Intra- and Interpattern Relations in Letter Recognition

Transcription

1 Jurnal f Experimental Psychlgy: Human Perceptin and Perfrmance 1991, Vl. 17, N. 4, Cpyright 1991 bv the American Psychlgical Assciatin. Inc I523/9I/S3.00 Intra- and Interpattern Relatins in Letter Recgnitin Thmas Sancki University f Suth Flrida Strings f 4 unrelated letters were backward masked at varying duratins t examine 3 majr issues, (a) One issue cncerned relatinal features. Letters with abnrmal relatins but nrmal elements were created by interchanging elements between large and small nrmal letters. Overall accuracy was higher fr letters with nrmal relatins, cnsistent with the idea that relatinal features are imprtant in recgnitin, (b) Interpattern relatins were examined by mixing large and small letters within strings. Relative t pure strings, accuracy was reduced, but nly fr small letters and nly when in mixed strings. This effect can be attributed t attentinal pririty fr larger frms ver smaller frms, which als explains glbal precedence with hierarchical frms, (c) Frced-chice alternatives were manipulated in Experiments 2 and 3 t test feature integratin thery. Relatinal infrmatin was fund t be prcessed at least as early as feature presence r absence. The present article cncerns the prcess f recgnizing letters, which are examples f highly familiar patterns. Of particular interest are several interrelated prperties f patterns and grups f patterns that can be referred t as spatialsize relatins. Such relatins exist between elements f patterns r between patterns themselves. In this article, I assume that primitive, elemental features crrespnd t cntinuus cllinear r curvilinear segments, r pieces f such segments, and that relatinal features invlve relatins between (cmbinatins f values n) elemental feature dimensins. Intrapattern relatins f interest here invlve the relative lcatins and sizes f elements. Interpattern relatins f interest here invlve the relative size f neighbring patterns. Intrapattern Relatins Little is knwn abut the prcessing f intrapattern relatins such as relative lcatin. Fr example, cnsider mdels in which features crrespnd t mutually exclusive, psitinspecific elements (e.g., McClelland & Rumelhart, 1981; Twnsend & Ashby, 1982). In these mdels, lcatin is reslved when a feature is detected because such infrmatin is assumed t be part f a feature's definitin. This is mre f an assumptin than an explanatin, hwever, and it has prblems.' Fr example, evidence suggests that withut fcal attentin, stimulus elements may be registered but nt lcalized (Treisman & Gelade, 1980). Other investigatrs have suggested that recgnizing patterns r bjects invlves detecting relatins between elements (e.g., Oden, 1979; Palmer, 1977). The relatins culd invlve fea- This research was supprted in part by a University f Suth Flrida Research and Creative Schlarship Award. I thank Gregg Oden fr helpful cmments n the research, and Carl Crumley, Rbert Edelman, Ian Rsen, and Cecilia Puccini fr their help in cnducting the experiments. Crrespndence cncerning this article shuld be addressed t Thmas Sancki at the Department f Psychlgy, BEH 339, University f Suth Flrida, Tampa, Flrida (Electrnic mail: Sancki@figment.csee.usf.edu.) 924 ture dimensins such as size, rientatin, and relative lcatin. Relatins between elements can be represented tgether with mre elemental features within a structural descriptin (e.g., Palmer, 1977). A structural descriptin is a hierarchy f structural units, each f which may include several elements and their interrelatins. The encding f relatins between elements culd prvide infrmatin abut the relative lcatins f elements (f curse), and the encding f multiple relatins culd prvide infrmatin abut the relative lcatin f each element within a pattern. Mdels that use structural descriptins gain explanatry pwer because the frmalism is bth pwerful and flexible; hwever, these qualities als make the mdels difficult t test. Indeed, althugh many investigatrs agree that structural infrmatin is imprtant in recgnitin (e.g., Massar & Schmuller, 1975; Oden, 1979; Palmer, 1977; Pinker, 1984), there is very little direct evidence that relatinal infrmatin is used in pattern recgnitin (but see Oden, 1979). Yet flexible representatins culd als becme rather precise under certain cnditins, and precisin culd enable certain types f empirical tests. The existence f letter fnts prvides a reasn fr precisin. Letters almst always appear tgether in the same fnt, having similar details and systematically related sizes. It wuld be adaptive fr the visual system t becme tuned fr fnt-specific details because such tuning wuld increase the efficiency f recgnitin within the fnt. That such tuning ccurs is supprted by the finding that recgnitin is mre efficient with target sets f n letters f a single fnt than with n letters f tw r mre fnts, fr which fnt-specific tuning wuld nt be pssible (Sancki, 1987, 1988). Given that precise representatins are used in letter recgnitin, it is pssible t test certain types f structural descrip- 1 The cited mdels have been designed t address prblems ther than spatial-size relatins, and their simplified feature assumptins may have been adpted fr cnvenience. The present cmments are nt meant t minimize the substantial cntributins f these mdels but t pint ut that there may be a set f imprtant issues hidden beneath assumptins f sme imprtant mdels.

2 PATTERN RELATIONS 925 tin mdels. The present experiments were mtivated by the hypthesis that perhaps because f years f experience reading, perceivers have develped representatins that include fairly precise infrmatin abut relatins that characterize cmmn letters. Such representatins might take different frms (see the General Discussin sectin), but the crucial characteristic f interest here is that they include fairly precise infrmatin abut relatins between elements f letters. Such representatins have implicatins quite different frm mdels that are based n simple elements nly (r frm structural descriptin mdels that d nt include precise relatinal infrmatin). In particular, if perceivers use precise relatinal infrmatin, then letters with relatins unlike typical letters ught t be less perceptible than letters with typical relatins because the "abnrmal" letters wuld nt match existing representatins as well as nrmal letters wuld. This ught t ccur even thugh abnrmal letters have the same elements (but in smewhat different relatins) as nrmal letters. The relatins f initial interest here were thse invlving the relative sizes f elements crrespnding t ascenders, descenders, and bdies f (lwercase) letters, r t pieces f these units. The assumptin that elements crrespnd t (r are cntained within) ascenders, descenders, and bdies is cnsistent with the frequent bservatin that ascenders and descenders are particularly imprtant fr lwercase letters. In Buma's (1971) analysis f letter cnfusins, the distinctin between small letters and letters with ascenders and descenders was amng the mst imprtant cues t letter identity, and such a distinctin is cnsistent with a number f ther feature sets that are based n lcal features (e.g., Keren & Baggen, 1981; McClelland & Rumelhart, 1981). Examples f the letters used in the present experiments are shwn in Figure 1. Nrmal letters can be fund in Rws 1 and 2. Abnrmal intraletter size relatins were created by switching the tps and bttms (ascenders and descenders) f ne size with the middles (bdies) f the ther size. The abnrmal letters are shwn in Rws 3 and 4. Nte that althugh these letters have abnrmal interpart size relatins, the elements themselves are the same as in the nrmal letters. A ptentially imprtant cnsequence f interchanging the parts is the resultant change in the amunts f cmmn and distinctive feature infrmatin. Cmmn features are shared by many items in the stimulus set, whereas distinctive features gqhnbdf t j pky gqhnbdtfjpky qqhnbdtt jpky _gqhnbdtf Jpky Figure 1. The tw nrmal fnts and the tw abnrmal fnts (largesmall and small-large), respectively. (These letters were used in Experiment 1.) are specific t a limited number f the items and are mre imprtant fr identificatin (e.g., see Keren & Baggen, 1981). In the present case, the bdies f the letters prvide mainly cmmn features, whereas the ascenders and descenders prvide mainly distinctive features. Adding large ascenders and descenders t small bdies (Rw 4) culd increase the relative amunt f distinctiveness and thereby increase perceptibility, whereas adding small ascenders and descenders t large bdies (Rw 3) culd reduce distinctiveness and decrease perceptibility. Mdels that emphasize precise relatinal infrmatin, hwever, predict that disrupting relatinal infrmatin ught t have effects in additin t thse f altering distinctiveness. Therefre, verall, the predictin is that letters with abnrmal relatins ught t be less perceptible than letters with nrmal relatins. Interpattern Relatins Als f interest here are interpattern relatins between different patterns within the field f view. The main relatin investigated here invlves the relative sizes f neighbring patterns. Rudnicky and Klers (1984) cmpared reading speeds fr texts cmpsed f same- and alternating-size letters and fund a decrement fr alternating size. Rudnicky and Klers attributed the decrement t wrd-based pattern-analyzing prcesses. Hwever, explanatins in terms f mre general mechanisms might be preferable, especially if sizemixing decrements can be btained with strings f unrelated letters mixed in size. An alternative accunt f size-mixing decrements fllws frm a literature n attending t frms at different size scales (e.g., Miller, 1981; Navn, 1977; Ward, 1982). In a typical experiment, a hierarchical frm (e.g., a large S r H cmpsed f smaller Ss r Hs) is presented fr subjects t identify at either the larger r smaller scale. The mst imprtant result is that within certain bundary cnditins, respnses at the smaller scale suffer mre frm respnse incnsistency at the larger scale than respnses at the larger scale suffer frm incnsistency at the smaller scale (see Kimchi & Palmer, 1985; Kinchla & Wlfe, 1979; Martin, 1979; Pmerantz, 1983). Navn (1977, 1981) attributed this effect t prcessing dminance f glbal features (larger frms) ver lcal features (smaller frms), which ccurs because visual prcessing resurces are allcated t glbal features befre lcal features. There are prblems with Navn's terminlgy and interpretatin, hwever. Navn never established that the larger and smaller frms are in fact perceptual features; the actual features used in this situatin are nt knwn. Therefre the mre cnservative terminlgy f frms at different size scales is used here (see Kimchi & Palmer, 1985). Furthermre, Navn's interpretatin in terms f feature prcessing nw has much less supprt than alternative explanatins in terms f attentinal pririty (e.g., Ber & Keuss, 1982; Miller, 1981; Paquet & Merikle, 1988; Ward, 1982). In these alternative explanatins, larger frms are assumed t be a higher pririty fr attentinal r decisinal prcessing, perhaps because f a greater eclgical imprtance r cnspicuity f larger frms (see Miller, 1981; Ward, 1983). Early visual prcessing is nt

