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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.

For years, psychologists attributed phantom-limb sensations to “wish fulfillment,” a purely psychological condition. Then, in 1984, a team led by Michael Merzenich, a neuroscientist at the University of California at San Francisco, conducted experiments that began to explain phantom limbs as a true physiological response. Merzenich and his colleagues first amputated the middle fingers from a group of adult owl monkeys and later stimulated the digits on the hand of each monkey that were adjacent to the amputation stump.

Placing microelectrodes, which detect electrochemical changes in actively firing neurons, into various areas of the monkeys’ brains, Merzenich found that the region of the cortex that originally fired in response to stimulation of the amputated finger was now triggered every time he touched the two adjacent fingers. The neurons had not responded to stimulation of these fingers before the amputation.

In 1991, Timothy Pons, a neuroscientist at the Laboratory of Neuropsychology at the National Institute of Mental Health, expanded on Merzenich’s findings. Working with adult macaque monkeys, Pons and his colleagues “deafferentated,” or cut, nerves that communicated sensory information between the cortex and the arm, forearm, hand, and rear of the head. The team then stimulated various body parts and found that the part of the cortex that had previously responded to the arm and back of the head now responded to stimulation of the face. Like ivy spreading over bare brick, Pons believes, surrounding neurons invaded the fallow cortical area corresponding to the deafferentated limbs, allowing it to respond to stimulation from other parts of the body.

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