Parkinson's and stroke patients benefit, neurologists believe, because the human brain is innately attuned to respond to highly rhythmic music; in fact, says Sacks, our nervous system is unique among mammals in its automatic tendency to go into foot-tapping mode. In Parkinson's patients with bradykinesia, or difficulty initiating movement, it's thought that the music triggers networks of neurons to translate the cadence into organized movement. "We see patients develop something like an auditory timing mechanism," says Concetta Tomaino, cofounder of the Institute for Music and Neurologic Function in New York City. "Someone who is frozen can immediately release and begin walking. Or if they have balance problems, they can coordinate their steps to synchronize with the music," improving their gait and stride. Slow rhythms can ease the muscle bursts and jerky motions of Parkinson's patients with involuntary tremors.