Visual selection mediated by location: Selecting successive visual objects

Transcription

1 Perceptin & Psychphysics 199,7 (4), Visual selectin mediated by lcatin: Selecting successive visual bjects KYLER. CAVE Vanderbilt University, Nashville, Tennessee and HALPASHLER University fcalifrnia, San Dieg, La Jlla, Califrnia In these experiments, each stimulus cnsists f a series f frames, each cntaining a target digit f ne clr and a distractr digit f anther clr. The task is t name the highest digit f the target clr. Subjects make fewer errrs when successive targets appear at the lcatin than when they appear at different lcatins, apparently because they select target bjects by using a mechanism that is based n lcatin. Whensuccessive targets appear at the lcatin, there is n need t "mve"the selectin mechanism t a new lcatin, leaving mre time t identify the stimuli. These experiments shw that lcatin-based selectin is used even thugh selectin by clr wuld be mre direct. They als demnstrate a methd f measuring lcatin-based selectin that can be applied t a variety f visual tasks. Further experiments reveal that althugh lcatin-based selectin is used t identify a digit in the presence f a digit distractr, it is nt used t identify a digit in the presence f a letter distractr, suggesting that this selectin mechanism is nt used in this situatin t prevent interference amng the basic features making up letters and digits, but t inhibit respnses assciated with the distractrs. Over the past r 1 years, many studies have prbed the nature f the "attentinal sptlight." Researchers have tried t determine its shape (c. W. Eriksen & St. James, 1986; Pdgrny & Shepard, 1983), measure its size (Cave & Ksslyn, 1989; Egeth, 1977; C. W. Eriksen & St. James, 1986; LaBerge, 1983; Larsen & Bundesen, 1978), assess the sharpness f its edge (Dwning, 1988; C. W. Eriksen & St. James, 1986; LaBerge & Brwn, 1989), clck its speed as it mves (Murphy & C. W. Eriksen, 1987; Remingtn & Pierce, 1984; Sagi & Julesz, 198; Tsal, 1983), detect its presence as it passes ver lcatins (Murphy & C. W. Eriksen, 1987; Shulman, Remingtn, & McLean, 1979), and test its ability t split int multiple sptlights (c. W. Eriksen & Yeh, 198; Psner, Snyder, & Davidsn, 1980). These studies are all based n ne central idea-that visual selectin is based n lcatin, r, in ther wrds, that infrmatin riginating This research was perfrmed while K.R.C. was a fellw at the Center fr Human Infrmatin Prcessing, University f Califrnia, San Dieg. Thanks t Alann Lpes fr prgramming and maintaining the cmputers used in these experiments; t Marlisa Andersn, Angelique Bruny, and Hanh Tran fr cnducting the experiments; and t Steve Luck, Jeff Schall, Claus Bundesen, and tw annymus reviewers fr knwledgeable advice. This wrk was supprted by Natinal Institute f Mental Health Training Grant MH-14268, awarded t the Center fr Human Infrmatin Prcessing, University f Califrnia, San Dieg, and by NCA2-22 and NCA2-4l4 frm NASA, NOOOI4-91-J-173 frm ONR, and l-r29-mh84-01 frm NIMH, awarded t H.P. Crrespndence shuld be addressed t K. R. Cave, Vanderbilt University, Department f Psychlgy, 301 Wilsn Hall, Nashville, TN 372 ( cavekr@ctrvax.vanderbilt.edu). frm ne particular lcatin r regin in the visual field is chsen fr special prcessing, while infrmatin frm all ther lcatins is excluded. Despite the imprtance f this spatial attentin assumptin t many current accunts f visual selectin, it has nt been tested very thrughly. Much fthe impetus fr the idea fselectin by lcatin came frm spatial cuing experiments, which shwed that subjects respnd faster t a stimulus when they knw its lcatin befre it appears (C. W. Eriksen & Hffman, 1974; Psner, Nissen, & Ogden, 1978; Psner et al., 1980). In general, cuing studies have prvidedimprtantdata cncerningvisual attentin. Hwever, these data by themselves are unlikely t present a cmplete picture fvisual selectin. First, the instructins in these studies suggest that lcatin is an imprtant prperty. The subject is led t expect each stimulus at a particular lcatin and may decide t select that lcatin and t exclude thers, even if ther methds f selectin are available r if selectin is nt required in this task. Secnd, in mst f these studies, stimuli have appeared n a mre r less blank display, with n distractrs t filter ut. Visual selectin is prbably far mre critical when distractrs are present (Shiu & Pashler, 1994). We are likely t see different aspects f the perfrmance f visual selectin ifwe test it under varying degrees flad, and the mst interesting prperties fselectin may nly appear under cnditins in which targets must be selected and distractrs must be excluded. A cmplete picture fvisual selectin requires experiments that include distractrs and that test whether Cpyright 199 Psychnmic Sciety, Inc.

