Temporal Target Integration Underlies Performance at Lag 1 in the Attentional Blink

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1 Journl of Experimentl Psychology: Humn Perception nd Performnce 212, Vol. 38, No. 6, Americn Psychologicl Assocition /12/$12. DOI: 1.137/2761 Temporl Trget Integrtion Underlies Performnce t 1 in the Attentionl Blink Elkn G. Akyürek, Snder A. H. Eshuis, Mrk R. Nieuwenstein, nd Jeft D. Sij University of Groningen Bernhrd Hommel Leiden University Deniz Bşkent University Medicl Center Groningen When two trgets follow ech other directly in rpid seril visul presenttion (RSVP), they re often identified correctly ut reported in the wrong order. These order reversls re commonly explined in terms of the rte t which the two trgets re processed, the ide eing tht the second trget cn sometimes overtke the first in the rce towrd conscious wreness. The present study exmined whether some of these order reversls might lterntively e due to mechnism of temporl integrtion wherey trgets ppering closely in time my e merged into single representtion. To test this integrtion ccount, we used n ttentionl link tsk in which the two trgets could e comined perceptully in meningful wy such tht the conjunction of the two trget elements constituted possile trget stimulus itself. The results showed tht when trgets ppered t 1, oservers frequently reported seeing only single merged trget stimulus, nd these reports occurred up to pproximtely three times s often s (rel) order reversls. When the possiility to report the integrted percept ws removed, order reversls consequently tripled. These results suggest tht integrtion my ctully e the primry cuse of order reversls in dul-trget RSVP tsks. Keywords: ttentionl link, temporl integrtion, 1, order reversls Our visul world is highly dynmic, nd we re exposed to continuous strem of visul informtion whtever we re wtching nd wherever we re moving. This poses high demnds on the processes underlying visul perception, which need to segregte the continuous strem of informtion into discrete events nd to discriminte etween relevnt events nd those tht cn e neglected. These kinds of processes hve een extensively studied y mens of rpid seril visul presenttion (RSVP) tsks, in which oservers identify or detect visul trgets ppering within rpidly presented sequence of distrctors. This rticle ws pulished Online First Mrch 19, 212. Elkn G. Akyürek, Snder A. H. Eshuis, Mrk R. Nieuwenstein, nd Jeft D. Sij, Deprtment of Psychology, Experimentl Psychology, University of Groningen, Groningen, The Netherlnds; Deniz Bşkent, Deprtment of Otorhinolryngology, University Medicl Center Groningen, Groningen, The Netherlnds; Bernhrd Hommel, Institute for Psychologicl Reserch nd Leiden Institute for Brin nd Cognition, Leiden University, The Netherlnds. Supported in prt y Roslind Frnklin Fellowship from the University Medicl Center Groningen nd VIDI grnt ( ) from the Netherlnds Orgnistion for Scientific Reserch (NWO) to Deniz Bşkent. We thnk Dvid Jungclussen for efficiently collecting the dt for Experiment 5, nd Brd Wyle nd two nonymous reviewers for their helpful comments on previous drft. Correspondence concerning this rticle should e ddressed to Elkn G. Akyürek, Deprtment of Psychology, Experimentl Psychology, University of Groningen, Grote Kruisstrt 2/1, 9712 TS Groningen, The Netherlnds. E-mil: e.g.kyurek@rug.nl Reserch using this prdigm hs yielded severl phenomen tht provide importnt insight into the temporl dynmics nd the limittions of mentl opertions underlying conscious visul perception. A key phenomenon in this domin is the ttentionl link (AB) the finding tht oservers frequently miss the second of two trgets if the second trget is msked nd presented t stimulus onset synchrony (SOA) etween 15 nd 5 ms (Brodent & Brodent, 1987; Rymond, Shpiro, & Arnell, 1992). Countless studies hve een conducted to investigte this link of the mind nd severl models hve een suggested to explin it (for review, see Dux & Mrois, 29). The present study, however, ws devoted to condition under which the AB my fil to occur: The second trget is often not missed if it ppers within less thn 15 ms from the first, which is n spect of the AB phenomenon tht hs een referred to s spring. 1 It is most commonly oserved in the so-clled 1 condition, in which the two trgets follow ech other directly without intervening distrctors, t n SOA of out 1 ms (for review, see Visser, Bischof, & Di Lollo, 1999). 1 The common term for this phenomenon is 1 spring, where 1 is used to indicte tht in studies using RSVP spring minly occurs when the second trget (T2) ppers in the seril position immeditely fter the first trget (T1) in the RSVP sequence. However, recent work hs shown tht spring is tied to the moment in time t which T2 ppers, not to its seril position (Bowmn & Wyle, 27; Nieuwenstein, Potter, et l., 29; Nieuwenstein, Vn der Burg, et l., 29), indicting tht spring is not necessrily confined the 1 position. To void confusion with constrints regrding T2 s seril position, spring will e used here. 1448

2 TEMPORAL TARGET INTEGRATION AT LAG Spring itself is not unitry phenomenon, lthough it my perhps seem so t first glnce. Spring hs een defined more or less formlly s n increse of t lest 5% in identifiction ccurcy of the second trget (T2), reltive to linked lgs (Visser et l., 1999). However, this increse is often otined only if prticipnts re to report the identity of the two trgets, while less or no spring is oserved if the correct trget order lso needs to e reported (Hommel & Akyürek, 25). In other words, wht looks like spring my seem more like trding order informtion for identity informtion. The loss of order informtion is especilly severe in short trget stimulus-onset synchronies, nd the report of the first trget (T1) is eqully ffected y the resulting order confusions (i.e., it is reported s T2; Hommel & Akyürek, 25). Indeed, in roder sense, 1 performnce cn lso e decomposed into multiple types of responses, nd even when the identity of T2 is preserved, differences my exist in other spects. For instnce, T2 my e reported (incorrectly) s T1, nd T1 my then e reported s T2, or T1 my not e reported t ll. The im of the present study ws to investigte the origin of this order-for-identity trde-off y contrsting two explntions of order reversls. The first concerns set of explntions tht derive from the precedence ccount first proposed y Reeves nd Sperling (1986; Olivers, Hilkenmeier, & Schrlu, 211; Wyle, Bowmn, & Nieuwenstein, 