A finer-grained analysis of within-session behavioral dynamics revealed that the stimulation-induced improvement in accuracy occurred relatively quickly, within approximately 12 minutes of HD-tACS delivery, and continued over the full 50-minute duration of post-stimulation blocks (Fig. 2b). During the first 4 minutes of stimulation no differences in performance accuracy between sham and active conditions were apparent. However, by time bin 2 (i.e., between 8–12 minutes from the start of stimulation) older adults began to show significant accuracy gains, which peaked by the first time bin of the post-stimulation period (i.e., time bin 4), and continued for the duration of the experiment. Stimulation-induced changes in RT lagged behind those in accuracy, and were short-lived (Fig. 2c). Significant RT speeding was first observed towards the end of the stimulation period (i.e., time bin 3), and then again offline only at time bins 4 and 6. The stimulation x time interactions on accuracy (F8,656 = 16.707, p < 0.01, partial η2 = 0.169) and RT (F8,656 = 3.655, p = 0.009, partial η2 = 0.043) were significant. The results suggest that stimulation rapidly improved
