Global cooperativity of the short-range processin apparent movement: Evidence obtained

with contour-containing stimuli

J. TIMOTI{Y PETERSIKRiPon Colaga RiPn' W's@ in

Previous research has demonshat€d thst the shorl-ralge process in apFrent movement' as

studied with Endomdot cinematosrl's, etlibits global @Perativitv: that is computatioM per-

formed by local eleDents inter&t DoDlileerlv &d arc pooled Oths res€dcb usrng,rlBplavs con_

taini"e Jrt""aea -"to""s has impli@t€d the sborrrrna€ Process' but has nevq deboBtrated

slobal i@perativi tj. ln the tust of fou.r eqeliE€ntr, iL was sbom Lhal uder certaD con(oirona

3i o.*"rti,ii-, . il"*-.anse motiot Pe;pt elhibitils appuent global cooPsal'iv-itv.can hP

obtained when cotlectioc ofrandostv locst€d coDt ur€ are rotated about the center or a dsplav'

a."olt" '" r."t tttot tt'. displacene;t of p€lilbsat contoG falls outsid€ the Dormal liuil of

ii"'"r'"rt-"r.e; p;*". rxi:rimenk 2-a'were ollructed t Pmvide turther evidence- thst the' observed notiolis shot-ra;ce (i e., it can be diswt€d b9 illuminatirg tbe Dt€rstiEulus inteF

val ctr wiih dichoptic viewinstard that tne perPt i.a clobalv clq€f,atiee ( i e- uasking tle c€nto

of the display, ;here sepe;tions betwer correqmdiDs €leDdts arss frah€- ee small$t'

rcsults in a decline ir the frequency of Ieports of tne shortEr€e FrPl') (nDtrol obslNanoa

sussesr t hsl Lhe effccl producad with mast!s Fs dot dG to a deqesg i_o the o umb€r ot element s

in -tf,e

ai"prav. rh. *ciment ihat tbe disPlav erbibits s shortrsngP plocss sith slobal coop€ra

tivity rs ftfther develoPed.

A p€rceptual process can be said to efibit coop€ranv-ity when spatially adjacent local el€ments of the processinteract nonlinearty with one another (Chang & Julesz,1985). The process is said to be global if the comPuta-tions performed by the local elements are pooled and resultin a coherent percepl Julesz (1971) showed how a globalcooperativ€ process could account for the percept of afigure in depth that results when random{ot lialnes con-taining conelated and displaced regioff are viewedstereoscopically. Chang andJulesz (1984, 1985) showedthat the figural p€rcept resulting when similai random-dot FatrIes are alt€mated in apparent movenent (AM) caItalso be accounted lor by a global cooperative ProcessSince the AM process underlying the se$egation of cor-rctated, disptaced regions in altemating random-dotframes is the "shot-range" process (so called because,among its propedes, it is limited to a relatively shon sPa-tial integation iange; Bmddick, 19?4), dle inferenc fro'nChang and Jutes's experiments is that the short-rangeprocess is global and cooperative.

P.rception & lsrchoPhNi$

I d g.atctul to Cathy C@fr'eri, B€th Ldfenberg. Amy Rc@r, ddSneny Schwddt fd their compre assistaM il d.tl colecijo., lheir\ugg6tio0\ Rgarditg erFmmtal pdeduE md th€n coN€Dr\reRardiry dan Drrp@tio0. I als rhad Nom t@mr ror b"i$ingwith slarisdcal d€cisiom, Mike Bnmlein fd p.dody emmsbg ddhetping re 10 consider a oft€sponde@-mtching elplMtion of therbta. and Bomie Wolff for her cmtul pEpMlion of lhe lmu$ript.Co@po.dence hat be ad,Crssed to J. Tinolhy Petersik, D@anmnlof Pstchology, Ripon Colleg€, P.O. Box 243, Ripon, wl 549?l

Copyright 1990 Psychonomic Society, Inc.

Curr€nt literature on apporent motiotr has shovm thatthe shod-range process can be reve3led in two type's ofdlsplays. One of these is lhe randon{ot (rn) displayssrudied by Bnddick and by Charg and Julesz. Fonnally,ri displays consist of targe anays (typicafy on the orderof 100 x 100) of black-and white pixels. When viewedindividually, lhe fi'ames of such displays show no globalcontours or forms. However, if a given area of randomdots is displaced a short distarce b€tween frames andreprcduc€d in both, altemation of ihe frames can rcsultin the app€arance oi a "figure" that stands out fiom itsbackground. In such r-d disPlays. lhe Slobaly coopera-rive shon-range process is responsible for tbe generationof the perceived form. The other type of display used tostudy the short-range AM process is bas€d on sets of largercontou$. For example, Petersik and Pande 0979) studieda display consisting of a row of three solid dots (about0.5' diam) that were uniformally disPlac€d horizontalyby their interdot distance from fiame to frane. Similarly,Cerbino (1984) srudied shon-mnge Phenomena using adisDlav consislina of irianales shose horizonBl and v€Fticil almensions-*ere alout 4 5' and 2". respeoivelv.These "form-containing" fiam€s yielded bistable per-ceDts: at short interstimulus inteflals (ISIS), some short-iange perc€pt is visible, and at long ISIS a long-tange Per-ceDt is seen (see Petersik, 1989, for a rcview). Th€s€ displays also di{fer ftom the r-d displays in that the short-range proc€ss does not gen€rate new foms; rather, itsp€rc€ptual manifestation is solely ofAM over sruI dis-tances or stationarity (e.g., Pande & Petersik, 19m).

3ffi

SHORT RANGE COOPERATIVITY 36I

Despite a ralher sizable body of literature on the short-range process (Petersik, 1989), its global cooperativityhas b€er demonstrated o y with rri displays. To date,no demonstration of the global cooFrativity of the shortrange process has apFsred wirh displays consisting offrames that contain large contours or forms. It is desir-able to determine v,hether the short-range motion observed with the altemarion of form-containing fiames ex-hibits global cooperadviiy because, although lbe AMproce"sses underlyitrg lhe generation of figures in r-d displays and the local motion p€rc€pts observed with bistabledisplays have both been caled "shortrange" Oe.ausetheir spatiotemporal coDsFaiots are sinilar), there hasb€en no hdepe.dent evi&rce !o demonstrate that the sameunderlying process is responsible for both phenomena.Evidence that ihe shon-range perc€pts obtained with theattemation of form-contaidng ftames re$lt ftom a globalcooperative process would suppon the argument that thesame short-range AM process opentes in bolh cases.

