This study is designed as a prospective, multi-centered, double-blind, randomized, delayed-stimulation/ sham-stimulation controlled 12-month study to evaluate the effectiveness and safety of bilateral stimulation of the subcallosal cingulate white matter (SCCwm) using the Infinity™ Deep Brain Stimulation (DBS) system as an adjunctive treatment of non-psychotic unipolar Major Depressive Disorder (MDD) for adults who are experiencing a Major Depressive Episode (MDE) with inadequate response to 4 or more antidepressant treatments.