The Processing of Human Emotional Faces by Pet and Lab Dogs

22 Pages·2016·3.29 MB·English

RESEARCHARTICLE
The Processing of Human Emotional Faces by
Pet and Lab Dogs: Evidence for Lateralization
and Experience Effects
AnjuliL.A.Barber*,DaniaRandi,CorsinA.Müller,LudwigHuber
ComparativeCognition,MesserliResearchInstitute,UniversityofVeterinaryMedicineVienna,Medical
UniversityofViennaandUniversityofVienna,Vienna,Austria
*[email protected]
a11111 Abstract
Fromallnon-humananimalsdogsareverylikelythebestdecodersofhumanbehavior.In
additiontoahighsensitivitytohumanattentivestatusandtoostensivecues,theyareable
todistinguishbetweenindividualhumanfacesandevenbetweenhumanfacialexpres-
sions.However,sofarlittleisknownabouthowtheyprocesshumanfacesandtowhat
OPENACCESS extentthisisinfluencedbyexperience.Herewepresentaneye-trackingstudywithdogs
emanatingfromtwodifferentlivingenvironmentsandvaryingexperiencewithhumans:pet
Citation:BarberALA,RandiD,MüllerCA,HuberL
(2016)TheProcessingofHumanEmotionalFaces andlabdogs.Thedogswereshownpicturesoffamiliarandunfamiliarhumanfaces
byPetandLabDogs:EvidenceforLateralizationand expressingfourdifferentemotions.Theresults,extractedfromseveraldifferenteye-track-
ExperienceEffects.PLoSONE11(4):e0152393.
ingmeasurements,revealedpronounceddifferencesinthefaceprocessingofpetandlab
doi:10.1371/journal.pone.0152393
dogs,thusindicatinganinfluenceoftheamountofexposuretohumans.Inaddition,there
Editor:KunGuo,UniversityofLincoln,UNITED
wassomeevidencefortheinfluencesofboth,thefamiliarityandtheemotionalexpression
KINGDOM
oftheface,andstrongevidenceforaleftgazebias.Thesefindings,togetherwithrecent
Received:December15,2015
evidenceforthedog'sabilitytodiscriminatehumanfacialexpressions,indicatethatdogs
Accepted:March14,2016 aresensitivetosomeemotionsexpressedinhumanfaces.
Published:April13,2016
Copyright:©2016Barberetal.Thisisanopen
accessarticledistributedunderthetermsofthe
CreativeCommonsAttributionLicense,whichpermits
unrestricteduse,distribution,andreproductioninany Introduction
medium,providedtheoriginalauthorandsourceare
Animportantperceptual-cognitivechallengeforalmostallanimalsistofindacompromise
credited.
between'lumpingandsplitting'whenconfrontedwiththevastamountofinformationarriving
DataAvailabilityStatement:Allrelevantdataare
attheirsenses[1].Animalsneedtocategorize,i.e.,totreatanobjectasbelongingtoageneral
withinthepaperanditsSupportingInformationfiles.
class,butbesidescategorizationofobjects,theyfurtherneedtorecognize,i.e.,totreatdifferent
Funding:Theresearchpresentedherewasfunded
imagesasdepictingthesameobject,suchasaspecificface,despitechangesintheviewingcon-
bytheViennaScienceandTechnologyFunds
ditions.Thebrainofmanyanimalssolvesbothtasksinanatural,effortlessmannerandwith
(WWTF),No.CS11-005;http://www.wwtf.at/)toLH.
anefficiencythatisdifficulttoreproduceincomputationalmodelsandartificialsystems.Still,
Thefundershadnoroleinstudydesign,data
collectionandanalysis,decisiontopublish,or howthisisaccomplishedisfarfrombeingfullyunderstood.
preparationofthemanuscript. Anenormouslyrichsourceofinformationandanimportantcategoryofvisualstimulifor
animalsinallmajorvertebratetaxaistheface[2].Inmanyspeciesfacesconvey,amongother
CompetingInterests:Theauthorshavedeclared
thatnocompetinginterestsexist. things,informationaboutdirectionofattention,age,gender,attractivenessandcurrent
PLOSONE|DOI:10.1371/journal.pone.0152393 April13,2016 1/22
ProcessingofHumanEmotionalFacesbyDogs
emotionoftheindividual(forreviewssee[3–5]).Almost50yearsagopsychologistshavedem-
onstratedthatfacialexpressionsarereliablyassociatedwithcertainemotionalstates[6,7].This
inturncanleadtobehavioraldecisionsofareceiverconcerninge.g.cooperation,competition,
consolidationorformationofrelationships.Someprimates,forinstance,seemtohaveevolved
specialabilitiesforreadingfacesduetotheircomplexsociallives(e.g.[8–10]).
