PsychologicalResearchPsychologischeForschungSpringer-Verlag2004DOI10.1007/s00426-004-0192-7

Original Article

How does practice reduce dual-task interference:Integration, automatization, or juststage-shortening?EricRuthruff()MarkVanSelstJamesC.JohnstonRogerRemington

E.RuthruffJ.C.JohnstonR.RemingtonMS2624,NASAAmesResearchCenter,MoffettField,CA94035,USA

M.VanSelstDepartmentofPsychology,SanJosStateUniversity,SanJos,CA95192,USA

E.RuthruffPhone:+1-650-6040343Fax:+1-650-6040801E-mail:eruthruff@mail.arc.nasa.gov

Received:29December2003/Accepted:31August2004

AbstractThepresentstudyassessedthreehypothesesofhowpracticereducesdual-task

interference:Practiceteachesparticipantstoefficientlyintegrateperformanceofataskpair;practice

promotesautomatizationofindividualtasks,allowingthecentralbottlenecktobebypassed;

practiceleavesthebottleneckintactbutshorterinduration.Thesehypothesesweretestedintwo

transfer-of-trainingexperiments.Participantsreceivedoneofthreetrainingtypes(Task1only,or

Task2only,ordual-task),followedbydual-tasktestsessions.PracticeeffectsinExperiment1

(Task1:auditoryvocal;Task2:visualmanual)werefullyexplainedbytheintactbottleneck

hypothesis,withouttaskintegrationorautomatization.Thishypothesisalsoaccountedwellfor

themajorityofparticipantswhenthetaskorderwasreversed(Experiment2).Inthiscase,however,

thereweremultipleindicatorsthatseveralparticipantshadsucceededineliminatingthebottleneck

byautomatizingoneorbothtasks.Neitherexperimentprovidedanyevidencethatpracticepromotes

efficienttaskintegration.

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IntroductionAlthoughpeopletypicallyhavedifficultyperformingtwochoice-responsetasksatthesametime,

thisinterferencecanbedramaticallyreducedwithpractice.Isthisreductionduetoaqualitative

changeintaskperformance?Specifically,doespracticeeliminatethecentralbottleneckwidely

believedtounderliedual-taskinterference?Thereareseveralwaysinwhichthismighthappen.

Ifthebottleneckoccursbecausecentraloperationsarecarriedoutbyscarcegeneral-purpose

resources,practicemightallowpeopletoperformtaskswithoutthoseresources(task

automatization).Forinstance,practicemightproducejumpercablepathwaysdirectlylinking

stimuliandresponses(Johnston&Delgado,1993).Alternatively,practicemightallowanefficient

integrationofthetwotasks.Forinstance,practicemightallowparticipantstore-organizetwo

tasksintoasinglesuper-task,thuseliminatingresourcecompetition.

Theplausibilityoftaskautomatizationandtaskintegrationprovidesmotivationtolookfor

qualitativechangesinperformancewithpractice,butitisbynomeansobviousthatsuchchanges

actuallyoccur.Itispossiblethatpracticeinsteadleavesthebottleneckintactbutshorterinduration

duetostage-shortening.Todeterminehowpracticereducesdual-taskinterference,thepresent

studyusedatransfer-of-trainingparadigm,comparingtheeffectsofsingle-taskpracticeand

dual-taskpracticeonsubsequentdual-taskperformance.Lookingahead,theresultsobtainedwith

thetaskorderadoptedbymanypreviousdual-taskstudies(Task1:auditoryvocal;Task2:

visualmanual)canbeexplainedentirelybythehypothesisthatpracticeleavesthebottleneck

intactbutshorterinduration.Withtheopposite(rarelystudied)taskorder,asubstantialmajority

ofparticipantsagainshowedanintactbottleneck.However,aminorityofparticipantssucceeded

ineliminatingthebottleneck,automatizingoneorbothtasks.

Background

The psychological refractory period

Manyearlydual-taskstudiesusedaccuracyastheprimarydependentmeasure(e.g.,Allport,

Antonis,&Reynolds,1972;Hirst,Spelke,Reaves,Caharack,&Neisser,1980).Accuracy,however,

maybeinsensitivetocapacitylimitations.Evenastrictinabilitytoperformcentralstagesofthe

twotasksinparallelwouldnotnecessarilycausemanyerrors.Peoplemightbeabletobuffer

informationononetask,switchtoasecondtaskforaperiodoftime,andlaterretrieveenoughof

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thebufferedinformationfromthefirsttasktorespondaccurately(seeShaffer,1975).Although

thisbuffer-and-switchstrategymightallowparticipantstoavoidmakingoverterrors,itwould

increaseresponsetime(RT)tooneorbothtasks(Pashler&Johnston,1989).Forthisreason,

recentdual-taskresearchhasemphasizedparadigmsthatallowprecisemeasurementofRT.

ByfarthemostwidelyusedRTparadigmindual-taskresearchisthepsychologicalrefractory

period(PRP)paradigm,inwhichthekeyindependentvariableisthestimulusonsetasynchrony

(SOA)betweenthetwotasks.LongSOAs,wheretemporaloverlapbetweentasksisminimal,

provideabaselinemeasurementofRTforeachtaskperformedseparately.ShortSOAs,where

temporaloverlapishigh,provideanopportunitytoobservewhetherparallelcentralprocessing

hasoccurred.ThetypicalPRPfindingisadramaticincreaseinRTtothesecondtask(RT2)at

shortSOAsrelativeto(baseline)longSOAs.ThisPRPeffectisremarkablyrobust,occurringeven

withverysimplepairsoftasksintheabsenceofinputoroutputmodalityconflicts(forreviews

seeLien&Proctor,2002;Pashler&Johnston,1998).

ThereissubstantialevidencethattheprimarycauseofthePRPeffectisacentralbottleneck,

requiringcentralstagessuchasresponseselectiontoproceedononlyonetaskatatime(Davis,

1957;Welford,1952).Figure1showsageneralizedbottleneckmodeloftheshortSOAcondition,

undertheassumptionthatTask1bottleneckoperationsareperformedbeforethoseofTask2.

Eachtaskisdividedintothreeprocessingstages:Pre-bottleneck(A),bottleneck(B),and

post-bottleneck(C).Byhypothesis,stagesAandCcanproceedinparallelwithanystageon

anothertask,butstageB(thebottleneckstage)proceedsononlyonetaskatatime.Thus,atshort

SOAs,theTask2bottleneckstage(2B)mustwaitfortheTask1bottleneckstage(1B)tofinish.

ThiswaitingtimeproducesaPRPeffectequalto1A+1B2ASOAshort(seePashler&Johnston,

1989).Becausethisequationdoesnotcontainatermforstage2B,thePRPeffectshouldnot

dependonthedurationoftheTask2bottleneckstage.Thisbottleneckmodelpredictionandmany

othershavebeenconfirmedinseveralstudies(e.g.,McCann&Johnston,1992;Pashler&Johnston,

1989).

Fig.1Stage-timediagramsforthe0-msstimulusonsetasynchrony(SOA),accordingtoageneralized

bottleneckmodel.Processingisdividedintothreestages,arbitrarilylabeledA,B,andC.StagesAandCof

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onetaskcanproceedinparallelwithanystageoftheothertask.StageB(thebottleneckstage),however,

canproceedononlyonetaskatatime.

AlthoughitisclearthatacentralbottleneckoccursinthePRPparadigm,MeyerandKieras

(1997a,1997b)havearguedthatthebottleneckmayreflectstrategicchoicesratherthanstructural

limitations(seealsoSchumacheretal.,2001).Ifthebottleneckisstrategic,thenthereshouldbe

conditionsunderwhichpeoplechoosetoperformtasksinparallel.Tolookforsuchconditions,

Ruthruff,Pashler,&Klaassen(2001)explicitlyencouragingprocessingoverlap,placedequal

emphasisoneachtask,askedparticipantstogrouptheirresponses(i.e.,emitthematthesame

time),minimizedinputandoutputconflicts,andpresentedbothstimuliatthesametime.Despite

theseefforts,dual-taskcostswereaboutaslargeasthosetypicallyobservedinthePRPparadigm.

Severalotherrecentstudieshavealsoarguedthatthecentralbottleneckisstructuralratherthan

strategic,atleastatrelativelylowpracticelevels(Levy&Pashler,2001;Pashler,1994;Ruthruff,

Miller,&Lachmann,1995,Experiment3;Ruthruffetal.,2001;Ruthruff,Pashler,&Hazeltine,

2003;Tombu&Jolicoeur,2000).

Can practice eliminate the central bottleneck?

