Critical period - Wikipedia
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In developmental psychology and developmental biology, a critical period is a maturational stage in the lifespan of an organism during which the nervous ... Criticalperiod FromWikipedia,thefreeencyclopedia Jumptonavigation Jumptosearch Maturationalstageinthelifespanofanorganism Forotheruses,seewindowofopportunity. Indevelopmentalpsychologyanddevelopmentalbiology,acriticalperiodisamaturationalstageinthelifespanofanorganismduringwhichthenervoussystemisespeciallysensitivetocertainenvironmentalstimuli.If,forsomereason,theorganismdoesnotreceivetheappropriatestimulusduringthis"criticalperiod"tolearnagivenskillortrait,itmaybedifficult,ultimatelylesssuccessful,orevenimpossible,todevelopcertainassociatedfunctionslaterinlife.Functionsthatareindispensabletoanorganism'ssurvival,suchasvision,areparticularlylikelytodevelopduringcriticalperiods."Criticalperiod"alsorelatestotheabilitytoacquireone'sfirstlanguage.Researchersfoundthatpeoplewhopassedthe"criticalperiod"wouldnotacquiretheirfirstlanguagefluently.[1] Someresearchersdifferentiatebetween'strongcriticalperiods'and'weakcriticalperiods'(a.k.a.'sensitive'periods)—defining'weakcriticalperiods'/'sensitiveperiods'asmoreextendedperiods,afterwhichlearningisstillpossible.[2]Otherresearchersconsiderthesethesamephenomenon.[3] Forexample,thecriticalperiodforthedevelopmentofahumanchild'sbinocularvisionisthoughttobebetweenthreeandeightmonths,withsensitivitytodamageextendinguptoatleastthreeyearsofage.Furthercriticalperiodshavebeenidentifiedforthedevelopmentofhearing[4]andthevestibularsystem.[1] Contents 1Strongversusweakcriticalperiods 2Criticalperiodmechanisms 2.1Criticalperiodopening 2.2Activity-dependentcompetition 2.2.1Oculardominance 2.2.2Axongrowth 2.3Microglia 2.4Spinemotility 2.5Excitatory-inhibitorybalance 2.6Criticalperiodclosure 2.6.1Perineuronalnets 2.6.2Myelin 2.7Neuromodulation 3Linguistics 3.1Firstlanguageacquisition 3.2Secondlanguageacquisition 4Vision 5Imprinting 6Auditoryprocessing 6.1Musicalability 7Vestibularsystem 8Memory 9Seealso 10References 11Externallinks Strongversusweakcriticalperiods[edit] Examplesofstrongcriticalperiodsincludemonoculardeprivation,filialimprinting,monauralocclusion,[5]andPrefrontalSynthesisacquisition.[6]Thesetraitscannotbeacquiredaftertheendofthecriticalperiod. Examplesofweakcriticalperiodsincludephonemetuning,grammarprocessing,articulationcontrol,vocabularyacquisition,musictraining,auditoryprocessing,sporttraining,andmanyothertraitsthatcanbesignificantlyimprovedbytrainingatanyage.[7][8] Criticalperiodmechanisms[edit] Criticalperiodopening[edit] Criticalperiodsofplasticityoccurintheprenatalbrainandcontinuethroughoutchildhooduntiladolescenceandareverylimitedduringadulthood.Twomajorfactorsinfluencetheopeningofcriticalperiods:cellularevents(i.e.changesinmolecularlandscape)andsensoryexperience(i.e.hearingsound,visualinput,etc).Bothneedtocoincideforthecriticalperiodtoopenproperly.Atthecellularlevel,criticalperiodsarecharacterizedbymaturationoftheinhibitorycircuits.[9]Moreprecisely,factorssuchasbrain-derivedneurotrophicfactor(BDNF)andorthodenticlehomeobox2(Otx2)contributetothematurationofamajorclassofinhibitoryneurons:parvalbumin-positiveinterneurons(PVcells).[9]Priortotheonsetofthecriticalperiod,modulationofthiscircuitishamperedbyearlyfactorssuchaspolysialicacid(PSA).[9]PSAacts,inpart,bypreventingOtx2interactionwithPVcells.[10]Soonaftertheopeningofthecriticalperiod,PSAlevelsdecrease,allowingPVcellmaturationbyactivatinginhibitoryGABAareceptorsthatfacilitateinhibitorycircuitremodeling.ArtificiallyremovingPSA,orexperimentallymanipulatinginhibitorytransmissioncanresultinearlyopeningofthecriticalperiod.[10][11]Whilethetimingofthesemoleculareventsseemstobepartiallyexplainedbyclockgenes,[12]experienceiscrucialassensorydeprivationexperimentshavebeenshowntointerferewiththepropertimingofcriticalperiods.[13][14][15] Activity-dependentcompetition[edit] Hebbiantheoryguidestheideaofactivity-dependentcompetition:iftwoneuronsbothhavethepotentialtomakeaconnectionwithacell,theneuronthatfiresmorewillmaketheconnection. Oculardominance[edit] Thisphenomenonofactivity-dependentcompetitionisespeciallyseenintheformationofoculardominancecolumnswithinthevisualsystem.Earlyindevelopment,mostofthevisualcortexisbinocular,meaningitreceivesroughlyequalinputfrombotheyes.