Learning disabilities are very common. They 【21】______ perhaps 10 percent of all children. Scientists now know there are many different kinds of learning disabilities and that they are 【22】______ by many different things. There is no longer any 【23】______ that all learning disabilities 【24】______ differences in the way the brain is organized. Since there is no 【25】______ sign of the disorder, some researchers began looking at the brain itself to learn what might be wrong. In one study, researchers 【26】______ the brain of a learning-disabled person. They found two unusual things. One 【27】______ cells in the left side of the brain, which control language. These cells 【28】______ are white, in the learning disabled person, 【29】______ , these cells were gray. The researchers also found that many of the nerve cells were not in a line the 【30】______ they should have been. The nerve cells were mixed together. The study was carded out 【31】______ the guidance of Norman Geschwind, an expert on learning disabilities. Doctor Geschwind 【32】______ that learning disabilities resulted mainly from problems in the left side of the brain, and this part of brain failed to develop normally. Probably, he said, nerve cells there did not connect 【33】______ they should. So the brain was like an electrical device in which the wires were 【34】______ . Other researchers did not examine brain 【35】______ Instead, they measured the brain's electrical activity and made a map of the electrical 【36】______ . Frank Duffy experimented with this 【37】______ and found large differences in the brain activity of normal children and those with reading problems. The differences appeared 【38】______ the brain. Doctor Duffy said his research is 【39】______ that reading disabilities involve 【40】______ to a wide area of the brain, not just the left side. 【21】