Lateralization of the analgesic effect of the transcutaneous vagus nerve stimulation (t-VNS®) in human

INTERVENTION: Intervention 1: verum left ‐ adjustment of the ear electrodes at both sides, left‐sided transcutaneous vagus nerve stimulation (t‐VNS): frequency 25 Hz, adjust stimulation intensity above perception threshold and below pain threshold Intervention 2: verum right ‐ adjustment of the ear electrodes at both sides, right‐sided t‐VNS: frequency 25 Hz, adjust stimulation intensity above perception threshold and below pain threshold Intervention 3: sham ‐ adjustment of the ear electrodes at both sides, no t‐VNS CONDITION: Healthy volunteers PRIMARY OUTCOME: psychophysical measurement of the pressure pain threshold (PPT) at the left and right hand ipsilateral and contralateral to t‐VNS; time of measurement: befor t‐VNS (PRE), during t‐VNS (PERI) und after t‐VNS (POST) SECONDARY OUTCOME: psychophysical measurement of the mechanical pain threshold (MPT) and the mechanical pain sensitivity (MPS) at the left and right hand ipsilateral and contralateral to t‐VNS; measurement of blood pressure and pulse; time of measurement: befor t‐VNS (PRE), during t‐VNS (PERI) und after t‐VNS (POST); sensitive sensations (self‐evaluation by the volunteer); questionnaire for vegetative symptoms (self‐evaluation by the volunteer): assessment of any vegetative changes during measurements (increase/decrease e.g. salivation, lacrimation etc.).; number of adverse events / incidents INCLUSION CRITERIA: healthy volunteers healthy in terms of medical history written informed consent form after volunteer information