Low Frequency Subthalamic Nucleus Deep Brain Stimulation to Improve Verbal Fluency

Rationale: Parkinson's disease patients with deep brain stimulation electrodes represent a unique opportunity to study the influence of basal ganglia on neurocognitive function. Intervention: Patients' deep brain stimulators will be turned off or on and the frequency will be changed to either theta or gamma. Objectives: To identify differences in higher cognitive functions with stimulation "on" and "off" and theta versus gamma frequency stimulation. Study population: 12 patients who had previously undergone bilateral STN deep brain stimulation implantation. Study methodology: Patients will undergo four sessions of neuropsychological testing (RNGT, verbal fluency, D-KEFS CWIT) at baseline, no stimulation, theta stimulation and gamma stimulation, in random order over one day. Study outcomes: Test results of RNGT, verbal fluency, D-KEFS CWIT. Follow-up: none Statistics: Test results will be analyzed using within-subjects statistical tests.