In lhe present paper, a display tfiat consists of frameswhich contain contoured foms and which shows short,range AM ph€nomena is described. It is arg'red that oneof the two percepts s€€n with lhis display exhibils globalcooperativiry. The display its€lfis a vadation ofones us€dpreviously by Bell and lappin (1979) and by Bischof lndGroner (1985) in their studies of shon- and long rangeAM phenomena. Howevei, th€s€ previous authors did notconsider th€ question of global coop€rativity. Briefly, thepre"sent display consists ofa s€t of40 dots randon y placedwilhin the invisible confmes of a larger circle (seeFigure l).t Frorn frame to frame oflhe AM display, thiscicle is rotat€d about its center bv some sD€cfied anount.

Fig@ l. An emple of rbe kitrd of dioulE u$d il tne Fe$tdD€rim€ r. Ih. p€.idet€. of tbe lrlg€r cirde * Dot visible intl* €rFridenls. ALo, tbe lmls of oFn Btimdu rlots ru esied.

Ifthe ISI is relatively short and rhe degree of rorarion nottoo larg€ (the two faclors interacr), subjects repon a globalback and-fonh rocking percept. However, if the ISI ismade too long or lhe rotation too lar8e, subjects reporta random. incoherent motion of the individual dots of thedisplay. As will be shown b€low. tle 'octinS p€rcepr l\thought to result from lhe influence of fie shon rangeproc€ss because its tempoml and spatial limils corcspondwell to those exhibited with other short-range AM displays (e-9.. Cerbino, 1984; Petersik & Pantl€, 1979).Although the incoherent motion has not been studied extensively, it is possible that it resulb in pan fiom unpooled(i.e., nonglobal, noncooperative) actions of the shon-rarge process and in part from the long-rdge process.

EXPERIMEI\'T I

M€thodSiimuli md Applrdus The slinnli w€re pfepared in $e lol

lowing way. Fnst, a tenplate wa prepared by @ttirg a crcle ou1of a pi@e of grapn paper. Fony cells of lhe circle were randomlychosen a.d holes were made at their lcations by a hole puncher,subject to tne followins @nsr.ain's: ( I ) €cb quadranr of rfie circlehad l0 a I holes. od (2) holes were noclose. than 0.5 cm (ll')edge to edge. The lenplale w6 lhen placed over the @.1er of astimulus @.d ( 17.5 x 12.5 cm) and .orared by a speifi€d amou.lif nffisiry. N. , tne cir@ofer€nce of qch dot on the renplarewa rac€d in india inl wllen vi€wed fiom a distance of 153 cn.the ouler<dge t@uter+dge dimeler of ach dot subtended 0.22'(or about 13.5 ). The line deM@tins rhe cirsmfoFnce of tiedot subte.d€d 2.3' Tte diameter ofthe (invisible) circle in whichthe dols lay subiendod 4.2' . O. the ceDter of @ch cifd qas dnwna solid black dol oi the same size as rhe other stinulus do1s. Ttissen.d 6 a firation point dunng the experinenr. When completed,there were sil sels of stinulus cards. one for qch of six anellarrotatjoN beteen the srimuli of ech sd. The dgular rctatios wse5 ' , 1 0 ' , 2 0 " , 4 0 " , 8 0 ' , a d 1 3 5 o .

Each of th6 two ftam6 of a display ea pr4enred throlgh onochamel of a tl're-chamel tachisloscope (Scientific Prototyp€,Model GB). Stimuli were dcwn in india irl on whit€ @ds, andthos. subtended a visual atrgle of 6.6o horizoni,lly and 4.6" venically. The lumirc of the whiie p$ts of ihe stimuli qd 4.2 c<VdP;of the black, 0-67 cdln'.

Puedu. Subj@ts vere run individually. Upon ariv.l, @cbssbjet w6 told tbe mturc ofthe ta3k ard ws shoen two diagmdat depicred the types of hotion ihat could be perceived du.ing$eexperimenl. Oneryp€of notionw6 called'r@king motionbequse a[ of rhe dots of rh€ display c@ld be sen to pivol backdd fort! arcund the fiiation point. Tle other typ€ of notior warefed€d to d random" modon b@use ech dot could be p€r-ceivd ro move h sone direcrion indeEndent of the othe6. For$epurposeofst blishing rspos€ cnLeda, Lhe.ubjds werc instructcd to idodry ody the displays in vhich all dors partjciparedin global pivoaing a teting. ' Displays with ey noti@ble devi-atiotrs ftom global @khg were 10 te called '1mdon. ' The sqb-jeals werc told that if they Frceived ione orher type of oorionduring tbe experinent they were to <tescnbe ii @retully- This @-cured ra.elv. and when il did the desc.iDtion ws Morded sdlle rirr w; rerun later h ile ssion.'�

'

The subj@is w* ne sbown to a lighr-pr@f obsenatlo. boxi. which €yepim had b@n plac€d for obsdation of lhetaclistGcopic dirplays. Tney wqe then shown rh.e ermplq of.@king nolion ed th@ of randod moIiotr s practie dd s @-choB for tuturc judgmenis. the tining of the pn tice displays w6ditrerc Aod dy used during the erperinenr proper.

o oo

O O U

o o o o

362 PETERSIK

The elpe.iment consisred of rhe p.esenrarion of 24 displays ihdtresulted from tie fadorial combination of the six rotlrion dglesand lbur IsIs (10, 20. 40. and 80 msed. Each sas viewed bbocularly five tinqs in each oftso exp€rimental sesions, makjng a tolal of 24O fials over two 60 din ssios (120 ri'lvse$ion). Trirtswere blcked in lhe fotlowing *ay. Fi.st, the s(muli oneslDnd-ing to a single rolatiod angle pere placed i! the tachistoscope. NeIt.bu r r ia l " core 'po ld rng to ea-h o f |ne toJr l s t r qere rn i r d rcndon ordcr. A new rotation angle was rhen randonly seleled, sub-ject to rhe constraint 1har no roration angle w6 repeated uniil uchof$e rcmining llve bad b@n used once. During trjals, de stinu-lus duration of @ch cdd wd 200 msa dd t]le ISI wd det. Stinuil pe.e shown for six cycles, Rhere one .ycle was Sdm-ulus l-Isl-Stimulus 2-ISl. FollowinC rhe comptete exposureof the sn6Ldl, the subje.t wa ro Bpond eithcr "r@ki!g , or .tan-don" as quickly 6 possible. No feedback was prorided.

sub j ( |s . I te .ubsa were 18 \o lu l rd r p r ) lhoro$ mr jo - ,Twelve of these we.e female, six nale. Each reporred notu3l vi,son,9.e4liring coftective lenses. All subjets were betwen l,\eages or 19 and 36 yean.

