Visual angle and apparent size of objectsin peripheral vision

L. R. NEWSOMEUniversity of Queensland, St. Lucia, Queensland, Austrialia, 4067

Measurements of apparent size were obtained by distance adjustment of aperipherally viewed stimulus to produce a match to a foveally viewed standard.As eccentricity increased, the peripheral stimulus was adjusted at distances ofprogressively greater visual angle, indicating that a continuous diminution inapparent size occurs with increased eccentricity. This effect was found to bestable for several conditions of illumination and for changes in the light adaptivestate of S. Apparent size diminution and apparent distance increase were alsofound for familiar objects viewed in an open field.

Fig.!..ApParatus used to provide a simultaneous display of two stimulussquares with one stimulus adjustable in distance and eccentricity.

level with the eye. A smooth perspexhandwheel mounted horizontallyabove the pivot and below S's chinallowed 8 to adjust the distance of theperipheral stimulus through a range of130 em to within 15 em of his eye. Topromote kinesthetic positionconfusion, movement of thehandwheel was by finger friction tothe top surface of the wheel.

The second black square served as astandard stimulus and was attached toa bar projecting out from a whitesemicircular background screen. Thestandard stimulus was located at adistance of 100 em from 8's right eyeand directly in front of it. Thebackground screen was of 155-emradius, was 65 em high, and extendedacross the full extent of 8's visualfield. The screen was illuminateduniformly to have a luminance of0.21 cd/m", An occluding shield wasprovided to cover S's left eye. Arepresentation of the apparatus isgiven in Fig. l.

Procedure. Each 8 was given sixtrials with the peripheral stimulus setat 80, 70, 60, 50, 40, 30, 20, and10 deg temporally (right visual field)and 10, 20, 30, and 40 deg nasally. Onhalf of these trials, the peripheralstimulus was presented at a distance of150 cm from 8's eye. For theremaining trials, the initial setting was30 em. Both the peripheral angle andthe initial setting were detenninedfrom a random order, a new orderbeing used for each 8. A trial consistedof 8's adjusting the peripheral stimulusin or out from the initial setting untilit reached an apparent equivalence

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selected as being right-eye dominantand were screened for defects in visualacuity, amplitude of accommodation,and ametropia.

Apparatus. The stimulus objectswere two 7.3-em squares of matteblack material, displayed to appear asequilateral diamonds. One squareserved as the peripheral stimulus andwas attached to the top of a30-cm-high thin metal rod. The rodwas carried by a sliding block on atrack mechanism. A 150'cm-long armcarrying the track mechanism waspivoted to the center of one edge of alarge table so that the arm lay acrossthe table. Above the pivot was aheadrest and temple clamparrangement. When an S was seated sothat his right eye was directly abovethe pivot, the peripheral stimuluscould be set by E to any point across8's field of view on a horizontal plane

EXPERIMENT 1Method

Subjects. Ten male students fromthe University of Queenslandvolunteered to serve as Ss. These were

Scattered through the literature ofp s y c hology are reports thatperipherally seen objects appeardiminished in apparent size (Collier,1931; Fraisse, Ehrlich, & Vurpillot,1956; Grindley, 1930; Helmholtz,1962; James, 1890; Piaget,Rutschmann, & Matalon, 1959;Salaman, 1929), and two papers haveattempted to explain visual illusions interms of spatial anisotropies of theperipheral visual field (Pearce &Taylor, 1962; Richards & Miller,1971). In spite of the frequentreference to the diminishment effect,there is no clear indication of either itsmagnitude or how magnitude varieswith viewing eccentricity. Stevens(1908) tried to obtain measurement ofthe apparent size decrease by usingsimultaneous comparisons of aperipheral stimulus with a fixedstimulus in foveal view but obtainedinconclusive results.

In the present study, the apparentsize of a peripheral object is measuredby manipulation of the distance of theperipheral stimulus until it matchesthe apparent size of a foveally viewedstandard. The technique of changingvisual angle by adjusting the distanceof the stimulus has been employed byother investigators (e.g., Thouless,1931; Joynson, 1949), and indicationswere obtained from a pilot study thatreliable measurements could beobtained with peripheral viewing.

