gibson (1952) exploratory experiments on the stimulus conditions for the perception of a visual...

8/12/2019 Gibson (1952) Exploratory Experiments on the Stimulus Conditions for the Perception of a Visual Surface

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EXPLORATORY EXPERIMENTS ON THE STIMULUSCONDITIONS FOR THE PERCEPTION OF AVISUAL S U R F A C E1JAMES J. GIBSONAND FREDERICK N. DIBBLE

Cornel l Universi tyIt is possible that the simplestvisual perceptions are those of surfaceand edge (1). These impressions can

beobtained whenthe eyesare fixated,i.e., without having to consider theeffect of the successionof overlappingretinal images norm ally obtained w henthe visual environment is scanned.They can probably be explainedwithout reference to the st imulat ionarising from the posture and themo vem ents of the eyes, head, andbodythe factor of equilibrium andorientation in space percept ion. Thevestibular-kinesthetic-tactual complexof stimuli has to be considered, ofcourse, but the visual s t imulus mightbe considered first. This approachto space perception issimplified aboveall by deferring consideration of themeaning of percepts u ntil thei r psy-chophysicalbasis has been established.The question to which this approachleads, then, is w h a t are the proximalstimuli, in terms of image-variables,fo r the elementary imp ressions ofsurface and edge?2A phenomenal surface has thePreliminary work on this problem over aperiod of two years was carried out by theauthors. The main experiment to be reportedwasperformed by Dickens W addellandW alterCarel,to whom especial acknowledgment is duefor undertakingto atropinize theiro wn eyes intheinterestsof theexperiment. Theresearchisp,art of a project carried ou t under ContractAF41(128)-42 between Cornell University and

the Air ForceSchoolofAviation M edicine.The impression of what the writer hascalled a corner, with its qualitiesofconvexityorconcavity, willnot bediscussed here althoughit also might be considered a basic spatialimpression(1,p. 93).

qualities of hardness, distance, slant,and illuminated color (2, p. 368 f.).8A phenomenal edge has propertiesof tw o sidedness, or rela t ive dis tance(a j ump in depth from one to theother side), of length, curvature , anddirection. An edged or bou nde d sur-facehas thepropertyofshapeor formalong with its slant, and size or areaalong with its distance. For allthese properties psychophysical exper-iments may be possible which willestablish the stimulus variables towhichthey correspond.According to what will here becalled the t ex ture-hypothes i s the stim-ulus for a visual surface is a ful lydifferentiated, sharp, or texturedretinal image. This, however, is acrude statement. It can be some-wha t refined by the statementthatasurface occurs in perception when theg r a d i e n t s o f luminous i n t ens i t y in thei m a ge b e t w e e n s m al l r e g i o n s o f differenti n t ens i t y a re max imal l y s teep (3).This phraseology implies that avisual surface is a thing. A su r facein visual experience is assumed to besomething distinct from the filmy orfoglike impression obtained when the

3The word hardness is not an adequate termfor the quality signified. It suggests tactualan d kinesthetic meanings which are notintended, such as the softness of fur and thehardness of marble. W hat is meant is only akind of visualdefiniteness which probably goeswiththe capacityof the eye to accommodate forthe surface in question. Inthis sense, fur andmarble are equally hard. Katzdescribed it assomething which seemed to stop the eye, ascontrasted with the penetrable character of afilmor fog (4).

414


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PERCEPTION OF A VISUAL SURFACE 415retinal image is homo geneous. Ac-tually, however, it should be con-sidered an open question whether asurface is best studied as an entityor as a variable of visual perception.The question is whether the charac-teristic quality of hardness, alreadyreferred to, is a matter of all-or-nothing, or whether it is at one endof a discriminable dimension withsom ething like softness at the otherend. The other qua lities of surface-ness are surely variables.

