This study aims to determine whether direct brain stimulation of specific regions and ranges improves cognition in people with Parkinsons Disease (PD) who already have a deep brain stimulator implanted. Research activities consist of 32 subjects undergoing stimulation changes to their device and being administered neurocognitive tests to evaluate the changes. An fMRI scan will also be done at baseline and at weeks 15 and 27. All subjects will undergo the stimulation changes in a randomized double blind crossover study. Evaluation of stimulation changes will be assessed through analysis of neurocognitive data.