3 926 THOMAS SANOCKI affected by size, hwever. Thus, large and small letters may activate letter cdes at the same rate, but this infrmatin becmes available t attentin r decisin prcesses sner fr large frms than fr small frms. Decrements due t size mixing in Rudnicky and Klers's (1984) reading experiments might ccur because the availability f infrmatin abut small letters in mixed strings is delayed. Hwever, because reading time is a glbal measure f perfrmance, differences between large and small letters within mixed strings wuld nt have been detected in their experiments. In the present experiments, perfrmance was measured fr individual large and small letters in mixed-size and cnsistent-size strings. Given previus findings f an advantage fr larger frms described earlier, a size-dminance effect is expected: The perceptibility f smaller letters ught t be reduced by the presence f larger letters. Hwever, this ught t ccur nly fr small letters in strings cntaining bth large and small letters; with pure-size strings, perfrmance fr small letters ught t be clser t that fr large letters. Time Curse f Prcessing T learn abut the time curse f the afrementined effects, a backward masking task was used, and prcessing time was varied. Explanatins f size-dminance effects in terms f resurce allcatin t large and small features predict differences early in the time curse f prcessing (see Navn, 1981), whereas explanatins in terms f attentinal pririty predict differences later in prcessing (see the fllwing). An additinal issue invlving prcessing time cncerns predictins abut the prcessing f relatins frm feature integratin thery (e.g., Treisman & Schmidt, 1982; Treisman & Suther, 1985). Treisman and Suther prpsed that infrmatin abut the presence r absence f elemental features is registered early in the time curse f prcessing, whereas relatins between elements are established in a later, attentindemanding prcess. In cntrast, the present emphasis n detecting relatins implies that relatinal infrmatin may be extracted directly frm a familiar pattern, tgether with feature presence r absence. These ideas are examined in Experiments 2 and 3. Experiment 1 The main purpse f this experiment was t assess the effects f abnrmality and size mixing n recgnitin. There were tw grups f subjects. Each grup received tw nrmal blcks f trials, ne with the large nrmal fnt and ne with the small nrmal fnt. Each grup als received tw abnrmal blcks. Fr the abnrmal-mixed grup, the sizes f the abnrmal letters were mixed. Fr the abnrmal-pure grup, the abnrmal letters were f cnsistent size within each abnrmal blck. Within each blck fr each grup, the target set always cnsisted f six items. In additin, the afrementined factrs were crssed with tw levels f prcessing time (stimulus nset asynchrny [SOA]) t prvide sme infrmatin abut the time curse f prcessing. Stimuli and Design Methd The target sets were drawn frm the nrmal and abnrmal sans serif letters shwn in Figure 1. The small (and nrmal) fnt was created first (Rw 2). In that fnt, large letters (thse with ascenders r descenders) were 0.65 f visual angle in height at the viewing distance used. The large (and nrmal) fnt was then created by magnifying the small fnt by a factr f 2 (Rw 1). Frm these tw fnts, tw abnrmal fnts were created: ne by cmbining the middle regins f the large letters with the tps and bttms f the small letters (the large-small fnt, Rw 3) and the ther in the ppsite manner (the small-large fnt, Rw 4). In this experiment, nly six letters frm the fnts (b, d,f, g, h, andy) were used. The size relatins in these fnts were guided by a nrmative study f fnts. Ten sans serif fnt families were selected frm a cllectin f text fnts (Digital Typeface Library, 1985). A 20- r 21-pint fnt was selected frm each family and prjected n a wall. Measurements were then made f the x height, the height f ascenders abve the x height, and the distance f descenders belw the baseline (bttm f x). The median ascender rati (ascender height/* height) was.38 (M=.42, SD =.127, range ), and the median descender rati (descender height/* height) was.40 (M =.40, SD =.072, range = ). Fr the nrmal fnts used here, these ratis were bth.33; fr the large-small fnt they were bth.17; fr the small-large fnt they were bth.67. The nrms were apprximated as clsely as pssible given display limitatins (e.g., ascenders n small letters were nly 2 pixels in height). There were tw grups f subjects. Fr each grup, there were tw nrmal blcks, ne with each f the nrmal fnts, and tw abnrmal blcks. Fr subjects in the abnrmal-pure grup, the abnrmal largesmall fnt and the small-large fnt were each used in ne blck. Therefre, each target string cntained letters f nly ne size and was "pure." Fr subjects in the abnrmal-mixed grup, there were als tw abnrmal blcks, but each had three large-small letters and three small-large letters. Because each target string had fur different letters chsen withut replacement frm the target set, it was mixed in size. Letters within target strings were always aligned alng their baselines. The rder f the fur blcks was cunterbalanced in a Latin square design. Within each blck, there was an equal number f trials at each f the tw SOAs (78 and 108 ms) that were used, Prcedure and Apparatus Subjects were tested individually in sessins lasting apprximately 50 min. Each blck cnsisted f 16 practice trials fllwed by 48 test trials. The first blck fr a subject (which varied with the rder f the blcks) was preceded by an additinal 16 practice trials in that cnditin. The experiment was cntrlled by an Apple IIGS micrcmputer. Stimuli were displayed n an 11 in. (27.9 cm) Apple mnitr, in white n black dt matrix letters. At the start f the experiment, the subjects were seated in frnt f the cmputer, with the backs f their heads resting cmfrtably against the wall f the sund-attenuated bth. The subjects were instructed t keep their heads against the wall s that viewing distance wuld remain cnstant, and cvert checks by the experimenter indicated that every subject cntinued t fllw this instructin. Fr each trial, the cmputer randmly selected fur target letters withut replacement frm the relevant target set. At the start f each trial, the mask appeared n the screen. A single nisy, patterned mask was used, cnstructed by repeated verlaying letter parts f varius types and sizes. When the subject was ready, he r she pressed a

4 PATTERN RELATIONS 927 "ready" key n the keybard, and the mask was immediately replaced by the fur target letters. Stimulus presentatin was accmplished by switching between screens f the cmputer's memry and ccurred within 17 ms. After the SOA elapsed, the target display was replaced by the mask. The mask was replaced 1 s later with fur clumns f tw letters each. Each clumn cntained the target frm the crrespnding psitin f the target string and a fil, randmly selected frm the five remaining items in the target set. (Nte that in the abnrmal-mixed cnditin, this meant that the fil was ften f a different size frm the target.) Psitin within the clumn (tp r bttm) was randmly determined. Each clumn was prbed in turn, frm left t right. The subject respnded by pressing either an "upper" key (fr the upper letter) r a "lwer" key n the keypad. Fllwing the last letter, the number crrect and the number wrng fr that trial were indicated by the cmputer in terms f high and lw beeps. During the frced-chice prbe, nly questin marks and xs were used; aside frm these characters and the stimuli, n ther characters appeared during the experiment. Subjects Fifty-ne students frm intrductry psychlgy curses at the University f Suth Flrida participated. They received extra curse credit fr their participatin. The data fr 3 subjects were discarded because their verall level f accuracy was less than a minimal criterin (65%), leaving 24 subjects in each abnrmality grup. Results Subjects' prprtins crrect were submitted t analyses f variance (ANOVAs), with variables f abnrmality grup (abnrmal-pure vs. mixed), abnrmality, SOA, and fnt size. Fr the fnt-size variable, the larger large-small fnt (large bdy, small ascenders and descenders) was treated as large, whereas the smaller small-large fnt was treated as small. Fr the abnrmal-mixed grup, each trial in the abnrmal blcks invlved bth large and small target letters. The data were separated by target size and then analyzed in the same way as the data fr the abnrmal-pure grup. Abnrmality Relative t the nrmal cnditins, perfrmance was lwer verall in the abnrmal cnditins, F(\, 46) = 12.85, p <.001. This is cnsistent with the hypthesis that fairly precise relatinal infrmatin is imprtant in letter recgnitin. The abnrmality decrement was abut the same magnitude fr the tw abnrmality grups, f\l, 46) < 1. These data are shwn in Table 1. Letter-Fnt Size With regard t size effects, the main predictin was that the relative perceptibility f small items wuld be reduced when they are mixed with larger items in the abnrmalmixed cnditin. Evidence f such a size-dminance effect is prvided by interactins invlving fnt size. There was n main effect f fnt size, F(l, 46) < 1, but there was an interactin f fnt size and abnrmality grup, F(l, 46) = 5.26, p <.05, and a Fnt Size X Abnrmality x Abnrmality Table 1 Mean Percentage f Crrect Respnses in Each Cnditin f Experiment 1 Cnditin Nrmal Abnrmal Difference Abnrmality grup Pure Mixed Grup interactin, F(l, 46) = 5.54, p <.05. The three-way interactin is shwn in Figure 2. The surce f these interactins is at least in part a disadvantage fr the small abnrmal letters (small-large fnt) that ccurs nly when they are mixed with larger letters, that is, nly fr the abnrmal-mixed grup. Perfrmance fr thse same letters in the pure blcks f the abnrmal-pure grup was higher than fr the larger largesmall fnt. This decrement fr small letters in the mixed grup is an example f size dminance fr larger frms ver smaller nes. These interactins als suggest that the abnrmality effect depended n bth fnt size and abnrmality grup. Within each abnrmality grup, perfrmance fr abnrmal letters was cmpared with that fr nrmal letters f the same size. In the abnrmal-pure grup, perfrmance fr large nrmal letters was higher than fr the large-small abnrmal letters, F(\, 23) = 6.35, p <.05, but perfrmance fr small nrmal letters was nt different frm that fr the small-large abnrmal letters (F< 1). In cntrast, in the abnrmal-mixed grup, perfrmance fr large nrmal letters was nt different frm that fr the large-small abnrmal letters, F(l, 23) == 1.01, p >.20, but perfrmance fr small nrmal letters was higher than fr the small-large abnrmal letters, F(l, 23) = 9.36, p Additinal Results Prcessing time (SOA) had a main effect, F(l, 46) = 15.84, p <.001. Als f interest was a marginal Grup X Abnrmality x SOA interactin, F(l, 46) = 3.13,.10 > p >.05. This is shwn in Figure 3. As can be seen, the rate f prcessing was similar fr nrmal and abnrmal letters when the abnrmal letters were in pure strings (F < 1 fr the Nrmality x SOA interactin in the pure grup). The rates differed, hwever, when the abnrmal letters were mixed in size, F(l, 23) = 4.59, p <.05, fr the mixed grup; the rate f prcessing was slwed by abnrmality. Identificatin accuracy varied with the psitin f items within strings, F(3, 138) = , p <.001. Accuracy was highest fr the leftmst psitin and decreased with rightness, as was expected n the basis f previus research (e.g., Sancki, 1988; Twnsend, 1981). The effect can be attributed t the distributin f visual prcessing capacity acrss the stimulus string (Twnsend, 1981). There was als a small Psitin x Size interactin, F(3, 138) = 2.67, p <.05. The relevant data are reprted in Table 2. Further infrmatin regarding interactins with size and psitin effects are prvided by the subsequent experiments.

