The experiment will take place in four sessions, carried out on different days. Consecutive test sessions will take place no earlier than 3 days, and no longer than 21 days apart. Before being recruited into the study, potential participants will go through a phone screen-a pre-experiment screening questionnaire (separate) to exclude subjects with neurological disorders, certain psychiatric disorders, who smoke or use nicotine products, or who use certain drugs (see exclusion criteria below) as they can affect PPI levels. If they are eligible per the phone screen, subjects will come in for screening (test day 1). They will be again asked about their tobacco/nicotine use and what medications they are currently taking. They will be asked about their caffeine history use. Caffeine and caffeine withdrawal both have effects on startle response but not on PPI so we are not restricting subject's caffeine use, but collecting this information. CO levels will be collected using a CO monitor to confirm that participants have not recently used a tobacco product. They will be asked questions about psychiatric symptoms, personality, and how they cope with stress. Participants' startle responses and PPI will be tested in the laboratory (see below).