[16]Normally,asdevelopmentprogresses,thevisualcortexwillsegregateintomonocularcolumnsthatreceiveinputfromonlyoneeye.[16]However,ifoneeyeispatched,orotherwisepreventedfromreceivingsensoryinput,thevisualcortexwillshifttofavorrepresentationoftheuncoveredeye.Thisdemonstratesactivity-dependentcompetitionandHebbiantheorybecauseinputsfromtheuncoveredeyemakeandretainmoreconnectionsthanthepatchedeye.[17] Axongrowth[edit] Furtherinformation:Axonguidance Axonformationandgrowthisanotherkeypartofplasticityandactivity-dependentcompetition.Axongrowthandbranchinghasbeenshowntobeinhibitedwhentheneuron'selectricalactivityissuppressedbelowthelevelofanactiveneighbor.[18]Thisshowsthataxonalgrowthdynamicsarenotindependentbutratherdependonthelocalcircuitswithinwhichtheyareactive(i.e.theactivityoftheotherneuronscompetingforconnections). Microglia[edit] Microgliainherentlyplayaroleinsynapticpruningduringadolescence.Asresidentimmunecellsofthecentralnervoussystem,microglia’smainroleisphagocytosisandengulfment.Studieshavefoundthatduringcriticalperiodsinthevisualcortex,neuralsynapsesbecomethetargetofmicroglialphagocytosis.[19][20]Neuronswhoreceivedlessfrequentinputfromretinalganglioncellsduringearlypostnatalperiodsweremorepronetobeengulfedandprunedbymicroglia,aspermonoculardeprivationexperiments.[19]SimilarresultswerefoundwhenmanipulatingG-coupledpurinergicreceptorsonmicroglialprocesses.Blockingthesereceptorsorperformingaknockoutexperimentsignificantlyloweredmicroglialinteractionsandsynapticpruningduringtheearlyvisualcortexcriticalperiod.[20]Morerecently,theexpressionofthecomplementcomponent4genehasbeenfoundtosignificantlycontributetoabnormallyhighlevelsofmicroglialsynapticpruningduringearlystagesofdevelopmentinschizophrenicneuronsandmicroglia,suggestingagenomicconnectionbetweentheimmunesystemandcriticalperiods.[21] Spinemotility[edit] Dendriticspinemotilityisthealteringofthedendriticmorphologyofaneuron,specificallytheappearinganddisappearingofthesmallprotrusionsknownasspines.Inearlypostnataldevelopment,spinemotilityhasbeenfoundtobeatveryhighlevels.Duetoitsmostpronouncedoccurrenceduringpostnataldays11through15,spinemotilityisthoughttohavearoleinneurogenesis.[22]Motilitylevelssignificantlydecreasebeforethestartofthevisualcortexcriticalperiodandmonoculardeprivationexperimentsshowthatmotilitylevelssteadilydecreaseuntilthecriticalperiodisover,hintingthatmotilitymightnotbeexplicitlyinvolvedinthisprocess.[23]However,binoculardeprivationbeforeeye-openingresultedinasignificantup-regulationofspinemotilityuntilthepeakofthecriticalperiod,[24]resultingincontroversialfindingsregardingtheroleofdendriticspinemotility. Excitatory-inhibitorybalance[edit] Anothercriticalcomponentofneuronalplasticityisthebalanceofexcitatoryandinhibitoryinputs.Earlyindevelopment,GABA,themajorinhibitoryneurotransmitterintheadultbrain,exhibitsanexcitatoryeffectonitstargetneurons.[25]However,duetochangesininternalchloridelevelsduetotheup-regulationofpotassiumchloridepumps,GABAthenswitchestoinhibitorysynaptictransmission.[25]ThematurationoftheGABAergicinhibitorysystemhelpstotriggertheonsetofcriticalperiods.[11]StrengthenedGABAergicsystemscaninduceanearlycriticalperiod,whileweakerGABAergicinputscandelayorevenpreventplasticity.[26][27]Inhibitionalsoguidesplasticityoncethecriticalperiodhasbegun.Forexample,lateralinhibitionisespeciallyimportantinguidingcolumnarformationinthevisualcortex.[28]Hebbiantheoryprovidesinsightontheimportanceofinhibitionwithinneuralnetworks:withoutinhibition,therewouldbemoresynchronousfiringandthereforemoreconnections,butwithinhibition,fewerexcitatorysignalsgetthrough,allowingonlythemoresalientconnectionstomature.[29] Criticalperiodclosure[edit] Perineuronalnets[edit] Criticalperiodclosurehasbeenshowntobemodulatedbythematurationofinhibitorycircuits,mediatedbytheformationofperineuronalnetsaroundinhibitoryneurons.[11]Perineuronalnets(PNNs)arestructuresintheextracellularmatrixformedbychondroitinsulfateproteoglycans,hyaluronan,andlinkproteins.[30]Thesestructuresenvelopthesomaofinhibitoryneuronsinthecentralnervoussystem,appearingwithagetostabilizematurecircuits.[30][31]PNNdevelopmentcoincideswiththeclosureofcriticalperiods,andbothPNNformationandcriticalperiodtimingisdelayedindark-rearing.