 1. Investigator-rated dyskinesia severity Levodopa challenge: Anti-parkinsonian medications will be held for 12 hours prior to testing. At the time of the assessment, patients will receive the morning dose of either topiramate or placebo and their usual morning levodopa dose. Patients will be videotaped prior to and following medication administration, and dyskinesia severity will subsequently be rated by a blinded investigator. Levodopa challenges will be conducted before each treatment arm baseline and at the completion of each treatment arm. 