Symmetry Search Psych Sci 92

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PSYCHOLOGICAL SCIENCE

Research ReportON THE ROLE OF SYMMETRY IN VISUAL SEARCH

Jeremy M. Wolfe and Stacia R. Friedman-HillMassachusetts Institute of Technology

Abstract// is known that the efficiencyoj visual search for a target item amongdistractor items increases when distrac-tors are similar to each other and de-creases nhen target and distractors aresimilar. Here we show that symmetry re-lations between targets and distractorscan alter search efficiency. When dis-tractors form a ba ckground texture sym-metrical about a vertical axis, sea rch iseasier than when they do not. In con-trast, when some distractors are sym-metrical with the target, search is moredifficult than when they are not. Theseresults suggest (1) that symmetry rela-tions are processed in parallel and canhelp to distinguish a target from a dis-tracting background and (2) that stimu-lus similarity can have several compo-nents even for a single feature (here, o ri-entation).

The ability to find a desired target in adistracting background is a central taskof vision. It is intuitively clear that thesearch for such a target becomes easieras the difference between that targetitem and the background items becomesgreater. Further, search is easier if all ofthe background items cohere into a back-ground texture so that the target can beseen as an interruption of that texture.These intuitions have been experimen-tally tested with the visual search para-digm. A visual search is generally veryefficient if the distractors are homoge-neous and if the target differs from thedistractors in some basic attribute suchas color, orientation, or motion (Treis-man & Souther. 1985). The time requiredto find the target in such a search islargely independent of the number of dis-tracting items. Based on this and otherfindings, searches for basic features areassumed to proceed with ail items pro-cessed in parallel.

Feature search becomes less efficientif the target and distractors are mademore similar to one another (e.g., Nagy& Sanchez, 1990) and when distractorsare less similar to each other (orienta-

tion, Moraglia. 1989; Wolfe. Friedman-Hill. Stewart & O'Connell, 1992; color,D'Zm ura, 1991; Dunc an, 1989). Similar-ity, in this case, is defined by the dis-tance between stimuli in a sensory space(color space, in the case of Nagy &Sanchez, 1990; angular separation in theorientation experiments). Either of thesemanipulations of the relations betweenstimuli can be sufficient to force an ap-parently serial, item-by-item searchthrough a display at a rate of about 40-60msec/item (Bergen & Jule sz. 1983).These findings can be summarized bysaying that the efficiency of visual searchincreases with increasing similarity be-tween distractor items and decreaseswith increasing similarity between thetarget and the distractors (Duncan &Humphreys. 1989).The notion that search performance isbased on similarity, defined as distancein some sensory (or even physical)space, has an attractive simplicity. How-ever, just as it has proven inadequate inthe categorization literature (e.g.. Me-din. 1989; Tve rsky, 1977), it turns out tobe inadequate in visual search. In thispaper, we deal with visual search for ori-entation. Certainly, the efficiency ofsearch for a target orientation is relatedto its angular separation from the dis-tractor orientations (Foster & Ward,1991a. 1991b). Here we show that it isalso necessary to consider the symm etryrelations among display elements. Ifbackground items are symmetrical witheach other and the target is not, the sym-metrical items seem to form a "good"background texture. Against this back-ground, the target stands out and visualsearch is facilitated. If target and distrac-tors are symmetrical, search is impededbecause the target is more likely to beseen as part of the background texture, aform of camouflage.Pascal (as cited in Corballis & Beale,1976) asserted that "symmetry is whatyou see at a glance." Were he writingtoday, he might propose that symmetrycan be detected "preattentively" (Ju-lesz , 1981; Jules z & Bergen, 1983;

Neisser, 1967). This assertion has besupported by the finding of Julesz (197that a 40-msec exposure allows percetion of symm etry in random dot p atternIf symmetry relationships are processin parallel, they should have an impaon visual search in the orientation dmain. Experiment 1 was designed to gsymmetry a chance to aid in visusearch.