[31]Forexample,PNNdigestionbyABCchondroitinaseinratsleadstoashiftinoculardominanceuponmonoculardeprivation,whichisnormallyrestrictedtoitscriticalperiodmuchearlierindevelopment.[32] Additionally,PNNsarenegativelycharged,whichistheorizedtocreateacation-richenvironmentaroundcells,potentiallyleadingtoanincreasedfiringrateofinhibitoryneurons,therebyallowingforincreasedinhibitionaftertheformationofPNNsandhelpingtoclosethecriticalperiod.[33]TheroleofPNNsincriticalperiodclosureisfurthersupportedbythefindingthatfast-spikingparvalbulmin-positiveinterneuronsareoftensurroundedbyPNNs.[33] Perineuronalnetshavealsobeenfoundtocontainchemorepulsivefactors,suchassemaphorin3A,whichrestrictaxongrowthnecessaryforplasticityduringcriticalperiods.[34]Inall,thesedatasuggestaroleforPNNsinthematurationofCNSinhibition,thepreventionofplasticaxonalgrowth,andsubsequently,criticalperiodclosure. Myelin[edit] Anothermechanismthatclosesthecriticalperiodismyelination.MyelinsheathsareformedbyoligodendrocytesintheCNSthatwraparoundsegmentsofaxonstoincreasetheirfiringspeed.[35]Myelinisformedintheearlystagesofdevelopmentandprogressesinwaves,withbrainareasoflaterphylogeneticdevelopment(i.e.thoseassociatedwith“higher”brainfunctionslikethefrontallobes)havinglatermyelination.[36]Thematurationofmyelinationinintracorticallayerscoincideswithcriticalperiodclosureinmice,whichhasledtofurtherresearchontheroleofmyelinationoncriticalperiodduration.[37] Myelinisknowntobindmanydifferentaxonalgrowthinhibitorsthatpreventplasticityseenincriticalperiods.[38]TheNogoReceptorisexpressedinmyelinandbindstotheaxonalgrowthinhibitorsNogoandMAG(amongothers),preventingaxongrowthinmature,myelinatedneurons.[38]Insteadofaffectingthetimingofthecriticalperiod,mutationsoftheNogoreceptorprolongthecriticalperiodtemporarily.[37]AmutationoftheNogoreceptorinmicewasfoundtoextendthecriticalperiodformonoculardominancefromaround20–32daysto45or120days,suggestingalikelyroleofthemyelinNogoreceptorincriticalperiodclosure.[37] Additionally,theeffectsofmyelinationaretemporallylimited,sincemyelinationitselfmayhaveitsowncriticalperiodandtiming.[36][39]Researchhasshownthatsocialisolationofmiceleadstoreducedmyelinthicknessandpoorworkingmemory,butonlyduringajuvenilecriticalperiod.[39]Inprimates,isolationiscorrelatedwithabnormalchangesinwhitematterpotentiallyduetodecreasedmyelination.[40] Inall,myelinanditsassociatedreceptorsbindseveralimportantaxonalgrowthinhibitorswhichhelpclosethecriticalperiod.[37][38]Thetimingofthismyelination,however,isdependentonthebrainregionandexternalfactorssuchasthesocialenvironment.[36][39] Neuromodulation[edit] Whilethepresenceorabsenceofsensoryexperiencesmostrobustlyshapesbraindevelopmentduringthecriticalperiod,thebehavioralcontext(i.e.theamountofattention,arousal,fearandrewardexperienced)concurrentwiththesensoryinputshavebeensuggestedtobeimportantinregulatingthebrainremodelingmechanisms.[41][42][43][44][45]Intermsofbrainconnectivity,thesebehavioralandcontextualinputsactivatetheneuromodulatorysystem,whichhavesubstantialconnectivitytothecortex[43][44][46][47]Themoleculareffectorsreleasedbytheneuromodulatorysystemarecalledneuromodulators,whichincludeacetylcholine,dopamine,andnoradrenalineamongothers.[46]Investigatingtheeffectofthesemolecules,aswellastheneuronsthatreleaseandbindthem,hasbeenoneapproachtoelucidatethebiologyofneuromodulation.Researchusingthisapproachhashighlightedtheroleofneuromodulationinsensoryprocessingduringthecriticalperiod.[11][43][44][45][47][48][49]Forexample,ontheonehand,inkittens,ashiftinoculardominanceresultingfrommonoculardeprivationduringthecriticalperiodisreducedbycombineddestructionofnoradrenergicandcholinergicneurons.[48]Inaddition,prenatalexposuretoselectiveserotoninreuptakeinhibitors(SSRI)causesashiftinperceptualnarrowingonlanguagetoearlierindevelopment.[50]Ontheotherhand,neuromodulatorystimulationhasbeenshowntoinducebrainplasticityinadultmice.[43][44]Whilebeingsubjectedtocholinergicordopaminergicstimulation,adultmicelisteningtoatoneofspecificfrequencyexhibitedexpansionofthetonotopicareaintheauditorycortexthatrespondsspecificallytosoundsofthatfrequency.[43][44] Mechanistically,neuromodulationisincreasinglybeingrecognizedforitsfine-tuningofthePVcell-mediatedinhibitionofexcitatorypyramidalneurons'soma.[45][49][51]CentraltotheneuromodulatoryregulationofPVcellactivityistheexistenceofdistinctsubsetsofinhibitoryneurons,whichareresponsivetoactivationbyneuromodulatorsandwhichinhibitPVcells.