2 422 CAVE AND PASHLER catin affects selectin even when lcatin is irrelevant t the task and selectin by lcatin is nt suggested by the instructins. If selectin is based n lcatin, and if we assume that it takes time and effrt t shift attentin frm ne lcatin t anther, tasks invlving multiple stimuli shuld be easier when the stimuli are at r near the lcatin. In ne relevant experiment by Hffman and Nelsn (1981), subjects searched fr target letters amng a rapidly changing array f fur letters (similar t displays used by Schneider & Shiffrin, 1977). At the time, they als watched fr a small U-shaped figure t appear near ne fthe letters and determined its rientatin when it did. On trials in which they crrectly identified which target letter appeared, they were mre likely t crrectly reprt the rientatin fthe U-shaped figure if it was psitined near the target letter. When they were wrng abut the letter, they were mre likely t be right abut the rientatinfthe U'-shaped figure if it was nt near the target letter. These results indicate that tw bjects can be prcessed mre effectively when they are clse tgether. This pattern wuld be expected if sme srt f facilitatin were applied t a particular regin at any given time. Whenever the area arund ne visual bject is chsen fr facilitatin, ther bjects near it shuld benefit, and bjects far frm it shuld suffer. Hffman, Nelsn, and Huck (1983) prvided additinal evidence fr this hypthesis.' These experiments shw that, at least under sme circumstances, tw targets are easier t identify when they bth appear at r near the lcatin. If selectin is based n lcatin, it might als be harder t prevent interference frm distractrs that are near a target. C. W Eriksen and Hffman (1972) used cnfiguratins f letters t shw that distractrs near a target (within abut 1 ) interfere with naming the target, whereas distractrs farther away d nt. This interference disappears if the target lcatin is cued at least 10 msec in advance. C. W Eriksen and Hffman (1973) and B. A. Eriksen and C. W Eriksen (1974) demnstrated that this interference ccurred at the respnse stage, because the interference was much greater when the nearby distractrs were assciated with a different respnse. (In a later experiment, Gatti & Egeth, 1978, fund Strp interference frm distractrs as far away as frm target.) Further evidence fr selectin by lcatin can be fund in the errrs that subjects make when they select the wrng stimulus in a multielement display. Snyder (1972) fund that when subjects misreprted the shape fa stimulus selectedby the criterinfclr, they were likely t reprt the prperties fa neighbring stimulus. In similar studies by Tsal and Lavie (1988, 1993), the stimuli mst likely t be reprted by subjects immediately after a search task were thse near the search target. Tsal and Lavie (1993) shwed that even when subjects culd ignre all aspects f a stimulus ther than its clr,.they were still faster t respnd t a secnd stimulus when it was lcated near the first, suggesting that they selected the target by lcatin even fr simple clr detectin. Additinal evidence cmes frm studies by LaBerge and Brwn (1986, 1989) shwing that allcatin fattentin t certain areas fr ne task can affect the prcessing f ther stimuli appearing at the lcatins. Nissen (198) demnstrated a special rle flcatin in selectin by using a different apprach. On the basis f Treismans feature integratin thery (Treisman & Gelade, 1980), she hypthesized that accessing infrmatin abut any visual prperty f a particular bject wuld first require determining the lcatin f that bject. The stimuli fr her experiments cnsisted ffur shapes f fur different clrs psitined at the crners fan imaginary diamnd. When subjects were given the lcatin and asked t reprt bth clr and shape, their accuracy fr these tw dimensins was independent, as Nissen expected. She predicted different results fr using clr t find shape and lcatin, hwever, since it shuld be impssible t find the shape fthe red bject withut first finding its lcatin. Using cnditinal prbabilities, she made quantitative predictins abut the percentage f trials in which subjects shuld make lcatin errrs, shape errrs, and errrs fbth types tgether. The data fit her predictins well (althugh Bundesen, 1991, ffers a different interpretatin). Each fthese experiments suggests that lcatin plays sme rle in visual selectin. Hwever, sme f them rely n rather indirect demnstratins, and nne fthem reveals whether selectin is based n lcatin in all circumstances. Fr instance, when Hffman and Nelsn's (1981) subjects searched fr a target amng characters in an array, they may have had t rely n sme type f selectin t islate each character fr identificatin. With n ther prperty such as clr r size differentiating the characters, it wuld be reasnable fr them t select by lcatin. Similarly, when Tsal and Lavie's (1993) subjects were trying t reprt as many characters as pssible frm a display f eight r nine, they als may have been unable t identify all the characters at nce. With n ther basis fr selectin, they may simply have chsen t select by lcatin, starting arbitrarily at the lcatin nearest the previus stimulus. Tsal and Lavie (1993) eliminated this prblem in Experiment 3, in which subjects were explicitly instructed t reprt nly the letter at a particular lcatin in the secnd task. Still, they were faster t reprt the selected letter when it was near the target fr the previus task, which was relevant nly in clr, and nt lcatin. Thus they apparently selected the clr stimulus by lcatin. Nevertheless, preparing t select by lcatin in the secnd task may have caused subjects t select the lcatin f the clr target in the first task. Tsal and Lavie (1993) addressed this alternative explanatin in their final experiment. Subjects n lnger reprted the letter designated by lcatin. Instead, nly tw letters were displayed in each trial, and subjects respnded if ne was an F. Again, respnses were faster fr letters near the previus target. Taken tgether, these last tw experiments prvide gd evidence that subjects selected by lcatin even

3 VISUAL SELECTION MEDIATED BY LOCATION 423 when the task did nt require r suggest lcatin selectin. One alternative, hwever, must still be cnsidered. Experiment 4 nce again intrduced an element f search that had been present in Experiments I and 2. One f the tw letters was either P r B, and the ther was either F r H. Subjects respnded nly when an F was present. With nly tw letters present, the need fr selectin was prbably much less than it was with the larger displays in Experiments 1 and 2. Nevertheless, subjects had t search fr an F between tw letters that had very similar shapes. Therefre subjects may still have fund it necessary t select each letter individually t identify it. The tw digits had differentclrs, s subjects culd select by clr rather than lcatin; but the clrs were assigned randmly, s selecting by clr in this experiment wuld be just as arbitrary as selecting by lcatin. With n ther basis fr selectin, they may have chsen t select by lcatin, starting with the ne near the previus stimulus. Thus, nne f these experiments tests whether selectin is based n lcatin even when all targets are marked by clr r sme ther simple visual prperty that makes lcatin-based selectin cmpletely unnecessary. In ne relevant experiment, cnducted by Bradbent (1977), subjects reprted ne clred wrd embedded in a series f successively presented black wrds. The wrds alternated between tw different lcatins. When subjects reprtedthe wrng wrd, they were mre likely t reprt a wrd that appeared at the lcatin as that f the target wrd than t reprt ne that ccurred immediately befre r after the target but at a different lcatin. In ther wrds, spatial prximity played a mre imprtant rle than tempral prximity. We set ut t perfrm ur wn, mre thrugh, test, under circumstances in which selectin by lcatin was nt explicitly required t separate target frm distractrs. Ifvisual selectin is based n lcatin, and if shifting