29). These ccounts will e contrsted with the temporl integrtion ccount proposed y Hommel nd Akyürek (25; Akyürek & Hommel, 25; Akyürek, Riddell, Toffnin, & Hommel, 27; Akyürek, Toffnin, & Hommel, 28; see lso Bowmn & Wyle, 27; Treismn, 1996). In the following sections, we will descrie these ccounts in detil. Precedence-Bsed Accounts The precedence ccount ws developed y Reeves nd Sperling (1986) in n ttempt to model report order in tsk tht required reporting of severl successive items in n RSVP strem. More specificlly, the tsk involved two simultneous RSVP strems shown to the left nd to the right of centrl fixtion cross. On detecting prespecified trget stimulus in the left strem, prticipnts hd to shift their ttention to the right-hnd strem s quickly s possile to encode the first four items occurring in tht strem. The results showed ell-shped distriution of reported items tht peked t out ms fter the trget. Anlyses of the order in which the items from this distriution were reported showed tht the item occurring pproximtely ms fter the trget, tht is, the item reported most often, ws lso the item tht ws most often reported first y prticipnts, with erlier items eing reported lter. In other words, there ws reltionship etween identifiction ccurcy nd report order, such tht the item tht could e identified most esily lso tended to e the one tht ws reported first. In constructing model tht could ccommodte these results, Reeves nd Sperling (1986) proposed n ccount in which order of report ws determined y the strength of n item s representtion in visul short-term memory (VSTM), with the strength of representtion eing function of the mount of ttention n item receives. More specificlly, Reeves nd Sperling proposed tht following detection of the trget in the left-hnd strem, n ttentionl gte ws opened to llow items from the right-hnd strem to enter VSTM. This opening of n ttentionl gte ws proposed to involve loction-specific ttentionl enhncement effect tht reched its mximl efficcy out ms fter the onset of the trget in the left-hnd strem. The strength of n item s representtion in VSTM derived from this ttentionl enhncement function, the ide eing tht items tht ppered t the time of mximl enhncement lso chieved the gretest strength of representtion in VSTM. With these ssumptions, Reeves nd Sperling were le to provide n ccurte simultion of the results otined cross numer of vrints of the tsk. In recent yers, the notion tht report order could e explined in terms of precedence effect tht is driven y ttention lloction hs een incorported into models of the AB nd spring. For instnce, in the episodic simultneous type seril token (estst) model proposed y Wyle, Bowmn, nd Nieuwenstein (29), report order ws derived from comintion of Reeves nd Sperling s notion of ttention-driven precedence nd ssumptions out the time course of working memory consolidtion. More specificlly, the estst model ssumes tht consolidting representtion in working memory is time-consuming process tht will tke longer for stimuli tht re more complex, or represented more wekly. The model further ssumes tht the order of report reflects the order in which the items complete the consolidtion stge. At this point, n item s representtion is ound to token through the estlishment of inding links etween the token nd the item s type representtion. By ssuming tht report order is determined y the order in which items complete the consolidtion stge, the predictions of the estst model entil tht report order depends not only on the mount of ttention n item receives (more ttention results in stronger representtion trce tht cn e consolidted more rpidly) ut lso on fctors tht ffect the rte of consolidtion, such s the intrinsic strength of the item nd the extent of competition etween simultneously ctivted trget representtions. With these ssumptions, estst ws shown to e cple of providing ccurte simultions of identifiction ccurcy nd report order in vrious RSVP tsks, including tsks similr to the tsk used y Reeves nd Sperling (1986), nd the dul-trget prtil report tsks typiclly used in studies of the AB. Another vrint of the precedence ccount ws recently proposed y Olivers et l. (211) who suggested tht report order could e explined in terms of the lw of prior entry (Titchener, 198; for review see Spence & Prise, 21). In ccordnce with oth estst (Wyle et l., 29) nd the precedence ccount proposed y Reeves nd Sperling (1986), the prior entry ccount holds tht order of report is determined y the mount of ttention n item receives. The ccount differs from the estst model, however, in tht it scries no role to the time tht is needed to consolidte n item s representtion in working memory. Insted, the prior entry hypothesis follows Olivers nd Meeter s (28) oost nd ounce model of the AB in ssuming tht consolidtion in working memory does not involve time-consuming process, nd therefore hs no effects on report order. In other words, while estst cn e sid to ssume tht report order reflects the order in which items complete the consolidtion stge, the prior entry hypothesis cn e sid to ssume tht report order reflects the order in which items enter the consolidtion stge. Integrtion Accounts The lterntive to precedence-sed ccounts of report order cn e found in the integrtion ccount proposed y Hommel nd

3 145 AKYÜREK ET AL. Akyürek (25). This ccount stems from considertions tht relte to the difficulty of segregting continuous, rpid strem of visul informtion into discrete events (see lso Zcks & Swllow, 27). The presumly most relile criterion to prse informtion nd to integrte it into the sme cognitive episode is time: The closer in time two pieces of informtion pper, the more likely they re prt of the sme event. Accordingly, if T1 nd T2 pper very close in time, s in the 1 condition, it could e tht they re integrted into the sme episodic trce (Akyürek et l., 27, 28; Bowmn & Wyle, 27; Hommel & Akyürek, 25). This would help to retin T2-identity informtion ut eliminte order informtion, exctly s ws found t 1. Furthermore, the resultnt order errors were shown to vry s function of the visile persistence of the stimuli, s would e expected if they were relted to temporl integrtion (Hommel & Akyürek, 25, Figure 9). Visul temporl integrtion is rther universl perceptul process in which stimuli pper within intervls of 2 ms or less. Across the shortest of these ( 2 ms), the limited temporl resolution of the visul system likely plys role (which might e referred to s fusion rther thn true