[9][45][49][51]Withinthesecells,somealsoinhibitspecificpyramidalcelldendrites.[45][49]ByinhibitingPVcellsactivity,theneuromodulator-sensitiveinhibitorycellssuchasthoseexpressingVasoactiveintestinalpeptide(VIP)orsomatostatin(SST)lifttheinhibitionofthepyramidalneurons;inotherwords,theactivityofVIPandSST-expressingcellsresultinthedisinhibitionofpyramidalneurons.[9][45][49][51]Then,byinhibitingonlycertaindendriticbranchesofthesenowdis-inhibitedpyramidalneurons,theneuromodulation-activatedcellsallowselectsensoryinputstoexcitethepyramidalneuronsandberepresentedinthebraincircuitry.[45][49]Thus,inalandscapeofglobalinhibitionbymaturinginhibitorysignaling,neuromodulationallowswindowsofdis-inhibition,temporallyandspatially,thatallowbehaviorallyimportantsensoryinputstheopportunitytoinfluencethebrain.[45][49] Linguistics[edit] Mainarticle:Criticalperiodhypothesis Firstlanguageacquisition[edit] Thecriticalperiodhypothesis(CPH)statesthatthefirstfewyearsoflifeconstitutethetimeduringwhichlanguagedevelopsreadilyandafterwhich(sometimebetweenage5andpuberty)languageacquisitionismuchmoredifficultandultimatelylesssuccessful.[52]ThehypothesisthatlanguageisacquiredduringacriticalperiodwasfirstproposedbyneurologistsWilderPenfieldandLamarRobertsin1959andpopularizedbylinguistEricLennebergin1967.Lennebergarguedforthehypothesisbasedonevidencethatchildrenwhoexperiencebraininjuryearlyinlifedevelopfarbetterlanguageskillsthanadultswithsimilarinjuries. Dr.MariaMontessoriwasoneoftheearliereducatorswhobroughtattentiontothisphenomenonandcalledit"SensitivePeriods",whichisoneofthepillarsofherphilosophyofeducation. ThetwomostfamouscasesofchildrenwhofailedtoacquirelanguageafterthecriticalperiodareGenieandtheferalchildVictorofAveyron.[53]However,thetragiccircumstancesofthesecasesandthemoralandethicalimpermissibilityofreplicatingthemmakeitdifficulttodrawconclusionsaboutthem.Thechildrenmayhavebeencognitivelydisabledfrominfancy,ortheirinabilitytodeveloplanguagemayhaveresultedfromtheprofoundneglectandabusetheysuffered.[52] ManysubsequentresearchershavefurtherdevelopedtheCPH,mostnotablyElissaNewportandRachelMayberry.Studiesconductedbytheseresearchersdemonstratedthatprofoundlydeafindividualswhoarenotexposedtoasignlanguageaschildrenneverachievefullproficiency,evenafter30yearsofdailyuse.[54]Whiletheeffectismostprofoundforindividualswhoreceivenosignlanguageinputuntilaftertheageof12,eventhosedeafpeoplewhobeganlearningasignlanguageatage5weresignificantlylessfluentthannativedeafsigners(whoseexposuretoasignlanguagebeganatbirth).Earlylanguageexposurealsoaffectstheabilitytolearnasecondlanguagelaterinlife:profoundlydeafindividualswithearlylanguageexposureachievecomparablelevelsofproficiencyinasecondlanguagetohearingindividualswithearlylanguageexposure.Incontrast,deafindividualswithoutearlylanguageexposureperformfarworse.[55] Otherevidencecomesfromneuropsychologywhereitisknownthatadultswellbeyondthecriticalperiodaremorelikelytosufferpermanentlanguageimpairmentfrombraindamagethanarechildren,believedtobeduetoyouthfulresiliencyofneuralreorganization.[52] StevenPinkerdiscussestheCPHinhisbook,TheLanguageInstinct.AccordingtoPinker,languagemustbeviewedasaconceptratherthanaspecificlanguagebecausethesounds,grammar,meaning,vocabulary,andsocialnormsplayanimportantroleintheacquisitionoflanguage.[56]Physiologicalchangesinthebrainarealsoconceivablecausesfortheterminusofthecriticalperiodforlanguageacquisition.[57]Aslanguageacquisitioniscrucialduringthisphase,similarlyinfant-parentattachmentiscrucialforsocialdevelopmentoftheinfant.Aninfantlearnstotrustandfeelsafewiththeparent,buttherearecasesinwhichtheinfantmightbestayingatanorphanagewhereitdoesnotreceivethesameattachmentwiththeircaregiver.Researchshowsthatinfantswhowereunabletodevelopthisattachmenthadmajordifficultyinkeepingcloserelationships,andhadmaladaptivebehaviorswithadoptedparents.[1] Thediscussionoflanguagecriticalperiodsuffersfromthelackofacommonlyaccepteddefinitionoflanguage.Someaspectsoflanguage,suchasphonemetuning,grammarprocessing,articulationcontrol,andvocabularyacquisitioncanbesignificantlyimprovedbytrainingatanyageandthereforehaveweakcriticalperiods.[7][8]Otheraspectsoflanguage,suchasPrefrontalSynthesis,havestrongcriticalperiodsandcannotbeacquiredaftertheendofthecriticalperiod.