You hear the same complaint all the time as people get older: “My memory is terrible.” Is it all in the mind, or do real changes take place in the brain with age to justify such grumbling (抱怨) The depressing answer is that the brain’s cells, the neurons, die and decline in efficiency with age. 　　Professor Arthur Shimamura, of the University of California at Berkeley, says there are three main ways in which mental function changes. The first is mental speed, for example how quickly you can react to fast-moving incidents on the road. Drivers in their late teens react quickly but tend to drive too fast, while the over sixties are more cautious but react more slowly. The near-inevitable slowing with age also partly explains why soccer players are seen as old in their thirties, while golf professionals are still in their prime at that age. This type of mental slowing results from a reduction in the efficiency with which the brain’s neurons work. 　　The fact that adults find it harder to learn musical instruments than children points to a second type of mental loss with age - a reduction in learning capacity. The parts of the brain known as the temporal lobes control new learning, and are particularly vulnerable to the effects of aging. This means that, as we get older, we take longer to learn a new language, and slower to master new routines and technologies at work, and we have to rely more on diaries and other mental aids. 　　“Working memory” is the third brain system which is vulnerable to the effects of aging. Working memory is the brain’s “blackboard”, where we juggle from moment to moment the things we have to keep in mind when solving problems, planning tasks and generally organizing our day-to-day life. Absent-mindedness occurs at all ages because of imperfections in the working memory system - so, for instance, you may continually lose your glasses, or find yourself walking into a room of your house only to find that you cannot remember what you came for. 　　Such absent-mindedness tends to creep up on us as we age and occurs because our plans and intentions, which are chalked up on the mental blackboard, are easily wiped out by stray thoughts and other distractions. Stress and preoccupation can also cause such absent-mindedness, in addition to age-related changes in the brain. The frontal lobes of the brain - located behind the forehead and above the eyes - are where the working memory system is located. Like the temporal lobes, which handle new learning, the frontal lobes are more vulnerable to the aging process than other parts of the brain. 　　The news, however, is not all bleak. Although neurons reduce in number with age, the remaining neurons send out new and longer connection fibers (dendrites) to maintain connections and allow us to function reasonably well with only relatively small drops in ability. 　　This and other evidence suggests that the principle “use it or lost it” might apply to the aging brain. Professor Shimamura studied a group of university professors who were still intellectually active, and compared their performance on neuropsychological tests with that of others of their age group, as well as with younger people. He found that on several tests of memory, the mentally active professors in their sixties and early seventies were superior to their contemporaries, and as good as the younger people. 　　Research on animals provide even stronger evidence of the effects of stimulation on the brain structure. Professor Bryan Kolb, of the University of Lethbridge in Canada, has shown that animals kept in stimulating environments show sprouting (生长) and lengthening of the connecting nerve fibers in comparison with animals kept in unstimulating environments. 　　The beneficial effects of continued mental activity are shown by the fact that older contestants in quiz shows are just as fast and accurate in responding to general knowledge questions as younger competitors, suggesting that at least part of their intellectual apparatus is spared the effects of aging because of practice and skill. 　　Such findings lead to the intriguing possibility of “mental fitness training” to accompany jogging and workouts for the health conscious. Research in Stockholm by Professor Lars Backman and his colleague has shown that older people can be trained to use their memory better, with the effects of this training lasting several years. 　　Just as people go bald or grey at different rates, so the same is true for their mental faculties. Why this should be the case for memory and other mental functions is not yet clear. If Professor Shimamura is right, then the degree to which people use and stretch their mental faculties may also have a role to play. 　　Questions: Scientists do not understand why people’s ________ decline at different rates.