The Role of Brain Dopamine in Chronic Pain

Inclusion Criteria: The following inclusion criteria must be met for all subjects to be considered eligible to participate: 1. 18 years old or older 2. Equal numbers of men and women, as well as racial and ethnic makeup representative of surrounding area 3. Able to speak, read, and understand English 4. In generally stable health 5. Sign informed consent document The following inclusion criteria must be met for chronic pain patients to be considered eligible to participate: 1. Patients must report chronic pain for more than 1 year. 2. The reported pain should be rated at ≥ 40/100 Exclusion Criteria: General exclusion criteria for all subjects include: 1. Significant other medical disease, such as unstable diabetes mellitus, congestive heart failure, coronary or peripheral vascular disease, chronic obstructive lung disease, or malignancy 2. History of traumatic brain injury (TBI) 3. Current misuse/dependence on substance(s), including alcohol, at the time of study enrollment 4. Major psychiatric disorder during the past 6 months 5. Significantly abnormal laboratory values, which include, but are not limited to, the following: 1. White blood cell (WBC) \< 1.5 or \>15.0 x 10\^3/μL 2. Hemoglobin (Hgb) \< 8 gm/dL 3. Hematocrit (Hct) \< 24% or \> 50% 4. Platelets (Plts) \<50 or \> 1000 x10\^3 5. Creatine \> 2 mg/dL 6. Glucose \> 125 mg/dL 7. Aspartate aminotransferase (AST) \> 250 U/L 8. Alanine Transaminase (ALT) \> 250 U/L 9. Bilirubin \> 3 mg/dL 6. Intra-axial implants (e.g. - spinal cord stimulators or pumps) 7. Inability to adequately perform the finger-span visual tracking task (training for rotating pain perception, see Brain Imaging Details). 8. All MRI exclusionary criteria: any metallic implants, brain or skull abnormalities, tattoos on large body parts, pregnancy, and claustrophobia. 9. In the judgement of the investigator, unable or unwilling to follow the protocol and instructions. 10. Gambling addiction self-reported during screening process (ensure computer-based games do not cause psychological or emotional problems) 11. Chronic pain patients with past history of allergic reactions to methylphenidate or levodopa/carbidopa. For healthy control subjects, current complaint(s) of pain, or a history of pain lasting \>4 weeks in the last year, will be excluded from participating. In addition, exclusion criteria for chronic pain subjects also includes: 1. Pain that is associated with any systemic signs or symptoms (e.g. - fever, chills) 2. Evidence of r…