The purpose of the first experimentwas to provide measurements ofapparent extent (size) for a number ofeccentric positions across the visualfield. Subsequent experimentsexplored the effect of varyingillumination conditions on suchmeasurements and the relationship ofapparent size to judgments of apparentdistance.

300 Copyright 1972, Psychonomic Society, Austin, Texas Perception &; Psychophysics, 1972, Vol. 12 (3)

Fig. 2. Mean visual angle subtended by the peripheral stimulus at the point ofsubjective size equality.

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PERIPHERAL ECCEHTRICITY IN DEGREES

Results and DiscussionFigure 2 shows the mean visual

angle necessary for equal apparentextent in the two stimuli to beachieved when the adjustable stimulus

EXPERIMENT 2In Experiment 1, although features

of the environment were keptconstant, it is possible that at leastpart of the observed effect was due tothe use by S of the juxtaposition ofthe movable stimulus object withextraneous background features suchas the surface texture of thebackground materials. It is knownthat, with foveal vision, size judgmentsoften change markedly whencontextual and backgroundinformation is removed. The presentexperiment tests this possibility forthe peripheral case by comparingmeasurements obtained under theconditions of Experiment 1 withsimilar measurements taken with theuse of luminous stimuli in anotherwise dark room.

results may be interpreted as showingthat, for a constant object size anddistance, viz, a constant visual angle, astimulus object appears progressivelydiminished in size as it is movedfarther out to the periphery of vision.This agrees with the earlier reports(James, 1890; etc.) that peripheralobjects appear diminished in size. Thepresent results, however, alsodemonstrate the magnitude of theeffect and show that the effectincreases with increased eccentricity ofviewing.

METHODSubjects. Ss were seven male

students selected for normal eyesight.Apparatus. The apparatus of

Experiment 1 was modified such thatthe background was black and the twostimulus squares comprised 7.6-cmsquares of greenish-white luminescentmaterial. In the absence of otherillumination, these squares had aluminance of 1.1 x 10-4 cd/m'".

Procedure. Procedure was similar tothat of Experiment 1. Trials wererestricted to the angles of 80, 60, 40,and 20 deg temporally and 20 and40 deg nasally for right eye only, andtwo trials were given per angle for eachcondition. All trials started with theperipheral stimulus positioned 30 emfrom S's eye. The first set of 12experimental trials was given undernormal room illumination conditions,the stim ul us squares appearinggreenish-white against the blackbackground. After a 30-min darkadaptation period, S was then given anadditional 12 trials using the luminoussquares in otherwise total darkness.

ResultsNo significant difference was

observed between the illumination andnonillumination conditions [F(1,6) =2.45, p > .05], but, as inExperiment 1, an analysis of variance

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was displayed at different locations inperipheral vision. An analysis ofvariance indicated that a progressiveincrease in visual angle was requiredwith increase in eccentricity [F(11,9)= 20.9, p < .01]. The effect of startingposition was significant [F( 1,9) =17.3, P < .01], but the interactionbetween angle and starting positionwas not (F < 1). Examination of theresults for individual Ss revealed thatthe starting position effect can beattributed to measures given by 4 ofthe 10 Ss.

Although Fig. 2 adequatelyrepresents the overall trend in settingsas a function of eccentricity, therewere wide individual trend variations.For instance, for 80 deg, the mostextreme eccentricity, individual meansranged from 21.3 to 55.5 em.However, the distribution of settingsfor any given individual WlUi relativelysmaller in range and did not increasemarkedly with eccentricity, the SDranging from 5.3 cm (approximately0.5 deg in terms of visual angle) to6.7 cm (1.7 deg) for the temporalvisual field and from 5.4 cm (0.6 deg)to 6.2 cm (0.9 deg ) for the nasal fieldin order of increasing eccentricity.

