Limitations
NRS
STILL
Concerning the SEP, in our setup we only used one cortical recording electrode at a time and only evaluated the N20-P25-amplitude and N20-latency for the upper extremity and only the N30-P40-amplitude and N30-latency for the lower extremity. From this follows that we cannot rule out effects of tsDCS on other SEP values which could have been recorded using different montages, and we cannot rule out effects of tsDCS on SEP values associated with nociception. Furthermore, we cannot definitely ensure that in every subject the SEP stimulation electrodes were in the exact same position for every measurement. We tried to keep the variation low in this regard by checking the correct positioning of the electrodes before every measurement, but it is still likely that the positioning of the electrodes was not exactly the same during the course of the experiment. We found neither a carry-over effect nor a group effect on the current intensity of the SEP measurements, which indicates that this variation did not introduce systematic error into the SEP measurements. Still, it is possible that this variation reduced the power of our experiment regarding the SEP variables.It should also be mentioned that the time interval between the end of tsDCS stimulation and PREP measurement for upper / lower limb varied depending on the group to which the subject was assigned (see Fig. The PREP parameters, especially the N1P1-amplitude, showed a considerable variability with standard deviations ranging from 36 to 51% of the mean for the N1P1-amplitude and from 13 to 23% of the mean for the N1-latency. These standard deviations lie within the range of previous studies on healthy subjectsAn important limitation of this interpretation is the scarcity of data on habituation effects during PREP using CE since this was so far only reported in one studyRegarding the change of N1P1-amplitude and NRS value during a session, there are some extreme values that fall outside of the range of 1.5× the interquartile distance, which is often used as a method to find possible outliers. Another method commonly used is the z-score-method. By this method, all of the possible outliers have a z-score of less than 3. We found no specific reason (e.g., technical errors) to exclude any of these data points. As can be seen from Figs. In our study, we used sham stimulation as control experiment. Additionally, to further explore the underlying mechanism other control conditions could also be used, such as applying cathodal tsDCS, applying tsDCS over other regions of interest or placing the return electrode at other locations. The additional use of a cervical electrode could have been more conclusive as to whether the effect of low-thoracic tsDCS was actually a local one. So far, we can only draw this conclusion on the basis of previous results.In summary, we could show an effect of low-thoracic tsDCS on N1P1-amplitudes of PREP using CE which was not inhibitory as expected, but rather facilitating. The neurophysiological mechanisms underlying these contrasting effects remain unclear and could not be investigated in this study. However, the fact that there was an effect, in contrast to SEP, suggests that PREP using CE are not the product of a Aß-fiber activation, but can rather exhibit a nociception specificity. Future studies should directly compare the effects on measurements utilizing different types of painful stimuli