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1、Chapter2TheCriticalPeriodHypothesisInthischapter,wewilldiscusstheCriticalPeriodHypothesis(henceCPH),whichplaysanimportantroleinourstudy.Inparagraph2.1thebiologicalconceptofacriticalperiodanditsfeatureswillbeintroduced.Subsequently,wewillverybrieflydiscussthenotio
2、nofacriticalperiodforlanguageacquisition(paragraph2.2)andintroducetheassumptionsassociatedwithit.Paragraphs2.3to2.5containareviewoftheliteraturerelatedtotheseassumptions.NeuroimagingstudiesthatcontributetotheCPHdiscussionwillbereviewedseparatelyinparagraph2.6.Inp
3、aragraph2.7otherfactorsthatmaycauseobservedageeffectsinsecondlanguageacquisitionwillbediscussed.Inthelastparagraph(2.8),wewilltrytoexplainhowourstudymightcontributetotheresearchthathasbeencarriedoutsofarwithrespecttotheCPHinsecondlanguageacquisition.2.1Criticalpe
4、riodsInbiology,therearemanyinstancesoflearningforwhichthereisacriticalperiod,i.e.“atimeduringthelifespanofanorganisminwhichtheorganismmaybeaffectedbysomeexogenousinfluencetoanextentbeyondthatobservedatothertimes”(Colombo,1982:261).Duringthisperiod,typicallyearlyi
5、nlife,thereisa(heightened)sensitivitytostimulithatarenecessaryforthedevelopmentoftheabilityconcerned.Afterthisperiod,thereisanon-lineardeclineinsensitivity.Iftherelevantstimuliarenotpresentduringthiscriticalperiod,theabilityconcernedwillnolonger(fully)developunde
6、rnormalcircumstances.Awell-studiedexampleofacriticalperiodinbiologyisthedevelopmentoforientationspecificityinthevisualcortexforcats(e.g.Baxter,1966):whencatsareonlyexposedtocertainpatterns(e.g.onlyCHAPTER2horizontalstripes)duringthefirstweeksoftheirlives,theywill
7、neverbeabletoperceiveotherpatterns(e.g.verticalstripes).Theendofthiscriticalperiodisratherabruptandabsoluteandithaslongbeenthoughtthatthesecharacteristicsweretypicalofallcriticalperiods.Therearemanycriticalperiods,however,afterwhichthedeclineinsensitivityandlearn
8、ingabilityis1muchmoregradual.Anotherexampleofacriticalperiodisimprintinginducklings(e.g.Ratner&Hoffman,1974).Whenducklingsareborn,theyfollowthefirstmovingobjec
