Cross-cultural and educational aspects ofthe Ponzo perspective illusion*

HERSCHEL W. LEIBOWITZThe Pennsylvania State University, University Park, Pennsylvania 16802

andHERBERT A. PICK, JR.

University of Minnesota, Minneapolis, Minnesota 44544

The magnitude of the Ponzo illusion was determined for two groups ofUgandans, university students and villagers. For the university population,increase in the depth cues in which the illusion was embedded produced anincrease in illusion magnitude similar to that observed among students in anAmerican university. For the villagers, however, no illusion was observed for anyof the stimulus conditions. Since the environmental ecology of the two groupswas similar, the lack of illusion among the villagers cannot be attributed todifferential past history of exposure to depth cues. Rather it is hypothesizedthat responses to two-dimensional stimuli are determined by both depth andflatness cues, and that the responses of the uneducated observers weredetermined primarily by flatness cues.

A large number of experimentalstudies have demonstrated thatmembers of various cultural groupsproduce differential responses onperceptual tasks. Some of the betterknown examples include thedemonstrations by Segall, Campbell,and Herskovits (1966) of cross-culturaldifferences on a number ofgeometric-optical visual illusions,Allport and Pettigrew's (1957) workwith the trapezoidal illusion, a seriesof perceptual studies by Deregowski(1967, 1968a, b, 1969), and Hudson's(1967) research with the perception ofdepth in pictures. These data areusually interpreted as resulting fromdifferential past histories of exposureto the critical perceptual cuesinvolved, either in the form ofexperience wi th real-life cues from thesurrounding environment, or with theconventional three-dimensional cuesused in pictures, photographs, andother two-dimensional representations.The influence of experience withreal-life environmental cues is the basisof the "ecological hypothesis," whichassumes that if the cues involved havea differential frequency of occurrencein the environments of differentcultural groups, they will acquiredifferential strengths that will bereflected in the responses made tothem on perceptual tasks.

Recently, the Ponzo perspective

*Supported by Grant MH08061 from theNational Institute of Mental Health. by agrant from the Carnegie Foundation toMakerere University and SyracuseUniversity. and by Grant 1 POI HD05027from the National Institute of Child Healthand Development. These data werepresented to the 1971 meeting of theEastern Psychological Association. Theauthors are indebted to Mr. Robert J. Millerfor help in the preparation of themanuscript.

illusion, Fig. 1, was presented touniversity students in Pennsylvaniaand Guam (Leibowitz, Brislin,Perlmutter, & Hennessy, 1969).Considering Fig. 1B, in which thePonzo illusion is embedded in railroadtracks, the overestimation of thehorizontal line nearer to the apex ofthe figure was 32% for thePennsylvania students. However, forthe Guamanians, who had much lessexposure to railroad tracks (only oneshort spur railroad exists in Guam) themagnitude of the illusion was only18%. This difference was interpretedas resuHing from the greatereffectiveness, stemming from differentpast history, of the depth cuerepresented by the converging lines ofthe tracks for the Pennsylvaniaobservers as compared with theGuamanians.1

The present experiment representsan extension of the original studyamong two groups of Ugandans,college students at MakerereUniversity and villagers from theKampala surround. The universitystudents represent a replication of thedata obtained from the Pennsylvaniastudents since they were of the sameeducational level and had hadexperience in a "carpentered" urbanenvironment very similar to thatexperienced by the Pennsylvaniastudents. In addition, the question ofinterpretation of instructions, attitude,and demand characteristics, variablesthat play a significant role inperceptual studies, will be minimal forthese Ss who are of the sameeducational level. On the other hand,rural Ugandans would be expected todiffer on a number of dimensions thatmight influence responses to thepictorial Ponzo stimuli. Their visualenvironments would not be expected

to differ significantly from those ofthe urban Ugandans or thePennsylvanians since the rural areasshow many examples of converginglines and perspective, mostconspicuously in the form of both dirtand tarmac roads, square homescontaining doors and windows, andrectangular farm plots reminiscent ofrural America during the early part ofthis century. However, differencesbetween the two groups in educationallevel and past experience withtwo-dimensional reproductions arepronounced: many of the villagers hadnot attended school and those whohad had not gone beyond the thirdgrade. Books were not typicallypresent in the home. Thus, the use ofthe same stimuli in these studiesprovides an opportunity to ascertainwhether differences exist betweengroups who live in similar visualecologies, but who differ significantlywith respect to educational level andex p o s ure to two-dimensionalreproductions.

