Prediction of the Therapeutic Response in Depression Based on Neuro-computational Modeling Assessment of Motivation

One hundred patients with a moderate to severe major depressive episode will be enrolled in this prospective multicenter study. Six visits will be scheduled within a year: * V0 (inclusion visit): verification of inclusion and exclusion criteria, information, and consent. * V1 (before randomization - baseline state): * Clinical evaluation using validated questionnaires for the severity of depression, quality of life, anhedonia, apathy, and cognitive dysfunction. * Neuro-cognitive evaluation using a battery of tests to explore motivation, emotion processing, belief construction, and their updating. Part of the tests will be performed during the functional MRI session. * Structural (anatomical) and functional MRI, ASL. * Blood samples. * Randomization and introduction of the new antidepressant will occur immediately after V1. To maximize acceptability by referring psychiatrists, dosage and co-prescriptions will be at the discretion of the psychiatrist in charge, but the assigned treatment will not be changed for 4 weeks (until V3). * V2 (7 days after the beginning of the new antidepressant - 'early response visit'): o Similar to V1. * V3 (28 days after the beginning of the new antidepressant - 'conventional response visit'): * Clinical evaluation using validated questionnaires for the severity of depression, quality of life, anhedonia, apathy, and cognitive dysfunction. * Blood samples * V4 (6 months after the beginning of the new antidepressant - 'remission visit'): * Clinical evaluation using validated questionnaires for the severity of depression, quality of life, anhedonia, apathy, and cognitive dysfunction. * Cognitive evaluation using a battery of tests to explore motivation, emotion processing, belief construction, and their updating. * Structural (anatomical) MRI, ASL * Blood samples * V5 (one year after the beginning of the new antidepressant - 'functional remission visit'): * Clinical evaluation using validated questionnaires for the severity of depression, quality of life, anhedonia, apathy, and cognitive dysfunction. 36 healthy volunteers without a history of neurologic or psychiatric disorder, matched for age, gender, and education will be included. They will perform V0-V2 (without MRI and blood sample at V2). Healthy volunteers will not receive any treatment as part of the research.