Fatigue and slowed thinking are very prevalent symptoms in people with Parkinson's disease (PD). To date there are no concrete effective treatment available for either symptom. This study will test transcranial direct current stimulation (tDCS) to ameliorate fatigue and slowed thinking in PD. tDCS is a noninvasive brain stimulation technique that is low-cost, relatively safe, and reproducible when conducted in repeat clinic visits.