Siamese brains. Pathological generosity. Viruses that eat patients' memories. Stroke victims who can't speak but can still sing. Until very recently, scientists had only one way to study the brain: wait for misfortune to strike and see how people's minds were transformed afterward. These people's lives laid the foundations of modern neuroscience, and their fascinating and dramatic stories expand our notions of what the brain is capable of — showing that when one part of the mind shuts down, something new and unpredictable and sometimes even beautiful roars to life.