Testing the S-R link hypothesis of P3b : the oddball effect on S1-evoked P3 gets reduced by increased task relevance of S2

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Biological Psychology

j ou rn a l h o m epa g e : w w w . e l s e v i e r . c o m / l o c a t e / b i o p s y c h o

Testing the S–R link hypothesis of P3b: The oddball effect on S1-evoked P3 gets reduced by increased task relevance of S2

Rolf Verleger

^a,c,∗

, Lin Marlena Hamann

^a

, Dariusz Asanowicz

^a,b

, Kamila ´Smigasiewicz

^a

aDepartmentofNeurology,UniversityofLübeck,Germany

bInstituteofPsychology,JagiellonianUniversity,Kraków,Poland

cInstituteofPsychologyII,UniversityofLübeck,Germany

a r t i c l e i n f o

Articlehistory:

Received8September2014 Accepted23February2015 Availableonline20March2015

Keywords:

P300 P3b Oddball Responseselection

a b s t r a c t

WehadpreviouslyreportedthattheoddballeffectontheP3bEEGpotentialevokedbyinfrequentvs.

frequentS1presentedinasequenceoftwostimuli,S1andS2,getsreducedina“combinationtask”.In thistask,responsesweredeterminedbythecombinationsofS1andS2ratherthanbyS1only.Wehad attributedthisreductionoftheoddballeffecttoincreasedtaskdifficulty.Thepresentstudyinvestigated possiblereasonsforthisreductionofS1-evokedP3binmoredetail,bymakingthecombinationtaskeasier inseveralrespects:allowingforformingassociationsfromS1toresponses(Experiment1),reducingthe complexityofstimulus–response(S–R)mappings(Experiment2),anddecreasingS2relevanceindefining responses(Experiment3).TheresultsshowedthatonlyS2relevanceaffectedtheoddballeffectonS1- evokedP3b.Namely,whenS2attainedsomerelevancebyinducingago/no-godecisionforS1-defined responses,theoddballeffectonS1-evokedP3bwasintermediatebetweenthelargeeffectinthesimple oddballtaskandthesmalleffectinthecombinationtask.Theresultsmaybeexplainedintermsofthe S–RlinkhypothesisofP3bwhichinterpretsP3basreflectingreactivationofwell-establishedS–Rlinks.

1. Introduction

Several lines of evidence have converged to show that the P3b component of the humanevent-related EEG potential lies at the interfacebetween stimulus (S)processing and response (R)preparation(Gerson,Parra,&Sajda,2005;Kelly&O’Connell, 2013;Connell,Dockree,&Kelly,2012;Ouyang,Herzmann,Zhou,&

Sommer,2011;Poli,Cinel,Citi,&Sepulveda,2010;Savilleetal., 2011; Verleger, Ja´skowski, & Wascher, 2005; Verleger, Schroll,

& Hamker, 2013). In this context, Verleger, Metzner, Ouyang,

´Smigasiewicz,andZhou(2014)haverecentlyspeciﬁedwhatmight beP3b’sfunction,referringtothewell-established“oddball”effect.

TheoddballeffectisthemassiveincreaseofP3bamplitudeswith infrequentstimuliwhentwostimuli,onerareandonefrequent, arepresentedinunpredictablerandomseriesandrequiredifferent responses(Duncan-Johnson&Donchin,1977;Johnson&Donchin, 1980;Ritter&Vaughan,1969;Squires,Squires,&Hillyard,1975).

Verleger,Metzner,etal.(2014)suggestedthatP3bingeneral,and theoddballP3inparticular,reﬂectsreactivationofsomealready

∗ Correspondingauthorat:KlinikfürNeurologie,UniversitätLübeck,D23538 Lübeck,Germany.Tel.:+494515002916;fax:+494515002489.

E-mailaddress:rolf.verleger@neuro.uni-luebeck.de(R.Verleger).

well-establishedS–Rlinkthatiscurrentlynotinanactivatedstate.

Theyarguedthatinmoststandardlaboratorytask(typicallycon- sistingoflongseriesoftrials)thereisnoparticularselectionofnew responsesineachsuccessivetrial.Rather,afewﬁxedS–Rlinksare establishedbyinstructionandpractice,mostfrequentlyonlytwo (e.g.,“frequentS→leftkey”,“rareS→rightkey”).Ifduringsome consecutivetrialsonlyoneoftheseS–Rlinksisused,theotherone, nothavingbeenusedforsometime,willhavetobereactivated whenthecorrespondingstimulusisperceived.Thisprocessofreac- tivatingwell-establishedS–R-linksisassumedtobereﬂectedinP3b amplitude.

