Parkinson's disease (PD) is the second most common neurodegenerative disease (after Alzheimer's disease) and affects approximately one million people in the United States. Mild Cognitive Impairment (MCI) is very common even in early stages of PD. In addition to cognitive impairment, patients with PD also suffer cognitive fatigue (defined as the general sensation of difficulty in initiating cognitive activity) and cognitive fatigability (defined as "deterioration in the performance of attention tasks over an extended period of time"). Cognitive impairment, cognitive fatigue, and cognitive fatigability affect quality of life in patient with Parkinson's disease.

Transcranial direct current stimulation (tDCS) is a noninvasive and safe brain stimulation technique that has been shown to be effective in improving cognitive function in subjects with Parkinson's disease. During tDCS, low-voltage, low amplitude current is passed through a pair of surface electrodes placed over the areas of brain of interest.

The specific aim of this study is to examine if atDCS to LDLPFC at 2 milliamps (mA) for 20 minutes daily for 5 days will improve cognitive function and reduce cognitive fatigue and fatigability in PD patients with MCI. The study will examine if the effects may last for two weeks.