It sets humans apart from all other species by allowing us to achieve the wonders of walking on the moon and composing masterpieces of literature, art and music. Throughout recorded time, the human brain—a spongy, threepound mass of fatty tissue—has been compared to a telephone switchboard and a supercomputer. But the brain is much more complicated than any of these devices, a fact scientists confirm almost daily with each new discovery. The extent of the brain’s capabilities is unknown, but it is the most complex living structure known in the universe. This single organ controls all body activities, ranging from heart rate and sexual function to emotion, learning and memory. The brain is even thought to influence the response to disease of the immune system and to determine, in part, how well people respond to medical treatments. Ultimately, it shapes our thoughts, hopes, dreams and imagination. In short, the brain is what makes us human. Neuroscientists have the daunting task of deciphering the mystery of this most complex of all machines: how as many as a trillion nerve cells are produced, grow and organize themselves into effective, functionally active systems that ordinarily remain in working order throughout a person’s lifetime.