## 2 Materials and Methods  
### The Experiment  
Ethics approval for the study was granted by the Health and Human Sciences Ethics Committee with the Delegated Authority of the University of Hertfordshire (UH)\(-\)Protocol number HSK/SF/UH/00124.  
Sixty-six participants were recruited as a convenience sample from among healthy staff and students at the University, local complementary health practitioners and other contacts. After the completion of some online questionnaires and an explanation of the procedures to be followed, participants attended their first session. They attended four sessions in all, a week or more apart (except for four participants who dropped out after only one session, and another who only completed three sessions). All sessions were conducted in the University Physiotherapy Laboratory, although despite our best efforts, this could not be completely soundproofed, or temperature controlled.  
Participants were seated upright in a comfortable chair, with both forearms supported. Informed consent was obtained, further paper questionnaires were completed, and the participants were then prepared for the session. This preparation, which took around 15 min, involved fitting an EEG cap with head movement sensors attached, and affixing electrocardiogram (ECG) electrodes to the forearms, as well as other sensors to the fingers of both hands. The EEG cap, ECG electrodes and other sensors were worn for the remainder of the session (usually around 60 to 90 min).  
Following an initial 5 min Baseline recording ('time slot 1'), TEAS was applied for 20 min to each hand, with a short pause halfway through to allow for further questionnaires to be completed and participants to rest briefly (Figure 2). Otherwise, during the whole procedure, participants were asked to keep their eyes open and focus gently on an object in front of them (to reduce eye movement artefacts). EEG recording continued during stimulation (Stim1-Stim4, or 'time slots 2-5'), which was between the acupuncture point LI4 (_hegu_), located on the dorsum of the hand between the 1 st and 2 nd metacarpal bones, and the ulnar border of _the same_ hand. In other words, the current only passed between the electrodes on each hand, and did not flow through the arms and torso, so that, in principle, it should not affect the heart\(-\)or brain\(-\)directly.  
After stimulation (and the completion of other questionnaires), the recording was continued for a further 15 min (Post1-Post3, or'slots 6-8') to assess post-stimulation changes (Figure 2). The electrodes and sensors were then removed, and further questionnaires were filled out before the participant left. After the laboratory experiment, 47 of the 66 participants completed further online questionnaires on several personality traits.  
A charge-balanced Equinox E-T388 stimulator was used in all four sessions (Figure 3), and in each session, was set at one of four different frequencies \(-\)2.5 alternating monophasic pulses per second (pps), 10 pps, 80 pps or 160 pps (the frequency or number of cycles of stimulation per second, in Hertz, was at half these values). For the three lower frequencies, the output amplitude was set to provide a'strong but comfortable' sensation for the participant. In contrast, 160 pps was applied as a'sham' treatment, with the device switched on (and a flashing light visible), but the output amplitude remaining at zero throughout \(-\) although a pretence was made of turning up the amplitude, out of sight of the participants. Nonetheless, the stimulation (at 80 and 160 pps) was visible as an interference pattern (envelope) on one of the screens, showing the recorded ECG (although hidden from participants' view so as not to distract them), and some participants were aware of a sensation in their hands at some moments during their sham session. The different stimulation frequencies for each participant were applied in a semi-randomised balanced order.