[6]Consequently,whenlanguageisdiscussedingeneral,withoutdissectionintocomponents,argumentscanbeconstructedbothinfavorandagainstthestrongcriticalperiodofL1acquisition. Secondlanguageacquisition[edit] Thetheory[58]hasoftenbeenextendedtoacriticalperiodforsecondlanguageacquisition(SLA),whichhasinfluencedresearchersinthefieldonbothsidesofthespectrum,supportiveandunsupportiveofCPH,toexplore.[59]However,thenatureofthisphenomenonhasbeenoneofthemostfiercelydebatedissuesinpsycholinguisticsandcognitivescienceingeneralfordecades. Certainly,olderlearnersofasecondlanguagerarelyachievethenative-likefluencythatyoungerlearnersdisplay,despiteoftenprogressingfasterthanchildrenintheinitialstages.ThisisgenerallyacceptedasevidencesupportingtheCPH.Incorporatingtheidea,"youngerequalsbetter"byPenfield,DavidSingleton(1995)statesthatinlearningasecondlanguagetherearemanyexceptions,notingthatfivepercentofadultbilingualsmasterasecondlanguageeventhoughtheybeginlearningitwhentheyarewellintoadulthood—longafteranycriticalperiodhaspresumablycometoaclose.Thecriticalperiodhypothesisholdsthatfirstlanguageacquisitionmustoccurbeforecerebrallateralizationcompletes,atabouttheageofpuberty.Onepredictionofthishypothesisisthatsecondlanguageacquisitionisrelativelyfast,successful,andqualitativelysimilartofirstlanguageonlyifitoccursbeforetheageofpuberty.[60]TograspabetterunderstandingofSLA,itisessentialtoconsiderlinguistic,cognitive,andsocialfactorsratherthanagealone,astheyareallessentialtothelearner'slanguageacquisition.[59] Overtheyears,manyexperimentershavetriedtofindevidenceinsupportoforagainstthecriticalperiodsfor secondlanguageacquisition. Manyhavefoundevidencethatyoungchildrenacquire language moreeasilythanadults,buttherearealsospecialcasesofadultsacquiringasecondlanguagewithnative-likeproficiency.Thusithasbeendifficultforresearcherstoseparate correlation from causation.[61] In1989,JacquelineS.JohnsonandElissaL.Newportfoundsupportfortheclaimthatsecondlanguagesaremoreeasilyacquiredbefore puberty,ormorespecificallybeforetheageofseven.[62]Theytestedsecondlanguagelearnersof English whoarrivedintheUnitedStatesatvariousagesrangingfromthreetothirty-nine,andfoundthattherewasadeclineingrammaticalcorrectnessaftertheageofseven.JohnsonandNewportattributedthisclaimtoadeclineinlanguagelearningabilitywithage.OpponentsofthecriticalperiodarguethatthedifferenceinlanguageabilityfoundbyJohnsonandNewportcouldbeduetothedifferenttypesofinputthatchildrenandadultsreceive;childrenreceivedreducedinputwhileadultsreceivemorecomplicatedstructures. Additionalevidenceagainstastrict criticalperiod isalsofoundintheworkofPallieretal.(2003)whofoundthatchildrenadoptedto France from Korea wereabletobecomenative-likeintheirperformanceof French evenafterthecriticalperiodfor phonology.[63]Theirexperimentmayrepresentaspecialcasewheresubjectsmustlosetheirfirstlanguageinordertomoreperfectlyacquiretheirsecond. Thereisalsosomedebateastohowonecanjudgethenative-likequalityofthespeechparticipantsproduceandwhatexactlyitmeanstobeanear-nativespeakerofasecondlanguage.[64]Whiteetal.foundthatitispossiblefornon-nativespeakersofalanguagetobecomenative-likeinsomeaspects,butthoseaspectsareinfluencedbytheir firstlanguage. Recently,aconnectionistmodelhasbeendevelopedtoexplainthechangesthattakeplaceinsecondlanguagelearningassumingthatsensitiveperiodaffectslexicallearningandsyntacticlearningpartsofthesystemdifferently,whichshedsfurtherlightonhowfirstandsecondlanguageacquisitionchangesoverthecourseoflearnersdevelopment.[65] Vision[edit] Inmammals,neuronsinthebrainthatprocessvisionactuallydevelopafterbirthbasedonsignalsfromtheeyes.AlandmarkexperimentbyDavidH.HubelandTorstenWiesel(1963)showedthatcatsthathadoneeyesewnshutfrombirthtothreemonthsofage(monoculardeprivation)onlyfullydevelopedvisionintheopeneye.Theyshowedthatcolumnsintheprimaryvisualcortexreceivinginputsfromtheothereyetookovertheareasthatwouldnormallyreceiveinputfromthedeprivedeye.Ingeneralelectrophysiologicalanalysesofaxonsandneuronsinthelateralgeniculatenucleusshowedthatthevisualreceptivefieldpropertieswascomparabletoadultcats.However,thelayersofcortexthatweredeprivedhadlessactivityandfewerresponseswereisolated.Thekittenshadabnormallysmalloculardominancecolumns(partofthebrainthatprocessessight)connectedtotheclosedeye,andabnormallylarge,widecolumnsconnectedtotheopeneye.Becausethecriticalperiodtimehadelapsed,itwouldbeimpossibleforthekittenstoalteranddevelopvisionintheclosedeye.Thisdidnothappentoadultcatsevenwhenoneeyewassewnshutforayearbecausetheyhadfullydevelopedtheirvisionduringtheircriticalperiod.Laterexperimentsinmonkeysfoundsimilarresultsconsistentwiththestrongcriticalperiod.