The statement of the texture-hypothesis in terms of gradients ofluminous intensity suggests thelatterinterpretation. Since th e steepnessof such gradients is a variable quan-tity, th e hardness of the impressionshould be a variable quality. A hardsurface would go with the greatestsharpness of texture and a soft filmwith theleast.A phenomenal edge wascharacter-ized as having a definite increase indepthor distance fromonesideto theother. The texture-hypothesis, byextension, can account for this fact.According to this explanation, depth(along with all other tridimensionalqualities) dependsin the first instanceon the re lat ive dens i t y of the texture.To this isadded th e relative disparity

of the texture when visionisbinocularand the relative displacement of thetexture when th e head moves (1).The step in relative distance isexplained by the step in density,disparity, and motion. The under-standing of such a phenomenon isofgreatpracticalaswellastheoreticalimportance, for it is approximatelywhat one sees at the edge of a cliff,at the bottomof the windshield whendriving a car, and at the line of thecowlingwhen landingan airplane.It shouldbenotedthat this explana-tion assumesthatanedge occursonlybetween two textured surfaces. If

any irreversible jump in phenomenaldepth couldbedemonstrated to occurwhen the margin in the retinal imageisone of intensity or wav e length only,the hypothesis would fail to thatextent.

T H E E X P E R IM E N T SThe adequacy of the texture-hypothesis maybetestedin a general way by making controlledobservations with devices which produce theretinal images supposedly necessary forphenom-enal surfacesand edges. Theexperiments to bereported are exploratory in this sense. They

were performed with only a few Os and inviterepetition by others.Appearance of an extended surface un d e rvariable i l luminat ion. A 20-ft. stretch of wallmade of a coarse-textured plasterboard andpainted a light gray could be mad e to fill nearlythe whole of 0's visual field if he were seatedabout 3 ft. in front of it. The wall was illu-minated by abankofceiling lights controlled bya rheostat. Even thoug h this devicewasmuchsimpler than that ofMetzger (6), a numberofhis observationscould bechecked with it.At full illumination, the experience was notonlysurfacelike in the sense ofhard,as Metzgerhas reported, but alsovisibly textured in a waydifferent from plywood,cloth,orconcrete. Th etexture of this type of plasterboard could beidentified and described. In addition to har d-ness, the perception also involved a definitedistance, azero slant to the line of sight, a graycolor,and an impression of definite illumination.These reports are consistent with Metzger's,but suggest that the variable of phenomenaltexture equality is not the same thing as hi shypothetical microstructure. Our surface hada coarse physical texture; hisplaster wall hadafine texture. It would seem, then, that thephenomenal q uality of texture or pattern mustbe distinguishedfromthe condition of the retinalimagesupposed toproducea surface.When the illumination of the wall wasgradually reduced, a level was reached at w hichall the properties mentioned were no longerreportable. The texture seemed to melt, theimpression of hardness softened, the definitedistance became foggy, and the impressionofcolorwas nolonger separablefromthe impression

of illumination. When the illumination wasraised, these properties of a surface seemed tobecome definite again. They appeared to be insome degree linked together, although it ispossible that different absolute thresholds fordifferent properties might be determinable withamoreelaboratesetup.


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416 JAMES J. GIBSON ANDFREDERICK N. DIBBLEWhether acontinuous dimension from hard-ness to softness could have been discriminatedin this situation is not possible to decide afterth e fact. A regularpsychophysicalexperiment

isneeded.The resultsa re consistent withthe hy pothesisthat the hardness of a surface isrelated to thesteepnessof the gradients of luminous intensityin the retinal image. f f e c t o f artificially focused an d unfocusedret inal images . A large lighting fixture orglobe, made of milk glass, was mounted infront of O's eyes so as to fill the entire field ofview. Th e shape of the outermost section wasthat of a flattened hemisphere. When moder-ately illuminated from outside, the visualfield

was homogeneous and no surface was visible.Th e translucent surface in front of the eyes,approximately 20 cm. distant, was sufficientlynear flat sothata lantern slide could be projectedon it fromoutside and the image would be infocus at the center but increasingly out offocus toward theperipheryof the field. Photo-graphicslideswere preparedofvariou s texturedsurfaces (a ploughed field, wallpaper, linepatterns) and the projector wasfocused for thecentral region ofthisscreen. In addition to theperipheral blur, the central image could also begradually blurred by altering the projector lens.When the projector beam was switched on ,O's impressionof aluminous foggaveway to animpressionofsomethingashort distancein frontofhiseyes. Presumablytheeyes accomm odatedand converged for the central portion of thephotographic image. The percept could not,however, be called a surface comparable to thewall described above. Distance was definite,therew asa certainhardness at the center of thefield, and the quality of texture was reportablebut noother surface properties were evident.The did not mistake the image for a realsurface, although this illusion is sometimespossible in experiments with an image on a flattranslucentscreen(2). The fieldbecame vagueran d perhaps softer toward the periphery as thegradients of intensity in the image becameshallower.When the center of the image was made toblur, th e periphery became even more blurred.The impression of softness then increased overthe wholefield.Theseresults are consistent with the generalidea that asharplytextured imagemakes for asurfacelike experience, bu t they also suggestthat this formula is insufficient. Th e phenom-enal texture observed was neither continuouslyextended, as in looking at a wall, no r bounded,as in looking at an object. Perhaps the alter-nativesofbeingeitherextendedorbounded are

