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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:[email protected]
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
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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
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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
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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
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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.
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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.,
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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,
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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).
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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
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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
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AcknowledgementsThisresearchwasfundedbytheAirspaceOperationsSystemsProjectof
NASAsAirspaceSystemsProgram.WethankMei-ChingLien,HalPashler,andRobertProctor
forhelpfulcommentsonanearlierdraftofthepaper.
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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
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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