The extent of the main effectdemonstrates that a peripherallyviewed object must be placed so as toachieve a greater visual angle thanwhen viewed directly to appear thesame in extent. Alternatively, these

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with the standard stimulus. S wasinstructed to maintain fixation of thecenter square (the standard stimulus)throughout each trial and to adjust theother square until it "appeared thesame size" as the center square.

Twelve preliminary trials weregiven, and S was shown how to findthe PSE by moving the peripheralstimulus back and forth through thezone of uncertainty. A check on themaintenance of fixation was made oneach trial by E by sighting along theedge of the apparatus just outside S'sfield of vision. It was found that evenrapid saccadic movements could easilybe detected, and if such movementsoccurred during preliminary trials, Swas admonished. Subsequent trialsobserved appeared generally to be freeof detectable eye movements. Duringthe initial instruction period, noattempt was made to hide theapparatus from S's view and S wasallowed ad lib inspection.

Measurements of the distance of theadjustable stimulus from S's eye weretaken after each setting. These weretransformed to give the visual angleachieved relative to that made by thefixated central stimulus.

Perception & Psychophysics, 1972, Vol. 12 (3) 301

Fig. 3. Mean visual angle subtended by A, at the PSE with AI' Relativepercentage distance of A, for the "equal distance" condition is given inparentheses for each eccentricity.

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MethodSubjects. Ten male students with

normal and unaided vision volunteeredfor this experiment.

Procedure. S was positionedstanding in the middle of a largeplaying field facing the back of a167-cm-tall (5 ft, 6 in.) male assistant(AI) standing 30.6 m (100 ft) away.Each trial began with a secondassistant of equal height (A, ) walkingeither towards or away from S alongone of three paths radiating from S'sposition. These paths took the angles20, 40, and 80 deg temporally to S'sfacing direction. S was allowedbinocular viewing and was told tofixate the center of AI's back. S's taskwas to call to A, to stop when,according to whichever conditionprevailed, A, appeared to be of thesame "size" at the same "distance" asAI' In between trials, S was told tolook down at the ground until Eindicated readiness for the next trial tobegin. As before, no furtherexplanation was given to S as to whatwas meant by "size" or "distance."Half of the Ss were given the"distance" task first, and for the otherhalf the "size" task came first. On

The latter instructional condition wasincluded to provide information onthe ability of an S to judge thedistance of an object using onlyperipheral vision.

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ExPERIMENT 4In the previous experiments, it has

been demonstrated that abstractstimulus objects can appear diminishedin size with peripheral viewing, but ithas yet to be demonstrated that thesame holds true for normal objects in afamiliar environment. The presentexperiment attacked the problem byusing two men as stimulus objects inthe context of a university playingfield. Added to a number of trialswhere S was required to judge whenthe two men appeared equal in extentwas a set of trials in which S wasrequired to equate apparent distances.

Results and DiscussionThere was no observable difference

between photopic and scotopicconditions (F < 1), although the angleeffect was still apparent as in the twoprevious experiments [F(5,20) = 4.67,p < .01]. This experiment was thusuna b I e to demonstrate thatadjustments could be influenced by aninduced change in retinal sensitivity.By reason of previous argument, it isconcluded that judged extensivity ofthe peripheral stimulus is not simply afunction of decline in nonfoveal,retinal sensitivity.

less dense filter for the standardstimulus in order to prevent itsdisappearance due to the lowersensitivity of the fovea.

MethodSubjects. These were five male

students selected for normaluncorrected vision.

