Participants will be recruited in Dr. Krystal's laboratory at Duke University which has proven infrastructure for clinical trials. The study team plans to enroll a total of 240 subjects healthy volunteers (age range: 21-65 years) in order to complete 100. This takes into account the expected rate of failure to: have a pupillary response in the auditory oddball task (AOT) (25%); have a sufficient fear response to CO2 challenge (50% - relevant for last 10 R21 and all R33 subjects); and to meet other screening criteria.