Critical Period In Brain Development and Childhood Learning

The critical period is a biologically determined stage of development where an organism is optimally ready to acquire some pattern of ... 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DevelopmentalPsychologyCriticalPeriodCriticalPeriodInBrainDevelopmentandChildhoodLearningByCharlotte Nickerson,publishedNov18,2021KeyTakeawaysCriticalperiodisanethologicaltermwhichreferstoafixedandcrucialtimeduringtheearlydevelopmentofanorganismwhenitisabletolearnthingswhichareessentialtosurvival.Theseinfluencesimpactthedevelopmentofprocessessuchashearingandvision,socialbonding,andlanguagelearning.Thetermismostoftenexperiencedinthestudyofimprinting,whereitisthoughtthatyoungbirdscouldonlydevelopanattachmenttothemotherduringafixedtimesoonafterhatching.Neurologically,criticalperiodsaremarkedbyhighlevelsofplasticityinthebrainbeforeneuralconnectionsbecomemoresolidifiedandstable.Inparticular,criticalperiodstendtoendwhensynapsesthatinhibittheneurotransmitterGABAmature.Incontrasttocriticalperiods,sensitiveperiods,otherwiseknownas“weakcriticalperiods”happenwhenanorganismismoresensitivethanusualtooutsidefactorsininfluencingbehavior,butthisinfluenceisnotnecessarilyrestrictedtothesensitiveperiod.Scholarshavedebatedtheextenttowhicholderorganismscandevelopcertainskills,suchasnatively-accentedforeignlanguages,afterthecriticalperiod.OverviewandHistoryThecriticalperiodisabiologicallydeterminedstageofdevelopmentwhereanorganismisoptimallyreadytoacquiresomepatternofbehaviorthatarepartoftypicaldevelopment.Thisperiod,bydefinition,willnotrecuratalaterstage.Ifanorganismdoesnotreceiveexposuretotheappropriatestimulusneededtolearnaskillduringacriticalperiod,itmaybedifficultorevenimpossibleforthatorganismtodevelopcertainfunctionsassociatedwiththatskilllaterinlife.Thishappensbecausearangeoffunctionalandstructuralelementspreventpassiveexperiencesfromelicitingsignificantchangesinthebrain(Cisneros-Francoet.al.,2020).ThefirststrongproponentofthetheoryofcriticalperiodswasCharlesStockhard(1921),abiologistwhoattemptedtoexperimentwiththeeffectsofvariouschemicalsonthedevelopmentoffishembryos,thoughhegavecredittoDarestefororiginatingtheidea30yearsearlier(Scott,1962).Stockhard’sexperimentsshowedthatapplyingalmostanychemicaltofishembryosatacertainstageofdevelopmentwouldresultinone-eyedfish.Theseexperimentsestablishedthatthemostrapidlygrowingtissuesinanembryoarethemostsensitivetoanychangeinconditions,leadingtoeffectslaterindevelopment(Scott,1962).Meanwhile,thepsychologistSigmundFreudattemptedtoexplaintheoriginsofneurosisinhumanpatientsastheresultofearlyexperiences,implyingthatinfantsareparticularlysensitivetoinfluencesatcertainpointsintheirlife.Lorenz(1935)lateremphasizedtheimportanceofcriticalperiodsintheformationofprimarysocialbonds(otherwiseknownasimprinting)inbirds,remarkingthatthispsychologicalimprintingwassimilartocriticalperiodsinthedevelopmentoftheembryo.Soonthereafter,McGraw(1946)pointedouttheexistenceofcriticalperiodsfortheoptimallearningofmotorskillsinhumaninfants(Scott,1962).Example:Infant-ParentAttachmentTheconceptofcriticalorsensitiveperiodscanalsobefoundinthedomainofsocialdevelopment,forexample,intheformationoftheinfant-parentattachmentrelationship(Salkind,2005).Attachmentdescribesthestrongemotionaltiesbetweentheinfantandcaregiver,areciprocalrelationshipdevelopingoverthefirstyearofthechild’slifeandparticularlyduringthesecondsixmonthsofthefirstyear.Duringthisattachmentperiod,theinfant’ssocialbehaviorbecomesincreasinglyfocusedaroundtheprincipalcaregivers(Salkind,2005).The20thcenturyEnglishpsychiatristJohnBowlbyformulatedandpresentedacomprehensivetheoryofattachmentinfluencedbyevolutionarytheory.Bowlbyarguedthattheinfant-parentattachmentrelationshipdevelopsbecauseitisimportanttothesurvivaloftheinfant,andthattheperiodfromsixtotwenty-fourmonthsofageisacriticalperiodofattachment.Thiscoincideswithaninfant’sincreasingtendencytoapproachfamiliarcaregiversandtobewaryofunfamiliaradults.Afterthiscriticalperiod,itisstillpossibleforafirstattachmentrelationshiptodevelop,albeitwithgreaterdifficulty(Salkind,2005).Thishasbroughtintoquestion,inasimilarveintolanguagedevelopment,whetherthereisactuallyacriticaldevelopmentperiodforinfant-caregiverattachment.Sourcesdebatingthisissuetypicallyincludecasesofinfantswhodidnotexperienceconsistentcaregivingduetobeingraisedininstitutionspriortoadoption(Salkind,2005).Earlyresearchintothecriticalperiodofattachment,publishedinthe1940s,reportsconsistentlythatchildrenraisedinorphanagessubsequentlyshowedunusualandmaladaptivepatternsofsocialbehavior,difficultyinformingcloserelationships,andbeingindiscriminatelyfriendlytowardunfamiliaradults(Salkind,2005).Later,researchfromthethe1990sindicatedthatadopteeswereactuallystillabletoformattachmentrelationshipsafterthefirstyearoflifeandalsomadedevelopmentalprogressfollowingadoption.Nonetheless,thesechildrenhadanoverallincreasedriskofinsecureormaladaptiveattachmentrelationshipswiththeiradoptiveparents.Thisevidencesupportsthenotionofasensitiveperiod,butnotacriticalperiod,inthedevelopmentoffirstattachmentrelationships(Salkind,2005).MechanismsforCriticalPeriodsBothgeneticsandsensoryexperiencesfromoutsidethebodyshapethebrainasitdevelops(Knudsen,2004).However,thedevelopmentalstagethatanorganismisinsignificantlyimpactshowmuchthebraincanchangebasedontheseexperiences.Inscientificterms,thebrain’splasticitychangesoverthecourseofalifespan.Thebrainisveryplasticintheearlystagesoflife,beforemanykeyconnectionstakeroot,butlesssolater.Thisiswhyresearchershaveshownthatearlyexperienceiscrucialforthedevelopmentof,say,languageandmusicalabilitiesandtheseskillsaremorechallengingtotakeupinadulthood(SkoeandKraus,2013;Whiteetal.,2013;Hartshorneetal.,2018).Asbrainsmature,theconnectionsinthembecomemorefixed.Thebrain’stransitionsfromamoreplastictoamorefixedstateadvantageouslyallowsittoretainnewandcomplexprocesses,suchasperceptual,motor,andcognitivefunctions(Piaget,1962).Children’sgestures,forexample,prideandpredicthowtheywillacquireorallanguageskills(Colonnesietal.,2010),whichinturnareimportantfordevelopingexecutivefunctions(MarcovitchandZelazo,2009).However,thisformationofstableconnectionsinthebraincanlimithowthebrain’sneuralcircuitrycanberevisedinthefuture.Forexample,ifayoungorganismhasabnormalsensoryexperiencesduringthecriticalperiod-suchasauditoryorvisualdeprivation-thebrainmaynotwireitselfinawaythatprocessesfuturesensoryinputsproperly(Gallagheretal.,2020).Oneillustrationisthisisthetimingofcochlearimplants-aprosthesisthatrestoreshearinginsomedeafpeople.Childrenwhoreceivecochlearimplantsbeforetwoyearsofagearemorelikelytobenefitfromitthanthosewhoareimplantedlaterinlife(KralandEggermont,2007;Gallagheretal.,2020).Similarly,thevisualdeprivationcausedbycataractsininfantscancausesimilarconsequences.Whencataractsareremovedduringearlyinfancy,individualscandeveloprelativelynormalvision;however,whenthecataractsarenotremoveduntiladulthood,thisresultsinsubstantiallypoorervision(MartinsRosaetal.,2013).Afterthecriticalperiodcloses,abnormalsensoryexperienceshavealessdrasticeffectonthebrainandleadsto-barringdirectdamagetothecentralnervoussystem-reversiblechanges(Gallagheretal.,2020). Muchofwhatscientistsknowaboutcriticalperiodsderivesfromanimalstudies,astheseallowresearchersgreatercontroloverthevariablesthattheyaretesting.Thisresearchhasfoundthatdifferentsensorysystems,suchasvision,auditoryprocessing,andspatialhearinghavedifferentcriticalperiods(Gallagheretal.,2020).Thebrainregulateswhencriticalperiodsopenandclosethroughregulatinghowmuchthebrain’ssynapsestakeupneurotransmitters,whicharechemicalsubstancesthataffectthetransmissionsofelectricalsignalsbetweenneurons.Inparticular,overtime,synapsesdecreasetheiruptakeofgamma-aminobutyricacid,betterknownasGABA.Atthebeginningofthecriticalperiod,outsidesourcesbecomemoreeffectiveatinfluencingchangesandgrowthinthebrain.Meanwhile,astheinhibitorycircuitsofthebrainmature,thematurebrainbecomeslesssensitivetosensoryexperiences(Gallagheretal.,2020).CriticalPeriodsvsSensitivePeriodsCriticalperiodsaresimilartosensitiveperiods,andscholarshaveattimesusedtheminterchangeably.However,theydescribedistinctbutoverlappingdevelopmentalprocesses.Asensitiveperiodisadevelopmentalstagewheresensoryexperienceshaveagreaterimpactonbehavioralandbraindevelopmentthanusual;however,thisinfluenceisnotexclusivetothistimeperiod(Knudsen,2004;Gallagher,2020).Thesesensitiveperiodsareimportantforskillssuchaslearningalanguageorinstrument. Incontrast,Acriticalperiodisaspecialtypeofsensitiveperiod-awindowwheresensoryexperienceisnecessarytoshapetheneuralcircuitsinvolvedinbasicsensoryprocessing,andwhenthiswindowopensandclosesiswell-defined(Gallagher,2020). Researchersalsorefertosensitiveperiodsasweakcriticalperiods.Someexamplesofstrongcriticalperiodsincludethedevelopmentofvisionandhearing,whileweakcriticalperiodsincludephenometuning-howchildrenlearnhowtoorganizesoundsinalanguage,grammarprocessing,vocabularyacquisition,musicaltraining,andsporttraining(Gallagheretal.,2020).TheCriticalPeriodHypothesisOneofthemostnotableapplicationsoftheconceptofacriticalperiodisinlinguistics.ScholarsusuallytracetheoriginsofthedebatearoundageinlanguageacquisitiontoPenfieldandRobert’s(2014)bookSpeechandBrainMechanisms.Inthe1950sand1960sPenfieldwasastaunchadvocateofearlyimmersioneducation(KrollandDeGroot,2009).Nonetheless,itwasLenneberginhisbookBiologicalFoundationsofLanguagewhocoinedthetermcriticalperiod(1967)indescribinglanguageperiod.Lennenberg(1967)describedacriticalperiodasaperiodofautomaticacquisitionfrommereexposure”that“seemstodisappearafterthisage.”Scovel(1969)latersummarizedandnarrowedPenfield’sandLenneberg’sviewonthecriticalperiodhypothesisintothreemainclaims:Adultnativespeakerscanidentifynon-nativesbytheiraccentimmediatelyandaccurately.Thelossofbrainplasticityatabouttheageofpubertyaccountsfortheemergenceofforeignaccents./li>Thecriticalperiodhypothesisonlyholdsforspeech(whetherornotsomeonehasanativeaccent)anddoesnotaffectotherareasoflinguisticcompetence.Linguistshavesinceattemptedtofindevidenceforwhetherornotscientificevidenceactuallysupportsthecriticalperiodhypothesis,ifthereisacriticalperiodforacquiringaccentlessspeech,for“morphosyntactic”competence,andifthesearetrue,howage-relateddifferencescanbeexplainedontheneurologicallevel(Scovel,2000).Thecriticalperiodhypothesisappliestobothfirstandsecondlanguagelearning.Untilrecently,researcharoundthecriticalperiod’sroleinfirstlanguageacquisitionrevolvedaroundfindingsaboutso-called“feral”childrenwhohadfailedtoacquirelanguageatanolderageafterhavingbeendeprivedofnormalinputduringthecriticalperiod.However,thesecasestudiesdidnotaccountfortheextenttowhichsocialdeprivation,andpossiblyfooddeprivationorsensorydeprivation,mayhaveconfoundedwithlanguageinputdeprivation(KrollandDeGroot,2009).Morerecently,researchershavefocusedmoresystematicallyondeafchildrenborntohearingparentswhoarethereforedeprivedoflanguageinputuntilatleastelementaryschool.Thesestudieshavefoundtheeffectsoflackoflanguageinputwithoutextremesocialdeprivation:theoldertheageofexposuretosignlanguageis,theworseitsultimateattainment(Emmorey,Bellugi,Friederici,andHorn,1995;KrollandDeGroot,2009).However,KrollandDeGrootarguethatthecriticalperiodhypothesisdoesnotapplytorateofacquisitionoflanguage.Adultsandadolescentscanlearnlanguagesatthesamerateorevenfasterthanchildrenintheirinitialstageofacquisition(SlavoffandJohnson,1995).However,adultstendtohaveamorelimitedultimateattainmentoflanguageability(KrollandDeGroot,2009). Therehasbeenalonglineageofempiricalfindingsaroundageofacquisition.Themostfundamentalofthisresearchcomesfromaseriesofstudiessincethelate1970sdocumentinganegativecorrelationbetweenageofacquisitionandultimatelanguagemastery(KrollandDeGrott,2009).Nonetheless,differentperiodscorrespondtosensitivitytodifferentaspectsoflanguage.Forexample,shortlyafterbirth,infantscanperceiveanddiscriminatespeechsoundsfromanylanguage,includingonestheyhavenotbeenexposedto(Eimasetal.,1971;Gallagheretal.,2020).Aroundsixmonthsofage,exposuretotheprimarylanguageintheinfant’senvironmentguidesphoneticrepresentationsoflanguage,andsubsequentlytheneuralrepresentationsofspeechsoundsofthenativelanguage,whileweakeningthoseofunusedsounds(McClellandetal.,1999;Gallagheretal.,2020).Vocabularylearningexperiencesrapidgrowthatabout18monthsofage(Kuhl,2010).CriticalEvaluationMorethananyotherareaofappliedlinguistics,thecriticalperiodhypothesishasimpactedhowteachersteachlanguages.Consequently,researchershavecritiquedhowimportantthecriticalperiodistolanguagelearning.Forexample,severalstudiesinearlylanguageacquisitionresearchshowedthatchildrenwerenotnecessarilysuperiortoolderlearnersinacquiringasecondlanguageevenintheareaofpronunciation(OlsonandSamuels,1973;SnowandHoefnagel-Hohle,1978;Scovel,2000).Infact,themajorityofresearchersatthetimeappearedtobeskepticalabouttheexistenceofacriticalperiod,withsomeexplicitlydenyingitsexistence.CountertooneoftheprimarytenetsofScovel’s(1969)criticalperiodhypothesis,therehavebeenseveralcasesofpeoplewhohaveacquiredasecondlanguageinadulthoodspeakingwithnativeaccents.Forexample,Moyer’sstudyofhighly-proficientEnglish-speakinglearnersofGermansuggestedthatatleastoneoftheparticipantswasjudgedtohavenative-likepronunciationinhissecondlanguage(1999),andseveralparticipantsinBongaerts(1999)studyofhighlyproficientDutchspeakersofFrenchspokewithaccentsjudgedtobenative(Scovel,2000).AbouttheAuthorCharlotteNickersonisamemberoftheClassof2024atHarvardUniversity.Comingfromaresearchbackgroundinbiologyandarcheology,Charlottecurrentlystudieshowdigitalandphysicalspaceshapeshumanbeliefs,norms,andbehaviorsandhowthiscanbeusedtocreatebusinesseswithgreatersocialimpact.Howtoreferencethisarticle:Howtoreferencethisarticle:Nickerson,C.