attentin frm ne lcatin t antherrequires a certain amunt feffrt and time, then prcessing a series fvisual stimuli shuld be easier when they all appear at the lcatin. Furthermre, as lng as the prcessing fthese stimuli requires attentin, this advantage fr -lcatin stimuli shuld hld even iflcatin is nt directly relevant t the task. In the experiments reprted here, this pssibility was explred by using tasks in which the lcatins ftarget and distractr stimuli were irrelevant t the respnse. If perfrmance is better when all target stimuli appear successively at the lcatin, we will cnclude that a lcatin-based selectin mechanism is at wrk. Of curse, such evidence wuld nt supprt the cnclusin that all selectin in this task is lcatin-based; there might be ther mechanisms simultaneusly selecting by ther factrs. EXPERIMENT 1 Selectin by Clr In each trial f this experiment, subjects viewed a series f clred digits. In each frame f the series, tw digits appeared, ne t the left and ne t the right fa fixatin crss. One fthe digits in each frame was red, and the ther was green. One grup f subjects was instructed t attend t the green digits and t ignre the red digits, while a secnd grup f subjects received instructins with the clr rles switched. At the end f each trial, the subject reprted the highest digit amng thse f the target clr in the series f frames fr that trial. In rder t be sure f the crrect respnse in the highest digit task, the subject must have identified all f the digits in the series with the target clr. Thus this task differed frm that used in many visual search experiments, in which the subject knws a single target in advance, and may nt need t identify the ther display elements. Each subject received a randm mixture f tw types ftrials. In a trial, the target digits all appeared at the psitin. In an trial, the psitin f the target digit alternated frm ne frame t the next. If attentin is allcated by psitin, and if there is sme cst in shifting attentin t a new psitin, we wuld expect perfrmance t drp in the cnditin relative t the cnditin. We have perfrmed many different experiments f this type, varying the number f frames in each trial. In this experiment, each trial will have nly tw frames, and nly 10 msec will pass between the appearance f the first digits and the disappearance f the last. This fast display will rule ut any pssibility that subjects might mve theireyes t the lcatin fthe first target after the presentatin fthe first frame. Methd Subjects. Twenty students frm the University f Califrnia, San Dieg, participated in the red-target versin f this experiment, and participated in the green-target versin. All were fulfilling a curse requirement. Nne knew the purpse f the experiment r the expected results befrehand. Mst subjects finished in less than an hur. Apparatus. The apparatus cnsisted f IBM PC and cmpatible micrcmputers equipped with Paradise VGA+ graphics cards and NEC Multisync r Multisync 2a mnitrs. Subjects' respnses were entered n the standard keybard. lcatin lcatin time + IFixatinI msec )+3 IFrame 1I )+3 17 msec 2+7 IFmme2! msec # # I Mask I # # Figure 1. The sequence f displays in a single trial frm Experiment 1. (Light digits represent green, and dark digits represent red.) I

4 424 CAVE AND PASHLER Stimuli. Each trial included a series f tw frames presented successively, fllwed by a mask. Each frame cnsisted f a fixatin crss at the center f the screen, and tw digits, ne n each side fthe crss, as shwn in Figure 1. Thus each frame included ne digit t be attended and ne t be ignred. The distance frm the center f ne character t the center f anther was 3 ern, and each digit was 2.3 cm high and I em wide. The fixatin crss was 1.6 em high and.8 em wide. The subject was apprximately 60 em frm the screen. The backgrund was white, and the fixatin crss was black. In each frame, ne digit was red and the ther was green. The mask at the end f each trial cnsisted ftw black "#" characters, abut the size as the digits, and psitined at the lcatins ccupied by the digits in the previus frames. Fr each subject, halfthe trials were trials, with bth target digits appearing n ne side and bth distractr digits n the ther. Of these trials, halfhad targets n the left and halfhad targets n the right. The remaining halffthe trials were trials, in which the target digit appeared n different sides in the tw frames. Halffthe trials started with a target digit n the left and half started with a target n the right. Prcedure. The sequence f displays fr a single trial is illustrated in Figure I. Each trial began with the presentatin fthe fixatin crss. This was the subject's cue t fixate n the crss and prepare fr the trial. After 1,000 msec, the tw stimulus frames appeared, each fr 7 msec. In this experiment and in all that fllw it, the presentatin f the first frame was synchrnized with the vide refresh. The secnd frame was fllwed by the mask, which remained visible until the subject respnded. The tw distractr digits were chsen randmly frm the digits I thrugh 9. The tw target digits were als chsen randmly, but with the cnstraint that the digit 9 never appear befre the last frame. (Ifsubjects saw a 9 befre the last frame, they wuld nt need t prcess the later digits.) Als, n digit appeared mre than nce in a trial, either as a target r as a distractr. At the end f the trial, subjects reprted the higher fthe target digits by typing the apprpriate key n the keybard. Subjects were encuraged t take as lng as they needed t respnd. If the respnse was incrrect, a tne sunded immediately, lasting 300 msec. A timed delay was added befre each trial t ensure a cnstant 0-msec intertrial interval, including the time fr the errr tne. Each subject received 672 trials, presented in 14blcks f 48 trials each. The subjects were encuraged t rest between blcks. Each subjectbegan with a single blck f48 practice trials. Mre practice was prvided if the subject requested it r if the experimenter thught it was necessary. Results In this experiment, we purpsefully did nt match shades fred and green fr brightness, because we were mre interested in prviding an effective cue fr attentin allcatin than in cmparing the effects f brightness and hue. The red stimuli used in this experiment may have been mre salient in sme way than the green stimuli, and thus might have interfered mre with ther stimuli appearing at the psitin. Fr green subjects in the cnditin, each target digit was either preceded r fllwed by a red digit, whereas in the cnditin, each target was nly preceded r fllwed by green digits. If red digits were mre effective masks than green digits, targets wuld be mre effectively masked in the cnditin. Fr red 'subjects, targets wuld be mre effectively masked in the cnditin. Therefre, we must analyze data frm the tw subject grups separately t be sure that any effect was due t spatial attentin and nt t masking. 0 3 ~ C30 CIl ~ 2 e20 (jj 1 Search fr Green Digits Figure 2. Errr rates frm Experiment 1 searches fr green targets. 2 Errr rates fr each grup f subjects were subjected t repeated measures analyses f variance (ANOVAs) with versus trial and left versus right startingpsitin as factrs. Subjects searching fr green targets made many mre errrs when the target switched psitins frm ne frame t the next [F(l,19) = 28.02, P <.001]. (See Figure 2.) In this case, there was n significant advantage fr trials beginning with targets n the left r right (F < 1). There were 32% errrs fr starting n the left and 31% fr starting n the right. There was n interactin between the tw factrs (F < 1). The pattern frm red-target subjects was different, hwever. With the target and distractr clrs switched, there was n significant difference between the number f errrs in the and cnditins [F(l,9) = 1.3, p >.2]. (See Figure 3.) Perhaps the red stimuli were indeed mre effective at masking ther search fr Red Digits 0 3 ~ C30 CIl ~ 2 e20 Gi 1 Figure3. Errr rates frm Experiment 1 searches fr red targets.

