Successes and failures in producing attentional object-based ...

Over 30 years of research using Posner's spatial cueing paradigm has shown that selective attention operates on representations of spatial ... Skiptomaincontent Advertisement SearchSpringerLink Search Successesandfailuresinproducingattentionalobject-basedcueingeffects DownloadPDF DownloadPDF AbstractOver30 yearsofresearchusingPosner’sspatialcueingparadigmhasshownthatselectiveattentionoperatesonrepresentationsofspatiallocations,leadingtospace-basedtheoriesofattention.Manipulationsofstimuliandmethodshaveshownthisparadigmtobesensitivetoseveraltypesofobject-basedrepresentations—providingevidencefortheoriesincorporatingobject-basedattentionalselection.Thispapercriticallyevaluatestheevidencedemandingobject-basedexplanationsthatgobeyondpositingspatialrepresentationsalone,withanemphasisonidentifyingandinterpretingsuccessesandfailuresinobtainingobject-basedcueingeffects.Thisoverviewofcurrentevidenceisusedtogeneratehypothesesregardingcriticalfactorsintheemergenceandinfluenceofobjectrepresentations—theirgeneration,strength,andmaintenance—inthemodulationofobject-basedfacilitatoryandinhibitorycueingeffects. Manydifferentexperimentalparadigmshavebeenusedtoinvestigatetheroleofobjectsinvisualselection(seeCave&Bichot,1999,andScholl,2001,forreviews).Throughconvergingoperations,strongevidencehasemergedrevealingthatobject-basedrepresentationscanmediateattentionalselection.Examplesofsuchparadigmsincludestudiesoftheattentionalblinkwithinandbetweenobjects(e.g.,Conci&Müller,2009),dividedattentionwithinandbetweenobjects(e.g.,Atchley&Kramer,2001;Awh,Dhaliwal,Christensen,&Matsukura,2001;Duncan,1984;Vecera,Behrmann,&Filapek,2001;Vecera,Behrmann,&McGoldrick,2000),responsecompetition(e.g.,Baylis&Driver,1992;Kramer&Jacobson,1991;Richard,Lee,&Vecera,2008),multiple-objecttracking(e.g.,Scholl,Pylyshyn,&Feldman,2001),negativepriming(e.g.,Mari-Beffa,Houghton,Estevez,&Fuentes,2000;Tipper,Brehaut,&Driver,1990),visualmarking(e.g.,Watson&Humphreys,1998),visualsearchinmultiple-objectdisplays(e.g.,Enns&Rensink,1990;Goldsmith,1998;Grossberg,Mingolla,&Ross,1994;Rensink&Enns,1995;Treisman,1982;Wolfe&Bennett,1997),andspatialcueing(e.g.,Egly,Driver,&Rafal,1994;Tipper,Driver,&Weaver,1991).Ineachoftheseliteratures,positingthatattentionoperatesonrepresentationsofobjectshasprovidedmorethoroughandsatisfyingexplanationsofdatathanwouldsimplyassumingthatattentionoperatesonspatialrepresentationsalone.Thepresentreviewconcentratesonevidenceforclaimsofobject-basedselectionfromstudiesusingthespatialcueingparadigm.Whyspatialcueing?Akeymotivationforfocusingonspatialcueingisthatclaimshaveemergedthatobject-basedspatialcueingeffectsarefragile:smallerinmagnitude,uneasilypredicted,andnotobservedunderaswidearangeofexperimentalmanipulationsasspace-basedeffects(forfacilitatoryeffects,see,e.g.,Kwak&Egeth,1992;Robertson&Kim,1999;forinhibitoryeffects,List&Robertson,2007;McAuliffe,Pratt,&O’Donnell,2001;Pratt&McAuliffe,1999;Schendel,Robertson,&Treisman,2001).Althoughallcandidaterepresentationsofattentionencodespaceandproducespace-basedeffects,itisthecontentionofthepresentreviewthatanassumptionthatattentionoperatesonrepresentationsofobjectsandtheirpartsisrequiredinordertofullyexplainevidencefromthespatialcueingparadigm.Surveyingthelargenumberofdatapointsavailablefromspatialcueingexperimentsmayrevealsomeimportantfactorsthatcontributetotheobservationandmagnitudeofobject-basedeffects.Apartfromempiricallyexaminingtherobustnessofobject-basedeffects,thereareatleastthreeotherreasonstofocusonthespatialcueingparadigmwhenlookingforevidenceinsupportofobject-basedrepresentations.First,spatialcueingisaseeminglysimpleparadigmthatoughttoyieldtractableinsightintothepropertiesofattentionalselectionbeforeconsideringbehaviourinmorecomplexparadigms.Second,despitetheconceptualsimplicityofspatialcueing,manyofitsdifferentaspectsandparametershavebeeninvestigated,yieldingalargedataset,fromoneofthehistoricallyearliestparadigmsusedtoprobeselectiveattention.Importantly,theseincludemanipulationsofperceptualfactorsthatmayinfluencethetypeofrepresentationsmediatingselection.Historically,thepresenceofconvergingevidenceproducedbyalargenumberofstudiesgeneratedfromdifferentlaboratoriesmeansthatresearcherscanhaveincreasedconfidencethattheyarenotbeingmisledbyTypeIorTypeIIerrors.Third,aswillbeclearlater,thespatialcueingtaskallowsfortheexperimentalexaminationoffacilitatoryandinhibitorymechanismsofattention,supplyingfoundationalevidencethatfacilitatoryandinhibitoryprocessescanoperateinparallelandondifferentrepresentationsproducedbythesamevisualdisplay.Theaimofthisreviewistorecountandevaluatereportedobject-basedspatialcueingeffectsinordertounderstandsuccessesandfailuresinobtainingthem,andconsequentlytomakepredictionsabouttheirlikelyoccurrenceandmagnitudeinfutureinvestigations.Indoingso,wecriticallyevaluateobject-basedeffectsandthenecessityofexplainingspatialcueingeffectsintermsofattentionoperatingonrepresentationsofobjects,orwhetherpositingmoreprimary,space-basedrepresentationsissufficienttoexplainthedata.Afterdescribingthespatialcueingparadigmanditsresultingfacilitatoryandinhibitoryeffects,wereviewevidencepertainingtotheinfluenceofspatialfactorsonthepresenceandmagnitudeofsucheffects.Evidencefortheroleofthegenerationandmaintenanceofobject-basedrepresentationsinthemodulationoffacilitatoryandinhibitorycueingeffectswillthenbereviewedfromspatialcueingstudiesusingmovingobjectstimuli,followedbyareviewofstudiesusingstaticobjectstimuli.Potentialfactorsinvolvedintheemergenceandmaintenanceofobjectrepresentationsduringspatialcueingtasksarediscussedineachsection.Finally,alternativespace-basedaccountsofthedataareevaluatedinlightofthecurrentevidence,followedbyasummaryandconclusions.CovertorientingandthespatialcueingparadigmImaginethatyouaredrivingalongacountryroad.Abirdswoopsthroughtheairaboveyouroncomingpath.Yourvisualsystemmayhaveprocesseditspresenceandyoumayhavedisregardeditasathreatwithouthavingmovedyoureyes,ormoreimportantly,withouthavingbegunsteeringtowardsthebird’slocation.Suchcovertorientingofattentionallowsustofiltersensoryinformationintheabsenceof,orpriorto,anyovertrealignmentofsensoryreceptorstowardsthestimulus.Giventhepresenceoflimitationsinprocessingcapabilities(sensoryandmotor),covertorientingisfundamentaltotheselectiveandefficientprocessingofsensoryinformation,anditplaysakeyroleinourabilitytomonitorandinteractwiththeenvironmentanditsphysicalcontents.Thespatialcueingparadigmhasbeenoneoftheprimarymethodsusedtostudycovertattentionalprocessingforthepast30 years.Atypicalspatialcueingtaskinvolvesthesimultaneouspresentationoftwoplaceholders(e.g.,squares)atequaldistancestotheleftandrightofacentralfixationpoint;seeFig. 1a.Oneplaceholderisbrieflyhighlightedbyaluminanceincrement,whichisreferredtoasa“peripheralcue”andispresumedtoorientattentiontothatplaceholderorcuedlocation.Thetargetispresentedashorttimelatereitherwithintheperipherallycuedplaceholder(thecuedorvalidlocationcondition)orwithintheoppositeplaceholder(theuncuedorinvalidlocationcondition).Whilespatialcueingofteninvolvesasimpletargetdetectionresponse,variantsincludetargetdiscriminationandtargetidentification—respondingbasedonaprespecifiedfeatureorcombinationoffeatures(e.g.,Brawn&Snowden,2000;Maylor,1985;Tanaka&Shimojo,1996).Fig.1 aTypicalspatialcueingtask(Posnercueingtask)andbtypicalpatternofRTresultsdependingoncue–targetstimulusonsetasynchrony(SOA).Adaptedfrom“InhibitionofReturn”byR.M.Klein,2000,TrendsinCognitiveSciences,4,pp.138–147.Copyright2000byElsevier.AdaptedwithpermissionFullsizeimage Spatialcueingtypicallyproducessignificantbenefitsorcostsforsubsequenttargetprocessingatthecuedlocation,dependingonthecue–targetonsetasynchrony(e.g.,Posner,1980;Posner&Cohen,1984)—seeFig. 