ThishypothesiswasputtotestbyVerleger,Baur,Metzner,and

´Smigasiewicz(2014).Intheirmodiﬁcationoftheoddballtask,two stimuliwerepresented(S1andS2)andresponsesweredeﬁned eitherbyonlyoneofthesestimuliorbytheircombination.S1was theletterXorU,onefrequentandonerare(80%vs.20%).Theletter wasaccompaniedorfollowed(aswillbethecaseinthepresent study)byablueoryellowframe(S2)thatsurroundedletterposi- tion.In thestandard oddballcondition,keyselection depended ontheS1letteronlyandS2servedjustas“go”signalforpress- ingthatkey,S2colorbeingirrelevant.Inthis easytask,a large oddballeffectwasobtainedontheS1-evokedP3amplitudes.In contrast,inthe“combinationtask”,S2colordeterminedthekey tobepresseddependingontheletter,e.g.,blueframemeant“left”

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andyellowframe“right”afterfrequentX,butbluemeant“right”

andyellow“left”afterinfrequent U.In thistask,theoddball-P3 evokedbyS1(inthisexamplebyinfrequent Us)wasmassively reduced.WhenconceivingofP3bbeingindependentofresponse processing(“stimulusevaluationhypothesis”,e.g.,Callaway,1983;

Duncanetal.,2009)difficultyofresponseselectionisnotexpected tomodifytheoddballeffectbecauseinanycasetheS1lettersare easilyidentifiedandaretask-relevant.When,ontheotherhand, P3bisassumedtoreflectdecisionprocesses(O’Connell,Dockree,

&Kelly,2012;Kelly&O’Connell,2013)then,ifanything,P3b is expectedtoincreasewhendifﬁcultdecisionshavetobemade.In contrast,Verleger,Metzner,et al.’s(2014)conception ofP3b as reﬂectingreactivationofreadilyavailableS–Rlinksmayaccount forthis reduction ofP3b byassumingthatsuchlinks werenot readilyavailablewiththeinfrequentstimuliinthecombination task.

Thequestionstillremainswhatactuallyarethefactorsrespon- sibleforunavailabilityofS–Rlinksinthiscombinationtask.Several possibilitieswillbetestedinthepresentstudy.Onefactormaybe theabsenceofanyassociationbetweenS1andresponses:when seeingXorUinthecombinationtask,participantsdidnotknow whetherleftorrightkey-presseswouldberequired.Ifthisfac- toriscritical,thenmakingS2predictablebasedonS1mayallow forcreatingsuchassociationsand foractivatingtheappropriate responsealreadybyperceivingS1,therebyforundoingthereduc- tionoftheoddballeffect.ThisaccountwillbetestedinExperiment 1.AnotherfactormaybecomplexityofS–Rmappings.Thiscom- plexitymayresult fromtheoverlapof thesame tworesponses withfourdifferentS1–S2combinations(4:2mapping):Perhaps, responsemappingfortheinfrequentU(e.g.,leftwithyellow,right withblue)cannotbeheldinreadinessbecausetheresponses“left”

and“right”arecontinuouslyrequiredforthefrequentX.Moreover, thisoverlapmayleadtoconflictsbetweenmappingofresponses toS2afterrareS1withsuchmappingsafterfrequentS1.E.g.,blue meansleftand yellowrightfor thefrequent X,butbluemeans rightandyellowleftfortheinfrequentU.Thisaccountintermsof complexitywillbetestedinExperiment2:Bygraduallydecreasing theS–RoverlapbetweenfrequentandrareS1andtheinterdepen- dencyofS1andS2indefiningtheresponses,theoddballeffect isexpectedtobecomegraduallyrestored.Alternatively,thecrit- icalfactormaynotbedifficultyofresponseselectionbutrather theincreasedrelevanceofS2fordeterminingtheresponses.This accountwillbe testedin Experiment 3by comparingtheodd- ballandcombinationtaskstogo/no-gotaskswhereS2colorwill indicatewhethertheselectedresponseshouldorshouldnotbe executed:WithS2beingmorerelevantinthego/no-gotasksthan intheoddballtaskandlessrelevantthaninthecombinationtask, theoddballeffectonP3bisexpectedtobesmallerinthego/no-go taskthanintheoddballtaskandlargerthaninthecombination task.

AlargenegativepotentialappearedintheERPsevokedbyinfre- quentstimuliinVerleger,Baur,etal.’s(2014)combinationtask.So itcouldbesuspectedthatP3bproperhadremainedunchanged andwasonlyapparentlyreduced byoverlapwiththisnegative potential.Butthisnegativitycouldbespatiallyandtemporallydis- sociatedfromP3b,becauseofitsfronto-centralfocus,whichwas distinctfromP3b’sparietalfocus,andbyitsbeingtime-lockedto onsetoftheS2colorframesfollowingtheinfrequentS1,ratherthan toS1onset.P3bremainedreducedinthatstudyevenwhen,with lengthenedintervalsbetweenS1andS2,theincreasednegativity occurredmuchlaterthanS1-evokedP3b.SothereductionofP3b wasgenuine.Thenegativepotentialwasinterpretedasacorrelate ofdifﬁcultresponseselection(cf.Hanslmayretal.,2008;Johnson, Henkell,Simon,&Zhou,2008;Lang,Obrig,Lindinger,Cheyne,&

Deecke,1990)andisexpectedtooccurinthepresentstudyaswell, wheneverresponsesaredifﬁculttoselect.