Apparatus and procedure. Theapparatus was the same as for theprevious experiment, except thatunder the scotopic condition, bothstimuli were covered with heavilyexposed and developed photographicnegative. When covered, neitherstimulus was visible to a number ofobservers unless the observer wasalmost completely dark-adapted.Measurements of the actual lightoutput of these stimuli could not beobtained. All Ss were given the sameprocedure as for Experiment 2 underthe illuminated condition first andthen dark-adapted for 30 min beforebeing given the remaining set of trialsfor the scotopic condition. In practice,it was found necessary to use a slightly

ExPERIMENT 3One of the more immediate

explanations of the results ofExperiment 1 is that the increase invisual angle required to maintain theperipheral stimulus at constantapparent size is a function of thediminished, or impoverished, qualityof peripheral vision. It has been shownthat the apparent size of an objectmay be decreased, and the apparentdistance increased, by either dimmingthe stimulus or reducing its contrast tothe ground (Johns & Sumner, 1948).The impoverishment explanationassumes that the effect of peripheralviewing is similar to a direct dimmingand reduction in contrast of thestimulus. Experiment 3 offers a test ofthe impoverishment hypothesis bycomparing results obtained underphotopic conditions with thoseobtained under scotopic conditions.Crawford (1937) has shown that thereare differential changes of foveal andperipheral retinal sensitivities betweenlight-adapted and dark-adaptedconditions. Mandelbaum and Sloan(1947) have shown that suchdifferential changes occur also forvisual acuity. If perception of apparentsize relates to the sensitivity andacuity of peripheral vision, factors thatcorrelate with dimmed andimpoverished quality of the viewedobject, then measurements obtainedunder scotopic conditions shoulddiffer from those obtained underphotopic conditions.

of the data showed that eccentricitywas again a significant factor indetermining set distance [F(5,30) =5.87, p < .01]. It may therefore beconcluded that background andcontextual information played no partin determining the results found inExperiment 1.

302 Perception & Psychophysics, 1972, Vol. 12 (3)

alternative trials, A 2 started either 6 or50 m away from S and moved eitheraway from or towards S accordingly.Two trials only were given for eachangle and each S.

Measurements of the distance fromS at which a halt was called were takenby A 2 from a steel tape laid alongsidethe paths and relayed via transceiver toan off-field data recorder. E stoodimmediately behind and to the offsideof S in order to monitor theproceedings and to give appropriateinstructions.

Results and DiscussionFigure 3 shows the mean visual

angle at which A 2 was judged "equalto A, " for both "size" and "distance"conditions. Distances relative to thestandard distance of A, are shown inparentheses for the "distance"condition. An analysis of variance onthe data obtained indicated that therewas no difference overall between thesize and distance tasks [F(1,9) = 1.24,p < .95] but that significant Task by Sand Angles by S interactions bothoccurred [F(9,60) = 6.62, p < .01; andF(18,60) 2.10, p < .05,respectively]. The angles' main effectwas also significant, as would beexpected from the trends in Fig. 2[F(2,18) 25.6, P < .01]. Meanvariability of set visual angle did notvary markedly with increasedeccentricity, the SD in terms of visualangle ranging from 0.49 to 0.76 degfor the size task and 0.11 to 0.13 degfor the distance task.

Inspection of the raw data revealedthe probable source of both interactiveeffects. In the "size" task, a numbe;ofSs found it necessary, at the mostextreme peripheral angle, to allow A 2to recede or approach to within about8 In of themselves in order to obtainthe appropriate perceptualequivalence. In the "distance" task, inno instance was A 2 told to stop atpoints closer than 13 m. Data fromthose Ss who were able to obtain asatisfactory "size" match at 15 m ormore showed little difference between"size" data and "distance" data. It issuggested here that in the case of the"distance" task, other cues to distancetended to predominate when A 2 wascloser than about 15 m, S would haveknown without reference to theextensivity of the proximal stimulusthat A2 was too close to equal thedistance of A,. This factor, then,could account for the attenuated datarange for the "distance" task at theangle of 80 deg and the subsequentinteraction effects.

The lack of difference betweentasks, apart from the interactiveeffects previously discussed, suggeststhat the same perceptual attributes ofthe stimuli were used for both types of

task. Furthermore, the closeresemblance of the data trends here tothe trends found in the previousexperiments suggests that a commonphenomenon is being exemplified byall four experiments. The compellingconclusions from the presentexperiment are that an illusorydiminution of extensivity can occurwith peripheral viewing of real objectsin the context of a normal visualenvironment, and that this may becoupled with an increase of theapparent distance of the object.