Whilethecategorization,interpretationandidentificationoffacesfromconspecificsis
likelyaconsequenceofthesociallife,whichmayhaveresultedinneuralspecializationforfaces
[11],itisafurtherchallengetoachieveatleastsomeoftheseabilitieswithfacesofheterospeci-
fics.Theconfigurationoftheface,theunderlyingfacialmusclesandtheresultingexpressions
aremoreorlessdifferentfromtheown,dependingonhowtaxonomicallydistanttheother
speciesis[12,13].Nevertheless,avarietyofanimalshavebeenshowntobeabletoidentifyand
categorizefacesandalsoemotionsofheterospecifics(e.g.macaques[14],sheep[15],horses
[16]).Severalinvestigationsoncon-andheterospecificfaceprocessingsuggestlearnedaspects
intheinformationtransferandspeed(c.f.faceexpertise,[17–22]).Herebyindividualsareable
torecognizeanddiscriminatebestbetweenfacessimilartothosethataremostoftenseenin
theenvironment.Forinstance,theinfluenceofindividualexperiencewiththeotherspecies
wasshownforurbanlivingbirdslikemagpies[23]andpigeons[24].Especiallyearlyexposure
tofacesfacilitatestheabilityoffacediscrimination[25,26].Althoughsomeindividualsareable
tolearnaboutheterospecificfacesalsolaterinlife(e.g.chimpanzees[27],rhesusmacaques
[28]),theywillnotreachthesamehighlevelofcompetence[12,21].Itremainsanopenques-
tion,however,whethertheimprovedabilitiestoreadfacesduetoearlylifeexposurearecaused
solelybyanacquiredearlysensitivityforfaces(innatemechanisms)orsimplybythelarger
amountofexperience(learnedresponses).
Anidealmodeltostudysuchquestionsaboutexperienceforheterospecificfacesisthe
domesticdog.Petdogsliveinanenduringintimaterelationshipwithhumans,oftenfrom
puppyageon.Asreviewedinseveralrecentbooks[29–32],dogshavedevelopedspecificsocio-
cognitivecapacitiestocommunicateandformrelationshipswithhumans.Mostlikelycaused
byamixtureofphylogenetic(domestication)andontogenetic(experience)factors,dogsliving
inthehumanhouseholdshowhighlevelsofattentivenesstowardshumanbehavior(reviewed
in[33]),followhumangestureslikenootheranimal,(e.g.[34]),exhibitahighsensitivityto
humanostensivesignals,likeeyecontact,namecallingandspecificintonation[35]andshow
anincreasedreadinesstolookatthehumanface[36].Incomparisontohand-rearedwolfpup-
pies,thelatterisalreadypresentindogpuppies([37];seealso[38]).Bymonitoringhuman
faces,dogsseemtoobtainacontinuousstreamofsocialinformationrangingfromcommuni-
cativegesturestoemotionalandattentivestates.Evenifthisdoesnotmeanthatdogsareread-
ersofourmindsbutonlyexquisitereadersofourbehavior[39],theyevaluatehumansonthe
basisofdirectexperiences[40]andaresensitivetowhathumanscansee(aformofperspec-
tive-taking;reviewedbyBräuer[41]).
Aspecialsensitivityforfacesindogsisfurthersupportedbytworecentfindings.AnfMRI
studyinawakedogshasrevealedthataregioninthetemporalcortexisfaceselective(it
respondstofacesofhumansanddogsbutnottoevery-dayobjects,[42]).Dogsalsohavebeen
foundtoshowagazebiastowardsfaces[43,44].Still,thereismixedevidenceforthelateraliza-
tiontowardsconspecificsandheterospecifics.Dogsshowedahuman-likeleftgazebiastoward
humanfacesbutnottowardmonkeyordogfaces[44].Thisleftgazebiaswasexhibitedonly
towardnegativeandneutralexpressions,however,butnottowardpositiveexpressionsof
humanfaces[43].Ithasbeenproposedthattherighthemisphereisresponsiblefortheexpres-
sionofemotionsandthattheassessmentofemotionsthereforeleadstoaleftgazebiastowards
suchstimuli(righthemispheremodel,[45,46]).Incontrasttotherighthemispheremodel,the
valencemodel[47,48]suggeststhatthelefthemispheremainlyprocessespositiveemotions
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ProcessingofHumanEmotionalFacesbyDogs
whiletherighthemispheremainlyprocessesnegativeemotions.Sofaravarietyofspecies(e.g.
humans[49,50],apes[51],monkeys[52],sheep[53],dolphins[54],dogs[43,55–57],etc.)have
beenshowntoshowlateralizationtowardsemotivestimuli(reviewedbyAdolphs[58]and
Salva[59]).