integrtion). At longer intervls, however, this resolution is not fctor, nd the perception of multiple stimuli is often mintined to some degree (i.e., it is seen s flicker), ut the resulting percept is nevertheless one of n integrted whole of the stimuli. Erly ccounts conceptulized the phenomenon s consequence of trveling perceptul moment (Allport, 1968), in which perceptul smple is uilt up cross running-verge intervl of fixed durtion (s opposed to discrete successive intervls, s previously proposed y Stroud, 1956). Experimentlly, some support for such n intervl hs come from so-clled missing element tsks (MET), nd conceptully similr letter-sed form-prt integrtion tsks. In MET tsks, regulr grid (e.g., 5 5) of dots or squres is presented cross two successive displys (e.g., 12 dots ech), with single empty position remining for the oserver to find. Thus, the missing element is only pprent from the comined percept, rther thn from either of the two individul displys (Di Lollo, 1977, 198; Hogen & Di Lollo, 1974). Similrly, performnce on the lettersed integrtion tsk depends on the ility to comine two visul noise displys, in order to revel letter shpes (Eriksen & Collins, 1967, 1968; Frisse, 1966). The principl finding from tsks like these is tht integrtion is inversely correlted with the totl durtion of the stimuli; shorter durtions led to incresed integrtion (nd incresing simultneity reports). The integrtion process opertes comprly cross meningless nd meningful stimuli such s dots nd letters, nd cts even when the tsk requires comining two distinct entities in time, such s string of letters nd delyed circle or r indicting one of them s the trget (Averch & Coriell, 1961). This is not to sy tht temporl integrtion is utomtic nd therey inflexile, such s would result from so-clled intrinsic persistence of visul stimuli. As proposed y Dixon nd Di Lollo (1994), the dt my e interpreted est in the frmework of temporl coding process, in which the perceptul system ttempts to mintin perceptul continuity (integrtion) ut lso needs to e le to detect rpid chnges (segregtion). Temporl coding is thought to operte on the correltion in time of successive stimuli, comprison tht could lso, in principle, e ised y the oserver, to ccommodte tsk demnds (i.e., to fvor or suppress integrtion in generl). In line with this ide, Visser nd Enns (21) provided some evidence tht temporl integrtion is modulted y the vilility of ttention, which ffords sustntil mesure of endogenous control. A recent electrophysiologicl study of the MET furthermore showed tht the N1, N2, nd P3 components of the event-relted potentil re implicted in temporl integrtion, ll of which re lso modulted y ttention (Akyürek, Schuö, & Hommel, 21). Although the involvement of the P3 my suggest tht working memory processes could lso ffect temporl integrtion, there is some evidence to suggest this is not the cse (Jing, Kumr, & Vickery, 25; see lso Brockmole, Wng, & Irwin, 22). Thus, modultions of the P3 in the MET my reflect differences in the consolidtion of integrted percepts rther thn chnges in ongoing integrtion, similr to P3 effects seen in the AB, which my e seen s consequence of erlier, ttentionl modultions, such s the N2pc (e.g., Dell Acqu, Sess, Jolicoeur, & Roitille, 26). How might these findings e relted to RSVP? One possiility s to how integrtion might occur in RSVP tsks cn e found in the STST model proposed y Bowmn nd Wyle (27), the predecessor to the estst model proposed y Wyle et l. (29). Like estst, the STST model ssumes tht report order is determined y comintion of how much ttention n item receives nd how long it tkes to consolidte the item s representtion in working memory. A crucil difference etween the two models lies in the fct tht, in STST, the consolidtion mechnism llows for two items to e merged in single episodic memory representtion, possiility ndoned in the estst model (see the next section for discussion of why this chnge ws implemented). In STST, integrtion occurs when there is sufficient degree of temporl overlp etween the ctivtion of the type representtions of the two trget representtions. In this cse, oth ctivted types cn e ound to single token insted of to two seprte tokens, resulting in loss of order informtion. Integrtion Versus Precedence: Evlution Bsed on Previous Reserch As lluded to ove in the discussion of the STST nd estst models, there hs een chnge of opinion out the possile role of integrtion in the order reversls tht occur in conditions of spring, with the originl STST model (Bowmn & Wyle, 27) proposing mechnism of integrtion to explin reversls while the more recently proposed estst model (Wyle et l., 29) ndoned this possiility. There re three min resons for this chnge of opinion. First, studies exmining order reversls in spring showed tht the rte of reversls is lower thn wht might e expected sed on strong version of the integrtion ccount. Notly, severl reserchers hve rgued tht the integrtion ccount predicts complete loss of order in the cse of spring, thus llowing them to rgue ginst the integrtion ccount ecuse report order is more frequently correct thn incorrect in the cse of spring. In our view, this does not present compelling rgument ginst integrtion ecuse it presumes tht integrtion occurs on every tril in which spring occurs, nd this need not e the cse. Insted, it seems plusile to ssume tht integrtion constitutes one of severl possile outcomes for trils in which spring occurs, with the outcome eing dependent on how close the rce etween consolidtion of T1 nd T2 is, nd who wins in cse the rce is not so close (cf. STST). Tht is, trils with very close finishes might

