This study aims at assessing efficacy and safety of DBS for treatment of patients suffering from resistant major depression, by means of a random, controlled and crossed study.

The first phase of the study will consist of implanting electrodes and applying continuous stimulation until the patients stabilise clinically. This period is anticipated between 6 and 9 months. The next phase will consist of the crossover study. Responders will be randomised to either of 2 groups for a period of 3 months: a stimulation-on group and a stimulation-off group. Patients will then be crossed over to the other group for a further 3 months. Fortnightly visits will be made during the entire study to evaluate treatment efficacy and patient tolerance

Inclusion Criteria:

1. Patients of both sexes aged between 18 and 70 years.
2. Patients diagnosed as having an MDD episode according to DSM-IV-TR criteria, resistant to pharmacological treatment with a score of 4 in the Thase-Rush index, with ECT contraindicated or having failed to produce a maintained response.
3. Patients with a HRSD-17 score of 18 or more.
4. Patients with an intellectual capacity that facilitates adequate communication and who are willing to cooperate with all the examinations and protocols of the study.
5. Patients who have not modified their antidepressant treatment in the month prior to the study.
6. Women of childbearing age using medically approved contraceptive methods.
7. Patients who have granted their informed consent in writing.

Exclusion Criteria:

1. Female patients who are pregnant or breastfeeding.
2. Patients with acute, serious or unstable illnesses.
3. Patients experiencing delirium or hallucinations, congruent or otherwise with their mood.
4. Patients with a history of substance abuse (other than tobacco or caffeine).
5. Patients with concomitant psychiatric disorders from axes I or II of DSM IV-TR.

h)Patients with general contraindications for DBS (pacemaker users, etc).