Results and DiscussionFigure 2 shows the results of the exper'nenr expressed

as the percentage of "random" movement reported ineach condition. A number of trends are apparent in rhedata. First, rotation angle was found to have a significantmain eflecr in a repedred mer'ures ANOVA [F(5.85) -1i4.79, p < -0011 such thai increases in roration anglewere associated with increases in repons of random move-ment. However, there were three groupings of dara basedon rotation angie. Data for 5' and I0' rorarions were notsignificandy different, tror were dara for 40". 80., andl.l5' rotation\ rboth p. L95.,. A\ cdr be ieen itFigure 2, data points for rhe 40', 80'. and 135' condi,trons overlapped substantialy (top line of fi_zure). Thethird group consisted ofthe data fo.the20'stiEluli. Posthoc comparisons showed these three groups io be signifi-candy different from one another (ail ps < .001).

Figure 2 also shows an effect ofISI: as ISI increases,the percentage of random movement repons increases foreacb grouping oldala. Overal. tiic main etfecr was srg-nificant in the ANOVA tF(3,51) = 14.16, p < 0oll.A variety of cuFe fitting routines we.e attempred withthe three collections of data, and the best fits were foundto be logarithmic. The best-fitting lines and their equa-tions are shown in Figure 2 for the averages of the con,ditions contained in each grouping- R1. or the proponionofvariability in the dara accounted for by the logaritlnicrelationship, is also shown for each grouping. In abso-Iute terms, variations in ISI had greater effects onresponses obtained with the 20' stimuli rhan wi.h thestimuli contained in the other two groups. Reasons forthis difference wili be considered below.

On the basis of the ISI effecl, the abrupt changeoverftom a coherent to an incoherenl p€rcept at some criticaiISI (a phenonenon visible in the data for individual sub-jects and discovered in pilot studies, but ror readily ap-parent in the averaged data), and the subjective globaliryof the percept, it can be tentatively held that ihe rockingmotion obse ed with the presenr displays is due to theshort-range process in AM- This argument is supporredby lhe finding of a significant main effect of rotation angle: as rotation increases, and thus as the distance betwe€ncorresponding dots from ftame to frame increases, thepercentage of rocking percepts decreases (cf. Bmddick,1974; Pantle & Petersik, 1980). However. .he ANOVAdl.o,howed a .ign;ficanr rotarion angle lSl interac-tion [F(1s,25s) = 1.13, p < .051.

Figure 3 shows the growth of the linear distance ber*een corresponding elements of the iwo toames ofrhesedisplals as a fincrion of their drslance (on a radiut fronthe cenrer of dre invisible circle. Distances corespond-rng lo four rotation angles are depicted: 5', 10', 20",and 4O'. This fisure can b€ used !o enimate the number

j - . 4 - . 1 . , 2 r . . 2 P 2 = a a a

: ( n s 0 . )

Fielr 2. R6ul(s of Experimmt 1. B@w gaphs itr studi6 tbrt lsy€ min€dIhe sbort-mnA€ p':N with contodontatuing sliml|i hv€ ldopted ine conventiotrol showing the Ftunlagc of rc@.ts ol th€ dterutive F.cept (i,e., tn€ mtr-sio.r-dng. p€rceF) otr tI€ ordiMae, heE dF Ffte as! ol .ndlom'Epo|s i, shovnes a lunction of ISI dmtiotr. The degN of robtion of condDoodiI: elehenrsacr6! fnm6 is th€ pardeter, B€st-fitting lin6 md loge hmic €qu.tiotr eshom for three eliedt colstios of dara.

of points whose shifts b€tween ftames fatl b€low andabove a hypothetical spaaia.l integration range ofthe shon-range process. For the prcsent displays, that hypothetical range falls b€tween a low of 15' (as conservativelyestnnated by Braddick, 1974, to be an absolute limit ofthe short-range process, a point of view no longer wellaccepted; s€€ Petersik, 1989) and a high of 16.6' (a lib€ralfigure based on 123% of the dot diameter, as estimatedby Petersit, Pufahl, & Krasnoff, 1983). Using the 15'figure only as a basis of comparison, it can tre seen fromFigure 3 rhat with a 5' rotation, all dots (regardless ofdistance from center) are shified tlirhin the putatire :pa-tial integration range of the short-range process. For thet0" rotatio! stimuli, dots fa ing within about 80'ofthecenter lie within the sho(-range spatial limit; thus, about25 doLs lie within .he spatial limit, about 15 outside it.The corresponding distances ftom the center that demar-cate the hypodEtical spatial linit of the short-rangeprocess for th€ 20" and 40' rotation stimuli are 60' (abort19 dots inside and 21 dots outside) and 20' (about 6 dotsinside and 3'4 dots outside). For rotations of 80' and 135 ",nearly a! shifted dots fal outsid€ the sparisl integarionrange of the short-range proce,ss. On the basis of this anal-ysis (along wirh the nonlinear effect of ISD, it is hypothe-sized that the shorl-rarge process exhibits global cooper-ativity in the present contour-cotrtaidng displays: byd€finition, rocking notion was reported only when all dotsofthe display were s€€n in a synchronized rocking move-ment. Thus, for example, when rocking motion was s€enwith the 20' rotation display, about 19 dots faling withinthe integmtion range of the short-Enge process influencedrhe perception of rhe 21 dots falling outside it.

SHORT,RANGE COOPERATIVITY

The existence of some reporc of rocking motion forstimuli rotated in the 40'- 135' range requiies additionalexplanation. For the 40' rotation stimuli. it is possiblethat the approximately 6 dots falling within the hypothet-ical spatial integration range of the shon-range processoir3sionaily irfluenced rhe 34 dots lying outside that rangesufficiently to produc€ a global per€ept of rocking move-ment. For the stimuli with larger rotations, it is possiblethat r€pons of rocking motion reflect a rcsponse bias onlhe pan of the subjects. Altematively, they may reflectguesses in situations in which the subjects were uncer-ra in about rhe Mrure ot their Frcepts. Under any c ircum-stances, it is clear that repons of both rocking and ran-dom motior occur with sone nonz€ro probability at any

ff lhe hypothesis that in the general cas€ rocking nove-ment represents global cooperativity of lhe shori-rangeprocess whereas random movement repres€nts the break-down of global caoperativity is true, this would explaintbe significant rotation angle x ISI interaction: ISI hada greater influence on the 20' rotation condition rhln onthe other t'ro groupings of daia. This may be due to thepossibility that, spatially, most ofthe dots at the 5' and10" rotations fall in or near the int€gration ]imit of theshort-range proc€ss, whereas most of the dots at the.!0" - 135 " rotations fall outside. Thus, ISI would have lit{eopportunity to affect these percepts. However, at the 20"rotation, it may be that a significant number of dots lieat or near the border of the short-range integration limit,and drus ISI can innuence the per€eF reponed.

