Free Phantoms in the Brain: Probing the Mysteries of the Human Mind by V. S. Ramachandran, Sandra Blakeslee

down his face, he exclaimed with considerable surprise that he could actually feel the warm water trickling down the length of his phantom arm. He demonstrated this to me by using his normal hand to trace out the path of the water down his phantom. In all my years in neurology clinics, I had never seen anything quite so remarkable—a patient systematically mislocalizing a complex sensation such as a "trickle" from his face to his phantom hand.
    These experiments imply that highly precise and organized new connections can be formed in the adult brain in a few days. But they don't tell us how these new pathways actually emerge, what the underlying mechanisms are at the cellular level.
    I can think of two possibilities. First, the reorganization could involve sprouting—the actual growth of new branches from nerve fibers that normally innervate the face area toward cells in the hand area in the cortex. If this hypothesis were true, this would be quite remarkable since it is difficult to see how highly organized sprouting could take place over relatively long distances (in the brain several millimeters might as well be a mile) and in such a short period. Moreover, even if sprouting occurs, how would the new fibers "know" where to go? One can imagine a higgledy−piggledy jumble of connections, but not precisely organized pathways.
    The second possibility is that there is in fact a tremendous redundancy of connections in the normal adult brain but that most of them are nonfunctional or have no obvious function. Like reserve troops, they may be called into action only when needed. Thus even in healthy normal adult brains there might be sensory inputs from the face to the brain's face map and to the hand map area as well. If so, we must assume that this occult or hidden input is ordinarily inhibited by the sensory fibers arriving from the real hand. But when the hand is removed, this silent input originating from the skin on the face is unmasked and allowed to express itself so that touching the face now activates the hand area and leads to sensations in the phantom hand. Thus every 30

    time Tom whistles, he might feel a tingling in his phantom arm.
    We have no way at present of easily distinguishing between these two theories, although my hunch is that both mechanisms are at work. After all, we had seen the effect in Tom in less than four weeks and this seems too short a time for sprouting to take place. My colleague at the Massachusetts General Hospital Dr. David Borsook9 has seen similar effects in a patient just twenty−four hours after amputation, and there is no question of sprouting's occurring in such a short period. The final answer to this will come from simultaneously tracking perceptual changes and brain changes (using imaging) in a patient over a period of several days. If Borsook and I are right, the completely static picture of these maps that you get from looking at textbook diagrams is highly misleading and we need to rethink the meaning of brain maps completely. Far from signaling a specific location on the skin, each neuron in the map is in a state of dynamic equilibrium with other adjacent neurons; its significance depends strongly on what other neurons in the vicinity are doing (or not doing).
    These findings raise an obvious question: What if some body part is lost other than the hand? Will the same kind of remapping occur? When my studies on Tom were first published, I got many letters and phone calls from amputees wanting to know more. Some of them had been told that phantom sensations are imaginary and were relieved to learn that that isn't true. (Patients always find it comforting to know that there is a logical explanation for their otherwise inexplicable symptoms; nothing is more insulting to a patient than to be told that his pain is "all in the mind.")
    One day I got a call from a young woman in Boston. "Dr. Ramachandran," she said, "I'm a graduate student at Beth Israel Hospital and for several