Thedog'sfollowingofthehumangazeand,moregenerally,thevisualscanningoffaces
requiretheapplicationofadvancedtechnicaltools.Themostcommonlyusedequipmentin
humanpsychophysicallaboratoriesistheeye-tracker[60,61].Besidesthegaze-followingstudy
byTéglásandcolleagues[62],twostudieshavesofarusedthistechnologyfortheinvestigation
ofthelookingbehaviorofdogs.Inakindofproof-of-conceptstudy,Somppiandcolleagues
[63]showedthatdogsarelookingwithahigherfrequencytotheinformativepartsofapicture
andthattheydirectmoreattentiontoconspecificsandhumanscomparedtoobjects.Inafol-
low-upstudyaddressingthelookingatfaces,theresearcherscoulddemonstratethatdogspay
moreattentiontotheeyeregioncomparedtotherestoftheface[64].Thefindingthatthe
meanfixationswerelongerforconspecificcomparedtoheterospecific(human)facessuggests
thatdogsprocessfacesinaspecies-specificmanner.Theuseoftheeye-trackerrevealedsubtle
differencesinlookingpatternsbetweendogslivingindifferenthumanenvironments:petdogs
andkenneldogs.Still,bothtypesofdogsshowedapreferencefortheownspecies,familiarpic-
turesandtheeyeregion.Overall,thesefindingssuggestthatthelivingconditionsofdogsaffect
thewaytheylookatus.However,itisnotclearwhetherthisisduetothelimitedtimeinclose
relationshipwithhumansortothedevelopmentofhumanfaceprocessingduringearlylife.
Thiseye-trackingstudyaddressesthisquestionbycomparingthelookingathumanfaces
betweenpetandlabdogs.Forbothtypesofdogsthehumanenvironmentduringtheirwhole
lifeisknown,especiallythedifferenceintheamountofexposuretohumansandthequalityof
thehuman-dogrelationship.Afurtherwaytoinvestigatetheroleofindividualexperiencewith
humanfacesistocomparethedogs'lookingatfamiliarandunfamiliarfaces.Itisalready
knownthatdogsareabletodiscriminatebetweenfamiliarandunfamiliarfaces,eitherimplic-
itlybyexhibitinglookingpreferences[65,66]orbymakingthediscriminationexplicitinatwo-
choicetask[67].However,itisnotyetknownifdogsshowpreferencesandrespondtodiffer-
encesbetweenfamiliarandunfamiliarfacesonlyifforcedtodosoorifdogsspontaneously
scanfamiliarfacesandunfamiliarfacesdifferently.
Afurtherinterestingbutnotyetinvestigatedinfluenceonthedog'slookingpatternat
humanfacesisthefacialexpressionasaresultofthehuman'semotion.Recentlywecould
show—confirmingandextendingthefindingsofNagasawaandcolleagues[68]–thatdogscan
learntodistinguishbetweentwofacialexpressionsofthesameunfamiliarhumanpersonsand
togeneralizethisabilitytonovelfaces[69].Importantly,ourstudysuggestedthisabilityisnot
dependentontheexploitationofsimplefeatureslikethevisibilityofteethbutratheronthe
memoryofhowdifferenthumanfacialexpressionslooklike.Thesememoriesarelikelybeing
formedineverydayinteractionswithhumans,especiallywiththedogs'humanpartner(s).
Therefore,inadditiontopossibleeffectsofexperience(petdogsvs.labdogs,familiarfaces
vs.unfamiliarfaces),thisstudyfocussesonthequestiononhowdogsprocessemotionalfaces
(angry,happy,sad,andneutral).Inlinewithapreviouseye-trackingstudy[64],weexpected
thatlabdogs,duetotheirlimitedexperiencewithhumans,wouldshowlongerlatencies,
shorterfixationdurationsandatendencytofixatemoreonfamiliarfacescomparedtopet
dogs.Furtherwehypothesizedthatthereisadifferencebetweentheprocessingofafamiliar
andunfamiliarfaceastheemotionalexpressionsofafamiliarfaceshouldbeprocessedfaster
duetopreviousexperiencewiththeface.Basedonthefindingsofourrecentstudy[69],we
expectedthatthefouremotionalexpressionsareprocesseddifferentlyandelicitadifferent
amountofattention(i.e.differingamountoffixationsandfixationdurations).Additionally,in
thispreviousstudydogsrequiredmoretrainingtodiscriminatehappyandangryfacesifthey
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hadtotouchtheangryface.Thisisincontrasttodogsthathadtotouchthehappyface.The
resultsindicatethattheyavoidedapproachingtheangryfaces.Thereforeweexpectedthatthey
fixatefacesthatareexpressingnegativeemotionsshorterandlessoften.Concerningthescan-
ningpattern,weexpectedthatmoreinformativepartsoftheface(eyeandmouthregion)are
fixatedmorefrequently.Finally,weexpectedthedogstoshowagazebiastowardsthefaces.
