Measurement setup of EP signals

The impedance and EP signals were measured using a BrainAmp DC system (Brain Products GmbH, Germany). The impedance value of each channel at 10 Hz was recorded automatically using a BrainVision system (Brain Products GmbH, Germany). The instrument only measures values between 0 and 100 kilohms. The sampling rate was 1000 Hz, and the band-pass filter was set at 0.3 to 70 Hz. A notch filter was applied at 50 Hz. The EIS test under a frequency range of 0.1 to 1000 Hz was conducted in an electrochemical workstation (CHI750E, CH Instruments Inc., USA) using a two-electrode system. The distance between the two-electrode centers on the scalp was 4 cm. The current was applied on the electrode pair to measure the contact impedance. The amplitude of the test signal for the impedance test was 5 mV, and the frequency of the test signal ranged from 0.1 to 1000 Hz. A commercial 64-channel EEG cap (EASYCAP GmbH, Germany) was used to mount the electrodes. The results of the average EIS were derived through six trials.