The designs of Experiment 1.

(A) Schematic representation of the modified Sternberg paradigm used in this study. For each trial, participants were shown a list of 7 consonants and were asked to indicate (by button press) whether the probe was part of the memory list. For trials with tACS application, EEG recording started from the onset of the retention subprocess, and stopped once tACS was triggered; for trials where tACS was not triggered, EEG recording lasted for 2.5 s since the beginning of the retention subprocess. (B) Participants completed 3 tACS sessions of different conditions (in-phase tACS, random-phase tACS, or anti-phase tACS). The order of the tACS conditions was counterbalanced across participants, and the 3 sessions of each participant were all separated by at least 3 days. (C) Experimental procedure for each session. Within each session, participants first completed 60 trials of the Sternberg task with EEG recorded during the retention subprocesses (“Baseline”). Subsequently, the IAF of this session was calculated using the Baseline EEG data. Then, 180 tACS-EEG trials of the Sternberg task were performed with tACS delivered specifically during the retention subprocesses (“Online tACS”). After Online tACS, participants completed 60 trials of the Sternberg task, while EEG data were recorded during the retention subprocesses (“Offline tACS”). (D) tACS was delivered in 3 different conditions: (1) in the in-phase condition, tACS was applied at the IAF with 0° relative phase to the endogenous alpha oscillations; (2) in the anti-phase condition, tACS was applied at the IAF with 180° relative phase to the endogenous alpha oscillations; and (3) in the random-phase condition, no phase alignment between the delivered tACS and the detected endogenous alpha oscillations was used, with the phase differences changing from 0° to 360° across trials. For all conditions, the onset phase of the sine-wave tACS waveform was invariably 0°. The illustrative waveforms in the figure do not represent the actual sizes. EEG, electroencephalogram; IAF, individual alpha frequency; tACS, transcranial alternating current stimulation.