1 IntroductionAn important problem in visual perception is how humans recover visual scale. Inother words, how does the visual system determine the absolute size and distance ofobjects? Some perceptual measures suggest that humans are very good at solving thisproblem. A class of distance judgments, characterized as visually directed walkingtasks (Loomis et al 1992), indicate that absolute distance estimations are performedaccurately given full-cue environments in the real-world. In these tasks, an observerfirst views a target and then attempts to walk to that target without vision. This spatialbehavior is carried out without systematic bias within the range of action space(Cutting and Vishton 1995) up to about 20 m (Loomis et al 1992, 1996; Philbeck andLoomis 1997; Philbeck et al 1997; Rieser et al 1990; Thomson 1983). A multitude ofcues, both binocular and monocular, have been defined and examined with respect totheir effectiveness for distance perception (Cutting and Vishton 1995; Gogel 1977).However, a computational analysis shows that, in the absence of familiar size andmotion parallax, none of the other classical visual cues provides direct information foraccurate absolute distance judgments to points in the environment beyond about 2 mfrom the viewer. The nature of the mechanisms used by the visual system to accuratelyscale perceptual space remains largely an open question.

Recent research on the use of head-mounted displays (HMDs) in immersive virtualenvironments has consistently shown compression of distance to targets on the floor,given full cues and visually directed action tasks (Durgin et al 2002; Loomis andKnapp 2003; Thompson et al, 2004; Willemsen and Gooch 2002). HMDs restrict fieldof view and provide an imperfect presentation of stereo information. Thus, it is reason-able to ask if these viewing-condition restrictions lead to inaccurate scaling of distance

The influence of restricted viewing conditions on egocentricdistance perception: Implications for real and virtual indoorenvironments

Perception, 2005, volume 34, pages 191 ^ 204

Sarah H Creem-Regehr, Peter Willemsenô, Amy A Goochô, William B ThompsonôDepartment of Psychology [ô School of Computing], University of Utah, Salt Lake City, UT 84112, USA;e-mail: sarah.creem@psych.utah.eduReceived 8 September 2003, in revised form 31 August 2004; published online 23 February 2005

Abstract.We carried out three experiments to examine the influence of field of view and binocularviewing restrictions on absolute distance perception in real-world indoor environments. Few ofthe classical visual cues provide direct information for accurate absolute distance judgments topoints in the environment beyond about 2 m from the viewer. Nevertheless, in previous workit has been found that visually directed walking tasks reveal accurate distance estimations infull-cue real-world environments to distances up to 20 m. In contrast, the same tasks in virtualenvironments produced with head-mounted displays (HMDs) show large compression of distance.Field of view and binocular viewing are common limitations in research with HMDs, and have beenrarely studied under full pictorial-cue conditions in the context of distance perception in the real-world. Experiment 1 showed that the view of one's body and feet on the floor was not necessaryfor accurate distance perception. In experiment 2 we manipulated the horizontal and the verticalfield of view along with head rotation and found that a restricted field of view did not affectthe accuracy of distance estimations when head movement was allowed. Experiment 3 showedthat performance with monocular viewing was equal to that with binocular viewing. Theseresults have implications for the information needed to scale egocentric distance in the real-worldand reduce the support for the hypothesis that a limited field of view or imperfections in binoc-ular image presentation are the cause of the underestimation seen with HMDs.

DOI:10.1068/p5144

perception in otherwise full pictorial-cue situations. Restricted viewing conditions mightact to either reduce the relative distance information used as the basis of an absolutedistance judgment or to interfere with accurate scaling of this relative information.

One explanation of how a standing observer visually recovers absolute distance totargets on a ground plane involves using the observer's eye height to scale relativedistance information arising from linear perspective (see figure 1). Studies suggest thatangle of declinationöthe angle between an observer's eye level and a visual targetöserves as a strong cue when scaled by eye height (Ooi et al 2001; Philbeck and Loomis1997). Angle of declination might be determined by several sources of information:proprioception defining the gravitational `vertical', a visual comparison between theviewing direction to the target and the implicit or explicit horizon of the surface onwhich the target is placed, or a visual comparison between the location of the target andthe location of one's feet in the optic array. Stoper and Cohen (1991) argue that thereference angle used for judging angle of declination is a combination of surface-relativeeye level, obtained from visual cues to the slant of the supporting surface, and gravita-tionally relative eye level, obtained from vestibular cues. Wu et al (2004) have proposed asequential surface-integration process hypothesis in support of the importance of the nearground surface in space perception. This hypothesis suggests that near depth cues areused to construct an initial ground surface representation and then adjacent surfacesare constructed by using optical slant information, and integrated with the initial frame-work of the ground surface. If the recovery of the angle of declination depends on avisual determination of the orientation of the supporting surface, then restricting theview of the ground under one's feet might well alter the resulting perceived distance.

