<qa>  What is the major aim of this 5-year study?The major aim of this 5-year study is to carry out a prospective, double blinded, randomized, clinical trial of DBS for medically intractable PD. The study will evaluate the effect of DBS in the Gpi and STN on motor, neuropsychological and psychiatric function, and quality of life in patients with PD. The study also will address two key issues: 1 whether there are differences between unilateral Gpi-DBS and STN-DBS and 2 which patients are the best candidates for bilateral DBS. 