Testing the arousal hypothesis of neonatal imitation in infant rhesus macaques

16 Pages·2017·1.36 MB·English

RESEARCHARTICLE
Testing the arousal hypothesis of neonatal
imitation in infant rhesus macaques
AnnikaPaukner1*,EricJ.Pedersen2,3,ElizabethA.Simpson2
1 EuniceKennedyShriverNationalInstituteofChildHealthandHumanDevelopment,NationalInstitutesof
Health,Poolesville,Maryland,UnitedStatesofAmerica,2 DepartmentofPsychology,UniversityofMiami,
CoralGables,Florida,UnitedStatesofAmerica,3 DepartmentofPsychologyandNeuroscience,University
ofColoradoBoulder,Boulder,Colorado,UnitedStatesofAmerica
*[email protected]
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a1111111111 Abstract
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a1111111111 Neonatalimitationisthematchingof(oftenfacial)gesturesbynewborninfants.Somestud-
iessuggestthatperformanceoffacialgesturesisduetogeneralarousal,whichmaypro-
ducefalsepositivesonneonatalimitationassessments.Hereweexaminewhetherarousal
islinkedtofacialgesturinginnewborninfantrhesusmacaques(Macacamulatta).Wetested
OPENACCESS 163infantsinaneonatalimitationparadigmintheirfirstpostnatalweekandanalyzedtheir
lipsmackinggestures(arapidopeningandclosingofthemouth),tongueprotrusionges-
Citation:PauknerA,PedersenEJ,SimpsonEA
(2017)Testingthearousalhypothesisofneonatal tures,andyawnresponses(ameasureofarousal).Arousalincreasedduringdynamicstim-
imitationininfantrhesusmacaques.PLoSONE12 uluspresentationcomparedtothestaticbaselineacrossallconditions,andarousalwas
(6):e0178864.https://doi.org/10.1371/journal.
higherinthefacialgesturesconditionsthanthenonsocialcontrolcondition.However,even
pone.0178864
aftercontrollingforarousal,wefoundacondition-specificincreaseinfacialgesturesin
Editor:KimA.Bard,UniversityofPortsmouth,
infantswhomatchedlipsmackingandtongueprotrusiongestures.Thus,wefoundnosup-
UNITEDKINGDOM
portforthearousalhypothesis.Consistentwithreportsinhumannewborns,imitators’pro-
Received:October21,2016
pensitytomatchfacialgesturesisbasedonabilitiesthatgobeyondmerearousal.We
Accepted:May19,2017 discussoptimaltestingconditionstominimizepotentiallyconfoundingeffectsofarousalon
Published:June15,2017 measurementsofneonatalimitation.
Copyright:Thisisanopenaccessarticle,freeofall
copyright,andmaybefreelyreproduced,
distributed,transmitted,modified,builtupon,or
otherwiseusedbyanyoneforanylawfulpurpose.
TheworkismadeavailableundertheCreative
CommonsCC0publicdomaindedication. Introduction
DataAvailabilityStatement:Allrelevantdataare Humannewbornsareresponsivetotheirenvironmentsbymodifyingthemovementsoftheir
withinthepaperanditsSupportingInformation
mouths:theyaltertheirsucklingpatternsforsweetsolutions[1],andtheyrespondtotheodor
files.
oftheirmother’sbreastmilkbymakingmouthingmovements[2].Newbornsalsousetheir
Funding:Thisresearchwassupportedbythe mouthstointeractwithvisualstimuli:MeltzoffandMoore[3–4]foundthatwithinminutes
NationalScienceFoundation(www.nsf.govBCS-
afterbirth,infantsrespondtofacialgestureswithmatchingfacialgestures.Themostcom-
1653737)andtheDivisionofIntramuralResearch,
monlytestedfacialgesturesaremouthopeningandtongueprotrusion,butnewbornsalso
NICHD.Thefundershadnoroleinstudydesign,
datacollectionandanalysis,decisiontopublish,or matchothervisualstimuli,e.g.lipprotrusion[5],emotionalfacialexpressions[6],headmove-
preparationofthemanuscript. ment[7],andevenfingermovements[8–9].Overall,resultshavebeenmixedwithsome
researchersreportingpositiveevidenceofmatchingbehaviors,andothersfailingtofind
Competinginterests:Theauthorshavedeclared
thatnocompetinginterestsexist.
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Arousalhypothesisofneonatalimitation
evidenceofwhathasbeentermedneonatalimitation.Therearemultiplereasonswhysome
studiesmayhavefailedtoreplicatepreviousfindings,rangingfromtestinginfantsoutsideof
windowsofasensitiveperiod[10]tomethodologicalshortcomings(foradiscussion,see[11]).
Ameta-analysisbyAnisfeld[12–13]foundthatthemostconsistently–andsomemayargue
theonly–matchedgestureistongueprotrusion,whichraisesthequestionofwhethertongue
protrusionmatchingisacaseofimitationorwhetheralternativeaccountsmaybesufficientto
explainthisphenomenon.