1b.At50-to300-mscue–targetonsetasynchronies(alsocalledstimulusonsetasynchronies,orSOAs),targetdetectionisspeededorfacilitatedbycueing:Thatis,responsetimes(RTs)arefastertotargetsappearingatcued(valid)ascomparedtouncued(invalid)locations.Incontrast,atcue–targetintervalsof300–3,000 ms(e.g.,Samuel&Kat,2003),RTsareslowerforcuedthanforuncuedtargets.ThislatereffectofslowingofRTstopreviouslyattendedlocationsisknownasinhibitionofreturn,orIOR(e.g.,Klein,1988;Posner&Cohen,1984;Posner,Rafal,Choate,&Vaughan,1985).Whatismeantbyfacilitationandinhibitionofreturn?ItisfrequentlyassumedthattheRTbenefitsatshortcue–targetintervals,andRTcostsatlongercue–targetintervals,reflectthecomplementaryprocessesoffacilitationandinhibition.Ontheonehand,facilitationasatheoreticalconstructmaybeconstruedasacognitiveprocessassociatedwithexcitationatthelocusofsensoryinformation(e.g.,Houghton&Tipper,1994;Moran&Desimone,1985).Similarly,inhibitionmaybeconsideredasaprocessthatsuppressesprocessingofsensoryinformationorresponsestoit(e.g.,Kingstone&Pratt,1999;Taylor&Klein,2000).Akeyresearchquestionishowthesetwofundamentalprocessesoperateandinteractduringattentionalselection.Thereisnowgeneralagreementthatspatialcueingcanproduceparallelfacilitatoryandinhibitorysignals,andthatresponsestosubsequenttargetsmayreflecteffectsarisingfromtheinteractionofthetwosignalsduringtargetprocessing.Thisviewissupportedbybothpsychophysical(e.g.,Ro&Rafal,1999;Tassinari,Aglioti,Chelazzi,Peru,&Berlucchi,1994;Tassinari&Berlucchi,1993)andneurophysiological(e.g.,Mevorach,Humphreys,&Shalev,2006)studies.Giventheindependenceoffacilitationandinhibition,therelativespeedingorslowingoftargetdetectionRTsmaynotnecessarilyreflecttheoperationofeitherafacilitatoryoraninhibitoryprocess,butratherthetemporalcombinationofthetwoparallelsignals(e.g.,Tassinarietal.,1994;seealsothereviewbyKlein,2000).Forthisreason,throughoutthisarticle,thetermsfacilitatoryandIOReffectsareusedtorefertotherelativespeedingorslowingofRTs,respectively,withoutassumingapriorithateacheffectderivesuniquelyfromeitherasinglefacilitatoryenhancementorinhibitorysuppressionofstimulusprocessing.Representationsmediatingspatialcueing:spaceandobjectsSpace-basedeffectsofspatialcueingThespatialcueingparadigmwasinitiallyusedtogatherevidencethatattentionoperatesonspace-basedrepresentations,wherebyinformationisrepresentedasorganisedaccordingtoitslocationinspace,resultinginbiasedsensoryprocessingforeventsatattendedlocationsrelativetootherlocations(e.g.,Downing&Pinker,1985;Eriksen&Hoffman,1972;Posner,Snyder,&Davidson,1980).Thenotionofaspatialrepresentation,however,isfarfromaunitaryconcept.Manydifferentspatialregionsandrelationsneedtoberepresented,suchasthespacebetweenobjects,thelocationthatanobjectoccupieswithintheworld,thespacewithinanobject’scontours(e.g.,ateacup),orthespacebetweenanobjectandtheobserver(seeRobertson,2004,foradiscussion).Similarly,differentcomponentsofspatialrepresentationsareimposedbytheneuralarchitectureofthevisualsystem.Irrespectiveofwhetherwearedealingwithspacewithinorbetweenobjects,orhowthisspaceisultimatelyimplementedneurally,inspace-basedrepresentationsspatialregionsandrelationsareexplicitlyencodedandareconsideredtoprovidetheroutetoselectionofinformationatthelocationoccupied,whileformorstructureisimplicitwithinthespatialmedium,onlyinfluencingselectionbywayofgroupinglocations(e.g.,Vecera&Farah,1994). Spatialcueingeffectsbetweenobjects Inearlyspatialcueingstudies,wherethecueandtarget(orplaceholderscontainingthem)weretheonlyobjectsinthedisplay,bothfacilitatoryandIOReffectswereinfluencedbywherethecueandtargetappeared,aswellasbytheirspatialseparation.Facilitatorycueingeffectsweakenascue–targetdistanceincreases,bothintwo-dimensionalspace(e.g.,Egly&Homa,1991;LaBerge,1983;LaBerge&Brown,1989;Shulman,Remington,&McLean,1979;Tsal,1983;Zimba&Hughes,1987)andindepth(e.g.,deGonzagaGawryszewski,Riggio,Rizzolatti,&Umiltà,1987;Downing&Pinker,1985).Similarly,IOReffectscanbeweakenedbycue–targetseparationintwo-dimensionalspace(e.g.,Bennett&Pratt,2001;Berlucchi,Tassinari,Marzi,&DiStefano,1989;Klein,Christie,&Morris,2005;Maylor&Hockey,1985;Snyder,Schmidt,&Kingstone,2001),aswellasbywhetherthecueandtargetappearonthesamesideofeithertheverticalorhorizontalvisualmeridians(e.g.,Berlucchietal.,1989;Tipperetal.,1997).Thesefindingsledtoanemphasisonselectiveattention’sspatialnature,withsuchmetaphorsasaspotlightmovingthroughspacelightinganareaofthevisualfield(e.g.,Eriksen&Eriksen,1974;Posneretal.,1980;seethereviewbyCave&Bichot,1999),zoomlenses(e.g.,Eriksen&StJames,1986;Eriksen&Yeh,1985),orspatialgradients(e.g.,Downing&Pinker,1985;LaBerge&Brown,1989). Studiesinvestigatingthespatialcoordinatesthatspatialcueingoperateswithin,haveshownspacetobecodedindifferentframesofreference(Fig. 2).Evidencehasshownthatinformationcanbecodedrelativetoanaxisdefinedinrelationtotheviewer(Fig. 2a)orbythepositionintheenvironmentrelativetoalandmarkinthevisualdisplay(e.g.,Maylor,1985;Posner&Cohen,1984).Laterstudiespresentingcuesandtargetsatdifferentlocationsin3-Dspaceprovidedevidencefortheviewer-centredcodingofspacebetweenobjects(cueandtarget;e.g.,Andersen,1990;Andersen&Kramer,1993;Arnott&Shedden,2000;Gawryszewskietal.,1987;Downing&Pinker,1985).Fig.2Illustrationofthedifferentframesofreferencethatspatialcueingeffectsmightoperateunder.Thefirstcolumnillustratestheinitialpositionsofthecueandtargetstimuli.Thesecondandthirdcolumnsillustratethecueandtargetpositionsfollowingplanerotation.Lightgreyareasindicatespatialcueingeffectsassociatedwiththeenvironmentallocationrelativetotheviewer(viewer-centredframesofreferenceyieldinglocation-basedspatialcueingeffects).Darkgreyareasindicatespatialcueingeffectsassociatedwiththeobject(scene-basedframesofreferenceyieldingobject-basedspatialcueingeffects)Fullsizeimage Spatialcueingeffectswithinobjects Laterspatialcueingstudiesstartedtointroduceobjectcontoursthatsurroundedboththecueandtargetandtoexaminetheeffectofsuchobjectcontourpresenceonthedistributionofspatialattention(e.g.,Eglyetal.,1994;Hollingworth,Maxcey-Richard,&Vecera,2011;Reppa,Fougnie,&Schmidt,2010;Robertson&Kim,1999).Thesestudieshaveshownthatwithin-objectlocationscanbecodedinobject-centredframesofreference(Fig. 2c),definedrelativetooneoftheobject’saxes(e.g.,elongationorsymmetry),andthatobject-associatedcueingeffectsremainunchanged,irrespectiveofchangesinother,viewer-centredframesofreference(e.g.,Gibson&Egeth,1994;Reppaetal.,2010;Umiltà,Castiello,Fontana,&Vestri,1995).Forinstance,Umiltàetal.showedthatfacilitatorycueingeffectsweretiedtothecuedlocationwithinanobject,irrespectiveofwhetherthecuedlocationhadchangedrelativetotheviewer. Whilewithin-objectlocationscanbecodedinobject-centredframesofreference,studiesinvestigatinghowattentionspreadsacrosstheboundedsurfaceofanobjecthaveshownthatwithin-objectshiftsaresimilartounboundedbetween-objectshifts.Forinstance,cueingeffectscanincreasewithincreasingcue–targetdistance,bothinthespacebetweendifferentobjects(e.g.,Downing&Pinker,1985)andinthespacewithinobjects(e.g.,Eglyetal.,1994;Hollingworthetal.,2011).Furthermore,justaswithspacebetweenobjects,locationsinthespacealonganobject’ssurfacecanbecodedinviewer-centredcoordinates,withshiftsofattentionawayfromtheviewerleadingtolargerfacilitatoryeffectsthandoesshiftingattentiontowardstheviewer(e.g.,Reppaetal.,2010).Therefore,shiftsofattentionacrossobjectspacecanresembleshiftsofattentioninthefeatureless,unboundedspacebetweenobjects.Thisfindinghasledsometheoriststoproposethatcueingeffects—facilitatoryandinhibitory—aresolelymediatedbyrepresentationsofspatiallocations.Theseaccountswillbediscussedinalatersection,butfirstevidenceforobject-basedspatialcueingeffectswillbediscussed. Object-basedeffectsofspatialcueingAlthoughspaceisallaroundus,oureverydayexperienceismostlyrelatedtothingsoccupyingspace—forinstance,objects.Inlinewithcommon-senseexperienceandincreasedabilitytopresentrealisticobjects,eventuallyspatialcueingstudiesstartedtoexaminethepossibilitythatattentionmayoperateonrepresentationsofobjects,asopposedtosimplyrepresentationsofthespacetheyoccupy.Indeed,theintroductionandmanipulationofobjectcontoursasstimuliinspatialcueingtaskshadsignificantconsequencesforthepattern,presence,andmagnitudeofspatialcueingeffects(e.g.,Eglyetal.,1994;Reppaetal.,2010;Robertson&Kim,1999;Tipperetal.,1991).StartingwiththeseminalworkofTipper,Driver,andWeaver(1991)andEgly,Driver,andRafal(1994)usingmovingandstaticobjectdisplays,respectively,spatialcueingstudieshaveshownthatatargetdetectionordiscriminationresponsefollowingacuecanbeinfluencedbythepresenceofobjectboundariesdefiningauniformcontour.Inmovingobjectdisplays,cueingeffectscanremaintiedtothecuedobject,irrespectiveofwhetherithaschangedlocationinspacedefinedinviewer-centredcoordinates(e.g.,Tipperetal.,1991;Tipper,Weaver,Jerreat,&Burak,1994).Instaticobjectdisplays,cueingeffectscanbetiedtotheselectionofobjectcontours:Targetsappearingwithinthesamecontourasthecuearerespondedtofasterthanaretargetsappearingwithinadifferentcontourfromthecue(e.g.,Eglyetal.,1994;He&Nakayama,1995;Jordan&Tipper,1999;Vecera,1994).Thesefindings,reviewedindetaillater,haveraisedthepossibilitythatvisualselectioncanbeobject-based,selectingrepresentationsofobjectsratherthanrepresentationsthatareorganisedstrictlyspatially,regardlessoftheobjectcontourspresentinthevisualinput.Thetermobject-basedis,initself,debated.Accordingtothemostcommonmeaningofobject-basedattention,attentionalselectionmightoperateondiscreteperceptualgroupsembodyingtheirownvisualproperties,includingthespacetheyoccupy,overothergroups.Itisthisusagethatthepresentreviewwillascribeto.Incontrast,itissometimesconsideredthatthenotionofspatiallyinvariantobject-centredrepresentations—thatis,representationsofobjectsindependentlyofthespacetheyoccupy—shouldbethehallmarkofobject-basedattention(e.g.,Vecera,1994;Vecera&Farah,1994).However,thehypothesisofspatiallyinvariantobjectrepresentationsisbasedononesetoftheoriesinobjectrecognition,namelyasubsetofstructuraldescriptiontheories(e.g.,Biederman,1987;Marr,1982),andhasreceivedlimitedempiricalsupport.Unsurprisingly,testsofthishypothesisintheselectiveattentionliteraturehavesimilarlyfailedtofindsupport(e.g.,Kramer,Weber,&Watson,1997;Schendeletal.,2001).Theearliestevidenceforobject-basedselectionandforthenotionthatobjectrepresentationsmayunderliesuchselectioninthespatialcueingparadigmcomesfromstudiesthathaveusedmovingobjectdisplays.Thesearereviewednext.Object-basedcueingeffectsinmovingobjectdisplaysMovingobjectdisplaysprocedureOnechallengingdesignissueforstudiesofobject-basedattentionhasbeendissociatingthecueingeffectsassociatedwithdisplayobjectsfromtheeffectsassociatedwiththelocationsthattheyoccupy.Instudiesusingmovingdisplays,thetermobjectreferstospatiallyandtemporallycontiguouscontours,suchasmightconstituteanobjectfileortoken(e.g.,Kahneman,Treisman,&Gibbs,1992).Onewaytoseparatetheeffectsduetoobjectversuslocationhasbeentocueanobjectoccupyingaspecificlocationandthentochangethelocationofthecuedobjectviarotational(e.g.,Tipperetal.,1991)ortranslational(e.g.,Christ,McCrae,&Abrams,2002;Ro&Rafal,1999;Soto&Blanco,2004)motion,thusmaintainingtheobject’sspatiotemporalcontinuity.Followingaback-to-centrecue,participantsaresubsequentlyaskedtodetectatargetthatappearsonthepreviouslycuedobjectatitsnewlocation,oratthecuedlocation,whichisnowoccupiedbyadifferentobjectthatwasnotpreviouslycued(seeFig. 3).Therationaleisthatifselectiononlyimplicatesrepresentationsofspatiallocationscodedsolelyrelativetotheviewer,thencueingbenefitsorcostsshouldnotbefoundfortargetsthatappearontheobjectafterithasmovedtoanewlocation(Fig. 2a).Conversely,ifselectionimplicatesrepresentationsofobjectsindependentlyoftheirspatiallocation,thencueingbenefitsorcostsshouldbeobservedevenifthepreviouslycuedobjecthasmovedtoadifferentlocation(Fig. 2b).Fig.3Examplesofmovingobjectdisplays.Inthe90°rotationcondition,thetargetappearsatanuncuedlocationbutinthecuedobject.Inthe180°rotationcondition,thetargetappearsatthecuedlocationandintheuncuedobject.IORinbothconditionswascalculatedastheRTfortheuncuedobjectminustheRTforthecuedobject.Adaptedfrom“Object-CentredInhibitionofReturnofVisualAttention”byS.P.Tipper,J.Driver,&B.Weaver,1991,QuarterlyJournalofExperimentalPsychology,43A,pp.289–298.Copyright1991byTaylor&Francis.AdaptedwithpermissionFullsizeimage Evidenceforobject-basedrepresentationsfrommoving-objectstudiesStudiesinselectiveattentionhaveusedthemoving-objectsmethodtoexaminehowobjectsinfluencespatialcueingeffectsindependentlyoftheirspatiallocation(e.g.,Abrams&Dobkin,1994;Behrmann&Tipper,1999;Christetal.,2002;Gibson&Egeth,1994;Kahnemanetal.,1992;Lamy&Tsal,2000;McCrae&Abrams,2001;Müller&vonMühlenen,1996;Ro&Rafal,1999;Soto&Blanco,2004;Tipper&Behrmann,1996;Tipperetal.,1991;Tipper,Jordan,&Weaver,1999;Tipperetal.,1994;Umiltàetal.,1995;Vivas,Humphreys,&Fuentes,2008;Weaver,Lupiáñez,&Watson,1998).ThismethodhasbeenprimarilyusedtoexamineIOReffects,withfewerstudiesexaminingfacilitatoryeffects(e.g.,Lamy&Tsal,2000;Soto&Blanco,2004).Intheiroriginalstudy,Tipperetal.(1991)usedavariantofthespatialcueingparadigminwhichtwosquareswerepresentedperipherallyoneithersideofacentral(fixation)placeholder.Oneofthetwoperipheralsquareswasbrieflycuedbeforebothsquaresstartedtorotatearoundthecentralplaceholderby90°or180°(Fig. 3).Subsequenttargetdetectionwasslowedforthecuedsquare,despiteitshavingmovedtoanewlocation,providingevidencecompatiblewiththeideathatIORcueingeffectscanbeobject-based.Thiswasthefirstdemonstrationofobject-basedIORshowingthatIORcanbeassociatedwithascene-basedframeofreference,whereeachobjectoccupiesalocationinascenethatmaybeseparatefromitslocationinother,viewer-centredframesofreference.Later,instudiesexaminingfacilitatorycueingeffects,similarresultsemergedshowingthatfacilitatoryeffectscanbeassociatedbothwithspace-basedandwithobject-basedrepresentationsofthesamedisplayaslongastheobjectsmaintainedtheirspatiotemporalcontinuity(e.g.,Lamy&Tsal,2000;Soto&Blanco,2004),asopposedtowhentheydidnot(e.g.,Schendeletal.,2001),andregardlessofwhetherornottheywererelevanttothetask(e.g.,Soto&Blanco,2004).Soonafterthefirstdemonstrationofobject-based(scene-based)IOReffectsbyTipperetal.(1991),studiesstartedshowingthatobjectscaninfluenceselectiveattentionbydefiningacoordinatesystemwithinwhichotherstimuli/eventscanbecoded(seeFig. 2c).Evidencethatselectiveattentioncanoperatewithinobject-centredframesofreferencehascomefromstudieswithbothneuropsychologicallyimpaired(e.g.,Behrmann&Tipper,1994,1999;Humphreys&Riddoch,1994,1995;Tipper&Behrmann,1996)andhealthy(e.g.,Gibson&Egeth,1994;Reuter-Lorenz,Drain,&Hardy-Morais,1996;Tipperetal.,1999;Umiltàetal.,1995)adults.Inthespatialcueingliterature,bothfacilitatory(e.g.,Umiltàetal.,1995)andIOR(e.g.,Gibson&Egeth,1994;Tipperetal.,1999)cueingeffectshavebeenobservedwithinobject-centredframesofreference.Thesestudieshaveshownthatthetwocueingeffectscanbeassociatedwithpartofanobjectasitmovesthroughspace,asopposedtobeingtiedtoafixedenvironmentallocationrelativetotheviewer.MultipletypesofrepresentationcanbeactiveinparallelBothfacilitatoryandIOReffectshavebeenshowntobemediatedbyrepresentationscodedinmorethanoneframeofreferenceduringthesameexperiment.Inspatialcueingstudieswithhealthyadults,viewer-andobject-centredframesofreferencewerefoundtobesimultaneouslyactiveandtoinfluencespatialcueing(seeFig. 2e),asdemonstratedbyGibsonandEgeth,(1994;seeTable 1).TheyuseddisplaysofasinglebrickrotatingindepthtoexaminewhetherlocationsdefinedwithrespecttothebrickwouldelicitIORsimilarinmagnitudetothatfromlocationsintheenvironmentfixedwithrespecttotheviewer.TheyfoundsignificantIORforlocationsdefinedbothwithrespecttotheviewer(viewer-centred)andtheobject(object-centred).Justasobject-andviewer-centredrepresentationsmaycoexist,scene-basedandviewer-centredrepresentationsmaybeactiveinparallelwithinasingleexperimentalsetting(Fig. 2d;e.g.,Tipperetal.,1999;Tipperetal.,1994;Weaveretal.,1998).Tipper,Weaver,Jerreat,andBurak(1994;seeTable 1)foundevidenceforscene-based(object-based)andviewer-centredcuecosts:IOReffectsweresignificantfortargetsappearingonthecuedsquareaswellasfortargetsappearingatthecuedlocation(nowoccupiedbyadifferentobject).Furthermore,IOReffectswerelargerwhentheobjectdisplayswerestaticasopposedtowhentheyweremoving,suggestingcumulativeIOReffectsarisingfromtheencodingoflocationsinbothviewer-centredandscene-basedframesofreference.Table1SummaryofIORcueingeffects(inmilliseconds)inmovingobjectstudiesreportedasafunctionofobjecttype,targetduration,stimulusonsetasynchrony(SOA),andmostrecentcue−to−targetinterval(MRCTI)Fullsizetable Notonlycandifferentframesofreferencebesimultaneouslyactive,buttheycanindependentlymediatefacilitatoryandIOReffects.Tipper,Jordan,andWeaver(1999)usedathree-squaredisplay(showninTable 1).Inthescene-basedcondition,thethreesquareswereunconnected,encouragingtheperceptofthreeseparateobjectsinascene.Intheobject-centredcondition,thesquareswereconnectedbylines,encouragingtheperceptofasinglelargeobject(e.g.,atriangle).Inthescene-basedcondition,asignificantIOReffectoccurredbothfortargetsappearingonthecuedobjectandforthoseappearingatthecuedlocation(nowoccupiedbyadifferentobject),replicatingpreviousevidenceforthesimultaneousoperatingofIORwithinscene-basedandviewer-centredrepresentations(e.g.,Tipperetal.,1991).However,intheobject-centredcondition—wherethesquareswereconnectedwithstraightlinestoencouragetheperceptofasingleobject—adifferentpatternofresultsemerged.WhiletherewasasignificantIOReffectforthecuedobjectpart(object-centredorpart-basedIOR),foranuncuedobjectpart(oneoftheuncuedsquaresappearingatthecuedlocation),ratherthanviewer-centred/location-basedIOR,therewasasignificantfacilitatoryeffect.TheTipperetal.(1999)studywasofcriticalimportancefortworeasons.First,itwasanearlydemonstrationthatfacilitatoryandinhibitoryprocessesmediatingcovertorientingcanoperateinparallelondifferentrepresentationsarisingfromthesamestimulusdisplay.Whenasingleobjectwasperceived,therewassignificantIORforthecuedpartoftheobject,suggestingthatIORresultedfromanobject-centred(part-based)representationofthedisplay.Meanwhile,therewassignificantfacilitationforthecuedlocationintheenvironment,suggestingthatthefacilitatorycueingeffectwasmediatedbyaviewer-centred(environmental-location-based)representation.ThesecondreasonTipperetal.(1999)wasimportantisthatenvironmental-location-basedcostswereeliminatedinthepresenceofsignificantobject-centredcosts.Thissuggeststhatwhenperceptualinformationencouragesadifferent,complexorganisationofthedisplay—thesceneisinterpretedasasingleobjectasopposedtothreeindependentobjects—earlierspatialrepresentationsmaynolongerbeconsultedorinfluenceselectiveattentiontothesameextent.InthecaseofTipperetal.