EXPERIMENT 1MethodsIn each trial, subjects were faced wan array containing distractor items two orientations. In the vertical symmtry condition, these were symmetricaround a vertical axis. (Note that for sigle line segments, symmetry about a vtical axis is accompanied by symmetaround a horizontal axis, as well.) In toblique symmetry condition, 50 deg wadded to each distractor orientatio

Thus, the distractors were still symmrical, but around an axis tilted 50 deg overtical. Various studies have showthat symmetry is far more salient aroua vertical axis than around an obliqaxis (Corballis & Roldan, 1975; Gomeier, 1972; Palmer & Hem enw a1978). Although the distractors wesymmetrical, the display as a whole wnot. It consisted of 4, 8, 12, or 16 lisegments randomly placed in an irregu4 x 4 array in an 11.3 by 11.3 deg fiewith a small central fixation point. Thewere an equal number of the two distrator orientations. The target, present 50% of the trials, was of a third oriention. Target and distractor orientatiovaried from trial to trial, picked at radom from a set of 16 possible tr ipletsorientations (listed in Table 1). Thsubjects could not look for specific targor distractor orientations or for a partular orientation category like "steep""tilted left" (Wolfe et al., 1992). Thneeded to examine the display and detmine whether an item of unique oriention broke the symm etrical texture of


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Jeremy M. Wolfe and Stacia R. Friedman

Table

12345678910111213141516Note:

1. Stimuli for Experime nt 1Vertical

target0- 1 0809050- 6 07020- 3 0409060300

- 6 0- 3 0For each condition

SymmetryD l30405060203040506070302070602070there are 16

D2- 3 0- 4 0- 5 0- 6 0- 2 0- 3 0- 4 0- 5 0- 6 0- 7 0- 3 0- 2 0- 7 0- 6 0- 2 0- 7 0triplets of 1

Obliquetarget

5040- 5 0- 4 0- 8 0- 1 0- 6 0702090- 4 0- 7 08050- 1 020

SymmetryD l

8090- 8 0- 7 0708090- 8 0- 7 0- 6 08070- 6 0- 7 070- 6 0

target and 2 distractors.

D220100- 1 03020100- 1 0- 2 02030- 2 0- 1 030- 2 0

distractors. Sample trials are shown inFigure la.It might have been desirable to have acondition without symmetry, but this isnot possible without important changesin other aspects of the experiment. Ifthere are two distractor orientations,those two orientations will always besymmetrical about some axis. To breaksymmetry would require a third distrac-tor orientation. This would make searchmore difficult by increasing d istractor in-homogeneity (Duncan & Humphreys,1989), making any com parisons problem-atical. In the present experiment, we relyon the comparison between salient ver-tical symmetry and far less salient ob-lique symm etry to reveal th e role of sym-metry in search.

Stimuli were straight lines 2.0 deg inlength and 0.3 deg in w idth. Stimuli werepresented on a standard TV monitor (640X 480 pixels). Antialiasing techniqueswere used to eliminate the jaggedness ofoblique lines. Displays were controlledby an IBM PC-XT with IBM-YODAgraphics.

Ten naive subjects with normal orcorrected-to-normal acuity were testedon 330 trials in the vertical and obliquesymmetry conditions. The first 30 trialsin each block were practice. Order ofconditions was pseudorandom acrosssubjects.

ResultsThe average results are shown in Fig-ure lb. The two lower curves (solidsquares) show target and blank trial re-action times (RTs) for the vertical sym-metry case. It is obvious that vertical

symmetry produces much faster RTsthan oblique symmetry (upper curves,solid circles) [ANOVA main effect ofcondition: target trials H I , 9) ^ 72.7, p< O.OOl, target-absent trials: F(l, 9) =23.2, p < 0.005]. For target trials, thetask actually becomes easier for largerset sizes [RT x set size slopes signifi-cantly less than 0.0 r(9) - 3.7, p 0.25].Error rates (given in Fig. 1 caption) arequite high but do not show evidence of aspeed-accuracy trade-off. Slower RTsare accompanied by more errors.

DiscussionThese RTs are long relative to thosein other, easier search tasks performedin our lab. Orientation search among ho-mogeneous distractors yields RTs ofabout 500 msec, independent of set size.This is hardly surprising. In this task,subjects do not know the identity of the

target or distractors on a given trial. The

distractors are heterogeneous and target has no unique categorical staAll of these factors have been shownimpair search efficiency (Wolfe et 1992). Under these circumstances, inoteworthy that RT, though long, is dependent of (or negatively related set size, suggesting a difficult but parasearch. Introspectively, subjects appto be sensitive to the texture createdthe distractor elements and detect target as a disruption of that texture. Tfaster search in the vertical symmecondition suggests that the distractform a better texture when they are symetrical about a vertical axis than whthey are symmetrical about an obliqaxis.

This conclusion is bolstered by negative slopes seen for the target trin the vertical symmetry condition.subjects are attempting to see the baground as a texture , 16 items make a bter texture than 4.Relatively slow but parallel extractof properties from a display is not precedented. For example, stereoscodepth supports parallel visual search often with RTs much longer than thfor other basic visual features (Nkayama & Silverman, 1986).

