Eligibility Details:  
        -  INCLUSION CRITERIA:
        Subjects must:
          -  Be able to give valid informed consent
          -  Be 18 60 years of age.
               -  Justification: Many neural processes change with age, and these changes could
                  introduce unwanted variability in both behavioral and MRI signals. In addition,
                  the risk of difficult-to-detect medical abnormalities such as silent cerebral
                  infarcts increases with age.
               -  Screening tool: History. Government-issued forms of identification (e.g. driver s
                  license, birth certificate) will be required when participant appears to be out
                  of age range.
          -  Be in good health.
               -  Justification: Many illnesses may alter neural functioning as well as fMRI
                  signals.
               -  Screening tools: Medical Assessment, Medical History and Physical Examination.
                  Medical assessments include: Vital Signs, EKG, oral HIV test, height/weight
                  measurements, urinalysis and blood sample. Tests on the blood sample include CBC,
                  complete metabolic profile, TSH, ESR, STS and HIV (if needed to confirm a
                  positive salivary test for HIV). The following individual laboratory results will
                  independently disqualify individuals: Cholesterol >250 mg/dl, Hemoglobin < 10.5
                  g/dl, WBC < 2400/ (Micro)l, LFTs > 3Xnormal, HCG positive, Casual serum glucose >
                  200 mg/dl, Urine protein > 1+. The MAI will retain discretion to exclude at less
                  extreme values, depending on the clinical presentation. (Serum glucose over 140
                  mg/dl will be followed up with a fasting serum glucose assessment. Those with
                  fasting glucose below 100 mg/dl may be considered for the protocol. Others will
                  be rejected and referred for work-up.) MAI will make the final judgment on any
                  questionable lab results.
          -  Right-handed.
               -  Justification: Using right-handed individuals will reduce variability in BOLD MRI
                  data.
               -  Screening tool: Edinburgh Handedness Inventory.
        EXCLUSION CRITERIA:
          -  Personal history of stroke, brain lesions, previous neurosurgery, any personal history
             of seizure or fainting episode of unknown cause, or head trauma resulting in loss of
             consciousness, lasting over 30 minutes or with sequela lasting longer than two days.
               -  Justification: Stroke or head trauma can lower the seizure threshold, and are
                  therefore contra-indications for TMS. Fainting episodes or syncope of unknown
                  cause could indicate an undiagnosed condition associated with seizures.
               -  Screening tool: TMS safety questionnaire, Medical History.
          -  First-degree family history of any neurological disorder with a potentially hereditary
             basis, including migraines, epilepsy, or multiple sclerosis.
               -  Justification: Neurological disorders can lower the seizure threshold, and are
                  therefore contra-indications for TMS. First-degree family history of certain
                  neurological disorders with a hereditary component increases the risk of the
                  subject having an undiagnosed condition that is associated with lowered seizure
                  threshold.
               -  Screening tool: TMS safety screening, Medical History.
          -  Cardiac pacemakers, neural stimulators, implantable defibrillator, implanted
             medication pumps, intracardiac lines, or acute, unstable cardiac disease, with
             intracranial implants (e.g. aneurysm clips, shunts, stimulators, cochlear implants, or
             electrodes) or any other metal object within or near the head that precludes MRI
             scanning.
               -  Justification: Any metal around the head is a contraindication for both MRI and
                  TMS, as both methods involve exposure to a relatively strong magnetic field.
               -  Screening tool: TMS safety screening, MRI safety screening, Medical History.
          -  Noise-induced hearing loss or tinnitus.
               -  Justification: individuals with noise-induced hearing problems may be
                  particularly vulnerable to the acoustic noise generated by TMS and MRI equipment.
               -  Screening tools: TMS safety screening.
          -  Current use (any use in the past 4 weeks, chronic use within 6 past six months) of any
             investigational drug or of any medications with psychotropic, anti or pro-convulsive
             action.
