, meaning each older adult (in Experiments 1 and 2) or younger adult (in Experiments 3 and 4) performed all levels of the independent variable (i.e., every stimulation condition) to eliminate confounds related to subject differences. Second, the experiments were sham-controlled. The sham stimulation condition followed the same procedure as the active theta-tuned frontotemporal condition, but stimulation only lasted 30 seconds, ramping up and down at the beginning and end of the 10-block period, simulating the tingling sensation that subjects typically experience and then quickly habituate to during active stimulation 67. Of note, while the sham condition implemented here is considered a gold standard in brain stimulation research, the present study benefited from several additional stimulation controls, such as the pre-stimulation baseline of Experiment 4 and the frontotemporal nontuned condition of Experiment 2. The latter condition stands as a particularly robust control because unlike the sham condition the electrical