Imitationhasbeendefinedinnumerousways,emphasizingthematchingofeithertheform
ofmovements,theoutcomes,ortheintentionsofanact(e.g.see[14]).Forthecurrentpur-
poses,wewillfocusonanewborn’sabilitytomatchthebehaviorsofothersinmotoricform
followingtheobservationofthesamebehaviorbyamodel,withoutmakinganyassumptions
regardingtheinfant’smotivationorintention.Behaviormatchingcanbeachievedviavisual-
visualmatching,orinthecaseoffacialgestures,visual-motormatching.Thatis,beingunable
toseetheirownfaces,newbornsmustmatchthevisualinputofsomeoneelse’sfacialgesture
withthekinestheticandproprioceptivefeedbackoftheirownfaces.Sincethisachievement
spansatleasttwosensorymodalities,MeltzoffandMoore[15]proposedthatinfantsareborn
withasupramodelrepresentationofacts,whichfacilitatesthisvisuo-motormatchingof
behaviors.Ifnewbornsareabletomatchnumerousgestures,thentheideaofageneral‘active
intermodalmapping’(AIM;[15]appearsplausible.If,ontheotherhand,newbornsincrease
thefrequencyofoneparticulargesture(suchastongueprotrusion)inresponsetovarious
visualstimuli,itwouldbemoreparsimonioustoconcludethattongueprotrusionispartofthe
newborn’sgeneralarousalresponse,whichbycoincidencehappenstoresembleamatching
responsewhenperformedafterseeingatongueprotrusionmodel[3].Itisthereforeimportant
tocomparetheresponsesinthematchingconditionwiththeresponsesinacontrolcondition,
andimitationshouldonlybeinferredwhentheperformanceinthematchingconditionis
greaterthanperformanceinthenon-matchingcondition[3];otherwise,arousalcannotbe
ruledoutasanalternativeexplanation.
Theideathattongueprotrusionismerelyoneofneonates’exploratoryorarousalresponses
hasbeenexploredbyJones[16–17].Jonespresentednewbornswithblinkinglights[16]and
music[17]andobservedthattongueprotrusionsincreasedinbothtestsituations.Nagyand
colleagues[18–19]testedinmoredetailwhethertongueprotrusiongesturesmaybelinkedto
infantarousal.NagyandMolnar[18]foundthatnewbornheartratesacceleratewheninfants
performtongueprotrusiongestureswhileviewingatongueprotrusionmodel,suggestingthat
theactofprotrudingtheirtonguesmayindeedinvolveanelementofarousalfornewborns.
However,Nagyetal.[19]alsomeasuredotherpotentialindicatorsofarousalsuchasarmand
fingermovementsandgeneralstateofalertness;noneofthesewerefoundtoincreaseoverthe
courseofthetestsession,orwhenpresentedwithatongueprotrusionmodelcomparedtoa
baselinemeasurement.Instead,theyreportthatnewbornsinthefirst5daysoflifeincreased
theirstrongtongueprotrusions(i.e.,fullyextendedtonguesbeyondthelips),butnottheir
weaktongueprotrusions(i.e.,tonguevisiblebutnotprotrudingbeyondthelips),whileobserv-
ingatongueprotrusiongesturecomparedtoabaselineperiod;thelattertypeofgesturemay
bemoreofanarousalresponse,buttheformerappearsselectivelyimitative[19].Thus,while
theevidencetodateindicatesthatarousalmaybeassociatedwithatongueprotrusion
response,itstillremainsunclearwhetherarousalonitsownissufficienttoexplainfacialges-
turematchinginnewborns.
Interestingly,otherspeciesalsoshowneonatalimitationresponses.Bard[20]reports
imitationofmouthopeninggestures(butnottongueprotrusiongestures)ininfantchim-
panzeesinbothastructuredandacommunicativetestingparadigm,andsuggeststhatthis
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Arousalhypothesisofneonatalimitation
propensitymaybelinkedtothespecies-typicalsmileorplayface.Similarly,infantrhesus
macaquesimitatelipsmackinggestures(rapidopeningandclosingofthemouth)[21],
whichisacommunicativegestureconveyingaffiliationbetweenmothersandinfants.
Macaqueinfantswhomatchlipsmackinggesturesalsoperformmorelipsmackinggestures
afteradelay[22],specificallywhenthelipsmackingmodelisthesamepersonratherthan
anewperson[23].Isitpossiblethatinnonhumanpopulations,matchingoffacialgestures
areanartefactofarousalandthatthematchwiththemodel’sgesturesismerely
coincidental?