Althoughthecentralbottleneckappearstobestructural,itmightneverthelessbepossibleto

overcomeitwithpractice.Untilrecently,onlyafewPRPstudieshadexaminedtheeffectsof

extensivepractice.ThesestudiesreportedsurprisinglylittleeffectofpracticeonPRPinterference

(e.g.,Bertelson&Tisseyre,1969;Davis,1956,1957;Dutta&Walker,1995;Hick,1948;Karlin

&Kestenbaum,1968;VanSelst&Jolicoeur,1997).KarlinandKestenbaum,forinstance,reported

residualPRPeffectsgreaterthan200ms,evenafterseventoeightpracticesessions.VanSelst

andJolicoeur(1997)laterreplicatedthisaspectofKarlinandKestenbaumsresults.

TheresultsoftheseearlystudiessuggestedthatPRPinterferenceisremarkablyresistantto

practice.VanSelst,Ruthruff,andJohnston(1999)arguedthatthisconclusionwaspremature,

however,becausethesepreviousPRPstudiesrequiredmanualresponsestobothtasks.Giventhe

likelihoodofstrongoutputinterferencebetweentwomanualresponses(eitherinprogrammingor

execution;seeDeJong,1993;Keele,1973),thesestudiesmighthaveprovidedlessthanideal

conditionsforobservingeffectsofpracticeoncentralinterference.Toaddressthisconcern,Van

Selstetal.(1999)conductedaPRPpracticestudyusingtaskpairsthatminimizedoutput(and

input)interference.Onetaskmappedauditorystimulitovocalresponses(high-andlow-pitched

tonestothespokenwordshighandlow)andtheothertaskmappedvisualstimulitomanual

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responses(alphanumericcharacterstokeypresses).Contrarytopreviousstudies,practice

dramaticallyreduceddual-taskinterference,from353msinthefirstsessiontoonly40msinthe

18thsession.Thismorerecentfindingdemonstratesthatpracticecanindeedsubstantiallyreduce

PRPinterference,providedthatinputandoutputmodalityconflictshavebeeneliminated(seealso

Hazeltine,Teague,&Ivry,2002;Schumacheretal.,2001).

Whatcausesthedramaticreductionindual-taskinterferencewithpractice?Inparticular,does

practiceactuallyeliminatethebottleneck?Belowweoutlinethreeplausiblehypotheses,eachof

whichfocusesonadifferentconsequencethatpracticemighthave.Aswillbeseen,thesemodels

makedistinctpredictionsaboutwhattypesofpractice(e.g.,single-taskversusdual-task)should

bemosteffectiveinreducingdual-taskinterference.

First,dual-taskpracticemightproduceanefficientintegrationofthetwotasks(see,e.g.,Hirst

etal.,1980;Spelke,Hirst,&Neisser,1976),increasingthedegreetowhichcentralstagescan

proceedinparallel.Asanextremeexample,participantsmightlearntocombinethetwotasksinto

asinglesuper-task(Kahneman,1973),sothatasinglecognitiveoperationjointlyselectsresponses

tobothtasksatthesametime.Accordingtothistaskintegrationhypothesis,integrationrequires

dual-taskpractice.Thus,dual-taskpracticeshouldproducePRPreductionaboveandbeyondwhat

ispossiblewithsingle-taskpractice.

Second,practicemightautomatizetheperformanceofindividualtasks.Forthepresentpurposes,

wedefinetaskautomatizationastheeliminationofdemandsonthelimitedcentralresources

responsibleforthecentralbottleneck.Forinstance,practicemightallowthedevelopmentof

jumpercablepathwaysdirectlylinkingstimuliandresponses(Johnston&Delgado,1993).If

eithertaskisautomatized,thenthereshouldbenoconflictforlimitedcentralresourcesandhence

nobottleneck.Accordingtothistaskautomatizationhypothesis,therefore,completebypassof

thebottleneck(i.e.,parallelprocessingofcentraloperations)shouldoccurfollowingeither

single-taskordual-taskpractice.

Third,practicemightcausenoqualitativechangeinperformancenotaskintegrationandno

taskautomatizationleavingthebottleneckintact.Eveninthiscase,practicewouldreduce

dual-taskinterferencebyshorteningstagedurations.Thisintactbottleneck(withstage-shortening1)

1Stage-shorteningisaninevitableconsequenceoftaskpractice.Itplaysacriticalroleintheintact

bottleneckhypothesisinfact,itisthesolecauseofPRPreduction.Althoughthetaskintegration

andtaskautomatizationhypothesesalsoallowforstage-shortening,itisnotassumedtobethe

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hypothesismakesthesamepredictionsforthePRPeffectthatallbottleneckmodels(seeFig.1)

make:PRPeffect=1A+1B2ASOA(Pashler&Johnston,1989).Thisequationindicates

thatthePRPeffectcanbereducedonlybyshorteningstages1Aor1B,whichcanoccureither

withdual-taskpracticeorwithsingle-taskpracticeonTask1.Single-taskpracticeonTask2should

notreducethePRPeffect(notethatreductionsin2AwouldactuallyincreasethePRPeffect).

Althoughthepredictionsofourthreemainhypotheseshaveneverbeentestedinanappropriate

transfer-of-trainingdesign,previouspracticestudiesprovidesomeusefulclues.VanSelstetal.

(1999)confirmedseveralbottleneckmodelpredictionsregardingfactorinteractionswithSOA

(seebelowformoredetails)afterpractice.Theirresultsprovidesomesupportfortheintact

bottleneckhypothesis.However,VanSelstetal.didnotincludeasingle-taskpracticecondition,

soadirecttestbetweenthecompetingmodels(taskintegration,taskautomatization,intact

bottleneck)wasnotpossible.

Ruthruff,Johnston,andVanSelst(2001)transferredthehighly-practicedVanSelstetal.

participantstonewdual-taskconditionsinvolvingeithertheoldTask2pairedwithanewTask1

(Experiment1),oranewTask2pairedwiththeoldTask1(Experiment2).Althoughtheirfinding

ofbettertransferwiththeoldTask1thanwiththeoldTask2providessomesupportfortheintact

bottleneckhypothesis,theirdesignhasseriouslimitations.Specifically,thetwotransferconditions

hadneitherTask1norTask2incommon.Hence,theresultsmightbeduetospecialproperties

ofthetesttasks(e.g.,onetaskpairmighthavebeenmoredifficultthantheother).Thisproblem

isanunfortunatebyproductofusingthesamecohortofparticipantsthroughoutalltheexperiments

(takingadvantageoftheirheroiclevelsofcumulativepractice).Asexplainedinthenextsection,

thepresentstudyavoidedtheseproblemsbyusingdifferentcohortsofparticipantsfordifferent

trainingconditions.

The present studyThegoalofthepresentstudywastoevaluatethetaskintegration,taskautomatization,andintact

bottleneckhypothesesofhowpracticereducesdual-taskinterference.Wetestedpredictionsof

thesethreehypothesesusingthetransfer-of-trainingparadigmsummarizedinTable1.Participants

receivedoneofthreetypesoftraining:

[Table1willappearhere.Seeendofdocument.]

principlecauseofPRPreduction(e.g.,ifthebottleneckisbypassed,theninterferencewillbesmall

regardlessofstagedurations).

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a)Single-taskpracticeonTask1

b)Single-taskpracticeonTask2

c)Dual-task(PRP)practice

Followingeighttrainingsessions,allthreegroupsthencompletedfourdual-task(PRP)test

sessions.Importantly,thedual-tasktestsessionswereidenticalforthethreetraininggroups.

Assessing the consequences of practice

Weanalyzedtwopossibleconsequencesofpractice:ReductionsinthesizeofthePRPeffect,and

eliminationofthebottleneck.AssessmentofPRPreductionisstraightforward,butassessmentof

bottleneckeliminationisnot.Somepreviousdual-taskstudieshaveusedtheamountofdual-task

costasaproxyforbottleneckelimination,assumingthatsmallPRPeffectsdirectlyindicatethe

eliminationofthebottleneckandlargecostsdirectlyindicatethecontinuedexistenceofthe

bottleneck.Unfortunately,theabsoluteamountofdual-taskinterferenceisnot,byitself,a

completelyreliablediagnostic.EventhebottleneckmodelpredictsthatPRPeffectsshoulddecline

withpractice.Infact,PRPeffectscouldapproachzero,ifTask1centraloperationsarefinished

veryquickly,beforeTask2centraloperationsaresettobegin.Insuchcases,thebottleneckissaid

tobelatent,producingnoobservableRTdelays(seeRuthruff,Johnston,VanSelst,Whitsell,

&Remington,2003).

Weusedseveraldifferentmeasurestoassesswhetherthebottleneckwasinfacteliminated.

First,asexplainedinmoredetailbelow,weexaminedwhetherthePRPeffectwasproportional

toRT1(accordingtothebottleneckmodel,PRP=RT11C2ASOA).Wealsoexamined

severalspecificbottleneckmodelpredictionsforhowtheeffectsofvarioustaskdifficultyfactors

shouldinteractwiththeeffectsofSOA(seebelow).Asanadditionalindicatorofwhethera

bottleneckoccurred,weexaminedtheorderofresponseswithinatrial.Accordingtothecentral

bottleneckmodel,theovertTask1responseshouldprecedetheTask2responseonalmostevery

trial.Butifparticipantsperformcentraloperationsinparallel(asallowedbythetaskintegration

andtaskautomatizationhypotheses),responsesmightoccurineitherorder.

