This study will compare brain changes in people with Parkinson's disease with those of normal control subjects while they learn motor skills. People with Parkinson's disease sometimes have trouble learning new skills, but it is not known why. This study will use repetitive transcranial magnetic stimulation (rTMS), nerve conduction studies, and electroencephaolography (EEG) to look for differences in the way the brain changes with learning in people with Parkinson's disease.