Inthepresentstudy,weaimedtodeterminewhetherfacialgesturesmaybetheresultof
arousalininfantrhesusmacaques,andconsequentlywhetherarousalcouldbetheresponsible
mechanismofso-calledimitativebehavior.Tomeasurearousal,wecountedyawningduring
theneonatalimitationtask.Althoughfingerandarmmovementsmaybeagoodindicatorof
arousalinhumaninfantstudies[19],andscratchinghasassociatedwitharousalinmacaques
[24],infantmacaquesarefrequentlyswaddledinsoftblanketsorclothsduringtesting,thus
fingerandarmmovementsmaybeinhibitedor,atleast,difficulttoobserve.Yawning,onthe
otherhand,isgenerallyassociatedwitharousalinavarietyofanimals.Forexample,when
administeredexperimentally,adrenocorticotropichormone(ACTH),apituitaryhormone
thatstimulatesthereleaseofcortisol,astresshormone,facilitatesyawningindogs,cats,rab-
bits,andmice[25].Innon-humanprimates,severalstudieshavesuggestedthatyawning
increasesinsituationsofstressorincreasedarousal[26–31].Developmentally,Nazkabooby
(Sulagranti)nestlingsshowincreasedyawningratesfollowingnaturalmaltreatmentbynon-
parentaladults[32]andinhumans,pre-terminfants’yawninghasbeenassociatedwith
increasedbehavioralarousal[33]aswellaspainfulproceduressuchasheellanceforbloodcol-
lection[34],suggestingthatyawningandarousalareassociatedfromearlyoninlifeandmak-
ingitasuitablemeasureforthecurrentstudy.
Belowwedescribeinfantmacaques’yawningresponsesandtheirassociationwithfacial
gestures(lipsmackingandtongueprotrusion)inaneonatalimitationtask.Infantswerepre-
sentedwiththreedifferentstimuli,alipsmackingcondition(LPS,anaffiliativefacialgesture),
atongueprotrusioncondition(TP,nocommunicativemeaning),andanonsocialcontrolcon-
dition(CTRL,awhiteplasticdiskwithhighcontrastorthogonalstripes).Todetermine
whetherfacialgesturesmaybetheresultofarousal,wefirstexaminedarousal(measuredby
yawningfrequencies)acrossconditionstodeterminewhethertheconditionscontainingfacial
stimuli(LPSandTP)maybemorearousingthantheCTRLcondition.Moreover,paststudies
haveinferredimitationinmatchingfacialgesturesifthereisanincreasebetweena(static/still)
Baselineanda(dynamic)Stimulusphase,whichislargerinthefacialgestureconditionthan
thespinning-diskcontrolcondition[20];wethereforeexaminedwhetherarousal(yawning)
wouldshowasimilarincreasebetweenBaselineandStimulusphasesspecificallyintheLPS
andTPconditionsbutnottheCTRLcondition.Wethenidentifiedsubsetsofinfantswho
showedanincreaseinlipsmackingresponsesbetweentheBaselineandtheStimulusphasespe-
cificallyintheLPScondition,calledLPSImitators,oranincreaseintongueprotrusionges-
turesbetweenBaselineandStimulusspecificallyintheTPcondition,calledTPImitators.We
comparedtheseinfants’levelsofarousalacrossconditionsandtoinfantswhodidnotshowan
increasefacialgestures(Non-imitators),hypothesizingthatiffacialgesturesarecausedby
arousal,ImitatorsshouldshowhigherlevelsofarousalthanNon-imitators,specificallyinthe
facialgestureconditions.Finally,giventheassumedassociationbetweenarousalandfacialges-
tures,weinvestigatedwhetherchangesinyawningbetweenBaselineandStimuluscould
explaintheincreasesintheselectivematchingoflipsmackingandtongueprotrusionresponses
ofImitatorsinthefacialgestureconditions.
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Arousalhypothesisofneonatalimitation
Materialsandmethods
Ethicsstatement
ThisstudywasreviewedandapprovedbytheEuniceKennedyShriverNationalInstituteof
ChildHealthandHumanDevelopmentAnimalCareandUseCommittee.Thestudywascon-
ductedinaccordancewiththeGuidefortheCareandUseofLaboratoryAnimalsandcom-
pliedwiththeAnimalWelfareAct.Duringthecourseofthisstudy,infantswereexclusively
fedwithSimilac1Advance1(AbbottLaboratories).Infants’housingwasenrichedbyaninan-
imatesurrogatemothercoveredwithfleecefabricaswellasblanketsandvariousplasticand
rubbertoys,whichwererotateddaily.Attheconclusionofdatacollectionforthecurrent
study,infantscontinuedtobehousedinthenurseryaspartofongoing,unrelatedresearch
studiesuntilca.6–8monthsofage,afterwhichtheyweretransferredtolargepeergroups.See
[35]forfurtherdetailsonhousing,enrichment,andfeeding.
Subjects
Subjectswere163infantrhesusmacaques(Macacamulatta,88males).Allinfantsweresepa-
ratedfromtheirmothersonthedaytheywereborn(typicallyby8am),andwererearedina
nurseryfacilityforongoing,unrelatedresearchstudies.Infantswereindividuallyhousedin
incubators(51cm×38cm×43cm)forthefirsttwoweeksoflifeandinmetalcagesthereafter.
Forthefirstmonthoflife,infantscouldseeandhear,butnotphysicallycontact,otherinfants
ofsimilarage.Humancaretakerswerepresentfor13heachdayandinteractedwithinfants
every2hforfeedingandcleaningpurposes.Imitationdatafrom126infantswerepreviously
publishedaspartofotherstudies[22,35–41].Weusedallavailabledataanddidnotexclude
infantsbasedonimitationoryawningperformance.
