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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 24rats.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 1975psychologist 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.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.

Laudenslager’s experiment showed that the immune system of those rats who could turn off the electricity () .

A.was altered
B.was strengthened
C.was weakened
D.was not affected

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The Vikings have left many traces of their settlement which are still visible today.Archaeology provides physical evidence of their conquests,settlement and daily life.The study of place names and language shows the lasting effect which the Viking settlements had in the British Isles,and DNA analysis provides some insights into the effect the Vikings had on the genetic stock of the countries where they settled.All of this provides valuable information,but the only reason that we have an idea of the ’Vikings’as a people is their appearance in the written sources.
Unfortunately,the value of the written evidence is limited.Not a lot of evidence survives,and much of what we have is either uninformative or unreliable.Many popular ideas about Vikings are nineteenth-century inventions.Others are the result of early historians accepting sources which modern scholars now regard as completely unreliable.In Scandinavia the Viking Age is regarded as part of prehistory because there are practically no contemporary written sources.Even in western Europe,the Viking Age is often seen as part of the ’Dark Ages’,from which comparatively few historical records have survived.
Surviving accounts of Viking activity were almost exclusively written by clergymen (神职人员).These monastic chronicles (编年史)outline broadly what happened,at what date.There are also sources of a more directly religious nature,such as the much-quoted letters of Alcuin,and Wulfstan’s famous ’Sermon of the Wolf ’,both of which chose to interpret the Viking raids as God’s punishment on the Anglo-Saxons for their sins.Even the chronicles reflect the fact that the Vikings often attacked monasteries for their wealth,which created an obvious bias against them,and the hostile tone of these contemporary accounts has done much to create the popular image of Viking atrocities.However,modern historians have noted that the same sources show Christian rulers behaving equally unpleasantly,but without being condemned on religious grounds.
We tend to think of the Vikings as a race of Scandinavian warriors,but the reality is more complex.Raids on the British Isles and the coasts of France and Spain were the work of Vikings from Norway and Denmark.The word Viking means one who lurks in a ’vik’or bay,in effect,a pirate.
The word ’Viking’has come to describe a whole new age in Europe between about 800and 1150.This is despite the fact that Vikings were not just pirates and warriors but also traders and colonists.But at the start of the Viking Age in the last decade of the 8th century,loot and adventure were the main goals of the Norwegians who raided in Scotland and Ireland and of the Danes who attacked England.Gold and silver treasures accumulated by the great monasteries could be converted into personal wealth and thus power,and captives could be sold as slaves.What better way for the young sons of good families to earn their way and see the world?

The author’s analysis of the Vikings mainly based on materials from ().

A.limited written sources
B.physical archaeological evidence
C.report on DAN analysis
D.the study of place names and language

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