Science explains the phantom limb phenomenon

Unlike other animals, humans are still in a process of development when they’re born. That’s why we’re so defenseless, we need our parents’ protection, and once we’ve grown we don’t remember anything from our first phase of life. The first years of growth are vital for the nervous system to form correctly, which implies the creation of new neurons and new connections between them. During the first stages of growth, while we are learning to walk or to control some of our physiological functions, the nervous system is also adjusting itself to gain control over these processes by establishing the correct connections between neurons. Proof of that is that, if you were to cover a baby’s eye at birth and uncover it a few years later, that eye, despite being healthy, would be blind, because the brain would not have learned to use it and wouldn’t be connected with it. In a similar but opposite sense, when your brain has learned to direct and locate a part of the body, if that body part disappears, the brain continues thinking that it’s there, and this is a real problem for people who have to deal with it.

The French doctor Ambroise Paré had already described “sensory ghosts” in the 16th century. René Descartes also makes reference to this phenomenon in one of his works. The first rigorous description is from Silas Weir Mitchell, the father of American neurology. After the Battle of Gettysburg, in the Civil War, many patients were brought to the hospital, the majority with amputations of limbs. Mitchell noted that, of 90 amputees, 86 described sensations in their missing extremity, and he detailed what type of sensation, not just pain or itchiness, but also shrinking or heat. It seems that his interest in this pathology grew from his friendship with the poet Walt Whitman. During the war, Whitman went to look for his brother, who was wounded in Fredericksburg, Virginia. Inspired by this visit, he spent three years volunteering in a field hospital assisting and accompanying wounded men, and he collected many stories of amputee soldiers who spoke of the ghosts of their own flesh that came back to torment them. In the 108th chapter of Moby Dick, Captain Ahab also describes how he still feels his amputated leg.

How does science explain these seeming ghosts? Inside of our brain, there are areas that regulate discrete functions. Certain brain injuries can make us lose our sense of taste, our ability to speak, or our ability to move a certain part of the body. In the same way, there are certain places that correspond to the sensory information that we receive from a certain arm or leg. What happens when we amputate that arm or that leg? The brain still has the area that corresponds to the signals from that limb. When it no longer receives signals from the limb, this part of the brain which no longer has a function generates spontaneous discharges that are interpreted as pain, itchiness or discomfort.

Is it possible to treat an illness with such a complex origin? Can we medicate a limb which isn’t there anymore? Traditionally, doctors tried interventions that dealt with the amputation site, but obviously the problem wasn’t there. Pain medicine has also been tried, but unlike pain provoked by a specific injury, chronic pain is difficult to treat. The Indian doctor Vilayanur S. Ramachandran devised an ingenious system. He began with the idea that people who had had their limb paralyzed prior to amputation were more likely to feel the phantom limb. This implied that during a time, the patients had tried to move the limb, without success, which had created a sensory feedback loop that the brain remembered. The pain was due to how the brain interpreted that it should be trying to move the paralyzed limb. Following this idea, therapy would consist in retraining the brain to eliminate this supposed paralysis. For this to work, doctors used a box with a mirror in which the healthy limb, through its reflection, imitated the amputated limb and the brain interpreted that it was no longer paralyzed. This therapy with the mirror box has had certain success, although many studies that describe its use don’t have the adequate methodology, for which reason the theory remains controversial. What’s clear is that the biggest box of surprises isn’t the mirror box of Ramachandran, rather our brains themselves.

J. M. Mulet is professor of Biotechnology.