Summary of key predictions

Thetaskintegrationhypothesispredictsthatdual-taskpracticeshouldbemuchmoreeffective

thansingle-taskpractice,reducingandpossiblyeveneliminatingthebottleneck.Thetask

automatizationhypothesispredictseliminationofthebottleneck,withlittleornoresidualPRP

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effect,followingTask1,Task2,ordual-taskpractice.Theintactbottleneckhypothesispredicts

thatthebottleneckshouldpersistwithpractice(albeitshorterindurationduetostage-shortening),

andthatPRPeffectsshouldbemuchsmallerfollowingeitherdual-taskorTask1practicethan

followingTask2practice.

Experiment 1AsinVanSelstetal.(1999),Task1requiredparticipantstosayloworhighinresponseto

low-pitchedorhigh-pitchedtonesandTask2requiredparticipantstopressakeycorresponding

totheidentityofanalphanumericcharacter(1,2,3,4,A,B,C,orD).Forhalfoftheparticipants,

thelettersweremappedcompatiblyontothefourresponsekeysandthedigitsweremapped

incompatibly.Fortheotherhalfofparticipants,theassignmentswerereversed.

FollowingVanSelstetal.(1999),wemanipulatedthreetaskdifficultyfactors:

a)Tonetaskdifficulty(whetherthetonepitchwasnearorfarfromthehigh/lowboundary)

b)Charactertaskcontrast

c)Charactertaskstimulusresponse(SR)compatibility

Ifthebottleneckremainsintactwithpractice:

a)Task1difficultyeffectsshouldcarryoverfullyontoRT2atshortSOAs

b)Task2characterintensityeffectsonRT2shouldbesmalleratshortSOAs(duetoabsorption

intocognitiveslack)thanatlongSOAs

c)Task2SRcompatibilityeffectsonRT2shouldberoughlythesameatallSOAs

Formoredetaileddiscussionofthesepredictions,seeVanSelstetal.(1999;seealsoPashler

&Johnston,1989,1998;Schweickert,1978).

Method

Participants

Theparticipantswere18studentsrecruitedfromcollegesanduniversitiesintheMountainView,

California,areaandfromwork-studyprogramsatNASAAmesResearchCenter.Eachofthe18

participantsperformed12sessions,foragrandtotalof216sessions.Theywerepaidfortheir

participation.

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Stimuli

ThestimulusforTask1wasoneoffourtonespresentedfor150ms.Thetwotoneshighestin

pitch(3,125and625Hz)wereclassifiedashightones,andthetwotoneslowestinpitch(400

and80Hz)wereclassifiedaslowtones.ThestimulusforTask2wasasinglealphanumeric

characterdrawnatrandomfromtheset{1,2,3,4,A,B,C,D}.Thecharacterswerepresentedin

TimesRomanfontataviewingdistanceofabout65cm.Allcharactersfittedintoarectangular

areaof1.41.94visualangle.Thebackgroundwaswhite;thecharacterswereblack(high

contrastcondition)onhalfofthetrialsandgray(lowcontrastcondition)ontheotherhalfofthe

trials.

Apparatus

StimuluspresentationandtimingwereperformedbyIBMPC-compatiblecomputersequipped

withaSchmitt-triggervoicekeyfordetectingspeechonsetandaVoiceConnexionsystem(external)

forrecognizingspeech.

Procedure

Participantswereinstructedtorespondtohightonesbysayinghighandtolowtonesbysaying

low.Theywereinstructedtorespondtotheidentityofthealphanumericcharacterbypressing

theh,j,korlkeysonastandardkeyboard,usingthefingersoftheirrighthand.Forhalf

oftheparticipantsineachtraininggroup,thelettersA,B,C,andDweremappedinalphabetical

orderontothefourresponsekeysfromlefttoright(i.e.,compatibly).Thenumbers,meanwhile,

weremappedinanincompatible,non-sequentialorder(3,1,4,2)ontothesamefourresponse

keys.Fortheremainingparticipants,numbersweremappedcompatibly(1,2,3,4)butletterswere

mappedincompatibly(C,A,D,B).Theinstructionsemphasizedtheimportanceofresponding

quicklyandaccurately.

Eachparticipantwasrandomlyassignedtooneofthethreetraininggroups.TheTask1training

groupperformedonlyTask1(thetonetask)duringtheeighttrainingsessionsandtheTask2

traininggroupperformedonlyTask2(thecharactertask).Thedual-tasktraininggroup,meanwhile,

performedbothTask1andTask2inaPRPdesign.Allthreegroupsperformedthesamenumber

oftrialspertrainingsession(560),althoughthedual-taskgroupmadetworesponsespertrialand

thesingle-taskgroupsmadeonlyoneresponsepertrial.Thus,thedual-taskgroupperformed

Task1asoftenastheTask1traininggroupandperformedTask2asoftenastheTask2training

group.

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Aftertheeighttrainingsessions,allparticipantsthenperformedfourdual-task(PRP)test

sessions(seeTable1).Thesedual-tasktestsessionswereidenticalforallthreegroups.Because

twoofthetraininggroupshadtolearnanewtaskandanewparadigm,thefirsttestsessionwas

consideredpractice.Participantswereinstructedtorespondtobothtasksquicklyandaccurately,

buttoplaceparticularemphasisonthespeedofTask1responses.

Eachsessionconsistedof35warm-uptrialsfollowedby525experimentaltrials.Thesession

wasbrokenintosevenblocksof80trials,separatedbyshortbreaks.Duringeachbreak,the

computerprovidedfeedbackontheaveragespeedofTask1andtheaccuracyofbothTask1and

Task2.Eachtrialbeganwiththepresentationofafixationcrossfor500ms,followedbyarandom

fore-period(100,150,200,or250ms).Inthedual-taskcondition,theTask1tonesoundedfor

150msandtheTask2alphanumericcharacterappearedafteravariableSOA(17,67,150,250,

450,or850ms).IntheTask1trainingcondition,onlythetonewaspresented.IntheTask2training

condition,onlythecharacterwaspresented.ThetimingoftheTask2characterinthissingle-task

conditionwasyokedtothatofthedual-taskcondition:Followingtherandomfore-periodthere

wasanadditionalSOAdelay.Ifeitherresponsewasincorrect,amessageindicatedthetaskon

whichtheerrorhadbeenmade.Also,iftheparticipantrespondedwithin100msofstimulusonset,

aTOOEARLYmessagewasdisplayed.Iftheparticipantfailedtorespondwithin2,500msof

stimulusonset,aTOOSLOWmessagewasdisplayed.Thefixationcrossforthenexttrialwas

presented750mslater.

Analyses

WeconductedseparateANOVAsonmeanRT1,RT2,Task1errorrate,andTask2errorratein

thetestsessions(sessions1012),usingthefactorsoftraininggroup(Task1only,Task2only,

Dual-Task),SOA,Task1difficulty,Task2SRcompatibility,andTask2contrast.Whenassessing

interactionsonRT2betweentask-difficultymanipulationsandSOA,wereliedonaseparate,and

moresensitive,ANOVAthatincludedjusttheshortestandlongestSOAsratherthanallsixSOAs

(whereadjacentSOAstendtoproduceverysimilarperformance).BeforeconductingRTanalyses,

wefirsteliminatedtrialscontaininganerroroneithertask.Then,forboththeerrorandRTanalyses,

weremovedalltrialswithanRToflessthan100ms,followedbytrialsinwhicheitherRT1or

RT2wasidentifiedasanoutlier(lessthan5.6%oftrialsineachexperiment)usingamodified

recursiveoutliereliminationprocedurewithmovingcriterion(describedinVanSelst&Jolicoeur,

1994).Allanalysesusedanalphalevelof.05.

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Results

Dual-task group only (training and test)

Wefirstexamineseparatelytheresultsfromthedual-tasktraininggroup,whichrepresentsa

replicationoftheVanSelstetal.(1999)experiment,albeitwithfewersessions(12vs.36)anda

differentsetofsixparticipants.TheresultsforthisgroupareshowninFig.2(unfilleddiamonds)

acrossthe12sessions,alongwiththeresultsofVanSelstetal.(1999;filledsquares).Eachpoint

representsthemeanPRPeffect(definedasRT2attheshortestSOAminusRT2atthelongest

SOA)forasessionplottedagainstthemeanRT1forthatsession.

Fig.2Meanpsychologicalrefractoryperiod(PRP)effectforeachsessionasafunctionofmeanTask1

responsetime(RT1)acrosssessionsinVanSelstetal.(1999;filledsquares),thepresentExperiment1

(unfilleddiamonds),andthepresentExperiment2(asterisks).Thebestfittingregressionlinewithaslope

of1.0isshownforeachexperiment.