4 TEMPORAL TARGET INTEGRATION AT LAG yield order reversls due to integrtion, while trils with less close finishes my yield either reversl due to prior entry or correctly ordered report in cse T1 wins the rce. Suffice it to sy, the finding tht the ccurcy of order of report does not rech chnce levels does not logiclly preclude the possiility tht integrtion plys role in the occurrence of order reversls. The second rgument ginst the integrtion ccount derives from findings tht show the occurrence of order reversls vries s function of mnipultions tht influence how much ttention is llocted to T1 nd T2. For instnce, it hs een shown tht incresing ttention for T1 y mens of precuing mnipultion leds to decrese in reversls while precuing T2 leds to n increse in reversls (Hilkenmeier, Olivers, & Schrlu, 212; Hilkenmeier, Schrlu, Wei, & Olivers, 212; Olivers et l., 211). Although we gree these findings demonstrte tht order reversls depend on the mount of ttention llocted to the two trgets, we disgree tht they provide solid rgument ginst the integrtion ccount. To wit, if we ssume tht the occurrence of integrtion depends on whether there is sufficient degree of temporl overlp etween the ctivtion of the type representtions of the two trgets (cf. STST), it follows tht ny mnipultion tht increses this temporl overlp will increse the occurrence of integrtion nd resulting order reversls while the converse would e true for mnipultions tht decrese temporl overlp. According to this view, precuing T1 reduces temporl overlp y giving T1 more of hed strt, while precuing T2 increses the proility tht there will e sufficient temporl overlp for integrtion to occur ecuse precuing T2 will increse the rte t which ctivtion of the T2 type will ccumulte. Finlly, the third reson for ndoning the STST integrtion in the estst model ws the oservtion tht when longer successive trget sequences re considered, order report (t lest in whole report) follows n U-shped function; order is lost most frequently for T2 nd T3, in series of four (Wyle et l., 29). If one ssumes tht integrtion strts t T1, nd then ends t some point in time (e.g., 2 ms) fterwrd, more imodl pttern might e expected, in which T1 nd T2 re exchnged most frequently, s well s T3 nd T4, in two successive integrted episodes. Whether such defult integrtion ehvior should e expected in these circumstnces is not entirely cler, however. Adptive control of integrtion does occur (Akyürek et l., 28), nd if prticipnts expect to see series of successive trgets, they my chnge their ehvior ccordingly. Oservers might decide to focus on the most recently viewed trgets, for instnce, ecuse they re most esily committed to nd mintined in memory, which could led to lte strt of integrtion. Similrly, prticipnts my extend the intervl cross which they integrte, so tht the imodl pttern might e oscured y trils on which events do not contin two trgets ech, ut perhps three nd one. It my furthermore e rgued tht with extended trget sequences such s these, memory-relted fctors will strt to ffect recll, nd indeed the order reports seen here follow typicl seril position pttern. Without these primcy nd recency effects, the underlying pttern of reversls is more diffuse, ut nonetheless still shows tht trgets re most often confused with their temporl neighors, which is comptile with n integrtion ccount. Tken together, lthough the considertions mentioned ove cst dout on the possiility tht integrtion plys role in order reversls, they do not logiclly preclude tht possiility. The ville literture furthermore provides some indictions tht uttress the plusiility of the integrtion ccount. First, s noted y some reserchers, the phenomenology of the 1 condition is tht the two trgets seem to overlp, s if superimposed (e.g., Ttgen, Juvin, Schipper, Borst, & Mrtens, 29). However, this oservtion hs not yet een verified in direct empiricl test, ecuse ll AB studies conducted so fr hve not llowed for reports of n integrted percept due to the fct tht these studies required seprte responses for the T1 nd T2 identities. Second, visul temporl integrtion hs een shown to occur t the temporl intervls used in RSVP in rnge of different tsks, such s form prt integrtion nd MET, which were mentioned previously. There seems to e little reson to ssume integrtion should not occur in RSVP, when its temporl dynmics re comprle. Indeed, s SOA increses in the MET, the ility to integrte its two displys decreses nd the proility of perceiving two distinct stimuli increses, which mirrors the time course of order reversls cross lgs seen in studies of the AB nd spring. The Present Study As is cler from the discussion ove, the findings used to rgue ginst the integrtion ccount nd in fvor of the precedence ccount do not rule out the possiility tht integrtion plys role in order reversls. The gol of the present study ws to conduct more criticl test of these ccounts y exmining prediction tht is unique to the integrtion ccount. This prediction reltes to the phenomenologicl spects of the 1 condition, where the integrtion ccount predicts tht prticipnts will often perceive the two trgets s eing superimposed, tht is, perceptully integrted. If so, it follows tht prticipnts should hve difficulty distinguishing etween the presenttion of two successive trgets nd the presenttion of single trget consisting of comintion of the two trget stimuli. To test this prediction, we modified the stndrd AB tsk in two wys. First, we used trgets tht could e comined in meningful wy such tht the compound of the two trgets constituted possile trget itself. For instnce, in one of the experiments, the trgets included the symols / nd, s well s the conjunction of these symols X. The primry issue of interest ws wht prticipnts would report in 1 tril in which the former two trgets were used s T1 nd T2: Would prticipnts report the two trgets seprtely, or would they report the integrted percept of the two trgets? To ensure tht prticipnts would consider reporting single trget s vile response, we lso included smll portion of trils with only single trget. Tken together, these design fetures enle dissocition etween correctly ordered reports of the two trget identities, reports of the illusory integrted percept, nd reports of the two trgets in the incorrect order (i.e., n order reversl not sed on integrtion). Experiment 1 In Experiment 1, the trgets consisted of one or more corners of squre (see Figure 1). Becuse these corners were never repeted etween the trgets, nd ecuse the trgets could consist of vrile numer of corners, this design llowed for the perceptul summtion of the two trgets. In other words, if prticipnts relly integrted the two trgets, this integrted percept constituted vlid nd reportle trget identity itself. To ensure tht reporting single trget lso constituted vile response, we lso included trils on which only

