PATIENTS  Inclusion Criteria:  Diagnosed with primary progressive multiple sclerosis Expanded Disability Status Scale of ≤ 6.5 No clinical relapse within last three months Have the ability to comply with all requirements of the study protocol, as determined by the investigator  Exclusion Criteria:  Pregnancy Pacemaker or other implanted electronic devices Claustrophobia Psychiatric disorder Administration of acute cortisol Changes in pharmacological treatment within the last 3 months Any contraindication to MRI Persons who do not wish to be informed about abnormal findings as part of the investigations  HEALTHY CONTROLS  Inclusion Criteria:  Able bodied Have the ability to comply with all requirements of the study protocol, as determined by the investigator  Exclusion Criteria:  Pregnancy Under medication at the time of the experiment (with the exception of contraceptive drugs) History of neurologic disease Pacemaker or other implanted electronic devices History of cerebral hemorrhage or brain damage Claustrophobia Psychiatric disorder Any contraindication to MRI Persons who do not wish to be informed about abnormal findings as part of the investigations