听力原文： Many of us believe that a person's mind becomes less active as he grows older. But this is not true, according to Dr. Jarvik, professor of psychiatry at the University of California. She has studied the mental functioning of aging persons for several years. (18)For example, one of her studies concerns 136 pairs Of identical twins, who were first examined when they were already 60 years old. As Dr. Jarvik continued the study of the twins into their 70's and 80's, their minds did not generally decline as was expected. However, there was some decline in their psychomotor speed. This means that it took them longer to accomplish mental tasks than it used to. But when speed was not a factor, they lost very little intellectual ability over the years. (19)In general Dr. Jarvik's studies have shown that there is no decline in knowledge or reasoning ability. This is true not only with those in their 30's and 40's, but with those in their 60's and 70's as well. (20)In the cases where the older person's mind really seems to decay, it is not necessarily a sign of decay due to old age. Often it is simply a sign of a depressed emotional state. How many pairs of twins did Dr. Jarvik's studies involved?