Methods
AllexperimentalprocedureswereapprovedinaccordancewithGPSguidelinesandnational
legislationbytheEthicalCommitteefortheuseofanimalsinexperimentsattheUniversityof
VeterinaryMedicineVienna(Ref:09/08/97/2012).Alldogownersgavewrittenconsenttopar-
ticipateinthestudy.Theindividualinthefiguresofthismanuscripthasgivenwritten
informedconsent(asoutlinedinPLOSconsentform)topublishthesecasedetails.
Subjects
Nineteenprivatelyownedpetdogsandeightlaboratorydogsparticipatedinthestudy(see
Table1).Thepetdogswereofvariousbreedsandtheiragerangedbetween1–12years.Dog
ownersofthepetdogsspentonaverage27h/weekactivelywiththeirdogs(walking,playing
andtraining).Withtheexceptionoftwodogs,alldogswereatleastonetotwotimesaweek
activeindogsportsactivitieslikeagility,dogdance,assistantdogtraining,mantrailingetc.
Thetworemainingdogshadonlytrainingasa“Begleithund”andobediencetrainingona
dailybasisbytheirowners.Theirownersgavewrittenconsenttoparticipateinthestudy.The
laboratorydogswereallbeagles,aged1–5years,andwerehousedonthecampusoftheUniver-
sityofVeterinaryMedicineViennainpacksofeither13(ClinicalUnitofInternalMedicine
SmallMammals(CU-IM),L1-L6inTable1)or4dogs(ClinicalUnitofObstetrics,Gynecology
andAndrology(CU-OGA),L7andL8inTable1).Noneofthelaboratorydogseverhaddone
anyactivityindogssportoreverparticipatedinprofessionalobediencetraining.Theywere
bornattheClinicalUnitofObstetrics,GynecologyandAndrologyattheUniversityofVeteri-
naryMedicineVienna.Afterweaningofffromthemotherattheageoftwomonth,thedogs
werelivinginpacks.Thehousingofbothgroupsconsistedofanindoorandoutdoorenclosure
(CU-IM:indoorenclosure=64qm,outdoorenclosure=248qm;CU-OGA:indoor
enclosure=18qm,3outdoorenclosures=~70qm),inwhichtheycouldmovefreelyduringthe
day.From22:00–6:00o´clockthedogswerekeptintheindoorenclosures.Theenclosures
wereenrichedwithdogtoys(chewing,squeakingtoysorballs).Theircontacttohumanswas
limitedtothedailyfeeding(onceadayinthemorning,wateradlibitum)andcleaningofthe
enclosuresoftheanimalkeepers.Additionallytheywereparticipatinginpracticalcoursesfor
thetrainingofstudentsofveterinarymedicine(approx.1–2timesasemester).However,none
ofthedogswassocializedinaclassicalway,astheywerelivingonlywiththeirconspecifics,but
notwithhumans.
Experimentalsetup
AllexperimentswereconductedattheCleverDogLaboftheUniversityofVeterinaryMedi-
cine,Vienna.Theexperimentalroomwasdividedbyawall,consistingofaprojectionscreen
(200x200cm)andtwodoors,intotwocompartments;asmallone(149x356cm)housingthe
computersystemoperatingtheeyetrackerandavideoprojector(NECM300XS,NECDisplay
Solutions,UnitedStates),andalargerone(588x356cm)withthechinrestdeviceandtheeye
tracker(seeFig1).Thestimuliwereback-projectedontotheprojectionscreen(sizeprojection
area110x80cm).WeusedtheeyetrackingsystemEyelink1000(SRResearch,Ontario,Can-
ada)torecordmonoculardatafromthesubjects.Thissystemisidealfordogresearchbecause
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ProcessingofHumanEmotionalFacesbyDogs
Table1. Listofparticipatingdogs.
