Effect of low-thoracic anodal tsDCS on N1P1-amplitudes of PREP using CE
pinprick pain
In contrast to the SEP, we were able to demonstrate an effect of low-thoracic tsDCS on N1P1-amplitudes of PREP using CE after stimulation of the lower limb. This difference suggests that signal transmission after electrical stimulation with CE must be different from that via Aß-fibers stimulated by SEP electrodes. Lenoir et al.Thus, the effect on PREP could also be interpreted as a further indication for spinothalamic transmission, which could be attributed to PREP using CE having a nociception-specific property. This result is particularly important in light of the fact that PREP with CE are not considered nociceptor-specific, mainly due to their shorter latency compared to LEPs in the signal transmission via the spinothalamic pathwayHowever, it should be emphasized that we could not prove an inhibitory effect of anodal tsDCS on nociceptive stimuli, as was described previously and as we expected. In our study, N1P1-amplitudes of PREP after stimulation of the lower limb (area innervated by the sural nerve) remained almost unchanged (− 0.4 µV) after anodal tsDCS at the thoracic level, but decreased (− 8.5 µV) after sham stimulation (see Table The different effect of anodal tsDCS on PREP with CE could be explained on the one hand by the fact that CE directly activates A-δ and C-fiber in the superficial skin layers (short electric stimuli applied through concentric planar electrodes)Another possible explanation for our unexpected results could be that tsDCS affects neurons responding to different stimulation qualities differently, resulting in an increased transmission of nociceptive signals through Aδ-fibers and a decreased transmission of nociceptive signals through C-fibers. However, this cannot easily explain all of the previous study results, since LEP and pinprick pain both rely mostly on Aδ-fibers.