A gene therapy treatment for Parkinson's disease appears to relieve symptoms by rewiring the brain circuits involved in movement. People with Parkinson's disease have tremors and muscle stiffness that are caused by overstimulation of a brain area called the subthalamic nucleus, which is responsible for coordinating the brain's motor regions. In a trial published in 2011, researchers at the Feinstein Institute for Medical Research in New York found that a gene therapy designed to turn down the activity of the subthalamic nucleus improved motor control for people with Parkinson's. Though the treatment reduced Parkinson's symptoms for at least a year, it was unclear how. To find out, the researchers have since used PET scans to compare the brains of 15 people who received the gene therapy with 20 who received a placebo.