Ifpracticeprimarilyreducesthedurationofcentralstages(forsupportingevidenceseeFletcher

&Rabbitt,1978;Mowbray&Rhoades,1959;Pashler&Baylis,1991;Welford,1976),theintact

bottleneckhypothesismakesanelegantpredictionregardingthesizeofthePRPeffectandRT1

acrosssessions.NotethatRT1equals1A+1B+1CandthePRPeffectequals

1A+1BSOAshort2A(assumingthatabottleneckdelayalwaysoccursattheshortestSOAbut

neveratthelongestSOA;Pashler&Johnston,1989).Theonlytermineachequationthat,by

hypothesis,canvarysubstantiallywithpracticeisstage1B.Hence,practiceshouldreducethe

PRPeffectandRT1byroughlythesameamountacrosssessions.Accordingly,agraphofthePRP

effectagainstRT1acrosssessionsshouldhaveaslopeofabout1.0.AsshowninFig.2,theVan

Selstetal.(1999;filledsquares)dataconfirmedthissimpleprediction(thebest-fittingslopewas

1.02),supportingabottleneckmodelwithcentralstageshortening.

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Infittingthedatafromthedual-tasktraininggroupofthepresentExperiment1,session1was

excludedbecausethemeanRT1wasverylong(1,091ms),indicatingaclearviolationofthe

assumptionthatabottleneckdelayneveroccurredatthelongestSOA(850ms).AsshowninFig.2

(unfilleddiamonds),declinesinthePRPeffectacrosssessionstrackeddeclinesinRT1

approximatelyoneforone,replicatingthefindingofVanSelstetal.(1999).Thebest-fitting

regressionlinewithaslopeof1.0,showninFig.2,wasPRP=RT1321.Allowingtheslopeto

takeonanyvalue,thebest-fittingregressionlinewasPRP=.86*RT1249,buttheadditional

varianceaccountedfor(beyondthemodelwiththefixed1.0slope)wasnotsignificant,

F(1,9)=2.43,MSE=387.6,p>.05.

AlthoughtherelationbetweenPRPandRT1isqualitativelysimilarbetweenthetwostudies,

therearetwonotablequantitativedifferences.First,meanRT1foragivenlevelofpracticewas

muchshorterinVanSelstetal.thaninthepresentstudy(e.g.,308vs.515msinsession12).Some

ofthisdifferencecanbeattributedtogreatermotivationand/orexperienceoftheparticipantsin

theVanSelstetal.study(1999),twoofwhomwereco-authorsofthepaper.Second,theVan

Selstetal.participantsshowedslightlymorePRPeffectforagivenRT1,asreflectedinthePRP

axisintercepts(243vs.321).Oneplausibleexplanationforthisdifferenceisthatthehighly

motivatedVanSelstetal.participantsreducedRT1inpartbyspeakingtheresponsesveryabruptly.

Theconsequentreductioninthedurationofstage1CwouldreduceRT1withoutreducingthePRP

effect,ineffectproducingmorePRPeffectforagivenRT1.

Training sessions

Table2showsmeanRT1andRT2acrosstheeighttrainingsessionsforthethreegroups.To

facilitatecomparisonswiththesingle-taskgroups,thedatafromthedual-tasktraininggroupare

basedononlythelongestSOA(theleastinfluencedbydual-taskinterference).Notsurprisingly,

RTdecreasedsteadilyacrosssessionsforeachtraininggroup.MeanRT1wasshorterforthe

single-tasktraininggroupthanforthedual-tasktraininggroup,butmeanRT2wasnot;aswillbe

seen,asimilareffectwasobservedinExperiment2aswell.Onepossibleexplanationisthat

dual-taskparticipantsinitiallyprepareforTask1aswellasthesubsequentswitchtoTask2(De

Jong,1995;Lien&Ruthruff,2004).Thisinitialspreadingofpreparationbetweenthetwotasks

couldslowdual-taskRT1,butwouldnotslowdual-taskRT2atthelongestSOA(atthisSOA,

participantscanprepareexclusivelyforTask2).

[Table2willappearhere.Seeendofdocument.]

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Task1 RT in test sessions

Figure3showsmeanRT1andRT2,averagedacrossthedual-tasktestsessions,asafunctionof

SOAandtraininggroup.MeanRT1wasshortestforthegroupthathadpracticedTask1only

(365.4ms),nextfastestforthegroupthathadpracticedbothTask1andTask2(522ms),and

slowestforthegroupthathadnotpracticedTask1(575ms).Thesedifferencesweremarginally

significantoverall,F(2,15)=3.2,MSE=3,183,749.1,.05

fortheTask2practicegroup,and221msforthedual-taskpracticegroup.Theinteractionbetween

traininggroupandSOAwasstatisticallysignificant,F(10,75)=2.22,MSE=74,977.8,p

bottleneck.Furthermore,thispersonhadaverylongmeanRT1(990ms),whichislongenough

toclearlyviolatethepreviouslynotedassumptionthatnobottleneckdelayoccurredatthelong

SOA(requiredforthelinearPRP/RT1relationshiptohold).Afurtherreasontodoubtthatany

individualinExperiment1eliminatedthebottleneckistheverylowrateofresponsereversals

(lowsingledigitsforeachparticipant).

Task1 carry-over prediction

ThedataconfirmedtheintactbottleneckpredictionthatTask1difficultyeffectsshouldcarry

overfullyontoRT2atshortSOAs,butnotatlongSOAs.Averagedacrossthethreetraining

conditions,theeffectonRT1was78ms.ThiseffectcarriedoverfullyontoRT2atshortSOAs

(85ms),butnotatlongSOAs(14ms),F(5,75)=11.0,MSE=10,036.0,p

monotonictrendinthepredicteddirection(witha56%reductionincontrasteffectsattheshortest

SOAcomparedwiththelongestSOA);however,thedecreasewasonlymarginallysignificantin

theANOVAcomparingjustthelongestandshortestSOAs,F(1,15)=3.53,MSE=6,148.1,

.05.2,orwhencomparingjusttheshortestandlongestSOAs,F(1,15)=2.2,MSE=14,089.3,

p>.1.Therewasalsonosignificantthree-wayinteractionbetweentraininggroup,Task2SR

compatibility,andSOA,F(2,15)

compatible(2.2%),F(1,15)=28.0,MSE=344.5,p

additiontothisspecificconcernitis,ofcourse,genericallyvaluabletoreplicateresultsunder

differentconditions.

Toaddresstheseconcerns,wereversedthetaskorderinExperiment2:Thecharactertask

becameTask1andthetonetaskbecameTask2.Inallotherrespects,thedesignwasidenticalto

thatofExperiment1.Notethatmostpreviousstudiesusingauditoryvocalandvisualmanual

taskshaveutilizedthetaskorderofExperiment1;studiesusingtheoppositeorderarerelatively

rare.Accordingtotheintactbottleneckhypothesis,thePRPeffectshouldstillbesmallerfollowing

practiceonTask1alonethanfollowingpracticeonTask2alone,eventhoughtheparticular

judgments(charactervs.tone)assignedtothesetaskshavebeenexchanged.Furthermore,we

shouldstillfindnoevidenceoftheeliminationofthebottleneck.

Withthetaskorderreversed,abottleneckwouldproduceadifferentpatternofinteractionswith

SOA.ThecharacterintensityandSRcompatibilitymanipulationeffectsarenowplantedinTask1

and,therefore,shouldcarryoverfullyontoRT2atshortSOAs.Thebottleneckmodelpredictions

forthedifficultyofthetonetask(nowTask2)arelessclear,becausetheprocessinglocusofthis

effectisnotknown.IfthisfactorprimarilyinfluencesthecentralstageofTask2(i.e.,stage2B),

theeffectsonRT2shouldbeconstantacrossSOAs.Ifthisfactorinsteadinfluencestheinputstage

ofTask2(i.e.,stage2A),theeffectsonRT2shouldbesmalleratshortSOAsthanatlongSOAs

(duetoabsorptionintocognitiveslack).

Method

Exceptforreversingtherolesofthetonetask(nowTask2)andthecharactertask(nowTask1),

themethodwasidenticaltothatofExperiment1.The18participantsinthisexperimentdidnot

participateinExperiment1.

Results

Dual-task group only (training test)

WefirstreporttheresultsfromthegroupinExperiment2thatperformeddual-tasksessionsduring

boththetrainingandtestphases,showninFig.2(asterisksymbols).DeclinesinthePRPeffect

trackeddeclinesinRT1approximatelyoneforoneacrosssessions,justasfoundinExperiment1

andVanSelstetal.(1999).Thebest-fittingregressionlinewithaslopeof1.0was

PRP=RT1218;thislineisshowninFig.2.Ifweinsteadallowtheslopetotakeonanyvalue,

thebestfittingregressionlineisPRP=1.06*RT1252.8.Allowingtheslopetovarydoesnot

18

accountforasignificantamountofvariancebeyondthatalreadyaccountedforwithaslopefixed

at1.0,F(1,9).05.Thesefindingsareconsistentwiththeintactbottleneck

hypothesis.