Fig.1.OutlineoftasksinExperiment1.Enteredarepercentagesofoccurrenceof eachstimulusineachtask.Eachtaskwasperformedtwice,oncewithfrequentXs andoncewithfrequentUs(intheﬁrstandsecondhalfoftheexperiment).Onlythe versionwithfrequentXsisdepicted,forsimplicity.Seetextforfurtherdescription ofthetasks.

2. Experiment1 2.1. Introduction

InExperiment1,weaimedatreplicatingthereductionofthe oddballeffectonS1-evokedP3binthecombinationtask(Verleger, Baur,etal.,2014)andatstudyingwhetherthereasonforthisreduc- tionistheabsenceofassociationsbetweenS1andresponses.The combinationtaskwaspresentedintwoversionsthatdifferedfrom eachotherbytheabsenceversuspresenceofassociationsbetween S1andS2,therebybetweenS1andresponses.Makingresponses predictablemightactivatetheS–R linkswithS1already, which mightrestoretheoddballeffectonP3b.Thestimuliconsistedof thelettersXandU(S1)andfollowingblueoryellowcolorframes (S2),separatedbyonsetasynchroniesof500ms.Oneofthetwo letterswasfrequentlypresented,theotherrarely(80/20%).Fig.1 displaysthestimuliandtheassignmentstoleftorrightkey-press responsesinthethreetasks.

Onetaskwasasimpleoddballtaskwithdelayedresponses.The letterXrequiredaleftresponse,Uarightresponse,andS2served asgo-signal,independentlyofitscolor.Thesecondtaskwasthe combinationtaskusedbyVerleger,Baur,etal.(2014),exceptthat stimulusonsetasynchronies(SOAs)betweenS1andS2hadbeen 0ms,100ms,400ms,or800msinthatstudy,while500msSOA wasusedhere.Thisisa“combinationtask”becauseinformation fromS1and S2hastobecombinedtodeterminetheresponse.

Here,itwillbetermed“combination50%”todenotethatblueand yellowS2s,andtherebyleftandrightresponses,wereequallyprob- able.Thethirdtaskwas“combination80%”because,whileresponse assignmentswereidenticaltocombination50%,blueandyellow S2werepresentedwithprobabilitiesof4/1,ratherthan1/1,such thatagivenletterasS1predictedtheresponsewith80%probability(cf.Fig.1).Thereby,combination80%mightbeconsidered midwaybetweenoddball,whereagivenS1predictedtheresponse with100%,andcombination50%,whereagivenS1predictedthe responsewith50%only.Therefore,ifreductionoftheoddballeffect onP3binthecombinationtask(Verleger,Baur,etal.,2014)wasdue todifferentpredictivevaliditiesofS1onresponses,thenprobabil- itiesof80%mightstillproduceasmalleroddballeffectthanthe 100%S1-responseprobabilitiesintheoddballtaskbutalargerone thanwiththe50%chanceprobabilitiesincombination50%.

We also expected to replicate the large S2-evoked negativ- itythat accompaniedthedifﬁcultresponse selectionsfollowing infrequentS1inVerleger,Baur,etal.’s(2014)combinationtask.

Sinceresponseselectionwillbeeasierincombination80%when S1allowedforpredictingtheprobableresponse,thisS2-evoked negativitymaybereducedinthistask.

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2.2. Materialandmethods

2.2.1. Participants

Twelveyoungadultsparticipated,elevenofwhichwereuniversitystudents.

Theywere7womenand5men,aged19–27years(mean=23,SD=2.2).Informed writtenconsentwasobtainedand15Dwerepaid.Participantsreportednormalor corrected-to-normalvision,nohistoryofneurologicaldisorders,andbeingright- handed,asquantiﬁedbyEdinburghHandednessInventoryscores(Oldﬁeld,1971) of70–100(mean92.5,SD9.65).

2.2.2. Stimuliandprocedure

Participantswereseatedinacomfortablearmchairinadarkenedroom,with about1.2mviewingdistancefromthecomputerscreen.Acomputerkeyboardwas putontheirlap.Responsekeyswereleftctrlandrightctrl,tobeusedwiththe indexﬁngers.ControlledbyaPresentation14.5program(www.neurobs.com)this computerpresentedthestimuli,recordedresponses,andsentstimulusandresponse codestothecomputerthatrecordedEEG.

Eachtrialstartedwithasmallblackﬁxationcross,presentedfor900msatthe centerofalightgray17screen.Then,oneofthetwoblacklettersXandU(Hel- vetica,35points)appearedasS1,followedafteranonsetasynchronyof500ms byablueoryellowrectangleasS2,surroundingtheletterposition(2.3cm×2.5cm width×height,linewidth3pixels).S1andS2werepresentedfor200mseach.Xand Uvariedacrosstrialsinrandomorderwithprobabilitiesof80/20%witharestriction againstimmediaterepetitionofrareletters.BlueandyellowS2salsovariedacross trialsinrandomorder,butwithprobabilitiesdifferingbetweentasks:50/50%in thecombination50%task,and80/20%bothinthecombination80%taskandinthe oddballtask.(Inthislattertask,thisvariationwasirrelevant.)Pressingthecorrect keyafterS2onsetterminatedthetrial,andthenextS1appeared900msafterthis key-press.Therewere250trialswithineachblock.Thus,withaverageRTsof,e.g., 400msandwithoutanyerrors,ablockwouldlastfor250trials×(900+500+400) ms=450s≈7½min.Anerrormessageappearedfor4sinred30pt.font(“pressed tooearly”,inGerman)whenkeyswerepressedbeforeS2onset,whichwasimpor- tantintheoddballtaskbecauseresponseswerefullydeﬁnedbyS1already.Blocks wereprecededbyinstructionscreens,informingabouttheS–Rassignmentinthe followingblock

