Deep brain stimulation (DBS) in the dorsal region of the subthalamic nucleus (STN) is very effective for reducing motor symptoms of Parkinson's disease (PD). Modeling studies suggest that this therapy may result in current spread into the ventral STN, causing altered cognitive processes. As a result, current stimulation parameters often lead to worsening in verbal fluency, executive function, and, particularly, cognitive control. There is evidence suggesting that low frequency oscillatory activity occurs across brain circuits important in integrating information for cognition. Preclinical studies and human recording studies indicate these low frequency theta oscillations drive cognitive control during cognitive tasks. Thus, the purpose of this study is to determine the safety, tolerability, and efficacy of low frequency stimulation (LFS) of the ventral STN alongside standard high frequency stimulation (HFS) of the dorsal STN in patients with PD.