Sometimes the signals get crossed, and any type of movement in the limb or in the face produces excruciating pain from the phantom appendage, he added.
Done With Mirrors
"These days the therapy for phantom pain involves a variety of painkillers and in some cases more extreme procedures, such as deep-brain stimulation, where an electrode is inserted into a region of the brain thought to be responding to phantom pain," said the University of Manchester's Pettifer.
Traditionally, an electrode is wired through the neck down to the chest. There, a small controller box generates a high-frequency signal to try and switch off the pain-causing part of the brain.
Pettifer and colleagues' less invasive virtual fix is modeled on Ramachandran's earlier work, which involved a cardboard box with a mirror on one side.
In that method, a person sits in front of the box and moves his or her arm. The reflection in the mirror appears to fool the person's brain into thinking the reflected limb is the missing appendage that is moving.
"The visual feedback," Ramachandran said, "seemed to alleviate pain."
But with the virtual system a patient wears a head-mounted display system with two tiny video screens, one for each eye. Each screen shows a virtual environment, providing a sense of three-dimensional vision.
The patient slips on a "data glove" with gauges that track finger movement. Three other sensors—one on the head and two on the remaining limb—detect the position and rotation of the user's body.
Once in the virtual environment, the user completes tasks such as touching a series of changing targets or batting away a ball floating in front of his or her face.
"These are all excuses to get them to move their limb," Pettifer explained.
"It is the good arm that is doing these movements. But in the virtual environment, they really see their missing arm perform the task," he said. "Like with the mirror box, it tricks a person's mind into thinking that the missing limb is moving."
Results vary, but researchers say a session on the system typically provides around two days of pain relief.
Four of the five people who have tested the system so far reported a significant reduction in phantom pain, and Pettifer's research team is planning large-scale clinical trials in February.
Albert Skip Rizzo is a psychologist at the University of Southern California's Integrated Media Systems Center. He said, "The surprising results provide a new way of understanding how the brain perceives [signals] from the body and how you can fool it with certain visual stimuli."
The University of California's Ramachandran thinks his and Pettifer's findings debunk a notion that there are fixed connections in the brain that one is born with, and that when these connections are damaged, there is little that can be done.
"It challenges the standard view of neurology over 50 years and argues that many of these neurological dysfunctions might just be a functional suppression that can be reversed, like touching a reset button," Ramachandran said.
The work, he added, may apply beyond phantom pain.
"People are using [the mirror system] for the rehabilitation of stroke victims [for regaining arm movement]. Even in something perceived as permanent, there might be partial recovery."
Ramachandran thinks Pettifer's virtual reality system may supplement existing therapies for phantom pain.
But several questions remain.
For starters, Pettifer admits, the device doesn't work for everybody.
"We don't know what type of subject it works on, whether there are different personalities that will respond or not, or which aspect of the virtual environment is most important in terms of helping the pain," he said.
"Once we understand more about these things, making a version of the system that can be put in clinics, or even in the home, is fairly straightforward," Pettifer said.
"It's the sort of thing that with a little modification could more or less be run from today's Xboxes or PlayStations."
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