Therearesixpossibleordersforpresentingthethreetasks(oddball,combination 50%,combination80%).Eachorderwasusedfortwoparticipants.Eachtaskwas presentedintwoversions,oncewithfrequentXandrareU(asdepictedinFig.1),and oncewithfrequentUandrareX.Thisway,unequalleft-rightresponseprobabilities werebalancedintheoddballandcombination80%tasks.Thefrequent-Xversions werepresentedasﬁrstthreeblockstohalftheparticipantsandaslastthreeblocks totheotherhalf.

2.2.3. EEGrecordingandprocessing

EEGwasrecordedwithAg/AgClelectrodes(Easycap,www.easycap.de)from60 scalpsites,including8midlinepositionsfromAFztoOzand26pairsofsymmetric leftandrightsites.Additionalelectrodeswereplacedatthenose-tipforoff-line referenceandatFpzasconnectiontoground.On-linereferencewasFz.Forartifact control,EOGwasrecorded,vertically(vEOG)fromabovevs.belowtherighteyeand horizontally(hEOG)frompositionsnexttotheleftandrighttailsoftheeyes.Data wereamplifiedfromDCto250HzbyaBrainAmpMRplusandstoredat500Hzper channel.Off-lineprocessingwasdonewithBrain-VisionAnalyzersoftware(version 2.03).Datawerere-referencedtothenose-tip,low-passfilteredat25Hz,andseg- mentedfrom100msbeforeS1onsetto1500msafterwards.Toeditforartifacts, trialsfirstwererejectedasgrossartifactswhenconsecutivedatapointsdifferedby morethan50␮VorwhenminimumandmaximumofvoltagesinanyEEGchannel differedbymorethan250␮V(exceptEOGandAF3,AFz,AF4,lesttrialswouldbe rejectedforblinks).Then,ocularartifactswerecorrectedbylinearregression,using themethodimplementedintheBrainAnalyzersoftware.Finally,datawerereferred tothemeanamplitudeofthefirst100msasbaselineineachchannel,andtrialswere rejectedwhenvoltagesexceeded±150␮VinanyEEGchannel.Theremainingtrials wereincludedwhenthefirstkey-pressresponseswerecorrectandmadeafterS2 onset.

2.2.4. Dataanalysis

Responsetimes(RTs)wereanalyzedforcorrectresponsesbetween50msand 1000msafterS2onset.Leftandrightresponses(i.e.,blockswithfrequentXand frequentU)werepooledandRTswereaveragedacrosstrialsineachofthethree tasksseparatelyforfrequentandrareS1sandforblueandyellowS2s(=frequentand rareS2sinoddballandcombination80%).Inthesameway,percentagesofwrong responsesweredetermined(includingtoolateresponses,i.e.,>1000ms).

ForanalysisofS1-evokedP3b,trialswereaveragedseparatelyforrareandfre- quentS1ineachtaskineachparticipant,poolingacrossblockswithfrequentXand frequentUandacrosstrialswithblueandyellowS2(becauseS1-evokedP3bcould notbeaffectedbyS2).ThemeannumberofincludedtrialsforinfrequentS1,sepa- ratelyforeachofthethreetasks,was75,withaminimumof49.Then,differences wereformedbetweenaveragesfromrareandfrequentS1.P3wasmeasuredinthese differencewaveforms.Lackingaclearpeakinthecombinationtasks,P3amplitudes weremeasuredinalltasksasmeanamplitudes300–600msafterS1onset.

Foranalyzingthecentralnegativityinrare-frequentdifferencewaveforms,data wereadditionallysplitbyS2colorbecauseresponseselection,reﬂectedbythis negativity,wasexpectedtobemoredemandinginthecombination80%taskwith infrequentthanfrequentS2.Themeannumberofincludedtrialsforthemostinfre- quentcategory(4%probability;infrequentS1followedbyinfrequentS2)was17 foroddball,14forcombination80%,withaminimumof12and8,and33for combination50%,minimum22.Thisnegativitywasquantiﬁedbymeasuringmean amplitudes250–450msafterS2onset(750–950msafterS1onset).

AllmeasurementsweretakenindataofthesevenmidlinechannelsFz,FCz,Cz, CPz,Pz,POz,Oz.