(1999),therepresentationoflocationsinaviewer-centredframenolongermediatedIOReffects,eventhoughitwasavailablefortheprocessesproducingfacilitatoryeffects.Someboundaryconditionsinfindingobject-basedeffectsinmovingobjectdisplaysThemajorityofstudiesexaminingspatialcueingeffectsinmovingobjectdisplayshavereportedsignificantobject-basedIOReffects.Nevertheless,ithasbeennotedintheliteraturethatcertainexperimentalparametersmayeitherfavourobject-basedfacilitatoryeffectsoverobject-basedIOReffects,oreveneliminatebotheffects(e.g.,Ro&Rafal,1999).Todate,theinfluentialboundaryconditionsaffectingthelikelihoodofobservingobject-basedIORinmovingobjectdisplayshaveincludedstimulusandpragmaticfactors.Anonsalientback-to-centrecue(Ro&Rafal,1999),lackoforambiguousspatiotemporalcontinuityofanobject(e.g.,Lamy&Tsal,2000;Schendeletal.,2001),andambiguousobjectcueing(Tipperetal.,1994)haveallbeenshowntodiminishobject-basedIORinmovingobjectdisplays.Similarly,highlevelsofexperienceorpractice(e.g.,Müller&vonMühlenen,1996;Weaveretal.,1998)havebeenfoundtoreduceobject-basedeffects.Finally,observationsimplicatingfacilitatoryandinhibitoryprocessesascoexistinghavesuggestedthattheirnetcontributionstoresponsetimemayconcealobject-basedeffects,asseemedtooccurinRoandRafal(1999).Inamoving-objectexperiment,Tipperetal.(1994,Exp.6;seeTable 1)foundthatifthecuedobject(oneofthestaticormovingsquares)wasambiguousduetoocclusionatthetimeofcueing,nosignificantIOReffectsemergedforthatobject.However,IOReffectsweresignificantforobjectsthatwerevisibleatthetimeofcueing.Thisfindinghasbeenreplicatedinlaterexperiments(e.g.,Takeda&Yagi,2000)andsuggeststhatcueingeffectsaremediatedbyexplicitrepresentationsabouttheobjectthatwascued.Similarfindingsandconclusionshavebeendrawnfromstudiesthathaveexaminedthefacilitatoryeffectofspatialcueing(e.g.,Lamy&Tsal,2000;Soto&Blanco,2004)bymanipulatingthespatiotemporalcontinuityoftheobject.Forinstance,LamyandTsal(2000,Exp.2)foundsignificantobject-basedeffectsonlywhentheobjectinwhichthecueappearedmaintaineditsspatiotemporalcontinuityintheintervalbetweencueandtargetpresentationbysmoothlymovingtoadifferentpositionbeforethetargetwaspresented.Incontrast,whenspatiotemporalcontinuitywasabsentandthestateofobjectfileswasambiguousbecausethecuedobjectabruptlyappearedintheuncuedlocation,noobject-basedfacilitatoryeffectoccurredfortargetsappearingwithinit(Lamy&Tsal,2000,Exp.1;seealsoSchendeletal.,2001).Thesefindingsreaffirmwhatoneintuitivelymightexpect:thatthepresenceandvisibilityofanobject—asasinglespatiotemporalentitydefininganobjectfile—isanecessaryrequirementforobject-basedeffectstoemerge.Althoughobjectcontoursclearlyplayanimportantroleinproducingcueingeffects,themerepresenceofanobject’soutlinecontourisnotsufficienttoelicitobject-basedeffectsinmovingdisplays.Onewell-documentedfactorinthereductionoreliminationofobject-basedIOReffectsinmoving-objectstudiesispracticeorrepeatedstimulusexposurewithinthespatialcueingtask.Weaver,Lupiáñez,andWatson(1998)showedthat,despiteremainingsignificant,object-basedandenvironmental-location-basedIOReffectsdecreasedinmagnitudesignificantlyandatsimilarratesafterthefirst170orsotrials,disappearingoverthecourseofover1,000trials.ThisfindingcouldpotentiallyexplainthepersistentlackofcuedobjectIORinMüllerandvonMühlenen’s(1996)experiments,whoseparticipantscompleted150unrecordedpracticetrialsbeforeproceedingtocompleteseveralhundredsofrecordedexperimentaltrials(seeTable 1forstudydetails).Thatpracticecaneliminateobject-basedIOReffectssuggeststhatthesimplepresenceofanobjectmaynotbesufficienttoelicitormaintainanew,nonspatialrepresentationofthedisplay.Underconditionsinwhichisitnotrelevantorusefultorepresentobjectsinthedisplay,consultingrepresentationsofobjectsmaybeeffectivelyabandonedinfavourofotherstrategies,suchasattentionaltracking(e.g.,Christetal.,2002;Müller&vonMühlenen,1996)orfilteringoutdetailastask-irrelevant(e.g.,Broadbent,1958)—instead,merelyrespondingtolow-spatial-frequencyinformation.Perhaps,undersuchfamiliarsituations,attentionalloadisdecreasedandrepresentationsofthevisualscenearemoresparse,leadingtononeedorbenefitforobjectinformationtoplayaroleintaskperformance.Tothispoint,wehavesurveyedthesuccessesandfailuresinobtainingobject-basedcueingeffectsinmovingobjectdisplays.Ithasemergedthatthenecessaryconditionsforobservingobject-basedcueingeffectsarethattheobjectbepresentandvisibleatthetimeofcueingandformasingle,unambiguousspatiotemporalentity.Thenextsectionwillreviewthesuccessesandfailuresinobtainingobject-basedcueingeffectsusingstaticobjectdisplays.Object-basedeffectsinstaticobjectdisplaysFactorsknowntoinfluencetheoccurrenceofobject-basedcueingeffectsinstaticobjectdisplaysarerelatedtobothproceduralandstimulusaspectsoftheexperiment.Todate,object-basedfacilitatoryeffectshavebeenobservedunderconditionsofbothexogenousandendogenouscueing(e.g.,Chen&Cave,2008;Goldsmith&Yeari,2003;butseeMacquistan,1997);indetection,discrimination,andidentificationtasks(e.g.,Chen&Cave,2008;Eglyetal.,1994;Moore,Yantis,&Vaughan,1998;Shomstein&Behrmann,2008;butseeBrawn&Snowden,2000);undertargetpositioncertaintyanduncertainty(e.g.,Chen&Cave,2008);whenattentionisnarrowlyorwidelydistributed(e.g.,Chen&Cave,2008;Goldsmith&Yeari,2003);andinthepresenceofhighandlowprobabilitiesoftargetlocation(e.g.,Shomstein&Behrmann,2008;Shomstein&Yantis,2004).Althoughrobustwithrespecttotheaforementionedmanipulations,factorsthatrelatetotheviability—generationandmaintenance—ofobjectrepresentationshavebeenshowntoinfluenceboththepresenceandmagnitudeofcueingeffects(e.g.,Avrahami,1999;Chen&Cave,2008;Law&Abrams,2002;Shomstein&Behrmann,2008).Thepresentandthefollowingtwosections(Determinantsofobject-basedfacilitatoryeffectsanddeterminantsofobject-basedIOReffectsinstaticdisplays)focusonthenotionofobjecthoodinanumberofdifferentspatialcueingstudiesandontherelationshipbetweentheviabilityofobjectrepresentationsandthereliableobservationofobject-basedcueingeffects.Thereislittleconsensusaboutwhatismeantbyaperceptualobjectoranobjectofattention(e.g.,Goldsmith,1998;Pylyshyn,2001).Nevertheless,thenecessary—albeitnotwhollysufficient—prerequisitefortheobservationofobject-basedeffectsisthepresenceofphysicalorapparentcontours(typicallyclosedregions)inthevisualdisplay.Displaysusedtodatehaveincludedoutlineandopaquerectanglesandsquares(e.g.,Brown&Denney,2007;Christetal.,2002;Eglyetal.,1994;Iani,Nicoletti,Rubichi,&Umiltà,2001;Jordan&Tipper,1999;List&Robertson,2007;McAuliffeetal.,2001;Müller&vonMühlenen,1996;Reppa&Leek,2003,2006;Shomstein&Behrmann2008;Theeuwes,Mathôt,&Kingstone,2010;Vecera,1994),hockey-stick-likefigures(e.g.,Haimson&Behrmann,2001),overlappingobjects(e.g.,Behrmann,Zemel,&Mozer,1998;Brawn&Snowden,2000;Lavie&Driver,1996;Law&Abrams,2002;Mooreetal.,1998),apparentrectanglesandsquares(e.g.,Han,Wan,Wang,&Humphreys,2005;Jordan&Tipper,1999;Mooreetal.,1998),thicklines(Robertson&Kim,1999),openparallellines(e.g.,Avrahami,1999;Marino&Scholl,2005),outlineribbons(e.g.,Avrahami,1999),outlineL-shapes(e.g.,Leek,Reppa,&Tipper,2003;Possin,Filoteo,Song,&Salmon,2009;Reppa&Leek,2003,2006),groupsofdotsformingrectangles(e.g.,Marrara&Moore,2003),andoutlinesof3-Dobjects(e.g.,Bourke,Partridge,&Pollux,2006;Gibson&Egeth,1994).SomeoftheaboveobjectdisplaysareshowninTables 2,3and4.Table2Summaryoffacilitatorycueingeffects(inmilliseconds)instudiesthathavevariedtimeparametersFullsizetable Table3Summaryoffacilitatorycueingeffects(inmilliseconds)instudiesusingvariantsoftheEglyetaltwo-rectangletaskwithdifferentobjectcontoursalienceFullsizetable Table4SummaryofIORcueingeffects(inmilliseconds)instaticobjectstudies.StudiesappearinchronologicalorderFullsizetable Ifselectionprocessesaremediatedbyrepresentationsofobjects,theobservationofobject-basedeffectsmightreasonablybeinfluencedbystimulusfactorsgivingstrongcluestoobjectstructure.Asempiricalstudieshaveshown,objectrepresentationstaketimetodevelopandarebuiltupincrementallyatdifferentspatialscales(e.g.,Sekuler&Palmer,1992;Ullman,1989).Itseemsreasonabletoproposethatthestrongerthecuesto“objecthood”ortheotherfactorsfacilitatingtheconstructionofobjectrepresentations(e.g.,stimulusexposureduration,figure–grounddistinctiveness,complexity,spatialscale,andspatialconfiguration),thefasterandmoreeffectivelyobjectrepresentationsmightformandinfluenceselectiveattention.Thenextsectionreviewsevidenceregardingtherolesoffactorsthatcontributetothegenerationandmaintenanceofobjectrepresentationsinproducingreliableobject-basedfacilitatorycueingeffects.Determinantsofobject-basedfacilitatoryeffectsFacilitatoryeffectsareinfluencedbyobjectpresenceSubstantialevidencefromspatialcueingstudieshasdemonstratedthatthepresenceofobjectcontourscanconstrainthespreadoffacilitatorycueingeffectsacrossthevisualfield.Usingatwo-rectangleparadigm,Eglyetal.