5 1452 AKYÜREK ET AL. Figure 1. Illustrtion of the procedure nd design of Experiment 1. Trgets consisting of one or more corners of squre ppered mong letter distrctors in rpid seril visul presenttion of 8 ms per frme (7-ms stimulus nd 1-ms lnk). Dotted frmes represent vrying numer of distrctors. A 1 tril is shown, for which the ctul identities of the trgets re shown s well s the jointly integrted percept (Int.) tht might e reported insted (see the first column of inset tle). The tle includes two more exmples of possile T1 T2 configurtions to illustrte the design. Stimuli re not drwn to scle. Resp. response; T1 first trget; T2 second trget. single trget ws present nd prticipnts were informed of this possiility efore the experiment egn. This design permitted us to seprte correct responses, integrtions, order reversls, nd other kinds of errors. Of note, the precedence ccount nd the integrtion ccount mke different predictions for the report of integrtions: While the integrtion ccount would predict sustntil numer of such reports, the precedence ccount would hve no ovious mechnism to explin their existence. Method Prticipnts. Twenty psychology students (ll women) t the University of Groningen prticipted in the experiment for course credit. Informed consent ws otined in writing, nd the study ws conducted in ccordnce with the Declrtion of Helsinki. Prticipnts were unwre of the purpose of the experiment, nd they reported norml or corrected-to-norml vision. Dt from two prticipnts were removed from the nlyses ecuse T1 performnce ws elow 3% correct overll, indicting tht the tsk ws not properly executed. Men ge ws 19 yers (rnge: 18 22). Apprtus nd stimuli. Prticipnts were individully seted in dimly lit testing chmer t distnce of out 5 cm from the screen. The 22-in. CRT screen ws driven y stndrd personl computer running the Microsoft Windows XP operting system, nd refreshed t 1 Hz with resolution of pixels in 16-it color. The experiment ws progrmmed in E-Prime Professionl, Version 2. (Psychology Softwre Tools). Responses were logged on stndrd keyord. A light gry ckground (RGB 192, 192, 192) ws mintined during the experiment, nd ll stimuli were presented in lck (RGB,, ). Distrctor stimuli consisted of upper-cse letters, drwn in 52-point, oldfce Courier New font. They were drwn rndomly without replcement from the full lphet for ech tril. The fixtion cross consisted of smll plus sign ( ) nd ws drwn in the sme font. As shown in Figure 1, the trgets consisted of one or more corners of squre tht ws pixels in size. The horizontl nd verticl sides of the corners were ech 23 pixels long nd 7 pixels wide, so tht the corners were seprted y gp of 8 pixels. Overll, the re within which the trget stimuli ppered ws comprle to tht of the distrctors. The trgets were chosen in such wy tht their fetures did not overlp with ech other (i.e., the sme corner ws never shown for oth T1 nd T2). This ws lso prerequisite to e le to distinguish correct, incorrect, nd integrtion responses (see Figure 1). With this constrint in plce, ll resulting possile comintions of T1 nd T2 (including different numers of corners, e.g., the upper left corner for T1 nd oth lower corners for T2) were used eqully often nd presented in rndom sequence. Procedure. The experiment comprised of totl of 688 self-pced experimentl trils with n optionl puse hlf-wy, nd strted with short lock of prctice trils tht were excluded from nlysis. The experimentl session lsted for out 7 min. At 1 ms fter the initition of ech tril, the fixtion cross ws displyed for 2 ms. Then the RSVP sequence of 19 stimuli commenced, ll of which were on screen for 7 ms nd followed for 1 ms y lnk screen (8-ms SOA). In most trils, two of these stimuli were trgets (i.e., T1 nd T2), while the others were distrctors. T1 ppered s either the fifth or the seventh item in the strem nd T2 followed T1 with either, 2, or 7 distrctors in etween, referred to s 1, 3, or 8 (31.2% of trils ech). There ws no T2 on smll portion of trils (just elow 6.4%), nd distrctor took its plce insted. These trils were lso excluded from nlysis. Prticipnts were sked to identify the trgets, nd they were told tht there would usully e two in the strem. At the end of the strem, 1-ms lnk dely ensued efore prticipnts were successively prompted to first enter the identity of T1 nd then tht of T2. Ech response ws given on leled section of the numer keypd. The upper left corner corresponded to the 4 key, the upper right corner to the 5 key, the lower left corner to the 1 key, nd the lower right corner to the 2 key. Thus, when trget consisted of multiple corners, it required multiple key presses to identify it. When sujects hd completed entering the identity of trget, they hd to finish y pressing the Enter key. Although prticipnts were encourged to guess, they were not forced to do so ecuse they were informed they could press Enter without entering response. Design. Repeted-mesures nlyses of vrince (ANOVA) with the single vrile of T1 T2 (1, 3, nd 8) were conducted to ssess tsk performnce. Anlyses were performed on men T1 nd T2 ccurcy, where T2 ccurcy ws computed s the percentge of correctly identified stimuli given tht T1 ws correct (T2 T1). For the min nlyses of oth mesures, response ws only considered correct if oth the identity nd the temporl position (order) were reported correctly. The frequency

6 TEMPORAL TARGET INTEGRATION AT LAG of integrtions (i.e., reports of the integrted percept) nd the frequency of order reversl errors (i.e., trils in which T1 ws reported s T2 nd vice vers) were lso nlyzed. Note tht response ws only ctegorized s n integrted percept if its fetures mtched the comintion of T1 nd T2 exctly. Prtil migrtions of individul fetures were not considered, even though they might well hve een indictive for integrtive processes. The primry reson for excluding prtil migrtions ws tht they ecome incresingly prone to chnce reports nd re therefore difficult to interpret. Furthermore, only responses in which the integrted percept ws the sole response (i.e., no second response ws entered t ll) were counted. This reduced the numer of trils during which integrtion ws considered to hve occurred, for instnce, excluding trils during which the integrted percept ws reported twice, ut the remining trils were those during which the oserver most clerly indicted hving seen only one (integrted) trget. For the nlyses nd line grphs of integrtions nd order reversls, their frequency ws computed reltive to the totl numer of trils during which oth trget identities were reported (cf. Chun & Potter, 1995). To lso llow for n ccurte ssessment of the entire response pttern t ech lg, the solute frequencies (i.e., reltive to the totl numer of trils) re shown in seprte cumultive r grphs (cf. Hommel & Akyürek, 25). In ll ANOVAs, degrees of freedom were djusted using the Greenhouse Geisser epsilon correction in cse there ws significnt heterogeneity of vrince etween conditions. Results nd Discussion Accurcy on T1 ws strongly ffected y lg, F(1.4, 