LOCALIZATION ON THE SKIN

Localizing a spot on the surface of the skin seems to be the very simplest of matters. An itch occurs on the hand and automatically we scratch it. Sometimes, it is true, the itch comes in the small of the back and we cannot quite reach it. But we can at least point out its approximate position to some other person. Both the scratching and the pointing seem to be as simple as reflex actions. This apparent simplicity led the psychologist Loetze to believe that every single spot on the body had a special attribute which he called a “local sign.” According to his doctrine we know exactly where we have been touched the instant that we are aware of the touch itself. Henri, another psychologist, discovered, however, that if he was stimulated on the skin when his eyes were closed and tried to locate the exact spot before he opened his eyes again, he made large errors of localization and the errors were as great on the finger as on the forearm. This discovery cast grave doubt on the doctrine of the local sign.

During the late war two psychologists, Gelb and Goldstein, were attached to a German army hospital. One day a patient was brought in, who had had both of his occipital lobes destroyed, but the rest of his nervous system was still intact. This meant that he had absolutely no visual cognitions but the rest of his mental functions were unimpaired. Gelb and Goldstein immediately began to experiment with him and they got some rather startling results.

If the patient was lying quietly with his eyes shut and was touched on some part of his body, perhaps his left foot, he knew that he had been touched but he had no idea of the place. He did not know whether it was on the leg or the forehead or the chest or the hand. It made no difference how intense the stimulus was or how long it was continued. As long as he remained motionless he could not tell where it was. If, however, he was allowed to move his body as much as he wished, he could localize the place touched almost exactly but he did it in a very strange way. He at first wriggled about so that he set his entire body in motion. Then gradually he would move less and less. If the stimulus was applied to his right index finger, he would, after a few squirms of his whole body, move only his trunk, then only his right arm, then just his forearm, his right hand, his fingers, and finally only the stimulated finger. Then he could say just where the pressure was felt.

If he was touched in two places at once, as long as he was quiet, he would feel only one. If the two stimuli happened to be very close together, for instance both on the same finger, even by wriggling he could not discover that they were two. But if they were rather widely separated he could by the proper movements locate first one and then the other.

When he was allowed to touch the place stimulated, his method was the same as when he wriggled his body. At first he would run his finger aimlessly over his skin, gradually bringing it closer and closer to the stimulated area until the exact spot was hit upon.

These results led Gelb and Goldstein to the conclusion, which was foreshadowed by Henri, that immediate localization is always a function of vision. And they agreed with the findings of some rather different experiments which had been carried on at Cornell University several years before. The subjects of these experiments had to be real scientific martyrs. They were required to swallow a rubber tube in which there was an electric coil. The tube reached from the mouth to the bottom of the stomach and at intervals there were openings in it through which the experimenter could send an electric shock. All of the subjects had the greatest difficulty in locating the shocks; they could feel them clearly enough but they could not discover in which part of their digestive tracts they were. A stimulation might be at the extreme end of the tube and the subject would report that it was in his thorax. A shock at the esophagus would be referred to the bottom of the stomach, and so on.

If localization is primarily a matter of vision, these errors are easily accounted for. No one has a very complete mental picture of the inside of his own digestive tract. When we localize sensations there we must do so by reference to the more familiar external body. But since the sensations are felt internally, not on the outside, it is only natural that the reference should often be erroneous. Doctors recognize this. They accept a patient’s statement that he has a pain but they are not so quick to take his word for its exact location. Instead they investigate. And when they thump about first at one place then at another, they are using the same method of localization which Gelb and Goldstein’s soldier employed when he ran his finger over his body. Fortunately, this method, though much slower than visual localization, is usually successful.