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 study would place them over FDA limits for annual radiation exposure. This guideline is an effective dose of 5 rem received per year * history of a bleeding disorder or currently taking anticoagulants (such as Coumadin, Heparin, Pradaxa, Xarelto) \*for subjects obtaining arterial line. * blood donation within 8 weeks of the start of the study * REM sleep disorder * brain injury (TBI, seizure/epilepsy, stroke, TIA) in lifetime (self-report and medical chart review) * electroconvulsive or ketamine therapy, or similar therapies that have rapid effects on brain neurochemistry within the past 6 months * high risk for stroke (above first quartile on Framingham Stroke Risk Profile) * current cancer