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.

　　Scientists now know there are many different kinds of learning disabilities 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 or she has a learning disability. There is no outward

　　sign of the disorder. So some researchers began looking at the brain itself to learn what might be

　　wrong.

　　In one study，researchers examined the brain of a learning-disabled person，who had died in

　　an accident. They found two unusual things. One involved cells in the left side of the brain，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，an early expert on

　　learning disabilities. 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. So the brain was like an

　　electrical device in which the wires were crossed.

　　Other researchers did not examine brain tissue. Instead，they measured the brain’s electrical

　　activity 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. The differences appeared throughout the brain. Doctor Duffy said his research is

　　evidence that reading disabilities involve damage to a wide area of the brain，not just the left side.

　　51.Scientists found that the brain cells of a learning-disabled person differ from those of a normal person in .

　　A. structure and function B. color and function

　　C. size and arrangement D. color and arrangement

　　52.Which of the following is NOT mentioned in the passage?

　　A. Learning disabilities may result from the unknown area of the brain.

　　B. Learning disabilities may result from damage to a wide area of the brain.

　　C. Learning disabilities may result from abnormal organization of brain cells.

　　D. Learning disabilities may result from problems in the left side of the brain.

　　53.All of the following statements are true EXCEPT that .

　　A. many factors account for learning disorder

　　B. a learning-disabled person shows no outward signs

　　C. reading disabilities are a common problem that affects 10 percent of the population

　　D. the brain activity of learning-disabled children is different from that of normal children