Procedure
Infantsweretestedforneonatalimitationthreetimesaday,everyotherday,inthefirstweek
oflife(days1–2,3–4,5–6,and7–8),foruptofourdays.Intotal,134infantsweretestedon4
days,25infantsweretestedon3days,and4infantsweretestedon2days.Therewasatleast
anhourbetweeneachtestsession.Infantsweretestedwithinanhouroffeedingtoensurethey
werenotfussyduetohunger.Infantshadtohavetheireyesopenandhadtoshowsignsof
activeorientationtowardsstimuliappearingintheirvisualfield(e.g.,theexperimenter
approachingtheincubator).Infantsweretransportedfromtheirincubatortoanearbytesting
room.Duringtransportationtothetestingroom,infantsclungtothearmoftheexperimenter,
andhadtoshowsignsofbodyreactivitytowardschangesinpostureandsupportinstability.
Failuretoshowthesesignsofalertnessresultedinadelayofthetestsessionuntil,atalater
timeinthesameday,infantsweresufficientlyalert.Oneexperimenterheldeachinfantonher
lap,oftenswaddledorlooselywrappedinadiaperpadorsoftfleece.Allinfantswereawake
andalert(butnotdistressed)atthebeginningofthetest.Ademonstratorpresentedinfants
withthreestimuli,oneduringeachsession,atadistanceofapproximately30cmateye-level
withtheinfant:alipsmackinggesture(LPS,rapidopeningandclosingofthemouth),atongue
protrusiongesture(TP,slowprotrusionandretractionofthetongue,ca.1gesturepersecond),
andanonsocialcontrolcondition(CTRL,awhiteplasticdiskwithorthogonalstripes—which
wereeitherblack/redorgreen/yellow—slowlyrotatedclockwiseandcounter-clockwise).
Eachstimulustypewaspresentedonceadaytoinfants;theorderofstimuluspresentations
remainedthesameforeachinfantbutwasrandomizedbetweeninfants.Ineachtestsession,
oneexperimenterheldtheinfant,asecondexperimenter—thedemonstrator—servedasthe
sourceofthestimuli,andathirdexperimenterwasthetime-keeperwhoensuredstimuliwere
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Arousalhypothesisofneonatalimitation
presentedforappropriatelengths.Allsessionswerevideotaped.Individualdemonstrators
wererandomlyassignedtoconditionsbutremainedconsistentacrossdayswithineachinfant.
TwentyseveninfantswereonlytestedintheLPSandCTRLconditions,nottheTPcondition,
and4sessionswerelostduetoexperimentererror,leavingatotalof1,745sessions.
Atthebeginningofatrial,a40secbaselinewasconducted,inwhichthedemonstratordis-
playedacalm,neutralfacialexpression(orthestilldiskincontrol).Duringthesubsequent
stimulusperiod,thedemonstratordisplayedafacialmovement(lipsmackingortonguepro-
trusion)orrotatedthediskfor20seconds,followedbyastill/neutralfacialexpression(still
diskinCTRL)periodfor20seconds.Thismovement-stillfacesequencewasrepeatedand
endedwithanadditional20secondmovementphase(totalof100secondstimulusperiod).
Seesupplementalmaterialsforvideoexamples.
Analyses
Infants’mouthmovementswerecodedoff-line,frame-by-frame((cid:21)30framespersecond),for
lipsmackinggestures,tongueprotrusiongestures,andyawns.Lipsmackingwasoperationally
definedasahighfrequencyopeningandclosingofthemouthwithoutsoundproductionin
whichthelipswererequiredtopartandthenrejoinwithin2seconds.Tongueprotrusions
weredefinedasforwardmovementsofthetonguesothatitcrossedtheinneredgeofthelower
lip.Yawnswerecodedwheninfantsperformedafullmouthstretchwhichcould(butdidnot
necessarilyhaveto)lastformorethan2seconds,sometimescombinedwithclosingofthe
eyes.Observerswereblindtothestimulus.Inter-observerreliabilitywasassessedbetweenan
anchorobserverandoneadditionalobserverfor345sessions(19.8%oftotalsessions).Inter-
observeragreementswerehighforallfacialgestures(lipsmacking:M =9.97,M =10.61,
1 2
intraclasscorrelationcoefficient(ICC)=.92,95%CI(.90,.93);tongueprotrusion:M =9.71,
1
M =10.24,ICC=.93,95%CI(.92,.95);yawns:M =1.18,M =1.12,ICC=.97,95%CI
2 1 2
(.96,.98).