part of the essential stimulus conditions for theimpressionof a visual surface.Observat ions with a bounded r egion in there t ina l i mag e .The texture-hypothesis in itscrude form might lead one to predict that an ybounded region of the visual field would be asurface when its image was differentiated or"speckled" and a film when its image wasundifferentiated. Th efacts,however,contradictsuch an inference. An ordinary object on abackground appears hard rather than filmy ineveryday experience even whenthe structureofits surface is sohom ogeneous, or its distance isso great, that the correspon ding retinal imagemust be homogeneous in effect. Likewise arotating color disk has a homogeneous retinalimagebutyieldstheimpressionof ahard surface.Katz noted facts of this sort (4), and the con-tradiction can be shownexperimentally.A simple device fo r inducinga surroundingfield of one kind of color and texture with asharply bounded region of another color andtexture is a large screen with an aperture orwindowin it. Underthe nameof a reduction-screen" having an aperture of small visualangle,thissetup is well knownin the ex perimentsoncolor perception.Several cardboard aperture-screens wereconstructed with circular holes at the centervarying in diameter from 1 cm. to 30 cm.The fixated the aperture at a distance of SOcm., usually with monocular vision and amotionless head. Behin d the screen any of anumber of different surfaces with differenttexturescouldbesetup, at afurtherdistanceof100-200cm.,insucha way as to fill theaperture.Exploratory observations demonstrated thatunder some conditions, especially with thelarger apertures, 0 saw a hard surface at somedistance behind the screen, with a definitesurface color and a definite texture. Theperception was similar to that of looking out awindow at, for instance, a surface of grass.Under other conditions, especially with thesmallerapertures, saw asoftpenetrable colorin the aperture itself, i.e.,filmcolor,orwhat hasbeen called a reduced color (4). The phenom -enon was not likethat of a window but merelylike a vague hole in a surface. Presumab ly inthis event wasaccommodating for the nearerrather than th e farther surface. There were,however, mo re alternatives tha n these. Un dersome small-aperture conditions 0 saw a hardsurface on the screeninstead of a film in theaperture. This resultseemedto be more likelywhen th e aperture was much darker or muchlighter than the screen, although the conditionsfor this,as for the other impressions, cannot bespecified since the number of variables in thesituationwaslarge. In such casesthe aperture


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PERCEPTION OF A VISUAL SURFACE 7was sometimes reported to look like a diskofblack or white paper .pasted on the screen.It.had np visible texture, but it neverthelessappeared to be a surface withrespect to hardness.

This result seems to contradict the texturehypothesis. Evidently the hypothesis cannotbe interpreted to mean that phenomenalhardness dependsonretinaltexturein the caseof a sharply bounded region of the visual field.However, it ispossiblethat the steep intensitygradient at th e contour may have been theeffective stimulus for the impression in thiscase rather than,the texturewithinthe contour;This interpretation becomes more likely if oneremembersthat a texture is itself composed ofmany small regions of the field and that itisthesharp marginsorcontours oftheseelementso f textuf? which, hypothetically produce thehardnessof acontinuous surface. It wouldno tbe surprising if the rule that held for a smallregion of the retinal image also heldfor alargerregionof it. Thesuggestionisthatthe stimulusconditions for texture and for contour arefundamentally alike.When the conditionsof thissetup were suchas to arouse the window type ofexperience,a clear impression of edge was obtained. Asmightbe expected, the jum p in distance appearedmore definitewithbinocularthanwith monocularvision, and when the head moved than when itwas motionless, If fixation of a single eyewith a motionless head were long continued,however, there semed to fee a tendency forthe farther surface:to come forward into align-ment with the newer surface, although retain-ing its surface qualities. The depth at theedge was then ambiguous. A fewcomparisonssuggested thatthis resultmight be more likelywhen the textureof the two surfaces was differ-ent, e.g.,whenonewaspatternedand theotherunpatterned, The situation obviouslydemandssystematicinvestigation. f f e c t o f a variable aperture o n t h eJudgment o fs u r f a c e or f i lm.In order to investigate furtherthe transitionbetween the impressionof surfaceand ' tha t of film, one of the variables in theaperture-screen setup was selected for moresystematicstudy the variableof-aperture size.An iris diaphragm of the type used m cameraswas mounted at the center of a large screen.Both thescreenand thediaphragm were paintedblack. Since the aperture was continuouslyvariable from adiameter of 2 mm. to aboutSOmm.and thescreen wassetat avariabledistanceof 10, 20, or 30 cm. from O's eye, a range ofaperturesizeswasobtainable from approximately1 degree ofvisual-angle to 24degrees. Th eOsat with head fixed in a headrest, fixated theaperture, and reported its appearance. Allvision was monocular. Five Os were used, all

