Anodal transcranial direct current stimulation over the lower limb motor cortex increases the cortical excitability with extracephalic reference electrodes

The aim of the present study was to investigate whether anodal transcranial direct-current stimulation (tDCS) of lower-limb primary motor cortex (M1) could increase cortical excitability when reference electrodes were placed at extracephalic positions. Ten healthy volunteers participated in this study. Anodal electrodes were placed over the left lower-limb M1, whereas reference electrodes were placed on the contralateral forehead (cephalic condition) or contralateral upper arm (extracephalic condition). Motor evoked potentials (MEPs) were recorded as a measure of cortical excitability before and after tDCS (2 mA, 10 minutes). Compared with a sham condition, MEPs significantly increased for both cephalic and extracephalic conditions, and this increase was maintained for approximately 60 minutes after stimulation. No side effects were reported. We conclude that tDCS over lower-limb M1 in conjunction with extracephalic reference electrodes can increase cortical excitability without any side effects.