Furthermore, the presumed eye height used to scale the angular declination might beacquired in a number of ways. Eye height could be informed by body posture (Wraga1999), or might be set with an adaptive calibration process. It is also possible that eyeheight could be influenced by perception of the distance to the surface that the viewer isstanding on, since distance cues such as binocular disparity paired with convergence doprovide effective absolute-distance information in this situation. Binocular viewing mayalso provide absolute scaling to other relative information. The effectiveness of binoculardisparity as an absolute-distance cue to isolated targets rapidly decreases beyond about 2 m,but it is an effective relative distance cue over much larger distances. It is possible that infull-cue viewing situations, binocular viewing of nearby points in the environment mighthelp scale disparity or other relative depth information associated with more distant points.

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Figure 1. The visual system can recover absolute distance d to targets on a ground plane usingeye-height h to scale angle of declination y, with the equation d � h cotany.

192 S H Creem-Regehr, P Willemsen, A A Gooch, W B Thompson

We conducted three experiments to investigate whether manipulations of field ofview and binocular viewing conditions affect the ability to accurately scale distances inotherwise full-cue, indoor viewing situations. Participants viewed a target on the floorat distances ranging from 2 to 12 m and attempted to walk without vision to thetarget. Experiment 1 restricted viewing of the observer's body and the floor to about1.5 m from where they were standing. In experiment 2 the horizontal field of viewwas restricted to 42 deg (32 deg vertical), and the head was free to rotate or not.In experiment 3 monocular and binocular viewing were compared. When head rotationwas encouraged, none of the restricted viewing conditions led to decrements in perfor-mance compared to the completely full-cue condition.

2 Experiment 1. Restricting vision of the ground and feetIn experiment 1 we examined whether the view of an observer's own feet and theground under his/her feet was necessary for accurate absolute distance estimations.Several factors support the conjecture that seeing the ground and one's feet couldcontribute to distance scaling. Sinai et al (1998) demonstrated that the surface ofthe ground is used as a reference frame for judging absolute distance, consistent withideas of Gibson (1950). They found that a continuous ground surface was importantfor accurate distance judgments by disrupting the homogeneity of the ground surfaceinformation (eg introducing a gap, or manipulating the texture gradient) and findingsystematic effects on distance estimations. Ooi et al (2001) provided evidence that thehuman visual system uses angular declination for egocentric distance judgments totargets on the ground with a series of studies using prisms. They found that increasingangular declination decreased distance estimations as revealed through visually guidedactions. As described above, information for angular declination and eye height to scaleangular declination could come from viewing the location of one's feet on the ground.It may be important to scan the surface from one's feet to the target (Wu et al 2004) orto perceive the distance to the surface on which the observer is standing. Furthermore,the view of an observer's own body is most often missing while viewing an environ-ment through an HMD, and some have suggested that seeing a `virtual body' increasesa sense of presence and may contribute to spatial awareness (see Draper 1995).

2.1 Method2.1.1 Subjects. Twenty-three participants (eleven male) from the University of Utahcommunity participated in partial fulfillment of a course requirement or for compensa-tion of $10. All participants had normal (20/20) or corrected-to-normal acuity andnormal stereo vision, determined by pre-testing before the start of the experiment.None had participated in a distance study before, and all were na|« ve to the design andpredictions of the experiment.

2.1.2 Design. We used a 2 (restriction)62 (sex)66 (distance) factorial design in whichviewing restriction and sex were between-subjects variables and distance was a within-subjects variable.

2.1.3 Stimuli and apparatus. A cardboard circular collar (56 cm diameter with 20 cmdiameter neck-hole) was created that rested on two foam cubes at the front and back(see figure 2). The collar occluded vision of the participant's body and the floor belowher/his feet to about 1.5 m. The experiment was performed in a wide hallway of anengineering building (see figure 3). The target stimulus was a foam-core circular disk(37 cm diameter) placed on the ground.