[66] Inafollow-upexperiment,HubelandWiesel(1963)exploredthecorticalresponsespresentinkittensafterbinoculardeprivation;theyfounditdifficulttofindanyactivecellsinthecortex,andtheresponsestheydidgetwereeitherslow-movingorfast-fatiguing.Furthermore,thecellsthatdidrespondselectedforedgesandbarswithdistinctorientationpreferences.Nevertheless,thesekittensdevelopednormalbinocularity.HubelandWieselfirstexplainedthemechanism,knownasorientationselectivity,inthemammalianvisualcortex.Orientationtuning,amodelthatoriginatedwiththeirmodel,isaconceptinwhichreceptivefieldsofneuronsintheLGNexciteacorticalsimplecellandarearrangedinrows.Thismodelwasimportantbecauseitwasabletodescribeastrongcriticalperiodfortheproperdevelopmentofnormaloculardominancecolumnsinthelateralgeniculatenucleus,andthusabletoexplaintheeffectsofmonoculardeprivationduringthiscriticalperiod.Thecriticalperiodforcatsisaboutthreemonthsandformonkeys,aboutsixmonths.[67] Inasimilarexperiment,AntoniniandStryker(1993)examinedtheanatomicalchangesthatcanbeobservedaftermonoculardeprivation.Theycomparedgeniculocorticalaxonalarborsinmonocularlydeprivedanimalsinthelongterm(4-weeks)toshortterm(6–7days)duringthecriticalperiodestablishedbyHubelandWiesel(1993).Theyfoundthatinthelongterm,monoculardeprivationcausesreducedbranchingattheendofneurons,whiletheamountofafferentsallocatedtothenondeprivedeyeincreased.Evenintheshortterm,AntoniniandStryker(1993)foundthatgeniculocorticalneuronsweresimilarlyaffected.Thissupportstheaforementionedconceptofacriticalperiodforproperneuraldevelopmentforvisioninthecortex.[68] Studiesofpeoplewhosesighthasbeenrestoredafteralongblindness(whetherfrombirthoralaterpointinlife)revealthattheycannotnecessarilyrecognizeobjectsandfaces(asopposedtocolor,motion,andsimplegeometricshapes).Somehypothesizethatbeingblindduringchildhoodpreventssomepartofthevisualsystemnecessaryforthesehigher-leveltasksfromdevelopingproperly.[69]Thegeneralbeliefthatacriticalperiodlastsuntilage5or6waschallengedbya2007studythatfoundthatolderpatientscouldimprovetheseabilitieswithyearsofexposure.[70] ExpressionoftheproteinLynx1hasbeenassociatedwiththenormalendofthecriticalperiodforsynapticplasticityinthevisualsystem.[71] Imprinting[edit] KonradLorenz Inpsychology,imprintingisanytypeofrapidlearningthatoccursinaparticularlifestage.Whilethisrapidlearningisindependentofthebehavioraloutcome,italsoestablishesitandcanaffectbehavioralresponsestodifferentstimuli.KonradLorenziswellknownforhisclassicstudiesoffilialimprintingingraylaggeese.From1935to1938,hepresentedhimselftoagroupofnewlyhatchedgoslingandtooknoteofhowhewasinstantlyaccepted,followed,andcalledtoasifheweretheonewholaidthemhimself.Asthefirstmovingobjecttheyencountered,Lorenzstudiedthephenomenoninhowquicklythegeesewereabletoformsuchanirreversiblebond.Throughhisworkhedemonstratedthatthisonlydevelopedduringabrief“criticalperiod,”whichwasaboutafewhoursafterhatching,suggestingastrongcriticalperiod.[72] Lorenzalsodiscoveredalong-lastingeffectofhisstudies,andthatwasashiftinthespecies'sexualimprintingasaresultfromimprintinguponafostermotherofasecondspecies.Forcertainspecies,whenraisedbyasecondone,theydevelopandretainimprintedpreferencesandapproachthesecondspeciestheywereraisedbyratherthanchoosetheirown,ifgivenachoice.[73] Imprintingservesasthedistinguishingfactorbetweenone'sownmotherandothermotherfigures.Themotherandtheinfantbothidentifywitheachother,thisisastrongbondingmomentforhumans.Itprovidesasortofmodelorguidetoadultbehaviorsinadditiontootherfactorssuchasnurture,protectionininfancy,guidance,andnourishment.Theimprintingprocess,Lorenzalsofound,broughtaboutasenseoffamiliarityfortheyounganimals.Whensuchastrongbondisformedatsuchanearlystage,itcreatesasenseofsecurityandcomfortforthesubjectandactuallyencouragestheimprintingbehavior. Pheromonesplayakeyroleintheimprintingprocess,theytriggerabiochemicalresponseintherecipient,leadingtoaconfirmedidentificationintheotherindividual.Ifdirectcontactbetweenmotherandinfantisnotmaintainedduringthecriticalimprintingperiod,thenthemothergoosemayrejecttheinfantbecausesheisunfamiliarwithhernewborn'sscent.Ifthatdoeshappen,thentheinfant'slifewouldbeinjeopardyunlessitwereclaimedbyasubstitutemother,possiblyleadingtoawkwardsocialbehaviorinlaterlife.