Locations and the stimulation electrodes on the auricular surface. Red spots indicate the locations for transcutaneous auricular vagus nerve stimulation (tVNS) and blue spots indicate the locations for sham tVNS. The left figure shows the two tVNS points being in the innervation area of the auricular branch of vagus nerve (green). The right figure indicates the clip/electrode used in the study. To achieve the design of double-blind, two paired electrodes were fixed to one ear clamp. But only one of them was connected with the wire that was imbedded in the clamp in order to keep the operation of double-blind. In the tVNS group, the upper electrodes are wired to the machine while in the sham tVNS group the lower electrodes are wired.