Neural Dynamics and Connectivity in Response Inhibition and Traumatic Brain Injury

Background: \- Previous research has shown that certain parts of the brain are involved in voluntarily stopping an ongoing motor response (movement); however, it is not known whether this same network is also involved in suppressing an urge to act. Traumatic brain injury (TBI) can significantly impair the brain's ability to voluntarily stop or inhibit certain actions. Using brain imaging (functional magnetic resonance imaging, or fMRI) and brain stimulation (transcranial magnetic stimulation, or TMS) to investigate how people perform activities that involve moving and suppressing movements, researchers hope to better understand how these brain areas might be affected in people who have had TBI. Objectives: * To determine the parts of the brain involved in suppressing an urge to act. * To determine the extent to which traumatic brain injury affecting certain parts of the brain is involved in problems with suppressing an urge to move and stopping movement. Eligibility: \- Individuals 18 to 40 years of age who have had mild or moderate TBI, or are healthy volunteers. Design: * This research study includes a screening visit and two study visits, each of which will last at least 2 hours. * Participants will be screened with a physical examination and medical history. Women who can become pregnant will have a urine pregnancy test before being allowed to participate in the study. * At the first study visit, participants will complete one of the following experiment tests in an MRI scanner. * Experiment 1: Participants will be shown arrows or images on a computer screen, and will press a button or not press a button depending on the image shown. Participants will practice the experiment tasks before performing them during MRI scans. * Experiment 2: Participants will be shown arrows or images on a computer screen, and will press a button or not press a button depending on the image shown. Participants will also have TMS while at rest, and will perform the experiment t