Inclusion Criteria:

• Men and women (all races and ethnicities) between 18 and 40 years old
• Women of childbearing age with a negative pregnancy test within 48 hours of scanning
• In good health
• Right-handed
• Normal or corrected-to-normal vision
• English as a first language
• Able to understand and provide signed informed consent
• No history of metal in their body or other reasons why they could not undergo an MRI
• No history of ischemic or hemorrhagic stroke, encephalopathy or encephalitis, minimal-cognitive impairment or dementia, movement disorder such as Parkinson's disease, head trauma causing loss of consciousness, cancer involving the central nervous system

Exclusion Criteria:

• Pregnant or breast-feeding women
• Individuals with implanted metal or electronic devices that would prevent them from MRI scanning.
• Individuals with a history of neurologic dysfunction that would prevent performance on attentional tasks including: history of transient ischemic attacks, history of cerebral infarction, history of Binswanger's disease (or a history of hypertensive encephalopathy), history of intracranial hemorrhage, history of head trauma with loss of consciousness, history of encephalitis, history of extended exposure to any known neurotoxin, history of acquired cognitive impairment, history of normal pressure hydrocephalus, history of a cancer metastatic to the central nervous system, history of Parkinson's or other basal ganglia disease, history of Guillain-Barré syndrome or chronic or relapsing polyneuropathy