Furlh€r evidence that the p€rception of rocking motionin lhe rotating displays used here is mediated by tho shon-

E

B

!

0 5 0 r 0 0 1 5 0

Dislance holn C€nler (arc min)

IiAG 3. G{Flh ot th. IiE! dis|lF b.t*en ffiFndirg eLfttrts ol rhetund6dlt AM dspLr 6. tudih.a &.n d--' nm tn€ di.plry @t4,Es! of rh€ f|[ liD6 sh.m rhir Ao*rtr 16 . rti|I.@r @a ru& ol .rr-ettor

0

SHORT RANGE COOPERATIVITY 363

!

5

Dislance lrom cenier tarc min)

Itg@ 3, GoFlh ot l!. lilEl 46c b.rrs @lclor.drs el.@nts of ttetro FG oari. AM.E$sy s a nrn nd.a ln.n ddrc nd tn. didly @IE,l*! ot the ftu litr6 shors lhis snwt[ ae s did€dr Esritu.L of..r.tion

of points whose shifts between ftaaes fall below andabove a hypothetical spaiial integration range oflhe sbort-Iange process. For the present disptays, that hypotheti-cal range falls betw€en a low of 15' (as conservativelyestimated by Braddick, 1974, to b€ an absolute limit ofthe short-range process, a point of view no longer wellaccept {i; s€e Petersik, 1989) and a high of 16.6' (a liberalfigur€ based on 123% of the dot diameter, as estirnatedby Petersik, Pufahl, & Krasnoff, 1983). Using the 15'figure only as a basis of conparison, it can be seen ftomFigure 3 that with a 5" rotation, all dots (regardless ofdistance lrom center) are shifted within the putative spa-tial integration range of the short-range process. For the10" rotation stimuli, dots falling within abour 80' ofthecenter lie within lhe short-mnge spatial limit; thus, about25 dots lie within the spatial limit, about l5 outside it.The co.responding distances ftom the c€nter that dernar-cate the hypolhetical spatial limit of the short rangeprocess for th€ 20" and 40" rolation stimuli are 60' (about19 dots inside and 2l dots outside) and 20' (about 6 dotsiffide and 34 dots ortside). For rotations of 80" and 135',nearly all shified dots fall outside the spatial integrationrange ofthe shon-rang€ proc€ss. On the basi! of this anal-ysis (along with the nonline"r eff€€t ofISD, it is hypothesized that the short-range process exhibits global cooper-ativity in the present contourrontaining displays: bydefi ition, rocking moiion was reported only when all dotsofthe display rvere seen in a synchmniz€d rockiry move-ment. Thus, for example, when rccking motion was s€enwith the 20" rotation display, about 19 dots falling withinthe integration range ofdrc short-mnge process itrflu€ncedthe perception of the 21 dots fa ing outside it.

Th€ existenc€ of some reports of rocking motion forstimuli rotated in the ,10'- 135' range requires additionalexplanation. For the 40' rotation stimuli, it is possiblethat the approximately 6 dors falling within the hypothet-ical spatial integration range of the shon-range processoccasiona y influenced lhe 34 doti lying outside that rangesufficiently to produce a global percept of rocking move-men . For the stimuli with larger rotations, it is possiblethat reports of rocking notion refl€ct a response bias onthe part ofthe subjects. Altematively, they may reflectguesses in situations in which fte subjects were uncer-rain about rhe nature ot their perceprs. U nder any circu m-stances, it is clear that repoft of both rocking and random motion occur with some nonzero probability at any

If the hypothesis that in the general cas€ rocking movement repres€nts global cooperativity of the short-rangeprocess whereas random movemedt represents the breakdown of global cooperativity is true, this would explainthe significant rotation angl€ x ISI interaction: ISI hada gieater influence on the 20" rotation condition than onthe other two groupings of data. This may be due to thepossibility that, spatialy, most ofthe dots at the 5" snd10' rotations fall in or near the integration limit of theshort-range prccess, whereas most of lhe dots at the,10'- 135' rotatioff fal oulside. Thus, ISI would have litdeopportunity to affect these percepts. However, at the 20o.otation, it may be lhat a significan number of dots lieat or near the border of the sho(-range integratioo limit,and thus ISI can influence the p€rcept reported

Further evidence that lhe perceplion of rocking motionin the rotatiog displays used here is mediated by the shon-

0

364 PETERSIK

range process is presenred in the next experiments. Fol-lowing that, additional evidence will be presented for thehypothesis that the perc€pt exhibits cooperativity.

EXPERIMENTS 2 AND 3

Braddick(1973, 1974), Pande and Picciano (1976). andPetersik and Pande (1979) have shown that the activityof the short-range process can be disrupted by brightlyiliuminating the ISI b€tween stimulus fiames or bypresenting stimulus frames dichoptically. At the sametime, Pande and Picciano have shown that long-r.nge AMis not affected by dichoptic presentation or illuminatedISIS. Therefore. these manipulations were made in thepre.enr exp€rimenls to delermine $helher rockrog motion would disappear with illuminated ISIS alld dichopticviewing. Positive findings wo d suggest that rocking rno-tion is mediated by the short-range process.

MethodStinuli $d App&alus. Tbe sdmuli for @ch exp€.inent cor'

sisred ofthe 10'and 80'rotation stinnli used in Exp€rimenr IThe viewing distance and srbsequent sizes of the stiouli *ere thesane as in Experimenl L In EiPerimenr 2, on half of rhe dials.ISIS wer€ illunituied to match lhe luninance of tbe vhite &ck-grounds of rbe stimuli (4.2 cd/n'). In Experine 3,theISI$6again dark. bnt on h,r ol the riais L\e stimuli were pFnI€d dichcP-tically by noa.s of polarizing filtes.

Subjecls. Ti. 'ubl*ri sere 20 volrnr*' p.rcroroS\ tujon.l0 of each gerd. A.l rep.ned nom"l \. on, - Frs coddirelenses. Eeh snbjei seNed id tDlh experimerts.

Ptued@. The subj@ls we.€ run individuauy. Each sulje1 etfir$ inroduced to exanples of rcting dd random moleEnt, atrdthis was followed by 10 praclice r.iah with stimuli pi*ntad ain Expe.inent I (i.e., bi.o@ld viewing Pitb a dart ISD. Nert.Eperiments 2 and 3 sere run in a redonrly detemined o.der.Experioent 2 consisted of eighl slimulus presentaions lesulti.giron the factorial combinadon of lwo lotatio.s (10" dd 80"), tsoIsIs (10 ard 80 mse), md rwo ilumination condnions of6e ISI, l iBh v . . da i l r . E ch . t iu lu " p re .enr , , ,on Lon. r red o t rven Lvcls of the t{o-frame stinuli. Trials were run in a random o.der,dd sdmulus duratiotr pas 200 ns4. Folloving *ch t.ial, lhe subjet rcsponded either 't@king norion or "random notion." Ex-perine.t 3 comisted of eight slinulus prdedations .$nlting ftontle factorial combination of the sme two rourioN and ISIS, aloDgwith two viewing condinons: binmdd vesus dichdplic Again.rials sere run in a rddom order od sdnulus duradon was200 nsec, The subjecfs lask was thc sde as in Exp€nnenr 2.