DogID(a) Breed Age(b) Sex(c) Status ValidSessions ValidTrials
P1 AustralianKelpie 1 m neutered 4 59
P2 AustralianShepherd 1 m intact - -
P3 AustralianShepherd 2 f neutered 2 14
P4 AustralianShepherd 3 m intact 5 56
P5 BerneseMountainDog 2 f neutered 2 27
P6 BergerBlancSuisse 5 m intact 2 15
P7 BorderCollie 1 m intact 5 76
P8 BorderCollie 6 m neutered 4 60
P9 BorderCollie 8 m neutered 5 57
P10 GermanShepherd 6 f neutered 4 68
P11 HungarianViszla 3 m neutered 5 60
P12 IrishWolfshound 1 f intact 3 31
P13 Mongrel(AmericanBulldog-Mix) 11 m neutered 3 37
P14 Mongrel(Beauceron-Husky-Mix) 3 m neutered 4 61
P15 Mongrel(Bracke-Shepherd-Mix) 8 m neutered 4 42
P16 Mongrel(Mudi-Schnauzer-Mix) 2 f neutered 5 65
P17 Mongrel(Spitz-Peke-Mix) 11 m neutered - -
P18 StandardPoodle 6 m intact 3 51
P19 Puli 1 f intact 5 68
L1 Beagle 3 m neutered 4 62
L2 Beagle 4 m neutered 5 70
L3 Beagle 1 m neutered 5 75
L4 Beagle 4 m neutered 5 57
L5 Beagle 1 m neutered 5 69
L6 Beagle 3 m neutered 5 64
L7 Beagle 1 m intact 5 69
L8 Beagle 1 m intact 4 51
aP=petdog,L=labdog.
bAgeinyears.
cm=male,f=female.
doi:10.1371/journal.pone.0152393.t001
Fig1.Experimentalsetup.(a)Lateralviewofchinrestdevice,projectionscreenandfeedingmachines
seenfromtherearoftheroomduringtrainingwithgeometricalfiguresand(b)frontviewofthechinrest
deviceincludingtheeye-trackingcamerawithIRilluminator.
doi:10.1371/journal.pone.0152393.g001
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ProcessingofHumanEmotionalFacesbyDogs
itcanbeusedwithachinrestorwithoutanyheadsupportandwitharemotecameraifhead
fixationisnotdesirable,buthighaccuracyandresolutionarestillimportant.Weusedacus-
tomizedchin-restdeviceforheadstabilization.Apillowwithav-shapeddepressionwas
mountedonaframetoallowverticaladjustmentofthechinresttotheheightoftheindividual
dog.Theframeconsistedofaluminiumprofiles(MayTecAluminiumSystemtechnikGmbH,
Germany)thatallowedtheeasilyadjustablebutstablefixationofadditionalequipment(e.g.
cameras).Thechinrestwaspositionedatadistanceof200cmfromtheprojectionscreen.The
eye-trackingcamerawiththeinfraredilluminatorwasmountedonanextensionofthechin-
restframe(seeFig1).Thisapparatuswasalignedhorizontallywiththechinrestatadistance
of50cm.Lightconditionsintheroomwerekeptconstantlyat75luxusingLED-lightbulbs
(9,5W,2700k,PhilipsGmbHMarketDACH,Germany).
Stimuli
Thestimulussetforeachsubjectconsistedofphotographsofthefaceofthedogs'owner(for
thelabdogsthedogtrainer,whotrainedthemfortheexperimentaltests)andofanunfamiliar
personexpressingfourdifferentemotions:neutral,happy,angryandsad(Fig2).Allowners
andallunfamiliarhumanswerefemale.Thephotographsweretakenbyexperimenter1(E1)in
astandardizedsetupattheCleverDogLabinfrontofawhitebackground.Foreachdogthe
facesoftheownerservedasfamiliarstimuliandthefacesofanotherdog´sownerparticipating
inthestudywereusedasunfamiliarstimuli.Fortheunfamiliarstimuli,apersonwaschosen
whodidnotdifferstronglyfromtheownerinappearance(e.g.haircolor).Thestimuliwere
projectedontothescreenwith1024x768pxresolution,resultingin900x850mmsizedpic-
tureswhichcorrespondsto25.36°x23.99°ofvisualangle(head:approx.400x300mmcorre-
spondingto11.42°x8.58°ofvisualangle).
Thestimuliwerevalidatedconcerningtheiremotionalexpressionandvalenceafterwardsby
67neutralobserversviaanonline-questionnaire.Allparticipantswererecruitedviatheinternet
(17male,50female,agerange16–55years).Theparticipantswereaskedforeverypictureto
ratetheemotionangry,happy,neutralandsadonscalefrom0–100%(e.g.Picturexy=0%
angry,0%happy,30%neutral,70%sad,thefournumbershadtoaddto100%).Foreverypic-
turedataofallratingsforthecertainemotionswerepooledanditwasdecidedifthepicturewas
identifiedcorrectly.Identificationofthecorrectemotionwasgivenwhenratedasthepredomi-
nantlyshownemotion,higheratleast15%thanthesecondhighestratedemotion,e.g.ifan
angrypicturewasratedwith62%angry,0%happy,23%neutraland13%saditwasconsidered
ascorrectlyidentified.Incontrast,ifthesamepicturewasratedwith52%angry,0%happy,38%
neutraland10%sadwouldnothavebeenconsideredascorrectlyidentified.Validationrevealed
thatfortheangrycondition74%,happy95%,neutral100%andsad60%ofthepictureswere
identifiedcorrectly.Thenegativeemotionswereusuallymistakenwithaneutralexpression
(angryin80%ofthecases,sadin88%ofthecases).Incorrectratedpictureswereleftintheanal-
ysisaswewantedthedogownerstoexpressemotionalexpressionsasnaturalaspossible.This
resultedinexpressionswhichcouldbeambiguousfornon-informedobservers.
