The BROADEN (BROdmann Area 25 DEep brain Neuromodulation) study is a study to evaluate the safety and effectiveness of deep brain stimulation in patients with a severe form of depression known as Major Depressive Disorder (MDD) who have not responded to multiple treatments. Stimulation to the brain is provided by a surgically implanted medical device called a deep brain stimulation (DBS) system. The system provides stimulation directly to an area of the brain known as Brodmann Area 25 (sometimes referred to as BA25). The study will build upon the depression work of a research team from the University of Toronto, led by neurologist Helen S. Mayberg, M.D. and neurosurgeon Andres Lozano, M.D., PhD, FRCSC.