Social Disconnection Study

Inclusion Criteria: * able to give written informed consent * age 55+ * English speaking Exclusion Criteria: * contraindication to MRI scanning including claustrophobia, high girth, moderate to significant white matter atrophy, and presence of ferromagnetic material in the body, including orthodontic braces or other non MR-compatible foreign bodies. All participants will be screened for metal objects by the same methods used for routine clinical MRI scanning * for women: pregnancy or breastfeeding * serious medical or neurological illness that in the opinion of the PI would interfere with the scientific goals of the study or participant safety * pervasive developmental disorders (PDD) or primary psychotic disorders * meet DSM-5 criteria for current severe substance use disorder (except marijuana or nicotine) * head injury that led to significant long term decline in cognitive abilities as seen by decline in grades or work performance * current psychosis, active significant suicidal (score of 6 on MADRS suicide item) or homicidal ideation * lifetime history of neurologic abnormality including seizure disorder, cerebrovascular, or neoplastic lesion, neurodegenerative disorder, or significant head trauma resulting in post-traumatic amnesia \>24 hours * full scale IQ lower than 70 * contraindications to PET (e.g. poor venous access for placement of venous lines) * history of prior radiation exposure for research purposes within the past year such that participation in this stu…