Experimental paradigm: Clinical, neuro-psychological, biological and imaging assessment will be performed 3 and one month before and 3, 6, 12 and 24 months after surgery. Bilateral electrodes (Medtronic 3389) will be implanted, under local anesthesia, by MR-guided frame-based stereotaxy, in the hypothalamic part of the fornix, before its entry in the mamillary body (well defined on T2 weighted sequences). Intraoperative stimulation will be used to search adverse effects or acute effects. Electrodes will be connected to the generator (Kinetra, Medtronic) under general anesthesia. Chronic high-frequency stimulation will be delivered immediately after surgery.