(1994)werethefirsttoshowthatfacilitatoryeffectsinselectiveattentioninducedbyacueappearingonanobjectcanextendtouncuedlocationsofthesameobjectrelativetoanuncuedobject.Sincethen,severalotherstudieshaveshownthatthepresenceofobjectsinavisualdisplaycaninfluencethedistributionoffacilitatorycueingeffects(e.g.,Abrams&Law,2000;Ariga,Yokosawa,&Ogawa,2007;Avrahami,1999;Brown&Denney,2007;Chen,1998;Chen&Cave,2008;Goldsmith&Yeari,2003;Haimson&Behrmann,2001;Hecht&Vecera,2007;Kravitz&Behrmann,2008;Lamy&Egeth,2002;Lamy&Tsal,2000;Law&Abrams,2002;Macquistan,1997;Marino&Scholl,2005;Marrara&Moore,2003;Mooreetal.,1998;Müller&Kleinschmidt,2003;Pratt&Sekuler,2001;Robertson&Kim,1999;Shomstein&Behrmann,2008;Shomstein&Yantis,2004;Vecera,1994).Realorillusorycontoursmustbepresenttoobserveobject-basedfacilitatoryeffects,andparticipantsneedtobeawareoftheirpresence(e.g.,Arigaetal.,2007).Thatdeepperceptualprocessingisrequiredfitswiththenotionthatobject-basedfacilitatoryeffectsoccurwellafterfigure–groundsegmentation,operatingonrepresentationsofunique,perceptuallycompleteobjects(e.g.,Chen,1998;Mooreetal.,1998;seealsoChen&Cave,2006).ThispointwascleverlyillustratedbyChenandbyMooreetal.InChen’sstudy,object-basedfacilitationdependedonhowparticipantswereinstructedtoperceivethevisualdisplay.WhenparticipantswerebiasedtoperceivealargeletterXasbeingcomposedoftwoadjacentVs,shiftsofattentionwithinthecuedVwerefasterthanshiftsbetweenthetwoVs.However,whenthetaskinstructionsbiasedperceptionofthesamedisplayasconsistingofasingleletterX,object-basedfacilitation(thedifferenceinRTsbetweentargetsappearingonthecuedVversusontheuncuedV)waseliminated.Moore,Yantis,andVaughan(1998;seealsoBehrmannetal.,1998)showedthatobject-basedfacilitatoryeffectscanbeobservedforpartiallyoccludedobjects,withnoreporteddifferenceinthemagnitudeoftheeffectfromthatobservedinnonoccludedobjects.Furtherevidenceforthelateoperationofselectiveattentionacrossobjects—wellaftertheoperationoffigure–groundandsize-scalingprocesseshaveoccurred—comesfromRobertsonandKim(1999).Inthisstudy,theperceivedlengthoftwophysicallyidenticalthinklines(resemblingthinrectangles)influencedtheallocationofselectiveattention,withlargersame-objectbenefitsforlinescorrespondingtotheperceivedlongercornerofanAmesroomwall,ascomparedtotheperceivedshortercornerofthewall.Sincethetwolineswerereallythesamelength,itwouldseemthatfacilitatoryeffectsofattentionemergedasaresultofconsultingadeeplyprocessedrepresentationofthedisplay.Insummary,theperceivedpresenceofanobjectanditsattributes,aswellasawarenessofitspresence,cancontributetotheobservationofreliablefacilitatoryobject-basedeffects.However,aswewillseebelow,presencealoneisnotalwayssufficient.Facilitatoryeffectsareinfluencedbythestrengthoffigure–groundsegmentationStudiesusingtheEglyetal.(1994)two-rectangleparadigmhaveshownthatobject-basedfacilitatoryeffectsaremorelikelytooccurwithmanipulationsthatfacilitatetheconstructionofperceptualobjectrepresentations.Objectpreviewtimeandperceptualcomplexityaretwofactorsthatcontributetoobtainingobject-basedcueingeffects. Objectpreviewtime Oneoftheearlieststudiesrelatingtemporalparametersandthemagnitudeofobject-basedeffectswasreportedbyAvrahami(1999).Shefoundthatobject-basedfacilitatoryeffectsweremorelikelytoarisewhenthetimebetweenthecueandthetargetwasincreased,presumablyallowingthescenetobesegmentedintoseparateperceptualgroupsorobjects,whichcouldtheninfluenceattention(butseeLamy&Egeth,2002,forthelackofaninfluenceofcue–targetintervalonfacilitatoryeffects). Themajorityofstudiesinvestigatingtemporalparametershavelookedattheroleofstimulusexposure,orpreviewtime,onthepresenceandmagnitudeofobject-basedeffectsbymanipulatingtheobserver’sexposuretotheobjectstimulipriortocueing.Table 2summarisesmeanfacilitatoryeffectsasafunctionofmanipulationsofexposuretime,andwithineachstudyitisclearthatincreasingthetimethatthestimulusdisplayisvisibledirectlyaffectsthemagnitudeoffacilitatorycueingeffects.Thesemanipulationswillnowbedirectlydescribed. LawandAbrams(2002)reportedaneliminationofobject-basedfacilitatoryeffectsbyreducingtheobjectpreviewdisplaydurationfrom186to129 ms.ChenandCave(2008)laterconfirmedtheinfluenceofpreviewtimeasafactormodulatingobject-basedcueingeffects.Theyusedathree-rectangledisplayintheformationofacrosstoexaminewhetheranendogenouscuewouldinduceobject-basedbenefitsfortargetsappearingonthecuedrectangle,asopposedtoononeofthesmaller,uncuedrectangles.Theyfoundsignificantobject-basedeffectswhenthetargetwasprecededbyanobjectpreviewtimeof1,005 ms.Whentheobjectpreviewtimewasreducedto120 ms,therewasnolongerasignificantsame-objectbenefit,leadingthemtoconcludethatcompletesegmentationoftheobjectsfromthebackgroundplayedaroleinobservingobject-basedbenefits. Objectpreviewtimehasbeenshowntointeractwithtargetprobabilityinmediatingfacilitatorycueingeffects.Inacuedtargetdiscriminationtask,ShomsteinandBehrmann(2008)foundthatwhenobjectswerepresentedforamere200 msbeforethecue–targetsequence,RTswereinfluencedbyprobabilityinformation,withthefastestRTsfortargetsappearinginhigh-probabilitylocations,andtheslowestfortargetsappearinginlow-probabilitylocations.Incontrast,whenobjectswerepresentedfor1,000 msbeforethecueandtargetsequence,object-basedfacilitatoryeffectsweresignificantregardlessofwhetherthetargethadahigherprobabilityofappearingonthesameoronadifferentobject.Thatobject-basedeffectswererevealedwithincreasedpreviewtimesuggeststhatanotherlevelofrepresentationwasestablishedandinfluencedattentionalallocation.Thisisanimportantfindingthatdemonstrateshowsubtlemanipulationscaninfluencewhichrepresentationwillbeactiveinmediatingselectiveattentionandtaskperformance. Salienceofobjectcontours Thesalienceandcomplexityofobjectcontoursinthedisplaycanmodulatethepresenceandmagnitudeofobject-basedfacilitatoryeffectsintwo-objectcueingtasks.Contoursaliencehastypicallybeenmanipulatedbyusinglinecontoursversusapparentcontours(e.g.,Mooreetal.,1998),byusingclosed-endedversusopen-endedrectangles(e.g.,Marino&Scholl,2005),bymaintaininguniformityoftheobject’ssurface(e.g.,Hecht&Vecera,2007;Watson&Kramer,1999),orbycolouringtwoobjectsdifferently(e.g.,Shomstein&Behrmann,2008).Table 3summarisesfacilitatoryeffectsasafunctionofcontourmanipulationsintwo-objectcueingstudies. Threestudieshaveallowedfordirectcomparisonofthemagnitudesoffacilitatoryobject-basedeffectsforobjectswithdifferentsalience(Hecht&Vecera,2007;Marino&Scholl,2005;Shomstein&Behrmann,2008,Exps.1,3,and4).HechtandVeceraexaminedtheinfluencesofsurfaceuniformity(e.g.,uniformityincolouracrossthecontour)andpartstructureonobject-basedfacilitatoryeffects(seeTable 3forstimulusexamples).Inagreementwithpreviousdemonstrationsoftheimportanceofsurfaceuniformityinobject-basedattention(e.g.,Watson&Kramer,1999),theyfoundsignificantobject-basedfacilitatoryeffectsforuniformlycoloured(Exps.1and3)butnotfornonuniformlycoloured(Exp.2)contours.Theyfurtherreportedthateveninthecaseofnonuniformlycolouredcontours,ifthechangeincolourcoincidedwithapartboundary(thusaddingadditionalcuestothepresenceofasingleboundedfigure),strongandsignificantobject-basedeffectswerereinstated. ShomsteinandBehrmann(2008)presentedtwodifferentlycolouredrectangles,theendsofeitherofwhichwouldbecuedfollowedbyatargetsearchdisplay.Despitethelowpreviewtimeof200 msandatargetlocationprobabilitymanipulation,object-basedfacilitatoryeffectsweresignificant.Meanwhile,thesamepreviewtimeof200 mswasnotsufficientforobservinganobject-basedeffectwhenthetwoobjectswereuncolouredoutlinerectangles.Presumably,themoresalientobjectcontoursofthecolouredrectanglesresultedinmoreeasilyparseddisplaysandagreaterlikelihoodthatobjectrepresentationswouldbecreated,consulted,andinfluencespatialcueing. ThedatafromShomsteinandBehrmann(2008)furtherdemonstratedthatobject-basedeffectsarecapableofoverridinghightargetprobabilities(seealsoShomstein&Yantis,2004).Facilitatoryeffectscanemergeeveninthefaceofprobabilitymanipulationsatveryshortobjectpreviewtimes,givenstrongcuestothepresenceofuniqueobjectsinthedisplay(e.g.,whenthetwocontoursaredifferentlycolouredintheexperiment). Increasingthesalienceofcuestoobjecthoodmaynotbeguaranteedtoinfluencethestrengthofobject-basedeffects(e.g.,Avrahami,1999;Marino&Scholl,2005).Forinstance,althoughclosureintuitivelyseemslikeitshouldbeanimportantfactorinthegenerationofobject-basedrepresentations,itisunlikelytobesufficienttoproduceobject-basedeffects(e.g.,Avrahami,1999;List&Robertson,2007).Explicitclosuremayalsonotbenecessary,becauseMarinoandScholl(2005,Exp.1)foundstatisticallysignificant,yetequivalent,same-objectbenefitsforopen-ended(5.6-msbenefit)andclosed-contour(7.2-msbenefit)rectangles.Thisnullresultmayreflectthevisualsystemmodallycompletingtheopen-endedlinedisplays,leadingtonofunctionaldifferencebetweenthetwodisplays.Surfacecontoursdonotneedtobeexplicitlyclosedtobeperceivedasobjectparts—forinstance,afoldinablanket.WhentheresultiscoupledwiththenotedlackofpowerinMarinoandScholl(2005),perceptualsaliencecannotbediscountedasanimportantfactorinproducingobject-basedeffects. TheremainingstudiesshowninTable 3eitherdidnotdirectlymanipulateobjectsalienceorsimplyqualitativelycomparedtheirfindingswiththoseofpreviousstudies.Nevertheless,itisnotablethatasobjectsappearsubjectivelymoresalient,facilitatoryeffectsemergewithincreasingmagnitudes(seetheSummaryandconclusionssection). Dorepresentationsofobjectsmediateobject-basedfacilitatoryeffects?Theaforementionedstudiessuggestthatobject-basedfacilitatoryeffectsrequiresufficienttimeandcuestofigure–groundsegmentationtoallowdistinctobjectrepresentationstoemerge.Furthermore,evidencehasshownthatobject-basedfacilitatoryeffectsareinfluencedbyperceptuallycompleterepresentationsofobjects,eitherfollowingmodalcompletion(e.g.,Mooreetal.,1998)orsubjectiveorganisationofthecontoursinthescene(e.g.,Chen,1998).Suchevidencesuggeststhatdisplaycharacteristicssuchaspreviewtimeandthesalienceofcontourscangiverisetorepresentationsofthedisplaythatarequalitativelydifferentfromearlyspatialrepresentations,andthatselectioncanoperatewithinthoseperceptuallycompleteobjectrepresentations.Neuropsychologicalevidencesimilarlysupportsthedissociationbetweenobject-basedandspace-basedmechanismsofselection,witheachoperatingonqualitativelydifferentrepresentations.UsingtheclassicEglyetal.