ThepointsforExperiment2generallylieabout100msabovethoseforExperiment1.Why

mightthishaveoccurred?Apriori,fasterinputprocessingforTask2(fasterstage2A)in

Experiment2thaninExperiment1(thatis,fastertoneclassificationthancharacterclassification)

wouldhavebeenexpectedbecauseauditoryinformationshouldgettothecortexfasterthanvisual

informationandbecauseaneasyphysicaldiscrimination(tonepitch)shouldbefasterthanone

basedonlearnedcategories(characteridentification).Suchadifferenceinthedurationofstage2A

wouldincreasethesizeofthePRPeffectforagivenRT1;hence,itwouldproducetheobserved

differencesinFig.2.

Training phase

Table3showsmeanRT1andRT2acrosstheeighttrainingsessionsforthethreegroups.Not

surprisingly,RTsdecreasedsteadilyacrosssessions.Also,meanRT1againtendedtobelonger

forthedual-tasktraininggroupthanforthesingle-tasktraininggroup,whichmightreflectacost

ofhavingtoinitiallypreparetwotasksratherthanjustone.

[Table3willappearhere.Seeendofdocument.]

Task1 RT in test sessions

Figure6showsmeanRT1andRT2asafunctionofSOAforeachtraininggroup.Averagedacross

thefourtestsessions,meanRT1wasfastestforthegroupsthathadpracticedTask1inthetraining

phase(550msfortheTask1traininggroupand528msforthedual-tasktraininggroup),and

slowestforthegroupthatdidnot(609msfortheTask2traininggroup).Withonlysixparticipants

pergroup,however,thisdifferencedidnotquitereachstatisticalsignificanceacrossparticipants,

F(2,15)=1.672350,MSE=904,083.5,p=.22.Afollow-upanalysisrevealedthattheeffectof

previoustrainingonTask1RTwaslargeduringtheinitialtestsession,butthenshrankoverthe

nextfewtestsessions.AsdiscussedintheTask1carry-oversectionbelow,meanRT1depended

onbothTask1contrast,F(1,15)=147.0,MSE=4,336.3,p

tointerpretthem.Thesesamethreefactorsalsointeractedsignificantlywiththetypeoftraining,

F(2,15)=3.8,p

butseveralclearlydidnot.Thislattersubsetofparticipants(filledsymbols)showsamuchsmaller

PRPeffectthanwouldbepredictedbasedontheirmeanRT1,andthereforearecandidatestohave

eliminatedthebottleneck.Wereturntothisissuelater,afterpresentingadditionalevidenceregarding

factorinteractionswithSOA.

Fig.7MeanPRPeffectversusmeanRT1foreachparticipantinExperiment2.Trianglesdual-tasktraining

group,squaresTask1traininggroup,circlesTask2traininggroup.Thepointsrepresentingthe4participants

suspectedofhavingbypassedthebottleneckarefilled;thepointsrepresentingtheother14participantsare

unfilled.

Task1 carry-over prediction

Thebottleneckmodelpredictsthatmanipulationsofbottleneckorpre-bottleneckstagesofTask1

shouldcarryoverfullyontoRT2atshortSOAsbutnotatlongSOAs.ThereweretwoTask1

variablesinExperiment2thatwecanusetotestthecarry-overprediction:ContrastandSR

compatibility.

TheeffectofTask1contrastonRTtoTask1itselfwas31.4msanditseffectonRT2was10.6,

20.3,23.1,18.0,22.9,and3.0msatthe17,67,150,250,450,or850msSOAsrespectively.Thus,

contrarytothecentralbottleneckmodelprediction,theeffectofcontrastonTask1didnotappear

tocarryoverfullyontoRT2attheshortSOAs.Furthermore,theinteractiononRT2between

contrastandSOA(shortvs.long)wasnotstatisticallysignificant,F(1,15).2.

BecausetheeffectofTask1SRcompatibilityonRT1(96ms)wasmuchlargerthantheeffect

ofTask1contrast(31ms),itprovidesanevenbetteropportunitytoevaluatetheTask1carry-over

prediction.Figure8showsthecarry-overofTask1compatibilityeffectsontoRT2foreachtraining

21

group(filledsymbols)aswellasthemeanTask1compatibilityeffectonRT1(averagedacross

SOAs;unfilledsymbols).Averagedacrossthethreetraininggroups,theeffectofTask1

compatibilitywas85.2,71.4,61.8,66.9,41.8,and10.3msatthe17,67,150,250,450,or850ms

SOAsrespectively.ThisinteractionbetweenTask1compatibilityeffectsandSOA(shortvs.long)

wassignificant,F(1,15)=14.9,MSE=20,326.6,p

compatibility,Task2tonedifficulty,andtrainingtypewasalsosignificant,F(2,15)=3.72,

MSE=51.8,p

Evidence of two distinct subgroups

Ourfirstindicationthatsomeparticipantshadeliminatedthebottleneckcamefromananalysisof

responseorder.BecausethebottleneckmodelassumesthatTask1centraloperationsareperformed

beforeTask2centraloperations,theresponsesshouldbeemittedinthissameorder;thisprediction

wasconfirmedinExperiment1andinmanypreviousPRPstudies.Withoutabottleneck,however,

itisentirelypossibleforTask2responsestobeemittedbeforeTask1responsesattheshortest

SOA;infact,becauseTask2tooklesstimethanTask1(whenperformedalone),responsereversals

shouldoccurmoreoftenthannot.Indeed,fourparticipantsinExperiment2reversedresponses

onaveryhighproportionoftrials(6698%),oftenrespondingtoTask2afewhundredmilliseconds

beforeTask1.Noneoftheother14participantsreversedresponsesonmorethan8%oftrials.

Thisenormousgapinthepercentageofresponsereversalsbetweenmembersofthetwogroups

stronglysupportsacategoricaldifferencebetweenthesegroups.

Thefourparticipantswithhighfrequenciesofresponsereversalsalsohappenedtoshow

considerablysmallerPRPeffects(range:3367ms)thananyoftheremaining14participants

(range:125505ms).Evenmoreimportantly,theyproducedsmallerPRPeffectsthanthebottleneck

modelwouldpredictbasedontheirmeanRT1,byroughly300ms.InFig.7,thesefourparticipants

correspondtothefourdatapoints(filledsymbols)thatliethefurthestbelowtheregressionline.

TherewereafewadditionalparticipantswithPRPeffectsbelowtheregressionline,butnoneof

themfrequentlyreversedtheresponseorder.Thus,thereareonlyfourparticipantswithclear

evidenceofhavingeliminatedthebottleneck.Forthesakeofbrevity,wecallthesefourindividuals

bypassersandtheremaining14participantsbottleneckers.

Asanadditionalindicatorofwhetherthebottleneckwasbypassed,wecomputedtheamount

ofTask1carry-overforthebypassersandbottleneckers.Ifthebottleneckhasbeeneliminated,

thereisnoobviousreasonforTask1difficultyeffectstostronglyinfluenceRT2.Butifthereis

abottleneck,theeffectsofthemanipulationsshouldcarryoverfullyontoRT2(asdiscussedabove).

Forthebypassers,theeffectofTask1SRcompatibilityeffectwas62msonTask1,butonly

13msonTask2.Forthebottleneckers,however,theTask1SRcompatibilityeffectonRT1was

103ms,andthiseffectcarriedoverfullyontoRT2(109ms).Thesefindingsfurthersupportour

classificationofparticipantsasbypassersandbottleneckers.Theyalsoexplainwhytheoverall

datashowedsignsofincompletecarry-over:Thedatarepresentedamixtureoffourparticipants

whobypassedthebottleneck(producingverylittlecarry-over)and14participantswhodidnot

(producingcompletecarry-over).Table4summarizesthedifferentialamountsofcarry-over

24

betweenthetwogroupsofparticipants,aswellastheotherprominentempiricaldifferences

mentionedabove.

[Table4willappearhere.Seeendofdocument.]

Attempts to reconcile the bottleneck model with small PRP effects

Torescuethebottleneckmodel,itmightbearguedthatthebypassersperformedTask1central

operationsveryquickly,beforeTask2centraloperationswereevenreadytobegin(i.e.,the

bottleneckwaslatent;Ruthruff,Pashler,&Hazeltine,2003).However,thisaccountisimplausible

giventherelativelylongmeanRT1(563ms).Anotherpossibilityisthatthebypassersgenerally

reversedthecentralprocessingorder(Task2beforeTask1).Thishypothesiscorrectlypredicts

thehighfrequencyofresponsereversalsandtheneareliminationofinterferenceonTask2.

However,itthenmustpredictdual-taskinterferenceonTask1.Contrarytothisprediction,mean

RT1was565,565,572,559,551,and564msatthe17,67,150,250,450,and850msSOAs

respectively.