E XP E R I M E NT 2MethodsIn a second experiment, we usquite different search paradigm to plore the role of symmetry. Here, demonstrate that symmetry between gets and distractors can slow vissearch. Unlike Experiment 1, the tarwas the same on all trials: a red line til40 deg left of vertical (henceforth -

deg). There were two conditions. In symmetrical condition, the distractwere green lines of the same -40 -dorientation and red lines that were symetrical, in relation to the vertical awith the target (40 deg). In the concondition, distractors were green and red + 10 deg, closer in orientatiothe target but not vertically symmetrwith it. In both searches targets wstandard conjunctions of color and entatio n. Set sizes we re V, 4, 8, anditems. In all other aspects, the methwere similar to Experiment 1. Ach


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Vertical Symmetry Condition: T; X deg. D: Y, -Y degTria l I ^ ^ ^ ^ ^ " Tria l 2 " "" "" Trial n

\\

\

/

/

\\

\

\

/Oblique Symm etry C ondition: T: X+50 deg, D: Y+50, -Y+50 deg

Tria l I ^ ^ ^ ^ ^ * " Tria l 2\

\

\

\

\

\

\

\

2200 -12000 -1800

E 16001400 -1200 1000 -

800

Oblique Symmetry Cond itionblank trialsslope=37.7 msec/itemtarget trialsslope=4.1 msec/ilem

Vertical Symmetry Conditionblank trialsslope=-L4 msec/itemtarget trialsslope=-22.7 msec/item

8 12set size

16

Fig. 1. Sample stimuli and data for Experiment L In both conditions, there are twodistractor orientations. On half the trials, a third target orientation is present. Sub-jects respond as quickly as possible to the presence or absence of the unique targetorientation. Target and distractor orientations vary from trial to trial. It is easier tofind the target if the distractors are symmetrical about a vertical axis (lower curves,solid squares) than if they are symmetrical about an oblique axis (upper curves, solidcircles). Error rates (set size, veriical sym, oblique sym): (4, 11.4%, 12A%), (8 .10.6%, 18.2%), (12, 5.9%, 15.5%), (16, 5.1%, 15.4%).matic examples of the stimuli areshown in Figure 2a.

ResultsResults are shown in Figure 2b.Slopes were computed without data from

set size I because with a single itemthere is no background and, thus, nosymmetry relationship. In Experiment 2,the symmetry of target and one distrac-tor type makes search more difficult. Fortarget trials, overall RTs are faster for

the control condition than for the symetrical condition [F(\, 9) = 7,7, p 0.025] and the slopes are shallower [t= 2.0, p < 0.05]. For target-absent als, overall RT is not different acroconditions IF (1, 9) = 1.7, p > 0though slopes of RT x set size functioare shallower in the control conditi[/(9) = 2.5, p < 0.02]. In neither contion do the target-absent and target trslopes appear to be in the 2:1 ratio pdicted by a serial, self-terminatisearch.' As in Experiment 1, error rat(given in Fig. 2 caption) are high but not show evidence of a speed-accuratrade-off. Longer RTs are accompaniby more errors.

DiscussionIn Experiment 2, symmetry interferwith search efficiency. The symmetry L It should not be surprising that the oentation X color conjunction search of Expiment 2 does not produce the pattern of sults associated with serial, self-terminatsearch. Though it was originally proposthat all conjunction searches would procein a serial fashion (Trelsman & Gelade , 198subsequent research has made it clear t

conjunction search is often more efficithan predicted by a serial model (Dehae1989; Egeth, Virzi, & Garbart, 1984; Nkayama & Silverman, 1986; Quinlan & Huphreys, 1987; Treisman & Sato, 1990; WoCave, & Franzel, 1989). We have proposthat attention is "guided" to conjunctions combining information from parallel, featuanalyzing modules lying earlier in the vispathway (Cave & Wolfe, 1990; Wolfe & C1989). Thus, in Experiment 2, a parallel comodule can activate all items of the corrcolor, while an orientation module can acvate all items of the correct orientati(though see Wolfe et al., 1992).The more similar an item is to other itemthe less activation it gets (Duncan & Huphreys, 1989). The activations are summand attention is directed to items in ordertheir magnitude of activation. In this conjution search, the item with the highest actition is likely to be the target item, as it has correct color and orientation. H owever, sithis process, like all neural processes, is graded by noise, other similar items may hrelatively high activations and may compfor attention with the target. Experimensuggests that the symmetry of targets and dtractors increases target-distractor similaand, thus, makes search less efficient.


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Symmetrical conditionT: black -40 degD: white -40, black 40 degControl conditionT: black -40 degD: wiiite -40, biack 10 deg

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Symmetry Search Psych Sci 92

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