Vagus Nerve Stimulation
Howland, Robert H. “Vagus Nerve Stimulation.” Current Behavioral Neuroscience Reports, vol. 1, no. 2, 2014, pp. 64–73., doi:10.1007/s40473-014-0010-5.
Vagus nerve stimulation has been an FDA-approved treatment for chronic treatment resistant depression since 2005, where it is safe and appears to have a modest effect. Currently the stimulation is done through an implanted device, but Howland believes that transcutaneous stimulation (using an external device) is a promising new way to stimulate the vagus nerve, and that more studies should be done to see its effectiveness.