(1994)two-rectanglespatialcueingtask,de-Wit,Kentridge,andMilner(2009)examinedspace-andobject-basedattentioninpatientDF.Followingbilaterallesionsinthevicinityofthelateraloccipitalcomplex(anareaassociatedwithobjectshaperepresentationandrecognition;e.g.,Kourtzi&Kanwisher,2001;Malachetal.,1995;Martínezetal.,2006),patientDFsufferedwithsevereacquireddeficitsinobjectrecognitionbuthadpreservedfigure–groundsegmentation.DFshowednormalspace-basedorientingeffectsofcuevalidity,withfasterRTstotargetsappearingatthesamelocationas,relativetoadifferentlocationfrom,thecue.Interestingly,DFshowednoobject-basedfacilitationeffects,withRTstotargetsappearingontheuncuedlocationofthecuedobjectbeingnodifferentfromthoseappearingontheuncuedobject.Thelackofobject-basedfacilitatoryeffectsinthepresenceofintactgroupingprocesses,butwithimpairedobjectrecognitionability,suggeststhatselectiveattentiontypicallyoperatesonobjectrepresentationsthatmediaterecognition,butwasunabletodosointhispatient.Sofar,evidencehasshownthatfacilitatorycueingeffectscanbeinfluencedbyfactorsrelatingtothepresenceofseparateobjectsinthedisplay.Increasingtheamounttimeobjectsareviewedandfacilitatingfigure–groundsegmentation(e.g.,bydifferentiatingobjectsinthevisualarrayandincreasingthesalienceofobjectcontours)contributestothegenerationofobjectrepresentations(e.g.,Rensink&Enns,1998;Sekuler&Palmer,1992).Theaforementionedfindingssuggestthatthesesamefactorscancontributetosignificantcueingbenefits,implicatingobjectrepresentationsasinfluencingselectionprocessesinattention.ThefollowingsectionrevealsthatasimilarpatternofevidenceexistsforIORcueingeffects.Determinantsofobject-basedIOReffectsinstaticdisplaysThesensitivityoffacilitatorycueingeffectstoobject-basedrepresentationsisrarelycontested.Somewhatmorecontroversialistheinfluenceofobjectsontheotherconsequenceofexogenousorienting—theIOReffect(e.g.,List&Robertson,2007;McAuliffeetal.,2001;Schendeletal.,2001).TheIOReffectisfrequentlyassumedtoreflecttheoperationofanovelty-seekingmechanismthatservestopreventrepeatedprocessingofinformationatpreviouslyattendedlocations(e.g.,Posner&Cohen,1984;Posneretal.,1985)andtofacilitatethecontinuousmonitoringoftheenvironmentfornovelvisualstimuli(e.g.,Klein,1988,2000).ImplicitinthisassumptionisthattheIOReffectislikelytoimplicaterepresentationsofobjects.Indeed,aboutadecadeafteritsdiscovery,findingsemergedshowingobject-basedIOReffects(e.g.,Tipperetal.,1991).SupportfortheconclusionthatIORcanbeobject-basedcomesfromfourdifferentlinesofevidence.First,inmovingobjectdisplays,IORforthecuedobjectissignificantdespitehavingmovedfromthecuedlocation(e.g.,Ro&Rafal,1999;Tipperetal.,1991;Tipperetal.,1999;Tipperetal.,1994).Second,instaticobjectdisplays,IORislargerfortargetsinobject-presentthaninobject-absentdisplays(e.g.,Jordan&Tipper,1998;Klein,1988;Leeketal.,2003;Possinetal.,2009;Takeda&Yagi,2000).Third,IORismodulatedbytheorganisationandsalienceofobjectcontoursinthedisplay(e.g.,Jordan&Tipper,1999;Leeketal.,2003;Reppa&Leek,2003,2006).Finally,IORismodulatedbyanobject’sinternalstructure(e.g.,Leeketal.,2003,Reppa&Leek,2003,2006;Possinetal.,2009).Theevidenceforobject-basedIORderivedfromstudiesusingmovingobjectdisplayswasdiscussedpreviously.Inthefollowingsections,evidencefortheremainingthreetypesoffindingsupportinganobject-basedIORinterpretationisreviewed.Footnote1 IOReffectsareinfluencedbyobjectpresenceObject-basedIOReffectshavebeenfoundintasksusingvariantsoftheEglyetal.(1994)two-rectangleparadigmwithlongerSOAs(e.g.,Jordan&Tipper,1999;Leeketal.,2003;List&Robertson,2007,Exp.1;Reppa&Leek,2003,2006;Possinetal.,2009).Table 4summarisesreportedIORfindingsbasedonthetwo-rectangleparadigm.ThefirstofthesestudieswasreportedbyJordanandTipper(1999),whoshowedthatIOReffectscanbeassociatedwiththeobjectthatthecueappearedon,asopposedtothelocationofcuealone.TargetsappearingatthecuedlocationofthecuedobjectyieldedasignificantIOReffect,replicatingthetypicallocation-basedIOReffect.Critically,IOReffectswerealsosignificantfortargetsappearingatuncuedlocationsoncuedobjects,suggestingthatIOReffectsareobservedacrossthecontoursofthecuedobjectina2-Dplane,andprovidinganotherlineofevidencethatIORcanbeobject-based.Similarly,Bourke,Partridge,andPollux(2006)showedthatIOReffectscanspreadacrossanobjectindepth.Theyfoundthatcueingthefrontsurfaceofasee-throughcubeyieldedsignificantIOReffectsnotonlyforthecuedsurfacebutalsoforanuncuedsurfaceofthecube.Object-basedIOReffectsareinfluencednotonlybythepresenceofobjectcontoursinthedisplaybutbythemembershipofsuchcontoursinaperceptuallycompletedobject.ReppaandLeek(2006)showedsignificantobject-basedIOReffectsbothforunoccludedandforpartiallyoccludedobjects,withnodifferenceinthemagnitudeofIORbetweenthetwotypesofdisplay.Thisfindingillustratesthat,aswiththefacilitatoryeffectsofattention,IOReffectscanbeinfluencedbyperceptuallycompletedobjectrepresentations.IORislargerinobject-presentversusobject-absentdisplaysStrongevidenceforthenotionthatIORcanarisefromobjectrepresentations—independentlyfrom,andinparallelwith,earlierspatialrepresentations—comesfromobservationsofadditivelocation-basedandobject-basedIOReffectsinstudiesusingbothmoving(e.g.,Tipperetal.,1999;Tipperetal.,1994)andstatic(e.g.,Jordan&Tipper,1998,1999;butseeTheeuwes&Pratt,2003)objectdisplays.Thisduallineofevidencesuggeststhatseparaterepresentationsofenvironmentallocationsandofobjectscanbeelicitedbythedisplayandcaninfluencecovertorientingindependentlyandinparallel.Instudiesusingstaticdisplays,evidencethatobject-basedandspace-basedIOReffectsareadditivecomesfromstudiesshowinggreaterIORwhenobjectcontoursarepresent,asopposedtowhentheyareabsentfromthedisplay(e.g.,Klein,1988;Leeketal.,2003;Possinetal.,2009;Takeda&Yagi,2000;butnotinMcAuliffeetal.,2001,andWeger,Al-Aidroos,&Pratt,2008)orwhencontoursdonotformanobject(e.g.,Jordan&Tipper,1998).Forinstance,usingavariantoftheoriginalPosnercueingparadigm,JordanandTipper(1998)firstshowedthatIOReffectssignificantlyincreasedwhenboundariesdefiningdistinctobjects(physicalorapparent)werepresent,ascomparedtowhentheboundariesinthescenedidnotdefineanobject.TheinfluenceofobjectpresenceonthemagnitudeofIOReffectswaslaterdemonstratedinstudiesusingavariantofthetypicaltwo-rectangledisplay(e.g.,Leeketal.,2003;Possinetal.,2009).Leek,Reppa,andTipper(2003)presentedcuesandtargetsinobject-presentandobject-absentdisplaysrandomlymixedwithinthesameblock.Thetargetsappearedwithequalprobabilityinsixdifferentlocationsoneitheroftwoobjects(object-presentcondition)orinemptylocations(object-absentcondition).IOReffectswerelargeroverallintheobject-presentdisplaysthanintheobject-absentdisplays(seealsoPossinetal.,2009).Finally,inBourkeetal.’s(2006)study,theobject-basedIOReffectobservedwhentwopartiallyoverlappingrectangleswerelinkedtoformasee-throughcubewasdoubleinmagnitudeascomparedtoIOReffectswhenthetworectanglesappearedasdifferentobjects.NeuropsychologicalevidencefromParkinson’sdisease(PD)patientsfurthersupportsthehypothesisthatspace-basedandobject-basedIOReffectsarisefromdifferentrepresentations.PDpatientshavepreviouslybeenreportedtohaveimpairedspace-basedinhibitoryprocesses(e.g.,Filoteoetal.,1997;Hsieh,Lee,Hwang,&Tsai,1997;Wright,Burns,Geffen,&Geffen,1990),inthepresenceofintactobject-basedinhibition(e.g.,Possin,Cagigas,Strayer,&Filoteo,2006).Unlikepreviousstudiesthathadseparatelyexaminedspace-andobject-basedinhibitoryprocessesindifferentPDpatients,Possin,Filoteo,Song,andSalmon(2009)examinedbothprocessesinthesamepatients.TheyusedthetaskemployedbyLeeketal.