5 928 THOMAS SANOCKI U 79 - Abnrmal-Pure Grup Abnrmal-Mixed Grup Nrmal Abnrmal 0> Q small large small large Size Size Figure 2. Percentage f crrect respnses as a functin f letter-fnt size in the tw mixing grups f Experiment 1. Discussin Abnrmal-Pure Grup There were tw majr findings in Experiment 1. First, the verall perceptibility f letters is reduced when they cntain abnrmal intrapattern relatins. This effect can be attributed t the relatins between elements rather than the elements themselves because the same elements were used in nrmal and abnrmal cnditins. This result is cnsistent with mdels in which precise relatinal infrmatin is represented and used, such as certain types f structural descriptin mdels. Hwever, the decrement fr abnrmal letters was restricted t certain cnditins, and it is imprtant t clarify the nature f these cnditins. This is accmplished in the next experiment. The secnd finding is a size-dminance effect; that is, there is an advantage fr large abnrmal letters ver small abnrmal letters nly when the sizes were mixed within strings. When the sizes f the abnrmal letters were nt mixed (abnrmalpure grup), the smaller nes are mre perceptible. Because f this interactin, the size-dminance effect cannt be attributed t differences in baseline discriminability (cf. Pmerantz, 1983). The effect may be caused by the same prcess that causes the glbal dminance effect (e.g., Miller, 1981; Navn, 1977). The advantage fr small abnrmal letters (small-large fnt) ver large letters in the abnrmal-pure grup is prbably related t the fact that the large ascenders and descenders f the small-large fnt are distinctive features within the set and therefre mre imprtant fr recgnitin. Hwever, the advantage is lst when the small-large fnt is mixed with the larger items f the large-small fnt in the abnrmal-mixed grup because f size dminance. An additinal finding is that abnrmality slwed the rate f prcessing when sizes are mixed but nt when sizes are cnsistent. Hwever, because the full time curse f prcessing was nt examined in this experiment, the exact nature f Abnrmal-Mixed Grup O r c 79-0) k. > Q Nrmal Abnrmal SOA SOA Figure 3. Percentage f crrect respnses as a functin f prcessing time (SOA) in the tw mixing grups f Experiment 1.

6 PATTERN RELATIONS 929 Table 2 Percentage f Crrect Respnses fr Each Letter Psitin and Size in Experiment 1 Size Large Small First Secnd Psitin Third Furth the time-curse difference is nt clear. The issue is examined further belw. Experiment 2 Experiment 2 had several majr purpses. First, it was designed t prvide mre infrmatin abut abnrmality and size-mixing effects and abut their time curse. T this end, five different SOAs were used, and size mixing was manipulated as a within-subject variable. The abnrmal blcks nw included tw types f stimulus strings: pure-size strings f abnrmal letters and mixed-size strings f abnrmal letters. This prvided a direct cmparisn f abnrmal-pure and abnrmal-mixed cnditins and a strng test f the pssibility that the time curse f prcessing is affected by mixing size within strings. The time curse f size-dminance effects with mixed strings has implicatins with regard t the surce f the effects. In particular, if visual prcessing resurces are allcated t larger frms r features befre smaller nes (Navn, 1977, 1981), then the advantage fr larger items ught t be evident early in prcessing because nly these items wuld initially receive a sufficient resurce allcatin. The small items may catch up later in prcessing, after accuracy has asymptted fr large items, because resurces wuld be released fr the small items (see Navn, 1981). In cntrast, large frms culd be a higher pririty than small frms fr pstperceptual attentinal r decisinal prcesses (e.g., Miller, 1981; Ward, 1983). In the present case, later prcesses may encde letter identities int a mask-immune frm such as a name cde (e.g., Estes, 1978). If pririty fr later prcessing is the cause f size-dminance effects, then cmpetitin (and thus the decrement fr smaller frms) ught t be greatest later in prcessing, when it is mre likely that cdes fr bth larger frms have been highly activated and are cmpeting with the smaller frms fr encding. An additinal manipulatin was intrduced in Experiment 2 t prvide further infrmatin abut the micrgenesis f the percept in the varius cnditins. The nature f the frced chices was manipulated t prvide infrmatin abut the availability f certain types f infrmatin at specific pints in prcessing time. Tw types f target-distractr pairs were embedded within each cnditin: featural pairs (e.g., hri), which differed by the presence r absence f an elemental feature (e.g., ascender), and relatinal pairs (e.g., bd), which had the same elements but in a different relatinship (e.g., side f ascender and bdy). Cnsistent with the idea that relatinal features invlve cmbined values n feature dimensins fr tw elemental features, crrect discriminatins between relatinal pairs required infrmatin abut the (relative) lcatin f tw elements. Fr each type f pair, ne member f the pair was used in the target string n a given trial and the ther was used as its distractr. If there are differences in the availability f infrmatin abut elemental features r abut feature relatins during prcessing, then there ught t be differences in the accuracy with which these types f discriminatins are made. This manipulatin was mtivated by feature integratin thery (Treisman & Schmidt, 1982; Treisman & Suther, 1985). In this thery, it is assumed that the presence r absence f elemental features is first registered n separate feature maps. At this stage, hwever, the lcatin f features is nt reslved, and neighbring features have nt been cnjined. Infrmatin abut lcatin and abut relatins between elemental features shuld nt becme available until after a subsequent, attentin-based prcess termed feature integratin. On the basis f this idea, ne might expect that at sme pint early in prcessing there wuld be infrmatin available abut the presence r absence f features but nt abut relatins between features. At that time, there ught t be an advantage fr featural pairs ver relatinal pairs. 2-3 The strng versin f the relatinal feature hypthesis is that bth elemental and relatinal features are extracted directly frm the stimulus and serve as integral parts f every letter's representatin. Therefre, n advantages fr featural ver relatinal discriminatins are predicted fr nrmal cnditins. Effects f abnrmality are mre difficult t predict in the alternative mdels, but in general, abnrmality ught t hurt bth types f discriminatins. 2 Further assumptins abut the frced-chice prcess are explicated here. The afrementined predictins pertain t when subjects make a respnse frm less-than-cmplete stimulus infrmatin. In ne case, subjects may use partial infrmatin (e.g., infrmatin abut features) in making frced chices. In this case, feature integratin thery implies that subjects are mre likely t have partial infrmatin abut feature presence r absence than abut relatins between features. In a secnd case, subjects might use partial infrmatin befre the frced-chice prcess. That is, they might use partial infrmatin during the stimulus display t arrive at an identificatin, even thugh it may be errneus. Feature integratin predicts that early in prcessing, infrmatin abut each f the features ught t be available, but infrmatin abut their relatinship ught t be lacking. Therefre, the errneus identificatins will ften be recmbinatins f the target letter's elements and ught t match the distractr f a relatinal pair mre ften than the distractr f a featural pair. 3 One ther issue cncerns the relative ease f different discriminatin types. Obviusly, ne type f discriminatin culd be made easier (e.g., by increasing physical differences), and this might influence the time curse f prcessing. Nte, hwever, that feature integratin thery is based n a fundamental distinctin n the basis f when infrmatin abut feature presence r absence and relatins is prcessed; specifically, at sme pint in time, infrmatin abut presence r absence is available, but n infrmatin abut relatins is available. Therefre, as lng as feature presence r absence is discriminable, different time curses will be predicted by feature integratin thery.

