• Robb G. Best

Phantoms, Magic Tricks and Video Games

In a fair and just world, losing a limb would be the end of feeling any discomfort in that particular region. For one thing, surely at that point, you’ve got enough on your plate. For another, it sounds like something that should go without saying. Once the nerve endings in that arm or leg are no longer connected to your body—and crucially, to your brain—it’s not as though you can clock accurate information about what that appendage is going through. (Nor would you want to.)

However, most post-surgery amputation patients report an unpleasant sensation sort of floating where their limb used to be. Sometimes this manifests as a burning pain, while sometimes it itches, or prickles like the departed body part has fallen asleep (an ironic expression, in this case.)

Sometimes, this only lasts a few seconds. Sometimes, it lasts days at a time. The Amputee Coalition explains that these attacks tend to decrease in length and frequency over the first six months, but warns, “many continue to experience some level of these sensations for years.” The Coalition also notes that “People are often reluctant to tell anyone” about arm or leg pain in an arm or leg that’s no longer there, “for fear that they will be considered ‘crazy.’”

Surely those individuals would be comforted to know that a full 80% of amputees experience some degree of phantom limb pain. Still, in another sense, this raises more questions than it answers. If that hand or foot is gone, just what are all these people feeling? Just what’s going on?

Medical science has yet to hit upon a definite answer for what causes this strange phenomenon, but it may be fallout from severed neural cords. Normally, data collected from nerve endings allow the brain to devise a sort of 3D model of what’s happening in the body, even absent any other source of information. When part of that data collection system suddenly goes offline, an amputee’s burning, itching, or prickling could be the brain attempting to “reprogram its signal network,” explains Bo Geng, a postdoc of the Faculty of Medicine at Denmark’s Aalborg University. Unfortunately, the brain seems to frequently interpret this disconnect as pain.

If phantom limbs are simply a glitch in the brain’s simulation, how can we debug it?

Possibly, with another simulation.

Over a decade ago, the podcast Radiolab ran an interview with neuroscientist V.S Ramachandran, who managed to “cure” a painful case of phantom limb using common household objects: “a mirror propped up in a cardboard box.” By reflecting the other side of the body onto the side with the amputation, Ramachandran could create for that individual the illusion of being able to move and control their missing part. That particular patient’s brain was pacified by this false input, and the pain went away.

However, “mirror therapy” has its limitations, says Bo Geng. The patient must be sitting in front of the mirror, performing repeated movements in a small space, while keeping their eyes on the glass. That’s why researchers at Aalborg University are looking to take that mirror system to the next level: VR headsets. "With Virtual Reality,” she says, “there is a much better chance of creating a convincing alternative reality."

In preliminary studies in China, patients donned VR goggles and scientists attached electrodes to the amputation site, stimulating the area with small amounts of electricity in an attempt to kickstart the sensation of that missing hand. (These studies focused on upper body amputations; the team at Aalborg intends to develop something similar for feet.) The patients then played Virtual Reality games requiring the involved use of both hands. Seeing those simulated objects respond the phantom hand seemed to fool the brain even better than the mirror trick.

Of the three initial subjects, two said their phantom limb pain was gone, while the third reported fewer attacks than before. Still, three is hardly a robust sample size. "Of course we need to do more tests,” says Bo Geng, “but the results so far look promising.”

Promising--and fascinating.


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