One month after losing his left arm in a car accident, Victor Quintero sat with his eyes closed in a brain-research laboratory as a scientist poked his cheek with a cotton swab.
"Where do you feel that?" asked Dr. Vilayanur S. Ramachandran, a neuropsychologist at the University of California in San Diego.
"On my left cheek and on the back of my missing hand," said the 17-year-old high school student.
Dr. Ramachandran touched a spot under Mr. Quintero's left nostril. "And where do you feel that?"
"On my left pinky. It tingles."
Eventually Dr. Ramachandran found points all over the young man's left face and jaw that evoked sensations in his amputated hand and arm.
When the scientist stroked the cotton swab on the right side of Mr. Quintero's face and body, the young man felt nothing in the phantom limb. But when he brushed Mr. Quintero's left shoulder just above the stump, the young man again felt discrete points on his missing hand and lower arm.
Finally, Dr. Ramachandran dribbled warm water down Mr. Quintero's left cheek. Both were amazed. "I feel it running down my arm," said Mr. Quintero, blinking his eyes to check that the limb was still gone.
This curious experiment sheds light on an emerging feature of the adult brain that is revolutionizing the way neuroscientists think about brain injury and everyday cognition.
Until recently, scientists believed that nerve cells in the brain died if the body part they were connected to was lost. And they thought that sensations in "phantom" limbs resulted from stimulation of nerves near the missing limb's stump.
Now, however, it seems that the brain does not have fixed circuits. Rather, in ways that are still unknown the adult brain appears to be capable of reorganizing and rewiring itself over incredibly large distances -- so that brain cells receiving inputs from the face and shoulder can trigger brain cells no longer receiving inputs from an arm.
Such rapid dynamic change is a property of healthy adult brains as well as injured ones, said Dr. Charles Gilbert, a neuroscientist at Rockefeller University in New York.
Cells in the visual cortex undergo extensive reorganization as people take in each new view of the world by moving their eyes from scene to scene, he said.
"We are just beginning to realize that the adult brain is more dynamic than static, said Dr. Michael Merzenich, a neuroscientist at the University of California at San Francisco. It continually "shapes and reshapes itself from experiences throughout life."
