Does Psilocybin Change Synaptic Density in Amnestic Mild Cognitive Impairment

The goal of this pilot, exploratory, clinical trial is to investigate the effects of psilocybin on synaptic vesicular density (SVD) as measured by the positron emission tomography (PET) radiotracer, 18F-SynVesT-1, in participants with amnestic Mild Cognitive Impairment (aMCI) and healthy participants. The investigators hypothesize that SVD levels in the brain will be higher following the ingestion of psilocybin in comparison to placebo, and that increases in SVD will be associated with improvements in cognition. 10 participants (6 with aMCI, and 4 sex and age matched healthy volunteers) will: * Be randomized to receive either: 1. Two 25 mg macrodoses of psilocybin separated by 1 week. 2. Two placebo doses separated by 1 week. * Receive a baseline 18F-SynVesT-1 PET scan, clinical, and neuropsychological assessments. * Receive a 18F-SynVesT-1 PET scan one week after the last dose of treatment. * Depending on available funds, receive a third PET scan at any time within 4 weeks of the screening visit to quantify tauopathy with the \[18F\]T807 radiotracer. * Receive clinical and neuropsychological testing 1, 4, and 12 weeks after the last treatment. Researchers will compare placebo vs. experimental groups to see if psilocybin will increase SVD, and if increases in SVD are associated with cognitive improvements.