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第PAGE\*MERGEFORMAT16页共NUMPAGES\*MERGEFORMAT16页 2021年6月大学英语六级阅读理解真题及答案 Questions11to15arebasedonthefollowingpassage.Wesometimesthinkhumansareuniquelyvulnerabletoanxiety,butstressseemstoaffecttheimmunedefensesofloweranimalstoo.Inoneexperiment,forexample,behavioralimmunologist(免疫学家)MarkLaudenslager,attheUniversityofDenver,gavemildelectricshocksto24rats.Halftheanimalscouldswitchoffthecurrentbyturningawheelintheirenclosure,whiletheotherhalfcouldmot.Theratsinthetwogroupswerepairedsothateachtimeoneratturnedthewheelitprotectedbothitselfanditshelplesspartnerfromtheshock.Laudenslagerfoundthattheimmuneresponsewasdepressedbelownormalinthehelplessratsbutnotinthosethatcouldturnofftheelectricity.Whathehasdemonstrated,hebelieves,isthatlackofcontroloveranevent,nottheexperienceitself,iswhatweakenstheimmunesystem.Otherresearchersagree.JayWeiss,apsychologistatDukeUniversitySchoolofMedicine,hasshownthatanimalswhoareallowedtocontrolunpleasantstimulidon’tdevelopsleepdisturbancesorchangesinbrainchemistrytypicalofstressedrats.Butiftheanimalsareconfrontedwithsituationstheyhavenocontrolover,theylaterbehavepassivelywhenfacedwithexperiencestheycancontrol.Suchfindingsreinforcepsychologists’suspicionsthattheexperienceorperceptionofhelplessnessisoneofthemostharmfulfactorsindepression.Oneofthemoststartlingexamplesofhowthemindcanaltertheimmuneresponsewasdiscoveredbychance.In1975psychologistRobertAderattheUniversityofRochesterSchoolofMedicineconditioned(使形成条件反射)micetoavoidsaccharin(糖精)bysimultaneouslyfeedingthemthesweetenerandinjectingthemwithadrugthatwhilesuppressingtheirimmunesystemscausedstomachupsets.Associatingthesaccharinwiththestomachpains,themicequicklylearnedtoavoidthesweetener.Inordertoextinguishthisdislikeforthesweetener,Aderreexposedtheanimalstosaccharin,thistimewithoutthedrug,andwasastonishedtofindthatthosemicethathadreceivedthehighestamountsofsweetenerduringtheirearlierconditioningdied.Hecouldonlyspeculatethathehadsosuccessfullyconditionedtheratsthatsaccharinalonenowservedtowe
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