R€sults and DiscussiotrThe results for Experiment 2 are shown in Table l. As

can be seen, illumination of $e ISI nearly eliminatedreports of rocking motion. It was observed that the elimi-nation of rocking motion required some time to develop(p€rhaps a light-adaptation process was al work); at thelo-msec ISI, this was about five cycles of the presenta-rion. After rcling morion c€a\ed under rumnadon conditions, random motion dominated.

The results of Experiment 3 3re shown in Table 2. Theestablishment of motion judgments proved to be difficultwith dichoptic viewing, and on many trials subjectsreported that they saw no moYement or movement lhat

I.ble IF.equ€my of Repo.rs of Shorl-Rdge iRakitg) Motion

ir ErPerinetrl 2 (n = 201

L elt lslDar[ ISI

Tlble 2F.€quercy of R€porls of Short-Rtnge (Rcling) Motior

h ExFrim€tt 3 (n = 20)

Rotation 10 rec ISI 8cns4 rSI lo_n€c ISI 80-ns{ Isl

zl 62

t710'80'

233

t 83

1 0 "80"

was nei6e. rocking nor random. Thus, rather than repeatmany irials in which a motion judgment could not bemade, a third response category was added. "other."'

As Table 2 shows, dichopiic viewing reduced the number of rocking-notion .esponses, particularly at thelGntsec ISI. However. random motion was only reportedon about 50% of these trials.

Because an illuminated ISI and dichoptic viewingproduced rather dramatic declines in the reports of rock-ing movement. and because these manipulations hrvepreviously been shown to feduce reports of short rangemovement, the results of these experiments turther sup-pora the argument that rocking notion is produced by theshon-range process in AM. It should b€ notei that at leastsome rocking motion was reponed in all conditions ofthese experiments (see Tables I and 2). It is not clearwhether such repo.ts of short-range movement undersuboptimal conditions represen. incomplete eliminationofthe short-range percept or response bias ln any case,it is not uncommon in studies of AM for subjects to reportshon-range percepts under conditions that favor the long-range process, and vice versa (e.g., Pantle & Picciano,1976; Petersik & Pantle, 1979). The conclusions offeredhere are based on the relatively large shifts in the proba-bility of reponing short-range movement over the experimental conditions. Nonetbeless. the existence ofreports of rocking modon in all conditions requires thatthe data be interpreted with caution- The final sn'dies inthis series provide additio,al support for the a.gurned thatrocking motion also exhibits global cooperativity.

EXPERIMENT 4

On the basis of the results of tbe first three exp€riments,the suggestion was made that the rocking motion perc€ivedin the presenr displays is based upon the short-rangeprocess in AM and that the percept exhibits glohi coop€rativity. Expenment 4 and the subsequent control study

i

SHORT,RANGECOOPERATIVITY 365

were designed to further test the hypothesis of globalcooperativity. It was reasoned that sinc€ dot elementscloser to the center of the imaginary circle have smallerlinear s€pararions b€twe€n fiames than do those closerto the circunference, they contribute lnore ro the rocking p€rcep Oy cooperative means) than do rhose further{iom the center. Th€refore, placing a mask over dots ne3rtlle center of the srimulus fram€s should reduce oreliminai€ reports of rocklng motion. depending upon rhesize ol fie mask. rhe degree of rorarion betwe€n framer.and the lsl. On lhe orher hand. ptacing an annurus ove,dols near the circumference ofthe invisibte ci,cle should.if an),tling, increase reports of rocking motion Oy remov-mg comp€ting random" signats).

M€lhodSltnuli md Appa.rrB. Tne appamtus wa the sme 6 usett

'n tne prvious erp€rinenrs. Th€ srimuii we.e rhe sme .s nsedp.€viously, with the Iollowing modifications. The number of tura,lons wa.educ.d ro four 10", 20.,,tO., and 80._ Thre sets ofstimuli wft eb6eqqent! prepared, two with eoter Mr.j ofdifferot sB aql one wnh e outs mulu mt. Tbe li6r se! of srinuliGmall msk) had a *hire circle osred over the enier of the stinn-lus dois. When vi.wed, rhis cncb had a dimeter of 1.4". The blac&fintion point was .edawn on $e cents of rhe l@t ru xludna fiuLion pomt The saond set ot stisuti narSe @kl $a \m.larly prepared. bur wittr d rust hlvrns a dimerer of2.9". Tjrerhird set had d outer mulus whos width subbtrd€d 0.96. _ Tlisleft visible r cents a@ of dors haviry a diderer of 2.28..

Thus, phen viewed, |lne slinuli wirb the smrl dask had e o erar@ h which appmxirotely 27 dors w* visibte . Tte ldce @kallo$el apprctirurel) l2 dots !o br vnrbte rhe smutus-pemilred apprc\irolelt 21 enra! dd\ ro be v'sibte.

S{bjEls. Two srcuF ofsubjer w€r. M in two ditrft oha6of ile experinenl ltirty subjers sed€d in the fiBt ph6e, in;hichthe nasked sdmuli were ued. Tweny strbjecrs seNed i. the seond ph6e , in which nnmked stimuli were ru. to obtai. bselinedata. All subjeds were volunte. psycholocy studenh, @porringnorulviqion. Tlere pere appro\mdrety equat nunb€^ of mtf.

h@dure. &ch d he rht rO qub|els $aq Srven a erptanaron or rfttung and radom movemenr 4 in rhe pre\iou e\p€ri.mentl. again followed by t0 phcrico t.ials. Nexr, the subjet wdrandonly expGed to 24 srinlu trials fomed by rhe fa€rorial @bbiDuon or four roktion angles. No ISts (lO dd 80 tuer, dolhe de nE* @ndiuotu. As Fehdsly, rhe ISI wa dft M ! Es-ingrd bincule. Sdnd6 duBlion wd 150 msec. Again fotowirg@ch rial. rhe subJd *s ro espond rl erder.c[n! or @dom no or had ben mr&iv.n

The sond phde of ihe experimenr @rsisred of the oflelio,of onirol dala fron 20 addiriorat subjft1s. IrE pnEd@ all.! vr*'ng condnioro weF Lhe sme as in Lhe fiat phaF. rtceor $rt no@rs qer. preseni in the stimuli.

