• INCLUSION CRITERIA:

Age: Participants will be males and females, 20-50 years of age.

IQ: IQ, as measured by 4 subscales from the Wechsler Adult Intelligence Scale-Revised (WAIS-R), must be greater than 80.

Medication status: No current use of any psychotropic medication or benzodiazepine.

EXCLUSION CRITERIA:

Because factors such as psychiatric disease, or CNS disease, can influence functional brain activity, these factors are exclusionary:

• Psychiatric History: Participants will be assessed using DSM-IV criteria via standardized psychiatric interviews conducted by trained examiners (SCID). All participants will be free of any current or past major affective disorder, psychotic disorder, substance dependence, anorexia, somatoform disorder, or anxiety disorders with the exception of specific phobias.
• Severe acute and chronic medical illnesses (e.g. cardiac disease, diabetes, epilepsy).
• CNS disease: Known history of brain abnormalities (e.g., neoplasms, subarachnoid cysts), cerebrovascular disease, infectious disease (e.g., abscess), other central nervous system disease, or history of head trauma which resulted in a persistent neurologic deficit or loss of consciousness greater than 3 minutes.
• Currently on regular medication that would interfere with study results. This includes alpha and beta adrenergic medications, other anti-hypertensive medications, glucocorticoid and mineralocorticoid medications, and medications causing sedation or stimulation. For example, current use of tylenol or ibuprofen is permitted, while current use of benadryl or methylphenidate is not. Use of oral contraceptive pills is permitted.
• Currently breast feeding or pregnant (as documented by pregnancy testing at screening or at days of the challenge studies).

Additional exclusion criteria for fMRI studies:

• Metal or electronic objects: Metal plates, certain types of dental braces, cardiac pacemakers, etc., that are sensitive to electromagnetic fields contraindicate MRI scans.
• Claustrophobia: participants will be questioned about potential discomfort in being in an enclosed space, such as an MRI scanner.
• Body weight - to reduce the likelihood of significant orthostasis from guanfacine, participants must weigh 60kg or greater. Those weighing less than 60 kg will be excluded.