5 VISUAL SELECTION MEDIATED BY LOCATION 42 stimuli appearing at the psitin. In the cnditin, each red target was subject t masking frm the ther red target, whereas in the cnditin each red target was nly subject t masking frm a green distractr. The ANOVA shwed n hint f an advantage fr either the left r the right starting psitin [F(l,9) = 1.9, p >.2], with 24% errrs fr starting n the left and 23% fr starting n the right. There was n interactin between starting psitin and versus cnditin (F < I). Discussin In this task, psitin f the digits was irrelevant t the crrect respnse. Ifsubjects nnetheless selected target digits by lcatin, their accuracy shuld have been higher ifbth f the target elements appeared at the psitin. Subjects searching fr green digits exhibited such an advantage fr trials ver trials, but the lack f a significant difference in the red subjects casts dubt n the rle flcatin-basedattentin in this experiment. It des nt, hwever, rule it ut. The masking effect that might have been prducing a cnditin advantage in the green subjects culd have been bscuring an attentinal cnditin advantage in the red subjects. Same trials might have been easier than trials fr red subjects because n attentin shift was required between Frames I and 2, but als mre difficult because f extra interference frm having tw red targets appear at the lcatin. The next experiment was an attempt t remve the masking advantage and determine whether any attentinal effect wuld persist. EXPERIMENT 2 MaskAfter Each Frame As in Experiment 1, each trial in this experiment cnsisted ftw frames, with tw digits each. Hwever, this time each frame was fllwed by a black mask identical t the mask presented at the end f each trial in the previus experiment. This mask, which cnsisted fa black "#" pattern at each f the tw digit lcatins, shuld thrughly mask each f the digits, regardless f whether it was preceded r fllwed by a red r green digit. With masking being equal in and cnditins, we culd nw attribute differences between these cnditins t attentin shifts rather than masking differences. In Experiment2, halfthe subjects attended t red digits, and half attended t green digits. We will examine the results f each grup separately, and f bth grups tgether. Methd Subjects. Seventeen subjects participated in this experiment. Thirteen were students frm the University f Califrnia, San Dieg, wh were fulfilling a curse requirement, and 4 were paid fr their participatin. Nne knew the purpse f the experiment lcatin time I lcatin IFixatin I + 100msec I Figure 4. The sequence f displays in a single trial frm Experiment2. r the expected results befrehand, and nne had participated in Experiment I. The subjects usually finished in less than an hur. Stimuli and Prcedure. In each trial, the mask appeared fr 0 msec aftereach frame, as shwn in Figure 4. Because the mask made identificatin f the digits mre difficult, the expsure times fr the digit frames were increased t 90 msec. Nine f the subjects attended t red and ignred green, and the remaining 8 had the clr rles reversed. As in Experiment I, each subject cmpleted 14 blcks f 48 trials each. The time between the appearance f the first digit pair and the disappearance f the secnd digit pair was 230 msec. Althugh this amunt f time might allw the initiatin and executin f a saccadic eye mvement, it is unlikely that subjects used saccades in this experiment. Because the subjects did nt knw the in advance, preparatinfr a saccade culd nt begin until after the first digit pair appeared. The eyes wuld nt actually mve fr apprximately 180 msec after that, lng after the first digit pair had disappeared. If the saccade were cmpleted at that time, then n half the trials it wuld actually mve the eyes away frm the target. On trials in which the saccade did fveate the secnd target, n mre than 30 msec f fveal prcessing wuld be pssible befre the mask appeared, and at least 20 msec f prcessing time wuld have been lst as the eyes were mving. Therefre, any subjects wh attempted t saccade wuld quickly find that it was mre effective t keep their eyes fixed n the crss thrughut each trial. This fact, cupled with the strng admnitins frm the experimenter t keep the eyes fixed, shuld have prevented saccades in Experiment 2. Results and Discussin The errr rates frm all subjects were included in a single ANOVA with starting psitin and trial type ( vs. ) as factrs. Errr rates were 8% lwer in the cnditin than in the cnditin [F(l,16) = 30.7,p <.001]. (See Figure.) Thus, when masking was cntrlled, perfrmance still drpped when mre attentin shifts were required. Starting psitin had n effect (F < 1), and there was n interactin (F = 1., p >.2). We als separated the data int tw I 100msec IFrame 11 +3!90msec I Mask I msec IFrame 2I msec I Mask I msec

6 426 CAVE AND PASHLER ~30 CD e 2 20 t: CD 1 Figure. Errr rates frm Experiment 2. grups by target clr and analyzed each separately. In each grup, the advantage fr trials was significant [F(l,7) = 12.,p =.01, fr green targets, andf(l,8) = 16.6,p <.01, fr red targets]. Once again, subjects made mre errrs when target lcatin switched frm trial t trial, just as the green subjects did in Experiment 1. In the previus experiment, sme fthis effect was prbably due t the less effective masking frm the green digits. In this experiment, hwever, each digit shuld have been thrughly masked by the black "#," prducing the degree f masking in the and cnditins. When the masking effect was eliminated, there were still mre errrs in the cnditin. Therefre, at least part f the effect measured in Experiment 1 was due t sme ther factr, presumably the shifting f attentin frm ne lcatin t anther. Fr the red subjects in Experiment 1, the attentinal effect was eliminated by the masking effect. A particular lcatin can be selected by facilitating the prcessing finfrmatin frm that lcatin, by inhibiting the prcessing f infrmatin frm ther lcatins, r by bth. Determining whether selectin is implemented by facilitatin r inhibitin frm experiments such as this is very difficult. In much f the discussin f the fllwingexperiments, lcatin-based selectin will be described as resulting in either facilitatin f target lcatins r inhibitin f distractr lcatins, but these descriptins merely refer t the relative effects f attentin n different lcatins, and nt t hw attentin is implemented. In Experiment I, the disadvantage fr the cnditin might have been due t sme perceptual difficulty stemming frm the changing clrs at each lcatin. Such an explanatin seems less likely fr Experiment 2, because bth the and the cnditins had a black mask appearing befre and after each digit. Thus bth cnditins had abrupt clr changes. One factr that must be cnsidered in using multipleframe displays with