Unlessotherwisestated,themainanalyseswereconductedusingBayesiangeneralizedlin-
earmixedmodels(GLMMs)withoverdispersedPoissondistributionstoproperlymodel
yawnsascounts[42–43].Thereasonforusingtheseanalyseswastheseverelypositiveskewed
distributionofyawnswhenaccountingforindividualtestdays,whichwouldnotbeappropri-
atelyhandledbyanalysesofvariance(ANOVA).WeranmodelsinRversion3.2.3[44]viathe
MCMCglmmpackage[45].Toaccountforrepeatedmeasures,thesemodelshaverandom
interceptsforsubjectsandrandomslopesfortestdayandcondition,andallwererunwith
non-informativepriors.Thecoefficientsforthemodelswerelogratesandwereinterpretedas
rateratios(RRs)whenexponentiated.Wereport95%highestdensityintervals(HDIs)forthe
coefficientestimatesandpMCMC(aBayesian“p-value”basedonMarkovchainMonteCarlo
estimation,definedastwomultipliedbytheprobabilitythattheparametervalueislessthanor
greaterthanzero,usingthesmalleroftheseprobabilities;[45]).Weconsideraresultstatisti-
callysignificantif(a)the95%HDIdidnotincludezeroand(b)pMCMCwaslessthan.05.
Results
Sexdifferencesinyawning
Sinceyawningratesarethoughttobesexuallydimorphicinadultrhesusmacaques[46],we
firstcheckedwhetherwewouldhavetocontrolforthisvariablebyexaminingwhethermale
andfemaleinfantrhesusmacaquesdifferedintheiryawnfrequencies.AGLMMpredicting
totalyawnspertestdaywithdummy-codedpredictorsforsex,condition,andtheirinterac-
tionsfoundnoeffectforsexandnointeractions(allpMCMCs>0.139).Sexwasthereforenot
includedinsubsequentanalyses.
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Arousalhypothesisofneonatalimitation
Yawningacrosstestdays
Neonatalimitationtestswereperformedoverthecourseoftheinfants’firstweekoflife,leav-
ingopenthepossibilitythatyawningratesmaychangeduringthisperiodoftime.Whilenot
themainvariableofinterestinthecurrentstudy,anychangeovertimecouldnonetheless
potentiallybiastheanalyses.Wethereforeexaminedyawnspersessionacrosstestdaysusinga
GLMMpredictingtotalyawnspersessionwithpredictorsforcondition,day,andtheirinterac-
tion.Daywasdefinedasacontinuousvariablewiththeinterceptsettobethefirsttestday
(day1or2oflife).Therewasasignificantamountofyawningonday1intheCTRLcondition
(i.e.,theintercept),b=-1.55,95%HDI[-1.86,-1.23],pMCMC<.001.Exponentiatingthe
intercept(e-1.55)returnedarateratio(RR)of.21,indicatingthatthemodel-predictednumber
ofyawnspersessiononday1was.21.Therewasalsoasignificanteffectforday,b=.16
(RR=1.17),95%HDI[.09,.22],pMCMC<.001,indicatingthateachadditionaldayleadtoa
17%increaseinpredictedyawnspersession.Daydidnotinteractwithcondition,indicating
thattheeffectofdaywasconsistentacrossconditions.Thus,yawnratessignificantlyincreased
withtestdayoverthefirstweekoflife(means:day1/2:0.48,day3/4:0.75,day5/6:1.04,day
7/8:1.16).Toaccountforthischangeovertimeinthemodel,inallsubsequentanalyses,unless
otherwiseindicated,weincludeddayasarandomeffect,butwedidnotincludeitasafixed
effectortestforinteractionswithothereffects.
Occurrencesofyawnsinrelationtotrialphasesandcondition
EachneonatalimitationtrialwasdividedintoastaticBaselinefollowedbyadynamic-static
(burst-pause)Stimulusphase.Sincethearousalhypothesispurportsthatfacialgesturesarethe
resultofarousal,wefirstexaminedlevelsofarousal(asmeasuredbyyawnfrequencies)during
theBaselineandStimulusphasestodeterminewhetheraparticularconditionmaybemore
arousingthantheothers.
Baselinephase. Weexploredwhetherthereweredifferencesacrossconditionsduringthe
BaselineperiodwithaBayesianlinearmixedmodelpredictingyawnsduringBaseline(aver-
agedacrosstestday;lowvarianceinthenumberofyawnsperbaselineperiodperdayledto
unstableparameterestimatesinacountmodelsimilartoourotheranalyses,thoughtheresults
weredirectionallysimilartowhatwereporthere),andincludedrandominterceptsforsubjects
andrandomslopesforcondition.BecausethestimulusduringtheBaselinephasewasidentical
fortheLPSandTPconditions(astillface),wejointlycomparedthesetwofaceconditions
againsttheCTRLconditionwithadummy-codedpredictor.Therewasasmallbutsignificant
amountofyawningintheCTRLconditionduringBaseline,b=.14,95%HDI[.10,.18],
pMCMC<.001.Therewasnotsignificantlymoreyawninginthefaceconditions,b=.03,
95%HDI[-.013,.067],pMCMC=.195,comparedtotheCTRLcondition(Fig1).Thus,the
datasuggestthatstaticfaceswerenotmorearousingthananonsocialcontrolobject.
