- Visits 2 through 5 (experimental sessions): Patients who have been taking antiparkinson medication for a long time are asked to not take their medication for about 12 hours (overnight withdrawal) before visits 2 through 5. They are off medication for about 14 hours total (until after the experiments are done). Patients may be admitted to the NIH Clinical Center for the overnight drug withdrawal if necessary. At the start of each session, participants are given either levodopa/carbidopa tablets or placebo (tablets identical in appearance but with no active medication). They perform the modified pegboard test before medication, after medication, and after brain stimulation with rTMS. During two of the sessions, they receive actual brain stimulation, and during the other two sessions they receive sham stimulation, which does not actually stimulate the brain.