23.6) , men squre error [MSE].8, p.1. Performnce verged 37% t 1, compred to 76% t 3 nd 81.1% t 8. The reltively low performnce t 1 ws prtilly due to the incresed frequency of integrtions nd order errors, which re nlyzed elow. If integrtions nd order reversls were counted s correct reports of T1 identity, nd if trils during which only T1 ws reported s T2 were treted likewise (i.e., ignoring report order nd using relxed report criterion, s in Hommel & Akyürek, 25, nd the lrge mjority of the AB studies), performnce t 1 rose to 67.8%. The incresed frequency of cses in which T1 identity ws lost ltogether nd superseded y the report of T2 identity s the first trget; (i.e., T2 identity-only reports) suggests some form of competition etween trgets t 1, which likely included fctors such s the strength of ckwrd (metcontrst) msking y T2 (see, e.g., Enns & Di Lollo, 2; McLughlin, Shore, & Klein, 21). Furthermore, the sptil discrepncy etween trget elements might hve plyed role. Performnce on T1 s function of lg is shown in the left pnel of Figure 2. T2 T1 performnce ws ffected y lg, F(1.5, 25.7) 29.61, MSE.2, p.1. Identifiction ccurcy verged 56.8% t 1, 75.9% t 3, nd 88.1% t 8. As expected, the AB ws evident during the short lgs. At 1, T2 performnce ws reduced y integrtions nd order reversls, just s T1 performnce ws. When report order ws ignored, performnce t 1 rose to 76.2%. The right pnel of Figure 2 shows verge performnce on T2, given tht T1 ws correct, plotted cross lg. In mny studies, the incresed contriution of order reversls (nd, hypotheticlly, integrtions) t 1 hs correlted with T1 ccurcy (%) spring, tht is, performnce t 1 hs een elevted over linked lgs, ut this ws not the cse here. Our results showed steep decrese from 8 nd 3 to 1 in the numer of trils on which oth trgets were identified correctly, thus lowering overll performnce t 1. An dditionl ANOVA on the frequency of this type of tril confirmed this pttern, F(2, 34) 11.52, MSE.12, p.1. Trils in which oth trgets were identified correctly verged 23.4% t 1, compred to 6.1% t 3 nd 72.6% t 8. Like T1 performnce, the drop in T2 T1 performnce t 1 might e ttriuted to the nture of the trget stimuli, which were presented centrlly ut nonetheless t different sptil loctions. Loction switches re lso known to prevent, or t lest to severely impir, the occurrence of spring, presumly due to the time required to shift ttention from one loction to the next (Breitmeyer, Ehrenstein, Pritchrd, Hiscock, & Crisn, 1999; Visser et l., 1999; for evidence tht this my require sptilly nrrow focus of ttention, see Jefferies & Di Lollo, 29; Kwhr & Ymd, 26; Lunu & Olivers, 21; Shih, 2). The trget stimuli were lso somewht complimentry msks of ech other something tht my hve produced metcontrst msking, which is known to ffect T2 performnce when the trgets re presented in direct succession (Seiffert & Di Lollo, 1997). However, no model would predict tht the presence of incresed competition etween trgets should led to incresed integrtion t 1. The primry reserch question of the present study ws whether integrtion occurred t 1. An nlysis of the frequency of reports of integrted percepts showed significnt effect of lg, F(1, 17.2) 29.86, MSE.43, p.1. 2 Integrtion ws 2 For completeness, the sme nlyses of integrtion frequency over lg were lso conducted on solute percentges. The pttern of results did not chnge meningfully s result nd integrtion continued to e most frequent t 1. Experiment 1, F(1, 17.1) 19.95, MSE.1, p.1; Experiment 2, F(1, 25.4) 37.6, MSE.15, p.1; Experiment 3A, F(1.2, 17.3) 22.43, MSE.2, p.1; Experiment 3B, F(1, 14.1) 16.58, MSE.12, p.1; Experiment 4, F(1, 23) 14.71, MSE.5, p.1. T2 T1 ccurcy (%) Figure 2. Experiment 1 tsk performnce on T1 in percent correct, plotted over T1 T2 (1st, 3rd, or 8th stimulus fter T1) (). T2 performnce on Experiment 1, given tht T1 ws identified correctly (T2 T1) in percent correct s function of lg (). Error rs represent 1 SEM. Dshed lines represent identifiction ccurcy when report order is ignored (using relxed ccurcy criterion). T1 first trget; T2 second trget.

7 1454 AKYÜREK ET AL. 5 1 Reltive frequency (%) Integrtion Reversl Frequency (%) Both correct Order reversl Integrtion T2 identity T1 identity Figure 3. Experiment 1 frequency of reporting of integrted percepts nd tht of order reversls s function of lg, s percentge of the totl numer of responses in which oth trget identities were reported correctly (). Experiment 1 distriution of responses for ech lg, s percentge of the totl numer of responses (). T1 first trget; T2 second trget. frequent t 1, occurring in 34.2% of identity-correct trils, compred to 2.2% t 3 nd.6% t 8. The frequency of order reversls ws lso ffected y lg, F(1.4, 24.6) 34.72, MSE.2, p.1. However, t 1, reversl errors were clerly less frequent thn integrtions, verging 12.8% t 1 (vs. 5% t 3 nd 1.5% t 8). The left pnel of Figure 3 shows the reltive frequency of integrtions nd order reversls s function of lg. The right pnel of Figure 3 shows the distriution of reports t ech lg (in solute numers, i.e., reltive to the totl numer of trils), trils during which oth trgets were correctly identified, integrtions, order reversls, nd trils for which the identity of only one trget ws reported (ut t the wrong temporl position). Trils for which nothing ws correct nd trils for which only T1 or T2 ws identified correctly (t the correct temporl position) re not shown to increse clrity. This distriution of responses gives comprehensive overview of performnce t 1, nd illustrtes the need to go eyond the notion of sence or presence of spring s n ritrry rise of performnce compred to other lgs. The results of Experiment 1 thus provided cler evidence: Integrtion plyed mjor role t 1, occurring in considerle numer of trils. One cvet should e noted, however: The trgets in the present tsk together formed recognizle shpe outline, which my e suspected s leding to effects resulting from Gestlt grouping. The possiility thus exists tht the Gestlt spect of the present tsk ws solely responsile for integrtion. If so, integrtion might not occur in typicl AB tsks tht do not feture Gestlt perception. Experiment 2 ws conducted to ddress this concern. Experiment 2 As mentioned, we conducted Experiment 2 to investigte the possile contriution of the Gestlt principle of visul completion to the occurrence of integrtion. In Experiment 1, visul completion my hve plyed role ecuse the stimuli used s trgets my hve formed recognizle squre shpe in cse they were