Itisalsoconceivablethatthelimitationunderlyingthebottleneckstillheld,butparticipants

circumventeditbyperformingcentraloperationsononlyonetaskpertrial(guessingrandomly

ontheother).Thishypothesispredictsthatthebypassersshouldshowadramaticincreaseinerror

rates.Inactuality,therewasonlyaslighttrendforthebypasserstomakemoreerrorsthanthe

bottleneckers(5.3vs.4.0%onTask1;4.2vs.3.6%onTask2),withconsiderableoverlapinerror

ratesbetweenmembersofthetwogroups.Becauseweseenowaytoreconcilethedatafromthese

fourparticipantswiththebottleneckmodel,weinsteadconcludethattheyfoundawaytoselect

andexecuteresponsestobothtasksatthesametime.

Factors that promote bottleneck bypass

ClearevidenceofbottleneckeliminationisextremelyrareinthePRPliterature(asdiscussedin

moredetailbelow).Itisimportant,therefore,toconsiderwhatfactorsmayhavefacilitatedbypassing

thebottleneckinExperiment2.Themostobviousenablingfactoristhehighlevelpractice(4,480

trainingtrialsplus2,240dual-tasktesttrialsperparticipant).Anotherpotentiallyimportantfactor

istheabsenceofinputmodalityconflictsandoutputmodalityconflicts.Athirdfactoristheuse

ofparticularinput/outputmodalitypairings(auditoryvocalandvisualmanual)thatminimize

dual-taskinterference(seeHazeltine,Ruthruff,&Remington,2004;Shaffer,1975;VanSelst&

Johnston,1997).Althoughthesefactorsarelikelytobeimportant,evennecessary,foreliminating

thebottleneck,theyclearlyarenotsufficient.Thesefactorswerepresentforallparticipantsinall

25

conditionsofbothexperiments,yetbottleneckeliminationoccurredinonlyafewcases.Below

weconsiderseveralotherfactorsthatmighthelpexplainwhenthebottleneckcanbebypassed.

Trainingconditiondoesnotappeartobeacriticalfactor.InExperiment2,atleast1outofthe

6participantsineachofthethreetraininggroupsappearedtobypassthebottleneck(1intheTask1

onlygroup,2intheTask2onlygroup,and1inthedual-taskgroup).Althoughmoreparticipants

eliminatedthebottleneckfollowingtrainingonTask2alone,thisdifferencewasnotstatistically

significant.Giventhehighcostsofrunningtheseexperiments(12sessionsperparticipant),it

wouldbeprohibitivetotestmuchlargersamplestodefinitivelyassessdifferentialeffectsoftraining

condition.However,toimproveourestimateoftheextenttowhichTask2trainingpromotes

bottleneckelimination,weinstructedanadditionalsixparticipantstoperformunderthiscondition.

Only1ofthese6additionalparticipantsshowedevidenceofbottleneckelimination,asinthe

originalTask1practiceanddual-taskpracticegroups.Thus,thereisnoevidencethatanyparticular

trainingconditioniscriticalforeliminatingthebottleneck.

Onefactorthatdidappeartostronglyinfluencethelikelihoodofeliminatingthebottleneckwas

taskorder.WiththetaskorderofExperiment1(Task1:Auditoryvocal;Task2:Visualmanual)

therewasnoevidencethatanyofthe18participantswereabletoeliminatethebottleneck.

Furthermore,noneofthesixparticipantsinVanSelstetal.(1999)wereabletoeliminatethe

bottleneckwiththistaskorder.Withtheoppositetaskorder(Task1:Visualmanual;Task2:

Auditoryvocal;Experiment2),however,therewasstrongevidencethat4outof18participants

wereabletoeliminatethebottleneck.Inthefollow-upexperimentdiscussedabove,usingthe

Task2trainingconditionofExperiment2,anotherparticipant(1outof6)alsobypassedthe

bottleneck.Combiningalloftheseresultstogether,0outof24participantseliminatedthebottleneck

withthetypicaltaskorder,but5outof24participantseliminatedthebottleneckwiththereverse

taskorder.2Thisfindingsuggeststhatfuturestudiestaketaskorderintoconsideration,although

itisnotyetclearwhetherthisfactorwillbeimportantwithothertaskpairs.

AnothercriticalfactormightbeanindividualsskilllevelinperformingTask1and/orTask2,

whichshouldbereflectedinthemeanRTs.ThemeanRT1was563msforbypassersand564ms

forbottleneckers.Thus,Task1skillappearsnottobeimportant.ThemeanRT2atthebaseline

longSOA,however,wasmuchsmallerforbypassers(358ms)thanforbottleneckers(572ms),

2Achi-squaredtestrevealedthatthisdifferenceisstatisticallysignificant,2(1)=5.581,p

t(16)=3.71,p

criticalempiricalquestionswerehowthedifferenttrainingtypeswouldinfluencethePRPeffect

andwhethertheywouldallowparticipantstoeliminatetheprocessingbottleneck.

Task integration hypothesis

Accordingtothetaskintegrationhypothesis,dual-taskpractice(butnotsingle-taskpractice)allows

participantstoefficientlyintegrateperformanceofthetwotasks,possiblyeliminatingthebottleneck.

Accordingly,thedual-tasktrainingconditionshouldproducethegreatestreductioninthePRP

effect.Contrarytothisprediction,dual-taskinterferenceinExperiments1and2wasnosmaller

followingdual-taskpracticethanfollowingsingle-taskpracticeonTask1.Hence,thepresentdata

providenoevidenceoftaskintegration.Itremainspossible,ofcourse,thatintegrationoccurs

underdifferentcircumstances(i.e.,withevenhigherlevelsofpracticeorwithtaskscloselytied

toacommongoal,suchasbrakingandsteeringacar).

Intact bottleneck hypothesis

Accordingtotheintactbottleneckhypothesis,practicedoesnotleadtotaskintegrationor

automatization.Althoughthebottleneckremainsintactwithpractice,itsdurationcandecline

(producingasmallerPRPeffect)asthedurationofcertainTask1stages(1Aand1B)become

shorter.Accordingly,Task1practiceanddual-taskpracticeshouldgreatlyreducedual-task

interference,butTask2practiceshouldnot.ThispredictionwasconfirmedinExperiment1

(Task1:Auditoryvocal;Task2:Visualmanual).Thedatafurtherconfirmedthepredictionsthat

Task1difficultyeffectsshouldcarryoverfullyontoTask2atshortSOAs,andthatthePRPeffect

andRT1shouldbelinearlyrelatedacrosspracticesessions(seeFig.2)andacrossindividuals(see

Fig.4).TherewasnoevidencethatanyindividualparticipantinExperiment1wasabletoeliminate

thebottleneck.Thus,withthistaskorder,weconcludethatpracticereduceddual-taskinterference

solelybyshorteningTask1stagedurations,leavingthebottleneckintact.Thisconclusionisthe

sameonereachedbyVanSelstetal.(1999),whoalsostudieddual-taskpracticeeffectswiththe

sametasksinthesameorder.

InExperiment2,weusedataskorderoppositetothatofExperiment1.Thisparticulartask

order(Task1:Visualmanual;Task2:Auditoryvocal)hasrarelybeeninvestigatedindual-task

studies,butweuseditheretounconfoundtheeffectsofTask1/Task2practicefromtheeffects

oftone/charactertaskpractice.Ontheonehand,wefoundevidencethatforthemajorityof

participants(14outof18)thebottleneckremainedintact.Ontheotherhand,wealsofoundclear

evidencethataminorityofparticipants(4outof18)hadcompletelyeliminatedthebottleneck.

28

AsdiscussedinExperiment2andsummarizedinTable4,thisdivisionofparticipantsinto

bypassersandbottleneckersissupportedbyseveralconvergingmeasures.First,thebypassers

showedverysmallPRPeffects(only55ms,comparedwith342msforthebottleneckers),which

wereroughly200mssmallerthantheintactbottleneckhypothesiswouldhavepredictedbased

ontheirmeanRT1(seethefourfilleddatapointsinFig.7).Second,eachbypasserrespondedto

thetasksinanorderoppositetothepresentationorder(oftenbyafewhundredmilliseconds)on

atleast66%oftrialsattheshortestSOA.Third,thebypassersshowedverylittlecarry-overof

Task1difficultyeffectsontoRT2.Thesefindingsstronglysuggestthat,contrarytotheintact

bottleneckhypothesis,thebypasserswereabletoselectresponsestothetwotasksvirtually

independently.

Task automatization hypothesis

Accordingtothetaskautomatizationhypothesis,practice(eithersingle-taskordual-task)allows

participantstolearnhowtoperformtaskswithoutlimitedcentralresourcesandthusbypassthe

centralbottleneck.Thus,allthreetypesoftrainingshouldproduceverysmallPRPeffects.Contrary

tothisprediction,weobservedlargemeanPRPeffectsforallthetrainingtypesinbothexperiments.

