Ghostly Pictures: What Phantom Limbs Can Tell Us About Your Brain

Ghostly Pictures: What Phantom Limbs Can Tell Us About Your Brain

Science June 13, 2013 / By The Connectome
Ghostly Pictures: What Phantom Limbs Can Tell Us About Your Brain

Recent research brings understanding of phantom limb pain almost within reach.

The accident had happened 35 years ago, but the pain kept worsening, even in the absence of the hand that felt it. The patient, a 57-year-old woman known as R.N., had been born with a malformed right hand, which had only three fingers and the stump of a thumb. The hand had been amputated after suffering severe damage in a car crash when R.N. was 18. And 35 years later, the pain in R.N.’s lost hand - as well as in the fingers she’d never had - was getting so unbearable that she traveled from specialist to specialist, searching for an expert who could understand her agony well enough to treat it.

 R.N.’s experience wasn’t as uncommon as you might think. As many as 80 percent of amputees experience pain in their lost limbs. Weeks after an amputation - years, sometimes - a patient returns to the hospital complaining of pain in an arm, hand, leg or foot that’s clearly long gone - except that in the patient’s brain, it somehow persists. Doctors have proposed a wide range of explanations over the decades, from inflamed nerve endings to persistent “hard-wiring” patterns within the brain itself. But the fact is, no one’s sure exactly why some people’s brains try so hard to resurrect the limbs they’ve lost.

 Part of the reason for this confusion is that pain can stem from a wide range of unusual and perplexing causes, even in limbs we clearly still have. To give you some idea, the website of one medical practice lists more than a dozen common causes of chronic hand pain and wrist problems alone - and those are just the cases treated at a typical doctor’s office; not necessarily the ones that are weird enough to inspire original research papers.

 But original research is exactly what came out of R.N.’s case. Neurologists V.S. Ramachandran and Paul McGeoch published a paper in the journal Neurocase, documenting their success treating R.N.’s phantom pain through use of a “mirror box:” A device that creates the optical illusion of a healthy hand in place of the missing one - a hand whose pain some patients say is completely cured once they’ve tricked their brains into believing it’s real. One ghost, it would seem, can be banished by summoning another.

 Not so fast, though. Ramachandran’s claims have drawn their share of sharp criticism over the years. The physician Marian Michielsen finds fault with Ramachandran’s hypothesis that the mirror box works by convincing certain brain areas that the mirrored limb actually exists; instead, she contends that the therapy works by increasing the brain’s awareness of the fact that the phantom limb is no longer attached. Rehabilitation specialist Andreas Rothgangel argues that mirror therapy has only been successful in a small percentage of cases; while phantom limb experts Melita Giummarra and Lorimer Moseley emphasize that the mirror technique only works for patients with very specific types of phantom pain - and that it may even be counterproductive in the early stages of rehabilitation. All in all, not exactly a ringing endorsement of the “ghost swap” idea - nor much of a consensus on the root cause (or causes) of phantom pain.

 As unclear as those ultimate causes may be, the latest research is getting us closer than ever to understanding just how phantom pain occurs in the brain. Take, for instance, a new study that paired patients’ subjective phantom limb reports with functional MRI scans of their brain activity. As the journal Nature Communications reports, a team of Oxford University researchers joined forces with David Henderson-Slater of the Nuffield Orthopaedic Centre, and assembled a group of 18 amputees who suffered from phantom pain after long-ago operations. After recording the patients’ detailed descriptions of their phantom pain, the researchers recorded videos of their brain activity in an fMRI scanner.

 The team discovered a striking correspondence: The more severe a patient’s phantom pain, the more neural activity persisted in brain areas that were once devoted to sensing its presence. “We were astonished to find that in amputees experiencing strong phantom pain, the brain's response was indistinguishable from that seen in people with intact limbs,” Makin said. What’s more, patients who experienced the strongest phantom pain had a lower-than-average amount of physical deterioration in areas of their bodies that were once connected to the phantom limbs. And although the local structure and function of the lost limb’s faithful brain areas persisted, those parts of the brain didn’t communicate in synch with other areas - a clue, perhaps, that there’s some truth to Michielsen’s “increased awareness” explanation of mirror-box therapy.

 Of course, none of this gives us a clear-cut reason why some patients experience less physical degeneration, or more persistent brain function, in response to an amputation than others do. Perhaps the cause can be emotional - a strong desire for the lost limb’s return, or the initial shock of losing it - and a patient’s genetics, along with his or her unique brain wiring, are also likely to play parts of their own. Even so, the more closely we’re able to peer at the brain’s ghostly images of lost limbs, the better we’ll understand which treatments work - and why they do.

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