## 2 Materials and Methods  
### Study Design and Data Collection  
The study received ethical clearance from the University of Hertfordshire's Health and Human Sciences Ethics Committee, which exercises Delegated Authority, under protocol number HSK/SF/UH/00124. The study was conducted with 48 participants, who each attended for four sessions. Sessions were structured into eight 5 min time slots, categorized as pre-stimulation (Slot 1), during stimulation (Slots 2-5), and post-stimulation (Slots 6-8). TEAS was administered using a charge-balanced Equinox E-T388 stimulator at frequencies of 2.5, 10, 80, or 160 pulses per second (pps), with the 160 pps frequency used as a'sham' treatment because it had a very low amplitude and many participants were unable to feel it. Exclusion criteria were carefully considered, to ensure the validity of the study. Participants were excluded if they had suffered a serious head injury, had epilepsy or diabetes, currently had cancer or a respiratory condition that impaired nasal breathing, or had an implanted electronic device. Individuals with impaired peripheral circulation (such as Raynaud's syndrome) or any shoulder, arm, or hand injury were also excluded. Additionally, those with severe physical or mental conditions or with learning disabilities that might prevent them from completing the study, or those dependent on prescribed or other psychoactive substances, were not included. Heavy users of caffeine, nicotine, or alcohol,defined as consuming more than 400 mg of caffeine, 20 cigarettes, or two alcoholic drinks daily, were also excluded. Individuals undergoing non-routine non-pharmacological or complementary medical treatments, women who were pregnant, and those with minimal understanding of English were not eligible to participate. The majority of the participants were local white British people from Hatfield in the United Kingdom, highlighting a limitation in sample diversity that may have influenced the results. The selection of the three TEAS frequencies used in this study--2.5, 10, and 80 pps--was guided by their prevalent use in clinical settings and the existing literature on TEAS [20]. The sequence of stimulation frequencies was semi-randomized across participants, to mitigate order effects. Figure 1a displays the Equinox stimulator, and Figure 1b shows the arrangement of sensors and electrodes, featuring the fingertip photoplethysmography (PPG) sensor, an ECG electrode positioned on the right forearm, and TENS (transcutaneous electrical nerve stimulation) electrodes located at the LI4 (Hegu) acupoint and along the ulnar edge of the hands. The ECG electrodes placed on the left forearm are out of view, and the thermistor attached to the left middle finger is visible in the figure. The electrodes were round, with a diameter of 32 mm. A new set was used for each participant. The manufacturer was Schwa-Medico, Ehringshausen, Germany. EEG data were recorded from 19 electrodes according to the international 10/20 system, with linked ears serving as the reference and a ground electrode placed anterior to Fz. Custom-fitted Electro Cap International (ECI) caps were used to ensure participant comfort and data quality. The EEG signals were amplified using a Mitsar EEG-202 amplifier and digitized at a sampling rate of 500 Hz. Preprocessing steps included the removal of noise and artifacts to produce clean EEG datasets for each participant, suitable for further analysis. The EEG slots were then divided into five 1 min segments, and each segment was split into two halves. Ten-second epochs were then extracted from the beginning of each half (at the 0 and 30 s marks). This method yielded ten epochs per slot, resulting in a comprehensive dataset that was used to assess the effects of the different TEAS frequencies on EEG activity. Figure 2 shows the first 1 s of each slot for better illustration, resulting in 8 s of data.