Perception is Inference

Ninety-eight percent (98%) of amputees experience phantom limb pain; it's not an off-the-wall, rare side effect of limb loss, but rather a complicated and common resulting pathology. One aspect of its complexity is that it's experienced as more than just pain: Dr. Atul Gawande explains that "People report not just pain but also sensations of sweatiness, heat, texture, and movement in a missing limb. There is no experience people have with real limbs that they do not experience with phantom limbs. They feel their phantom leg swinging, water trickling down a phantom arm, a phantom ring becoming too tight for a phantom digit." (Gawande, 2008)

The traditional understanding of phantom pain was likening it to neuropathic pain--where nerve damage causes mixed-up signals to be sent to the brain, causing chronic pain. Yet the description above shows that phantom limb pain includes sensations far too detailed and realistic to be reasonably caused by the "random firings of a bruised nerve". (Gawande, 2008) The theory proposed by Dr. Atul Gawande and Dr. V. S. Ramachandran, among others, revolves around the maxim that Perception is Inference. Increasing evidence leads scientists to believe that "perception is the brain's best guess about what is happening in the outside world. The mind integrates scattered, weak, rudimentary signals from a variety of sensory channels, information from past experiences, and hard-wired processes, and produces a sensory experience full of brain-provided color, sound, texture, and meaning." Gawande, 2008) That is to say, phantom limb pain may originate from the brain attempting to fill in the gaps of the missing sensory input from the amputated limb, not from a direct problem with peripheral input as such.

Dr. Gawande employs an analogy to clarify the "sensor syndrome" of phantom limb pain: "When your car's dashboard warning light keeps telling you that there is an engine failure, but the mechanics can't find anything wrong, the sensor itself may be the problem." (Gawande, 2008) Usually sensations of pain serve a protective function, but when the brain lacks sensory input from a missing limb, the brain can go haywire.

Dr. Ramachandran's "Tales of the Tell-Tale Brain"

Follow the link above to NPR's Fresh Air segment!

Treatment with the usual drugs, such as tranquilizers, antidepressants, and narcotics, may ease symptoms for a short time, but they usually stop working. Dr. Ramachandran invented mirror treatments that trick the brain into thinking the missing limb is indeed intact (despite the patient knowing that this is an illusion), which have provided both immediate relief and long-term results. The dramatic success of this treatment in a published randomized trial at Walter Reid Medical Center supports the theory of brain confusion instead of faulty peripheral input as the cause of phantom limb pain.

Later posts consider which segments of the population are most affected, so for now it will suffice to say that while combat Veterans and trauma victims are often the population groups that first come to mind when thinking of amputees, highly prevalent diseases, most especially diabetes, can commonly and unfortunately lead to limb loss as well.

Sources:

Gawande, A. (2008, June 30). The Itch. The New Yorker. Retrieved from http://www.newyorker.com/magazine/2008/06/30/the-itch

Ramachandran, V. S. (2011, February 14). Tales of the Tell-Tale Brain. In NPR Books. Retrieved from http://www.npr.org/2011/02/14/133026897/v-s-ramachandrans-tales-of-the-tell-tale-brain