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  3. We sometimes think humans are uniquely vulnerable to anxiety, but stress seems to affect the immune defenses of lower animals to

We sometimes think humans are uniquely vulnerable to anxiety, but stress seems to affect the immune defenses of lower animals to

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问题    We sometimes think humans are uniquely vulnerable to anxiety, but stress seems to affect the immune defenses of lower animals too. In one experiment, for example, behavioral immunologist (免疫学家) Mark Laudenslager, at the University of Denver, gave mild electric shocks to 24 rats. Half the animals could switch off the current by turning a wheel in their enclosure, while the other half could not. The rats in the two groups were paired so that each time one rat turned the wheel it protected both itself and its helpless Partner from the shock. Laudenslager found that the immune response was depressed below normal in the helpless rats but not in those that could turn off the electricity. What he has demonstrated, he believes, is that lack of control over an event, not the experience itself, is what wakens the immune system.
   Other researchers agree. Jay Weiss, a psychologist at Duke University School of Medicine, has shown that animals who are allowed to control unpleasant stimuli don’t develop sleep disturbances or changes in brain chemistry typical of stressed rats. But if the animals are conditioned to confront with situations they have no control over, they later behave passively even when faced with experiences they can control. Such findings reinforce psychologists’ suspicions that the experience or perception of helplessness, is one of the most harmful factors in depression.
   One of the most startling examples of how the mind can alter the immune response was discovered by chance. In 1975 psychologist Robert Ader at the University of Rochester School of Medicine conditioned (便形成条件反射) mice to avoid saccharin (糖精) by simultaneously feeding them the sweetener and injecting them with a drug that while suppressing their immune systems caused stomach upsets. Associating the saccharin with the stomach pains, the mice quickly learned to avoid the sweetener. In order to extinguish this dislike for the sweetener, Ader reexposed the animals to saccharin, this time without the drug, and was astonished to find that those mice that had received the highest amounts of sweetener during their earlier conditioning died. He could only speculate that he had so successfully conditioned the rats that saccharin alone now served to weaken their immune systems enough to kill them.
The reason why the mice in Ader’s experiment avoided saccharin was that ______
选项 A、they associated it with stomachach
B、it affected their immune systems
C、it led to stomach pains
D、they disliked its taste
答案A
解析 由第三段可知，Robert Ader使老鼠形成了回避糖精的条件反射。他在喂老鼠糖精(致甜剂)的同时，给老鼠注射压抑它们免疫系统并导致肠胃不适的药。这样，老鼠就会将糖精与胃肠不适联系起来，因而回避糖精。
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