multiple targets is the "attentinal blink" described by Bradbent and Bradbent (1987) and Raymnd, Shapir, and Arnell (1992). In thse studies, the presence fne target in the sequence fdisplays impaired the ability t detect a secnd target appearing as much as 00 msec after the first. The inhibitin in the attentinal blink did nt take effect immediately after the first target's appearance, but built ver time. If there was an attentinal blink in the present experiments that fllwed the time curse as that measured by Raymnd et ai., the secnd target wuld have appeared befre the inhibitin had becme very strng. Even ifan attentinal blink was exerting a majr effect n perfrmance in the present experiments, hwever, the cnclusins wuld remain the. Bradbent and Bradbent and Raymnd et al. did nt test whether the inhibitin applied t stimuli at all lcatins r nly t the target lcatin. Ifit applied t all, it wuld apply equally in the and cnditins, with n net effect n the result. Ifit is lcatin-specific, it wuld actually be wrking against the effect demnstrated in the present experiments, by raising the errr rate nly in the cnditin. If a lcatin-specific attentinal blink was affecting these results, the advantage fr a target appearing at a selected lcatin is actually larger than the results suggest. Anther pssibility is that the attentinal blink is just ne aspect fa lcatin-specific inhibitin that can als ccur cncurrently with a target t blck distractrs at ther lcatins. Ifs, the attentinal blink is just part f the lcatin-specific selectin mechanism that these experiments investigate. EXPERIMENT 3 Letter Distractrs Experiment 2 demnstrated hw cmparing perfrmance in and cnditins can serve as a measure flcatin-basedvisual selectin. This methd makes it pssible t explre what srts f visual prcessing tasks elicit selectin by lcatin, and what stimulus prperties can cause infrmatin frm a lcatin t be selected. In Experiment 3, we tested whether selectin by lcatin wuld be used when targets were defined by alphanumeric categry rather than by clr. If the categry f a stimulus is determined early in prcessing, attentin might be directed t the lcatin fa character with the crrect categry t help in its final identificatin. In Experiment 3, subjects viewed tw digits and tw letters in each trial. They ignred the letters and reprted the higher digit. The clr f the characters was irrelevant t the crrect respnse. If in this experiment we shuld find the advantage fr the cnditin as that fund in Experiment 2, we wuld knw that subjects can allcate spatial attentin accrding t alphanumeric categry, as they d accrding t clr. If nt,

7 VISUAL SELECTION MEDIATED BY LOCATION 427 then either categry infrmatin cannt direct spatial selectin, r the different type fdistractrs makes spatial selectin unnecessary. Methd There were three different variatins fthis experiment, as described belw. Subjects. Thirty-ne subjects participated in Versin A fthis experiment, 12 in Versin B, and 20 in Versin C. All were UCSD students wh were fulfilling a curse requirement. Nne knew the purpse fthe experiment r the expected results befrehand, and nne had participated in the previus experiments. Mst subjects finished in less than an hur. Stimuli and Prcedure. The displays in all three versins f this experiment included tw frames with ne digit and ne letter in each. Digits were chsen as they were in the previus experiments. Letters were chsen randmly frm the set fall 26 letters with replacement. In Versin A, bth the digits and the letters were black. Each frame was visible fr 0 msec. As in Experiment I, there was n mask between the first and secnd frames, and the mask afterthe secnd frame remainedvisible until the subject's respnse. Versin B was mre like Experiments I and 2 in that each character was green r red. The clr was assigned randmly t each character, hwever, s that clrprvided n infrmatin abut which character was the target (digit) and which was the distractr (letter) in each frame. In this versin, a mask appearedfr 0 msec between the tw frames, and the mask after the secnd frame remained visible until the respnse. In Versin C, red and green clrs were als assigned randmly t the letters and digits, but with the cnstraint that ne character in each frame be red and the ther be green. In this versin, each frame was visible fr 90 msec, and each was fllwed by a 0-msec mask. All three versins cnsisted f 14 blcks f48 trials each. Results The errr rates frm Versin A are shwn n the left side ffigure 6, and thse frm the ther tw versins are n the left side ffigure 7. The errr rates frm each f the three versins were subjected t separate ANOVAs with starting psitin and trial type ( vs. ) as factrs. Fr all three versins, the errr rate remained the, whether bth targets appeared in the lcatin r nt [fr Versin A, F(l,30) = 2., p >.1; fr Versin B, F(l,II) < 1.0; fr Versin C, F(l,19) = 1.4, P >.2]. The errr rate als remained cnstant regardless fwhether the first digit appeared t the left r t the right [fr Versin A, F( I,30) < 1.0; fr Versin B, F(I,II) = 2.,p>.1; fr Versin C, F(l,19) = 2.9,p >.1]. The interactin between trial type and starting psitin shwed a surprising and incnsistent pattern acrss the three versins fthis experiment. Versin A shwed n hint fan interactin [F(l,30) < 1]. In the ther tw versins, errr rates were lwer when the digit in the secnd frame was n the left side f the display [fr VersinB,F(I,II) = 6.9,p<.0;frVersinC,F(l,19) = 20.7,p <.001]. What this pattern reflects is unclear, but in bth cases, fewer errrs ccurred when the last target digit in the sequence was n the left. Nte that the expsure times in Versin C were exactly thse used in Experiment 2 with digit distractrs. If the mre accurate respnses in Experiment 2's cnditin had been due t saccades, there shuld have been a similar advantage in Experiment 3, Versin C. The lack fany effect here indicates that ur assumptin abut fixatin in Experiment 2 was crrect. Discussin Whatever the cause f the interactin, all three versins fthis experiment shwed that when targets and distractrs were differentiated nly by alphanumeric categry, and nt by clr, the relative lcatin fthe tw targets did nt affect accuracy. Subjects apparently did nt use selectin by lcatin in this case, because their perfrmance was n wrse when lcatin shifts were necessary. This result might suggest that the lcatinbased selectin mechanism can be directed by simple visual prperties such as clr, but nt by higher level prperties such as alphanumeric categry. By the time a character has been sufficiently prcessed t determine its categry, there may be nthing t gain by subjecting 0 3 0' ~3O CD e 2 ~ 20 CD 1 Experiment 3A Experiment 4A Figure 6. Results frm Versin A fexperiments 3 and 4.