Thus,thetaskautomatizationhypothesis,byitself,isinconsistentwiththeoverallresults.Looking

atindividuals,thishypothesisisinconsistentwithresultsfromalloftheparticipantsinExperiment1

and14of18participantsinExperiment2.However,itcanexplaintheresultsofthe4participants

inExperiment2whobypassedthebottleneckdespitehavingpracticedonlyoneofthetwotasks.

Which task was automatized?

Wenotedabovethatautomatizationofeithertaskcould,inprinciple,allowparticipantstobypass

thebottleneck.Therefore,itispossiblethatthebypassersinExperiment2hadautomatizedjust

oneofthetwotasks.Consistentwiththispossibility,thebypassersrespondedmuchmorequickly

thanthebottleneckerstoTask2(evenatthelongSOA)butnottoTask1.Thisobservationsuggests

thatthebypassershadautomatizedTask2(thetonetask)butnotTask1(thecharactertask).

Thetonetaskmighthavebeenmoresusceptibletoautomatizationbecauseithadlessneedof

centralbottleneckresourcestobeginwith,eitherbecausethestimulusclassificationwasbasedon

aphysicalproperty(tonepitch)ratherthananabstract,learnedproperty(characteridentity),or

becausetheSRmappingwashighlycompatible(sayhightoahightone)ratherthanincompatible

(thevisualtaskhadanincompatiblemappingforhalfthestimuli).Adifferentpossibilityisthat

auditorystimuliarebetterthanvisualstimuliatgainingaccesstothebottleneckbypassroute(e.g.,

29

ajumper-cableroute)becauseauditorystimulimorereadilyattractattentiontothemselves

(cf.Posner,Nissen,&Klein,1976;Wickens&Liu,1988).Furtherresearchisneededtodetermine

whichofthesefactors(easeofstimulusclassification,SRcompatibility,differenceinalerting

ability)isthekeytofacilitatingtaskautomatization.Along-termgoalistodeterminehowtasks

canbeautomatizedandwhysomepeoplecanaccomplishthisbetterthanothers.

What caused the effects of task order?

Whenwepresentedtheauditoryvocaltaskfirstandthevisualmanualtasksecond,noneofthe

24participants(poolingtogetherExperiment1andVanSelstetal.,1999)producedclearevidence

ofhavingbypassedthebottleneck.Usingthissametaskorderandasampleofsixyoungeradults

andsixolderadults,Maquestiaux,Hartley,andBertsch(inpress)alsofoundthatparticipantshad

bypassedthebottleneckafterpractice.Whenweusedtheoppositetaskorder,however,5outof

24participants(poolingtogetherExperiment2andtheassociatedcontrolexperiment)produced

clearevidenceofhavingbypassedthebottleneck.AsnotedintheDiscussionsectionof

Experiment2,thiseffectoftaskorderwasstatisticallysignificant.Interestingly,asimilareffect

oftaskorderwasobservedinJohnstonandDelgado(1993)whenanunpracticedbutexceptionally

easytaskwasusedaseitherTask1orTask2(seebelowforfurtherdiscussionoftheirfindings).

Howcanweexplaintheeffectsoftaskorderontheprobabilityofeliminatingthebottleneck?

Aboveweproposedthatthetonetaskwasautomatizedforsomeparticipants.Because

automatizationofeitherTask1orTask2shouldeliminateconflictforthecentralbottleneck

resource,thishypothesisincorrectlypredictsbottleneckeliminationwithbothtaskorders(notjust

whenthetonetaskservedasTask2).Clearly,someadditionalassumptionsareneededtoexplain

taskordereffects.

Oneattractivehypothesistoexplaintaskordereffectsisthattasksrecruitavailablecentral

resourcesevenwhentheyarenotneededtoperformthattask.Toillustratethisgreedyresource

usagehypothesis,considertheanalogyofabankteller(abottleneckmechanismthatcanserve

onlyonecustomeratatime).Supposethattwocustomersarriveatthebank,onewhorequiresthe

teller(e.g.,toperformawiretransfer)andonewhodoesnot(e.g.,towithdrawcash).Iftheperson

whodoesnotrequirethetellerarrivesfirsthemight,forconvenience,engagethetellerevenif

thereisanearbyautomatedtellermachine(ATM).Oncethiscustomerhasengagedtheteller,the

othercustomer(whorequirestellerassistance)mustwait(abottleneckdelay).Nowconsiderthe

oppositearrivalorder,wherethecustomerrequiringtellerassistancearrivesfirstandengagesthe

teller.Whentheothercustomerarrives,hecannotimmediatelyusetheteller.Insteadofwaiting,

30

however,hecouldsimplyutilizethenearbyATMtowithdrawcash,thusavoidingabottleneck

delay.Followingthisanalogy,thegreedyresourceusagehypothesispredictsanasymmetryinthe

PRPparadigm,suchthatabottleneckdelaycanbeavoidedonlywithonetaskorder(wherethe

taskthatcanusethebypassrouteservesasTask2).

Ifwenowcombinethegreedyresourceusagehypothesiswiththeaboveconclusionthatthe

tonetaskismoreamenabletoautomatizationwithpracticethanthelettertask,itfollowsthat

bottleneckbypasswouldbemorelikelytooccurwhenthetonetaskservesasTask2(asin

Experiment2)thanwhenitservesasTask1(Experiment1).Thus,thishypothesiscanexplain

thetaskordereffectsobservedinourstudy(andthoseofJohnston&Delgado,1993).Atthesame

time,weacknowledgethatthishypothesisispost-hocandrequiresfurthertesting.

Practice can (sometimes) eliminate the bottleneck

Oneofthemostimportantfindingsofthepresentstudyisthatitisindeedpossibletoeliminate

thecentralbottleneckwithpractice.Priortothisstudy,therewaslittlepublishedevidencethat

thiswaspossible.ManyearlystudieshadfoundthatpracticehadverylittleinfluenceonthePRP

effect.VanSelstetal.(1999)latershowedthatpracticecandramaticallyreducePRPeffectswhen

inputandoutputconflictswereeliminated.Interestingly,oneparticipantshowednoPRPeffect

all.ThisparticipantsmeanRT1wassoshort(~250ms),however,thateventhecentralbottleneck

couldhavebeenlatent,producinglittleornoobservablePRPinterference.Ruthruff,Johnston,

VanSelst,Whitsell,&Remington(2003)laterconfirmedthelatentbottleneckaccountofthis

participantsdata.WhentheSOAbetweentaskswasadjustedtobringthecentraloperationsof

thetwotasksmorecloselyintoalignment,PRPinterferencereturned.

Smalldual-taskinterferenceeffectshavealsobeenobservedafterpracticeinanon-PRPparadigm

whereparticipantsperformpureblocksthatcontainjustonetaskortheotherandmixedblocks

thatcontainsomesingle-tasktrialsandsomedual-tasktrialswitha0-msSOA(Hazeltineetal.,

2002;Levy&Pashler,2001;Schumacheretal.,2001).However,thesesmalldual-taskeffects

wereobtainedwithanunusuallyeasytask:Participantssimplypressedabuttonthatcorresponded

spatiallywiththelocationofavisualstimulus.Consequently,meanRTsweretypicallylessthan

300ms.Itisquitepossiblethatthebottleneckwasnoteliminatedinthesestudies,butinsteadwas

latent(seeHazeltineetal.,2002forrelevantdiscussion).Interestingly,afewstudiesusingthis

dual-taskparadigmbutwithmoredifficulttaskshaveoftenfoundlongermeanRTsandlarger

amountsofdual-taskinterference(Levy&Pashler,2001,Experiment2;Schumacheretal.,2001,

Experiment3;butseealsoHazeltineetal.,2004,Experiment3).

31

Inconsideringthelatentbottleneckhypothesis,itiscriticalthatthebypassersinthepresent

Experiment2showedsolittlePRPeffectdespitehavingrelativelylongmeanRT1s(between492

and631ms).LongRT1sareusuallytheresultoflongcentralstagedurations.Accordingly,itis

implausiblethatinterferencewasavoidedsimplybecausethetwotasksneverdemandedcentral

operationsatthesametime,especiallygiventhatweusedmultipleSOAsacrossawiderange(17

to800ms).Itisalsoimportanttonotethat,unlikemanypreviouslypublishedstudies,wedidnot

baseourinferencessolelyonthemagnitudeofPRPeffects.Instead,wealsoexaminedthePRP/RT1

relationship,severalfactoreffects,andthefrequencyofresponsereversals.Futurestudiesdesigned

tolookforbottleneckbypassshouldalsousetheseconvergingindicators.

