Several studies have demonstrated that direct currents delivered through the skin at the level of the low-thoracic spinal cord can influence spinal cord function. In human volunteers, anodal low-thoracic transcutaneous spinal direct current stimulation (tsDCS) alters spinal processing of nociceptive inputs. Whether cervical tsDCS is able to do the same is less well known. In this double-blinded, sham-controlled and cross-over trial, the investigators will compare the effects on the nociceptive processing of healthy volunteers of cervical and low-thoracic tsDCS.