Inclusion Criteria:  Aged 60-90 years-old Major Depressive Episode according to DSM-5 criteria, confirmed by the MINI - Mini International Neuropsychiatric Interview MADRS score greater than 20 (Montgomery Asberg Depression Rating Scale) Patient resistant to at least one well-conducted antidepressant treatment, as documented by the MGH-ATRQ scale Patient who can undergo N2O diffusion via a facial mask Patient who has signed an informed consent Person affiliated with a social security scheme  Exclusion Criteria:  Bipolar disorder, schizophrenic disorder, neurodegenerative disease, documented by the MINI and the MMSE (non-inclusion if MMSE < 24/30), addictive disorder Unstable somatic pathology (including unstable neurological or cardiological diseases at risk of interfering with N2O diffusion) Presence of active and significant psychotic symptoms, at investigator's discretion Contraindications to EMONO (50%N2O/ 50%O2) : pneumothorax, emphysema, bowel obstruction, intracranial hypertension, chronic deficiency in vitamin B12 or B9 Contraindications to MRI, including claustrophobia Legal incapacity and/or other circumstances unabling the patient to understand the nature, purpose or consequences of the study A person participating in a drug clinical trial or during a period of exclusion from any clinical study