graduate students with training inthistypeofobservation.Behind the screenat a distance ofsomethingover 2 m, was set a textured surface of .highreflectance. For two < 9 sthis consistedofcottonsheeting; for three others it was plasterboardwith a coarser texture. In order to controlaccommodation, the eyes of all O s had beendosedwith atropin (5% solution of homatropin)40min.beforethe experiment and the distanceof the critical surface was adjusted for each 0sothatitw as necessarilyinfocus. Theeffectofthis procedure was to force accommodationfor the surface and prevent accommodation forthe aperturescreen. It canprobablybeassumedthat the transitions from surface to film werenot accompanied bychanges in accommodation,but were produced solely by a decrease in thesize of the retinal image. The room was fullyilluminated. At the larger apertures 0 simplysaw awhite textured surface through acircularhole in the black screen, the edges of the holebeingblurred becauseof the faraccommodation.The intention of this experiment was todetermine the minimum angular extent of agiven texture whichis required to see a surface.The aperture was either decreased from itsmaximumor increasedfrom itsminim um at anygiven distance until reportedatransition fromsurface to film or from film to surface. Tenascending and ten descending judgments wereobtained at each of the three distances fromthe screen. Each 0 was allowed to deter-mine for himself the criterion for making hisjudgment^ and each had been given a consider-abledegree of practice in making the distinction.The resultsof the experiment were not quiteas expected. Although the judgment wasdifficult to make, every 0 could do so and hisangular threshold could be determined for theascending and descending series,and for thethree different distances. Each individual 0showed some consistency but the different Osgave widely different thresholds. The lowestthreshold obtainedfor an0wasin the neighbor-hoodof 2 degrees and the highest in the neighbor-hoodof 20degrees. Thesedifferences were notrelated to' th e coarseness of the texture used.The suggestion wasthat each had developedadifferent subjective standard ofwhathecalleda surfacelike_impression. The only fact thatemerged clearly was that the impression of asurface becomes more likely when the apertureisMncreased in size, and less likely-when it isdecreased. The implicationisthat this impres-sion is notan entity but a variable) andthatthere is noclear point ofdifference between theimpression of -surface and that of film. It ispossible that the accommodation response ofthe eye is a matter of all-or-nothing, but that


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418 JAMES J. GIBSON AND FREDERICK N. DIBBLEthe retinal stimulation usually correlated withthe responseis avariable.The aspect ofstimulation beingvaried herewasarea,ornumberofadja cen t visible elementsoftexture. Steepnessof intensitygradientsforthese elements washeld constan t. The resultsindicatethatthe quality of hardnessisa functionof the extent of a differentiated imageas well asitsintensitive differentiation.The contour of the aperture was alwaysblurred in this experiment rather than sharpas it was in the previoussetup,i.e., a shallowintensity gradient rather than a steep one.Accordingly,it isnoteworthythatthe film in theaperture was never reported tolook likeahardsurface on the screen as it sometimes waspreviously.