7 930 THOMAS SANOCKI Stimuli and Design Methd The same fnts as in Experiment 1 were used, but they were increased t 12 letters each. The 12 letters are shwn in Figure 1. There were tw types f blcks: nrmal blcks with the small r large nrmal fnt and abnrmal blcks in which all 24 abnrmal letters were used as the target set. The abnrmal blcks had tw types f trials: pure strings f the small-large r large-small abnrmal fnt and mixed-size strings with tw letters frm each abnrmal fnt. Given the purpses f this experiment, and given the results f Experiment 1, it was apprpriate t have mre target items and mre string types in the abnrmal blcks than in the nrmal blcks; cmparisns f perfrmance fr nrmal and abnrmal letters with these factrs equated were btained in Experiment 1. In each sessin f the present experiment, a subject received the tw nrmal blcks (ne with each size) and tw lgically identical abnrmal blcks. Order was cunterbalanced acrss subjects and the tw sessins. Tw pairs f letters were used fr featural discriminatins (hn and gq). As nted previusly, the h and n differ by the presence r absence f an ascender. The g and q differ by the presence r absence f a hrizntal element. The relatinal pairs (bd and tf) differ by the relative psitins f the same elements. 4 Several steps were taken t verify that feature presence r absence was discriminable in the featural pairs. First, elements frm the hn and bd pairs were presented t a separate grup f subjects under the same cnditins as the experiment. The presence-absence discriminatin f the hn pair was tested in tw ways: with a shrt line and n line (crrespnding t the ascender) and with a lng line and a shrt line (crrespnding t the entire vertical strke). The relatinal discriminatin f the Wpair was als tested in a third cnditin with a lng line that varied in hrizntal psitin. Sizes and psitins f the elements crrespnded t the apprpriate letter, presented between the secnd and third letter psitin. Stimuli were presented individually at the three intermediate SOAs. Discriminatin was near perfect fr all three f the discriminatins. A secnd step was that in analyzing Experiment 2, a separate analysis was cnducted fr featural and relatinal pairs frm the small-large fnt in the abnrmal-pure cnditin. These letters had relatively large ascenders and descenders and thus larger features. Fr example, the rati f the lengths f the vertical strkes f n and h was 6:10. Treisman and Grmican (1988) shwed that pput effects indicative f a preattentive feature can be btained with smewhat smaller differences (5:8 rati; Experiment 1, easy cnditin). Finally, nte that the hrizntal element that distinguishes g frm q ught t be discriminable in all cases because it differs frm the ther elements in rientatin, which is a preattentive feature (e.g., see Treisman & Grmican, 1988). On each trial in each cnditin, stimuli were selected randmly within the same cnstraints. One target item was chsen frm a featural pair, ne was chsen frm a relatinal pair, and tw were chsen withut replacement frm the remaining letters (i.e., frm the unused featural pair and relatinal pair and the ther fur letters). Fr the frced chices, the fils fr the ne featural target and the ne relatinal target were the ther members f the pair; fr the ther tw items, the fils were chsen frm the remaining unused letters. N letters were repeated within trials (as targets r distractrs), and the frequency f featural and relatinal items was equated. In the abnrmal blcks, the fils were chsen t be the same size as their crrespnding targets. Thus, fnt-size infrmatin wuld nt aid the frced chices. Within each blck the five SOAs were used equally ften and were randmly assigned t trials. The SOAs were 17, 51, 85, 119, and 153 ms. In the abnrmal blck, SOA was crssed with the fur size types f trials: (a) all targets large, (b) all targets small, (c) featural and relatinal targets large and thers small, and (d) featural and relatinal targets small and thers large. Size types were randmly assigned t trials. Prcedure, Apparatus, and Subjects Subjects participated individually in tw sessins lasting apprximately 50 min each. A different rder fr the fur blcks was used in each sessin. Each blck cnsisted f 10 practice trials fllwed by 60 test trials; the first blck f a sessin was preceded by an additinal 20 trials f practice. In ther regards, the apparatus and prcedure were the same as in Experiment 1. Sixteen subjects frm the same ppulatin as Experiment 1 participated fr extra curse credit. Results Overall perfrmance fr individual subjects averaged between 64% and 79%. The 65% minimum perfrmance criterin was nt used in the present experiment because sme shrt SOAs were used, resulting in lwer verall accuracy levels. Abnrmality and Mixing The mst general questins cncern differences between the three main cnditins (nrmal, abnrmal-pure, and abnrmal-mixed). Cnditin had a main effect, F(2, 30) = 19.81, p <.001, and interacted with SOA, F(B, 120) = 2.68, p<.01. These data are shwn as the pints in Figure 4. T describe the effects mre precisely, each subject's prprtins f crrect respnses were fit t an expnential apprach t asymptte (Wickelgren, 1977). Because f the.50 flr n perfrmance, the functin was p =.50 + (a -.50) (1 - e~ rt ), where a dentes the asymptte, r dentes the rate, and t dentes the amunt f time since the delay (Wickelgren, 1977). Asymptte values were limited t the range.7 t 1.0, and rate values were limited t the range.01 t.09; the delay was set at 34 ms, which prvided the best fit fr each cnditin. Within these cnstraints, parameter values were selected t minimize the sum f the squared deviatins. Functins fr the grup data are shwn in Figure 4. The average rt mean squared deviatin fr individual subjects was 4 If features are very precise, then the g differs by having a diagnal segment and a hrizntal tail; the g has a slightly lnger vertical descender (see Figure 1). These are multiple presence r absence differences and d nt qualify the manipulatin. If features are very precise, hwever, then the large versins f t and/als differ by the directin f the diagnals that cnnect the hrizntal and vertical segments (see Figure 1). This difference may invlve the presence r absence f different diagnal features. Therefre, the relatinal nature f this pair is suspect, and the data will be examined by individual letter pair. A suspect pair was included in the experiment nly because there are very few (if any) ther relatinal discriminatins in the lwercase alphabet (aside frm pq, which was nt included because f its similarity t bd).

8 PATTERN RELATIONS Differences amng the rate parameters in the three cnditins were marginally reliable, F(2, 30) = 3.12, p =.06. Planned cmparisns indicated that as predicted, the rate was slwer in the abnrmal-mixed cnditin (.039) than in the ther tw cnditins, F(l, 15) = 10.65, p <.01. The rates in the nrmal (.052) and abnrmal-pure (.054) cnditins did nt differ (F < 1). This indicates that sme factr assciated with mixing the sizes f adjacent abnrmal letters slws the rate f recgnitin. The asymptte parameters differed between cnditins, F(2, 30) = 6,56, p <.01. Planned cmparisns indicated that the asymptte was higher in the nrmal cnditin (.85) than in the ther tw cnditins, F(l, 15) = 15.43, p <.01, but did nt differ between the abnrmal-pure (.82) and abnrmal-mixed (.81) cnditins (F< 1). Thus, abnrmality reduces the asymptte t which perfrmance is driven. The failure f the asympttes t apprach unity was due mainly t lw asympttes at rightward string psitins, a finding that is cnsistent with previus research (e.g., Sancki, 1988; Twnsend, 1981). The asymptte levels are cnsidered tgether with size-dminance effects in the fllwing discussin. Because mre analytic analyses reduced the numbers f bservatins in each cnditin, further statistical analyses were limited t ANOVAs. In sme cases, grup means were fit as abve fr interpretive purpses. Letter-Fnt Size The data fr large and small letters qualify the asymptte effects reprted abve and are relevant t the issue f sizedminance effects. The main effect f size was marginally reliable, F(l, 15) = 4.27,.10 > p >.05. Mre imprtant, size interacted with cnditin, F(2, 30) = 24.40, p <.001, and with SOA, F(4, 60) = 2.61, p <.01. The data are shwn in Figure 5 (pints) tgether with functins fit t the grup data. Asymptte and rate parameters are als given in the figure and are useful fr cmparing cnditins. These data qualify the finding that asympttes were lwer in the abnrmal-pure cnditin. The data fr that cnditin are shwn in the middle panel. As can be seen, there is a marked difference between large and small letters Q Q SOA (ms) 153 Nrmal Abnrmal-pure Abnrmal-mixed Figure 4. Prprtin f crrect respnses as a functin f prcessing time (SOA) fr the three main cnditins f Experiment 2. (Data are dented by pints, and best fitting functins are dented by lines.) Relative t nrmal letters (Figure 5, tp panel), the asymptte is lwer fr large abnrmal letters but nt small nes. Therefre, the lwer asymptte btained abve fr the abnrmalpure cnditin is due entirely t effects n large letters (largesmall fnt). Reasns fr this are given in the Discussin sectin. In cntrast t the abnrmal-pure cnditin, asympttes are lw in relatin t nrmal cnditins fr bth large and small abnrmal letters in the abnrmal-mixed cnditin (Figure 5, bttm panel). In additin, rates are reduced in bth cases. Thus, the asymptte and rate reductins in the abnrmal-mixed cnditin hld fr bth large and small letters. There is als evidence f a size-dminance effect. Fr pure strings in the abnrmal-pure cnditin (Figure 5, middle panel), there is an advantage fr small abnrmal letters ver large nes, F(l, 15) = 26.76, p <.01. In cntrast, fr mixed strings in the abnrmal-mixed cnditin (Figure 5, bttm panel), perfrmance is reduced fr the small letters (relative t the abnrmal-pure cnditin), and the advantage ver large letters is eliminated, F(l, 15) = 1.43, p >.20. Thus, perfrmance fr smaller items is reduced by the presence f larger nes. As can be seen by examining rate and asymptte parameters, size dminance reduced bth rate and asymptte parameters fr small letters. This result culd be cnsistent with explanatins f the size-dminance effect in terms f resurce allcatin (which predict early effects such as rate effects) r attentinal pririty (which predict later effects such as asymptte effects). Less ambiguus evidence was btained in the next experiment. Featural and Relatinal Discriminatins Nrmal letters. On each trial there was ne featural discriminatin, ne relatinal discriminatin, and tw filler items. The present analyses cncern nly the tw discriminatins. The data fr nrmal letters are shwn in Figure 6. There was n effect f discriminatin type, F(l, 15) < 1, and n interactins invlving type apprached significance. As can be seen, the functins fr these tw types f discriminatins are similar; it appears that infrmatin necessary fr a featural discriminatin des nt becme available earlier than infrmatin necessary fr a relatinal discriminatin. This result is incnsistent with the predictin frm feature integratin thery but cnsistent with the idea that relatinal infrmatin is extracted directly frm the stimulus. Discriminatin type and abnrmality. Further analyses included the three main cnditins. There was an verall advantage f relatinal ver featural discriminatins, F([, 15) = 18.97, p <.001; hwever, type interacted with cnditin, F(2, 30) = 3.43, p <.05. This interactin is shwn in Table 3. As nted abve, fr nrmal letters there was n difference due t discriminatin type. The small advantage fr relatinal 5 Fits with delay varied and rate fixed were als successful and prduced a pattern f effects similar t that reprted abve. Therefre, the present rate differences culd als be viewed as differences in delay. Fits with bth delay and rate varied were nt appreciably better than fits with nly ne f these tw parameters varied.

