GPi stimulation may be used in patients who do not qualify for STN stimulation. Especially postural instability, age over 70 years, and mild to moderate cognitive deficits are commonly considered to be exclusion criteria for STN stimulation but not for GPi stimulation. Therefore, the investigators propose a prospective randomized controlled trial of GPi stimulation in patients with PD and motor complications who have relative or absolute contraindications for STN stimulation.