Stimulusphase. WenextexploredtheStimulusphase,inwhichinfantsvieweddynamic
stimuli(facialgesturesorrotatingdisk),withaGLMMpredictingthenumberofyawnsduring
theStimulusphase.UnlikeintheBaselinephase,thestimuliweredifferentineachcondition,
soweusedseparatedummycodesfortheLPSandTPconditions.Therewasasignificant
amountofyawningduringtheStimulusphaseintheCTRLcondition,b=-1.12(RR=.33),
95%HDI[-1.32,-.93],pMCMC<.001.ThereweresignificantlymoreyawnsintheLPScon-
ditioncomparedtoCTRL,b=.23(RR=1.26),95%HDI[.07,.39],pMCMC=.005,andthere
weresignificantlymoreyawnsintheTPconditioncomparedtoCTRL,b=.23(RR=1.26),
95%HDI[.06,.40],pMCMC=.009.Re-runningthemodelwithdummyvariablesrecodedto
setLPSastheinterceptshowedthattherewasnosignificantdifferencebetweenLPSandTP,
b=.01,95%HDI[-.17,.16],pMCMC=.998.Overall,duringtheStimulusphaseinfants
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Arousalhypothesisofneonatalimitation
Fig1.BoxplotsoftheaveragerateofyawningperminuteduringBaseline(white)andStimulus
(grey).LPS=lipsmackingcondition,TP=tongueprotrusioncondition,CTRL=controlcondition.Boxplots
depict1stand3rdquartile,linedepictsmedian,whiskersdepict1.5timesinter-quartilerange(IQR).Circles
representmildoutlierslargerthanQ3+1.5*IQR,asterisksrepresentextremeoutlierslargerthanQ3+3
*IQR.
https://doi.org/10.1371/journal.pone.0178864.g001
yawnedapproximately26%moreduringLPSandTPthanCTRLwithnodifferencebetween
LPSandTP(Fig1),suggestingthatdynamicfacialdisplaysmaybemorearousingthannon-
socialdynamicdisplays,regardlessofthespecificfacialgesture.
ChangefromBaselinephasetoStimulusphaseacrossconditions. Thepreviousanalysis
suggeststhatdynamicfacialstimulimaybemorearousingthanthedynamiccontrolobject,
whichpotentiallyholdsimplicationsforneonatalimitationoffacialgestures.However,neona-
talimitationiscommonlyassessedbyevaluatingtheincreaseinmatchingresponsesbetweena
BaselineandaStimulusphase.Ifanincreaseinarousalwereresponsibleforanincreasein
facialgestures,infantsshouldalsoshowarelativelylargerincreaseinyawningbetweenBase-
lineandStimulusintheLPSandTPconditionscomparedtotheCTRLcondition.Totestthis
hypothesis,weranaBayesianlinearmixedmodelwithdummycodesforcondition(LPSand
TPwerecodedseparately),predictingadifferenceintherateofyawnsperminuteduring
StimulusversusBaseline(averagedacrosstestday),andincludedrandominterceptsforsub-
jects.Therewasasignificantincreaseintherateofyawns/minintheCTRLcondition,b=.16,
95%HDI[.10,.22],pMCMC<.001.NeitherthedummycodeforLPS(b=.06,95%HDI
[-.01,.13],pMCMC=.115)northedummycodeforTP(b=.03,95%HDI[-.04,.11],
pMCMC=.420)weresignificant,indicatingthattheincreasesinyawningbetweenBaseline
andStimulusintheLPSandTPconditionswerenotdifferentfromtheincreaseinyawning
betweenBaselineandStimulusintheCTRLcondition.Theseresultssuggestthateventhough
infantsyawnedmoreoverallduringtheStimulusphaseofLPSandTPcomparedtoCTRL,the
changeinyawningbetweenBaselineandStimulusdidnotdifferacrossconditions.Giventhat
imitationisinferredbasedonthedifferenceinfacialgesturesbetweenBaselineandStimulus,
thisresultdoesnotsupporttheideathatarousalincreasesspecificallyintheLPSorTPcondi-
tionsandcouldtherebyleadtoincreasedfacialgestures.
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Arousalhypothesisofneonatalimitation
Isarousalsufficienttoexplainfacialgesturesinallinfants?Imitatorsvs.
