Statistical analysis The impact of STN DBS on non-motor symptoms of PD has not been extensively studied and only limited data are available. Based on these data, we anticipate that in order to detect a between-group change of the Non-motor Symptoms Scale (NMSS) of at least 23.3 points the sample size should be of at least 20 patients per each study arm, assuming a power of 0.80 and an alpha value of 0.05. Such a difference is not as large as it seems as the NMSS total score ranges from 0 to 360. Furthermore, this estimation was computed on a pilot study with a small sample size. Beyond that, the NMSS also includes a subscore for each of the 9 domains. Those will be analyzed as well. Presurgical differences between treatment groups will be evaluated with parametric or non-parametric tests, as appropriate. Mean changes in the continuous variables between baseline and 3-month follow up after STN DBS will be compared within and between treatment groups using repeated measures analysis of variance with "treatment group" and "time" as independent categorical factors. The interactions between dependent variables and any other variable of interest (e.g. age, MDS-UPDRS-III, stimulation parameters) will be entered into the model. Given the nature of our study, the standard non corrected significance α level of p<0.05 will be used.