8 428 CAVE AND PASHLER ~30 CD ~ 2 g20 CD 1 Experiment Experiment 48 0 Experiment 3C 0 Experiment 4C ~30 30 CD ~ 2 2 g20 20 CD Figure 7. Results frm Experiments 3 and 4, Versins Band C. it t lcatin-based facilitatin r inhibitin. Hwever, the next experiment will indicate that selectin by lcatin might nt have been necessary in this particulartask at all. EXPERIMENT 4 Letter Distractrs With Clr Infrmatin Selectin by lcatin might be s helpful t visual identificatin that subjects will use it whenever they have the pprtunity. They might have been denied that pprtunity in Experiment 3, because they culd nt identify the alphanumeric categry f each stimulus quickly enugh t direct selectin. On the ther hand, spatial selectin may nt have been as useful in this task as it was in Experiment 2. The distractrs in Experiment 2 interfered with identificatin fthe target, even thugh they were a different clr. This interference culd have ccurred at a number fdifferent prcessing levels. Fr instance, ifthe targetwas identifiedby detectingthe set f features that it included, the presence ftherfeatures at ther lcatins might have interfered. This feature-level interference is predicted by a number f current attentinal theries, including feature integratin thery (Treisman, 1988; Treisman & Gelade, 1980) and guided search (Cave & Wlfe, 1990; Wlfe, Cave, & Franzel, 1989). Experiment3 raises anther pssibility. The targets in that experiment were defined by alphanumeric categry, and subjects did nt use lcatin-based selectin but culd still perfrm the task very well. Perhaps there is a detectr fr every pssible digit that respnds independently f lcatin, and each f these detectrs tends t elicit a respnse, whether the digit that stimulates it is the target r a distractr. In this case, selectin is necessary in rder t prevent interference at the respnse level, rather than at the feature level. In Experiment 4, the targets were digits fne clr, and the distractrs were letters f anther clr. Bth clr and categry infrmatin were available fr finding. Ifletterdistractrs interfere with the identificatin f digits enugh t make the selectin prcess wrthwhile, this experiment shuld shw better perfrmance in the cnditin, as in Experiment 2.

9 VISUAL SELECTION MEDIATED BY LOCATION 429 We tried a variety f different versins f this experiment, matching the different versins f Experiment 3. Methd Subjects. Twenty-six subjects participated in Versin A f Experiment 4, 12 in Versin B, and 1 in Versin C. All were UCSD students. Nne knew the purpse f the experiment r the expected results befrehand, and nne had participated in the previus experiments. Mst subjects finished in less than an hur. Stimuli and Prcedure. The displays in all three versins f this experiment had tw frames per trial, each with ne digit and ne letter. Digits and letters were chsen as they were in the previus experiment. Versin A was similar t Experiment 3, Versin A, with 0 msec/frame and a single mask at the end. Digits were green and letters were red. Versins Band C bth had a 0 msec mask between the tw frames t equate masking acrss the tw cnditins. In bth versins, apprximately half the subjects searched fr red digits amng green letters, and the ther half searched fr green digits amng red letters. Versin B crrespnded t Experiment 3, Versin B, with 7 msec/frame and a final mask that remained until the respnse. Versin C crrespnded t Experiment 3, Versin C, with 90 msec/frame and a final mask f 0 msec. All three versins cnsisted f 14 blcks f 48 trials each. Results The errr rates frm Versin A were subjected t an ANOVAwith starting psitin and trial type as factrs. Errr rates were significantly lwer when bth targets appeared at the lcatin [F(1,2) = 17.2, P <.001]. This pattern des nt necessarily indicate selectin by lcatin, hwever, because this experiment has n mask between the tw frames, allwing the type f masking differences as was seen in Experiment 1. A cmparisn fthe cnditins in Versin A f Experiments 3 and 4, shwn in Figure 6, makes it clear that the green targets are masked mre by red characters at the lcatin than by green. In Experiment 3, Versin A, digits and letters were all black, whereas in Experiment 4, Versin A, digits were green and letters were red. Even with the clr infrmatin cntributing t the selectin f targets, there were mre errrs in the cnditin f Versin A fexperiment 4 than in that f Versin A, Experiment 3. Versins Band C included masks after each frame and thus gave a mre accurate indicatin fthe use flcatin-based selectin in these tasks. These data are presented in Figure 7. The data frm each f these experiments were analyzed in separate ANOVAs with starting psitin, trial type, and target clr as factrs. These analyses shwed a pattern like that fund in Experiment 3, Versins B-C. Errr rates were abut the, whether the tw targets in each trial shared the lcatin r nt (F < 1.0 in bth cases). Even thugh subjects nw had clr infrmatin t direct lcatin-based selectin, they apparently identified the targets withut selecting their lcatins. In nne fthe three versins was there any significant effect f starting psitin [fr Versin A, F(1,2) < 1; fr Versin B, F(1, ) = 2.9, P >.1; fr Versin C, F( 1,13) < 1]. There was n interactin between trial type and starting psitin in Versin A [F(I,2) < 1]. Versin C shwed the advantage fr trials with the secnd-frame target n the left that was seen in Experiment 3, Versins B-C [F(1,13) = 9.4,p =.01]. A similar trend was present in Experiment 4, VersinB, althugh it did nt reach significance [F(1,lO) = 3.0,p >.1]. Discussin Subjects in Experiment 4 enjyed a ptential advantage ver thse in Experiment 3, in that they culd use clr infrmatint differentiate targets and distractrs. Nevertheless, the results frm Experiment 4, Versins B-C, indicate that subjects did nt rely upn selectin by lcatin, as they did when clr infrmatin was available in Experiment 2. Apparently, when digits are being sught and letters are present in the display, the letters d nt interfere with digit identificatin enugh t make lcatin-based selectin wrth the effrt. Selectin by lcatin may still be necessary if a larger number f distractrs is present. Althugh subjects did nt use spatial attentin in Experiment 4, errr rates were generally lwer than in Experiment 3, in which clr infrmatin was nt available. The difference between Versins C fexperiments 3 and 4 was cnfirmed with an ANOVA [F(1,33) =.1,P <.01]. Althugh subjects were nt selecting by lcatin, they may have had sme ability t select directly