Section D Learning disabilities are very common. They affect perhaps 10 percent of all children. Four times as many boys as girls have learning disabilities. Since about 1970, new research has helped brain scientists understand these problems better.【72】and that they are caused by many different things. There is no longer any question that all learning disabilities result from differences in the way the brain is organized. You cannot look at a child and tell if he has a learning disability. There is no outward sign of the disorder.【73】. In one study, researchers examined the brain of a learning-disabled person who had died in an accident. They found two unusual things.【74】, which control language. These cells normally are white. In the learning-disabled person, however, these cells were gray. The researchers also found that many of the nerve cells were not in a line the way they should have been. The nerve cells were mixed together. The study was carried out under the guidance of Norman Geschwind,【75】. Doctor Geschwind proposed that learning disabilities resulted mainly from problems in the left side of the brain. He believed this side of the brain failed to develop normally. Probably, he said, nerve cells there did not connect as they should.【76】. Other researchers did not examine brain tissue. Instead,【77】and made a map of the electrical signals. Frank Duffy experimented with this technique at Children's Hospital Medical center in Boston. Doctor Duffy found large differences in the brain activity of normal children and those with reading problems.【78】. Doctor Duffy said his research gives evidence that reading disabilities involve damage to a wide area of the brain, not just the left side. Sentences: