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EGETH & YANTISVISUAL ATTENTIONAnnu.Rev. Psychol. 1997.48:269–97Copyright© 1997by AnnualReviewsInc. All rightsreserved

VISUAL ATTENTION: Control,Representation, and TimeCourse

Howard E. Egeth andSteven YantisDepartment of Psychology,TheJohnsHopkinsUniversity, Baltimore,Maryland 21218

KEY WORDS: attention, cognition, human subjects, perception, psychophysics, reaction time,vision, visualsearch

ABSTRACT

Threecentral problemsin therecent li teratureonvisualattentionarereviewed.The first concerns the control of attention by top-down(or goal-directed) andbottom-up (or stimulus-driven) processes. The second concerns the repre-sentationalbasisfor visualselection, including howmuchattentioncan besaidto belocation- or object-based.Finally , weconsiderthetimecourseof attentionasit is directedto onestimulusafteranother.

CONTENTSINTRODUCTION..................................................................................................................... 269STIMULUS-DRIVEN AND GOAL-DIRECTED CONTROL OFATTENTION.................. 270

FeatureSingletons and Attentional Capture....................................................................... 271Abrupt VisualOnsetsand Attentional Capture................................................................... 274Interactionof Goal-Drivenand Stimulus-DrivenCapture ................................................. 275

THE REPRESENTATIONAL BASISOFVISUAL SELECTION......................................... 277Overlapand Grouping ........................................................................................................ 278Motion.................................................................................................................................. 281

THE TIME COURSEOFATTENTION.................................................................................. 285DirectingAttention.............................................................................................................. 285TheDwell Time ofAttention................................................................................................ 286TheMovementof Attention.................................................................................................. 292ConcludingRemarks........................................................................................................... 294

INTRODUCTIONWe review the literatureon threefundamentalaspectsof attentionthat havebeenthefocusof muchrecentresearch.Thefirst concernsattentionalcontrol,or theextentto which thedeploymentof attentionis a resultof theobserver’s

deliberatestateof attentional readiness(calledtop-downor goal-directedcon-trol), or whetherattentionis capturedby certainaspectsof the imageinde-pendentlyof current perceptualgoals (called bottom-up or stimulus-drivencontrol). We emphasizestimulus-drivencontrol and the interactionbetweenthe two attentional control modes.The secondtopic concernsthe repre-sentationalbasisfor visualselection.In particular,we examinetheconditionsunder which attention may be said to be directed to regions of space,asspace-basedtheoriesassert,or to preattentively definedperceptualobjects,assuggestedby object-basedtheoriesof attention. Finally, we examinerecentevidenceconcerningthe time courseof attention, both as it movesthroughspaceand as events occurring sequentially in time are selected. In thissection, several different estimatesof the time scaleon which attentionalevents occur are reviewed.

A single chaptercannotcompletely cover the broad and active field ofattention.Readersinterestedin pursuingtheseandotheraspectsof attentionmay consultthree recent tutorialvolumes (Dagenbach& Carr 1994, Krameretal 1996,Pashler1996a)or severalrecentmonographs(LaBerge1995,Pashler1996b,van der Heijden 1992). In addition, thereare severalrecentAnnualReviewchapterson attention(Desimone& Duncan1995,Johnston & Dark1986,Kinchla1992).

STIMULUS-DRIVENAND GOAL-DIRECTEDCONTROLOFATTENTION

WhenWil liam James(1890) first delineatedthe varietiesof attention over acentury ago,one major categorical boundarywas the distinction betweenpassiveand active attention. The modernterms are usually bottom-upandtop-downor thelessmetaphoricalstimulus-drivenandgoal-directed.Theideais thatthedeploymentof attentionmaysometimesdepend onthe propertiesofthe imagealmostexclusively(e.g.suddenmovementin the periphery);othertimes it may be under strict supervisionaccordingto the observer’s goals.Mountingevidencehasrevealedthat thesetwo domainsof attentionalcontrolalmostinvariablyinteract.With a few possible exceptions,boththepropertiesof the imageand the expectationsand goalsof the observerdeterminetheattentionalconsequencesof a givenperceptualepisode.(We considerthetimecourse of attentional controlin the lastsectionof thischapter.)

The past 25 yearshave yielded ample evidencethat the distribution ofattentioncanbecontrolledby theintentionsof theobserver.Helmholtz (1925,p. 455)first notedthis ability in the pastcentury,but it was notuntil the 1950sthat the perceptualconsequencesof the deliberatedeployment of attentionwere first studiedsystematically by Mertens (1956). Much of the modernevidencefor top-downcontrolhasbeenreviewedpreviously(e.g.Johnston&

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Dark 1986).Significant advancesin our understanding of the top-downde-ployment ofattentionbeganwith aseminalseriesof studiesby Eriksenandhiscolleagues(e.g.Eriksen& Hoffman 1972,1973).Subjectsidentified a letterindicatedby a barmarkerandattemptedto ignoreotherlettersin thedisplay.The amountof interferencecausedby the to-be-ignoredletters provided ameasureof the efficiency andtime courseof attentionaldeployment.Studiesby Posnerandcolleagues(e.g.Posner1980,Posneret al 1980)furtherexam-ined top-downattentionalcontrol.

The evidenceconcerningthe captureof attention(i.e. bottom-upcontrol),unlike thatfor top-downcontrol, ismorerecentandis thefocusof thissection.Two major categoriesof stimulus propertiesthat could in principle captureattentioncanbedistinguished:stimuli thatdiffer substantially in oneor moresimple visual attributes(e.g. color, orientation, or motion) from their back-grounds—hereafter calledfeaturesingletonsor simply singletons—andabruptvisualonsets.We considertheevidenceabouteachof thesecategoriesin turn,andwe returnto this topic in thesectionentitled“The Time Courseof Atten-tion.”

FeatureSingletons andAttentional Capture

In consideringwhat stimulus propertiesmight captureattentionregardlessof(or in spiteof) theobserver’s stateof attentional readiness,featuresingletonsappearto be likely candidates.Featuresingletons are judgedas subjectivelysalient,andthereis ampleevidencethatsuchstimuli canbe foundefficientlyin visual search.For example,Neisser(1967) observedthat curved letterscould be found easilyamongstraightletters,andusinga somewhatdifferentparadigm,Egethet al (1972)drew a similar conclusion. Treisman& Gelade(1980) also showed,using a visual searchparadigmlike that of Egethet al(1972), that variousfeaturesingletons could be efficiently detectedin visualsearch.Many exampleshave been cataloguedby Treisman & Gormican(1988;see also Bravo& Nakayama 1992).

Thesedemonstrationsprovideno directevidence,however,aboutwhetherfeaturesingletonscaptureattention,becausein all thecitedcasesthestimulusin questionwas itself the targetof searchand thereforepresumablyelicitedtop-down, deliberate deployment of attention. Therefore,one must designexperimentsthat explicitly dissociatethe observer’s attentionalset from thepropertiesof the stimulus array.Severalsuchstudieshavebeenreported,butthey haveyielded different conclusions.Wefirst reviewpapersthatmight leadoneto concludethat featuresingletonsdo captureattention.We nextconsiderstudiesthat suggestotherwiseandendthe sectionwith discussion of a paperthatprovidesa possiblereconciliation.

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SINGLETONS CAPTURE ATTENTION Pashler (1988, Experiment 7) had sub-jects searchfor a slash(/) in anarrayof manyOs or for an O among/s. Theidentityof thetargetwasnotknownin advance.Onsometrials,twoof theitemsin thearraywereuniquelycolored.Thesewerealwaysirrelevantto thetask,andsubjectsweretold to ignorethem.Pashlerfoundthatreactiontime(RT) to locatethe targetshapewas prolongedon thosetrials in which the color singletonsappeareddespitesubjects’ intentionsto thecontrary.Becauseof theseresults,one mighttentativelyconclude thatfeature singletons do captureattention.

Theeuwes(1991a,1992) further exploredthe conditionsunderwhich fea-ture singletonscontrolledthe deploymentof attention.In his tasks,subjectstypically searchedfor an easy-to-detecttarget(e.g.a diamond)in an arrayofdistractors(e.g. circles). Each stimulus had inscribed in it a line segment,eitherhorizontalor vertical (in the target)or oblique (in the distractors).Todemonstratesuccessfultargetacquisition, thesubjecthadto indicatewhetherthe line in the targetwas horizontalor vertical. Subjectswere told that thecritical line would alwaysbecontainedwithin theshapesingleton.On half thetrials,all thestimuli hadthesamecolor (e.g.red),andon theremainingtrials,oneof thestimuli—neverthe targetshape—wasuniquein color (e.g.green).Subjectswere instructedto ignore the color variation.Additionally, the totalnumberof elementsin the display varied (5, 7, or 9). Reactiontime wasindependentof displaysize,which suggestsparallelprocessing.With respectto the issueof attentionalcapture,this experimentis similar in manywaystothatof Pashler(1988,Experiment7). Results weresimilar aswell: RTs wereprolongedon thosetrials containing a color singletoncomparedwith thetrialswithout a color singleton. Theeuwes(1991a)concludedthat when subjectssearchfor a targetin parallel,which is possiblewhen the targetis a featuresingleton,thentop-downcontrol is not possible andattentionis capturedevenby singletons knownto be irrelevantto thetask.

Another example of stimulus-driven capture was reported recently byJoseph& Optican(1996).Subjectswererequiredto searchfor anL embeddedin anarrayof T stimuli; this targetarraywasflashedbriefly andthenmasked.Subjectswererequiredto reportthe location(oneof four quadrants)in whichthe L appeared.Immediately precedingthe targetarray,a cuearraywaspre-sentedbriefly. The cue array consistedof vertical line segmentsin which asingle horizontalsegmentwas embedded.Subjectswere correctly informedthat the locationof theorientation singletonin thecuearraywasuncorrelatedwith the locationof theupcoming target.(To call anuninformativestimulus acue may seemproblematic; we accedehereto what appearsto be commonusagein the field.) Nevertheless,responsesweremoreaccuratewhenthecueappearedin thelocationsubsequently occupiedby thetarget,which suggested

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thatattentionwasdrawnto thecueeventhoughit wasknownto beirrelevantto thetask.

SINGLETONS DO NOT CAPTURE ATTENTION These preceding studies all sug-gest that featuresingletons,evenonesthatareknownto betaskirrelevant,docapturevisual attention. However, there is also evidencefor the oppositeconclusion.Jonides& Yantis(1988)reportedthatcolor andbrightnesssingle-tonsdo not captureattention. Observerswererequiredto searchfor a letter inanarrayof multiple letters.On eachtrial oneletterdifferedfrom all therestincolor for somesubjects andin brightness forothers. Subjectsweretold thatthetargetwouldoccasionallybetheuniqueelement,butonly onrandomlyselectedtrials.Thatis, thefeaturesingletonprovidednohelpin solvingtheprimarytask,which wasto find thetargetletter.At issuewaswhetherRT to find thetargetdifferedwhenthetargetdid anddidnothappentobetheuniqueelement.Yantis& Jonidesfoundthatit madenodifference:Responseswerenofasterwhenthetargetwas the singleton thanwhen it was not. This resultwas subsequentlycorroborated byTheeuwes (1990).

Hillstrom & Yantis (1994) reportedthat not evenvisual motion capturesattentionunderall circumstances.They had subjectssearchfor a rotatedTamong rotated Ls. One of the stimuli on each trial exhibited one of fivedifferenttypesof visualmotion. In all cases,theposition of thetargetelementwas uncorrelated with the position of the moving element.At issue waswhetherRT differedaccordingto whetherthetargethappenedto correspondtothemovingelement.If motioncapturesattention, thenonewould expectmorerapidRTsto moving targetsthanto stationary ones.Hillstrom & Yantisfound,however, thatRT did not differ for thetwo conditions.

ATTENTIONAL CONTROL Thus,severalstudiessuggestthatsingletons do cap-tureattention,whereasseveralotherssuggestthattheydo not.Bacon& Egeth(1994)proposedareconciliationby suggestingthattheseconflictingresultsaremanifestationsof two differentattentionalstrategiesadoptedbysubjects.Undersomecircumstances,subjectsentersingletondetectionmode,in whichattentionis directedto thelocationin thearrayexhibiting thelargestlocalfeaturecontrast(for further evidence, see Bravo& Nakayama 1992;Nothdurft1992,1993).Insingletondetectionmode,thelocationof thegreatestcontrastcan be accessed,butnot theidentity of thedimension(s)onwhichthestimuli differ. Thus,whenoneis searchingfor, say,a shapesingleton,an irrelevantcolor singleton may“win out” becauseof itsgreaterlocalfeaturecontrast.Otherstimulusconditionsmight leadsubjectsto adoptfeaturesearchmode,in whichattentionis directedto locations that matchsometask-definedvisual feature(e.g.“red” or “verti-cal”). Bacon& Egeth(1994)supportedthis proposalby showingthatcapturebyato-be-ignoredfeaturesingletononlyoccurredwhenthetaskcouldbecarried

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out in singletondetectionmode.Whenthatstrategywasmadeineffective,thencaptureby irrelevant featuresingletonsdid not occur. In one experiment,singletondetectionmodewasmadeineffectiveby including severalidenticaltargetshapes,thusensuringthatnooneof themcouldserveasasingletontarget.In anotherexperiment,severaluniquestimuli werepresentthatwerenontargets,againensuringthatthetargetcouldnot befoundefficiently simply by lookingfor a singleton.

AbruptVisualOnsetsand AttentionalCapture

A categoryof stimuli that behavessomewhatdifferently than most featuresingletonsis abruptvisual onset.Early studiesof whetherabruptonsetscap-ture attention were motivatedby the cuing techniqueintroducedby Eriksenandhis colleagues(e.g.Eriksen& Hoffman1972).Jonides(1981)showedthata peripheralattentionalcue (i.e. a bar marker presentednear the locationsubsequentlyoccupiedby a to-be-identified letter) drawsattention“automat-ically,” whereasacentralarrowheadcuerequiresadeliberateshift of attention.Theautomaticity of theperipheralcuewasdemonstratedin his Experiment2by showingthatperipheralcuesdrewattentionwhethertheywereinformativeaboutthe locationof the targetor not, while centralcuesonly controlledthedeploymentof attention whentheywereinformative. Remingtonet al (1992)found thata peripheralcuecapturesattention evenwhenit is knownnevertoindicatethe targetlocation.

Yantis & Jonides(1984) proposedthat peripheralcueslike the onesem-ployed by Jonides(1981) might captureattentionbecausethey haveabruptonsets.The magnocellularvisual pathwayis known to be quite sensitivetohigh temporal frequency,and one of its functions might be to signal thelocation to which attention should be directed (Breitmeyer & Ganz1976originally proposedthis idea).Yantis& Jonides(1984)pursuedthis ideawitha visualsearchtaskin which observerssearched fora prespecifiedtargetletterembeddedin anarrayof nontargetletters.The letterswereformedby illumi-natinga subsetof the segmentsof a figure eight ason a digital alarmclock(borrowing a techniquedevisedby Todd & Van Gelder 1979). Each trialbeganwith anarrayof completefigure-eightplaceholders.Thesewereactuallyletters that were “camouflaged” with irrelevant line segments.The figureeightsremainedon the screenfor 1 s. The camouflagingline segmentswerethen removedfrom the figure eightsto revealletters(theseweredesignated“no-onset”letters),andsimultaneouslya singleletterappearedin a previouslyblank location (the “onset” letter). The target letter was presenton half thetrials. On the target-presenttrials, the targetwastheonsetletteron 1/n of thetrials (wheren is thetotal numberof lettersin thedisplay).Becausethetargethadan abruptonsetonly rarely,therewasno incentive to deliberatelyattend toit.

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Even though therewas no incentive to attendto the abruptonsetletter,Yantis& Jonides(1984)found thatRT to find thetargetwhenit happenedtobetheonsetletterwasfastanddid not vary with thenumberof elementsto besearched,whereasRT to find thetargetwhenit wasoneof theno-onsetletterswasslowerandincreasedlinearlywith thenumberof elementsto be searched.They concludedthat the onsetletter capturedattentionon eachtrial. If thetargethappenedto be the onsetletter,a responsewasmadeimmediately andnofurthersearchingwasrequired,but if theonsetletterwasnot thetarget,thenan attentionally demandingsearch had tobe initiated.Yantis& Jonides(1990)foundthatthis capture,which occursin theabsence ofany relevant attentionalset, is preventedwhensubjectsare inducedto focusattentionon a differentspatiallocationin advanceof eachtrial. This finding wascorroboratedusingdifferent approachesby Theeuwes(1991b)andby Koshinoet al (1992)andJuolaet al (1995).

At leasttwo potentialmechanismscouldaccountfor attentional capturebyabruptonsets(Yantis & Hillstrom 1994).One,mentionedabove,is that theluminanceincrementactivatesvisual pathwaysthat respondto high temporalfrequency,which in turn direct attention to the eliciting object. A secondpossibility is that the appearanceof a perceptualobject,which requiresthecreationof an episodicperceptualrepresentation,elicits a shift of attention.This secondpossibility might be a “hard-wired” responseto the needto rap-idly identify newobjectsenteringthevisual field. Yantis & Hillstrom (1994)performeda seriesof experimentsthat permittedthemto determinewhich oftheseaccountswas correct.They usedstimuli that were equiluminant withtheir backgrounds (e.g. random-dot stereogramsin which the letters werecomposedof dotsexhibitingbinoculardisparity againsta zero-disparityback-ground).Thesedisplaysthusexhibitedno change inmean luminance, buttheydid include the appearanceof a new perceptualobject.The experimentpro-videdclearsupport forthenew-objectaccount:Attention wascapturedby newperceptualobjectseventhoughthey did not exhibit a luminanceincrement.Hillstrom & Yantis (1994) corroboratedthis finding by showing that whilemotion per se doesnot captureattention when it is task irrelevant(asnotedabove),when motion segmentsan object from its background(as when themotionof a moth’s camouflagingwingssegmentit from a tree’s bark),atten-tion is captured.Fora recentdebateaboutthenew-objectsaccount,seeGibson(1996a,b) andYantis& Jonides(1996).

Interactionof Goal-Drivenand Stimulus-DrivenCapture

Thestudiesreviewedsofar in this sectionprovideevidencethatundercertaincircumstancesattentionis drawnto objects(e.g. featuresingletonsor abruptonsets)without a deliberateintent to direct attentionthere.However,in each

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case,top-downcontrol playsa role. Oneexampleof top-downcontrol is thatirrelevantfeaturesingletons captureattentiononly whensubjectsentersingle-tondetectionmode,where featurecontrastgradients controlthedistribution ofattention.Anotherexampleis thatattentional captureby abruptonsetscanbeprevented or atleast modulated byfocused attentionelsewhere in thedisplay.

Folk et al (1992)proposeda theoretical framework forthe interactionbetweengoal-drivenattentionalcontrol and stimulus-drivenattentionalcap-ture.Theyarguedthatanygivenperceptualactentailsan “attentionalcontrolsetting,”which is part of theexplicit or implicit set of perceptual goals held bythe observerat that moment. Thesegoals might be a result of instructionsprovidedby an experimenter(e.g.“searchfor the red vertical bar”), or, moreoften, bythe individual’s currentplan ofaction in everyday life(e.g. searchingfor thecarkeys).Thevisual featuresthatareof currentinterest(e.g.“red” or“vertical”) will controlthedistribution of attention.

They providedevidencefor this idea by showingthat the deploymentofattentiondependscritically on what the subjectis setfor. In oneexperiment,each trialconsistedof a fixationdisplay,followed in rapidsuccessionby a cuedisplay and a targetdisplay.Eachelementshown in the targetdisplay waseither an x or an =. Two types of target displayswere used.Color targetdisplaysconsistedof threewhite elementsandonered element,andthe taskwasto identify the red elementasquickly aspossibleaseitheran x or an =.Onsettargetdisplaysconsistedof only one element,and so the targetwascharacterizedasbeingtheonly elementwith anabruptonset.Again, the taskwas to identify the targetas being an x or an =. Immediately precedingthetargetdisplay,a cuedisplayappeared;this could consistof eithercolor cues(in which onelocationwassurroundedby red dotsandthe other threeloca-tionswere surroundedby white dots)or onsetcues(in which onelocationwassurrounded bysuddenly onsetwhite dots andthe remaining locations re-mainedblank). Eachtype of targetdisplaywascombinedwith eachtype ofcuedisplay.Cuevalidity wasmanipulatedbetweenblocks; in oneconditionthecuewas100%valid (It alwaysindicated thelocation oftheto-be-identifiedtarget element),and in anothercondition it was 100%invalid (It alwaysindicateda nontargetelement).Folk et al (1992)found thatwhenthecueandtargetwereof the sametype, i.e. both color or both onset,cuevalidity hadalarge effect. In particular,subjectscould not ignore invalid cues.However,whenthecueandtargetwereof different types(e.g.a color cueandanonsettarget),thenthecuehadlitt le or no influenceon responsetimes.This resultisconsistentwith the ideathat the stateof attentional readinessadoptedby theobserver determineswhatsorts offeaturesingletons willcapture attention.

A similar idea motivatedWolfe’s (1994) GuidedSearchmodel (seealsoCave& Wolfe 1990, Wolfe et al1989). According to Guided Search,attention

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is directedto objectsserially in orderof priority. Attentionalpriority is deter-minedjointly by two things.Oneis top-downactivation,that is, how closelyan object matchesthe currentattentional set. For example,if the subjectissearchingfor a red verticalbar, thenall red things andall vertical thingswillreceivehigherpriority thanthingsthatareneitherrednor vertical.Thingsthatarebothredandverticalwill, of course,receivethemostactivation. Theotherdeterminantis bottom-upactivation,that is, how mucha given objectdiffersfrom neighboring objectswithin any given perceptualdimension. For exam-ple, a red object surroundedby greenobjectswill have greaterbottom-upactivation than will a red object surroundedby orangeobjects.Thesetwosourcesof activationarecombinedto producean “attentionmap” that deter-minesthe order inwhich objectsarevisited duringvisualsearch.

Both thesetheoriesincorporatea principle thatWilli amJamesrecognized:Thedeploymentof attentiondependsjointly onpropertiesof theimageandthegoalsand expectationsof theobserver.

THE REPRESENTATIONAL BASISOFVISUALSELECTION

In an important1981paper,Kahneman& Henik asked,“If attention selectsastimulus,what is thestimulusthat it selects?”(p. 183).Until thatpoint—withsomenotableexceptions—attention was viewed (implicitly or explicitly) assimilar to a spotlight directedto regionsof space,“illum inating” the objectslocatedthere(e.g.Eriksen& Hoffman1972,Hoffman& Nelson1981,Posneretal 1980).Theevidenceconsistedprimarily of demonstrationsthatthespatialseparationbetweenelementssignificantly modulatedattentionaleffects.Forexample,Hoffman & Nelson (1981) requiredsubjects to identifya target letterthatappearedin oneof four locationsin thevisual field, andthento identify asecondaryshapethat was either near the target letter or elsewherein thedisplay. Theyfoundthat identification accuracywasmuchbetterwhenthetwostimuli were adjacentto eachother,which suggesteda spatial limitation individing attention. Downing& Pinker(1985)cuedsubjectsto attendto oneoften boxesarrangedin a horizontal row (five on either side of fixation) inanticipationof a luminanceincrementin oneof the boxes,mostoften in thecuedbox. DetectionRT wasfastestwhenthetargeteventoccurredwithin thecuedbox, andit slowedmonotonically asthedistancebetweenthe targetandthe attendedlocation increased.This stronglysuggesteda spatialgradientofselectiveattention.

Kahneman& Henik (1981)urgedreaders,however,to considerthepossi-bility that attentionmight be directednot only to spatiallocationsbut alsotoperceptualobjects.The idea is that the raw retinal image providesonly a

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fragmentedrepresentationof the scenebecauseof occlusion, yet perceptualexperience is coherentand “smooth.” Therefore,someearly visualmechanismis requiredto constructrepresentationsof objects.Discoveringthe principlesby whichobjectrepresentationsareconstructedwasamajorgoalof theGestaltpsychologists. Kahneman & Henik suggestedthat often the object repre-sentationsresulting from perceptualorganizationserveastherepresentationalbasisfor visualselection.

Becauseobjectsoccupy locations,experimentsdesignedto provide evi-dencefor object-basedaccountsmustdemonstratethata given finding is dueto allocationof attentionto a locationally invariantobjectrepresentationandnot to a spatiallocation. Severalstrategieshavebeenadoptedthataccomplishthis.Evidencefor object-basedtheories ofattentionfall into two broadcatego-ries.Locationsor featuresin an imagecanbe probedthat differ accordingtotheir relationship to objectstructure,but that donot differin spatiallocation orseparation.Attentionmayalsobe directedto movingobjects,which by defini-tion involves continuously changingspatial locations.Within eachof thesetwo categories,manyspecific techniqueshave been devised.

Overlap and Grouping

Oneof the earliestdemonstrationsof object-specificattentional benefitswasreportedby Rock & Guttman (1981) in an experimentshowingthat subjectscan selectivelyattendto one of two objectsappearingin the samespatiallocation.Observersvieweda sequenceof 10 pictures.Eachpictureconsistedof two superimposedoutline drawingsof novel shapes,onedrawnin red inkandonein green.Thesubjectwasaskedto makeanaestheticjudgmentof theobjectdrawnin red ink (or, for half thesubjects,greenink) andto ignoretheobject drawn in the other color. The judgmenttask was merely a cover toinducesubjectsto processthe items selectively.After viewing all the draw-ings, subjects weregiven asurprise recognitiontest. Onethird of thetestitemshadbeenattendedduring the judgment task,one third hadbeenunattended,andone third werenew. Subjectsweremuchmore likely to reportattendeditemsasold thanunattendedor newitems.Their judgmentsfor unattendedandnew itemsdid not differ. This resultshowsthat attentionneednot be purelylocation-based,but that it is possible to selectively attend to one of twospatiallycoincidentperceptual objects.

Duncan(1984) laidoutexplicitly the distinction between space-basedtheo-riesandobject-basedtheoriesof attentionandexploredthedistinction empiri-cally with a perceptualversion of the memory task employedby Rock &Guttman.Subjectsvieweda display consisting of a rectanglewith a tilted linedrawnthroughthe middle. Eachobjectcould takeon two valuesfor eachoftwo attributes:The line could be tilted right or left, andit could be dottedor

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dashedin texture. The rectanglecould be tall orshort,andit hadasmall gapinits contouron the right or left side.The displaywasflashedbriefly andwasfollowed by a mask.Subjectswereaskedto reporttwo attributeson each trial.On sometrials, thetwo attributesbelongedto thesameobject(e.g.thetilt andtextureof theline), while onothertrials theybelongedto differentobjects(e.g.the height of the rectangleand the tilt of the line). Responseswere moreaccuratewhen the attributesbelongedto the sameobject.This was takenasevidencethat observersattendto objectsasa whole:Whenjudgmentshadtobe madeaboutboth objects,a costwas incurredbecauseof the needto shiftattentionfrom one objectto the other.

Vecera& Farah(1994)verified that Duncan’s resultsreflectedan object-basedeffect (ratherthan what they termeda spatial“groupedarray” effect).Theynoted that a spatial account predicts that themagnitudeof theobject-spe-cific benefitshouldbelargerwhenthetwo objectsarespatiallyseparatedthanwhen they are spatially superimposed.They found no evidenceof such apattern.However, whenthe task required judgments that did not involveaccessingthe shapesof the objectsbut requiredonly the detectionof a smalldot at variouslocationson the objectcontours,thenonly space-basedeffectswere observed. They concluded thatsimple detectiontasks may accessastrictly spatiallevelof representation,while shapediscrimination tasksrequireobject-basedrepresentationsand thereforeyield object-basedattentional ef-fects.

Thestudiesjust reviewedemploying two overlappingobjectsrevealedthatonecanselectivelyattendto anobjectoccupyingthesamespatiallocationasanotherobject,asobject-basedtheoriespredict.A related techniqueis to showthatwhenattentionis directedto one partof anobject, other partsof theobjectenjoyanattentionalbenefit,whereasequallydistantlocationsin otherobjectsdonot.Severalexamplesof this approach havebeenreported.Baylis & Driver(1993)showedthat judging the relativelocationsof two “corners”of a com-plexstimuluswasmoredifficult whentheybelongedto two objectsratherthanone.This wasthecaseevenwhen theone-objectand two-objectdisplayswerephysically identical, with instructions determining how many objectswereseenin thedisplay.Baylis & Driver found,like Duncan,that judgments abouttwo partsof a single object weremadefasterthan thoseaboutpartsof twodifferent objects(for further discussion of this procedure,seeBaylis 1994,Gibson1994).

M Behrmann,RS Zemel & MC Mozer (unpublishedmanuscript)docu-menteda similar object-specificbenefit using a perceptualmatching task.Subjectswereshowna displayin which two rectangles,oneorientedat +45°andtheotherat −45°, overlappedsuchthatonerectanglewasseenasbeinginfront of theother.Two of therectangleendshadeithertwo or three“bumps”

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on them,andsubjectswererequiredto reportwhetherthe numberof bumpswasthesameor differenton the two ends(theothertwo endswerestraight).Of greatestinterestwaswhethertheendsto be judgedwerepart of the sameperceptualobject.For example,the rectangleendsto be judgedcould be ateitherendof the partly occludedrectangle,or onecould be at oneendof theoccludingrectangle,andthe othercould be at oneendof the partly occludedrectangle.The main result was that judgmentsmadeabouttwo partsof thesameobject were faster than judgmentsmadeabout parts of two differentobjects,evenwhentheobjectin questionwaspartly occluded.This finding isrelatedin many ways to the result reportedby Duncan(1984). In this case,however,thepartlyoccludedobjects werefragmentedin theimage.Perceptualorganizationmechanismswererequiredto put the object fragmentstogetherinto coherent objectrepresentations.

Egly et al (1994)hadsubjectsview a displaycontainingtwo vertically (or,on other trials, horizontally) orientedrectanglespresentedsideby side.Oneend of one of the rectangleswas cued(its local contourwas briefly bright-ened),andafter a short delayoneendof oneof the rectangleswas filled in(this was the “target”). Subjectswere to pressa button when the targetap-peared(asimpledetectiontask).Theyweretold thatthetargetwouldappearatthecuedlocationon 80%of thetrials (thevalid condition), at theotherendofthe cuedobject on 10% of the trials (the same-objectcondition), and at thesame end of theuncued object on 10% of thetrials (the different-objectcondition).The latter two locationswereequallydistantfrom the cuedloca-tion, but they differed in their relationto the cuedobject.The authorsfoundthatmeanRT in thevalid condition wasfasterthantheothertwo conditions.More revealingwas the presenceof an object-specificbenefit: RTs in thesame-objectconditionwerefasterthanin the different-objectcondition. Thisoutcomeis consistentwith an object-basedaccount.

A relatedstudy was carried out by Yantis & Moore (1995). They usedrectanglepairs like thoseusedby Egly et al, but in someconditionstheyplacedanoccludingsurfacein front of therectangles.At issuewaswhethertheobject-specificbenefitsdocumentedby Egly et al persistedwhenthe objectscontaining thetarget eventswere partly occludedand requiredperceptualorganizationto becompleted[asin theBehrmannandcolleagues(unpublishedmanuscript)study].Theresultsrevealeda robustobject-specificbenefit.Yan-tis & Moore(1995)wenton to showthatwhentheperceptualtaskrequiredofsubjectswas a temporal-orderjudgment,no object-specificbenefit was ob-served;insteadonly location-basedeffectswere observed.This latter resultparallelsthe similar findingby Vecera & Farah (1994) notedearlier.

Severalstudieshaveshownthattheattentionaleffectsof imagefeaturescanvary as a function of how they are perceptuallygrouped,ratherthan where

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they are locatedin the image.Suchresultssupportobject-basedtheoriesofattention,on the assumption that the function of perceptualgrouping is tocreateobjectrepresentations.For example,Driver & Baylis (1989;Baylis &Driver 1992) askedwhetherinterferencefrom to-be-ignoredstimuli duringtarget identification dependedonly on relative spatial location or on morecomplexgroupingprinciples.They employedthe flankerstaskdevelopedbyEriksen& Eriksen(1974)in which subjectsarerequiredto reportthe identityof a centrallylocatedtargetletterandignoreadjacentnoiseletters.Eriksen&Eriksen found that when the noise letterswere assignedto a responsethatconflictedwith the responseassociatedwith the targetletter, responsesweresignificantly slowed,which suggeststhat attentioncould not be completelyfocused onthe targetletters.

Baylis & Driver (1992) constructeddisplays in which the stimuli werecoloredletters.For example,in their Experiment2, five letterswerearrangedin a row. The first, third, and fifth letterswereone color (e.g. red), and thesecondandfourth letterswereanothercolor (e.g.green).The lettersX andYwereassignedto oneresponse(e.g.presstheright button),andC andS wereassignedto anotherresponse(e.g.pressthe left button).H andT wereneutrallettersnot assignedto a response;theyneverservedastargetletters.Subjectswere supposedto press thebuttonassignedto theidentityof the middle (third)letterin thestring.Baylis & Driver (1992)foundthattheidentity of thelettersthatmatchedthetargetin color,andnot thelettersthatwerespatiallyclosesttothe target,had the greatestinfluence(facilitation and inhibition of RT). Forexample,thestringXr Hg Sr Hg Xr (wherethesuperscriptindicatesthat letter’scolor)producedlongerRTsthandid thestringHr Xg Sr Xg Hr, eventhoughtheresponse-incompatible Xs arecloserto the targetin the secondstring thaninthe first string.Driver & Baylis (1989)obtainedqualitatively identicalresultsusinggroupingvia commonmotion.

Theseresultshavebeencorroboratedby Kramer& Jacobson(1991),whofoundthattheextentto which flanking elementsinterferedwith theidentifica-tion of a targetdependedon whetherthe flankerswerejoined via connectingline segmentsto the target(producinglarge interferenceeffects)or to otherobjects(producingsmallereffects).

Motion

A secondapproachto exploringtherepresentational basisof visualselectionisto separateobjectsfrom their location via motion. Kahnemanet al (1992)introduceda priming techniquethat produceswhat they term an object-spe-cific “re-viewing” effect. We heredescribea simplified versionof their Ex-periment4. Eachtrial beganwith theappearanceof a squareanda triangleonoppositesidesof the display (e.g.aboveand below fixation) for 500 ms. A

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capitalletterthenappearedwithin eachshapefor 1 sandthendisappeared;thisinitial display constituted the “preview field.” The empty shapessmoothlymovedto new positions to the left andright of fixation over a periodof 590ms,at which time theystoppedanda targetletterappearedwithin oneof theshapes.Thesubjectswererequiredto namethetargetlettervocally asquicklyaspossible after it appeared.Thetargetlettercouldeitherbeoneof thetwo inthe previewfield, or it couldbe new.Whenit was one of thepreview letters, iteitherappearedwithin thesameobjectasit did in thepreviewfield, or in theotherobject.This led to threepossibletrial types.For example,if thepreviewfield consistedof anS in thesquareanda P in thetriangle,thena same-objecttrial would consistof anS in thesquareduring the targetdisplay,a different-objecttrial wouldconsistof anSin thetriangleduringthetargetdisplay,andano-matchtrial wouldconsistof a V ineither shape.

Kahnemanet al (1992)found that naminglatenciesweremuchslowerforno-matchtrials thanfor theotherconditions.More importantly, however,RTsweresignificantly fasterfor the sameobject condition than for the differentobjectcondition.Theyinterpretedthis finding asfollows. Whenavisualobjectappearsin the visual field, an object file is createdfor it. An object file is atemporaryepisodicrepresentationof a visualobject,containinga recordof itslocation, its variousattributes(including, perhaps,its name),and its recenthistory(Kahneman& Treisman 1984). In thecaseof theexperimentdescribedabove, object files forthe square(and the letter appearingwith it in thepreviewdisplay)andfor thetriangle(andits previewletter)arecreatedat thestartof the trial. Whenthe targetletterappearsafter theshapesmoveto theirnew locations,thenthe object file is reaccessed,and if the object file corre-spondingto theshapein which thetargetappearscontainsa traceof thetargetletter(asit would in thesame-objectcondition)thennaminglatencyis speededrelativeto theno-matchcondition.This is a form of object-specificperceptualpriming.

Another study in which motion was usedto separateobjectsfrom theirlocationswas reportedby Yantis (1992), who useda multi-element visualtracking proceduredevisedby Pylyshyn& Storm (1988).On eachtrial, tenelements(small plus signs)appearedon the screen,usually in randomloca-tions.A subsetof these(n = 1 to5) was flashedseveral times.This constitutedthetargetset.Thetargetsstoppedflashing,and all10 elementsbeganto moveabout the screen independently, changing direction at random times andbouncingoff oneanotherandtheedgesof thescreen.After 7 s, theelementsstoppedmoving, and one was flashed.Subjectswere to report whetherthisprobeelement wasamemberof thetargetsetor not.Pylyshyn& Storm(1988)hadshownthat this taskcouldbe carriedout with reasonablygoodaccuracy,andto explainthis they invokeda theoryof visual indexing (Pylyshyn1989)

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accordingto which eachtarget elementis independentlyindexedat the startofthe trial. When the probe appears,it is queriedto determinewhetherit isamongthe indexedset.

Yantis (1992)suggestedinsteadthat this taskwascarriedout by groupingthe targetelementsinto a “virtual polygon,” an object representationthat isanalogousto an object file (Kahneman& Treisman1984, Kahnemanet al1992). Evidencefor selectiveattention to the target elementssupportsanobject-basedrepresentation forselection, becausethe target and nontargetelements could not bedistinguishedon thebasisof spatiallocation (Theywerespatiallyintermixedastheymovedsothatno convexspatialregioncontainedall and only targets).The experimentsweredesignedto show that taskper-formancewas modulatedby factors that influencedhow easily perceptualgroupscould be createdand maintained.In Experiment4, for example,theconfigurationof targetelementswaseitherunconstrainedor wasconstrainedto remainconvexduring motion. The convexity constraintensuredthat thevirtual polygonwould remaincoherentthroughoutmotion (i.e. theorderingofverticesalongthe perimeterremainedconstant),andthis permitted observersto usea single object representationthroughoutmotion. Performancein theconstrainedcondition wassignificantly betterthanin theunconstrainedcondi-tion.

Severalstudieshave attemptedto dissociateobjectsand locationsin aninhibition of return (IOR) paradigm.Summoningcovertattentionto a spatiallocationwith a task-irrelevantperipheralcuecaneitherspeedor slow detec-tion of asubsequenttarget.Whenthetargetfollows theonsetof thecueby 150ms or less, RTs are usually faster when targetsare displayedin the cuedlocationthanin anuncued location(e.g.Maylor 1985,Posner & Cohen 1984).This facilitation hasbeenthoughtto reflectoneconsequenceof attendingto alocation.Whenthetargetfollows thecueby morethan300ms,however, timeto detecta target is often faster for targetspresentedat previously uncuedlocationsthan at previouslycuedlocations.It is this effect that Posneret al(1985)calledinhibition of return.We considerherejust oneaspectof inhibi-tion of return, namely whetherit is associatedwith a spatial location or aperceptualobject.(For discussions of otheraspectsof this phenomenon, seee.g. Abrams& Dobkins 1994, Klein & Taylor 1994, Kwak & Egeth1992,Rafal etal 1989.)

EarlyexperimentssuggestedthatIOR wasassociatedwith spatiallocations,specificallyspatial locations definedin fixed environmental coordinates(May-lor & Hockey1985,Posner& Cohen1984).However,theselocationsoftenhappenedto coincidewith objects.For example,in Posner& Cohen’s (1984)study, displays consisted of squares. When brightened or dimmed, thesesquaresservedas cues.A targetcould then appearin a cuedor an uncued

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square.However,thesquaresappearedin fixed locationson a screen, and thusit was not possibleto say whetherthe resulting IOR was associatedwith aparticular location, a particularsquare, or both.

In aneffort to distinguishamongthesepossibilities,Tipperet al (1991)setthesquaresinto motion. Subjectsfixateda centrallocationaroundwhich twodiametrically opposedsquaresrevolved in a clockwise direction along thecircumferenceof an imaginary circle. At a certaintime during this circularmotion onesquarewasbriefly cued(brightened).Both squarescontinuedtorevolvefor a variabletime until the target(a dot) wasshowninsideeitherthecuedor theuncuedsquare.Consider thecasein which a squarewascuedasitreachedthe leftmostpoint of thecircle, andthetargetdot waspresentedafterthe pair of squareshad completedan additionalhalf-turn (i.e. 180°) aroundfixation. If RT wereslowerwhenthetargetappearedat theleft location,whichis the samelocation in environmental coordinatesas the original cue, thanwhen it appearedat the right location, a location-basedaccountwould besupported. The pattern of resultsobservedby Tipper et al (1991) clearlyfavored the opposite outcome.RT was slowerwhen the targetappeared withinthe previouslycuedobject,which suggestedthat IOR is object-basedundertheseconditions.In a subsequentstudyTipperet al (1994)foundevidenceforbothlocation-basedand object-basedIOR.

We have so far distinguished between object-based and location-basedrepresentations.This simpledistinctionmight leadusto thinkthat whenatten-tion is paidto anobject,thentheentireobjectbenefits(or, in thecaseof IOR,suffers)equally.We concludewith a brief descriptionof oneadditional studythatsuggestsrepresentationsmaybemorecomplexthanthat.Gibson& Egeth(1994) arguedthat the conceptionof an object as independentof locationshouldnot be understoodto imply that an object is devoidof location. (Seealso Baylis & Driver 1993,Farahetal 1990.)

An outcomeof visual objectprocessingappearsto be a structuraldescrip-tion that includesan explicit specificationof relative locations of parts orsurfaceswithin an object (e.g.Hummel& Biederman1992).Thus,althoughobjectsaredistinct from thespatiallocationsthattheyoccupy,thereexistotherintraobjectlocationsthatmaybefixed with respectto theoverallobject.Thatis, anobjectcanbeconstruedasa “microenvironment” within which specificlocationsmay be taggedby the mechanisms that produceIOR or attentionalfacilitation. To test thesenotions,Gibson& Egeth(1994)employed a com-puter-generateddepictionof a “brick” thatrotatedin depth inthetimebetweenthepresentationof acue andasubsequent target.Theresultsof aseriesof fourexperimentsshowedthat IOR wasassociatedbothwith locationson thebrickthatremainedfixed with respectto thebrick aswell aswith locationsthatwerefixed in referenceto the unmoving environment.

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THE TIME COURSEOFATTENTION

The deployment of attentionfrom one stimulus to anotheris by no meansinstantaneous.A substantialbodyof researchhasexploredthetemporalchar-acteristics ofattentional deployment. In this sectionwe examine(a) howquickly attention canbe directedto a particularstimulus, (b) how long atten-tion remainsdirectedat a particularstimulus(thedwell time of attention),and(c) howattentionmovesfrom locationto location.

Directing Attention

There is a substantial literature concerninghow attention may be covertlydirectedto a particularstimulusor to a locationin thevisual field, which wascoveredin a recentAnnualReviewof Psychologyarticle(Kinchla 1992).Onlybasic findings are recountedhere.In studiesby Eriksenand his colleagues(e.g.Eriksen & Collins1969,Eriksen &Rohrbaugh1970)stimuli werebrieflydisplayedletterson thecircumference ofanimaginary circle.A cue indicatingthe locationof the to-be-reportedletter couldbe shownin advance of theletterdisplay. Accuracy of report increasedwith increasingstimulus-onsetasyn-chrony (SOA) betweenthe cue and the target letter. Therewas substantialimprovementwith just a 50-msSOA,andtheeffectof theprecuewasasymp-totic by about 200 ms. However, the story is not quite as simple as thatdescriptionmight suggest.As discussedin the sectionon attentional control,apparentlytwo different mechanismscan direct attention to a stimulus orstimulus location—onethat is stimulus-drivenand anotherthat is goal di-rected.We review herestudiesthat reveal the time courseof thesemecha-nisms.

In a study by Müller & Rabbitt (1989), subjectsfixated the centerof adisplay while four boxeswere presentin the peripheryof the display at thecornersof a largerimaginarysquare.Subjectshadto discriminatetheorienta-tion of a T presentedin oneof theboxes;theremainingthreeboxescontainedplussigns.Beforethepresentationof thesecharacters,subjectsreceiveda cuethatwaseitherthebrief brightening of oneof the four boxes(a peripheral cue)or thepresentationof anarrowat thecenterof thedisplaythatpointedat oneof theboxes(a centralcue).Thesecueswerepartially valid. Half of the timetheyindicatedthebox thatcontainedthecritical T-shape,andhalf of thetimethey indicatedoneof the boxesthat containeda plus sign. Performancewasexaminedasa function of the SOA betweenthe cueandthe characters.Theresultsshowedthattheperipheralcuehada fast-actingeffecton performance.For example,with a valid peripheralcue,performancewasquitegoodevenattheshortestSOA (100ms).It improvedasSOA increasedto 175msandthendeclinedsomewhatto a stablelevel for SOAsbeyond400 ms. In contrast,avalid centralcuewas virtually ineffectiveat 100 ms; performanceincreased

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steadilyuntil, at 400ms,it reached approximately the samestablelevel as thatachievedby theperipheralcue.Thus,theperipheralcuewascharacterizedashaving a fast, transientresponse,and the central cue was characterizedashaving a slow, sustainedresponse.More specifically, central cueselicit adeliberateshift of attentionthat is characterizedby a monotonic rise to anasymptote,while peripheralcuesproducea quick riseandthenfall to a lowerasymptotic level (and,perhaps,inhibition of return at still longer intervals).Similar findings havebeenreportedby Kröse& Julesz(1989),Nakayama&Mackeben (1989),and Cheal& Lyon (1991).

The DwellTimeof Attention

VISUAL SEARCH In muchof therecentresearchon attentionthevisualsearchparadigmhasbeenusedto probethe mechanismsof attention.Among otherthings,this paradigmhasbeenusedto estimatethe amountof time spentperitem in thevisualdisplay.Let us takeasa startingpoint searchfor a T in anyorientationin a backgroundof Ls in anyorientation(seee.g.Bergen& Julesz1983,Egeth& Dagenbach1991,Wolfe etal 1989).Thistaskis demanding andmay well require serial processing(a requirementfor any straightforwardestimateof time per item). If one plots meanRT againstdisplay size, theresultingtarget-absentandtarget-presentfunctionsarenearlylinearandhavesubstantialslopesthatstandin roughlya2:1ratio.Forexample,in thestudybyWolfe et al (1989,Experiment4) thepresentandabsentslopesfor onesetofconditionswere 19.2and41.6msper item,respectively,and for anotherset ofconditionswere 24.9and 60.9msper item,respectively.Takentogether,thesetwo datasetssuggesta serial searchthat inspectsnontargetsat the rate ofapproximately50msperitemuntil thetargetis found,with theshallowerslopeof the target-presentfunction due to the subjectterminatingthe searchuponfinding thetargetafterhalf of thestimuli (onaverage)havebeeninspected.Theslopeof the target-absentfunction canbeconstruedasthe time that attentiondwells on an item beforemoving to thenext item. Obviouslythis dwell timewill dependonmanyfactors,suchasthedifficulty of thediscriminationbetweentargets andnontargets(see e.g. Cheal& Lyon 1992,Palmer 1994).

Useof the slopeof a searchfunction asan estimate of how long attentiondwellsonastimulushassubstantial face-validity,but it is notwithout interpre-tive problems(e.g.Palmer& McLean1995;Townsend1971,1990).Perhapsthebiggestproblemis that theunderlying serialmodelmaybe inappropriate.Suppose, forexample, that nontargets wererejectedin parallelby a limited-ca-pacityprocess(suchthatit takeslongerto workthe moreelementsthere are inthe display). One could still computea slope,but it would not accuratelyreflect the time courseof attentionacrossdiscreteitems in the display.Forexamplesof modelswith varyingdegreesandkindsof parallelism,seeDuncan

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& Humphreys(1989), Grossberget al (1994), Hoffman (1979), Palmer&McLean (1995),and Wolfe(1994).

RAPID SERIAL VISUAL PRESENTATION (RSVP): WHOLE REPORT If we movebeyondthesearchtask,wefind otherparadigmsthatcastlight onthetimecourseof attention. Severalinvolve the sequentialdisplay ofstimuli asopposed tothesimultaneousdisplayof thevisualsearchtask.(For someearlyapplicationsofsuchdisplaysto the issueof whetherprocessingis parallelor serial,seee.g.Eriksen& Spencer1969,Shiffrin & Gardner1972,Travers1973.)Someof thesequentialdisplay procedureshaveyielded estimatesof dwell time that areshorterthantheroughly50msestimateswegetfromvisualsearchstudies,whileothers haveprovidedsubstantially longerestimates.

Saarinen& Julesz(1991)presentedtwo, three,or four numeralsin randompositions on a ring surroundingfixation. Each numeralwas followed by amaskin thesamelocation.Eachnumeralandeachmaskwaspresentedfor 33ms, and there was a blank interval of variable duration (0, 33, or 67 ms)betweeneachnumeral and its followingmask. Eachmaskappearedsimultane-ously with the appearanceon screenof thenextnumeral.Thus,SOAsin thisexperimentwere 33,67,or 100ms.At the end ofthe stimulus presentationthesubject wasto type in all thenumerals inthesequencein thecorrectorder.Notsurprisingly, asthenumberof numeralsin thesequenceincreased,thepropor-tion of trials on which the subjectcould correctly identify all of them de-creased. The authors emphasized, however, that performance was abovechanceevenwhentherewerefour numeralsin thesequenceandtheSOA was33ms.Theyconcluded that the speedof focalvisual attention canbequitefast(at least50 msper item),with performance stillrespectable at 33msper item.

In a subsequentexperiment,exposuredurationsasshortas16.7 ms wereused,in addition to a condition in which stimuli were presentedsimultane-ously(Hunget al 1995).Again performancewasbetterthanchanceevenwithfour-numeralsequencespresentedat theshortestSOA(i.e.16.7ms).Accuracyof report in the correctorderin that condition wasapproximately 0.2%; theirestimateof chance inthatconditionwas0.02%.

Thereis a problem,however,with basingthis argumenton the fact thatperformancewasabovechance.Supposesubjects alwayssawthefirst numeralclearlybut, becauseof capacitylimitations,sawnoneof the following items,which they would haveto guessrandomly. Even suchminimal informationwould leadto above-chanceperformance,butsuchperformancecouldnot thenbe convertedinto a meaningfulestimateof dwell time per item. By our owncalculations,performancein the Saarinen& Juleszstudy appearsto be toogood to be accountedfor in termsof subjectsseeingone item andguessingthree.If anything, it is morelike seeingthreeclearlyandguessingone.Thus,thework of Juleszandhis colleaguesstrongly suggestsa high speedfor focal

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attention,but it is not clear whethera preciseestimate of thatspeedis possiblebased onthis technique.

Theresearchof Saarinen& Julesz(1991)andHunget al (1995)attemptedto estimate dwelltime by presentingstimuli sequentiallyand determininghowquickly they canbe presentedwhile still maintaining above-chanceperform-ance.A different approachto estimating dwell time asksinsteadhow slowlystimuli needto bepresentedto keepreportaccuracyatahigh level.SomeearlyresearchusingRSVP wasconcerned withreadingand soused letter sequencesthat formedwords.Kolers & Katzman(1966)presentedsix lettersoneaftertheotherin thesamespatiallocation;theyfoundthatit tookanSOAof 375msfor accuratereport (over 90% correct)of the lettersin a sequence.Haber&Nathanson(1969) useda similar display format and presentedwords thatvariedin lengthfrom four to eight letters.They found that the SOA requiredfor asymptotic performanceincreasedwith word length.For four-letterwordsthey estimated the critical SOAto be65 ms, andfor eight-letterwords110ms.Haber& Nathansongaveseveralreasonsfor believing that the relationshipthey found betweenword length and SOA may be artifactual.For example,therewasno maskbeforethefirst letteror afterthelast letter.Thustwo of thefour lettersin a four-item list areparticularlyeasy,but only two of the eightitemsin aneight-itemlist wereparticularlyeasy.Thus,the65 msestimateofrequiredprocessingtime is probablytoo short.It is possiblethat the 110 msestimateis alsotoo short,at leastif we considertheresultsof Travers(1973).In a conditionin which thelettersof a word werepresented sequentially in thesamespatial location (with the string precededand followed by a mask)anexposureduration of 375 ms yielded between80 and 85% of words (notletters) correctlyidentified.

RAPID SERIAL VISUAL PRESENTATION: PARTIAL REPORT One problem withinterpretingtheaforementionedstudiesis that theuseof wordscreatesoppor-tunitiesfor all sortsof guessingstrategiesto occur.To avoid theseproblems,onemight presentrandomletterstrings(e.g.Kolers& Katzman1966,Travers1973). However, this creates problemsof its own. In particular,memoryrequirementscometo dominatetaskperformance.Onesolution to thisproblemis toeliminatetheneedfor wholereport(Sperling1960).A varietyof interestingdesignshaveadoptedthisapproach.Theyhavein common therequirementthatsubjectsreportthestatusof just oneor two items,calledtargetitems,thataredifferentiatedfrom theother itemsin thestream insomeway.

In a seriesof four experiments,Broadbent& Broadbent(1987) distin-guishedtargetsfrom nontargetsin severaldifferentways.In onetask,subjectshad to report two uppercasetargetwords presentedin an RSVP streamofotherwiseall lowercasewords.Subjectswereunableto reportboth targetsifthey were presentedin temporallyadjacentpositions. Moreover,this deficit

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persistedevenwhenwordswereseparatedby one,two, or threeinterveningnontargets.(At theexposuredurationof 80 ms,threeintervening itemstrans-latesinto an SOA of 320 ms betweensuccessivetargets.)In anotherexperi-mentusinga somewhatmoredifficult discrimination(targetsweredesignatedby thepresenceof a hyphenon eitherside),thedeficit in reportingbothwordswaspresentfor temporalseparationsof up to 480ms.Thedifficulty of report-ing both targetswas not limited to situations in which targetand foil weredistinguishedby a simple physical feature.Similar resultswere obtainedinanothertask in which all items were lowercaseand subjectshad to reportanimal names. Thislengthy refractoryperiod is consistentwith Duncan’s(1980) claimthatit is difficult to process twotargets atthe sametime.

The RSVPstudiessupplyan appreciationof what thephrase“at thesametime” means.Roughly speaking,poor performanceon the secondtargetmaybeviewedasreflectingthedurationof processingof thefirst target.However,this is a simplification.Whenitemsfollow oneanotherrapidly, subjectsoftenprocessthemin the “wrong” order.For example,in the Broadbent& Broad-bent (1987) experimentusing uppercasetargets,when the two targetsweretemporallyadjacentthe probabilities of reportingthe first andsecondtargetswere 0.46and 0.35respectively,but theprobabilityof reportingbothcorrectlywasonly 0.075.Thus,apparentlyon manytrials subjectswereableto reportthesecondbut not thefirst target.In contrast,whenthetargetswereseparatedby three interveningitems, the correspondingprobabilities were 0.45, 0.14,and0.075.Herethedeficit would appearto bemostly,but not entirely,duetoprolongedprocessingof thefirst target.Reeves& Sperling (1986)and Weich-selgartner& Sperling(1987)provideddetailedtemporalanalyses ofresponsesin multitargettasks.

The RSVPstudieswehavereviewedsuggestthat theallocation of attentionto a targetin a stimulus streamproducesa fairly protracteddeficit. It is notclearwhatthenatureof this deficit is. In studiesthatrequiredword identifica-tion thedeficit maybein word identification, or itmaybe insomelower-levelvisualprocess.Similarly, in theWeichselgartner& Sperling(1987)studythedeficit may be in memorymechanisms, or it may be in perceptualor atten-tional processes.Raymondet al (1992)suggestedthat what we areseeinginthesestudiesis a suppressionof visualprocessing.They wrotethat“these datasuggestthat the mechanisms involved in targetidentificationaretemporarilyshut down after use. It is as if the perceptualand attentional mechanismsblink” (p. 851).

Raymond et al (1992) designeda dual-taskRSVP experimentin whichresponserequirementsweresomewhatsimpler thanin precedingstudies.Let-terswerepresentedoneat a time at a rateof 11 per second.One letter waswhite; all therestwereblack.Subjectshadto identify thewhite letter.On half

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of the trials therewasan“X” somewherein thestream(but neverprior to thewhite letter).On theotherhalf of thetrials therewasno X. After reportingtheidentity of thewhite letter,thesubjectwasto indicatewhetherthestreamhadcontainedan X. (Note that on sometrials the white letter was an X.) Bothresponseswereunspeeded,andthe memoryload wasminimal. The focusofthis study was on the consequencesassociatedwith paying attention to atarget;for thisreason,the white letter wasreferredto as thetarget,while theXwas referredto as the probe. In the control condition, the subjectwas in-structedto ignore the white letter and just indicatewhetherthe probewaspresentor not. Note that the stimuli were identical in the experimentalandcontrol conditions. This allows oneto determinewhetherposttargetperform-ancedeficits aredueto sensoryfactorssuchasmaskingof the probeby thetarget or totheattentionaldemandsof identifying the targetletter.

Therelevantdataarethepercentagesof correctdetectionsof theprobeasafunction of the position of the probein the series.When the subjectdid nothaveto identify the white letter, probedetectionwas very good, averagingabout90%correct,anddid not vary asa functionof probeposition.However,whensubjectsdid haveto identify thewhite letter,probedetectionprobabilitywassimilar to control performanceatposition 0,i.e. when theprobeandtargetcoincided,anddeclinedsteadilyuntil, at position 3, probedetectionwaslessthan50%.Performancethenrecoveredgraduallyuntil, by position6, it onceagaindid not differ from control performance.This substantialandextendeddip in performanceof theexperimentalconditionwasreferredto by Raymondetal (1992)astheattentionalblink. In thatstudy, itwasstatistically significantin theposttargetinterval from 180 to 450 ms.We areconcernedherechieflywith documenting the existenceand extentof the attentionalblink. Severalattemptsto provide theoreticalaccountsof the phenomenonhavebeenpro-posedrecently(e.g.Chun& Potter1995,Grandisonetal 1996,Raymond etal1995,Seiffert& DiLollo 1996).

RAPID SERIAL VISUAL PRESENTATION: MINIM AL SEQUENCES The partial reportprocedureis clearly a simplification of the whole report method.However,RSVP with a multi-item sequenceis a dauntingtask nevertheless,requiringselectionof stimuli presentedathighspeed.Onefurthersimplificationhasbeenintroduced.Duncanet al (1994;seealsoWard et al 1996)presentedjust twostimuli sequentially and had subjects identifyboth. Thestimuli werepresentedin two different locationsandwereboth postmasked.Whenthe stimuli werepresentedclosetogetherin time, the first stimulus interferedwith thesecond.The investigators measuredthe intervaloverwhich the interferencepersisted.Theytook this to beanindexof thetimecourse of thefirst object’s attentionaldemand(or,astheyputit, of thedwell timeof attention).Theresultisconsistent

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with otherestimatesfromtheRSVPliterature.Dwell timeswereseveralhundredmilli seconds. Asummaryestimateof 500mswould notbe far off the mark.

Thesimplicity of theexperimentalparadigm makes this perhaps thestrong-estpieceof evidencethat searchcannotshift betweenobjectsfast enoughtoaccount for thereaction timesthat are obtainedin typical visualsearch tasks.

WHY THE DISCREPANT RESULTS? We take the high speed (17–33 ms) esti-mates of dwell time derivedfrom thestudiesof Saarinen& Julesz(1991)andHung et al (1995)to beprovocativebut, for the reasonsstatedabove,not yetsufficientlysecureto serveasthebasisfor furthertheoreticalspeculation.Whatremains,then,is to considerwhy theresultsfrom searchtasks(50msperitem)andawidevarietyof RSVPtasks(500msperitem)yield suchdifferentresults.This sectionmustbeconsideredspeculativebecauselittle work hasaddressedthis question.

Moore et al (1996)notedthat theexperimentsby Duncanet al (1994)andWardetal (1996)used masked stimuli. Typically, masked stimuli arenot usedin visual searchtasks.If dwell time dependson thespecificstimuli andtasksused,thenperhapsthe discrepancyis moreapparentthanreal.More specifi-cally, attention may remainfocusedlongerfor a difficult discrimination thanfor an easy discrimination,and itis reasonable tothink thatmaskingmayhavemade thediscrimination difficult and thusled to an unusually longdwell time.

The Moore et al(1996) experimentwas very similar in designto theexperimentsby Duncanandhis colleagues.Theonecrucialdifferenceis thatin oneconditionboth thefirst andsecondstimulus werepostmaskedimmedi-ately after their exposure(asin Duncanet al), while in anotherconditionthefirst andsecondstimuli weremaskedat thesametime (immediately after theexposure of the second stimulus). The critical question was whether thechangein maskingstatusof the first stimulus would affect the dwell time ofattentionon that stimulus. It did. Dwell time was reducedto about200 ms.Althoughstill longerthanthetimesderivedfrom visualsearch tasks,this timeis lessthanhalf of theestimatein Duncanet al (1994).It seemspossible thatother differences between themethodsmight reduce the time still further.

Bennett& Wolfe (1996)havemadea furthereffort to bridgethemethodo-logicalgapbetweenvisualsearchandRSVPprocedures.Subjectssearchedfora rotatedT amongrotatedLs, a task that would seemlikely to elicit serialsearch.Stimuli were presentedone at a time at randomlocations in a largefield. SOAs varied acrosstrials: 26, 52, 78, or 104 ms. Oncepresented,astimulus remainedin view until theendof the trial. Reactiontime wasmeas-uredfrom the onsetof the trial until the subjectpresseda key. What wasofinterestwashow well subjectscould “keep up” with the sequentialpresenta-tion of items.If subjectscouldmoveattentionfrom stimulusto stimulusat thesameratethat they werebeingshown,thenthe slopeof the function relating

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meanRT to thetime in thesequencewhenthetargetwasfoundshouldbe1.0.The seven subjectskept up with stimulus presentation at SOAsof 104, 78, and52 ms,andfell behindonly at 26 ms.This suggeststhatsubjectscandiscrimi-nate rotatedTs andLs at a rate ofabout 50msper item.

With this interesting paradigmand result we have comefull circle. Al -thoughpresentationwassequential, theestimateof dwell time wasasshortasthosefrom simultaneouspresentationsin classicalstudiesof visualsearch.It isby no meansclearjust what it is aboutthesevariousparadigmsthat leadstosuchdiffering results;this remainsa problemfor further research.Neverthe-less,it seemsreasonableto conclude,following MM Chun,JM Wolfe & MCPotter(unpublishedmanuscript) that “while it ispossible to tie upattention forseveralhundredms after a targethasbeendetected,suchcommitment by nomeansrepresentsa mandatoryminimum dwell time for successfulprocess-ing.”

The Movement of AttentionIt is widely acceptedthatattention canbeshiftedfrom onelocationto anotherin the visual field without any concomitantmovement of the eyes.However,thenatureof theshift is lessclear.Doesattention movein ananalog,continu-ous fashion,or is the shift of attention accomplishedabruptly, without anyactual movement?

Severalinvestigatorshaveobtainedresultsthattheytookto supporttheideathat, like a spotlight, attention movescontinuously throughspace(e.g.Shul-manet al 1979)andthusrequiresmoretime to movea greaterdistance(e.g.Tsal 1983).For example,Tsal(1983) hadsubjects rapidly discriminatean“X”from an“O”. Stimuluspresentationswere4°, 8°, or 12° to the left or right offixation. At a variabletime beforethe letterwaspresented,a cuewasbrieflyflashedat thelocationin which theletterwasto appear.Tsalreasonedthatthecueshouldbe beneficial.On the basisof the assumptionthat attentiontakestime to move,hereasonedfurtherthatthemaximum benefitof thecueshouldoccurprogressivelylater in time the further awayfrom fixation the stimulusappeared. Thiswas the patternhe observed.

This researchsuggesting that attention moves in ananalog fashionhasbeencriticizedin somedetailby Eriksen& Murphy (1987)andYantis (1988).Forexample, oneproblemwith the study by Tsalis that it didnot includeacontrolfor generalarousalor alertness.Yantis (1988,p. 205)concludedthatboth theTsal (1983) andShulmanet al (1979)experiments“are simply inconclusiveaboutwhetherattention shifts havecontinuousor discretedynamics.”How-ever,therearesomeotherapproachesthatavoidtheproblemsof theaforemen-tionedstudies.

Sagi& Julesz(1985) showedevidence foranabrupt relocation ofattention.Subjectshadto decidewhethertwo simultaneouslypresentedstimuli werethe

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sameor different.Thestimuli—rotatedTs andLs—werepresentedat varyingseparationsandwere followedby a maskto limit processingtime.The findingof chief interestwas that at any given stimulus-maskonsetasynchrony,dis-crimination accuracywas independentof distance,which they took as evi-dence for “fast, noninertialshiftsof attention” (p. 141).

Converging evidenceof distance-independentrelocationof attention wasprovidedby Kwak etal (1991).Subjectsmadesame-differentjudgmentsaboutpairs of Ts and Ls that varied in separation.In their experiments,stimuliappearedat varyingseparationson an imaginarycircle, to controlacuity.Thedependentvariableof chief interestwasreactiontime.As in theSagi& Juleszstudy, performancewas independentof separation.This was true for bothupright Ts andLs (Experiment1) androtatedTs andLs (Experiments2, 3).Remington & Pierce(1984)reported a similar result.

In both the Sagi& Julesz(1985)andKwak et al (1991)experiments,it isimportant to establishthat the tasksactually requiredattention.If the taskswere accomplishedpreattentively,there would be littl e reasonto speakofreallocationof attention. Wefocushereontheanalysisprovidedby Kwak etal(1991,Experiment3). To establishthat the two stimuli were examinedoneafter theother,theyuseda diagnostic basedon theadditivity of a within-dis-play visual quality manipulation (Egeth& Dagenbach1991).This diagnosticappliesto situationsin whichprocessingof stimuli mustbe exhaustive. Thisisthe casein a same-differentmatching task because bothstimuli must beexaminedto makea correct response.

The two lettersin the display were, independently, either high or low incontrast.If the lettersareprocessedsequentially,thenthe slowing causedbypresentinglow-contrastcharactersshouldbe additive.That is, if makingonecharacterlow in contrastincreasesmeanRT by 20 ms, then making bothcharacterslow in contrastshould increasemeanRT by 40 ms. However,ifprocessingis parallel,thenthe 20 ms slowing producedby onelow-contrastletter shouldnot be exacerbatedby making the other letter low in contrast.That is, the effects of reducing the contrastsof the two letters should besubadditive.Theresultsshoweda clearpatternof additivity for therotatedTsandLs. Thus,attention presumablyhadto moveserially from oneitem to theother.Sagi& Julesz(1985)usedsomevery differentexperimentaldiagnosticsthatalsopointedto serialprocessing.Thus,it seems reasonableto suggestthatit is appropriateto speakof the (null) resultsof the distancemanipulation inthesestudiesas indicating that it doesnot take longer for attentionto movegreater distances.

Sperling& Weichselgartner(1995)haveindependentlyreportedevidencethat longer movementsof attentiondo not requiremore time. They furthershowedthat attention canskip over an interveningobstaclewithout any time

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cost.Togethertheseresultssuggestthatthemovementof attentionis “quantal”rather than analog.

Concluding RemarksOur review revealssignificantrecentadvancesin the understandingof atten-tion. Of course,manyof the issuesandsomeof the mechanismsthatoccupyjournalpagestodaywereanticipatedto somedegreeaslong asa centuryagoby William Jamesandothers.Nevertheless,manyempiricaldetailshavebeenclarified, richer theoretical frameworkshave evolved, and important newideas,suchasthe distinction betweenobject-basedandlocation-basedselec-tion, havebeenadvancedanddeveloped.Thereis noreasonto believethatthisrecentprogresswill subside any time soon;behavioralstudies of attention,augmentedby neuroimaging studiesof the functioning brain and neuropsy-chologicalstudiesof brain-damagedpatients,promisenew insights into themechanismsof visualattention.

LiteratureCited

AbramsRA, Dobkins RS. 1994. Inhibition ofreturn: effects of attentional cueing on eyemovement latencies. J. Exp. Psychol.:Hum.Percept.Perform. 20:467–77

Bacon WF, EgethHE. 1994. Overridingstimu-lus-driven attentional capture. Percept.Psychophys.55:485–96

Baylis GC. 1994. Visual attention andobjects:two-object cost with equal convexity. J.Exp.Psychol.: Hum.Percept. Perform. 20:208–12

BaylisGC,DriverJS.1992. Visual parsingandresponse competition: the effect of group-ing factors. Percept. Psychophys. 51:145–62

Baylis GC, Driver JS. 1993. Visual attentionandobjects:evidence for hierarchical cod-ing of locations. J. Exp. Psychol.: Hum.Percept.Perform. 19:451–70

Bennett SC, Wolfe JM. 1996.Serial visualsearchcan proceedat 50 msecper item.Invest. Ophthalmol . Vis. Sci . 37:298(Abstr.)

Bergen JR,JuleszB. 1983. Parallel versus se-rial processingrapid patterndiscrimination.Nature303:696–98

Bravo MJ, Nakayama K. 1992. The role ofattention in different visual-searchtasks.Percept.Psychophys.51:465–72

Breitmeyer BG, GanzL. 1976. Implicationsofsustained and transient channels for theo-ries of visual pattern masking, saccadicsuppression, andinformation processing.Psychol. Rev.83:1–36

Broadbent DE, Broadbent MHP. 1987. Fromdetection to identifi cation: responseto mul-

tiple targetsin rapid serialvisualpresenta-tion. Percept. Psychophys.42:105–13

CaveKR, Wolfe JM. 1990. Modeling the roleof parallel processing in visual search.Cogn. Psychol. 22:225–71

Cheal ML, Lyon DR. 1991. Central and pe-ripheral precuing of forced-choice dis-crimination. Q. J. Exp. Psychol. 43A :859–80

ChealML, Lyon DR. 1992. Attention in visualsearch: multiple search classes. Percept.Psychophys.52:113–38

Chun MM, Potter MC. 1995. A two-stagemodel for multiple targetdetection in rapidserialvisualpresentation. J. Exp. Psychol.:Hum.Percept. Perform. 21:109–27

DagenbachD, Carr TH, eds.1994. InhibitoryProcessesin Attention, Memory,and Lan-guage.SanDiego: Academic. 461 pp.

Desimone R, DuncanJ. 1995. Neuralmecha-nisms of selective visual attention. Annu.Rev.Neurosci.18:193–222

Downing CJ, PinkerS.1985. The spatial struc-ture of visualattention. SeePosner & Ma-rin 1985, pp. 171–87

Driver JS, Baylis GC. 1989. Movement andvisual attention: the spotli ght metaphorbreaksdown. J. Exp. Psychol.: Hum.Per-cept. Perform. 15:448–56

Duncan J. 1980. The locus of interferenceinthe perception of simultaneous stimuli.Psychol. Rev.87:272–300

DuncanJ. 1984. Selective attention and the or-ganization of visual information. J. Exp.Psychol.: Gen.113:501–17

DuncanJ, HumphreysG. 1989. Visual search

294 EGETH& YANTIS

and stimulus similarity. Psychol. Rev.96:433–58

Duncan J, Ward R, Shapiro K. 1994. Directmeasurementof attentional dwell time inhumanvision. Nature369:313–15

Egeth H, Jonides J, Wall S. 1972. Parallelprocessing of multidimensional displays.Cogn. Psychol. 3:674–98

Egeth HE,DagenbachD. 1991. Parallel versusserial processingin visual search:furtherevidencefrom subadditive effectsof visualquality. J. Exp. Psychol.: Hum. Percept.Perform. 17:551–60

Egly R, Driver J,RafalRD. 1994. Shift ing vis-ualattentionbetweenobjectsandlocations:evidencefrom normal and parietal lesionsubjects.J. Exp. Psychol.: Gen.123:161–77

Eriksen BA, Eriksen CW. 1974.Effects ofnoiselettersuponthe identification of a tar-get letter ina nonsearchtask.Percept. Psy-chophys.16:143–49

Eriksen CW, Coll ins JF. 1969. Temporalcourse of selective attention. J. Exp. Psy-chol. 80:264–61

EriksenCW, Hoffman JE.1972. Temporal andspatial characteristicsof selective encodingfrom visualdisplays.Percept. Psychophys.12:201–4

EriksenCW, HoffmanJE.1973. Theextentofprocessingnoise elements during selectiveencoding from visual displays. Percept.Psychophys.14:155–60

EriksenCW, Murphy TD. 1987. Movement ofattentional focus acrossthe visual field: acritical look at the evidence.Percept. Psy-chophys.42:299–305

EriksenCW, Rohrbaugh JW. 1970. Some fac-tors determining efficiencyof selective at-tention. Am.J. Psychol. 83:330–42

EriksenCW, Spencer T. 1969. Rateof infor-mation processing in visual perception:some results and methodological consid-erations.J. Exp.Psychol. 79:1–16

FarahMJ, Brunn JL, Wong AB, WallaceMA,Carpenter PA. 1990. Framesof referencefor allocating attention to space:evidencefrom the neglect syndrome. Neuropsy-chologia 28:335–47

Folk CL, Remington RW, Johnston JC. 1992.Involuntary covert orienting is contingenton attentional control settings. J. Exp.Psy-chol.: Hum. Percept. Perform. 18:1030–44

GibsonBS.1994. Visual attention and objects:one versus two or convex versus concave?J. Exp. Psychol: Hum. Percept.Perform.20:203–7

Gibson BS. 1996a. Visual quality and atten-tional capture: a challenge to the specialrole of abrupt onsets. J. Exp. Psychol.:Hum.Percept.Perform. In press

Gibson BS. 1996b. The masking account of

attentional capture: a reply to Yantis andJonides (1996). J. Exp. Psychol.: Hum.Percept. Perform. In press

Gibson BS,Egeth H. 1994. Inhibition of returnto object-basedandenvironment-basedlo-cations.Percept.Psychophys.55:323–39

Grandison TD, Ghirardell i TG, Egeth HE.1996. Masking of the target is sufficient tocausethe attentional blink. Percept.Psy-chophys.In press

Grossberg S, Mingolla E, Ross WD. 1994. Aneural theoryof attentive visualsearch:in-teractionsof boundary, surface,spatial, andobject representations. Psychol. Rev.101:470–89

HaberRN, Nathanson LS. 1969. Processingofsequential ly presented letters. Percept.Psychophys.5:359–61

Helmholtz H von. 1925. (1866). Treatise onPhysiological Optics, Vol. 3, ed./transl.JPCSouthhall. Washington, DC: Opt. Soc.Am. 3rd ed.

Hil lstrom AP, Yantis S. 1994. Visual motionand attentional capture. Percept. Psycho-phys.55:399–411

HoffmanJE.1979. A two-stage model of vis-ual search.Percept. Psychophys. 25:319–27

HoffmanJE,Nelson B. 1981. Spatial selectiv-ity in visual search.Percept. Psychophys.30:283–90

Hummel JE, Biederman I. 1992. Dynamicbinding in a neural network for shape rec-ognition. Psychol. Rev.99:480–517

Hung GK, Wilder J, Curry R, JuleszB. 1995.Simultaneous better than sequential forbrief presentations. J. Opt. Soc. Am.A 12:441–49

JamesW. 1890. The Principlesof Psychology.NewYork: Holt

Johnston WA, Dark VJ. 1986. Selective atten-tion. Annu. Rev.Psychol. 37:43–75

Jonides J. 1981. Voluntary versus automaticcontrol over themind’s eye’s movement. InAttention and Performance,ed. JB Long,AD Baddeley, pp. 187–203. Hil lsdale,NJ:Erlbaum. 9th ed.

Jonides J, Yantis S. 1988. Uniqueness ofabrupt visual onset in capturing attention.Percept. Psychophys.43:346–54

JosephJS,Optican LM. 1996. Involuntary at-tentional shiftsdue to orientation differ-ences.Percept. Psychophys.58:651–65

Juola JF,Koshino H, WarnerCB. 1995. Trade-offs between attentional effects of spatialcuesand abrupt onsets.Percept. Psycho-phys.57:333–42

KahnemanD, Henik A. 1981. Perceptual or-ganization andattention. In Perceptual Or-ganization, ed. M Kubovy, JR Pomerantz,pp. 181–211. Hil lsdale,NJ:Erlbaum

Kahneman D, Treisman A. 1984.Changingviewsof attention andautomaticity. In Va-

VISUAL ATTENTION 295

rietiesof Attention, ed.R Parasuraman, DADavies,pp. 29–61. New York: Academic

KahnemanD, TreismanA, Gibbs BJ. 1992.The reviewing of object fil es: object-spe-cific integration of information. Cogn. Psy-chol. 24:175–219

Kinchla RA. 1992. Attention. Annu. Rev.Psy-chol. 43:711–43

Klein RM, Taylor TL. 1994. Categoriesof cog-nitive inhibitionwith reference toattention.See Dagenbach& Carr1994, pp. 113–50

Kolers PA, KatzmanMT. 1966. Naming se-quentially presented letters and words.Lang. Speech9:84–95

Koshino H, Warner CB, Juola JF.1992. Rela-tive effectiveness of central, peripheral,andabrupt-onsetcuesin visualsearch.Q.J. Exp. Psychol. 45A:609–31

Kramer AF, Coles MGH, Logan GD. 1996.Converging Operations in the Study of Vis-ual Selective Attention. Washington, DC:Am. Psychol. Assoc.

KramerAF, Jacobson A. 1991. Perceptual or-ganization and focused attention: the roleof objectsandproximity in visualprocess-ing. Percept.Psychophys.50:267–84

KröseJ,JuleszB. 1989. The control and speedof shifts of attention. Vis. Res. 23:1607–19

Kwak HW, DagenbachD, EgethH. 1991. Fur-ther evidence for a time-independent shiftof the focus of attention. Percept. Psycho-phys.49:473–80

Kwak HW, Egeth H. 1992. Consequencesofallocating attention to locations and toother attributes. Percept. Psychophys. 51:455–64

LaBerge D. 1995. Attentional Processing: TheBrain’s Art of Mindfulness. Cambridge,MA: HarvardUniv. 262 pp.

Maylor E. 1985. Facilitory andinhibitory com-ponents of orienting in visual space.SeePosner & Marin 1985, pp. 189–204

Maylor E, Hockey R. 1985. Inhibitory compo-nents ofexternally controlledcovert orient-ing in visualspace.J. Exp.Psychol.: Hum.Percept.Perform. 11:777–87

MertensJJ.1956. Influencesof knowledge oftarget location upon the probabil ity of ob-servation of peripherally observable testflashes.J. Opt. Soc. Am.46:1069–70

MooreCM, Egeth H, BerglanL, Luck S.1996.Are attentional dwell it ems inconsistentwith serial visual search?Psychon. Bull.Rev.In press

Müller HJ, Rabbitt PMA. 1989. Reflexive andvoluntaryorienting of visualattention: timecourseof activation andresistanceto inter-ruption. J. Exp. Psychol.: Hum. Percept.Perform. 15:315–30

Nakayama K, Mackeben M. 1989. Sustainedand transient components of focal visualattention. Vis. Res.29:1631–47

NeisserU. 1967. Cognitive Psychology. NewYork: Appleton-Century-Crofts.351 pp.

Nothdurft HC. 1992. Feature analysis and therole of simil arity in preattentive vision.Percept. Psychophys.52:355–75

Nothdurft HC. 1993. Saliency effects acrossdimensions in visual search.Vis. Res.33:839–44

Palmer J. 1994. Set-size effects in visualsearch: The effect of attention is inde-pendent of the stimulus for simple tasks.Vis. Res.34:1703–21

PalmerJ, McLean J. 1995. Imperfect, unlim-ited-capacity parallel searchyields largeset-size effects.Presented at Soc. Math.Psychol., 28th, Irvine,CA

PashlerH. 1988. Cross-dimensional interactionand texture segregation. Percept.Psycho-phys.43:307–18

Pashler H, ed.1996a.Attention. London: Univ.Coll . London Press

PashlerH. 1996b. ThePsychologyof Attention.Camgridge,MA: MIT Press

Posner MI. 1980. Orienting of attention. Q. J.Exp. Psychol. 32:3–25

Posner MI, Cohen Y. 1984. Components ofvisualorienting. In Attention and Perform-ance, ed. H Bouma, DG Bouwhuis, pp.531–55. Hil lsdale,NJ:Erlbaum. 10th ed.

Posner MI, Marin O, eds.1985. Attention andPerformance.Hil lsdale, NJ: Erlbaum. 11thed.

Posner MI, Rafal RD, Choate L, Vaughan J.1985. Inhibition of return: neural basisandfunction. Cogn. Neuropsychol. 2:211–28

Posner MI, Snyder CRR, Davidson BJ. 1980.Attention and the detection of signals. J.Exp. Psychol.: Gen.109:160–74

Pylyshyn ZW. 1989. The role of location in-dexesin spatial perception: a sketch of theFINST spatial-index model. Cognition 32:65–97

Pylyshyn ZW, Storm RW. 1988. Trackingmultiple independent targets: evidence fora parallel tracking mechanism. Spat. Vis.3:179–97

RafalRD, CalabresiPA, BrennanCW, ScioltoTK. 1989. Saccadepreparation inhibits re-orienting to recently attended locations. J.Exp. Psychol.: Hum.Percept.Perform. 15:673–85

Raymond JE, Shapiro KL, Arnell KM. 1992.Temporary suppression of visual process-ing in an RSVPtask: an attentional blink?J. Exp. Psychol.: Hum. Percept. Perform.18:849–60

Raymond JE, Shapiro KL, Arnell KM. 1995.Similarity determinesthe attentional blink.J. Exp. Psychol.: Hum. Percept. Perform.21:653–62

ReevesA, Sperling G. 1986. Attention gatingin short-term visual memory. Psychol. Rev.93:180–206

296 EGETH& YANTIS

Remington RW, Johnston JC, Yantis S. 1992.Involuntary attentional capture by abruptonsets.Percept.Psychophys.51:279–90

Remington RW,Pierce L.1984. Movingatten-tion: evidence for time-invariant shifts ofvisualselective attention. Percept.Psycho-phys.35:393–99

Rock I, GuttmanD. 1981. The effect of inat-tention on form perception. J. Exp. Psy-chol.: Hum.Percept.Perform. 7:275–85

SaarinenJ, JuleszB. 1991. Thespeedof atten-tional shifts in the visualfield. Proc. Natl.Acad. Sci. USA88:1812–14

SagiD, JuleszB. 1985. Fastnoninertial shiftsof attention. Spat. Vis.2:141–49

Seiffert AE, DiLollo V. 1996. Low-level mask-ing in the attentional blink. J. Exp. Psy-chol.: Hum.Percept.Perform. In press

Shiffr in RM, Gardner GT. 1972. Visual proc-essingcapacity and attentional control. J.Exp.Psychol. 93:72–82

Shulman GL, Remington RW, McLean JP.1979. Moving attention through visualspace. J. Exp. Psychol.: Hum. Percept.Perform. 5:522–26

Sperling G.1960. The information available inbr ief visual presentations. Psychol .Monogr. 74 (WholeNo. 498)

Sperling G, Weichselgartner E. 1995. Episodictheoryof the dynamicsof spatial attention.Psychol. Rev.102:503–32

Theeuwes J. 1990. Perceptual selectivity istask-dependent: evidence from selectivesearch.ActaPsychol. 74:81–99

Theeuwes J.1991a.Cross-dimensionalpercep-tual selectivity. Percept. Psychophys. 50:184–93

Theeuwes J. 1991b. Exogenous and endo-genous control of attention: the effect ofvisualonsetsandoffsets. Percept.Psycho-phys.49:83–90

TheeuwesJ. 1992. Perceptual selectivity forcolor and form. Percept. Psychophys. 51:599–606

Tipper SP, DriverJS, WeaverB. 1991. Object-centeredinhibitionof return of visualatten-tion. Q. J. Exp. Psychol. 43A:289–98

Tipper SP,Weaver B, JerreatLM, Burak AL.1994. Object-andenvironment-basedinhi-bition of return of visualattention. J. Exp.Psychol.: Hum. Percept. Perform. 20:478–99

Todd JT, Van Gelder P. 1979. Implications ofa transient-sustained dichotomy for themeasurement of human performance. J.Exp. Psychol.: Hum. Percept.Perform. 5:625–38

Townsend JT.1971. A note onthe identifi abil-ity of parallel andserialprocesses.Percept.Perform. 10:161–63

TownsendJT.1990. Serial vs.parallel process-ing: Sometimesthey look likeTweedledum

andTweedledeebut they can(andshould)bedistinguished.Psychol. Sci.1:46–54

Travers JR. 1973. The effectsof forcedserialprocessing on identifi cation of words andrandom letterstrings. Cogn. Psychol. 5:109–37

Treisman AM,GeladeG. 1980. A feature-inte-gration theory of attention. Cogn. Psychol.12:97–136

Treisman AM, Gormican S. 1988. Featureanalysis in early vision: evidence fromsearch asymmetries. Psychol. Rev. 95:15–48

Tsal Y. 1983. Movements of attention acrossthe visual field. J. Exp. Psychol.: Hum.Percept. Perform. 9:523–30

van der Heijden AHC. 1992. Selective Atten-tion in Vision. New York: Routledge,Chapman& Hall. 310 pp.

VeceraSP,FarahMJ. 1994. Doesvisual atten-tion selection objectsor locations?J. Exp.Psychol.: Gen.123:146–60

WardR, DuncanJ,Shapiro K. 1996. The slowtime-courseof visualattention. Cogn. Psy-chol. 10:79–109

Weichselgartner E, Sperling G. 1987. Dynam-ics of controlled visual attention. Science238:778–80

Wolfe JM. 1994. Guidedsearch2.0: a revisedmodel of visualsearch.Psychon. Bull. Rev.1:202–38

Wolfe JM, Cave KR, Franzel SL. 1989.Guidedsearch:analternative to the featureintegration model for visualsearch.J. Exp.Psychol.: Hum.Percept.Perform. 15:419–33

Yantis S. 1988. On analog movements of vis-ual attention. Percept. Psychophys. 43:203–6

Yantis S. 1992. Multi-elementvisualtracking:attention and perceptual organization.Cogn. Psychol. 24:295–340

Yantis S, Hil lstrom AP. 1994. Stimulus-drivenattentional capture: evidence from equilu-minant visual objects. J. Exp. Psychol.:Hum.Percept. Perform. 20:95–107

Yantis S,JonidesJ.1984. Abrupt visual onsetsandselective attention: evidence from vis-ualsearch.J. Exp. Psychol.: Hum.Percept.Perform. 10:601–21

Yantis S,JonidesJ.1990. Abrupt visual onsetsand selective attention: voluntary versusautomatic allocation. J. Exp. Psychol.:Hum.Percept. Perform. 16:121–34

Yantis S, Jonides J. 1996. Attentional captureby abrupt visualonsets:newperceptual ob-jects or visual masking? J. Exp. Psychol.:Hum.Percept. Perform. In press

Yantis S, Moore C. 1995. Spread of visual at-tention behind an occluding surface. Pre-sentedat Annu. Meet. Psychon. Soc., 36th,Los Angeles

VISUAL ATTENTION 297