2.1.4 Procedure. Participants were provided with both written and verbal instructionsabout the task. They were given approximately 5 min of practice walking withoutvision in which the experimenter verbally instructed the participant to start and stop

Restricted viewing conditions and egocentric distance perception 193

walking and to turn. This practice was intended to build trust between the participantand the experimenter and to familiarize the participant with walking without vision.In all conditions, participants wore headphones that introduced broadband maskingnoise to minimize auditory distance cues. The headset was also connected to a wirelessmicrophone that allowed participants to binaurally hear instructions from the experi-menter. Participants were instructed to face the target and to form a `good image' ofthe target and the hallway surroundings. They were encouraged to rotate their head tolook to the sides of the hallway as well as the target in front of them. When ready,they were instructed to cover their eyes with a blindfold and to walk purposely anddecisively to the target without vision, visualizing the environment as they walked

Figure 2. A participant wearing the occluding collar used in experiment 1.

Figure 3. The hallway setting used in experiments 1 ^ 3.

194 S H Creem-Regehr, P Willemsen, A A Gooch, W B Thompson

through it. One experimenter walked next to the participant to ensure that she/he wouldnot walk into a wall. A second experimenter removed the target before the participantapproached. The distance walked from the starting position to the participant's stoppingposition was measured. The participant was then walked back to the starting positionalong an indirect path while remaining blindfolded.

Participants performed in one of two conditions, blind-walking while wearing theoccluding collar, and blind-walking under full-cue conditions. The target was placedat distances of 2, 3.5, 5, 8, 10, and 12 m. Each distance was repeated three times.

2.2 Results and discussionDistance walked while wearing the collar did not differ from the no-collar condition.In both conditions, performance was close to accurate (see figure 4). A 6 (distance)62 (collar)62 (sex) mixed ANOVA was performed on the mean distance walked. Distancewas the only significant effect (F5 95 � 549:35, p 5 0:001). Distance walked increasedlinearly with actual distance to the target. The average distances walked were fit wellby a linear function for each condition. The slopes for collar and no-collar conditionswere 0.98 and 0.93, respectively (R 2 � 0:99 for both conditions). Variable error (within-subject variability) was also assessed for each condition by calculating the standard deviationof the mean of three trials for each distance for each subject. A 6 (distance)62 (collar)ANOVA was performed on the mean variable error and again the only significant effectwas distance (F5 105 � 13:44, p 5 0:001).Variable error increased with increasing distance,but did not change as a function of wearing the occluding collar.

In all, this experiment demonstrated that there was no effect of wearing theoccluding collar on accuracy of distance judgments. These results indicate that, givenotherwise full-cue conditions, viewing the ground that one is standing on is not neces-sary for accurate distance scaling in an indoor environment. These findings do notnecessarily negate the relevance of eye height or angle of declination for recoveringabsolute distance; rather they suggest that, if eye height scaling is used, it does not requirea view of the ground directly beneath one's body. Future studies that directly manipulateeye height and prevent viewing of the ground in real and virtual environments couldhelp to address this question. It is also possible that viewing a continuous groundplane would be more important when other visual information is missing.

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Figure 4. Mean distance walked (�1 SE)as a function of actual distance forthe collar and no-collar conditions inexperiment 1.

Restricted viewing conditions and egocentric distance perception 195

3 Experiment 2. Limiting field of view (FOV)Experiment 1 demonstrated that seeing the ground and one's feet on the ground wasnot necessary for accurate distance judgments in an otherwise full-cue indoor environ-ment. In our next study, we examined the restriction of horizontal and vertical FOV.There are mixed experimental results about the importance of a large FOV for accurateegocentric distance perception. Several researchers have reported that the world appearssmaller with restricted FOVs (Alfano and Michel 1990; Dolezal 1982; Hagen et al 1978).We might expect a restricted FOV to have a detrimental effect on spatial judgmentsbecause of its diminishing effect on peripheral information used in spatial behavior(Dolezal 1982). An FOV restriction could work to reduce environmental context andtexture-gradient information, and to form an artificial frame around the world. WhenFOV has been manipulated in real-world distance studies, some have found a compres-sion of perceived distance (Dolezal 1982; Hagen et al 1978) and size (Alfano and Michel1990) while others have found little compression (Knapp and Loomis 2004) or decrementsas a function of the extent of the restriction (Wu et al 2004).

Given that compression of distances to targets on the floor has now been foundin a number of studies with HMDs and that HMDs typically have reduced FOVs, therestricted FOV has logically been proposed as a factor influencing distance perception(Kline and Witmer 1996; Psotka et al 1998; Witmer and Kline 1998; Witmer andSadowski 1998). Using virtual environments, Kline and Witmer (1996) and Psotka et al(1998) found that manipulations of FOV size led to changes in distance estimations.FOV has been shown more consistently to have an effect on other types of spatialtasks such as visual search, walking, navigation, and spatial memory after turning,using large projection screens, HMDs, or flight simulators (Arthur 2000; Piantanidaet al 1992; Riecke et al 2001, 2002; Wells and Venturino 1990).

Loomis and Knapp (2003) commented that one factor that has not been explicitlycontrolled for in FOV distance-estimation studies has been the extent to which observerswere free to rotate their heads while in a restricted FOV setting. For example, Klineand Witmer (1996) used a BOOM stereoscopic display so that natural head movementwas restricted, and assessed perception of distance using verbal magnitude estimations.They found that verbal estimations were consistently overestimated with a more narrowFOV (60 deg) compared to a wider FOV (140 deg). Knapp and Loomis (2004) createda restricted FOV condition in the real-world that constrained viewing to 58 deg hori-zontal and found that walking and verbal judgments of distance were no different thanin a full-FOV condition. The present experiment involved three conditions in which theinfluence of restricted FOV combined with the presence or absence of head rotation onegocentric distance judgments in a real-world indoor environment was directly examined.In the restricted conditions, FOV was limited to 42 deg horizontal and 32 deg verticalto be consistent with the FOV in the HMD used in our previous studies (Thompsonet al 2004; Willemsen and Gooch 2002). Observers performed in one of three conditions:a full-FOV, a restricted FOV in which they were free to rotate their heads, and anFOV restriction along with a neck brace that restricted rotation and tilt of the head.

3.1 Method3.1.1 Subjects. Forty-eight students (twenty-five male) from the University of Utahparticipated in partial fulfillment of a course requirement. All participants had normal(20/20) or corrected-to-normal acuity and normal stereo vision, measured before theexperiment began. None had participated in a distance study before, and all were na|« veto the design and predictions of the experiment.

3.1.2 Design. We used a 3 (restriction)62 (sex)65 (distance) factorial design in whichviewing restriction and sex were between-subjects variables and distance was a within-subjects variable.

196 S H Creem-Regehr, P Willemsen, A A Gooch, W B Thompson

3.1.3 Stimuli and apparatus. The target stimulus was the same as in experiment 1. Torestrict FOV, we used a pair of viewing goggles with conically shaped occluders for eacheye (see figure 5). The occluders were constructed from black foam-core board andmounted into a frame built from LEGOs. They restricted horizontal FOV in each eyeto approximately 38 deg. Worm-gears in the frame allowed lateral adjustment of theposition of the individual occluders in order to control the binocular FOV to approx-imately 42 deg. Vertical FOV was approximately 32 deg. The viewing goggles werecovered with black fabric to enclose the viewing cones and to prevent light from enteringthe device. In one condition, we used a surgical neck brace that adjusted to the size ofthe neck with a Velcro strap to restrict head rotation and viewing of the feet and groundbelow one's feet (see figure 5). The experiment was performed in a similar hallway as inexperiment 1, in the same engineering building.

3.1.4 Procedure. The training and testing procedures were the same as in experiment 1,except for the changes in the viewing conditions and the target distances. Participantsperformed in one of three viewing conditions: full-FOV, restricted viewing goggleswith free head rotation (FOV-rotate), restricted viewing goggles and restricted headrotation (FOV-not rotate). In the restricted conditions, participants were fitted with theviewing goggles and tested to ensure that their binocular FOV was close to 42 deg(�1 deg). The initial lateral spacing of the occluders was set to roughly match the inter-ocular distance of the participant. The participants were then placed 2 m from a posterwith marks designating FOV increments. Participants were told to center themselveson the zero-degree mark of the poster and focus on that mark, viewing with both eyes.Working on one side of the zero-degree mark at a time, experimenters slowly moveda black rectangular marker in towards the zero-degree mark. Participants indicatedwhen the marker first appeared in their vision. The lateral position of the occluderswas adjusted as necessary to obtain the intended binocular horizontal FOV. Thefull-FOV and FOV-rotate conditions were performed as in experiment 1. In the FOV-notrotate condition, participants viewed the target as in the other conditions, but they wereprohibited from rotating their head to the left and right and up and down by theconstraint of the neck brace. Target distances were 4, 6, 8, 10, and 12 m from the observer.

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Figure 5. The FOV restricting goggles (a) worn byan observer and (b) with the neck brace used inexperiment 2.

Restricted viewing conditions and egocentric distance perception 197

3.2 Results and discussionPerformance with the restricted viewing goggles did not differ from the full-cue conditionwhen head rotation was allowed; both conditions demonstrated near-perfect performance.However, restricting head rotation led to underestimated distance judgments (see figure 6).A 5 (distance)63 (restriction)62 (sex) ANOVA was performed on the mean distancewalked, with distance as a within-subjects variable and restriction and sex as between-subjects variables. The ANOVA indicated an effect of distance (F4 168 � 870:92, p 5 0:001),restriction (F2 42 � 8:41, p 5 0:001), and a distance6restriction interaction (F8 168 � 2:61,p 5 0:05). Scheffe a posteriori tests showed that the FOV-rotate condition was notdifferent from the full-FOV condition (mean difference � 0.45 m, p � 0:34) and that theFOV-not rotate condition differed from both the full-FOV (mean difference � 1.27 m,p5 0:001) and the FOV-rotate (mean difference � 0.81 m, p5 0:05) conditions. Estima-tions increased linearly with distance in all conditions. Figure 6 shows that the averagedistances walked were fit well by a linear function for each restrictor condition. Theslopes for the full-FOV, FOV-rotate, and FOV-not rotate were 0.93, 0.92, and 0.78,respectively (R 2 � 0:99) for all conditions. Variable error (within-subject variability)was also assessed for each condition as in experiment 1. A 5 (distance)63 (restriction)ANOVA was performed on the mean variable error and the only significant effect wasdistance (F4 180 � 18:63, p 5 0:001). Variable error increased with increasing distance,but did not change as a function of viewing restriction.

Our results showed that, given the freedom to rotate one's head, a narrow horizontalfield of view of 42 deg did not impair distance estimations in the real-world indoorsetting, supporting the findings of Knapp and Loomis (2004) and Wu et al (2004).In the present study, the FOV-not rotate condition explicitly addressed the question ofwhether decrements in performance that have been found in HMD distance estimationtasks (eg Kline and Witmer 1996) could have resulted from the prevention of headmovement. We found that, when observers were prohibited from rotating their head, theyunderestimated distances. However, the inter-trial variability remained constant acrossconditions. These results suggest that distances were consistently perceived as closerwithout full information from the periphery gained through head movement.

The distinction between the lack of effect of FOV restriction (when head movementwas allowed) in the present study and the compression seen in earlier studies of Hagen

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Figure 6. Mean distance walked (�1 SE)as a function of actual distance forthe full-FOV, FOV-rotate, and FOV-notrotate conditions in experiment 2.

198 S H Creem-Regehr, P Willemsen, A A Gooch, W B Thompson

et al (1978) and Dolezal (1982) could be a result of several methodological differences.Hagen et al compared free monocular viewing with viewing through a peephole(2 mm diameter), a rectangular truncation (4 cm66 cm), and photographic slidesconsistent with the truncated view, and assessed exocentric distances between two objectswith a verbal-report task. Dolezal (1982) restricted FOV to 12 deg with two view-ing tubes, a much larger restriction than in the present studies. In all, experiment 2indicates that a full FOV is not necessary for accurate distance perception in thereal-world when head rotation is allowed and encouraged. Although we cannot makegeneralizations to restrictions greater or less than 42 deg, we suggest that the commonrestriction of FOV in HMDs cannot fully account for the large compression effectsfound in virtual-environment studies

4 Experiment 3. Monocular and binocular viewingA wealth of research has been devoted to the effectiveness of binocular viewing ondistance perception (Foley 1980; Gogel 1977). Accommodation and convergence havebeen shown to be effective cues at distances less than 2 m when consistent with eachother (Gogel 1961). Binocular disparity as a cue to depth is also most effective fornear spaces (see Cutting and Vishton 1995). Binocular disparity provides informationabout relative depth, but when paired with convergence can provide absolute scaleinformation. Studies in which the effects of binocular viewing with targets in reduced-cue environments were examined have revealed little influence of binocular informationon distance perception beyond short distances (Philbeck and Loomis 1997).

A question exists, however, about the relevance of binocular information beyond2 m in full pictorial-cue environments. It is possible that effects of binocular disparityand the pairing of accommodation and convergence for near distances could provideinformation for scaling farther distances. Wu et al (2004) have suggested that cuesfor the near ground surface are important for perceiving farther distances. In our finalexperiment we tested the possibility that binocular viewing was necessary to accuratelyscale distances beyond 2 m in a well-lit indoor environment.