integrted. In Experiment 2, the potentil contriution of visul completion ws eliminted y replcing the squre corners with digits tht would not form fmilir visul shpe in cse they were integrted. Method Prticipnts. Twenty-eight new prticipnts (21 women, 7 men) were recruited using the sme procedures nd criteri s used in Experiment 1. Dt from two women were removed from the nlyses, using the sme exclusion criterion s used in Experiment 1. Men ge ws 19.7 yers (rnge: 18 24). Apprtus nd stimuli. The experimentl setup nd stimuli were identicl to those of Experiment 1, with the exception tht the trgets now consisted of the digits 4, 5, 2, nd 1, drwn in 36-point old Courier font. Compred to the corners used for T1 nd T2 in Experiment 1, these digits in Experiment 2 were displced slightly towrd the center of the screen (y nture), ut still ppered in their respective loctions, similr to the corners in Experiment 1. They mpped directly to their response keys (cf. Experiment 1). Procedure nd design. The experimentl procedure nd nlyses were identicl to those of Experiment 1. Results nd Discussion T1 ccurcy ws ffected y lg, F(1.4, 34.6) , MSE.8, p.1. Performnce ws 41.1% t 1, nd improved to 82% t 3 nd to 88.1% t 8. If T1 performnce ws ssessed using the relxed criterion (ignoring order), performnce t 1 incresed to 74.2%. Performnce on T1 s function of lg is shown in the left pnel of Figure 4. Predictly, T2 T1 performnce ws lso ffected y lg, F(1.3, 32.6) 35.75, MSE.19, p.1. Identifiction verged 66.2% correct t 1, 84.4% t 3, nd 91.9% t 8. Performnce t 1 verged 81.5% when report order ws

8 TEMPORAL TARGET INTEGRATION AT LAG T1 ccurcy (%) ignored. As in Experiment 1, the frequency of trils during which oth trgets were reported correctly ws lowest t 1 (29% vs. 7.8% t 3 nd 81.6% t 8), F(2, 5) , MSE.9, p.1. The right pnel of Figure 4 shows T2 T1 performnce plotted over lg. As in Experiment 1, integrtion occurred more frequently t 1 thn t lter lgs, F(1, 25.6) 43.94, MSE.45, p.1. At 1, the report frequency of the integrted percept verged 37.1%, compred to 4.1% t 3 nd 1.6% t 8. The frequency of order reversls lso showed significnt effect of lg, F(1.2, 29.5) 51.62, MSE.3, p.1, with 11.5% of the responses eing reversls t 1, compred to 2.2% t 3 nd.4% t 8. The left pnel of Figure 5 shows the reltive frequency of integrtions nd order reversls s function of lg, nd the right pnel of Figure 5 shows the distriution of prtil reports t ech lg. Tken together, the results of Experiment 2 were cler-cut, nd they represented virtully perfect repliction of Experiment 1: T2 T1 ccurcy (%) Figure 4. Experiment 2 tsk performnce on T1 in percent correct, plotted over T1 T2 (). T2 performnce on Experiment 2, given tht T1 ws identified correctly (T2 T1) in percent correct s function of lg (). T1 first trget; T2 second trget. Integrtion ws s frequent t 1 s it ws in the previous experiment, indicting no cler influence of Gestlt grouping on tsk performnce in Experiment 1. If nything, the tsk used in Experiment 2 seemed slightly esier, lthough this might lso hve reflected group differences (see, e.g., Mrtens, Munneke, Smid, & Johnson, 26). Experiment 3A The experiments reported so fr included sptil component. In Experiment 1, the sptil lyout of the corners ws closely tied to the identifiction tsk for prticipnts. The sptil discrepncy etween the trgets might hve modulted performnce t 1, or it might hve creted unusul trget msking conditions. Experiment 2 chnged the visul ppernce of the stimuli, therey removing the perceptul Gestlt, ut the tsk ws still rther sptil in nture ecuse the trgets still occupied distinct loctions in the disply. Moreover, lthough prticipnts were sked to identify the digits, the digits did not vry in position, so tht loclizing the stimuli my hve een sufficient to ccomplish the tsk. The possiility thus exists tht temporl integrtion requires such sptil component to occur, even though the comined frequency of order reversls nd integrtions oserved in Experiments 1 nd 2 seemed lrgely consistent with the frequency of order reversls seen in previous reports (see lso the Generl Discussion section elow). To exmine whether the occurrence of integrtion generlizes to conditions more typicl of those used in other AB studies, Experiment 3 used n RSVP prdigm in which ll stimuli ppered in the sme sptil loction. Method Prticipnts. Sixteen new prticipnts (13 women, 3 men) were recruited using the sme procedures nd criteri s used in Experiment 1. Men ge ws 2.6 yers (rnge: 18 24). Apprtus nd stimuli. The experimentl setup nd stimuli remined mostly identicl to those of Experiment 1. The trgets 5 1 Reltive frequency (%) Integrtion Reversl Frequency (%) Both correct Order reversl Integrtion T2 identity T1 identity Figure 5. Experiment 2 frequency of integrtions nd order reversls s function of lg, s percentge of the totl numer of identity-correct responses. Experiment 2 solute distriution of responses for ech lg (). T1 first trget; T2 second trget.

9 1456 AKYÜREK ET AL. now consisted of ll possile comintions of the upper-cse letter O nd the forwrd (/) nd ckwrd slsh ( ) symols, s shown in Figure 6A. These symols were presented in lck, in the sme font nd size s the distrctors. To void undue confusion with the distrctors, the letters O nd X were removed from the distrctor set. Prticipnts could identify the trget symols y mens of leled keys on the numer keypd (keys 2, 4 9 underneth the lels). Procedure nd design. Becuse of the new trget items, nd the corresponding chnge in the numer of response lterntives, the numer of trils now totled 68 (31.6% for ech lg). The percentge of trils during which no T2 ws shown lso chnged slightly to 5.3% (previously, 6.4%). The design nd nlysis were otherwise identicl to those of Experiment 1. Results nd Discussion T1 performnce ws ffected y lg, F(1.4, 2.4) 27.22, MSE.6, p.1. Performnce ws 41.9% t 1, 54.6% t 3, nd 57.7% t 8. When order ws ignored, T1 performnce incresed most strongly t 1, to 64.7%, lthough performnce t s 3 nd 8 lso incresed (to 66.1% nd 67.8%, respectively). Performnce on T1 s function of lg is shown in the left pnel of Figure 7. T2 T1 performnce ws lso ffected y lg, F(1.4, 21.3) 21.54, MSE.15, p.1. Performnce ws 22.5% t 1, 22.6% t 3, nd highest t 8, with 43.4%. Of note, when order ws ignored, performnce t 1 incresed gretly to 48.6%. At s 3 nd 8, the increses were more modest (34.7% nd 52.2%), respectively). 