Non-imitators
Theresultsabovesuggestthatarousalmayincreaseoverthecourseofaneonatalimitationtest
asevidencedbythehigherrateofyawningduringtheStimulusphase,particularlyintheLPS
andTPconditions.However,theincreasesinyawningbetweenBaselineandStimuluswere
similaramongallthreeconditions.Sinceimitationisinferredifafacialgestureincreases
betweenaBaselineandaStimulusphasespecificallyintheconditionwherethefacialgesture
ispresentedtoinfants,thearousalhypothesiswouldpredictanincreaseinarousal(i.e.,yawn-
ing)alsospecificallyinthematchingcondition,whichwedidnotobserve.Thus,whilethe
resultssuggestthatarousalmaycontributetofacialgesturesbeingperformedintheStimulus
phase,arousalisinsufficienttoexplainwhysomeinfantsincreasefacialgesturesspecificallyin
thematchingcondition.Theseinfants,whichwehavecalledImitatorsinthepast,areidenti-
fiedbyaveraginglipsmackingortongueprotrusiongesturerates(per60sec)fromBaseline
andStimulusacrossalltestdaysineachcondition.InfantsareImitatorsiftheyproducean
increaseinlipsmackingortongueprotrusionfromtheBaselinetotheStimulusphaseinthe
LPSorTPconditiontoagreaterextentthantheyincreaselipsmackingortongueprotrusion
fromtheBaselinetotheStimulusphaseintheCTRLcondition(seealso[22;23]).Ifarousal
wasresponsibleforthisincreaseinfacialgestures,thenwewouldpredictahigherfrequencyof
yawninginImitatorscomparedtoNon-imitators.
Toexplorethisideafurther,weassessedtheimitatorstatusofthecurrentsample.Weiden-
tified79lipsmackingImitatorsand84lipsmackingNon-imitators,and49tongueprotrusion
Imitatorsand87tongueprotrusionNon-imitators.Wethenrepeatedouraboveanalysesto
includeImitator/Non-imitatorasabetween-subjectfactortoexplorewhetherImitatorsand
Non-Imitatorsdifferedintheirgeneralarousallevel(asseenbyadifferenceinyawningrates).
Baselinephase—LipsmackingImitators. Poolingacrossallconditions,weexplored
whetherImitatorsandNon-imitatorsdifferedintheiryawnratesduringtheBaselineperiod
usingaBayesianlinearmixedmodelwithadummycodeforlipsmackingimitatorstatuspre-
dictingyawnsduringBaseline(averagedacrossdays;seefootnote1)withrandomintercepts
forsubjectsandrandomslopesforcondition.Theinterceptofthemodelwassignificant,
b=.14,95%HDI[.10,.19],pMCMC<.001,indicatingthatNon-imitatorsyawnedasignifi-
cantamount.TherewerenodifferencesbetweenImitatorsandNon-imitators,b=.02,95%
HDI[-.03,.08],pMCMC=.418.Next,weaddedadummycodeforthefaceconditions(LPS
andTPcombined,sincetheywereindistinguishable,i.e.bothstillfacesduringthebaseline)
versustheCTRLconditionintothemodel,aswellasaconditionbyimitatorstatusinteraction.
Theinterceptofthemodelwassignificant,b=.11,95%HDI[.06,.17],pMCMC<.001,indi-
catingthatNon-imitatorsintheCTRLconditionyawnedasignificantamount.However,
Non-imitatorsdidnotyawnsignificantlymoreintheLPSandTPconditionsthaninthe
CTRLcondition,b=.05,95%HDI[-.005,.11],pMCMC=.064.Thedummycodedpredictor
forimitatorstatuswasnotstatisticallysignificant,b=.06,95%HDI[-.02,.14],pMCMC=.14,
norwastheimitatorstatusbyconditioninteraction,b=-.05,95%HDI[-.13,.03],pMCMC=
.190,indicatingthatImitatorsdidnotyawnmorethanNon-imitatorsineitherthefacecondi-
tionsortheCTRLcondition(Fig2).
Baselinephase—TongueprotrusionImitators. Next,were-ranthesamemodelswitha
dummycodefortongueprotrusionimitatorstatusratherthanlipsmackingimitatorstatus.In
thefirstmodelpoolingacrossconditions,theinterceptwassignificant,b=.15,95%HDI[.11,
.18],pMCMC<.001,indicatingthatNon-imitatorsyawnedasignificantamount.Therewere
nodifferencesbetweenImitatorsandNon-imitators,b=-.01,95%HDI[-.08,.04],pMCMC=
.639.Next,weaddedadummycodeforthefaceconditions,aswellasaconditionbyimitator
PLOSONE|https://doi.org/10.1371/journal.pone.0178864 June15,2017 8/16
Arousalhypothesisofneonatalimitation
Fig2.BoxplotsoftheaveragerateofyawningperminuteduringBaseline(white)andStimulus(grey)
forlipsmackingNon-imitators(top)andImitators(bottom).LPS=lipsmackingcondition,TP=tongue
protrusioncondition,CTRL=controlcondition.Boxplotsdepict1stand3rdquartile,linedepictsmedian,
whiskersdepict1.5timesinter-quartilerange(IQR).CirclesrepresentmildoutlierslargerthanQ3+1.5*IQR,
asterisksrepresentextremeoutlierslargerthanQ3+3*IQR.
https://doi.org/10.1371/journal.pone.0178864.g002
statusinteraction.Theinterceptofthemodelwassignificant,b=.13,95%HDI[.08,.18],
pMCMC<.001,indicatingthatNon-imitatorsintheCTRLconditionyawnedasignificant
amount.Thedummycodedpredictorforimitatorstatuswasnotstatisticallysignificant,
b=-.02,95%HDI[-.11,.06],pMCMC=.550,norwastheimitatorstatusbyconditioninterac-
tion,b=.02,95%HDI[-.07,.10],pMCMC=.727,indicatingthatImitatorsdidnotyawn
morethanNon-imitatorsineitherthefaceconditionsortheCTRLcondition(Fig3).