(2003)toexaminespace-andobject-basedIORinPDpatientsandagroupofage-matchedhealthycontrols.BoththehealthycontrolgroupandthePDpatientgroupshowedtypicalpatternsofobject-basedIOR.However,thePDpatientsshowedanattenuatedspace-basedIOReffectrelativetothehealthycontrols.Thereversepatternofresultshasbeenfoundinolderadults,whofailtoshowobject-basedIORinthepresenceofintactspace-basedIOReffects(e.g.,McAuliffe,Chasteen,&Pratt,2006;McCrae&Abrams,2001).Suchadoubledissociationsuggeststhatduringthespatialcueingtask,multiplerepresentationsofthedisplaycanemerge,anyofwhichmightbeconsulted,dependingonthenatureoftheattentionalnetworkintheindividual.InadditiontoevidencefromspatialcueingstudiesthatIOReffectsareinfluencedbyrepresentationsofobjects,furthersupportcomesfrom“probe-following”studies,whereavisualsearchtaskisfollowedbyaluminancedetectionprobetask.Probe-followingfindingssupportthehypothesisthatIORcanbeassociatedwiththeobjectsinascene,asopposedtounoccupiedspatiallocations(suggestedbyTipperetal.,1994,andconfirmedinameta-analysisbyWang&Klein,2010).Intheprobe-followingstudybyKlein(1988),participantstookpartineasyordifficultsearchesandweresubsequentlyaskedtodetectthepresenceofaluminanceprobe(target).Thecriticalmanipulationinvolvedthelocationoftheprobe,whichwouldappeareitheratthelocationthatwasoccupiedbyadistractorduringthesearchtask(“on”probe)orinanunoccupiedlocation(“off”probe).TherationalewasthatifIORservestopreventattentionfromreturningtopreviouslyinspectedobjects,probedetectionwouldthenbeslowerfor“on”probesthanfor“off”probes.ThefindingsconfirmedgreaterIORfor“on”thanfor“off”probesinthedifficultsearchtask.Critically,thisdifferenceinIORmagnitudebetween“on”and“off”probesdependedonthemaintenanceofthevisualsceneafterthesearchtaskwascompletedandduringtheprobedetectiontask(e.g.,Klein&MacInnes,1999;Müller&vonMühlenen,2000;Takeda&Yagi,2000;Wolfe&Pokorny,1990;seealsothereviewbyWang&Klein,2010).IfIORarisesfromspace-basedrepresentationsalone,itshouldhavebeenobservedregardlessofthedisappearanceoftheobjects,asthespatiallayoutoftheenvironmentremainedaccessible.Apparently,IORwascruciallytiedtothemaintenanceofrepresentationsofthesceneandofobjectswithinit,becauseremovalofsuchinputobliteratedtherepresentationsinvolved,takingtheproductionofIOReffectswithit.IORisinfluencedbythesalienceofobjectcontoursJustaswithfacilitatoryeffects,itappearsthatIORcanbemodulatedbycontoursalience,andinparticularbycontourcomplexity(e.g.,Jordan&Tipper,1999;Leeketal.,2003;Reppa&Leek,2006).Forinstance,object-basedIOReffectsforKanizsaapparentrectanglesinJordanandTipper(1999)weresmallandnonsignificant(5 ms).However,largeandsignificantIOReffectswereobservedforoutlinerectangles(12 ms),andlargeragainforoutlinerectanglesthatweresurroundedbyKanizsacircles(21 ms),amanipulationprovidingredundantcuestoimagesegmentation.Furthermore,object-basedIOReffectsweresusceptibletopracticeonlyfortheapparentrectangles(3-msfacilitatoryeffectinthesecondblockoftrials),butnotfortheKanizsa-plus-outlinerectangles(17 msinthesecondblockoftrials).Similarly,object-basedIOReffectsweresmaller(11–12 ms;Jordan&Tipper,1999;Reppa&Leek,2003,Exp.1)whentheobjectswereoutlinerectangles,ascomparedtowhentheobjectsweresegmentedL-shapes(26 msinReppa&Leek,2003,Exp.2;and30 msinLeeketal.,2003)orcolouredrectangles(29 msinReppa&Leek,2006).Althoughtherelationshipbetweenobjectcontourcomplexityandmagnitudeofobject-basedIOReffectsremainstobesystematicallymanipulatedandexaminedinfuturestudies,asurveyofTable 4suggestsatrendforlargereffectswithincreasingcontourcomplexity(seetheSummaryandconclusionssection).Thesuggestionthatperceptualsaliencecandeterminethemagnitudeofobject-basedIOReffectsis—albeitindirectly—supportedbyastudybyMcAuliffe,Pratt,andO’Donnell(2001).TheyreportedafailuretofindlargerIOReffectsfromobject-presentdisplays.Theypresentedcuesandtargetsoneitheroftwopossiblelocationsalignedeitherhorizontallyorverticallyaroundacentralfixationcross.Inhalfofthetrials,eachlocationwassurroundedbyaclosedcontour(object-presentcondition),andintheotherhalfofthetrialstherewerenoobjectcontours(object-absentcondition).Inthreeexperiments,theymanipulatedtheSOAfrom400to1,000 msandwhethertheobject-presentandobject-absenttrialswereblockedorrandomlyintermixed.Whentheobject-presentandobject-absenttrialswerepresentedindifferentblocks,therewasnodifferenceinthemagnitudeofIORbetweenthetwodisplays.However,significantobject-basedIOReffectswereobservedwhenobject-absentandobject-presenttrialswererandomlyintermixedinthesameblocks.Thefailuretoobserveobject-basedIORintheblockeddesignmighthaveresultedfromtherepeatedpresentationoftwotask-irrelevantoutlinesquaresinthesamelocationsforhundredsoftrials,whichislikelytohaverenderedthemuninterestingandunnecessarytorepresent.IORisinfluencedbyobject-internalstructureStructure-basedmodulationofIOR,ortheSBMeffect(e.g.,Leeketal.,2003;Possinetal.,2009;Reppa&Leek,2003,2006),providesanotherlineofevidence,highlightingtheroleofobjectsasperceptuallycompleteandinternallystructuredentitiescapableofmodulatingIOR.TheSBMeffectreferstothefindingthatthemagnitudeofIORchangesdependingonthepresenceofanobject-internalcontourappearingbetweenthecueandthetarget.ObservationsoftheSBMeffectinstudiestodateareshowninTable 4.InthefirstdemonstrationoftheSBMeffect,ReppaandLeek(2003)presentedparticipantswithtwoL-shapedoutlinestimulisimultaneouslyontheleftandrightsidesofacentralfixationcross(seeTable 4).ThecueappearedatthecentreofoneoftheL-shapesandwasnotpredictiveofthesubsequentlocationofthetarget.Thecriticalmanipulationwaswhetherornottherewasalinediscontinuity(i.e.,apartboundary)betweenthecueandthetarget.Inthesegmentedobjectdisplays,thetargetcouldappearatthecuedlocation,onthecuedobjectpart,oronadifferentpartfromthecue(uncuedpart).Thekeyfinding,andahallmarkoftheSBMeffect,wasthatwhilesignificantIORwasfoundinboththesame-partanddifferent-partconditions,IORwasgreaterintheuncued-partthaninthecued-partcondition(seeTable 4).Later,Possinetal.(2009)replicatedtheSBMeffectinagroupofPDpatients,whodespiteimpairedspace-basedIORshowedsignificantobject-basedIOReffects.Moreover,inalaterstudy,ReppaandLeek(2006)confirmedthattheSBMeffectisonlytriggeredbyobject-internalstructuraldiscontinuities,notbydiscontinuitiesthatarepartofanoccludingobject.Combined,theevidencefromhealthyadultsandPDpatientssuggeststhatIOReffectscanarisefrominternallystructuredrepresentationsofobjects(Leek,Reppa,&Arguin,2005),asopposedmerelytoglobalforms.Areobject-basedIOReffectsinstaticdisplaysreliable?Object-basedIOReffectshavenotalwaysbeenobservedintwo-objectstaticdisplays,raisingtheopportunitytogainabetterunderstandingoftheboundaryconditionsoftheeffect.Inonestudy,ListandRobertson(2007)examinedtheinfluenceofcue–targetSOAandback-to-centrecueingonspace-basedandobject-basedIOReffects.Infiveexperiments,space-basedIORwasconsistentlysignificant,survivingmanipulationsofSOAandtheremovalofback-to-centrecueing(centralfixationbrighteningafterthecuewaspresentedbutbeforethetargetappeared).Incontrast,object-basedIORwasobservedonlywhenaback-to-centrecuewaspresentandwhenthemostrecentcue-(peripheralorcentral-fixation)to-targetinterval(MRCTI)wasapproximatelybetween600and1,200 ms.Intheremainingconditions,smallandnonsignificantobject-basedfacilitationeffectswereobservedinthepresenceofsignificantspace-basedIOReffects.Thisledtheauthorstoconcludethatobject-basedeffectswerefragileandsensitivetoback-to-centrecueingandtoanMRCTIwithinaveryconstrictedrange.TherolethattheMRCTIplaysinobtainingobject-basedIOReffectsisclearlyinneedoffurtherexamination,giventhatdatareportedintheliteratureviolatetheconclusionsdrawnabouttherangeatwhichobject-basedeffectscanbeobserved(Leeketal.,2003;Reppa&Leek,2003,2006;Possinetal.,2009;asreportedinTable 