METHOD AND PROCEDUREThe method and procedure were

identical to that followed in theprevious study (Leibowitz, Brislin,Perlmutter, & Hennessy, 1969) andutilized the same stimuli as illustratedin Fig. 1. For all four stimulusconfigurations, the upper horizontalline, or board, subtended the samevisual angle while the lower boards orlines varied systematically in theirhorizontal dimension. The Ss wereasked to indicate which line or boardwas larger. The subjective equalityvalue was defined as the midpoint ofthe region of uncertainty bounded bythe sizes of the lower line or board,which were consistently called "largerthan" and "smaller than" the upperline or board. The magnitude of theillusion is the overestimation, inpercent, of the upper line or board.For the photographs, Fig. 1A and 1B,the upper board was actually 182.9 x10.2 em and located 18.29 m from thecamera, which had a 90-mm lens. Thenegatives were enlarged 6.7 times,producing an image width of the upperboard of 6.0 em. The lower boardswere chosen so as to produce an imagesize on the prints varying from 4.7 to10, em, in equal steps. For thegeometric stimuli, Fig. 1C and 1D, thedimensions of the upper and lowerlines were identical to those on thephotographs.

The S was first shown a stimulusconfiguration for which the lower linewas clearly larger or smaller than theupper and asked to indicate which onewas the larger. The next stimulus waschosen to elicit the "larger than"response for the other line. Thisprocedure served to indicate whether

430 Copyright 1972, Psychonomic Society, Inc., Austin, Texas Perception & Psychophysics, 1972, Vol. 12 (5)

Fig. 1. The stimuli used in this study. The extent of the horizontal lines in these examples is the same.

the S understood the instructions. Theother members of the series were thenpresented to S. At the end of theseries, large differences were againpresented in order to confirm that Ssmaintained the same instructional set.Order of presentation for the fourtypes of stimulus was determined by aLatin-square design.

The S population consisted of 20Makerere University students and 120villagers. All Ss were tested by a nativeresearch assistant under the closesupervision of the junior author, avisi ting professor at MakerereUniversity. For the villagers,the instructions were translated intoLuganda and checked by translationback to English by a second speaker.

RESULTSThe value of the upper line, which

matched the lower, was determined byinterpolation as the midpoint of theregion at which the S's responsesc hanged. The magnitude of the

illusion, or the percentageoverestimation of the upper line, isplotted in Fig. 2 for the four stimulusconfigurations. For comparisonpurposes, data from the previous studyfor the Pennsylvania and GuamanianSs have also been plotted on thisgraph.

It will be noted that, for theUgandan college students, theoverestimation is lowest for the plainbackground and increases system­atically with addition of the geo­metric converging lines, the plowedfield photographic background, andthe railroad tracks. This function issimilar to that obtained from thePennsylvania college students.However, the data for the villagers isstriking in that no illusion oroverestimation is observed for any ofthe four sets of stimuli, either forgroups or for individuals.

DISCUSSIONThe similarity of the college

Ugandans as compared withPennsylvania college students was notunexpected in view of the similaritiesof their backgrounds and visualexperiences. In effect, the data fromthe college Ugandans represent areplication of the previous study withPennsylvania college students.

The lack of response among thevillagers is very striking. None of thestimuli produced any illusionwhatsoever. Normally, when depthcues are ineffective in cross-culturalstudies, one looks for differences inthe past history of exposure of Ss tothe cues in question. However, this isnot appropriate in the presentsituation since the rural environmentin which the villagers live is, in termsof depth cues, similar to thatexperienced by both the Ugandan andPennsylvania college students. Thereare many examples of buildings, roads,plowed fields, etc., in theirenvironment and it is thus mostunreasonable to assume that the depth

Perception & Psychophysics, 1972, Vol. 12 (5) 431

Perception & Psychophysics, 1972, Vol. 12 (5)

UgandaVillage

Penna.College

UgandaCollege

(Received for publication March 27. 1972;revision received July 19,1972.)

NOTE1. The Pennsylvania Ss also produced a

larger illusion (21%) than the Guamanians(12%) on the texture photograph (Fig. lC).However. with the geometrical figure (IA),the Guamanians produced a slightly greaterillusion. This implies that the criticaldifferences do not result from linearperspective per se, but from the presence oftexture and other monocular cues in thephotographs.

DEREGOWSKI, J. A pictorial perceptionparadox. Acta Psychologia, 1969, 31.365-374.

GREGORY. R. L. Eye and brain: Thepsychology of seeing. New Yolk:McGraw-Hill. 1966.

HARDY, A. L., &< PERRIN. F. H. Theprinciples of optics. New Yolk:McGraw-Hill. 1932.

HUDSON. W. The study of the problem ofpictorial perception amongunacculturated groups. InternationalJournal of Psychology. 1967. 2. 90-107.

LEIBOWITZ. H. W.. BRISLIN. R.,PERLMUTTER. L., &< HENNESSY. R.Ponzo perspective illusion as amanifestation of space perception.Science. 1969. 166. 1174-1176.

