The Acute Effects of Psilocybin on Cognition, Memory, and Brain Function

Inclusion Criteria: * 21 to 45-years-old. * 3-30 lifetime psychedelic uses. * English as a first language. * High school education (or equivalent) * Psychiatrically healthy (as assessed by the SCID-5). * Medically healthy (as assessed by a physical examination and ECG). * Willingness to attend all study sessions and complete all procedures. * BMI between 19 and 30. Exclusion Criteria: * Current or past diagnosis of psychiatric disorders except panic attacks, depressive disorder, or anxiety, or disorder from ≥1 year prior. * Current or past medical conditions that might interfere with study participation or be contraindicated for psilocybin administration (e.g., hypertension, history of stroke, cardiovascular disease, etc.) * Current daily medications except birth control (females). * Pregnant, nursing, or planning to become pregnant (assessed with urine pregnancy test). * Ingestion of a psychedelic \<2 months prior to an experimental session (with the exception of psilocybin administered in the context of the current study's repeated measures design). * History of serious adverse event with a psychedelic and/or self-reported hypersensitivity to psychedelics. * Inability to abstain from alcohol 48 hours prior to an experimental session. * Use of other psychoactive drugs (other than caffeine or nicotine) 1 week prior to an experimental session. * Positive urine drug screening for drugs of abuse during experimental sessions. * Self-reported ferrous metal, metallic implants, or implanted medical devices that would preclude participation in MRI procedures, including but not limited to cochlear implants, implanted brain stimulators, and aneurysm clips. * Self-reported past penetrating brain injury or any head injury resulting in a loss of consciousness for 30 minutes or more or post-concussive symptoms for more than seven days following a head injury. * Self-reported claustrophobia (prohibiting MRI acquisition). * Any other factors such as unstable housing or life-threatening circumstances, erratic behavior, etc. that are judged by the investigators to be a significant barrier to participation in the study protocol and/or to establishing rapport necessary for safe administration of psilocybin. * Participant unwillingness to not ingest or use additional serotonergic psychedelics outside the context of study procedures for the duration of the study. * Resting blood pressure \>140/90 mm hg at study entry. * Lifetime history of cardiomyopathy, stroke, heart disease, heart attack, tachycardia, elongated QT-interval corrected by Friderichia (\> 450ms for men and \> 470ms for women); clinically significant cardiac arrhythmia within 1 year of study entry; and/or abnormal electrocardiogram on study entry. * Left-handedness (given that functional lateralizations may differ from those of right-handed individuals).