Procedure
The subjects were randomly assigned to eight groups according to the order of testing by limb (foot first/hand first), by test method (PREP first/SEP first) and by order of stimulation types (sham stimulation first/anodal tsDCS first). Subjects and examiner were blinded to the order of stimulation types, but not to order of testing by limb and test method.The experiment was divided in two sessions, which were at least one week apart. On each subject, anodal tsDCS was performed in one session and sham stimulation in the other session. Before and after application of tsDCS in each session, SEP and PREP using CE were performed on right hand and right foot of each subject. The sequence of recording PREP and SEP after electrical hand and foot stimulation was randomly assigned to 4 different groups (block of tests). The different measurements in each session were performed immediately after each other and also immediately after the end of tsDCS, only with a delay of a few minutes necessary to prepare the next measurement. Setup and timeline of the experiment are visualized in Fig. Experiment design. Each subject participated in a sham session and an anodal session, which each consisted of one block of tests before and after sham stimulation or anodal tsDCS. The order of measurements in a block of tests depended on the group the subject was in. The top part of the figure shows the timeline for one block of tests. The middle part shows the timeline for one session, which consisted of either sham or anodal tsDCS, preceded and followed by one block of tests. The bottom part shows the timeline for the whole experiment, which consisted either of one session of anodal tsDCS, followed by an interval of at least one week and then one session of sham tsDCS, or of one session of sham tsDCS, followed by an interval of at least one week and then one session of anodal tsDCS.During the experiment, the subjects were seated in an armchair. They were instructed to keep their gaze fixed on a mark at a distance of about 1.5 m in front of them during all measurements, in order to reduce artifacts due to eye movements, similar to previous studies, e.g. 