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  3. Fear helps animals, including humans, to survive since it allows them to avoid predators and dangerous situations. Having too mu

Fear helps animals, including humans, to survive since it allows them to avoid predators and dangerous situations. Having too mu

admin2009-06-24  55
问题     Fear helps animals, including humans, to survive since it allows them to avoid predators and dangerous situations. Having too much fear, or not being able to control it can, however, harm them. It can freeze animals into inaction, which is hardly an effective defence tactic, and it can cause a variety of debilitating disorders, such as phobias, pathological anxiety and the increasingly fashionable diagnosis of post-traumatic stress disorder.
    Understanding how fear is formed in the brain may shed light on these disorders and help to develop ways to erase unwanted fears. In a paper published in the current issue of Cell, Gleb Shumyatsky, of Rutgers University in New Jersey, and his colleagues have achieved just that, in mice at least.
    Dr. Shumyatsky was interested in the role of a gene called stathmin. His interest was piqued because this gene, though present in every cell in the body (as are all genes), is active only in cells of a part of the brain called the amygdala. It was established a few years ago that the amygdala is the area that governs fear. Rare individuals whose amygdalas are damaged are, literally, fearless.
    To investigate the role of stathmin, Dr. Shumyatsky and his team established a strain of so-called knock-out mice who had had the gene removed from their DNA. They then conducted a series of experiments on instinctive and learned fear.
    The team found that their knock-out mice showed neither form of fear. They would, for example, venture, insouciantly into environments that normal mice avoid, such as open spaces and elevated platforms where they could easily be seen by predators. They were also less prone to freeze up in response to events that would normally induce fear, such as seeing cats.
    In addition to this lack of instinctive fear, the knock-out mice seemed to have weaker memories for past aversive experiences. The researchers tested this using the famous experimental method called conditioning, which was developed by Ivan Pavlov over a century ago. The essence of a neutral one such as a sound and a significant one, such as an electric shock, that produces a strong and consistent response. If an animal is given the shock immediately after heating the sound, it will associate the latter with the former and show fearful behavior when it hears the sound.
    Using this sort of set-up, Dr. Shumyatsky discovered that mice with stathmin knocked out found it hard to make the association. They could not, in other words, learn to be afraid. To be sure this was not due to changes in other features that might result from lack of the gene, he tested the animals’ hearing and pain sensitivity. Both were normal. So was their spatial memory. And although he did not try tests where the learned association was with pleasant rather than a fearful stimulus, he is reasonably confident that stathmin’s effect is specific to fear because it is confined to the amygdala.
选项 A、is essential for animals to survive in dangerous situations.
B、is harmful to animals if they cannot control it.
C、is an effective defence strategy for animals.
D、is helpful in combating post-traumatic disorders.
答案B
解析
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