Phantom Limb: The Science of Impossible Sensation Part 2

An amputee receives mirror therapy.

Source: Donna Miles/Air Force Photos

By Alexander Metz

Searching for Explanations

In recent years, and particularly with the advent of things like neuroimaging, scientists have been able to posit more plausible hypotheses on the origins of phantom limb pain/syndrome.

Scientists have already shown that the somatosensory cortex of the brain contains a map of all the bodies’ surfaces. That is to say, that sensation in the foot stimulates the “foot area” of the brain, sensation in the hand, the hand area, and so on. It is also known that the size of these areas is related, not to the relative sizes of their corresponding body parts, but to the relative number of nerve fibers those parts contain. Using modern imaging technology, scientists have been able to construct the Penfield map of the somatosensory cortex.

Phantom-Limb-The-Science-of-Impossible-Sensation-Part-2 Phantom Limb: The Science of Impossible Sensation Part 2

Pinfield diagram which shows how the body maps onto the somatosensory cortex.

Source: Ramachandran, V. S., & Hirstein, W. (1998). The perception of phantom limbs: The D. O. hebb lecture. Brain, 121(9), 1603–1630.

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Keeping this in mind, it phantom sensation begins to make more sense. How can those born without certain limbs experience phantom sensation? Because those patients have the same body map in their brains as someone born with their arms and legs intact. This also presents a plausible explanation for the “remapping phenomena” as the face and hand areas of the brain are both very large and very close together.

Can science then give a complete account of phantom sensation? Not quite. The precise mechanism by which the phantom emerges is still shrouded in mystery. Why do some patients present immediately after amputation, while others present only after several years? Why do some phantoms fade and other not? Why are some controllable and others not? Why do some experience the phantom consistently and other only intermittently? None of these have definite answers. What we do seem to know is that phantom sensation is the result of our brain’s retention of a sensory map, even after the subject of that map has been removed.

Further, phantom pain may be caused by the body’s adverse reaction to a loss of input from the limb. The brain does not understand how to interpret this complete loss of signal. So, as a sort of evolutionary fail-safe, the patient experiences the sensation (or lack thereof) as pain.


Traditional Treatments

As one might expect, the relative vagueness of the causes of phantom limb pain, as well as the wide range of symptoms with which the condition presents has made treatment difficult. For many years, treatment of amputees focused almost entirely on prosthetics, the vast majority of which were rigid facsimiles of the missing limb attached at the amputation site. More modern treatments include, over the counter pain medication, opioids, anti-depressants, anti-convulsant, and NMDA receptor antagonists (a class of anesthetics which work by binding to their namesake receptors). Medical therapies include acupuncture, Repetitive transcranial magnetic stimulation (rTMS), and spinal cord stimulation.

Mirror Box Therapy

By far, the most well-known treatment for phantom limb pain is known as the mirror box. Developed in the early 1990s by neuroscientist Vilayanur S. Ramachandran, the mirror box is a deceptively simple solution to an enormously complex and poorly understood problem. As the name suggests, the mirror box requires a mirror and a box. The mirror is placed inside the box, dividing it into two separate compartments. Holes in each compartment allow the patient to place their intact limb on one side of the mirror, and their phantom limb on the other. By angling their heads to one side, patients can see the reflection of their working limb imposed onto the space where they feel their phantom limb. By then progressing through a series of symmetrical exercises, patients are able to “see” their phantom limb moving as if it were really there.

This procedure in some, but not all, cases provides relief for those who suffer from phantom pain. How and why does the mirror box achieve this effect? This too is not entirely understood. Ramachandran theorizes that visual input serves as a stand-in for the sensory information the body typically receives.

Virtual and Augmented Reality

Today, scientists have taken the concept of the mirror box and furthered it with the introduction of augmented and virtual reality. Patients wear a sensory glove on one hand and, with the help of 3D graphics software, can observe a real-time rendering of their phantom hand. This procedure has several obvious advantages over the traditional mirror-box, the primary one being the ability to treat phantom pains in limbs that are not easily inserted into the traditional apparatus. Another advantage of the virtual interface is its capacity to simulate non-symmetrical movement. This may enhance the mirror illusion by allowing the brain to see that the image of the phantom hand is not just a refection, but a distinct entity. Finally, “virtual mirror box” therapy may allow clinicians to tailor the experience to particular patients specific needs. This might be helpful for those whose phantoms present atypically, are telescoped, or are stuck in an odd position.

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