Analysesofvariance(ANOVAs)wereusedforstatisticalanalyses.Pair-wisecom- parisonswereconducted,ontheonehand,betweentheoddballandcombination 50%tasksinordertotestwhetherVerleger,Baur,etal.’s(2014)resultscouldbe replicated,and,ontheotherhand,betweenthecombination50%and80%tasksin ordertotestwhetherthereductionoftheoddballeffectcouldbeundonebythe80%

associationfromS1toresponses.RepeatedmeasurementfactorsforanalysisofRTs andoferrorrateswereS1Frequency(frequentvs.rare),Task(oddballvs.combina- tion50%inoneanalysis,combination50%vs.combination80%intheother),andS2 Color(bluevs.yellow;colorvariationwasirrelevantintheoddballtaskandrelevant inthetwocombinationtasks,whereblueandyellowwerepresentedwith80/20 probabilitiesinthecombination80%task,andwith50/50probabilitiesinthecom- bination50%task).ForanalysisofP3,factorswereRecordingSite(Fz,FCz,Cz,CPz,Pz, POz,Oz)andTask(asabove).Foranalysisofcentralnegativity,factorswereRecor- dingSite,Task,andS2Color(alldeﬁnedasabove).Tointerpretinteractions,ANOVAs wereconductedseparatelyforthelevelsofeachoftheinteractingfactors.Degrees offreedomoftheRecordingSitefactor(beingarepeated-measurementfactorwith morethantwolevels)werecorrectedwiththeGreenhouse–Geissermethod.

2.3. Results

2.3.1. S1-evokedP3

GrandmeanwaveformsaredisplayedinFig.2.Theoddballeffect wasobviouslylargerinoddballthanincombination50% (Task:

F1,11=25.7, p<.001). Crucially,this reduced effectdidnotdiffer betweenthetwocombinationtasks(Task:F_1,11=0.0,n.s.).

The oddball effect onP3 was largestat Pz (cf. themaps of P3’stopographicaldistributionatthebottomofFig.2;effectsof RecordingSite:F_6,66=16.3,p<.001inoddballvs.combination50%, andF_6,66=8.1,p=.001incombination50%vs.combination80%).

Topographiesoftheoddballeffectdifferedbetweentheoddballand combination50%tasks(Task×RecordingSite:F6,66=8.7,p=.002) butthisdifferencedisappearedaftervectorscaling(i.e.,normal- izingtheamplitudesinordertocomparethetopographicproﬁles independentlyofamplitudedifferences),F6,66=0.4,n.s.,whichmay speakagainstatruechange(McCarthy&Wood,1985,thoughsee Urbach&Kutas,2006).Topographiesdidnotdifferbetweenthe twocombinationtasks(Task×RecordingSite:F6,66=0.8,n.s.).

2.3.2. S2-evokednegativity

2.3.2.1. Oddballvs.combination50%. Negativitywaslargeratante- riorthanposteriorsites(RecordingSite:F_6,66=5.4,p=.008)butwas restrictedtoFzandFCzintheoddballwhileextendingfromFzback- wardstoPzincombination50%(Task×RecordingSite,F6,66=3.8, p=.02)suchthatnegativitywaslargerincombination50%thanin oddballatCz,CPz,andPz.(EffectofTaskinseparateanalysesat theserecordingsites:F1,11≥6.2,p≤.03).TherewasnoeffectofS2 Color(F≤2.1,p≥.13).

2.3.2.2. Combination 50% vs. combination 80%. Negativity was appreciablylargerafterthe20%probability(yellow)S2incombi- nation80%thanafterthe80%probability(blue)S2,whichdidnot differfromthe50%probabilityS2sinthecombination50%task, resulting ina Task×S2 Color interaction,F_1,11=9.6, p=.01, and maineffectsofTask,F_1,11=9.7,p=.01,andofS2Color,F_1,11=9.2, p=.01.NegativityextendedwithaflattopographyfromFztoPz afterblue(frequent)S2s,andincreasedaboveallfromFCztoPz withyellow(rare)S2s.Theeffectwasreflectedinamaineffectof RecordingSite,F6,66=6.3,p=.002,andanS2Color×RecordingSite interaction,F_6,66=3.3,p=.03,unmodifiedbyTask.

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Fig.2. GrandmeansofERPsfromPzinExperiment1.Unitofx-axisisms.Letters werepresentedattime-point0ms,andcoloredframesat500ms.Unitofy-axis is␮V,withnegativepolarityplottedupwardsandscalesspanning30␮Vequally inbothpanels.GreenlinesdenotedatafromtheOddball4/1task,blacklinesfrom Combination1/1andpurplelinesfromCombination2/1.Intheupperpanel,dataare displayedseparatelyforfrequentS1(thinlines)andrareS1(boldlines).Difference waveformsrare-frequentS1aredisplayedinthelowerpanel,withdatafromthe Combination2/1tasksplitbyS2color,withthefrequentS2colorinboldandtherare S2colorasthinline.Topographiesofthedifferencepeaksaredisplayedformean amplitudes300–600msafterS1(belowthewaveforms)and250–450msafterS2 (abovethewaveforms).Colorsandstylesofarrowsdenotetheconditions.Positive polarityisred,negativepolarityisblue,scalingisfromminimumtomaximum, differingbetweenmaps.(Forinterpretationofthereferencestocolorinthisﬁgure legend,thereaderisreferredtothewebversionofthearticle.)

