The Phantom Limb Phenomenon
“"Centuries later, the American physician Silas Weir Mitchell coined the term phantom limb to describe the same phenomenon in Civil War soldiers. His many case studies established that phantom limbs are the rule, not the exception…today many neuroscientists believe…phantom limbs are caused by a remapping of the cortex" (Seung, 29)
The phantom limb phenomenon is as old as the process of amputation itself. It begins with an ailment that requires a part of the body to be amputated; then, soon after amputation, the patient begins to feel varying levels of sensation (usually pain) in the region of the body that has been chopped off. Sometimes it is onset by a change in temperature or pressure. However, often it is purely sporadic and can continue for long periods of time. Almost immediately after the loss of a limb, 90 to 98% of all amputees experience sensation of a phantom limb (Ramachandran p3). Treatments of the phantom limb pain are limited. Some try the mirror trick, where a mirror is placed next to the one working limb to create the perception that the limb actually exists. It has not been proven to alleviate the pain absolutely but it can help. Although significant empirical evidence has been collected that points to cortical remapping as the cause, scientists have still not proven what exactly happens inside the brain during the phantom phenomena.
As I read Sebastian Seung’s Connectome, the topic of phantom limbs jumped out at me immediately. I was completely unaware that this phenomenon was indeed a legitimate scientific ailment. I enjoy history and have studied the Civil War extensively, and, at one time or another, I stumbled upon an article about wounded troops. The article, written in the late 1800s, was about wounded Civil War veterans reporting pain in their missing limbs, citing a Dr. Silas Mitchell as the physician who wrote of this. Naturally I found the concept to be too far-fetched to be real, so I dismissed the article and the idea altogether. ‘How could someone feel pain in a missing limb?’ I thought. I attributed those instances to complications of post-traumatic stress disorder and read on about other topics. So when I read Seung’s background information on the phenomenon, I was more than surprised. My interest was immediately aroused, and I embarked on my own personal quest to discover as much as possible about this and perhaps come up with a theory of my own. The fruits of my labor are summarized below.
The man who named this phenomenon was Silas Weir Mitchell himself during the Civil War. He was studying cases of causalgia, which is severe burning pain in a limb caused by an injury to a peripheral nerve. At the time, the common treatment for causalgia was amputation, so he would observe his causalgia patients undergo the process and help them recover afterwards. It was then that he noticed the phantom limb phenomenon. He wrote, “Nearly every man who loses a limb carries about with him a constant or inconstant phantom of the missing member, a sensory ghost of that much of himself, and sometimes a most inconvenient presence, faintly felt at time, but ready to be called up to his perception by a blow, a touch, or a change of wind.”
His remedial experiments consisted of faradizing patients who had amputated arms and asking his patients where they reported pain. Most reported that their entire arm was on fire, including the hands. Some even stated that the hand pain was the most distinct and painful. He speculated that, like Descartes had in the 1600’s, the pain was caused by memories of activity with the amputated body part. It was all just assumptions, due to lack of modern technology. Modern advances in neuronal networking could make the answer crystal clear, as Seung suggested, but the majority of research up to date has been based on reactions to the cause not the cause itself.
A study was done in 1998 by V.S. Ramachandran and William Hirstein to possibly allocate the cause of the phantom limb phenomenon. They observed phantom limb patients and used Penfield’s sensory map of the brain to make general assumptions that were only tested by a simple Q tip. In his paper, Ramachandran proposes that the phantom limb phenomenon is caused by the remapping of area 3 (the sensory homunculus). Due to the lack of stimulation from the amputated body part, the area of the brain associated with that body part changes its source of stimulation to the adjacent areas. In other words, the other areas swallow up the amputated area. He tested this by using a Q tip to stimulate the faces of amputees. The amputees reported sensation not only from their faces, but also from their amputated lower arms. His experiments proved that cortical remapping was indeed the answer; however, he still did not show the actual brain processes that accompany it. He did not have the technology to show the nuances of the neuronal wiring that accompany cortical remapping. Developments such as the MRI and fMRI are making that more and more possible, however without achieving Seung’s dream of a fully mapped connectome, we may never actually know. A few universities have undertaken this near impossible task of attempting to display legitimate evidence of cortical remapping. The University of Sussex in the United Kingdom hypothesizes that “phantom limb sensations reflect a principled (rather than random) reorganization of brain circuits supporting movement and perception that were in operation prior to amputation” (USussex). They are attempting to confirm this through a mixture of classical Ramachandran-like experiments and observations of brain activity through the utilization of EEGs, MRIs, and fMRIs. The primary researcher is Dr. Jamie Ward, a senior lecturer at the university.
Although their research is promising, they will never truly allocate the cause of phantom limbs. The University of Sussex researchers approach the topic more from a theoretical and psychological background. They are by no means concerned with displaying neuronal networks that make up thought and demonstrating how they change over time. This would be an incredibly difficult and work intensive undertaking. Not to mention it would be very expensive. Unfortunately, until we can completely map a brain, it seems that we will never truly know.
In that regard, I suppose the main question has been answered. Why do amputees experience phantom limbs? The answer is: their cortexes have remapped themselves. But the question remains: how? Nearly every question about the brain boils down to that. Studies of schizophrenics, anorexics, and others with “brain abnormalities” show that their brains are wired differently; however, we cannot show how. It’s almost as if neurology has reached a full stop in front of the immense wall of the connectome. But there is hope. Technology is advancing rapidly, and within our lifetimes it is very possible that this question could be answered, and many neurological diseases could be cured in the process.
On this topic, my best hypothesis centers on the connections between neurons. It is general knowledge that action potentials are initiated in series from one neuron to the next in a sequence to transmit thoughts and messages. The neurons form extremely complicated networks with millions of axons connected to millions of dendrites in very cramped space. If a limb is chopped off and the neuron chains that are associated with transmitting information to and from the limb are inactive for a long period, the synapses would weaken and the chain would “wither away”. However, neurons must always remain active. So they establish new connections with other neurons. Due to the close proximity of the neurons which operate other parts of the brain, they naturally connect with the neurons close to them. Thus the “face” neurons connect with the “chopped off arm” neurons. So when the “face” neurons fire, the “chopped off arm” neurons also fire and the person experiences sensation in the phantom limb. Of course, this is not an absolute truth. I have not tested if a neuron would simply reconnect randomly with another neuron when left idle. However I believe that it’s a start. Perhaps with the advancement of technology and the modification of hypotheses, we will finally be able to put the phantom limb to rest.