Rene Descartes' explanation of pain has long been acknowledged in medicine. He proposed that pain is a purely physical phenomenon - that tissue injury makes specific nerves send a signal to the brain, causing the mind to notice pain. The phenomenon, he said, is like pulling on a rope to ring a bell in the brain. It is hard to overstate how deeply fixed this account has become. In medicine, doctors see pain in Descartes' terms-- as a physical process, a sign of tissue injury. The limitations of this explanation, however, have been apparent for some time, since people with obvious injuries sometimes report feeling no pain at all. Later, researchers proposed that Descartes' model be replaced with what they called the gate control theory of pain. They argued that before pain signals reach the brain, they must first go through a gating mechanism in the spinal cord . In some cases, this imaginary gate could simply stop pain signals from getting to the brain. Their most _ suggestion was that what controlled the gate was not just signals from sensory nerves but also emotions and other "output" from the brain. They were saying that pulling on the rope need not make the bell ring. The bell itself--the mind-- could stop it. This theory led to a great deal of research into how such factors as mood, gender, and beliefs influence the experience of pain. In a British study, for example, researchers measured pain threshold and tolerance levels in 53 ballet dancers and 53 university students by using a common measurement: after immersing your hand in body-temperature water for two minutes to establish a baseline condition, you put your hand in a bowl of ice water and start a clock running. You mark the time when it begins to hurt: that is your pain threshold. Then you mark the time when it hurts too much to keep your hand in the water: that is your pain tolerance. The test is always stopped at 120 seconds, to prevent injury. The results were striking. On average female students reported pain at 16 seconds and pulled their hands out of the ice water at 37 seconds. Female dancers were almost three times as long on both counts. Men in both groups had a higher threshold and tolerance for pain, but the difference between male dancers and male nondancers was nearly as large. What explains that difference? Probably it has something to do with the psychology of ballet dancers--a group known for self-discipline, physical fitness, and competitiveness, as well as by a high rate of chronic  injury. Their driven personalities and competitive culture evidently accustom them to pain. Other studies along these lines have shown that outgoing people have greater pain tolerance and that, with training, one can reduce one's sensitivity to pain. There is also striking evidence that very simple kinds of mental suggestion can have powerful effects on pain. In one study of 500 patients undergoing dental procedures, those who were given a placebo  injection and promised that it would relieve their pain had the least discomfort-- not only less than the patients who got a placebo and were told nothing but also less than the patients who got actual drug without any promise that it would work. Today it is abundantly evident that the brain is actively involved in the experience of pain and is no more bell on a string. Today every medical textbook teaches the gate control theory as fact. There's a problem with it, though. It explains people who have injuries but feel no pain, but it doesn't explain the reverse, which is far more common-- the millions of people who experience chronic pain, such as back pain, with no signs of injury whatsoever. So where does the pain come from? _ . The author refers to "chronic back pain" as an example of something that is  _
costly, because it troubles millions of people puzzling, because it sometimes has no obvious cause disappointing, because it does not improve with treatment worrying, because it lies beyond the reach of medicine