Olfactory Function and Model-Based Behavior in People Living With HIV and SUD

* INCLUSION CRITERIA: In order to be eligible to participate in this study, an individual must meet all of the following criteria: For all participants: * Stated willingness to comply with all study procedures and availability for the duration of the study * Aged 18-65 years old. Justification: prevalence of olfactory impairments increases with age; after age 53, the prevalence is 24.5 percent, increasing to 62.5 percent in people ages 80-97. * Agreement to adhere to Lifestyle Considerations throughout study duration. * Ability of subject to understand and the willingness to sign a written informed consent document. For participants in the HIV+ and SUD+ and HIV+ and SUD- groups: -HIV positive. For participants in the HIV+ and SUD+ and HIV- \& SUD+ groups: -Substance Use Disorder (SUD) other than Alcohol Use Disorder (AUD) or Tobacco Use Disorder (TUD). EXCLUSION CRITERIA: An individual who meets any of the following criteria will be excluded from participation in this study: For all participants: * History of neurological illnesses or neurosurgery including but not limited to cerebrovascular accident, Parkinson disease, Alzheimer disease, Huntington disease, central nervous system (CNS) tumor, significant head trauma with sequelae, multiple sclerosis or other demyelinating diseases, epilepsy, movement disorders. The MAI will also retain discretion to exclude based on a history of a neurological illness or trauma that may compromise data integrity. * History of current (past 12 months) uncontrolled major DSM-5 psychiatric disorder including major affective disorder, obsessive-compulsive disorder, schizophrenia, or posttraumatic stress disorder, excluding SUD. * Candidates who recently (12 hours prior to a study visit) used medications and/or substances that are psychoactive or otherwise affect alertness will have to pass a clinical assessment for intoxication on the day of the study visit. Based on participant preferences and MAI/PI judgment, participants who fail the clinical assessment for intoxication will either be withdrawn or rescheduled. * History of clinically significant anaphylaxis, e.g., due to severe asthma or food and nonfood allergies (e.g., latex, detergents, soap, etc.). The MAI will retain discretion to exclude a participant based on this criterion. * Uncorrected impairments in visual acuity. * Non-English speaking. Justification: The data integrity of some of the behavioral tasks used in this study would be compromised as they have only been validated in English. * Pregnancy. * Any other condition that in the judgment of the investigators is incompatible with participation. For participants in the HIV+ and SUD- and HIV- and SUD- groups: * History of current (past 12 months) SUD, excluding tobacco use disorder. * Pattern of alcohol and drug use in the past 12 months that is indicative of harmful use, loss of control over use, or physical dependence. * Daily alcohol or drug use (excluding caffeine and nicotine) for at least 4 continuous weeks in the past 12 months. For participants in the HIV- and SU+ and HIV- and SUD- groups: -HIV positive. For the Optional MRI Segment: -Unable to undergo MRI scanning due to certain metallic or magnetic devices or implants in the body, or claustrophobia.