R€suItrThe results ofboth phases of the experiment are shown

in Figure 4, where the data are ptotted separately for thelo-lnsec ISI (Panel a) and ae iO msec r3t pairer u;r asa tunction of rotation angle. S€parate curves .re shownfor each mask condition. The first findinq of nole is thatthe data fron th€ control conditions coneaDond well withthe data from Experiment I Cigure 2), wirh the followmg exceptions: for the lGmsec, 80. condition, there were

(b)

Rotation Anste (des)

Iiaurc t. Raulii oa ErFrnnqt 4 vith tt€ lerent ge oa sm-ilm' cpoM sh.vn a r lumriotr ol dt. rl€sc ot ror.don betw@ onalmdin3 ft.Im oltlE AM @ir. S€pd.L w6 shotrbe Bd.s obtriEd witb c.nt.d EAt ! ot drtftrur sic. u dtstdulu, ed unmsk.d (cortrcl) stinuli. (s) R6uli! obt ired witnr IGnF ISl. {b) Rqutll obr.ircd rith m glM rsr.

fewer rep,orts of mndom movement in the present experi-mentt for the 8o-msec, 10" condition, there were som€-what more reports of random movement in the Dresentexperimenl. These differencescould b€ due to rhe difier-ence in pro(€dure in rhe rwo experirEms. Under any circumstance, the trends shown in the ilara are verv simitarfor $e rwo experimenrs.

Exanining the data for the l0-msec ISI snown mFigure 4a, it can be s€€n that the.e was no differenc€ mthe .eports of random movement between the small andlarge masks. Howerer. the di\rributions of response frequencres upon whicb rhese da|a are based wer€ !ho*n robe significandy different for the average ofthe two maskconditions and the control condition lx'el = '7.43,p < .051. On the other hand, ihe controt data were no.distributed differendy from the data obtained with the

366 PETERSIK

outer annulus tx'(3) : 2.10, p > .051. To evaluatedifferences between nask and ainulus conditions (and inview of the fact rhat rhe data are from relaied samples),the McNemar lest (Sieget & Castellan, 1988) was appliedto lhe liequencies of random and rocking responses fo.each (ondi l lon dr each rokl ion. Since lhere $ere no sig-nificant difierences between the large- and smal-annulusconditions, the present statistical tests were conducted ondata obtained with the large annulus stimuli only. In theoveral analysis, this seemed to provide the most conservatjve test olthe null hypothesis. w}ten expected ftequencies were ,Inall. fie binomial re't s!s applieo Frequencies between mask and annulus conditions weresignificantly different for data obtained with the 20' rotation stimuli [/(1) : 16.06,p < .0O1, McNemar tesr]and with the 80' stimuli (p : .035, binomial test). Theftequency difference with the 10' stimuli approached sig-nificance (p : .062, binonial teso and with the 40'stimuli, failed 1o reach significance (p = -50, binomial

Chi-square tests were also conducted on the frequencies that correspond to the percentages shown mFigue 4b. Again, it was found that although the data werein the predicted direction, the daia for the controi condi-tion were not distributed differently from the data in rheannulus condition lxl(3) = 3.34 and 2.74, respectively,both ps < .051. Again, McNemar and binomial testswere applied to the frequencies obtained with each levelof rotation for the So-msec ISI (large-annutus stimulionly). Frequency differences were significant for the 10'stimuli tf(l) : 8.64, p < .01, McNemar testl and forthe20'st imul i lx ' ( l ) :8.64.p < .01. McNemar test l .Differcnces failed to reach significance for the 40' and80' stimuli (p : .3'77 and .112, respectively, bothbinomial tests).

On the basis ofthese data, it can be conclud€d that thereis evidence that masking the central dots in the apparent-rotation display d€creases the ftequency ofrNking-motionpercepts rela.ive to the absence of such masking. At thesame time, this experiment was not able 10 discriminateeffects due to the sizes ofthe ftrsk. Also, there was sometrend in the dal,a to suggest that an outer annulus increasesthe frequency of reports of rocking movement; however,this effecr was nor significant. Thus, it seems likely thatplacing a mask over dots fiat have the least linear separation f.om frame to frame feduces the strength of theshort-mnge motion percept (compared to a no-mask con-dition). This, in tum, coutd be due.o an elirnination orreduction of cooperalivity nomally produced by thosestimulus elements.

CONTROL STI]DY

The argument could be rn de dlat the de.line in reportsof rocking movement found with the central masks usedin Experiment 4 could be due to a decrease in the num-ber of elements available with which to solve the cor-respondence problem. This argument do€s not seem ten

able because the use of an outer anflius in Erpenment 4did not reduce the number of repolts of rocking move-ment, and even increased them somewhat qith lhe80-mse. ISL Nonetheless, as a control, a rc* sel ofstimuli was made in which drere were oniy 12 dots (corresponding to the approximate number of dots visible sithihe large mask of Experinent 4) . These stimuli were prepared for rotations of 20' and 40'. Three experiencedobservers subsequendy viewed the new stimuli, as wellas the original unnasked stimuli of Exp€riment I (for 10'and 20'rotations), at two ISIS, l0 and40 msec. Twentyobservations were made fo. each stimulus set at each rorduon and lsl. The frequency ol repod\ of rocking motion for each condition for each subject are shown inTable 3. As can be seen. the decrease in dot density ledto more frequent repons of rocking motion (pe.haps byelim;nating the number of possible fatse matches), contr!.y to predictiofls based on an absolute numb€r" ar-gumem. Funhermore. the 3 subjects observed no obvi-ous dilferences in the quatity of AM with the two setsof stimuti. Therefore, it seems unljkely that the declinein the absolute number of stimuius elements accounts forthe de.line in reports of rocking movement in Experinent 4 Out see Generat Discussio').

