Many patients with Parkinson disease are currently treated with electrode implants on each side of their brain that can dramatically improve their motor symptoms and allow a reduction in medications that cause negative side effects. However, while most of the patients do very well with improvement in movements, they may lose some mental abilities such as being able to name objects. We are not certain if this is due to the structures that are penetrated by the electrodes on the way down to the target, or by the effects of the stimulating electrodes at the target spots in the brain. In order to understand this, we wish to assess the activity at the target site with recordings of the cells and activity around the cells and test those sites using a mental task that does not involve any movements. We can find out which sites are most involved in these cognitive processes and perhaps avoid stimulating these sites for the final therapy. These studies will shed light on the role of this part of the brain in mental function not involving movements and may enhance treatment outcomes.