(2021,Nov18).CriticalPeriodInBrainDevelopmentandChildhoodLearning.SimplyPsychology.www.simplypsychology.org/critical-period.htmlReferencesBongaerts,T.(1999).UltimateattainmentinL2pronunciation:ThecaseofveryadvancedlateL2learners.Secondlanguageacquisitionandthecriticalperiodhypothesis,133-159.Cisneros-Franco,J.M.,Voss,P.,Thomas,M.E.,&deVillers-Sidani,E.(2020).Criticalperiodsofbraindevelopment.InHandbookofClinicalNeurology(Vol.173,pp.75-88).Elsevier.Colonnesi,C.,Stams,G.J.J.,Koster,I.,&Noom,M.J.(2010).Therelationbetweenpointingandlanguagedevelopment:Ameta-analysis.DevelopmentalReview,30(4),352-366.Eimas,P.D.,Siqueland,E.R.,Jusczyk,P.,&Vigorito,J.(1971).Speechperceptionininfants.Science,171(3968),303-306.Emmorey,K.,Bellugi,U.,Friederici,A.,&Horn,P.(1995).Effectsofageofacquisitionongrammaticalsensitivity:Evidencefromon-lineandoff-linetasks.AppliedPsycholinguistics,16(1),1-23.Knudsen,E.I.(2004).Sensitiveperiodsinthedevelopmentofthebrainandbehavior.Journalofcognitiveneuroscience,16(8),1412-1425.Hartshorne,J.K.,Tenenbaum,J.B.,&Pinker,S.(2018).Acriticalperiodforsecondlanguageacquisition:Evidencefrom2/3millionEnglishspeakers.Cognition,177,263-277.Kral,A.,&Eggermont,J.J.(2007).What'stoloseandwhat'stolearn:developmentunderauditorydeprivation,cochlearimplantsandlimitsofcorticalplasticity.BrainResearchReviews,56(1),259-269.Kroll,J.F.,&DeGroot,A.M.(Eds.).(2009).Handbookofbilingualism:Psycholinguisticapproaches.OxfordUniversityPress.Kuhl,P.K.(2010).Brainmechanismsinearlylanguageacquisition.Neuron,67(5),713-727.Lenneberg,E.H.(1967).Thebiologicalfoundationsoflanguage.HospitalPractice,2(12),59-67.Lorenz,K.(1935).Derkumpaninderumweltdesvogels.JournalfürOrnithologie,83(2),137-213.Marcovitch,S.,&Zelazo,P.D.(2009).Ahierarchicalcompetingsystemsmodeloftheemergenceandearlydevelopmentofexecutivefunction.Developmentalscience,12(1),1-18.McClelland,J.L.,Thomas,A.G.,McCandliss,B.D.,&Fiez,J.A.(1999).Understandingfailuresoflearning:Hebbianlearning,competitionforrepresentationalspace,andsomepreliminaryexperimentaldata.Progressinbrainresearch,121,75-80.McGraw,M.B.(1946).Maturationofbehavior.InManualofchildpsychology.(pp.332-369).JohnWiley&SonsInc.Moyer,A.(1999).UltimateattainmentinL2phonology:Thecriticalfactorsofage,motivation,andinstruction.Studiesinsecondlanguageacquisition,21(1),81-108.Gallagher,A.,Bulteau,C.,Cohen,D.,&Michaud,J.L.(2019).NeurocognitiveDevelopment:NormativeDevelopment.Elsevier.Olson,L.L.,&JaySamuels,S.(1973).Therelationshipbetweenageandaccuracyofforeignlanguagepronunciation.TheJournalofEducationalResearch,66(6),263-268.Penfield,W.,&Roberts,L.(2014).Speechandbrainmechanisms.PrincetonUniversityPress.Piaget,J.(1962).Thestagesoftheintellectualdevelopmentofthechild.BulletinoftheMenningerclinic,26(3),120.Rosa,A.M.,Silva,M.F.,Ferreira,S.,Murta,J.,&Ca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