1 spring ws thus clerly otined with this prdigm. T2 T1 performnce s function of lg is shown in the right pnel of Figure 7. Integrtion frequency showed the sme pttern s tht oserved in the erlier experiments. There ws significnt effect of lg, F(1.4, 2.9) 33.25, MSE.2, p.1, with integrtion eing more frequent t 1 (43%) thn t 3 (22.7%) or 8 (9%). It seemed tht this tsk elicited higher seline of integrtion responses thn oserved previously. However, this cn e ttriuted to the incresed difficulty of the tsk, which resulted in fewer trils in which the identities of oth trgets were correctly reported. As cn e seen y compring the reltive frequencies with the solute reports (see the right pnel of Figure 8), the reltively high level of the former could e ttriuted to the somewht rtificil infltion cused y tsk difficulty. The reson for this increse in difficulty my hve een the removl of the sptil spect of the tsk or the incresed power of the distrctors to /\O /\\O/O \ / O T1 ccurcy (%) induce erroneous responses. Although the letters most likely to e confused were removed from the tsk (X nd O), others remined tht might lso hve hd some effect (e.g., Q or K). In ny cse, the most importnt result ws tht the numer of integrtions t 1 remined sustntil, nd ws clerly elevted ove the seline level. The numer of order reversls ws lso ffected y lg, F(2, 3) 1.6 MSE.4, p.1. Order reversls verged 18.5% t 1, 1.8% t 3, nd 8% t 8, ll of which seemed to e slightly inflted y tsk difficulty s well. Integrtion nd reversl frequency s function of lg is shown in the left pnel of Figure 8. There were lso chnges in the distriution of errors other thn integrtions nd reversls. As cn e seen from the right pnel of Figure 8, there were mny trils during which only the identity of T2 ws retrieved (i.e., reported s T1, nd the identity of T1 ws not reported s T2), cross ll lgs. The sme ws true, to lesser degree, for T1 identity-only reports. These errors might lso e explined y the incresed difficulty of the tsk. In this experiment, the trgets were more similr to the distrctors, which likely constituted n dditionl source of difficulty. Of note, the frequency of these errors did not vry meningfully with lg, F 2.1 for T1 identity, F 2.9 for T2 identity. Another spect of interest tht emerges from the right pnel of Figure 8 is the more modest increse in trils for which oth trgets were reported correctly from 1 to 3 (from 9.7% to 12.7%). Experiment 3B Experiment 3B ws conducted to more directly investigte the possile effects of tsk difficulty on the different performnce mesures. Becuse Experiment 3A seemed to e more difficult overll thn Experiments 1 nd 2, Experiment 3B ws designed to mke the tsk esier. To this end, the trgets received unique color. T2 T1 ccurcy (%) Figure 7. Experiment 3A tsk performnce on T1 in percent correct, plotted over lg (). Experiment 3A tsk performnce on T2 T1 (). T1 first trget; T2 second trget. Method Figure 6. Trget stimuli used in Experiment 3 (). In the experiment, the stimuli were presented without repetition s T1 nd T2. In Experiment 3A, they ppered in lck; in Experiment 3B, in lue. Trget stimuli used in Experiment 4 (). Prticipnts. Fifteen new prticipnts (1 women, 5 men) were recruited using the sme procedures nd criteri s used in Experiment 1. Men ge ws 19.2 yers (rnge: 18 22).

10 TEMPORAL TARGET INTEGRATION AT LAG Reltive frequency (%) Integrtion Reversl Frequency (%) Both correct Order reversl Integrtion T2 identity T1 identity Figure 8. Experiment 3A reltive integrtion nd reversl frequency in percent s function of lg (). Experiment 3A distriution of responses y lg (). T1 first trget; T2 second trget. Apprtus nd stimuli. The experiment ws repliction of Experiment 3A, except tht the trget stimuli now ppered in lue (RGB,, 255). The distrctors remined in lck. Procedure nd design. The procedure nd design were identicl to those of Experiment 3A. Results nd Discussion T1 performnce ws strongly ffected y lg, F(1.2, 16.3) 98.87, MSE.14, p.1. Performnce ws low t 1 (45.3%), ut improved t 3 (82.6%) nd remined high t 8 (86.8%). When report order ws ignored, performnce ws 75% t 1, 89.5% t 3, nd 91.6% t 8, gin clerly showing the most sustntil increse t the shortest lg. The left pnel of Figure 9 shows T1 performnce s function of lg. T2 T1 performnce ws lso significntly ffected y lg, F(2, 28) 27.31, MSE.26, p.1. Performnce t 1 ws 37.4%, compred to 57.3% t 3 nd 8.9% t 8. As ws consistently the cse efore, ignoring report order improved performnce most t 1 (63.3%, 65.3%, nd 85%, t s 1, 3, nd T1 ccurcy (%) T2 T1 ccurcy (%) Figure 9. Experiment 3B tsk performnce on T1 in percent correct, plotted over lg (). Experiment 3B tsk performnce on T2 T1 (). T1 first trget; T2 second trget. 8). The right pnel of Figure 9 shows T2 T1 performnce s function of lg. Crucilly, integrtion frequency ws once gin dependent on lg, F(1, 14.3) 25.98, MSE.45, p.1. Reports of the integrted percept were frequent t 1, verging 36.3%, ut not t 3 (3.2%) or 8 (.5%). As efore, order reversls followed the sme pttern, F(1.3, 17.8) 44.66, MSE.3, p.1, though t lower rtes overll. Order reversls verged 17.3% t 1, 5.5% t 3, nd 2.1% t 8. The left pnel of Figure 1 shows integrtions nd order reversls s function of lg. As shown in the right pnel of Figure 1, there ws one slient chnge, s compred to Experiment 3A: The numer of trils in which oth trgets were identified correctly ws reltively high t 3 (48.8%). Becuse the AB mgnitude ws comprle etween Experiments 3A nd 3B (computed s T2 T1 performnce t 8 minus 3, link mgnitude ws 2.8% in Experiment 3A nd 23.6% in Experiment 3B), this chnge must e ttriuted to other fctors tht ffected the overll level of performnce, which did indeed differ sustntilly etween these experiments. It seems tht the lue color of the trgets set them prt from the distrctors, which mde them esier to discern from the distrctors (ut not necessrily from ech other). Thus, Experiment 3B replicted the primry result of the other experiments: Integrtion took plce frequently t 1. The induced chnge in the overll level of performnce did not seem to cuse meningful chnges in the frequency of integrtions or order reversls. Experiment 4 Experiment 4 ws conducted to further support the generliztion of the present results to clssic AB studies tht use RSVP prdigms in which trgets typiclly consist of letters or digits. First, with such trgets, there is more visul overlp etween trgets, nd one might suspect tht incresing overlp could decrese integrtion frequency: Overlpping prts do not netly complement ech other, which my e cue for the perceptul