GENERAL DISCUSSION

The present exp€riments present evidence supponingtbe following conclusions. (l) The "rocking" motion ob-served in the rotary motion displays is rnedialed by thesame short range process previously described withresp€ct io bislable. contour containing displays (€-9.,Braddick & Adlard, 1978; Gerbino, 1984j Pafltle & Picciano, 1976; Petersik & Pande, 1979). This conciusionis supported by the fact that rocking motion is dominantat shon ISIS and smali separations between co.respond-ing elements across frames (Experiment l), aIId by thefact that rocking motion is disrupted by illuninated lslsand dichoptic viewing (Experiments 2 and 3). (2) Therocking rnotion observed in the present displays exlibitsgiobal cooperativity. This is supported by the observation that rocking motion is perceived at the perimeter of

T&bte f,Pererrrg€ ol R€po.b of Short-Range (Rdkind Morioni.-!Eie!qeL!!q llleqsg __

Roction lGnsec ISI 4{ ms& ISI lo-ns* IsI 4+Na lsl

20',10'

20'

20'

90

0

1020

lLrol0

8520

10025

lo0100

Il]o95

100

tm100

l(x)95

the circular stimulus areas (ad, inde€d, rhmushour theenlire stimdus area). even *hen correspondin[rdmuJuselements across fiames are s€parated by disrances out-side the presumed spatial integration limit of the short,nnge Focess (ExlEriment l). In addition, maskinq areasin lhe center of rhese circular disptays (areas in whichstimulus-€lement separations are within the short ranseinteg?tion limi

decreases rcporrs of rockmg morion fd-penment 4). (l) By interence, on $e basis offie firsr rwoconclusions, and because certain similaities berween rhesnort-Iange process observed in r{ displays and contourconlarning displays have airead) been reponed {e.g..Petersik. lg89: Perers* & Pande, tc7qr, rican be con-cluded that the same shorr-range pracess mediates formp€rception in attemating r d disptays and movement (orstationarity) p€rception in certain bistable conmurcontaining displays.

An ailemative inrerpreradon of rbe presenr e\perimenrsrs ba$d upon "correspondence marching (Uurnan,1979. 1980). That is. A,M disptays conraining mutripteelements presenr a iiruarjon in which rhe visual syslemmust choose, for each element in rhe first frame. a cor,responding partner in the second ftame ro which AM isimparted. Among the "guidelines" U nan (1979) iden_tified wirh d€ conespondence process are an afEniry mea,sule partially bas€d upon the distance betwe€n elementsin succ€ssive frames: all other infiuences being equal, cor-respondence matches across ftames should be befweennearest neighbors. In terms of th€ present exp€riments,tne greater the rotation of stimulus etemenrs Derweenframes, the grarer should be the liketihood of falsernatches, that is, the greater wil be the chance that thesecond-frame nearest neighbor of any first-fi-ame dot wilbe different ftom that dot's own rotated Dosition inFrame 2. Since such matches should be random {r.e..based upon rhe originai random pos'rjoning ofrhe dorr).the resulttng AM should aisogive rhe app€arance of ran-domness. Thus. the increase ij repons ofradom mov€.ment with increases in angle of rotation coutd reflecr aninuease in fals€ co.respandence matches rather than thebr€3tdo*n of the short mtrge process. This interprela-trcn N supported by the fact thar in the control srudv. areducrion in $e densiry ot srimulus dors. which w;rddalso decreas€ the number ofpotential frlse con€,spondencematches, resulted in a d€crsse ill the relative nuBb€r ofrandom-movement rcports_

The position raken her€ is that co.respondence natch-Ing carljllo| be studied indepetrdetrtty of t-he perceptualrnerhanisrns rhar underlie the hyporhedcal sbon- ard long_range process€s. For €xample, conespondence matchesare strongly influenced by adaptation of short-mnge orroog range percep6, or by 'naniputarions hvotving ilu_nxnated ISls or dichoptjc viewing (aI rlree are effecrjveover a wide range of ISIS; petersik & pa le, 1979). In_deed, the fact that, in the present experiments, p€rc€prsoI rmdom movement were increas€d with i umimted ISISand with dichoptic viewing suggests a role for the short-mtrge process. What cannot be logica y determined fiom

SHORT-RANGECOOPERATIVITY 361

the pr€sent data is whether rhe facilitation of randommovement with increasing angles of rotation reflects anbherent limitation of the short-range process or an en-banced contribution of false correspondence narches.Probably the two are not indepndenti AM percepts arestrongly influenced by constraints on possible conespon-dence rnatches (e-9. , tflInan, 1980), and corresDondencenntches are infiuenced b) rhe nature ofthe AM processfavored by such pararneren as lst. stim'jus durad;n, andviewing cotrditions. The latrer notion is evidenc€d bv tllefad tbat correspo ence matches change drasricaliy inboth r{ displays oeading from percepts of the coherentmovement ofa co.related targer area under the influenceof the short-range proces! to perceprs of incoherent rnove,ment when the shon mtrge process is not favored) andcontaur-containing displays (e.g., the change ftom end-to-€nd movement to group movement in the Temus dis-play) as ISI pmgresses from shon to long. Therefore, anystudy of €orrespondence matching must simultaneouslyconsider the AM process favored by the disptays. Sincethe pr€sent expe.iments w€r€ designed to test AM per-cepts over conditions both favorabte ard unfavorabt€ tothe shon-range process. lhe effect of angle of robdon catprobably b€st b€ interpreted as reflecting a dominance ofthenearest-neighborprinciple of cor€spondeoce rnatch-ing under cooditjons that do not favor rbe global cooper-arivity {and consequeol rocking movement ouerwrs€produced by the short-rang€ process.

Th€ above discussion has ofiered a correspondence-natching explanation of random movement. tn keepingwith the emphasis on inrerprerations rhat atso consider A[,tpro€esses, I now consider a process-bas€d exptanation ofrandom movement. Since the present exFriments wereprimarily designed to examine the role ofthe short-ranseprffess in rocUng mo!ement. and since no experimenLltests of hypotheses relating to fandom motion have Deenconducted, the folowing argum€nrs are based solety uponobserva.ion. Some AM displays tllat contain global con-tours yield altemative motion percepts. For eiample, theTemus display used by Braddick ard Adlard (1978), pan-tle and Picciano (1976), and Petersik and pantte (1979)yields percepts of "end-to-€nd movement" and ..groupmovemenl. " The former is considered to be mediated bvlhe shon{ange process. wh€rejs lhe Laner is rhouqht t;be mediated by fie long-range process in AM (cl.

-Brad-

dick, 1974).' "Random morion" do€s not s€€m o oesolely mediated by the long range percept for the folow-ing r€asons. First, there is no common motion ve{rtor inwhich a1l elements partake, as is seen in, for example,grcup movemenr rPaDtle & Picciano. 1976,, where theelements move in toto. Second. stimulss matches overframes seem to be random and chaotic: some dols moveover short differences, some over long. Fudhemore,some dots se€m to change their motion paths over differ-ent cycles of a display. This chaotic motion seems alsoto be inconsisrent with th€ coherent motion that is offenproduced wilh the globally cooperative shon rangeprocess (e.g-, rccking motion or the establishment of

368 PETERSIK

figures in alternating r-d displays). Since global cooper-at;vity is thought to be establish€d by the spatiat poolingof shof-range activity (Chang & Julesz, 1985), the sug-gestion is made here that "random motion" is the productof the independent activity of the long-range process andunFroled shon range acriviry. This would account for boththe chaotic appearance and the range of distances overwhich motion is perceived in what has been called random modon, and is consistent with the previous observa-tion that short-range and long-range percepts can coexistin the same AM display (Braddick & Adlard, 1978).