[74]Inrelationtohumans,anewbornduringthecriticalperiodidentifieswithitsmother'sandotherpeoples'scentssinceitsscentisoneofthemostdevelopedsensesatthatstageinlife.Thenewbornusesthispheromoneidentificationtoseekthepeopleitidentifieswith,whenintimesofdistress,hunger,anddiscomfortasasurvivalskill.[75]InferencescouldbemadefornewbornsbaseduponLorenz'sstudies.Whenimprintingontheirmothers,newbornslooktothemfornourishment,asenseofsecurity,andcomfort.Humannewbornsareamongthemosthelplessknownwithorangutangnewbornsrankingsecond.Newbornsofthesespecieshaveaverylimitedarrayofinnatesurvivalabilities.Theirmostimportantandfunctionalabilityistoformbondswithcloseindividualswhoareabletokeepthemalive.Imprintingisacrucialfactorofthecriticalperiodbecauseitfacilitatesthenewborn'sabilitiestoformbondswithotherindividuals,frominfancytoadulthood. Auditoryprocessing[edit] Manystudieshavesupportedacorrelationbetweenthetypeofauditorystimulipresentintheearlypostnatalenvironmentandthedevelopmentonthetopographicalandstructuraldevelopmentoftheauditorysystem.[4] Firstreportsoncriticalperiodscamefromdeafchildrenandanimalsthatreceivedacochlearimplanttorestorehearing.Approximatelyatthesametime,bothanelectroencephalographicstudybySharma,DormanandSpahr[76]andanin-vivoinvestigationofthecorticalplasticityindeafcatsbyKralandcolleagues[77]demonstratedthattheadaptationtothecochlearimplantissubjecttoanearly,developmentalsensitiveperiod.Theclosureofsensitiveperiodslikelyinvolvesamultitudeofprocessesthatintheircombinationmakeitdifficulttoreopenthesebehaviorally.[4]Theunderstandingofthemechanismsbehindcriticalperiodshasconsequencesformedicaltherapyofhearingloss.[78]M.Merzenichandcolleaguesshowedthatduringanearlycriticalperiod,noiseexposurecanaffectthefrequencyorganizationoftheauditorycortex.[79] Recentstudieshaveexaminedthepossibilityofacriticalperiodforthalamocorticalconnectivityintheauditorysystem.Forexample,ZhouandMerzenich(2008)studiedtheeffectsofnoiseondevelopmentintheprimaryauditorycortexinrats.Intheirstudy,ratswereexposedtopulsednoiseduringthecriticalperiodandtheeffectoncorticalprocessingwasmeasured.Ratsthatwereexposedtopulsednoiseduringthecriticalperiodhadcorticalneuronsthatwerelessabletorespondtorepeatedstimuli;theearlyauditoryenvironmentinterruptednormalstructuralorganizationduringdevelopment. Inarelatedstudy,Barkat,PolleyandHensch(2011)lookedathowexposuretodifferentsoundfrequenciesinfluencesthedevelopmentofthetonotopicmapintheprimaryauditorycortexandtheventralmedicalgeniculatebody.Inthisexperiment,micewererearedeitherinnormalenvironmentsorinthepresenceof7 kHztonesduringearlypostnataldays.TheyfoundthatmicethatwereexposedtoanabnormalauditoryenvironmentduringacriticalperiodP11-P15hadanatypicaltonotopicmapintheprimaryauditorycortex.[80] Thesestudiessupportthenotionthatexposuretocertainsoundswithinthecriticalperiodcaninfluencethedevelopmentoftonotopicmapsandtheresponsepropertiesofneurons.Criticalperiodsareimportantforthedevelopmentofthebrainforthefunctionfromapatternofconnectivity.Ingeneral,theearlyauditoryenvironmentinfluencesthestructuraldevelopmentandresponsespecificityoftheprimaryauditorycortex.[81] Musicalability[edit] Mainarticle:Absolutepitch Absolutepitchmanifestsitselfalmostalwaysbeforeadolescenceandrarelyifeveramongindividualswhoarefirstexposedtomusicaftermid-childhood,suggestingthatexposuretomusicorsimilarphenomena(e.g.,tonallanguages)inearlytomid-childhoodisanecessaryconditionforitsdevelopmentorrefinement.Studiesthataskmusiciansandnon-musicianstosingorhumwell-knownpopularsongsthathavedefinitiverecordings(andhencearesunginstandardizedkeys)showthat—onaverage—participantssingwithinasemitoneofthestandardizedkeybutthatoutsidethesmallsubsetofparticipantswithabsolutepitchthereisbroadvariation(the"bellcurve"thatreflectsthedegreeofapproximationtothestandardkeyisbroadandflat).