9 932 THOMAS SANOCKI Nrmal Cnditin SOA Abnrmal-Pure 100 SOA Abnrmal-Mixed Cnditin Cnditin Large (.86/.055) I Small (.84/.042) 200 Small (.85/.047) Large (.76/.050) 200 Small (.80/.038) I Large (.78/.038) f all letter pairs, there was a marginal advantage fr therelatinal pairs, F(l, 15) = 4.33,.10 >p >.05. In an analysis fcusing n the hn and bd pairs, there was n difference between the featural and relatinal pairs, F(l, 15) = 1.04, p >.20. The lack f an advantage fr featural discriminatins in the latter case was f particular interest because there are data indicating that the line lengths discriminating between h and n in the small-large fnt are preattentively discriminable (Treisman & Grmican, 1988, Experiment 1). Psitin and Size Psitin had a main effect, F(3, 45) = 93.31, p <.001, and it interacted with size and with cnditin: Size x Psitin interactin, F(3, 45) = 3.24, p <.05; Cnditin x Psitin interactin, F(6, 90) = 2.28, p <.05. The Size X Cnditin x Psitin interactin was marginal, F(6, 90) = 1.92,.10 > p >.05. The main effect f psitin is nt f much interest (it may be determined by prbe rder), but the interactins with size are. The interactins can be seen in Figure 7. The middle panel shws psitin effects fr small abnrmal letters (and small nrmal letters fr cmparisn). As can be seen, there is a reductin in perfrmance fr small items in mixed strings (abnrmal-mixed cnditin), but at Psitins 3 and 4 nly. The difference between the abnrmal-pure and abnrmal-mixed cnditins was nt reliable at Psitins 1 r 2 (ps >.10), but the difference was reliable at Psitin 3, F(l, 15) = 12.18, p<.01, and marginal at Psitin 4,^(1, 15) = 4.54, p =.05. This indicates that the size-dminance effect f large frms ver small frms is restricted t the rightmst psitins. There was als a Psitin x SOA interactin, ^(12, 180) = 18.64, p <.001. This interactin simply reflects the fact that there were n psitin effects when perfrmance was near the flr, at the shrtest SOA. Psitin effects at the fur lngest SOAs were virtually identical. These data are reprted in Table 4. Psitin and SOA did nt interact with size r cnditin (ps <.10) Q SOA 200 Figure 5. Prprtin f crrect respnses as a functin f prcessing time (SOA) fr each size within the three main cnditins f Experiment 2. (Shwn are data [pints], best fitting functins [lines], and asymptte and rate parameters, respectively.) O discriminatins in the abnrmal-pure cnditins was nt reliable, F(l, 15) = 1.84, p >.10, but the larger advantage in the abnrmal-mixed cnditin was reliable, F(l, 15)= 13.68, p <.01. This implies that abnrmality and mixing interfere mre with the determinatin f feature presence r absence than with the determinatin f relatins between features. One further set f analyses was cnducted n the data fr the small-large fnt in the abnrmal-pure cnditin, which had relatively large ascenders and descenders. In an analysis SOA Figure 6. Percentage f crrect respnses as a functin f prcessing time (SOA) fr featural (dtted lines) and relatinal (slid lines) letter pairs in the nrmal cnditin f Experiment 2.

10 PATTERN RELATIONS 933 Table 3 Percentage f Crrect Respnses fr Featural and Relatinal Discriminatins in Each Cnditin f Experiment 2 Type Featural Relatinal Abnrmality Nrmal Cnditin Abnrmal-pure Discussin Abnrmal-mixed In Experiment 2, the verall decrement btained fr abnrmal letters in Experiment 1 was replicated and extended. The findings prvide further supprt fr the idea that precise relatinal infrmatin is imprtant in letter recgnitin. The data frm the abnrmal-pure string cnditin indicate that fr pure string items, abnrmality has an adverse effect n the large-small fnt but nt the small-large fnt. (This result was als btained in Experiment 1.) The decrement fr the large-small fnt was a reductin in the asymptte t which perfrmance is driven. One way t explain the decrement is t assume that there are detectrs fr relatins between letter bdies and ascenders r descenders. A simple way t imagine ne type f relatin detectr might be as a template fr the upper r lwer prtin f a letter, including part f the bdy. Because the ascenders and descenders f the largesmall fnt are relatively small, they wuld nt activate such detectrs as strngly as larger nes. In cntrast, increasing ascender r descender size beynd that f nrmal fnts (small-large fnt) des nt appear t aid perceptibility. It is pssible that relatin detectrs have a limited "receptive field" (limited t the size f nrmal relatins) and that extensin f ascenders and descenders beynd the receptive field might nt affect such detectrs. In the abnrmal-mixed cnditin, perfrmance was lwer (relative t nrmal cnditins) fr bth abnrmal fnts. The decrement fr the small-large fnt seems t be a size-dminance effect (see the fllwing). Fr bth fnts, bth asymptte levels and rates were lwered. Hwever, as nted (Ftnte 5), the rate effect culd als be interpreted as a difference in delay, which crrespnds t when accuracy begins t rise abve chance levels. A pssible mechanism will be cnsidered in the General Discussin sectin after additinal relevant data are reprted and interpreted. 6 Size Dminance A size-dminance effect was btained in Experiment 2 fr mixed strings, althugh the present negative effect f larger frms n smaller frms cnsisted nly f eliminating an advantage that ccurred fr small letters in pure strings. Size dminance reduced bth the rate and asymptte f prcessing. This culd be cnsistent with the idea that resurces are allcated t large letters befre small nes (Navn, 1977, 1981) r with the idea that large letters are a higher pririty fr pstidentificatin prcesses (e.g., Miller, 1981). The relative lss fr small frms ccurred nly at Psitins 3 and 4. These were als the psitins in which perfrmance asymptted at levels cnsiderably belw 1, a result btained in ther experiments (Sancki, 1988; Twnsend, 1981). An interpretatin f bth f these findings fllws frm the idea that perfrmance in this situatin is limited by attentinal capacity (e.g., Shibuya & Bundesen, 1988; Twnsend, 1981). Perhaps because f left-t-right reading habits, subjects allcate mre attentin t leftward psitins, and perfrmance levels at rightward psitins are thus limited by reduced capacity. Indeed, Twnsend (1981) fund that perfrmance levels fr rightward psitins remained at abut 75% thrughut 100 ms f asympttic prcessing time, a result that rules ut interpretatins in terms f encding r scanning time. The size-dminance effect may be greater at rightward psitins because f the capacity f limitatins. Such a limitatin culd arise if capacity is allcated t large frms befre small frms (Navn, 1977) r if large frms are a higher pririty fr pstidentificatin prcesses that require attentin (e.g., Miller, 1981), such as encding items int a mask-immune frm (see Estes, 1978). An alternative interpretatin f the Psitin X Size interactin ught t be cnsidered because accuracy was always prbed in a left-t-right rder. Namely, the variatin in the size-dminance effect culd result frm memry r respnse strategies determined by prbe rder. Hwever, this interpretatin runs int a prblem. The size-dminance effect depends n a superficial prperty f the frms (size). It is usually assumed, hwever, that letters in a masking task are encded int an abstract, mask-immune cde fr which size infrmatin is irrelevant. Therefre, size shuld nt affect memry r respnse prcesses that use the mask-immune cde; it shuld nly affect identificatin r encding prcesses that precede memry. Thus, effects invlving size (e.g., size dminance) are mre likely t ccur at a stage preceding memry r respnse stages. Althugh changes in prbe rder may alter the relatin between size dminance and psitin, this culd happen because prbe rder affects the allcatin f attentin rather than because f a direct relatin between size and memry r respnse prcesses. Featural Versus Relatinal Infrmatin There was n evidence in the present experiments f an advantage f featural ver relatinal discriminatins. This is 6 The finding f higher perfrmance in the abnrmal-pure cnditin than in the abnrmal-mixed cnditin cntrasts with Experiment 1 (in which, relative t the nrmal cnditins, there were equal decrements fr the tw abnrmal cnditins). The difference between experiments prbably stems frm differences in the cnstraints n the frced chices. In Experiment 1, fils fr the frced chices were randmly chsen frm the six target items, s in the abnrmal-mixed cnditin the chice was ften between a large and small letter, and fnt-size infrmatin wuld be relevant. Fnt size was nt relevant t frced chices in the ther cnditins f Experiment 1 r in the abnrmal-mixed cnditin f Experiment 2 (in which the size f the target and fil was always equated in frced chices). Therefre, the use f fnt-size infrmatin in the abnrmal-mixed cnditin f Experiment 1 may have reduced the abnrmality decrement fr that cnditin.