Procedure
Training. Priortotheexperiment,thedogsweretrainedfortheexperimentalprocedure
(for2–6months).TrainingwasdonebyE1andE2.Weusedoperantconditioningwithaposi-
tivereinforcement(foodrewards)andaclickerasasecondaryreinforcer.Thedogswere
trainedonceortwiceaweek,dependingontheiravailability.Theownerofthedogwasallowed
tostayintheroomduringthewholeprocedure.Shewaspositionedonachairatthebackof
theroomandinstructedtoavoidinteractionwiththedog.
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ProcessingofHumanEmotionalFacesbyDogs
Fig2.Examplesoftheexperimentalstimuli.
doi:10.1371/journal.pone.0152393.g002
Thetrainingconsistedofthreesteps.Inthefirststepthedogwastrainedtogotothechin
restdeviceandtoresttheheadonthechinrestforatleast10sec(seeFig1a).Afterthedoghad
learnedtostayinthechinrestitwasconfrontedwithatwo-choiceconditionaldiscrimination
taskwithgeometricalfigures(GFs).Withthistaskweaimedtoincreasethedog’sattentionto
thescreen(upto20sec)andtomakethetrainingmoreinteractive.Furthermore,thisstepwas
includedtopreparethedogforthecalibrationprocedureduringwhichthedoghadtofollow
thepositionof3dotsonthescreen(seebelow).Inthelasttrainingstep,picturesoflandscapes,
animals,architectureandhumans(butnotofhumanfacesexpressingemotions)wereinter-
spersedbetweentheGFstohabituatethedogtothepresentationofpicturesandtoincrease
thedurationthedogcouldleaveitsheadmotionlessonthechinrestfurther.Thesepictures
variedinsizebetween50x50and100x100cm.Additionallyweintroducedthepresentation
ofsmallanimatedvideoswithsound(interspersedwithintheblocksofpictures)whichwere
usedasattentiontriggersduringthetestphase.Trainingwasconsideredsuccessfulifthedog,
in4outof5trials,stayedmotionlessonthechinrestwiththeirheadandorientedtowardsthe
screenforatleast30secondswhilethestimuliwerepresented.Orientationwasdeterminedby
theexperimenterviavisualinspectionaidedbytheon-the-flyoutputoftheeyetracker.
Testing. TestingwasdonebyABandDR.Duringthewholeproceduretheownerwas
allowedtostayatthebackoftheroomandwasinstructedtoavoidinteractionwiththedog.At
thebeginningofatestsession,thedogwasallowedtoexploretheroomforapproximately5
min.Followingthat,theeyetrackingsystemwascalibratedwithathreepointcalibrationpro-
cedure.Therebythreesmalldotswerepresentedoneaftertheotheronthescreen(sizeofthe
dot:50mm,coordinates(x,y):(512,65),(962,702),(61,702))whilethedoghaditsheadonthe
chinrest.Whenthedogfocusedonthedotforatleastasecond,thepointwasacceptedbyE1
ontheoperatingcomputer.Aftercalibrationofthethreepoints,avalidationoftheaccepted
calibrationpointsfollowed.Forthispurposeallthreecalibrationpointswereshownagain.If
thevalidationrevealedanaveragedeviationofmorethan3°ofvisualanglebetweenthetwo
repetitions,thecalibrationwasrejectedandrepeateduntilthiscriterionwasmet.Calibration
wasdonebeforeeverysession.Followingsuccessfulcalibration,thedogwaspresentedwiththe
stimuliasfollows:
1. Beforethepresentationofeachpicture,anattentiontrigger(smallanimatedvideo)waspre-
sentedonthescreen.Thepositionofthetriggerwasrandomlyassignedtooneofthefour
cornersofthescreen(x,y:(150,150),(150,620),(875,150),(875,620)).Thesizeofthetrigger
was5cmonthescreenwhichcorrespondsto1.43degreesofvisualangle.Thepresentation
softwareoftheeyetracker(ExperimentalBuilder1.10.1241,SRResearchLtd.,Ontario,
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ProcessingofHumanEmotionalFacesbyDogs
Canada)wasprogrammedsothatpresentationofthestimulifollowedimmediatelyand
automaticallyoncethedogwasfixatingonthetrigger.
