People in good health do not perceive the future in the same way as those suffering from depression. This difference in perception affects behaviour and emotions and warrants further investigation, both before and after treatment with antidepressants. Research shows that past negative experiences with medication can reduce the effectiveness of treatment, whilst positive expectations can increase it. It is important to gain a better understanding of how antidepressants influence the way in which elderly people with major depressive disorder (MDD) change their beliefs, and to determine whether an improvement in their depressive symptoms is linked to these changes in their beliefs. This study aims to understand whether antidepressants can alter the way people view the future and whether this contributes to their effectiveness. It also seeks to determine whether patients' expectations regarding treatment influence clinical outcomes, and whether changes in their mood may in turn affect these expectations. The study will involve 31 patients aged 65 and over who are suffering from a depressive disorder and require a change of antidepressant, and 31 healthy subjects, also aged 65 and over. The participants' involvement will last for 12 weeks.

During a medical consultation, once the doctor has checked that the participant meet all the study's eligibility criteria, they will ask for the participant consent to take part in the study. The participant will then be asked to answer one question about what you hope to gain from the new antidepressant treatment that the doctor will prescribe for the participant. Finally, the participant will carry out a belief-updating exercise, which involves : presenting the participant with a series of events and ask them to indicate how likely they think each of them is to occur, and then. Once they have received further information about these events, ask them to reassess the likelihood of them occurring. This task will be carried out again 12 weeks after the participant start taking their antidepressant, during a consultation required as part of their individual care plan. From the first week of antidepressant treatment until week 12, the participant will be contacted by telephone once a week to answer two short questions about their expectations regarding the treatment and any changes in your mood.

Inclusion Criteria for patients with major depressive disorder :

• Patient over 65 years old in outpatient care
• Patients with major depressive disorder or patients with bipolar major depressive disorder as evaluated by the Mini International Neuropsychiatric Interview (MINI) semi-structured interview for the group suffering from depression
• Absence of cognitive impairment as evidenced by an MMSE (Mini Mental State Examination) ≥ 24
• MADRS ≥ 20
• MMSE ≥ 24
• Change of antidepressants
• Person affiliated with a social security scheme
• Patients who have received informed information about the study and have signed a consent form to participate in the study

Exclusion Criteria with major depressive disorder :

• mental disorders other than mood disorders : attention deficit dissorder, borderline personality disorder, schizophrenia, autism spectrum disorder, anorexia-bulimia disorder, obsessive-compulsive disorder, anxiety disorders and substance use disorder (except tobacco).
• Inability to understand task instructions and perform the behavioral task
• Neurological or neurosurgical comorbidities
• Major visual impairment
• Adults under state medical aid
• Adults under legal protection (legal guardianship, curatorship, trusteeship), persons deprived of their liberty

Inclusion Criteria for patients with major depressive disorder :

• Absence of psychiatric disorders attested by administration of the MINI
• Absence of cognitive impairment as evidenced by an MMSE ≥ 24

Data about study protocol and clinical information will be available on request from other researchers.