2.3.3. Responsetimesanderrorrates

MeanRTsanderrorratesaredisplayedintheleftpanelsofFig.3.

2.3.3.1. Oddballvs.combination50%.Responsesweremuchslower andmoreerrorswerecommittedincombination50%thaninthe oddball,F_1,11=84.4,p<.001 forRT,F_1,11=32.0,p<.001 forerror rate.Itwasonlyincombination50%thatresponseswereslower (byabout100ms)andmoreerrorswerecommitted(about16%

more)afterrarethanafterfrequentS1,reﬂectedbymaineffects ofS1Frequency,F1,11=57.2,p<.001forRT,F1,11=32.2,p<.001for

errors,andbyTask×Frequencyinteractions,F_1,11=59.0,p<.001 forRT,F1,11=24.8,p<.001forerrors,andconsequentlybyeffects ofFrequencybeingsignificantinthecombination50%taskonly, F_1,11=89.0,p<.001forRT,F_1,11=29.3,p<.001forerrors,notinthe oddballtask,F1,11≤0.2,n.sforRTanderrors.EffectsofS2Color werenotsignificantforerrors, F_1,11≤1.7,p≥.22, butRTswere, unexpectedly,slowerinthecombinationtasktoyellowthanto blueS2s(althougheithercolorhad50%probability),reflectedby amaineffectofS2Color,F_1,11=8.9,p=.01,aTask×S2Colorinter- action,F_1,11=6.9,p=.02,andtheeffectofColorbeingsignificant inthecombination50%task,F1,11=8.7,p=.001,notintheoddball task,F1,11=0.6,n.s.

2.3.3.2. Combination 50% vs. combination 80%. As reﬂected by maineffects ofS1Frequency,responseswereslowerand much more errors were committedafter rare than after frequent S1, F1,11=133.9, p<.001 for RT, F1,11=50.7, p<.001 for errors. The effectsofS2Color,F1,11=34.4,p<.001forRT,F1,11=14.6,p=.003for errors,andofTask×S2Color,F_1,11=28.7,p<.001forRT,F_1,11=11.2, p=.007forerrors,reﬂectedlargeeffectsofColorinthecombination 80%task,F1,11=44.4,p<.001forRT,F1,11=23.7,p<.001forerrors.

Theseeffectsmeantthat,afterblueS2,responsesbecamefasterand errorratessmallerincombination80%thanincombination50%

(80%vs.50%probabilityofblueS2),F1,11=23.5,p=.001forRTand F_1,11=5.6,p=.04forerrorsfortheeffectofTaskinseparateanaly- sisonblueS2.TherewasnodifferencebetweentaskswhenS2was yellow(20%vs.50%probabilityinthesetwotasks),F1,11≤0.6,n.s.

forRTanderrors.ForRTs,thespeedingofresponseswithblueS2in combination80%waslargerafterfrequentthanafterinfrequentS1 (67msvs.38ms),asindicatedbythetripleinteractionofTask×S2 Color×S1Frequency,F1,11=18.3,p=.001,resolvingtoasigniﬁcant interactionofTask×S1FrequencywithblueS2,F_1,11=5.5,p=.04, andnotwithyellowS2,F_1,11=1.7,n.s.Thistripleinteractionwas notsigniﬁcantforerrors,F1,11=1.5,n.s.

2.4. Discussion

IncreasingtheprobabilityofoneS2color,inthecombination 80% task, speeded up RTs and decreased error rates compared tothecombination50%task.Sinceresponsestothis colorwere deﬁnedbythespeciﬁcS1-S2(letter-color)combinationratherthan bytheS2color,thisresultshowsthatassociationsbetweenlet- terS1stimuliandresponsesweresuccessfullyestablishedinthe combination80%task.Nevertheless,theS1-evokedoddballeffect onP3amplitudeswasasreduced inthistaskasintheoriginal combination(50%)task.Therefore,noevidencewasfoundforthe assumptionthatthereductionoftheoddballeffectinthecombina- tiontaskisduetotheabsenceofassociationsbetweenS1stimuli andresponses.

Itcameunexpectedthattherewasasimilar,albeitmuchsmaller effectofS2coloronRTsinthecombination50%task,withresponses beingfastertobluethantoyellowframes.Thismighthavebeen duetoaprimacyeffectininstructions,becauseparticipantswere alwaysgiven theinstruction onblue S2ﬁrst (e.g.,“Xand blue meansleft,Xandyellowmeansright”).Alternatively,itmightbe duetocarryoverwhencombination50%wasprecededbycombi- nation80%.Totestforthislatterpossibility,additionalanalyses distinguishedbetweenthose participantswhohad combination 80%beforecombination50%,andviceversa,andbetweenthever- sionsofthecombination50%taskpresentedwithinthelastthree blocks(suchthatatleastonecombination50%taskpreceded)vs.

theﬁrstthreeblocks.Butnoneoftheseorderfactorsinteracted withS2Color.Thus,theprimacyeffectininstructionsmighthave beendecisive,cf.Kotchoubey(2014)forarelatedﬁndingandmore thoroughdiscussion.

