Trigeminal Nerve Stimulation for Depression: Dose Finding

This study will aim to examine the use of external trigeminal nerve stimulation (TNS) as an adjunctive treatment for adults with major depressive disorder (MDD) when added onto antidepressant medications. Our primary objective is the examination of two different "doses" of TNS, in terms of pulse frequency. To accomplish our specific aims, the investigators will test the following specific hypotheses: 1. Subjects will show greater improvement in ratings of mood and other symptoms of depression during the six-week of high frequency stimulation than during low frequency stimulation periods. 2. Subjects will show greater improvement after 12 weeks of high frequency stimulation than after six weeks of high frequency stimulation. 3. Subjects will show improvement in ratings of life functional capacity and quality of life with TNS. 4. Subjects will report the TNS treatments to be acceptable in terms of side effects and burden of using the device. 5. Subjects will show significant differences in regional brain function at the end of the high frequency stimulation period compared with baseline, and significant differences between high and low frequency stimulation conditions.