Impact of Subthalamic Nucleus Deep Brain Stimulation on COGnitive and PSYchiatric Symptoms in Parkinson's Disease

Change in composite verbal fluency score for left versus right hemispheric stimulation. To calculate the composite verbal fluency score, the standardized z-scores for phonemic and semantic verbal fluency task will be averaged. For the semantic fluency task, participants generate orally as many category names as possible during 2 minutes (German category 1: professions, category 2: hobbies; French category 1: furniture; category 2: fruit). During the phonemic fluency task, participants generate orally as many words as possible beginning with a certain letter while respecting the following rules: no repetition and no word with the same root, and no proper nouns (German: P-words, M-words; French: R-words, V-words). During each condition (right side stimulation OFF and left side stimulation OFF), one semantic fluency task and one phonemic fluency task will be performed (making sure the same letter and cateogry are only assessed once).

Change for left versus right hemispheric stimulation in composite impulsivity score. The impulsivity score will be calculated by averaging the standardized z-values for a computerized Go/No-Go task (reaction time and mistakes) and a Stroop Task (inhibition time and mistakes). The Go/NoGo subtests measures selective attention and motor impulsivity. Subjects have to react as quickly as possible when they see a cross on the screen (x), but not when they see a plus sign (+). Both speed and number of mistakes are measured. The interference subtest of the Stroop test measures inhibitory control and selective attention. In the task, participants see color words (e.g., "RED," "BLUE," "GREEN") printed in a conflicting ink color. They are instructed to name the ink color rather than read the word. Time and number of mistakes are measured.

Change for left versus right hemispheric stimulation in Neuropsychiatric Fluctuation Scale (NFS). The Neuropsychiatric Fluctuations Scale for Parkinson's Disease is a self-report questionnaire. The NFS includes 10 items evaluating OFF neuropsychiatric symptoms and 10 items evaluating ON neuropsychiatric symptoms. Each item is rated from 0 to 3. The total score of the NFS ranges from 0-60, with higher scores indicating better mood. The NFS will be admistered once during each intervention (stimulation right side OFF and stimulation left side OFF).