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Fig.3. MeanresponsetimesanderrorratesinExperiments1–3.Upperpanelsareforresponsetimes,lowerpanelsforerrorrates.Eachcolumndepictsdatafromone experiment.y-Axisdenotesresponsetimesinmsintheupperpanelsandpercentagesofcorrect-responsetrialsinthelowerpanels.Boldandthinlinesshowvaluesforrare andfrequentS1,respectively.Intheleftpanels,graylinesdisplayvaluesforyellow(rare)S2,blacklinesforblue(frequent)S2.(Notethat“rare”and“frequent”inExperiment 1donotapplytoS2inthecombination50%taskwhereblueandyellowS2wereequiprobable.)Graydashedlinesinthelowerrightpaneldisplayerrorratesinno-gotrials (i.e.,falsealarms),withboldandthinlinesusedforrareandfrequentS1,respectively.

3. Experiment2 3.1. Introduction

Experiment2investigatedwhetherthereductionoftheoddball- P3binthecombinationtaskofVerleger,Baur,etal.(2014)wasdue tocomplexityofS–Rmappings.Threetaskswereimplementedthat reducedthiscomplexitycomparedtotheoriginalcombinationtask.

Bystepwisedecreasingcomplexityacrosstaskstheoddballeffect isexpectedtobecomegraduallyrestored.UnlikeinExperiment1, stimulusprobabilitieswereidenticalacrossthefourtasks.Fig.4 displaysthestimuliandtheirresponseassignmentsinitsupper part,andprovidesasurveyonthefactorspresumedtocontribute toreductionoftheoddballeffectinitslowerpart,asdetailedinthe following.

ThecombinationtaskusedbyVerleger,Baur,etal.(2014)and inExperiment1channelsthefourpossiblecombinationsofS1×S2 totworesponses(thereforewillherebecalled“combination4:2”):

Left-handresponsesarerequiredbothbyblueS2afterXandby yellowS2afterU,andright-handresponsesarerequiredbothby yellowS2afterXandbyblueS2afterU.Ifthis4:2mappingiscriti- calforreducingtheoddballeffectthentheoddballeffectshouldbe smallerwiththistaskthanwiththethreeothertasks,inwhichfour ratherthantwoﬁngerswereusedforresponding.Tobebettercom- parabletotheothertaskswheremiddleﬁngersandthumbswere usedforresponding,this“combination4:2”taskwasruntwice,

once withthumbsand oncewithmiddle ﬁngersas responding agents.

Amongthethreeothertasks,mostsimilartotheoriginaltask wasthe“combination4:4”task:Thecriticaldifferencewasthat overlapofresponseswasremovedbyassigningdifferentresponses toeachS1–S2combination.Therefore,asjustnoted,ifoddball-P3b wasreducedintheoriginalcombinationtaskbecauseresponsesto rareS1cannotbeindependentlystoredinworkingmemorydueto theiroverlappingwithresponsesassignedtofrequentS1,thenthis reductionshouldnotoccuranymoreinthecombination4:4task.

In the “independent S1” task, letters provided independent response-relevantinformation.E.g.,anXmaymeanthatthemid- dleﬁngerwillbeused,andaUmaymean“thumb”.Whichhandto usewasspeciﬁedbythefollowingS2,stilltobecombinedwithS1 informationandstillhavingoppositemeaningsforrarevs.frequent S1(e.g.,blueS2means“left”afterXand“right”afterU).Therefore, ifoddball-P3bwasreducedinthecombinationtaskbecausethe infrequentS1didnotprovideanyindependentinformation,then thisreductionshouldnotoccuranymoreintheindependent-S1 task.

Sofar,inalltasks,S2informationhadoppositemeaningswith thetwoS1s.Thisfeaturewasabolishedinthe“independentS1&S2”

task.Here,letterandcolorprovidedindependentinformation,such that,e.g.,afterhavingspeciﬁedthemiddleﬁnger(likeinthepre- vioustask),theXinformationdidnotneedtobecombinedwith theS2information.(Thus,thistaskisapre-cueingtaskasdevised

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Fig.4.OutlineoftasksinExperiment2.Intheupperpart,therequiredresponsesarecompiled,with“3”and“1”denotingmiddlefingerandthumb,and“L”and“R”denoting leftandrighthand.Stimulusfrequenciesremainedconstantacrosstasksandarecompiledontheleft.Eachofthefourtaskswasperformedtwice:Thestandardcombination task(“combination4:2”)wasperformedoncewiththethumbsandoncewiththemiddlefingersasresponseagents,andtheotherthreetaskswereperformedwithS1 determiningoncetherespondingfingerandoncetherespondinghand.HalfofparticipantsperformedthetaskswithfrequentXsandrareUs,viceversatheotherhalf.Only theversionwithfrequentXsisdepicted,forsimplicity.Inthelowerpart,asurveyisprovidedonthefactors,variedbetweentasks,presumedtocontributetoreductionof theoddballeffect.Seetextforfurtherdescription.

byRosenbaum,1980.)Therefore,ifoddball-P3bwasreducedinthe combinationtaskbecauseinformationfrominfrequentS1stillhad tobestoredforfurthercombinationwithS2information,thenthis reductionshouldnotoccuranymoreintheindependentS1&S2 task.

