Transcutaneous spinal direct current stimulation (tsDCS)
For tsDCS, two saline-soaked sponge electrodes (7 × 5 cm) were fixed to the volunteers' skin. The cathode was fixed to the right shoulder, while the anode was fixed to the spinous process of thoracic vertebra 10 (Th10). The stimulation device was a DC-stimulator produced by neuroConn GmbH, Ilmenau, Germany. For anodal tsDCS, a direct current stimulation increased gradually over a timespan of 10 s to an intensity of 2.5 mA, at which it was kept up for 20 min and then was gradually reduced to zero over a timespan of 20 s. For a sham stimulation, the current also increased gradually over a timespan of 10 s, but was immediately after reduced to zero during a timespan of 20 s. In both conditions, the electrodes were removed after the device signaled that 20 min of stimulation were over. Both stimulations do not differ in evoked sensation, so that neither the subjects nor the examiner could differentiate sham stimulation from anodal tsDCS. Before starting tsDCS, the device checked the impedance between the electrodes and started not before the impedance was reliable below 50 kOhm. The tsDCS parameters were identical to those used by Lenoir et al.