LEIBOWITZ. H. W., BUSSEY. T •• &<McGUIRE. P. Shape and size constancyin photographic reproductions. Journal ofthe Optical Society of America, 1957, 47.658-661.

SCHLOSBERG, H. Stereoscopic depth fromsingle pictures. American Journal ofPsychology. 1941,54.601-605.

SEGALL. M. H., CAMPBELL, D. T.. &<HERSKOVITS. M. J. The influence ofculture on visual perception.Indianapolis: Bobbs-Merrill. 1966.

SMITH, 0 .. &< GRUBER. H. Perception ofdepth in photographs. Perceptual &<Motor Skills. 1958,8.307-313.

Control

30

~20

c0III:::J

...0

"10

'0:::J-C.,.C~

0

Geometric Photograph PhotographFigure Texture Tracks

Viewing Condition

Fig. 2. The magnitude of the Ponzo illusion for the various conditions of theexperiment. In each case, the illusion represents the percentage overestimationof the upper member of the pairs of horizontal lines illustrated in Fig. 1. ThePennsylvania college and Guam college data are from a previous experiment.

fortunately testable. If the samestimuli are presented under conditionsthat eliminate both perceptual andcognitive cues to flatness, the depthdependent illusory response among thevillagers would be expected toincrease. This could be accomplishedby monocular viewing oftransparencies under conditions forwhich depth would not be ruled outby the experimental arrangement, i.e.,presentation of the stimuli in awindow. If observation underconditions that minimize flatness cuesproduces a greater response on thepart of the rural Ugandans than in thepresent experiment, the present datawould be consistent with the proposedinterpretation, which accounts for themagnitude of the illusory responses interms of the relative strengths of depthand flatness 'cues.

REFERENCESALLPORT. G. W•• &< PETTIGREW. T. F.

Cultural influence on the perception ofmovement: The trapezoidal illusionamong Zulus. Journal of Abnormal &<Social Psychology. 1957.55.104-113.

AMES. A. The illusion of depth from singlepictures. Journal of the Optical Societyof America. 1925. 10. 137-148.

DEREGOWSKI. J. The horizontal-verticalillusion and the ecological hypothesis.International Journal of Psychology.1967. 2. 269-273.

DEREGOWSKI. J. Difficulties in pictorialdepth perception in Africa. BritishJournal of Psychology. 1968a. 59.195-204.

DEREGOWSKI. J. Pictorial recognition insubjects from a relatively picturelessenvironment. African Social Research.1968b. 5. 356-364.

cues present in the stimuli have noprevious association whatsoever amongthese Ss. Any explanation of their lackof response must assume that thesedepth cues are familiar to them but forsome reason are not effective in thepresent situation.

Following this line of reasoning, it isinstructive to examine the conditionsunder which the stimuli are presented.All of the cues are incorporated intwo-dimensional presentations andareviewed binocularly. As Schlosberg(1941) has pointed out,two-dimensi onal representationsinvolve not only depth cues, butflatness cues as well. The absence ofstereopsis while viewing atwo-dimensional representationbinocularly, the visibility of thesurface and edge of the stimuli, thecontext in which they are viewed, andthe cognitive awareness that there isno depth in a piece of cardboard allprovide cues to flatness. When flatnesscues are reduced, the effectiveness ofdepth cues is enhanced, as evidencedby the plastic depth effect that occursw hen viewing photographs underconditions that eliminate or reduceflatness cues (Ames, 1925; Gregory,1966; Hardy & Perrin, 1932;Schlosberg, 1941; Smith & Gruber,1958). Similarly, the tendencytowards size constancy in photographshas been shown to be greater whenviewed monocularly than when viewedbinocularly (Leibowitz, Bussey, &McGuire, 1957). Presumably binocularobservation of a two-dimensionalreproduction introduces flatness cues,since binocular disparity that normallyaccompanies binocular vision is absent.Furthermore, freedom of headmovement provides motion parallaxinformation that the reproduction is infact flat.

If cues to flatness can attenuate theeffectiveness of these cues to depth foreducated Ss in the United States, apossible explanation of the Ugandanvillage data would follow by assumingthat the flatness cues present in thetesting situation are even moreeffective for them. The perceptuallearning process is assumed to involvenot only increased strength of depthcues, but also the ability to disregardor inhibit flatness cues. It seemsreasonable to assume that frequentexposure to two-dimensionalpresentations in photographs, books,newspapers, cinema, and television,which is a normal part of theeducational and day-to-day livingprocesses, results in acquisition of theability to disregard flatness cues andwould be consistent with a differentialresponse on the part of the educatedcollege Ugandans as opposed to theuneducated villagers.

This explanation, although bothqualitative and speculative, is

432