DISCUSSIONTwo conclusions, at least, aresuggested bytheseexploratoryobser-vations, although many questions stillremain in doubt. In the first placeit is probably a mistake to assumethat a phenomenal surface is anelementary impressionofvisual space.It is not a thingat all, but a variable

of things. The ha rdnes so f a surfacelieson ad imension havingthe qual i tyof softness at the other extreme.In the second place the visualhardness of a surface does not seemto be in psychophysical correspond-ence with texture as such. Thetexture-hypothesis as usually form-ulated is inadequate. A sharp con-tour in the retinal image seems toyield hardness whether it delimits asmall speckoflightor alargepatchoflight. The formula of the steepnessof gradients of lum inous intensity be-tween regions of the image, however,gives promise of being valid.4 The

4G radientsofintensity are suggested inthisstatement as a first formulation although, inaddition,gradients of wavelengthm ight havetobe considered. The ineffectiveness of a differ-encein huewithout a difference inbrightness toproducea visual contour (3), however, suggeststhat this complication may be minor. Thea m o u n t of intensity difference as well as thegradient of this difference between adjacentregionsmayalso haveto beconsidered.

size of the regions referred to isunspecified,and is am at te r foremp ir i-cal study. The investigator mustlearn to think of the retinal image interms of l ight rather than of objectswhich reflect light if he is to explainour perceptions of the latter. Fromthispointof viewthe speckled imageof a so-called textureand the contourimage of a so-called figure differ onlyinthat the fo rmer has m any elementsinstead of one.The problem of the st imulus con-ditions for an edge is s imilar to thatfo r surface character except that morevariables are involved. Pe rha ps edgeis best conceived as a variable, notan entity. A true phenomenal edgebounds a phenomenal surface and itmay be the case that one can neverelicit the former without the latter.6Only a beginning has been made inthe study of edge character duringthe present experiments, but theydo suggest ways in which an investi-gator can go about m ak ing systematicexperimen ts. There seemto be threebasicwayso fprodu cing re t inal imageswh ich aro use an edge. First is asetup which bisects the visualfield,i.e., produces an unclosed edge inexperience. Second is asetup whichyields what might be called thew i n d o w p h e n o m e n o n a closed edgewith depth increasing from outsideto inside the contour. Third is asetup which yields a closed edge withdepth increasingf rom insideto outside

8The quality ofedgeis, in fact, asortofupperthreshold for thequalityofslant. It isprobablythe impressionwhichslant approachesas alimitwhen it increases from zero slant (a surfaceperpendicular to the line ofsight) to an infiniteslant (a surface parallel to the line of sight).As one fixates the flat face of an object, a cubefor instance, an d walks around it, the surfacegets increasingly slanted (and the form getsincreasingly foreshortened) until suddenly th esurface disappears and becomes an edge. Thisrelationship will be further discussed in asubsequentreport.


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PERCEPTION OF A VISUAL SURFACE 419the contour. This is allied to theclassical f i g u r e - g r o u n d p h e n o m e n o n .All these setups involve the impres-sion of two planes of distance. Theevidence suggeststhe hypothesis thatunder some circumstances an edgemay degenerate into a mere marginor contour, in which case the twoplanes of dis tance become indis t inct .The phenomenon of figure-on-groundhas apparently been studied hereto-fore under these c ircumstances .Surface character can be studiedwithout the complicating effect ofedges or of any boundaries savethose of the visual field itself. Thewall experiments demonstrate thispossibility. Themostinterestingsur-faces, however, are thoseexem plifiedby the flat face of an object and theground behindthe object (and variouscombinations of these). Experimen-tation with these requiresthe controlof a large nu m ber of variables. In

order to do so, it is wise for theexper imenter first to spend sometime taking an unpre judiced look a tthe visual world about him.(Manuscript received September24, 1951)

R E F E R E N C E S1. G I B S O N J. J. The perception of the v i s u a lworld. Boston:H oughton-Mufflin,19SO.2. GIBSON, J. J. The perception of visualsurfaces. A m e r . J . Psychol . 1950, 63,367-384.3. G I B S O N J. J., & W A D D E L L D. Homogene-

ou s retinal stimulation and visual per-ception. A m e r . J Psychol . (In press.)4. KATZ, D. Th e world o f colour. London:Kegan Paul, Trench, Trubner and Co.,193S.5. L I E B M A N N S. Uber das Verhalten farbigerFormen be i Helligkeitsgleichheit vonFigur und Grund. Psychol . Forsch.1 9 2 7 9,300-353.6. MBTZOBR,W . Optische Untersuchungen amGanzfeld. II. Zur Phanomenologie deshompgenen Ganzfelds, Psychol . Forsch.1930,13,6-29.

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