Study Interventions Bilateral simultaneous STN implants will be performed in all patients using the standard stereotactic technique adopted at Toronto Western Hospital. According to the standard of practice currently adopted at Toronto Western Hospital, DBS patients will undergo a pre-surgical evaluation (including a videotaped L-dopa challenge to verify the response of motor symptoms to L-dopa, a neuropsychological evaluation, a psychiatric assessment and a visit with the neurosurgeon). Then, they will be seen one month after surgery for the first programming visit (when the "microlesional" effect is supposed to be gone) and then once a week for the subsequent 8 weeks (in order to slowly adjusts DBS parameters). The optimal stimulation settings will be progressively adjusted according to the patient's response. The standard pulse setting will be 60 μsec in duration at 130 Hz, with voltage adjusted to the individual patient. As regards antiparkinsonian treatment, patients will be randomly assigned to L-dopa monotherapy or DAs monotherapy during the first programming visit. During the 8-weeks adjustment period, L-dopa or DAs (depending on the study arm) will be slowly tapered off and dose of the drug left in monotherapy will be adjusted according to the patient's clinical condition. According to the standard of practice, the drugs dosage will be adjusted according to the patient's needs and will be maintained to the minimum sufficient level to achieve optimal motor control in absence of side effects throughout the duration of the study. In keeping with the standard of care at Toronto Western, patients will then be seen after 3 months since surgery for the completion of the study visit. However, unscheduled visits will be possible in case of unexpected clinical conditions (i.e. occurrence of side effects or worsening of motor conditions).