GENERAL DISCUSSIONAll four experiments offer evidence

that there is a reduction in apparentsize of objects when viewing becomesperipheral, and that the reductioncontinues monotonically withincreased eccentricity. The results ofExperiment 4 indicate that visual fieldanisotropies occur for perceiveddistance also.

Although the anisotropic effectsdemonstrated by these experimentswere large, there are some reservationsto be stated about the data and themethod used.

First, while eye movements weremonitored in the first experiment, noobjective record of eye position waspossible to guarantee that undetectedmovements did not occur. In view ofthe present results, the most likelyeffect of the occurrence of eyemovements would be to reduce theinfluence of peripheral diminution onthe settings made. This suggests thatthe magnitude of the anisotropiesindicated could be conservative.

A second reservation is that withthe method used for varying visualangle, there is no control of thepossibly perturbing effects of distancejudgments independent of visual angle.This applies to Experiment 1 as well asto Experiment 4. Further work isrequired where visual angle is varieddirectly while distance is heldconstant.

Exploration of the peripheralfield other than the horizontal planeportion is also required. Until theparameters of peripheral change inapparent size have been morethoroughly researched, it is perhapspremature to attempt an extensiveexplanation of the effect. However,several tentative explanations can bediscussed.

An immediate suggestion is theimpoverishment hypothesis alreadydiscussed in Experiment 3. The resultsof Experiment 3 failed to indicatechanges in perception with conditionsinducing changes in stimulusimpoverishment. The suppositionmade in Experiment 3, that loweredsensitivity and acuity is the same ineffect as impoverishment by dimming,

is a tenuous one. But unless objectivespecifications can be attached to whatis meant by impoverishment inperipheral vision, the impoverishmenthypothesis adds nothing by way ofexplanation except a semantictranslation in description. Anotherdifficulty with this hypothesis is to saywhich is cause and which is effect.Does a peripheral object look smallerbecause it appears vague or dimmed,or does the object look dimmedbecause it looks smaller?

A second approach to explainingthe effect might be to relate apparentsize to the structural properties of theeye. This has some attractiveness, sincethe untransformed results (inverse ofdata in Fig. 2) of Experiment 1plotted as a function of eccentricitybear some resemblance to the declinein retinal receptor density as providedby Q>sterburg (1935 ).

A second structural explanation isthat the peripheral decline in apparentsize follows the diminished image sizethat results from the reduced distanceof the peripheral retina from the lens.The optical explanation is importantto the density theory in that it posesthe problem of defining therelationship between the stimulus fieldand the topography of the retina:there is yet no clear specification ofthe relationship. Both structuralexplanations suffer the samedifficulty. There is no apparent reasonwhy perceived size should beisomorphic with either receptortopography or the geometricaldimension of the retinal image.

A further feature of the visualsystem of possible relevance to thepresent data is the mechanismconcerned with the generation of thebinocular horopter, the locus ofbinocularly perceived points in spaceof common depth. The horopterdefines a concave surface havingsomething of the same shape acrossthe horizontal plane as the curve forthe raw distance data of the presentexperiments. A concise definition of acausal relationship between the twocurves would be difficult, as thehoropter refers to conditions ofbinocular fixation of the stimulusobject. The practical horopter mustalso remain confined to within about40 deg of the midfrontal plane. Whileit may still be possible that the locusof equal distance points follows thehoropter when viewing is monocularand peripheral, and dictates the pointsof isotropic size, the continued declinein setting distances shown by thepresent data for points considerablyoutside the 40-deg limits remainsunaccountable in these terms.

None of the explanations provides asatisfactory and compelling reasonwhy visual field anisotropies for

Perception &; Psychophysics, 1972, Vol. 12 (3) 303

apparent size should occur. Nor dothey suggest how these anisotropiesaffect the adaptive function of thevisual system. Further knowledge ofthe parameters of peripheral sizeperception may be required before acomprehensive explanation of suchanisotropies can be formulated.

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(Accepted for publication May 17, 1972.)

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