2. Faceswerepresentedforfivesecondsinthecenterofthescreen(x,y:(512,382)).
3. Faceswerepresentedinblocksoffourtrials(eachtrialconsistedofthepresentationofa
triggerfollowedbythepresentationofafacepicture).Theblockscontainedpicturesof
eitheronlypositive(happyandneutral)oronlynegative(angryandsad)facialexpressions.
Theorderofthepresentationwasrandomizedwithintheblock,buteveryblockcontained
bothcorrespondingemotionalexpressions(eitherpositiveornegative)ofthefamiliarand
theunfamiliarperson.
Aftertheendofeachblock,thedogreceivedafoodreward.Dependingonthedog´smoti-
vationandconcentrationashortpause(upto1min)wasmadebeforethedogwasaskedto
repositionitselfinthechinrestdevice.Duringthispausethedogwasallowedtorestorto
movefreelyintheroom.
4. Withinonesessionfourblocksofpictureswerepresented.Twoblockscontainedpositive
andtwoblockscontainednegativefacialexpressions.Eachoftheeightpicturesofadog’s
stimulussetwasshowntwiceineachsession.
Everydogcompletedfivesessions(onesessionperweek)withtheexceptionofthreedogs
forwhichwediscontinueddataacquisitionduetoalackofmotivation(forexampleifthedog
didnotwanttogobacktothechinrestanymoreordidnotstayinthechinrestduringpicture
presentation).Forthisreasondataacquisitionwithonedogwasstoppedaftertwosessionsand
fortwootherdogsafterfoursessions.Theavailabledataofthesedogswasnevertheless
includedintheanalyses(seedata-preparation).
Dataanalysis
Tobeincludedintheanalyses,datahadtofulfilfollowingcriteria:Trialswereonlyincludedin
theanalysisifthedogfixatedatleastonceintotheface.Additionally,asessionwasonly
retainedifthedoghadfixatedatleastonceonafaceofbothemotionalconditions(positive
andnegative).Thedataofadogwerediscardedcompletelyifitdidnotincludeatleasttwo
validsessions(forfurtherdetailsseeTable1).
Eachfacewasdividedintofiveareasofinterest(AoI):eyes,forehead,mouth,facerest(nose,
cheeksandchin)andpicturerest(hairandbackground,seeFig3).Sizeandpositioningofthe
AoIswasbasedonthefollowingrules:
1. AoIforehead:Theforeheadareawasarectangulararearangingupwardsfromthetopofthe
eyebrowstothehighestpointofthehairlineboundedbythefacecontourstotheleftand
theright.
2. AoIeyes:Theeyearearangedfromthemiddleofthepupiltotheeyebrow.Thisareawas
mirroreddownwardsandboundedbythefacecontourstotheleftandtheright.
3. AoImouth:Themouthregionrangedfromthemiddleofthemouthtothebaseofthenose.
Thisareawasmirroreddownwardsandboundedbythefacecontourstotheleftandthe
right.
4. Facerest:IncludedallpartsofthefaceareanotbelongingtoAoI(1)-(3).
5. PicturerestincludedallremainingpartsofthepicturethatdidnotbelongtoAoI(1)-(4)
(e.g.blankparts,hair,shoulderetc.)
PLOSONE|DOI:10.1371/journal.pone.0152393 April13,2016 8/22
ProcessingofHumanEmotionalFacesbyDogs
Fig3.Schemeshowingtheareasofinterest(AoI)forforehead,eyes,facerest,mouthandrestofthe
picture.
doi:10.1371/journal.pone.0152393.g003
FixationsintotheAoIs(1)-(4)aresummarizedas“in-face”fixations.Raweyemovement
datawasanalysedusingDataViewer2.1.1(SRResearch,Ontario,Canada).Eyemovement
eventswereidentifiedbytheEyeLinktracker´son-lineparser.Asthereistodatenovalidated
literatureonthedefinitionofeyemovementeventsindogs,wewereworkingwithrawdata
withoutanythresholdsforfixationduration.Fromtherawdataweextractedfixationdura-
tions,numberoffixationsandlatenciestothefirstfixation(fixationstart).Notethatfixation
durationswerenottrimmedanddurationscouldthereforeexceedthepresentationtimeofa
trialifadogwasholdingafixationlongerthanthepicturepresentation.Onthebasisofthese
datawecalculatedmeanvaluesforfixationdurationsandnumberoffixationsovereachtrial.