11 934 THOMAS SANOCKI incnsistent with the idea frm feature integratin thery that feature presence r absence is registered befre relatins between features are frmed (e.g., Treisman & Suther, 1985) but cnsistent with the idea that relatinal infrmatin is directly encded. An additinal finding was that the accuracy f featural discriminatins was hurt mre by abnrmality and mixing than the accuracy f relatinal discriminatins. This result was nt predicted, but it suggests that there might be interesting differences in the mechanisms fr encding infrmatin abut presence r absence and infrmatin abut relatins. These differences ught t be explred in future research Nrmal Cnditins first secnd third furth Nrmal(Large) Nrmal(Small) Small Letters in Three Cnditins 50 first secnd third furth Nrrnal(Small) Abn-Pure(Small) Abn-Mixed(Small) Large Letters in Three Cnditins Experiment 3 The previus experiments have prvided strng evidence that abnrmal letters are less perceptible than nrmal letters. The effect is bserved as a reductin in asympttic levels f perfrmance fr the large-small letters. Furthermre, in the abnrmal-mixed cnditin, there were tw effects f size mixing: The rate f prcessing was reduced (r prcessing was delayed), and smaller items in mixed strings suffered frm the presence f larger items (the size-dminance effect). These tw size-mixing effects ccurred nly with abnrmal letters, hwever. Wuld these same effects als ccur if nrmal letters were mixed in size? In Experiment 3, there were nrmal blcks with large and small letters and a mre hetergeneus (mixed) blck, as in Experiment 2. Hwever, in Experiment 3, all f the items in the hetergeneus blck were nrmal letters. There again were tw types f strings in the hetergeneus blck (pure and mixed), but nw the pure strings cnsisted f small r large nrmal letters (nrmal-pure cnditin), and the mixed strings cnsisted f tw small and tw large nrmal letters (nrmal-mixed cnditin). If size mixing per se caused the rate reductin and size-dminance effects in Experiment 2, then similar effects ught t be bserved in Experiment 3. In cntrast, if abnrmality als cntributes t these effects, then the effects ught t be reduced r eliminated in Experiment 3. In additin, the cmparisn f featural and relatinal discriminatins was repeated in Experiment 3. Methd The methd was the same as in Experiment 2, except that large and small nrmal letters replaced the large and small abnrmal letters in the hetergeneus blcks. The three main cnditins were nrmal (nrmal blcks), and nrmal-pure and nrmal-mixed (frm the hetergeneus blck). Eight new subjects frm the same ppulatin as the previus experiments participated. Results Overall perfrmance fr individual subjects averaged between 64% and 80%. 90 Nrmal(Large) Abn-Pure(Large) Abn-Mixed(Large) Nrmal and Hetergeneus Cnditins As in Experiment 2, the initial analyses cncerned differences amng the three main cnditins (nrmal, nrmal secnd third furth Psitin Figure 7. Percentage f crrect respnses as a functin f string psitin in Experiment 2. (The tp panel cntains data fr small and large letters within the nrmal cnditin, whereas the middle and bttm panels cntain data fr small and large letters, respectively, in each f the three main cnditins.) Table 4 Percentage f Crrect Respnses fr Each Psitin at Each Stimulus Onset Asynchrny (SOA, in Millisecnds) in Experiment 2 SOA First Secnd Psitin Third Furth

12 PATTERN RELATIONS 935 pure, and nrmal-mixed). In the verall ANOVA, the effect f cnditin was nt reliable, F(2, 14) = 2.40, p <.10, and there was n interactin with SOA, F(8, 56) < 1. T describe the time-curse functins mre accurately, each subject's prprtins f crrect respnses were fit t an expnential apprach t asymptte, as in Experiment 2. Functins fr the grup data are shwn in Figure 8, tgether with the data (pints). The search space was limited t the same range f asymptte and rate parameters as befre; the value fr delay was set at 17 ms, which prvided the best fit fr each cnditin in the present data. The rt mean squared deviatin fr individual subjects averaged.028. There was n main effect f cnditin n the rate parameters, F(2, 14) < 1. Planned cmparisns were cnducted as in Experiment 2. There were n reliable differences in rate between the nrmal-mixed cnditin (.028) and the ther tw cnditins r between the nrmal (.031) and nrmalpure (.0029) cnditins (Fs < 1). Fr asymptte parameters, the main effect f cnditin was marginal, F(2, 14) = 3.28,. 10 > p >.05. Planned cmparisns indicated that there was n difference between the nrmal (.88) and the nrmal-pure and nrmal-mixed cnditins, F(l, 7) < 1, but there was a difference between the nrmal-pure (.88) and nrmal-mixed (.85) cnditins, F(l, 7) = 7.88, p <.05. These results cntrast with thse f Experiment 2. First, there were n reliable rate differences in the present experiment, in cntrast t the effect in the abnrmal-mixed cnditin f Experiment 2. This indicates that simply mixing size des nt create rate differences; pattern abnrmality and mixing are bth necessary. The secnd cntrast is that a reliably lwer asymptte was btained fr the nrmal-mixed cnditin. In Experiment 2, lwer asymptte values were btained, but nly in cnditins with abnrmal letters. This latter effect is clarified belw. Fnt-Letter Size There was an verall advantage fr large letters, F(l, 7) = 18.90, p <.01, but size interacted with cnditin, F(2, 14) = 13.48, p <.001, and with SOA, F(4, 28) = 4.33,p <.01. The data are shwn in Figure 9 (pints) tgether with functins fit t the grup data and with rate and asymptte values. These data qualify the finding abve that asympttes were lwer in the nrmal-mixed cnditin. As can be seen in Figure 9, in the nrmal-mixed cnditin (bttm panel) the asymptte was reduced nly fr the small nrmal letters; the asymptte fr large nrmal letters was unaffected. This indicates that the lwer asymptte reprted abve stems frm negative size-dminance effects n small letters. There was a strng size-dminance effect in the present data. The results fr large and small letters were quite similar in the nrmal and nrmal-pure cnditins (tp and middle panels f Figure 9). In particular, there was n effect f size in the nrmal-pure cnditin, F(l, 7) = 3.00, p >.10. Hwever, in the nrmal-mixed cnditin (bttm panel), perfrmance is markedly reduced fr the small letters but nt fr the large nes, and the difference between large and small letters is reliable, F( 1,7) = 40.72, p<.01. This is a particularly striking example f the size-dminance effect. Furthermre, c = 60 a '5 SOA (ms) Nrmal l Nrmal-Pure Nrmal-Mixed Figure 8. Prprtin f crrect respnses as a functin f prcessing time (SOA) fr the three main cnditins f Experiment 3. (Data are dented by pints, and best fitting functins are dented by lines.) nte that the decrement fr small letters ccurs mainly at the lnger SOAs. The asymptte was reduced in the mixed cnditin, but the rate was nt. This is incnsistent with the idea that the size-dminance effect is due t resurces being allcated t large items befre small nes (Navn, 1981) but cnsistent with explanatins in terms f attentinal pririty fr larger items (e.g., Miller, 1981). Smaller items may be a lwer pririty fr an attentinal prcess that encdes them int mask-immune frms. The lwer rate fr small letters in mixed strings f Experiment 2 can be attributed t particularities f that cnditin (see the General Discussin sectin). Featural and Relatinal Discriminatins Nrmal cnditin. The data fr featural and relatinal discriminatins in the nrmal cnditin are shwn in Figure 10. As can be seen, there is an advantage fr relatinal discriminatins. There was a main effect f type, F( 1, 7) = 6.07, p <.05, and a marginal interactin f type and SOA, F(4, 28) = 2.53,.10 > p >.05. Each relatinal pair was at least as perceptible as each featural pair. Thus, these results, like the results f Experiment 2, prvide n supprt fr the idea frm feature integratin thery that infrmatin necessary fr a featural discriminatin becmes available earlier than infrmatin necessary fr a relatinal discriminatin. All three main cnditins. When all three cnditins are included in the analysis, there is again an verall advantage f relatinal discriminatins ver featural discriminatins, F(\, 7) = 14.11, p <.001, and the Discriminatin Type x SOA interactin is reliable, F(4, 28) = 6.05, p <.01. These effects had the same frm as in Figure 10. The Discriminatin Type x Cnditin interactin btained in Experiment 2 was nt btained in the present experiment. The relevant data are reprted in Table 5. This indicates that abnrmality was a necessary factr in the previus interactin. Psitin and Size Psitin had a main effect, F(3, 21) = 29.15, p <.001, and interacted with size and cnditin: Size x Psitin interactin, F(3, 21) = 4.27, p <.05; Cnditin x Psitin interactin,

13 936 THOMAS SANOCKI Nrmal Cnditin Large (.87/.028) Small (.87/.025) 0) O Nrmal-Pure Cnditin 200 Large (.87/.029) I Small (.87/.020) SOA Figure 10. Percentage f crrect respnses as a functin f prcessing time (SOA) fr featural (dtted lines) and relatinal (slid lines) letter pairs in the nrmal cnditin f Experiment 3. c a. nt, F(l, 7) = 2.40, p >.10. This replicates the finding in Experiment 2 that the size-dminance effect f large frms ver small frms is restricted t the rightmst psitins (and Psitin 3 in particular). The effect is independent f abnrmality because it ccurred fr nrmal letters. There was als a Psitin X SOA interactin, F(\2, 84) = 11.31, p <.001; it had the same frm as in Experiment 2 (Table 4). Interactins invlving psitin and SOA and size r cnditin did nt apprach reliability (ps >.20). 0.9 Nrmal-Mixed Cnditin Large (.87/.031) Small (.80/.023) General Discussin Majr Findings SOA Figure 9. Prprtin f crrect respnses as a functin f prcessing time (SOA) fr each size within the three main cnditins f Experiment 3. (Shwn are data [pints], best fitting functins [lines], and asymptte and rate parameters, respectively.) F(6, 42) = 2.14,.10 > p >.05. The interactin f these variables can be seen in Figure 11. These interactins are cnsistent with the pattern f results btained in Experiment 2. The middle panel shws psitin effects fr small letters in pure and mixed strings and, fr cmparisn, fr small letters in the nrmal blck. There was a reductin in perfrmance fr small items in mixed strings, but at Psitins 3 and 4 nly. Fr these letters, there were n differences between the nrmal-pure and nrmal-mixed cnditins at Psitins 1 and 2 (ps >.10). The difference at Psitin 3 was reliable, F(\, 7) = 11.98, p <.05, but the difference at Psitin 4 was 200 Intrapattern Relatins The main finding cncerning intrapattern relatins was that verall, letters cntaining nrmal interelement size relatins were mre perceptible than letters having abnrmal relatins. This finding supprts the claim that fairly precise relatinal infrmatin is used in letter recgnitin. Relatinal infrmatin may be prcessed by detectrs that have becme specialized fr relatins that characterize cmmn letters. As nted belw, such detectrs are cnsistent with several additinal cnstraints prvided by the results. The first cnstraint is that the decrement fr abnrmal relatins was asymmetrical, ccurring fr the large-small fnt, which had abnrmally small ascenders and descenders, but nt fr the small-large fnt, which had abnrmally large ascenders and descenders. Table 5 Percentage f Crrect Respnses fr Featural and Relatinal Discriminatins in Each Cnditin f Experiment 3 Cnditin Type Nrmal Nrmal-Pure Nrmal-Mixed Featural Relatinal