[citationneeded]Theseresultssuggestthatalmostallhumanshavesomeinnateaptitudeforabsolutepitchrecognition—thoughotherfactorsmayenhanceorlimitthelevelofthataptitude.Also,theresults'conjunctionwiththeaforementionedchronologicalobservationssuggeststhatearlytomid-childhoodexposuretoenvironmentswhoseinterpretationdependsonpitchisadevelopmental"trigger"forwhateveraptitudeanindividualpossesses. Vestibularsystem[edit] Inourvestibularsystem,neuronsareundevelopedatneuronalbirthandmatureduringthecriticalperiodofthefirst2-3postnatalweeks.Hence,disruptionofmaturationduringthisperiodcancausechangesinnormalbalanceandmovementthroughspace.Animalswithabnormalvestibulardevelopmenttendtohaveirregularmotorskills.[82] Studieshaveconsistentlyshownthatanimalswithgeneticvestibulardeficienciesduringthiscriticalperiodhavealteredvestibularphenotypes,mostlikelyasaresultofinsufficientinputfromthesemicircularcanalsanddopaminergicabnormalities.Moreover,exposuretoabnormalvestibularstimuliduringthecriticalperiodisassociatedwithirregularmotordevelopment.Childrenwithhypofunctioningvestibularreceptorsfrequentlyhavedelayedmotordevelopment.Theresultsofthestudiesdoneonferretsandratsreinforcedtheideathatthevestibularsystemisveryimportanttomotordevelopmentduringtheinitialneonatalperiod.Ifthevestibularreceptorsarepresentduringtheinitialsixmonthstoayearwhentheinfantislearningtositandstand,thenthechildmaydevelopmotorcontrolandbalancenormally.[83] Thevestibulo-ocularreflex(VOR)isareflexeyemovementthatstabilizesimagesontheretinaduringheadmovement.Itproducesaneyemovementinthedirectionoppositetoheadmovement,thuspreservingtheimageonthecenterofthevisualfield.StudiesinfishandamphibiansrevealedasensitivityintheirVOR.Theylaunchedintospaceflightfor9-10,somewithdevelopingVOR'sandotherswithalreadydevelopedreflexes.Thefishwithdevelopingreflexesdevelopedanupwardbendintheirtails.Thealteredgravityresultedinashiftoforientation.Thosewhowerealreadymaturedwiththereflexwereinsensitivetothemicrogravityexposure.[84] Memory[edit] Recentstudiesalsosupportthepossibilityofacriticalperiodforthedevelopmentofneuronsthatmediatememoryprocessing.Experimentalevidencesupportsthenotionthatyoungneuronsintheadultdentategyrushaveacriticalperiod(about1–3weeksafterneuronalbirth)duringwhichtheyareintegraltomemoryformation.[85]Althoughtheexactreasoningbehindthisobservationisuncertain,studiessuggestthatthefunctionalpropertiesofneuronsatthisagemakethemmostappropriateforthispurpose;theseneurons:(1)Remainhyperactiveduringtheformationofmemories;(2)aremoreexcitable;and(3)MoreeasilydepolarizableduetoGABAergiceffects.Itisalsopossiblethathyperplasticitymakestheneuronsmoreusefulinmemoryformation.Iftheseyoungneuronshadmoreplasticitythanadultneuronsinthesamecontext,theycouldbemoreinfluentialinsmallernumbers.[85] Theroleoftheseneuronsintheadultdentategyrusinmemoryprocessingisfurthersupportedbythefactthatbehavioralexperimentshaveshownthatanintactdentategyrusisintegraltohippocampalmemoryformation.[85]Itisspeculatedthatthedentategyrusactsasarelaystationforinformationrelatingtomemorystorage.Thelikelihoodofacriticalperiodcouldchangethewayweviewmemoryprocessingbecauseitwouldultimatelymeanthatthecollectionofneuronspresentisconstantlybeingreplenishedasnewneuronsreplaceoldones.Ifacriticalperioddoesindeedexist,thiscouldpossiblymeanthat:(1)Diversepopulationsofneuronsthatrepresenteventsoccurringsoonafteroneanothermayconnectthoseeventstemporallyinthememoryformationandprocessing;OR(2)Thesedifferentpopulationsofneuronsmaydistinguishbetweensimilarevents,independentoftemporalposition;OR(3)Separatepopulationsmaymediatetheformationofnewmemorieswhenthesameeventsoccurfrequently.[85] Seealso[edit] Behavioralcusp Childdevelopment Criticalperiodhypothesis Developmentalpsychology Malleableintelligence Universalgrammar 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延伸文章資訊
- 1Critical period - Wikipedia
In developmental psychology and developmental biology, a critical period is a maturational stage ...
- 2Critical/Sensitive Periods | Encyclopedia.com
A critical or sensitive period is defined as a period when certain experiences are particularly i...
- 3Critical Period - an overview | ScienceDirect Topics
Sensitive periods are special time-windows in early development where experience has a profound e...
- 4Critical Period In Brain Development and Childhood Learning
In contrast, A critical period is a special type of sensitive period - a window where sensory exp...
- 5Sensitive and critical periods in visual sensory deprivation
This observation of a critical period of susceptibility to deprivation was among the first to rev...