Effects of bilateral anodal transcranial direct current stimulation over the tongue primary motor cortex on cortical excitability of the tongue and tongue motor functions

Transcranial direct current stimulation (tDCS) is a non-invasive tool for inducing cortical excitability changes [1]. Numerous studies have investigated the effects of tDCS over the primary motor cortex (M1) for exploring mechanisms underlying cortical plasticity and motor functions of the limbs. However, little is known about the effects of tDCS on the tongue region. Here, we investigated the effects of anodal tDCS over the bilateral tongue M1 representation relative to those of unilateral anodal tDCS on tongue region cortical excitability and tongue and jaw motor functions. Fifteen healthy volunteers (11 men; mean age, 24.5 years) participated in this randomized, double-blind, sham-controlled, crossover study. Participants provided written informed consent; procedures were approved by the local ethics committee. Three sessions of either anodal tDCS over both hemispheres (bi-tDCS), anodal tDCS over the left hemisphere (lt-tDCS), or sham stimulation were performed with an inter-session interval of at least 7 days for each subject. tDCS was applied for 20 min with a StarStim stimulator (Neuroelectrics, Barcelona, Spain) using two 24-cm2 target electrodes and one 72-cm2 return electrode. Target electrodeswere placed over the tonguemuscle “hotspot” region; return electrode was placed over the forehead. Bi-tDCS session current intensity was 2 mA (0.0833 mA/cm2) and 4 mA for the target and