The goal of this observational study is to learn how the brain's information processing changes during and following administration of serotonergic psychedelics (psilocybin, N,N-Dimethyltryptamine/DMT, Lystergic Acid Diethylamide/LSD, etc.) for people with and without mental illness receiving serotonergic psychedelics through any clinical trial at Yale University. The main questions it aims to answer are: 1. Do serotonergic psychedelics cause the brain to rely on new information more than previously learned information while under the influence? What about 1 day, 5-14 days, and 4-6 weeks after use? 2. Do serotonergic psychedelics cause long-lasting side-effects in how people perceive (see, hear, feel, etc.) the world and how easily people change their beliefs? 3. How does the brain's electrical activity change after using serotonergic psychedelics? How does the balance between excitation and inhibition change while under their effect? 4. Can changes in how the brain uses information predict who will benefit from a psychedelic and who will have side effects from psychedelics? Researchers will compare with people given placebos to see what changes in brain processing are unique to serotonergic psychedelics. Participants will have the opportunity to do some combination of the following: 1. Online computer assessments consisting of games and questionnaires that probe how participants think. 2. Magnetoencephalography (MEG) or electroencephalography (EEG) with eyes closed and with repeated clicks, images, or sensations delivered. 3. A magnetic resonance imaging (MRI) scan. 4. Semi-structured qualitative interviews about their experience after taking a serotonergic psychedelic recorded via Zoom.