3.2. Materialandmethods

DifferencesfromExperiment1willbedescribedonly.

3.2.1. Participants

Fourteenuniversitystudentsparticipated,buttwoofthemhadtobeexcluded duetomalfunctionsofexperimentalcontrol.Theremaining12participantswere 7womenand5men,aged19–26years(mean=22.6,SD=2.0).Informedwritten consentwasobtainedand5Dbaseremunerationwaspaidplus1ct.foreachcorrect response,addingtogethertoamountsaround20D.Participantsreportednormalor corrected-to-normalvision,nohistoryofneurologicaldisorders,andbeingright- handed,asquantiﬁedbyEdinburghHandednessInventoryscoresof80–100(mean 88,SD9.4).

3.2.2. Stimuliandprocedure

ResponsekeyswereTabfortheleftmiddlefinger,LeftCtrlfortheleftthumb, Enterfortherightmiddlefinger,andRightCtrlfortherightthumb.S2probabili- tieswereidenticalinalltasks,50/50.Becausealltasksweredifficult,blockswere precededbyinstructionscreens,denotingthestimulus–responseassignmentinthe followingblock,followedbytenpracticetrials.Correctresponseswererewarded by1ct.each.Feedbackscreensafterevery50trialsinformedaboutthenumberof correctresponsesandtheamountofmoneyearned.

Therewerefourtasks,asdescribedabove:combination4:2(asinExperiment1 andinVerleger,Baur,etal.,2014),combination4:4,independentS1,andindepen- dentS1&S2.Eachtaskwaspresentedintwoversions,asdetailedinFig.4.

Thetwoversionsofeachtaskwereprincipallypresentedinconsecutiveblocks, exceptthatthecombination4:2taskwasalwayspresentedinthemiddle,inthe4th and5thblocks,therebysplittingthetwoversionsofthemiddleoneofthethreeother taskstobecomeblocks#3and#6.Theorderofthesethreeothertaskswasfully balanced:Eachofthe6possiblepermutationswasusedintwoparticipants,withone participanthavingeachtask’s“finger”versionfollowedbyits“hand”version,vice versaintheotherparticipant.Crossedwiththissplitting,halftheparticipantshad XasfrequentandUasrarestimuli(asdepictedinFig.4)andhadthemiddle-finger versionofthecombination4:2taskbeforethethumbversion,andtheotherhalf hadUasfrequentandXasrarestimuliandthethumbversionofthecombination 4:2taskbeforethemiddle-fingerversion.

3.2.3. EEGrecordingandprocessing MethodswereidenticaltoExperiment1.

3.2.4. Dataanalysis

RTsanderrorratesofthetwoversionsofeachtaskwerecomparedtoeachother inpreliminaryanalyses,andnodifferenceswerefound.Forthemainanalyses,data werepooledacrossthetwoversions,andpooledacrosstrialswithblueandyellow S2.Likewise,forERPanalysis,trialswereaveragedseparatelyforrareandfrequent S1ineachofthefourtasksineachparticipant,poolingacrosstrialswithblueand yellowS2andacrossthetwoversionsofeachtask.Themeannumberofincluded trials,separatelyforeachtask,was71forinfrequentS1,withaminimumof34.P3 amplitudesweremeasuredasmeanamplitudes350–600msafterS1onset.

RepeatedmeasurementfactorsinANOVAswereS1Frequency(frequentvs.

rare)andTask(combination4:2,combination4:4,independentS1,andindepen- dentS1&S2)foranalysisofRTsandoferrorrates,andRecordingSite(Fz,FCz,Cz, CPz,Pz,POz,Oz)andTask(asabove)foranalysisofP3andCentralNegativityinthe differencewaveformsofrareminusfrequentS1.

3.3. Results

3.3.1. S1-evokedP3

GrandmeanwaveformsaredisplayedinFig.5.Theoddballeffect onP3waslargestatPzandPOz(RecordingSite:F_6,66=9.8,p<.001).

Therewasnodifferencebetweentasks,maineffectF_3,33=1.1,n.s., Task×RecordingSite,F18,198=1.2,n.s.

3.3.2. S2-evokednegativity

Theoddballeffectonresponseselection,reﬂectedinS2-evoked negativity,waslargestatFCzand Fz(RecordingSite:F6,66=5.7, p=.02,Task×RecordingSite,F18,198=1.1,n.s.).Ofinterest,negativ- itydifferedbetweentasks(maineffectofTask:F_3,33=3.7,p=.04).

Pair-wiseANOVAsoneachtaskpairshowedthatnegativitywas largerincombination 4:2thanin thetwo“independent” tasks, F_1,11≥8.8,p≤.01,andweaklytendedtobesocomparedtocom- bination4:4,F_1,11=2.5,p=.14.Thethreeothertasksdidnotdiffer fromeachother,F1,11≤1.8,p≥.21.

3.3.3. Responsetimesanderrorrates

MeanRTsanderrorratesaredisplayedinthemiddlecolumn of Fig. 3. Responses were much slower and more errors were