               -  Justification: The use of certain medications or drugs can lower seizure
                  threshold and is therefore contraindicated for TMS.
               -  Screening tools: MRI safety screening questionnaire, Medical history, Medical
                  Assessments: Urine toxicology analyzes for presence of a broad range of
                  prescription and nonprescription drugs.
          -  Lifetime history of major depressive disorder, schizophrenia, bipolar disorder, mania,
             or hypomania.
               -  Justification: The population of interest here is a healthy control population
                  with no psychiatric disorders. In subjects with depression, bipolar disorder,
                  mania or hypomania, there is a small chance that TMS can trigger (hypo)manic
                  symptoms.
               -  Screening tools: MINI Screen Patient Questionnaire. Potential diagnoses will be
                  further evaluated by a counsellor.
          -  Current use of nicotine or history more than 20 cigarettes lifetime or history of
             daily smoking.
               -  Justification: The population of interest here is a healthy control population
                  with no substance use disorder and therefore a minimal cigarette exposure history
                  in the control group is required.
               -  Screening tools: Self-report, urine cotinine (e.g. NicAlert) level of 2, CO < 6.
          -  Meet current DSM-5 criteria for any substance use disorder, smoke daily, or urine
             toxicology positive for any illicit substance inconsistent with history given.
               -  Justification: The population of interest here is a healthy control population
                  with no substance use disorder. Current use of illicit substances could lower
                  seizure threshold and is therefore contraindicated for TMS.
               -  Screening tools: MINI Screen Patient Questionnaire. Potential diagnoses will be
                  further evaluated by a counsellor, Drug Use Survey (DUS), Substance Use Disorder
                  Evaluation, Medical Assessments: urine qualitative drug screen is performed for
                  methadone, benzodiazepines, cocaine, amphetamine/methamphetamine, opiates,
                  barbiturates, and tetrahydrocannabinol. Participants who test positive at
                  screening (under protocol 06-DA-N415) will be evaluated with a neuromotor exam to
                  further assess for current intoxication. For participants who are not found to be
                  currently intoxicated, screening staff will assess for SUD and coherence of their
                  drug use history and toxicology, with particular attention to substances for
                  which they are positive and may require a return screening visit to demonstrate
                  ability to produce a negative urine before allowing them to proceed to clearance
                  for this study.
          -  Have met DSM-5 criteria for any substance use disorder in the past.
               -  Justification: the population of interest here is a healthy control population
                  with no present or past substance use disorder.
               -  Screening tools: MINI Screen Patient Questionnaire. Potential diagnoses will be
                  further evaluated by a counsellor. Drug Use Survey (DUS), Substance Use Disorder
                  Evaluation.
          -  History of myocardial infarction, angina, congestive heart failure, cardiomyopathy,
             stroke or transient ischemic attack, or any heart condition currently under medical
             care.
               -  Justifications: the risk of TMS for individuals with a heart condition is
                  unknown.
               -  Screening tool: physical assessment (EKG), medical history.
          -  Pregnant women or women with reproductive potential who are sexually active and not
             using an acceptable form of contraception.
               -  Justification: it is unknown whether TMS or MRI poses a risk to fetuses.
               -  Screening tool: Medical assessments (urine pregnancy test) at the beginning of
                  each visit that involves TMS or MRI.
          -  Participation in an rTMS session less than two weeks ago.
               -  Justification: in order to limit exposure to TMS, we will not enroll subjects who
                  have received TMS less than two weeks ago.
               -  Screening tool: TMS safety screening questionnaire.
          -  Learning disability, ADHD or cognitive impairment
               -  Justification: Cognitive impairment and learning disabilities are associated with
                  alterations in brain regions and may introduce significant variably into the
                  data.
               -  Screening tool: self-report of special education classes, history of specific
                  learning disability or mental retardation, Wechsler Abbreviated Scale of
                  Intelligence (WASI), Medical history and Adult ADHD Self-Report Scale with follow
                  up clinical interview.