Part II Reading Comprehension (35 minutes)1999年6月
Directions: There are 4 reading passages in this part. Each passage is followed by some questions or unfinished statements. For each of them there are four choices marked A), B), C) and D). You should decide on the best choice and mark the corresponding letter on the Answer Sheet with a single line through the center.
Questions 11 to 15 are based on the following passage.
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.【doc】历年大学英语六级真题及答案(完整版)(免费下载)
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 weakens 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.【doc】历年大学英语六级真题及答案(完整版)(免费下载)
But if the animals are confronted with situations they have no control over, they later behave passively 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 (使形成条件反射) 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.【doc】历年大学英语六级真题及答案(完整版)(免费下载)
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.