Stimulusphase—LipsmackingImitators. WeranaBayesianGLMMpredictingyawns
duringtheStimulusphasewithadummy-codedpredictorforlipsmackingimitatorstatus.
Non-imitatorsyawnedasignificantamountduringtheStimulusphasewhencollapsedacross
conditions,b=-1.09(RR=.34),95%HDI[-1.31,-.86],pMCMC<.001.Imitatorsdidnot
yawnmorethanNon-imitators,b=.22(RR=1.25),95%HDI[-.08,.51],pMCMC=.138.
Addingdummy-codedpredictorsforcondition(LPSandTPcodedseparately)tothemodel,
alongwithconditionbyimitatorstatusinteractions,didnotrevealanysignificantcondition
byimitatorstatusinteractionsinyawningduringtheStimulusphase,pMCMCs>.501.Thus,
ImitatorsdidnotyawnmorethanNon-imitatorsintheStimulusphase,andImitatorsand
Non-imitatorsyawnedsimilarlyacrossconditions(Fig2).
Stimulusphase—TongueprotrusionImitators. Next,were-ranthesamemodelswitha
dummycodefortongueprotrusionimitatorstatusratherthanlipsmackingimitatorstatus.
Non-imitatorsyawnedasignificantamountduringtheStimulusphasewhencollapsedacross
conditions,b=-1.04(RR=.35),95%HDI[-1.27,-.82],pMCMC<.001.Imitatorsdidnot
yawnmorethanNon-imitators,b=.04(RR=1.04),95%HDI[-.29,.39],pMCMC=.822.
Addingdummy-codedpredictorsforconditiontothemodel,alongwithconditionbyimitator
statusinteractions,didnotrevealanysignificantconditionbyimitatorstatusinteractionsin
yawningduringtheStimulusphase,pMCMCs>.135.Imitatorsdidnotyawnmorethan
PLOSONE|https://doi.org/10.1371/journal.pone.0178864 June15,2017 9/16
Arousalhypothesisofneonatalimitation
Fig3.BoxplotsoftheaveragerateofyawningperminuteduringBaseline(white)andStimulus(grey)
fortongueprotrusionNon-imitators(top)andImitators(bottom).LPS=lipsmackingcondition,
TP=tongueprotrusioncondition,CTRL=controlcondition.Boxplotsdepict1stand3rdquartile,linedepicts
median,whiskersdepict1.5timesinter-quartilerange(IQR).Circlesrepresentmildoutlierslargerthan
Q3+1.5*IQR,asterisksrepresentextremeoutlierslargerthanQ3+3*IQR.
https://doi.org/10.1371/journal.pone.0178864.g003
Non-imitatorsintheStimulusphase,andImitatorsandNon-imitatorsyawnedsimilarly
acrossconditions(Fig3).
ChangefromBaselinephasetoStimulusphaseacrossconditions. WeranBayesianlin-
earmixedmodelswithdummycodesforcondition,imitatorstatus,andtheirinteractionspre-
dictingadifferencescoreofyawnsperminuteduringStimulusversusBaseline(averaged
acrosstestday),includingrandominterceptsforsubjects.Therewasasignificantincreasein
therateofyawns/minforlipsmackingNon-imitatorsintheCTRLcondition,b=.15,95%
HDI[.07,.23],pMCMC<.001.Nootherterms,includinginteractionsbetweencondition
andimitatorstatus,weresignificant(pMCMCs>.155),indicatingthattherewerenosignifi-
cantdifferencesintheincreaseinyawnratesbetweenBaselineandStimulusinanycondition
orbetweenlipsmackingImitatorsandNon-imitators.Thatis,therewasmoreyawninginthe
StimulusphasethanintheBaselinephaseinallconditions,withnodifferenceintheincrease
ofyawningbetweenImitatorsandNon-imitators.
Therewasalsoasignificantincreaseintherateofyawns/minbetweenBaselineandStimu-
lusfortongueprotrusionNon-imitatorsintheCTRLcondition,b=.17,95%HDI[.10,.26],
pMCMC<.001.ThedummycodefortheLPSconditionwasalsosignificant,b=.10,95%
HDI[.001,.20],pMCMC=.045,indicatingthatNon-imitatorsintheLPSconditionincreased
theiryawns/minmorethandidNon-imitatorsintheCTRLcondition.Nootherterms,includ-
inginteractionsbetweenconditionandimitatorstatusweresignificant(pMCMCs>.204),
indicatingthattherewasnosignificanteffectofimitatorstatusinanycondition.Inother
words,therewasnoevidencetosupporttheideathattongueprotrusionImitatorsincrease
theiryawningratebetweenBaselineandStimulusintheTPconditionmorethantonguepro-
trusionNon-Imitators.
PLOSONE|https://doi.org/10.1371/journal.pone.0178864 June15,2017 10/16