14 PATTERN RELATIONS Nrmal Cnditins first secnd third furth Nrmal(Large) Nrmal(Small) Small Letters in Three Cnditins first secnd third furth Nrmal(Small) Nrm-Pure(Small) Nrm-Mix(Small) reduced by the presence f large letters. This effect can be explained in terms f the hypthesis that larger frms are a higher pririty fr pstrecgnitin attentinal r decisin prcesses than smaller frms are (e.g., Ber & Keuss, 1982; Miller, 1981). In the present situatin, larger frms may be a higher pririty fr a limited-capacity prcess that cnverts visual letter cdes int abstract mask-immune cdes. The size-dminance effect cannt be attributed t differences in baseline discriminability (cf. Pmerantz, 1983) because the effect cnsists f an interactin f size and cnditin (in pure string cnditins, perfrmance fr small letters was equal t r greater than that fr large letters). The results als seem incnsistent with an explanatin in terms f resurces being allcated t larger frms befre smaller nes (e.g., Navn, 1981) because time-curse functins in Experiment 3 indicated that size mixing causes lwer asympttes fr small letters rather than effects early in prcessing. Size mixing did cause a slwer rate f (r delay in) prcessing fr abnrmal letters in Experiment 2, but this result appears limited t mixed abnrmal letters because it did nt replicate with mixed nrmal letters in Experiment 3. A further result was that the size-mixing effect was restricted t rightward letter psitins, and Psitin 3 in particular. If psitin effects are attentinal, then the relatin between size dminance and psitin is cnsistent with the attentinal pririty interpretatin. Explanatins are cnsidered in mre detail later in this sectin. Large Letters in Three Cnditins first secnd third Psitin Nrmal(Large) Nrm-Pure(Large) Nrm-Mix(Large) furth Figure 11. Percentage f crrect respnses as a functin f string psitin in Experiment 3. (The tp panel cntains data fr small and large letters within the nrmal cnditin, whereas the middle and bttm panels cntain data fr small and large letters, respectively, in each f the three main cnditins.) Secnd, the time-curse functins in Experiment 2 indicated that the decrement fr the large-small fnt was a reductin in the asymptte t which perfrmance was driven. This effect can be attributed t lwer activatin levels f letter cdes caused by increased featural mismatches fr abnrmal letters. Interpattern Relatins The main finding cncerning interpattern relatins was a size-dminance effect: The perceptibility f small letters was Featural Versus Relatinal Infrmatin Experiments 2 and 3 als prvided infrmatin abut the time curse f featural discriminatins, which require infrmatin abut the presence r absence f elements, and relatinal discriminatins, which require infrmatin abut relatins between elements. In the experiments, there was n evidence that infrmatin abut elements was available earlier than infrmatin abut their relatinships. In fact, there were advantages fr relatinal infrmatin in the abnrmalmixed cnditin f Experiment 2 and with nrmal letters in Experiment 3. These results are incnsistent with the idea frm feature integratin thery that infrmatin abut elements is registered befre their relatins are registered (e.g., Treisman & Schmidt, 1982; Treisman & Suther, 1985) but cnsistent with the idea that relatinal infrmatin is extracted directly frm the stimulus. The results may be handled within feature integratin thery by adding the assumptin that certain relatinal features are registered n their wn feature maps. In fact, Treisman and Patersn (1984) prvided evidence that certain relatinal features (which they refer t as "emergent" features) play a rle in the recgnitin f familiar patterns. Hwever, the additin f feature maps fr relatinal features may cmprmise the nature f the thery because the number f feature maps wuld increase, and they wuld becme mre cmplex in nature. Fr example, relatinal features might be encded as hlistic cnfiguratins r shapes, but such entities wuld be mre cmplex than simple lines, rientatins, r clrs, which define the mre prttypical feature map (see Treisman & Grmican, 1988; Treisman & Schmidt, 1982; Treisman & Suther, 1985). The additin

15 938 THOMAS SANOCKI f feature maps als cmplicates the feature integratin prcess, which must cmbine infrmatin frm the separate maps. Why Use Redundant Infrmatin? The hypthesis that subjects use fairly detailed, partially redundant relatinal infrmatin is cnsistent with the mre general claim that humans use multiple surces f infrmatin in recgnitin (e.g., Oden, Rueckl, & Sancki, 1991; Sancki, 1988). By using additinal infrmatin, the recgnitin prcess gains efficiency because redundant infrmatin can be pled, and missing r inaccurate infrmatin can be cmpensated fr. In the case f letter recgnitin, subjects appear able t use fairly detailed visual infrmatin abut the current fnt t increase perceptual efficiency (Sancki, 1988). This view cntrasts with an assumptin underlying much f the wrk n pattern recgnitin, which is that nly a minimal set f features is used. Minimal feature-set mdels can explain the present results nly if it is assumed that the minimal feature set cntains relatinal features crrespnding t the relatins manipulated here. Althugh such an assumptin culd be made, the need t sharpen and enlarge the minimal feature set seems t reduce the appeal f the idea. Sensitivity t Fnt Typicality An imprtant implicatin f the present results is that the letter-recgnitin prcess seems t be fairly sensitive t the "typicality" f the letters, that is, the similarity between stimulus letters and letters encuntered in the subject's prir histry. This suggests an imprtant qualificatin f the assertin, standard in many textbks, that letters are recgnized acrss a wide variety f fnts. Althugh this may be the case, it appears that letters f atypical fnts will nt be recgnized as efficiently as letters f typical fnts. A further implicatin f this finding is that the prcesses underlying the recgnitin f typical and atypical letters may be different and that careful explratin f such differences culd be a fruitful apprach t learning abut the recgnitin prcess. Further explratin f the scpe f typicality effects is needed. Typicality effects als qualify the assertin that additinal, partially redundant infrmatin is used t increase the efficiency f recgnitin. When additinal infrmatin des nt fit existing prcessing structures (e.g., small-large fnt), it may nt be useful. Differing Lci fr Intra- and Interpattern Effects? The cnfiguratins f effects are cnsistent with the idea that intrapattern relatins affected early prcesses invlved in identifying individual letters, whereas interpattern relatins affected later prcesses such as the encding f identities int mask-immune frms. Intrapattern abnrmality had very cnsistent effects, affecting the abnrmal large-small letters in all cnditins and at all letter psitins. These effects were independent f ther effects, such as psitin, which may be related t later, mre attentin-demanding prcesses. The interpattern size-dminance effects ccurred fr nrmal and abnrmal letters but were mre cntext dependent: They ccurred nly fr small letters, nly when they were mixed with large letters, and nly at rightward psitins, in which prcessing may be limited by attentin. The apparent links between abnrmality effects and identificatin prcesses n ne hand, and between size-dminance effects and mre attentin-demanding pstidentificatin prcesses n the ther hand, wuld be cnsistent with a distinctin between early, verlearned bligatry prcesses and later, mre attentindemanding strategic prcesses (e.g., Lgan, 1980; Psner, 1978; Shiffrin & Schneider, 1977). Extracting Relatinal Infrmatin There are at least several different ways in which the relatinal infrmatin implicated here might be encded. In ne mdel utlined previusly, the present results arise at the level f feature detectrs. The set f feature detectrs assciated with a letter might include detectrs fr distinctive relatins, fr example, relatins between letter bdies and ascenders r descenders. Thus, b might be distinguished in part by the feature ascender abve and n left side f bdy. As nted earlier, a simple way t imagine such a detectr might be as a template fr the upper prtin f the letter (r as a receptive field with ff regins n each side f the n template). Abnrmal letters with small ascenders r descenders (largesmall fnt) wuld nt activate such detectrs as highly as nrmal letters because part f the ascender r descender wuld be missing. In an activatin mdel such as McClelland and Rumelhart's (1981) that was augmented with the apprpriate feature detectrs, this culd result in lwer verall activatin levels fr letters that cntain such features and cause lwer asympttes. In cntrast, abnrmally large ascenders and descenders wuld extend beynd the range f such detectrs (e.g., beynd their receptive fields). The ut-f-range prtins shuld nt affect the detectrs, whereas the within-range prtins shuld prvide nrmal activatin f the detectr. This explains why perfrmance fr the small-large fnt was similar t perfrmance fr nrmal fnts. Alternatively, the results culd be explained in terms f specialized letter units that encde relatinal infrmatin implicitly. Fr example, in a secnd mdel, it is assumed that units fr letters are size specific in that each letter unit receives inputs frm feature units f a limited size range. Infrmatin abut interpart size relatins is represented implicitly by the size-specific units, and relatinal features are nt necessary; the features culd be simple, mutually exclusive but sizespecific segments (e.g., McClelland & Rumelhart, 1981). In this mdel, an abnrmal letter wuld activate bth large and small feature units, which in turn activate letter units at different size scales. If the letter units are independent f each ther, then the activity frm an abnrmal letter wuld be divided amng at least tw different-size letter units. In this mdel, the asymmetrical effects in pure string cnditins can be explained if we assume that large ascenders and descenders activate distinctive features at bth size scales. This assumptin seems reasnable because mst types f small distinctive features wuld be cntained within the large distinctive features f the larger fnts. Thus, letters f the small-large fnt