4).Meanwhile,theimportanceofback-to-centrecueinginobservingobject-basedIOReffectshighlightstheneedtotakeintoaccountfactorsthatmayinfluenceobjectperceptionwhenlookingforobject-basedeffects(seealsoRo&Rafal,1999).Itseemsplausiblethataback-to-centrecuemayhavecontributedtothemaintenanceandinterpretationofthedisplayasobject-basedbyservingasareminderoftheobjects’presence,strengtheningthegroupingbetweenthetarget’scurrentlocationandthepreviouslycuedlocation—andtherebystrengtheningobject-basedIOReffects.Whilefutureworkisneededtosystematicallyexaminetheroleoftheemergenceandmaintenanceofobjectrepresentationsinobtainingrobustandreliableobject-basedIOReffects—ashasstartedtohappenwithfacilitatorycueingeffects—thestudiesreviewedabovesuggestthatspace-basedandobject-basedIOReffectsarisefromfundamentallydifferentrepresentationsofthestimulusdisplayandinfluencethepresenceandmagnitudeoftheeffects.Apartfromthenumerousinstancesinwhichsignificantobject-basedIORhasbeenreportedinstaticdisplays,theeffecthasbeenshowntobemodulatedbyobject-internalstructureandtooperateonperceptuallycompleterepresentationsofobjectsinthedisplay.Althoughobject-basedIOReffectsmaybeobservedundermorespecificcircumstancesthanspace-basedeffects,itisthecontentionofthepresentreviewthatthosespecificcircumstancesrelatetothegenerationandmaintenance(throughouttheexperiment)ofobjectrepresentations.Justaslocation-basedIOReffectshavebeenshowntodisappearinthepresenceofcuestoanalternative,object-basedinterpretationofthescene(e.g.,Tipperetal.,1999),similarly,object-basedIOReffectscandisappearwhensuchcuesdonotexistoraretooweaktoencouragetheconstructionandmaintenanceofobjectrepresentations.Space-basedinterpretationsofobject-basedeffectsInresponsetoclaimsthatobjectsplayaroleinattentionalselection,space-basedexplanationshaveemergedattemptingtoparsimoniouslyaccountforallspatialcueingeffectsusingasingle,space-basedrepresentationalmedium.Thebasictenetisthatthereisnoneedtorecruitrepresentationsofobjectsinordertoexplainapparentobject-basedeffects,whichcaninsteadbeaccountedforintermsofselectionofspacethatis“structuredbyobjects”inthedisplay.Twokindsoffindingshavebeenpresentedasevidencethatspace-basedrepresentationscanaccountforobject-basedeffects.Thesefindingsarereferredtoas“objectsinspace”and“objectswithspace.”The“objectsinspace”findingisthat,althoughobjectrepresentationsmaybeselected,thedistancebetweentheminspacecanstillinfluencecueingeffects(e.g.,Vecera,1994;Vecera&Farah,1994).Thisfindingwasexplainedusingthegrouped-locationsarrayrepresentationhypothesis(e.g.,Vecera,1994;Vecera&Farah,1994),wherebyobject-basedeffectsreflecttheselectionoflocationsgroupedviaperceptualgroupingprocessesratherthanselectionofobjectsperse(e.g.,Ho&Atchley,2009;Mozer,2002;Mozer&Vecera,2005;Vecera,1994;Vecera&Farah,1994).Suchgroupingprocessesarepostulatedtooperatewithinaviewer-centredspatialreferenceframe,resultinginrepresentationsof“proto-objects”(e.g.,Driver,Davis,Russell,Turatto,&Freeman,2001;Rensink,2000;Rensink&Enns,1995)or“groupedarraysoflocations”(e.g.,Müller&O’Grady,2009;O’Grady&Müller,2000;Vecera,1994;Vecera&Farah,1994).Theserepresentationsarestillspace-based,inthesensethattheymakethelocationinspaceofagrouporobjectexplicitanddriveselection,whileobjectformandstructureareimplicit.ManystudiesusingtheEglyetal.(1994)two-objectparadigmhavereportedlargercueingeffectsfortargetsappearingonthecuedlocationofthecuedobject,relativetothoseappearingontheuncuedlocationofthecuedobject.This“objectswithspace”findingsuggeststhatthespacewithinobjectsisexplicitlyrepresented,andithasbeensuggestedthatthedatamayreflectselectionnotofobjectsperse,butofperceivedspace.Thatis,objectsinfluencecueingeffectsbystructuringspaceandalteringthewayitisperceived(e.g.,Robertson,2004;Robertson&Kim,1999).Thereareseveralproblemswithattemptingtoeliminateobject-basedrepresentationsfromexplanationsofapparentobject-basedeffects.First,irrespectiveofhowmuchevidenceoneaccruesregardingtheadequacyofspace-basedexplanations,onefailstodisprovethatobject-basedrepresentationsexistandareconsulted,orthattheyhaveactivelyplayedaroleinattentionalselectionwithinotherexperiments.Allobject-basedrepresentationspossesscontours,whichinandofthemselvesarerepresentedspatiallyandwouldpresumablyhavesimilarpropertiesboundedbytheobjectrepresentationofwhichtheyarepart.Selectionofanobjectrepresentationneednotexcludeselectionofthespacewithinit(e.g.,Hollingworthetal.,2011;Reppaetal.,2010).Asecondproblemwiththeoreticalapproachesthatattempttoeliminateobject-basedexplanationsinspatialcueingisthat,despitetheirattractivenessonthegroundsofparsimony,grouped-arrayrepresentationsandspace-basedexplanationscannotcomfortablyaccountformanyoftheobject-basedfindingsrecountedinthepresentreview—letaloneinthemanydomainsofpsychologicalinquiryoutsideofspatialcueing,forwhichobject-basedexplanationsseemrequired.Forinstance,datashowingmodulationoffacilitatoryandIORspatialcueingeffectsbycontourcomplexity,distinctiveness,andinternalstructure,orthoseshowingadditivespace-basedandobject-basedeffects,allseemtoimplicateobjectrepresentationsthatalongsiderepresentationsofspatiallocationsareactiveandcanmediatebehaviourinparallel.Toaccountforsuchfindings,space-basedaccountswillneedtomakeanunprecedentedappealtothenotionoftheobjectanditsperceptualcharacteristics,whichwouldpresumablymakethemhardtodifferentiatefromobject-basedaccounts(seeGoldsmith,1998).Thetaskofdeterminingthecontentandscopeofactiverepresentationsremainsimportant,ofcourse,butspace-basedexplanationsofobject-basedeffectsneednotnegatetheroleofobjectrepresentationsinmediatingspatialcueingeffects.SummaryandconclusionsSpatialcueinghasbeenproventobeaneffectivetoolforelucidatingtheorganisationandstructureofrepresentationsthatmediatefacilitatoryandinhibitoryprocessesresultingfromtheallocationofselectiveattention.Clearly,muchempiricalandtheoreticalprogresshasbeenmadeoverthepast30 yearsinourunderstandingoftherolethatobjectsandtheirresultingrepresentationsplayinthecovertorientingofattention.Whilehistoricallyearlyapproachestospatialcueingassumedandtestedspace-basedrepresentations,laterresearchexpandedintoquestioningwhethertherepresentationalmediumofselectiveattentionmightbeobject-based,eitherinadditionto,orinsomecasesinsteadof,simpleunstructuredspatialrepresentations.TheevidencepresentedheresuggeststhatbothfacilitatoryandIORcueingeffects,inmovingandstaticobjectdisplays,aresensitivetothepresenceofobjectsinthedisplay.Cueingeffectsareobservedforperceptuallycompletedobjectsandaremodulatedbyobjectsalienceandstructure,suggestingthatobjectrepresentationscanbeselectedandcaninfluencebehaviour.Apartfromevidenceshowingthatbothspace-andobject-basedrepresentationsandorientingeffectsresultingfromsuchrepresentationscanoccurandinfluenceperformance,thereisevidencethatthoserepresentationsexistinparallel(e.g.,Leeketal.,2003;Reppa&Leek,2003;Tipperetal.,1999).Whatwilldetermineifobjectrepresentationswillbeselectedandinfluenceorientingeffects,asopposedtorepresentationsofspatiallocations?Emergingfromthisreviewofsuccessesandfailuresinobservingobject-basedfacilitatoryandinhibitoryeffectsareseveralpracticesthatclearlyplayaroleinencouragingobject-basedrepresentationstobecreatedandconsultedviavisualattention.Fromthissummaryofresearch,itseemsthatthestrongerthecuestoobjectsegmentationinthedisplay,themorelikelythatobjectrepresentationswillbeimplicatedinprocessingtoproducestrongandreliableobject-basedeffects.Somestudieshaveaddressedthishypothesisdirectlybymanipulatingcontoursalience,asconveyedbymeansofobjectcontouruniquenessinthedisplay(e.g.,Shomstein&Behrmann,2008)oraddedcuestoimagesegmentation(e.g.,Hecht&Vecera,2007).Evenwheresuchcomparisonshavenotdirectlybeenmade,findingsreportedintheliteraturesupporttheroleofobjectcomplexityinmodulatingobject-basedcueingeffects.Forinstance,lookingatTable 3,itappearsthatthemorecomplextheobjectstimuliusedacrossstudies,thegreaterthemagnitudeofobject-basedcueingeffectsthatemerged.Toexaminethismoreconcretely,Fig. 4plotsthemagnitudeofobject-basedcueingeffectsasafunctionofobjectcomplexityforbothfacilitatory(toprow)andinhibitory(bottomrow)effects.Complexityissimplyquantifiedintermsofthenumberoflinesegmentsorofthenumberofverticesintheobjects.Evenwithsuchsimplemeasures,theoutcomeisclear:ForbothfacilitatoryandIORobject-basedeffects,increasesinobjectcomplexityarepositivelyrelatedtoincreasesinthemagnitudeofobject-basedcueingeffects.Fig.4Meanobject-basedfacilitatory(toprow)andIOR(bottomrow)effectsfromTables 2and3(forfacilitatoryeffects)andTable 4(forIOReffects),plottedhereasafunctionof(a)thenumberoflinesegmentsand(b)thenumberofverticespersingleobjectintwo-objectdisplays.NreferstothenumberofstudiesperdatapointFullsizeimage Inadditiontoobjectcomplexity,sufficientprocessingtimeandthecontinuingrelevanceoftheobjectsthroughoutthesubject’staskseemnecessaryprerequisitesforobjectrepresentationstoinfluenceperformance.Whathasbeenhighlightedinthepresentreviewisthatthenotionofobjecthood—thefactorscontributingtoitsgeneration,strength,andmaintenanceofanobjectfileortoken—needstobecarefullyconsideredinstudiesofobject-basedattention.Insteadofrejectingtherolethatobjectrepresentationsmightplayinobtainingapparentobject-basedeffects,onthebasisoffindingsthatshowanabsenceofsucheffects,amorefruitfulapproachmaybetoexaminethefactorsthatcontributetogenerationandmaintenanceofobjectrepresentationsinselectiveattentiontasks.Assuggestedbypastreviewsoftheliterature(e.g.,Cave&Bichot,1999),whetherselectiveattentionismediatedbyspace-orobject-basedrepresentationsneednotbeaneither/orissue.Instead,whenanobjectrepresentationhasbeenconstructedandconsulted—forinstance,whentherepresentationhasinformationinaformatfromwhichtaskperformancemightbenefit—thenobject-basedeffectswillbeobserved.However,ifcuestoobjecthoodormotivationstoconsultobject-levelrepresentationsareabsent,itwouldappearthatsimplerrepresentationsmediateperformance.Conceptually,thisisasignificantsteptowardsagroundedunderstandingofobject-basedcueingeffectsandobject-basedselection. 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