by clr, strengthening signals frm bjects fthe target clr and attenuating thse frm bjects f the distractr clr. Ifselectin by clr is pssible, it must be fairly limited, r selectin by lcatin wuld nt have been used in any f these experiments. (See Tsal & Lavie, 1993, fr further evidence f selectin by clr.) Perhaps the letter distractrs in Experiment 4 interfered with the digit targets enugh t elicit clr selectin, but nt enugh t elicit lcatin selectin, which may take mre effrt, and may cause prblems n trials. In Experiment 3, any interference frm letter distractrs wuld nt be cntrlled by either clr r lcatin selectin, because fthe lack fclr cues designating targets. The uncntrlled interference in Experiment 3 culd explain why the errr rate in Versin C f that experiment is higher than in either Versin C fexperiment 4 r Experiment 2. Earlier we suggested an alternative explanatin fr Experiment 1 based n increased perceptual difficulty due t changes in clr at each lcatin. Ifclr changes did make it mre difficult t identify the target digits, perfrmance shuld have been lwer in the cnditin f Experiment 4, which had the clr differences at each lcatin. Because perfrmance was equivalent between and cnditins in Versins Band C f Experiment 4, any explanatin based n clr differences seems unlikely. GENERAL DISCUSSION Tw imprtant cnclusins arise frm this series f experiments. The first cncerns hw selectin is accmplished in the visual system, and the secnd cn-

10 430 CAVE AND PASHLER cerns why selectin is necessary. The first tw experiments (especially Experiment 2) demnstrate that when subjects must visually identify target digits defined by clr in the presence f distractr digits f a different clr, the task is mre difficult when the target digits appear at different lcatins in the visual field. This lcatin effect ccurs despite the fact that the relative lcatin f the tw (r mre) targets is nt relevant t the crrectrespnse; the respnse will be the wherever the targets appear. Subjects in this task apparently treat ne lcatin preferentially ver anther. Once a particular lcatin has been selected fr this preference r activatin, it is easier t prcess new stimuli that appear at the lcatin than t shift it t a different lcatin. It is nt pssible t tell whether the selectin in these experiments was based n ther visual prperties in additin t lcatin. Fr instance, fr subjects attending t red, prcessing fnly red stimuli might have been facilitated at the selected lcatin, r perhaps nly the detectin f shape features used t distinguish digits was facilitated at the selected lcatin. Whatever attentinal excitatinr inhibitin is ccurring in these tasks, hwever, at least sme f it is lcatin specific. Other experiments (Cave, 1994) have shwn that attentin t a clred digit such as thse used in these displays can speed the detectin f anther stimulus at the selected lcatin with a different clr, shape, and size. Thus the lcatin-based selectin seems t be very general. Anther plausible explanatin fr these results des nt include selectin by lcatin.> Subjects might have stred as many digits as pssible alng with their lcatins int shrt-term memry, using selectin directly by clr t increase the chances that target digits were stred. When the distractrs were digits (Experiments I and 2), they then had t use infrmatin abut clr that was als stred in shrt-term memry t determine which digits were targets. Matching clr with digits was easier when bth digits at ne lcatin were f the clr, leading t fewer errrs when bth targets were at the lcatin. While this accunt is cnsistent with these experiments, it des nt explain results frm Cave (1994). In thse experiments, attentin t a digit in the presence fa distractr digit fantherclr speeded the respnse time t a small dt appearing at the target digit's lcatin. Apparently the spatial attentin allcated t a lcatin fr the target digit enhanced detectin fa very different type fstimulus (the dt) that appeared at the lcatin. Thus selectin by lcatin can accunt fr bth results. The accuntfvisual selectin perating accrding t lcatin is cnsistent with many ther current accunts f visual attentin, including "sptlight theries." The experiments described here, thugh, are amng the few that demnstrate selectin by lcatin under circumstances in which lcatin is nt an explicit part f the task r the instructins. The fact that it ccurs under these circumstances underscres the imprtance f lcatin in the rganizatin f visual infrmatin. Figure 8 gives a rugh sketch fhw a lcatin-based selectin mechanism might wrk. (This cnceptin is similar t that described in Treisman, 1986 and 1988, and Treisman & Grmican, 1988.) Prcessing starts at the bttm, with tw "map" representatins. The ne n the left encdes the presence fthe clr green, which in this example is the simple visual feature that determines what lcatin shuld be selected. The secnd map cntains whateverfeatures are used t distinguish characters frm ne anther. The clr infrmatin, via a general lcatin map, determines which lcatin is selected. Frm features frm the selected lcatin make their way t the mechanisms that distinguish different characters, while features frm the lcatin nt selected are blcked." Experiments I and 2 nt nly demnstrate selectin by lcatin, but they prvide a tl fr measuring when selectin by lcatin is used. In Experiments 3 and 4, that tl was used t inquire why this selectin mechanism was necessary, and they led t a secnd cnclusin: Althugh lcatin-based selectin ccurs when a digit is identified amng distractr digits, it des nt ccur when a digit is identified amng letters. If we assume that letter distractrs cntain many f the features fund in digits, then any feature-level interference amng stimuli must nt be strng enugh t make lcatin-based selectinnecessary in these experiments. The letter distractrs, hwever, d nt pse the threat f respnse-level interference as d the digit distractrs, because letter distractrs (assuming they are accurately identified) will nt elicit incrrect digit respnses in the way that digit distractrs culd. If there are letter and digit detectrs as in Figure 9, the letter distractrs will generally activate nly the letter detectrs. Because subjects knw that the respnse must be a digit name, the letter detectrs will nt be allwed t generate respnses, and n interference will ccur. Therefre, these data tgether suggest that lcatinbased selectin is used t amelirate respnse-level inabstract identitydetectrs, I -@- " I' Figure8. A lcatin-based selectin mechanism driven by clr infrmatin.