Previous evidence of the elimination of the bottleneck

HavingfoundclearevidenceofbottleneckeliminationinExperiment2,wenowtakeastepback

andconsiderhowthisfindingfitsinwiththepreviousevidence(albeitscarce)ofbottleneck

eliminationinRTparadigms.Indoingso,itishelpfultoclassifychoiceRTtasksaccordingtothe

compatibilityoftheSRtranslationand,hence,theirdemandoncentraloperations.Theeasiest

tasksareideomotorcompatibletasks,suchasshadowing(repeatingaloudwhateverwordyou

hear)wherethestimuluscodescloselyresembletheresponsecodes(Greenwald,1972).Intermediate

indifficultyarecompatibletasks,wherethestimuluscodesandresponsecodesdonotresemble

eachotherbuthavesomenaturalrelationship(e.g.,inourtonetaskparticipantssaidlowto

low-pitchedtonesandhightohigh-pitchedtones).Thenexthighesttierofdifficultywould

includearbitrarytasks,wherethereisnopre-learnedassociationbetweenstimuliandresponses.

Thehighesttierofdifficultyincludesincompatibletasks,wherethemappingopposessome

pre-learnedassociation.

Fortheeasiesttaskcategory(ideomotorcompatible),thebottleneckmightbeexpectedto

generallybebypassed.Interestingly,severalstudieshavereachedtheoppositeconclusion(e.g.,

Lien,McCann,Ruthruff,&Proctor,inpress;Lien,Proctor,&Allen,2002).Otherstudieshave

reportedsmallPRPeffects(e.g.,Greenwald,1972;Greenwald&Shulman,1973;Greenwald,

2003),butbecauseRTsweresoshortitisdifficulttoruleoutalatentbottleneckaccount(see

Lien,Proctor,&Ruthruff,2003).Pashler,Carrier,andHoffman(1993;Experiments1and2),

however,didfindstrongevidenceforbottleneckbypasswhenTask1wastheusualtonetaskand

Task2requiredaneyemovementtoavisualstimulus(anideomotorcompatibletask).Interestingly,

thepatternofresultswassimilartothatofthebypassersinExperiment2:AlthoughthePRPeffects

weregreaterthanzero,participantsgenerallyrespondedtoTask2wellbeforeTask1atshort

32

SOAs.JohnstonandDelgado(1993)foundsimilarevidenceofbottleneckbypasswithacompatible

Task1andanideomotorTask2thatrequiredcontinuoustrackingofavisualstimulususinga

joystick.Atthesametime,theyfailedtofindclearevidenceofbypasswithseveralothertracking

tasks,soideomotorcompatibilityisapparentlynotasufficientconditionforbottleneckbypass.

Interestingly,JohnstonandDelgadoalsofailedtofindclearevidenceofbottleneckbypasswhen

thecontinuoustrackingtaskservedasTask1.Thisfindingprovidesfurthersupportforourgreedy

resourceusagehypothesis,whichpredictsthatbottleneckbypassshouldoccuronlywhenthe

automatizedtaskservesasTask2.

Weareawareofnopreviousstudieswithclearevidenceoftheeliminationofthebottleneck

witheithercompatible,arbitrary,orincompatibletasks.Thepresentstudy,however,didfindsuch

evidence.Ourtonetaskusedacompatiblestimulusresponsemapping,whereasourcharactertask

usedacompatible/incompatiblemapping(dependingonwhetherthecharacterwasadigitora

letter).Itshouldbenoted,however,thatbottleneckeliminationoccurredonlyforasubsetof

participants,onlywithonetaskorder,andonlyafterextensivepractice.

SummaryThepresentstudyprovidednoevidencethatdual-taskpracticeallowsparticipantstoefficiently

integratetaskpairs.Instead,theprimaryeffectofpracticewastoshortenstages,leavingthe

bottleneckintactbutshorterinduration.However,practice(single-taskanddual-task)alsoallowed

afewparticipantstoautomatizeoneorbothtasksandavoidthecentralbottleneck.

Thesedatarepresentthestrongestevidenceyetthatpracticecaneliminatethebottleneck.This

findingisanimportantleadforfurtherresearchaimedatuncoveringthenatureoftheprocessing

bottleneckandhowtocircumventit.Atthesametime,weshouldnotlosesightofthefactthat

thisintriguingfindingrequiredthecombinationofseveralfavorableconditions.Theseconditions

includeextensivepractice,relativelyeasytasks(especiallycomparedwithmanyreal-worldtasks),

anabsenceofinputconflictsandoutputconflicts,andpreferredmodalitypairings(auditoryvocal

andvisualmanual).Evenwiththesefavorableconditions,wefoundevidenceofbottleneck

eliminationonlywithoneofthetwopossibletaskorders.Evenwiththemorefavorabletaskorder,

itoccurredonlyforaminorityofparticipants(4outof18).Thus,atthispoint,eliminationofthe

bottleneckisstillarareexception.

33

AcknowledgementsThisresearchwasfundedbytheAirspaceOperationsSystemsProjectof

NASAsAirspaceSystemsProgram.WethankMei-ChingLien,HalPashler,andRobertProctor

forhelpfulcommentsonanearlierdraftofthepaper.

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36

Tab

le1Task(s)performedineachsessionofExperiments1and2asafunctionofthetraininggroup(Single/Task1,Single/Task2,Dual-task).Eachtrainingconditionusedadifferent

groupofparticipants.InExperiment1,Task1wasthetonetask(auditoryvocal)andTask2wasthecharactertask(visualmanual)task.InExperiment2,thetaskorderwasreversed

Trainingcondition

Session

Dual-task

Single/Task2

Single/Task1

Task1/Task2

Task2

Task1

1Training

Task1/Task2

Task2

Task1

2Task1/Task2

Task2

Task1

3Task1/Task2

Task2

Task1

4Task1/Task2

Task2

Task1

5Task1/Task2

Task2

Task1

6Task1/Task2

Task2

Task1

7Task1/Task2

Task2

Task1

8Task1/Task2

Task1/Task2

Task1/Task2

1Test

Task1/Task2

Task1/Task2

Task1/Task2

2Task1/Task2

Task1/Task2

Task1/Task2

3Task1/Task2

Task1/Task2

Task1/Task2

4

37

Tab

le2MeanresponsetimeforTask1(tone)andTask2(character)duringtheeighttrainingsessionsforeachtraininggroupinExperiment1.Forthedual-taskpracticegroup,mean

responsetimesarebasedonthelongstimulusonsetasynchrony(SOA;baseline)conditiononly

Trainingcondition

Session

Dual-task(longSOA)

Single/Task2

Single/Task1

Task2character

Task1tone

Task2character

Task1tone

Task2character

Task1tone

993

1,090

1,095

440

1722

767

718

365

2669

697

672

336

3629

647

644

321

4595

602

639

311

5569

581

600

320

6585

617

602

308

7566

566

624

297

8

38

Tab

le3MeanresponsetimeforTask1(character)andTask2(tone)duringtheeighttrainingsessionsforeachtraininggroupinExperiment2.Forthedual-taskpracticegroup,mean

responsetimesarebasedonthelongSOA(baseline)conditiononly

Trainingcondition

Session

Dual-task(longSOA)

Single/Task2

Single/Task1

Task2tone

Task1character

Task2tone

Task1character

Task2tone

Task1character

726

845

597

592

1633

664

564

554

2556

620

549

522

3509

588

549

501

4500

589

595

525

5516

567

533

459

6509

558

522

450

7538

558

503

424

8

39

Tab

le4DifferencesinresultsbetweenthetwosubgroupsofparticipantsinExperiment2.PRPpsychologicalrefractoryperiod,RT2responsetimetoTask2

Nobottleneckgroup(n=4)

Bottleneckgroup(n=14)

55ms

342ms

PRPeffect

82%

2%Responsereversals

21%

106%

Carry-overofTask1compatibilityeffectsontoRT2

40

IntroductionBackgroundThe psychological refractory periodCan practice eliminate the central bottleneck?

The present studyAssessing the consequences of practiceSummary of key predictions

Experiment 1MethodParticipantsStimuliApparatusProcedureAnalyses

ResultsDual-task group only (training and test)Training sessionsTask1 RT in test sessionsPRP effect on Task2 in test sessionsPRP effect vs. RT1 for individualsTask1 carry-over predictionTask2 absorption predictionTask 2 SR compatibilityTask1 error ratesTask2 error rates

Discussion

Experiment2 (reverse task order)MethodResultsDual-task group only (training test)Training phaseTask1 RT in test sessionsPRP effect on Task2 in test sessionsPRP effect vs. RT1 for individualsTask1 carry-over predictionTask2 tone difficulty effectsTask1 error ratesTask2 error rates

DiscussionEvidence of two distinct subgroupsAttempts to reconcile the bottleneck model with small PRP effectsFactors that promote bottleneck bypassRe-examining PRP effects as a function of training group

General discussionTask integration hypothesisIntact bottleneck hypothesisTask automatization hypothesisWhich task was automatized?What caused the effects of task order?Practice can (sometimes) eliminate the bottleneckPrevious evidence of the elimination of the bottleneck

SummaryReferences