Summer Research Description: Introduction: Bilateral deep brain stimulation (DBS) of the subthalamic nucleus (STN) is an FDA-approved treatment of advanced Parkinson’s disease (PD). Despite minimal cognitive side-effects, some advanced PD patients treated with DBS suffer a mild decline in verbal fluency—the speed of controlled word generation. Background: Preliminary evidence from a two-year pilot study of DBS in early stage PD patients shows that DBS is well tolerated. However, the pilot results suggest a mild decline in verbal fluency in subjects treated with DBS after the first two years. A three-year extension phase of this study, which included annual administration of phonemic (FAS) and semantic (animals) verbal fluency tests, was recently completed. The long-term effects DBS has on verbal fluency in patients with early PD are currently unknown. Hypothesis: Patients in both DBS and optimal drug therapy (ODT) groups will have a decline in the mean scores of both phonemic and category fluency compared to baseline. DBS scores will be numerically but not statistically lower than the ODT scores. Methods: Mean verbal fluency scores for each group at 3,4,and 5 years will be plotted. A two-sample t test model will be used to compare verbal fluency scores between the DBS and ODT groups.