The goal of this clinical trial is to identify reliable markers of apathy in elderly subjects with bipolar disorder, age between 70 and 85 years, in order to accurately identify subjects at high risk of progressing to dementia by measuring motor activity (actimetrics), recorded language and analysing brain changes (MRI).

Actimetry is the measurement and recording of body movements using an actimeter. This device is worn on the wrist and contains sensors capable of measuring and recording all movements, including those of very low intensity. An automated speech analysis using artificial intelligence is used to detect low-intensity anomalies, and we want to test whether individual differences correspond to individual differences in brain anatomy and function.

Researchers will compare elderly subjects with bipolar disorder and healthy volunteer, age between 70 and 85 years.

Participants will be asked to:

• Perform an MRI
• Complete 3 cognitive tests: verbal memory, verbal fluency and an emotional storytelling task, in which you will be asked to describe a memory orally using positive, negative and neutral emotions.
• wear an actimeter on your wrist for 4 days.

Inclusion Criteria:

1. Population: Age between 70 and 85 years-old, living at home (Participants living in nursing homes are not included).
2. Condition: OABD type 1, type 2 and type 3 assessed by the DSM5 criteria
3. Stable: no MDE or hypomanic state within the last 6 months
4. Ambulatory setting only
5. General condition: Successful Gait speed test from the Short Physical Performance Battery (SPPB): beingable to walk 4 meters in 4 seconds (SPPB NIH Toolbox)44
6. Person affiliated to a social security regime
7. Patients who have given their free, informed and written consent to take part in the study

Exclusion Criteria:

1. Psychiatric conditions and or co-morbidities

   1. Unipolar depression
   2. Recurrent unipolar depression
   3. Substance use disorder according to DSM5 criteria. Benzodiaepine and/or z-drugs dependence are accepted.

2. Neurological and cerebral co-morbidities

   1. Major Cognitive Disorder: significant cognitive decline characterized by extensive cognitive tests or at least a standardized clinical evaluation AND at least loss of autonomy in complex instrumental daily living function, not related to delirium (DSM5 criteria)
   2. Medical history of known degenerative disorders: Alzheimer's disease, Lobar Degenerative Fronto-temporal disorders, Lewy Body disease, corticobasal degenerative disorder, Supranuclear Palsy, epilepsy.
   3. Medical history of known Parkinson's disease (according to the Movement Disorder Society (MDS)45 criteria)
   4. Medical history of known stroke
   5. Severe Parkinsonism (defined by MDS-Unified Parkinson's Disease Rating Scale46 > 20)

3. MRI contra-indications: metallic implants, severe claustrophobia

4. Adults under legal protection (safeguard of justice, curatorship, guardianship), persons deprived of their liberty.

5. Hospitalized at inclusion

Patients treated in the old-age psychiatry department of the Pôle Hospitalo-Universitaire de Psychiatrie Adulte (PHUPA) for OABD may be offered participation in the study. Eligible patients according to the inclusion criteria will be recruited during standard visits at CHGR. Informed consent along with a document describing the study will be presented during visit.

Healthy volunteers will be recruited from the patient's relatives and/or via posters and/or the CHGR's internal networks.