Aswewereinterestedinfaceprocessing(ratherthanpictureprocessing)onlyfixationsintothe
faceregionwereincludedintheanalysesunlessstateotherwise.Forthisreason,weextracted
thefixationdurationofthefirstfixationdirectedintooneofthefourin-faceAoIs,thenumber
offixationsdirectedintoin-faceAoIs,theidentityoftheAoIofthefirstin-facefixation,the
PLOSONE|DOI:10.1371/journal.pone.0152393 April13,2016 9/22
ProcessingofHumanEmotionalFacesbyDogs
vector(angle)betweenthefirstandsecondfixation(gazedirection)andthelatencyofthefirst
fixationintooneofthein-faceAoIs.
StatisticalanalyseswereconductedwithSPSS22(IBMCorp.,Armonk,NewYork,United
States)andR3.1.1[70].Linearmixedeffectmodels(LMM)wereusedtoanalysemeanfixation
duration,fixationdurationofthefirstfixation,overallfixationcountpertrialandthelatency
tothefirstfixation(time).AsfixationcountsintothefourdifferentAoIsdidnotfulfillthe
assumptionsofaPoissondistribution(inparticularzeroswerestronglyoverrepresented),we
usedzero-inflatednegativebinomialmodelstoanalyzethesedata(numberoffixationsintothe
differentAoIs),usingtheR-packageglmmADMB0.8.0[71].Likewise,thefixationnumberof
thefirstfixationintooneofthefourin-faceAoIswasanalysed(latencyoffirstfixationintoin-
faceAoI,notethatthefirstfixationwastherebylabelledaszero).Generalizedlinearmixed
models(GLMMs)wereusedtoanalyseifthefirstfixationwasdirectedintothefaceregion
(SPSS,binomialGLMM)andwhichAoIwasfixatedfirst(firstAoI,SPSS,multinomial
GLMM).UsingtheR-packagelme41.1.7[72],weanalysedifthefirstfixationwasdirectedto
theleftorrighthalfoftheface(binomial,GLMM).Additionallywecalculatedtheproportion
(cid:1)
ofthefixationtimedwelledonthelefthalfoftheface((averagedurationleft 100)/(average
durationleft-averagedurationright),negativebinomialGLMM).
Wecouldnotfindanydifferencebetweentheemotionsangryandsadaswellashappyand
neutral.Forthisreason,andtoincreasestatisticalpower,the“positive”emotionalexpressions,
happyandneutral,andthe“negative”emotionalexpressions,angryandsad,werepooled.All
linearmodelsincludedemotiontype(positivevs.negative),familiarity(familiarvs.unfamil-
iar),dogtype(petvs.labdog)andwhereapplicableAoI(eye,forehead,mouth,facerest),as
wellasall2-wayinteractionsbetweenthem,asfixedeffects.Where2-wayinteractionsturned
outsignificant,thedatasetwassplittoexplorethenatureoftheinteraction.Dogtypeandses-
sionnumbernestedwithindogtypewereincludedasrandomfactorsinallmodels.Addition-
allywetestedifthetrialnumber(nestedwithinsession)andtheageofthedogcouldexplaina
significantproportionofthevariationinthemodelsbyaddingthesevariablesasrandomfac-
tors.Bothvariablesdidnothaveasignificantinfluenceonthedata(decisiononbasisofAkaike
´sinformationcriterion(AIC)).Wealsotestedforaneffectofbreedonthedata.Forthispur-
pose,wecombineddataofdogsthatareknowntobelongtothegroupofherding(P1-5,P7-9,
P14,P19),hunting(P11,P12,P18,P15,L1-8)orprotectiondogs(P5,P10,P13;categorization
onbasisoftheiruseasworkingdog).Wecomparedthegroupoflaboratorybeagles(hunting
dogs)tothethreebreedgroupsofthepetdogs.Wedidnotfindsignificanteffectconcerning
thebreedgroup.Thisfactorwasdischargedfromfurtheranalysis.Modelreductionwasdone
backwardsonthebasisofAkaike´sinformationcriterion(AIC).Predictorswereassumedtobe
significantwithα(cid:3)0.05.
Results
Thefinaldatasetincludeddatafrom17petdogs(11maleand6female)withatotalof66ses-
sionsand847trials(percentagetrialswithpositiveemotion48.4%,mean50trials/dog,ranging
from14to76)and8labdogswithatotalof38sessionsand517trials(percentagetrialswith
positiveemotion50.7%,mean65trials/dog,ranging51to74).
FixationDuration
Totalfixationtimepertrial. Therewasasignificantdifferencebetweenpetandlabdogs
concerningthetotaltimetheyspentfixatingthepicture(LMM:F =7.27,p=0.014).The
1,19.6
labdogsspentmoretimefixatingthepicture(5545+/-98ms)thanthepetdog(4701+/-
72ms,seeFig4).Alsotherewasasignificantinteractionbetweendogtypeandemotiontype
PLOSONE|DOI:10.1371/journal.pone.0152393 April13,2016 10/22