In one video,(link is external) an elderly gentleman who normally can barely walk pedals a bike effortlessly down an Amsterdam street. In another video(link is external), a man who typically can’t hold a mug without sloshing the water out of it catches a ball as he jogs across the room and tosses it back to the person who threw it. This phenomenon of swift, smooth movement in patients whose motor functions are impaired by Parkinson’s disease remains a mystery to the scientists who study the disorder and the part of the brain it affects.

“It’s called paradoxical kinesia, which means paradoxical movement,” said Scott Grafton(link is external), a UC Santa Barbara neuroscientist who studies how people organize movement into goal-oriented action. “People with Parkinson’s can be frozen most of the time, and then every once in a while whatever brakes are on in their motor system are released and they move normally.”