An electrode is permanently implanted to stimulate the subthalamic nucleus or, alternatively, the globus pallidus internus. Through a mechanism that is still not well understood, stimulation reduces the abnormal discharge that underlies many of the symptoms of Parkinson’s disease. The implant is secured to the skull and connected under the skin to a stimulator and battery pack placed in the chest. An external programming device is used to adjust the frequency following surgery to optimize symptom relief and minimize side effects. Figure reproduced from ref. 25.