Altbough there is no coherent overall ih€oretical structure that integrates notions of short range and long-rangeprocessing, there is a sizable body of literature that sup-pons 6e two-pr&ess distinction (Petersil, 1989) lndeid,the Dre.ent exDerimenr\ dorelail nicel) sifi fie heuri'_uc iheorerical perspetriue offered by Peret.* { 1989r.which suggests that the short_range plocess can generateglobally cooperative percepts through the spatial poolingof hypothetical "shon range units "

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BELL. H. H., & L^m, J. S. (19t9) The deE tior ot rolaud'n ratuddor pattems. P..c.ption & PsrchoPr$t.s, 26, 415 417

BN.Hots, W. F. , & GRotEr, M. ( 1985) Betond the displaceme.t lmtAn a.alysis of shorljmee pr@osss in apPate nolion vitian

Fd?ar.r, 25, 839-848.BMDDTC(, O. i. (1973). Tbe Msk.e of aP!a!e.r notion in rddom

dol Dattems. I/irnn R.rcarcr, 13, 355 369BkD;cR, o. J. (19?4). A sbon-rm8€ prdess in apparent molion I/i

sion Research, 14, 519'521 .BMDDrcx. O J., & ADL^D, A. J. (1973) ApPm norion md the

morion dete.tor. ln J. Amington, J Kiauskopt, & B. R W@ren

GAs.\, tawn psrth.Pbsrt uld Prttiol,Sl (PP 4U 426) New York:

Chr \u ,J I s l r I r / .B I 'o8r ' c@pr tu t r \ e phenore tu i ' ' ppar

eni movenent perePtion of tmdonnot cinedalograns ltui'n

Re! earch, 24, r't 3r - r1 48.CEANG. J. J., &.tulEs, B. (1985). copentve md non-cmpe&lre

pl@ss or aPpaEnt novmnt of *ndonrlot cinemtosld t'iizl

GEturNo, w. (1984). bR-level ed hiSrh level Pr6e$es in the !'Fceptual otgeization of dEninensional apPsen! notion Pel"p-

IrAsz, R. ll91l). Fawdatia6 of CJclopean percePtim Ch;cngo:

UniveBity of Chicaeo PBs.PANTLE, A. J.. & P!rE{I(, t. T (1980). Etrats of sPatial pdamelers

on the p€.cepdal otgdLation of a bistable notion displ'v P?rcep'non & Pst haphlsrs,21,lo1 ilz

p { N L a . l . ; h , , r { N d . L r ' a 7 o ' A n u l u u b l c m n \ ' m e n r d hplayi Evidence for two sQarate notion svsem n nnmr vs'o''

S.i.n.., 193, 50G502

PErErsrx, J. T. (1989). The iwcpMess dtulinction in app,Fnr m.\tm. Psl.holasicdl Bullenn, fi6. 107 121

PErEsrx,r. T.. &l^NrLE, A. J. (1979).IactoG onholling th€ competing senslions produed by a bi$able stroboscoPic nolio. dlsplavt/kion Res earch, 19, 143' 154

PErE6(, t. T,, Puts^nl, R.. a (ssNos, E (1983) Faihte bnndd absoluE rc1inal limit or a pulatile shon_r&ge pr@6s in appdenlnotion. Ydi,, Xesezr.r. 23, 1661 16?0

SEGEL, s., & C^5rELL{N, t. N . rs (l9a$ NohParMtnc stu n,.sfat tu beha!turul :rpn.6. New York: McCFw Hill

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LrLdAN, S. (1980). The efrect of sinilaritt between line segdents on

rhe coresponljence streigib in aPpaeit norioa Per..ption 9

611-626.

NOTES

t These franes diffcr from .lssical ril flfies in teo ways Fnst,rh€ slinulus elenenls of thc present displavs are oudinc co.tou( mtnertha tuled pfiels. Seond, od nor€ inporrddv. lns conton6 e tandonty saftred o. a honogeneous *hite backgrcund. In rn framesevery Dnel is a rtential sllnulus baving a polentiai parh€r in the ale;; fi'de. rheErore. i. Fd franes, n is nqninsless to thin* ofI ouresmdbdkeround'rnttefrme' tlearpnor @lh.rrdlkmation*-"* " r.- *i..-e" a* o he rfl \'D olthe.hon rMee p'de"ln lhe p't$r ftalc, dE fom N cldly defitEd, 6 is rhe b3ckgl@d,and rhey re visible prior 10 altem.oon.

2. Tbe Esponse cri@tia were designe! to retuce rhe likelinood otfalely idendtins conditions thal yield a globol pecfl The fi61 5 sub-

iels *e€ qn;sdoned rcgardins $eir ahilitv to use rh€ above c eria:lney reDoned lnal the ssk Nas generally asv dd that P€@pG w'$odiou; nofteii.s nolioi had been call€d 'Jddon. Fou!ofthe5subjers als €Frned ista@ of being uNE how to €spond, pri@ilv

beause durine lhe bri€f period of pEsendlion ooe or rwo ' contou's''nav hove rode rondon notions i' orbeRise Nking pelcepls Insuch-insdces. and ror the emlnins l3 slbj*6, de subjcs weE n$rocled to use rneir be$judsnent reSdding ho* to mne rhe Peeplln nos cases, such displays *ere slbFquendv identified $ NkngMy bei inpre$io. is $at rhe* insi2nces accoun&d for less thm l0 %

L lhn corclsion wd lwb€d an r ln€ 66t 2 subiets fNnd it dimcul

Lo caremtue sotre dnhoou. mold d$Lvs l olo$tn8 fii' $. erpein;r "a nod,fied h;. ude be ohr' cdtrlofr dd'nctr r'h

iai6 wer€ run ro replc the inilial 2.4. chmclerislics of rhe lo.g_ma€ preess are condonlv lbougbt

b include . ldge spatial range over which mtches aE mdc (up toseverol desres): a prcference lor relativelv lons ISh (usualrv 8ut€rltlm 80 IIM)i a pEfetene for longei slinulus duralioN d'd lne snorl

rmge pllBi a hk of Ftuitivity lo sdnulus conttdt, iluminat€d lsh.or dissimilarity of foms; aod d anenhve, seirar pr@ssrg n@e (sa

P€!eBi*, i989, ior a re!'ee)

lMmlscriPr reeived MaY 25, 19391Evision acceptet for publicalion Odob€r 16, 1989 )