Phantom arms, legs, fingers and toes: seemingly the stuff of horror movies. Yet for nearly 70 percent of the 4 million amputees in the United States, vivid sensations in missing body parts-such as pressure, tingling, warmth, cold, and pain that can be both constant and excruciating-are all too real.

Phantom limbs have puzzled scientists for years. But recent studies have shed light on possible mechanisms underlying the phenomenon, including evidence that neurons in the brain that receive input from a limb may rewire themselves to seek input from other sources after the limb is amputated. These findings challenge the long-standing belief that the brain is immutable beyond a certain age and are leading researchers to develop new therapies for victims of phantom-limb pain and some spinal-cord injuries.