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A second, though no less eerie question, is inherently neurobiological: how does the brain function so as not to register the missing limb? Can it be that the brain creates a perception that is not true to reality? Or, in other words, "what is 'me' might not necessarily be" (4).
In order to begin to explain the phenomenon of phantom limbs, it is necessary to gain a general understanding of the organization of the brain as it relates to various parts of the body. As determined by V. Sussman in 1995, phantom limb sensations in the part of the brain called the sensory cortex (1). The sensory cortex bears a sort of map of the rest of the body, organized in a 'topographic' manner so that each part corresponds to a specific section of the body, thus earning the map the name "homunculus" or "little man" (1). Sensations originate in various body parts and make their way to the brain for recognition by the homunculus. Thus, according to Richard Sherman, a pinch on the left index finger tip stimulates a location on the homunculus representing the left index finger tip (2).
This is all well and good if every part of the body and its corresponding portion of the homunculus is in full operation. However, following an amputation the direct relationship between left index finger and homunculus is interrupted. As a direct result the brain changes, too. The brain has been noted to alter over a lifetime based upon various experiential factors. For example, blind Braille readers are found to have an increased corticol representation for their index fingers (3). Much the same 'plastic' effect of the brain is at work in the creation of phantom limb sensations.
According to Toni Ray in his article "Helping Phantom Limb Pain", when the body loses an appendage, the brain compensates for the signals it is no longer receiving by rewiring nerve impulses in the sensory cortex to travel down "previously untraveled pathways" (1). To think of this in terms of the 'little boxes' we discussed in class, we determined that very few inputs to the nervous system originate from sensory neurons (i.e. a pinch on the index finger). The lack of an index finger does not shut down the pathways from the finger to the brain, it merely prevents sensory neurons from doing their work. If signals are created inside the nervous system, as we know they are, they can still run along the same nerve paths to the homunculus and the resulting sensation may seem to originate in the missing finger. Thus, Sherman concludes, the brain has no way of actually knowing that the finger tip is not present (2).
This explanation via little boxes is supported by the monkey experiments performed by Merzenich. Merzenich found that by amputating the index finger of a monkey, within a month the cortical neurons could receive inputs from the back of the hand (4). These studies suggested that neurons within the homunculus rewired themselves to pathways that were only exposed once the original pathways were destroyed via the amputation of the finger. Therefore, the reorganization of the sensory cortex can be attributed as producing phantom limb sensations (4).
Treatments of phantom limbs are as intriguing as the condition itself. According to Sherman, success rates for curing phantom limbs have historically been dismal, with only about one percent of treated sufferers obtaining lasting relief for longer than one year (2). Different symptoms of phantom limbs can be treated individually. For instance, cramping limbs respond well to treatments for cramping in the remaining limb, even if the pain is not felt in that limb. Sufferers who report burning often respond well to increasing blood flow in the remaining limb. Both of these treatments suggest a rewiring of the homunculus so that treatment in one location--the remaining limb--corresponds to feelings in the phantom, or missing, limb.
Most interestingly, it seems the brain can be beaten at its own game. If the sensory cortex believes, due to inputs, that the limb exists some treatments bank on the notion that it can be fooled visually in quite the same way. Using a method called the Mirror Virtual Reality Box, the missing limb is visually simulated using mirrors in order to give the appearance of the phantom limb realized. Oftentimes, participants report a lessening of the tension and pain as if they are actually moving and stretching the missing appendage (3).
Phantom limb sensations provide the unique experience in which the brain might be able to conceptualize its own deception. Which is, to say the least, a disarming possibility.
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