16 PATTERN RELATIONS 939 wuld activate small features fr their bdies and fr ascenders and descenders as well as large features fr ascenders and descenders. Because a cmplete set f small features is activated, a small letter unit culd becme as highly activated as in nrmal cnditins. In cntrast, letters f the large-small fnt wuld activate large features fr their bdies but small features fr the ascenders and descenders. Neither large nr small letter units wuld receive as much activatin as they receive in nrmal cnditins, resulting in lwer asympttic levels. This mdel seems capable f handling the present abnrmality effects by using simple segment features and an implicit scheme fr encding relatins. The mdel is generally cnsistent with the claim that fairly detailed infrmatin is used in recgnitin in that feature-letter cnnectins are size specific. Nte, hwever, that this mdel des nt prvide a slutin fr the prblem f lcalizing and interrelating mutually exclusive features that was nted in the intrductin. One way t slve such a prblem might be t assume that relatinal features are detected and serve as additinal inputs t the size-specific letter units. One ther alternative is that relatinal infrmatin is encded implicitly in hlistic template representatins. Nrmal letters may be mre perceptible than abnrmal nes because they match existing templates better; hwever, the asymmetry in pure string cnditins prvides a prblem fr template mdels. Existing templates shuld nt match either the largesmall fnt r the small-large fnt. Therefre, decrements ught t ccur in bth cases, and the asymmetry btained is incnsistent with the template mdel. In additin, evidence frm similar situatins is incnsistent with the idea f templates. In particular, the advantage fr target sets f n letters f ne fnt ver sets f n letters f tw fnts (e.g., Sancki, 1988) presents difficulty fr template mdels because the number f templates required in the tw cnditins is equal, and therefre there is n reasn fr the advantage in singlefnt cnditins. Thse results seem t require an explanatin in terms f underlying similarities between cmpnents f same-fnt letters (e.g., Sancki, 1987, 1989). Explaining Size Dminance The size-dminance effects btained have been generally cnsistent with the attentinal pririty explanatin utlined in the intrductin, althugh an alternative explanatin als ught t be cnsidered. In additin, tw further aspects f the results present prblems fr a simple attentinal pririty explanatin. Lateral masking. A pssible alternative explanatin stems frm the phenmenn f lateral masking (e.g., Estes, 1978; Wlfrd & Hllingswrth, 1974). In the present situatin, lateral masking is assumed t refer t the reductin in the availability f featural infrmatin abut a frm because f the presence f ther neighbring frms and the imperfect spatial reslutin f the sensry system. Interir letters, which have tw neighbrs, are affected mre than exterir letters, which have ne neighbr, and larger frms have strnger negative effects n smaller frms because larger frms cntain mre infrmatin and therefre greater ptential fr interference. Size-dminance effects culd be attributed t the strnger masking effects f large letters. Hwever, a majr prblem fr this explanatin stems frm the finding that sizedminance effects ccurred mainly at Psitin 3 and t sme extent at Psitin 4, but nt at Psitin 2. Masking is expected t be strngest at Psitins 2 and 3 and weakest at Psitins 1 and 4. Additinal evidence against certain masking explanatins stems frm examinatin f size-mixing effects fr small abnrmal letters with ascenders and descenders. Masking culd be expected t have mre adverse effects n letters whse distinctive features (i.e., ascenders r descenders) were in regins that culd be bscured by neighbring letters. Because letters were aligned alng their baselines, the ascenders f the small abnrmal letters (small-large fnt) were quite lw in relatin t the ther fnt; that is, they barely extended abve the bdies f the larger letters. Thus, a large letter culd bscure the ascender f a small letter. In cntrast, the descenders f small abnrmal letters extended belw the descenders f the larger letters. Therefre, descenders f the small letters were nt bscured by large letters; if anything, they ught t be mre salient because they prtruded. If masking caused the mixing decrement fr smaller letters, then the negative effects ught t be strnger fr small abnrmal letters with ascenders than fr small abnrmal letters with descenders. In Experiment 1, the mst relevant and direct cmparisn cncerns perfrmance fr letters in the nrmal and the abnrmal-mixed cnditins f the abnrmal-mixed grup. These data are shwn in Table 6. As can be seen, the decrement in the abnrmal-mixed cnditin was slightly smaller fr small letters with descenders than fr small letters with ascenders. The difference is very small, hwever, and culd accunt fr nly a fractin f the size-dminance effect. In Experiment 2, the mst relevant and direct cmparisn is between perfrmance fr certain pairs f small abnrmal letters in the abnrmal-pure and abnrmal-mixed cnditins. In Table 6, data frm a featural pair with descenders and a pair with an ascender are shwn. In additin, data frm a relatinal pair with ascenders are shwn. As can be seen, the pair f letters with descenders suffered mre than the ther pairs in the mixed cnditin relative t the pure cnditin. This is cntrary t the predictin by the lateral masking explanatin. Attentinal pririty. The majr effects are cnsistent with an explanatin in terms f attentinal pririty fr large frms ver smaller frms (e.g., Ber & Keuss, 1982; Miller, 1981). The idea is that larger frms are a higher pririty fr pstrecgnitin attentinal r decisinal prcesses, and it is used in the fllwing mdel. Feature infrmatin is assumed t activate letter units at the fur letter psitins in parallel. The mask is assumed t terminate prcessing, and cnsequently there are tw main prcesses that must be cmpleted fr a letter t be recgnized. First, enugh featural infrmatin must be extracted t activate the apprpriate letter unit; secnd, the letter must be encded int an abstract maskimmune name cde (Estes, 1978). This encding prcess requires attentinal resurces, and there are nt enugh resurces fr all fur items (see Shibuya & Bundesen, 1988; Twnsend, 1981). Resurces may be required t create an

17 940 THOMAS SANOCKI Table 6 Sizes f Decrements (in %) fr Small Letters With Ascenders and Descenders in Experiments 1 and 2 Letters Experiment/Cnditin gj bdfh gq hn bd Experiment 1 Nrmal Abnrmal-mixed Decrement Experiment 2 Abnrmal-pure string Abnrmal-mixed string _ Decrement Nte. A dash indicates that these letters were nt used in the experiment. bject file fr each letter than includes infrmatin abut identity and lcatin (e.g., Kahneman & Treisman, 1984). Perhaps because f reading habits, subjects allcated sufficient resurces t leftward letter psitins, and perfrmance is limited nly by the availability f featural infrmatin, asymptting at clse t 100% (Sancki, 1988; Twnsend, 1981; present data). In cntrast, there are nt enugh resurces fr rightward items, and perfrmance asympttes at levels belw 75% (e.g., Sancki, 1988; Twnsend, 1981; present data). In this mdel, size-dminance effects arise at rightward letter psitins, in which there are nt enugh resurces t ensure encding f activated letter units. Larger frms may warrant higher pririty fr resurces because they are mre eclgically significant (Navn, 1977), mre cnspicuus (Ward, 1982), r because they are larger and attentin is allcated acrss space (therefre, items that take up mre space receive mre resurces). In any case, the effect ught t be strnger later in prcessing, when perfrmance depends mre n encding resurces than n the availability f featural infrmatin. Such an expectatin is cnsistent with the finding that size dminance resulted in lwer asympttes in Experiments 2 and 3. Hwever, further aspects f the results seem t require a smewhat mre cmplicated explanatin. First, nte that if larger items have pririty ver smaller nes, then there ught t be an advantage fr large items amng small items relative t large items amng ther large items because small items wuld prvide less cmpetitin fr resurces. There was n evidence f such an advantage. Therefre, it may be necessary t add a further assumptin t the attentinal pririty mdel. One ptentially reasnable assumptin is that different-size letters require different prcessing channels and that switching between size channels requires additinal time. Thus, the psitive effects fr large letters in mixed strings (i.e., thse having less resurce cmpetitin) wuld be ffset by negative effects f having t switch channels. In studies f the perceptin f hierarchical frms, Miller (1981) and Paquet and Merikle (1988) btained evidence cnsistent with the idea that large and small frms ccupy different prcessing channels (althugh size differences were much greater in thse studies). The idea f pririty and channel switching als prvides a partial explanatin f ne last puzzling result. That result is the reductin in rate f (r delay in) prcessing fr bth sizes in the abnrmal-mixed cnditin f Experiment 2. It is nt clear hw this result wuld fllw frm a simple attentinal pririty explanatin; hwever, such a result can be at least partially explained in terms f the channel-pririty mdel. Because f the pririty fr large channels, the encding f items in small channels will smetimes be delayed until after encding f items in large channels. On the trials in which encding the small channels was delayed, the delay will be greatest when the large frms are recgnized relatively slwly. This was the case in Experiment 2 because the larger, largesmall fnt was the least perceptible fnt (relative t same-size nrmal letters, the rate and asympttes were reduced in the pure string cnditin). Thus, there will be negative effects n the small-large letters even at shrt SOAs because their encding will be delayed by the presence f less perceptible large-small letters in higher pririty channels. Difficulties frm switching channels culd als cntribute t the slwer rates fr larger letters in abnrmal-mixed cnditins. Hwever, channel pririty prbably depends n a weighted cmbinatin f several factrs. When psitins have sufficient resurces allcated t them (i.e., leftward psitins), items in thse psitins seem t have a high pririty. The pririty fr leftward psitins may be mdified t sme extent by instructins (Wlfrd & Hllingswrth, 1974), and size-dminance effects may vary accrdingly. Salience prbably affects pririty. Salience culd favr items at the ends f strings and explain the fact that size dminance was greater in Psitin 3 than 4, even thugh the amunt f resurces allcated t thse psitins, as indexed by perfrmance, was abut equal. Other physical qualities such as brightness might als be relevant. The prperty f being a target rather than a distractr is relevant (e.g., Shibuya & Bundesen, 1988). In additin, if attentin switching takes time, then cnsistency with adjacent channels might be an additinal factr. Further research is needed n the channel-pririty explanatin as well as n the relatins amng the present phenmena and glbal dminance and size-mixing effects in reading. References Ber, L. C., & Keuss, P. J. G. (1982). Glbal precedence as a pstperceptual effect: An analysis f speed-accuracy tradeff functins. Perceptin & Psychphysics, 31, Buma, H. (1971). Visual recgnitin f islated lwer-case letters. Visin Research, 11, Digital Typeface Library. (1985). Digital typeface library. New Yrk: Authr. Estes, W. K. (1978). Perceptual prcessing in letter recgnitin and reading. In E. C. Carterette & M. P. Friedman (Eds.), Perceptual prcessing (pp ). New Yrk: Academic Press. Kahneman, D., & Treisman, A. (1984). 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