11 VISUAL SELECTION MEDIATED BY LOCATION 431 terference in this task, and nt the feature-level interference predicted by feature integratin thery and guided search. The visual system must be able t detect and cmbine features efficiently enugh t identify tw well-spaced characters simultaneusly withut spatial selectin. Feature-level interference may still trigger lcatin-based selectin in ther tasks with mre distractrs r with distractrs that share mre features with the targets. Mzer's (1991) mdel f recgnitin illustrates hw a system might be able t perfrm the simple letter discriminatins in this experiment withut spatial selectin, but culd still have prblems with interference when mre distractrs are present. Mzer's mdel als illustrates anther imprtant pint, that even thugh lcatin-based selectin is used t prevent interference at a high level f prcessing, the selectin itself may wrk much earlier, at a level belw the level f the ptential interference. There must als be sme ability t gauge the difficulty f perceptual tasks and decide whether spatial selectin is necessary. Styles and Allprt (1986) used a different apprach t shw that selectin was necessary t identify characters amng distractrs f the categry. Their subjects reprted letters that appeared in the presence f either ther letters r digits. In their experiments, when all the characters in the stimulus array were letters, accuracy was lwer fr thse in the inner psitins fthe cnfiguratin, surrunded by ther letters n bth sides, and higher fr thse at ne edge fthe cnfiguratin. Hwever, when subjects were reprtinga letteramng digits, accuracy did nt change with psitin. Styles and Allprt cncluded that the disadvantage fr inner psitins was due t a need fr selectin, and nt t decreased accuracy in feature detectin. When the distractrswere digits and the target was a letter, selectin was nt necessary. After perfrming ther experiments with masked stimuli visible fr different intervals, they als cncluded that the identities f the characters in the display were Figure 9. Lcatin-based selectin is nt used with a digit target and a letter distractr. determined early, but that the identities were nt linked with lcatins r with clrs until later in prcessing. Of curse, selectin mechanisms besides the spatial attentin demnstrated in ur experiments might be (and prbably are) at wrk in differentparts fthe visual system, and they may be based n prperties ther than lcatin. In rder t disentangle different types f visual selectin, we must first learn mre abut hw this lcatin-based mechanism perates and why it is necessary. Elsewhere we have reprted tests fwhether it is used in tasks therthan character identificatin, whether it is capable f selecting nncntiguus regins, and whether selectin allcated fr ne type fstimulus generalizes t ther types (Cave, 1994; Cave & Pashler, 1994). The mst imprtant questins raised here are prbably thse cncerning the cnditins under which this type f selectin is necessary. Answering these questins will determine when interference ccurs in the visual system and will uncver imprtantfacts abut the general architecture f the visual system. REFERENCES BROADBENT, D. E. (1977). Clur, lcalisatin, and perceptual selectin. In G. Olern (Ed.), Psychlagie experimentale et cmparee: Hmmage apaul Fraisse (pp. 9-98). Paris: Presses Universitaires de France. BROADBENT, D.E.,& BROADBENT, M. H. P. (1987). Frmdetectin t identificatin: Respnse t multiple targets in rapid serial visual presentatin. Perceptin & Psychphysics, 42,-113. BUNDESEN, C. (1991). Visual selectin f features and bjects: Is lcatin special? A reinterpretatin f Nissen's (198) findings. Perceptin & Psychphysics, SO, CAVE, K. R. (1994). Measuring the allcatin fspatial attentin. Manuscript submitted fr publicatin. CAVE, K. R., & KOSSLYN, S. M. (\989). 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(1989). Guided search: An alternative t the feature integratin mdel f visual search. Jurnal fexperimental Psychlgy: Human Perceptin & Perfrmance, 1, NOTES 1. Hffman and Nelsn (1981) used an incnsistent mapping f stimuli t respnses, which Schneider and Shiffrin (1977) claimed wuld require cntrlled prcessing. Hffman et al. (1983) used a cnsistent mapping, which accrding t Schneider and Shiffrin shuld result in autmatic prcessing ver time. 2. The length f the errr bars n all the graphs was calculated as VMSl/n, where MSI is the mean square fr the subject X / interactin (errr term fr the / factr), and n is the number fvalues cntributing t each mean. This prcedure was suggested by Lftus (1993). 3. This alternative was pinted ut by Claus Bundesen. 4. Our current data lead t n cnclusins abut the nature fthe abstract identity detectrs, and the diagrams shuld nt be interpreted as making any such claims. In fact, there may nt be a separate mechanism dedicated t each character (see Mzer, 1991). Likewise, the data presented here allw n cnclusins abut the shape f the selected area (discussed in Cave and Pashler, 1994), r the sharpness f the bundary between the selected and unselected areas. (Manuscript received July 6, 1993; revisin accepted fr publicatin Nvember, 1994.)