Transcutaneous vagus nerve stimulation for the treatment of depression: a study protocol for a double blinded randomized clinical trial oleh Rong Pei-Jing, Fang Ji-Liang, Wang Li-Ping, Meng Hong, Liu Jun, Ma Ying-ge, Ben Hui, Li Liang, Liu Ru-Peng, Huang Zhan-Xia, Zhao Yu-Feng, Li Xia, Zhu Bing, Kong Jian