4.1 Method4.1.1 Subjects. Sixteen volunteers (eight male) were tested on the monocular viewingcondition. All participants had normal (20/20) or corrected-to-normal acuity and normalstereo vision. None had participated in a distance study before, and all were na|« ve tothe design and predictions of the experiment.

4.1.2 Design. We used a 2 (restriction)62 (sex)65 (distance) factorial design in whichviewing restriction (monocular versus binocular) and sex were between-subjects variablesand distance was a within-subjects variable. The full-FOV condition from experiment 2was used as the binocular control condition in the present experiment.

4.1.3 Stimuli. The target and hallway were the same as in experiment 2. An eye patchwas used to cover the participant's non-dominant eye.

4.1.4 Procedure. The same general procedure was used as in experiments 1 and 2. Targetdistances were the same as in experiment 2. Participants were tested for eye dominanceand wore an eye patch over their non-dominant eye throughout the entire experiment.

4.2 Results and discussionThere was no difference in performance between the monocular and binocular viewingconditions. A 5 (distance)62 (restriction)62 (sex) ANOVA was performed on the meandistance walked with distance as a within-subjects variable and restriction and sex asbetween-subjects variables. The only significant effect was of distance (F4 116 � 537:92,p 5 0:001). Figure 7 shows that the average distances walked were fit well by a linear

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Restricted viewing conditions and egocentric distance perception 199

function for both viewing conditions. The slopes for the binocular and monocular condi-tions were 0.93 and 0.96, respectively (R 2 � 0:99 for both conditions).Variable error wasalso assessed for each condition as in the previous experiments. A 5 (distance)62 (restriction)ANOVA was performed on the mean variable error and the only significant effect wasdistance (F4 124 � 11:75, p 5 0:001). Variable error increased with increasing distance,but did not change as a function of monocular or binocular viewing.

Consistent with studies in real-world reduced-cue environments, binocular viewinghad little effect on the accuracy of distance judgments in well-lit action space. Theseresults address the question whether accommodation, convergence, and binoculardisparity as near depth cues might contribute to scaling of farther distances in afull-pictorial-cue environment. However it is that the scaling of distance judgments isdone in full viewing situations, it does not require information from binocular viewing.Furthermore, if stereopsis is not required for accurate distance judgments over theranges explored in this experiment, then the intrinsic limitations on accurate displayof stereo imagery inherent in HMDs (Wann et al 1995) may not be the limiting factoron virtual-environment distance judgments.

5 General discussionIn three experiments we investigated the influence of restrictions of FOV and binocularvision on visually directed walking tasks in full-pictorial-cue indoor environments.There is a scarcity of cues that can specify absolute distance in action space. Torecover absolute distance to targets on the ground plane, the visual system may scalerelative-distance information by the viewer's eye height. We examined restricted viewingconditions that might interfere with accurate scaling of this relative information.The present manipulations were consistent with limitations of virtual environmentspresented through HMDsöthe missing view of the observer's feet, a reduced fieldof view, and imperfect stereoscopic viewing. In all, we found that if observers wereencouraged to look around, none of the restricted viewing manipulations had a negativeeffect. Walking without vision remained accurate at all distances tested. These resultshave implications both for the cues that are necessary for absolute distance perceptionin the real-world, and the factors involved in creating veridical perception of space invirtual environments.

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Figure 7. Mean distance walked (�1 SE)as a function of actual distance for themonocular and binocular viewing condi-tions in experiment 3.

200 S H Creem-Regehr, P Willemsen, A A Gooch, W B Thompson

5.1 Absolute distance perception in a well-lit real-world environmentEgocentric distance perception has been investigated with respect to the contribution ofbinocular and monocular cues at ranges of near to far distances (Cutting and Vishton1995), and the extent to which different measures of perceived distance reflect perceiveddistance (Loomis et al 1992; Philbeck and Loomis 1997). The examination of cues fordifferent ranges of distances has a long history, much of which has focused on reduced-cue environments. The logic of studying reduced-cue environments is to isolate and test acertain cue by making other cues unavailable to the visual system. A number of elegantstudies have demonstrated that cues of accommodation, convergence, binocular disparity,and motion parallax are weak cues to absolute egocentric distance beyond 2 m (Beallet al 1995; Gogel 1961, 1977; Philbeck and Loomis 1997). However, angular declinationscaled by eye height has been shown to be a stronger cue (Ooi et al 2001; Philbeckand Loomis 1997; Sinai et al 1998). We chose to manipulate our viewing restrictions ina well-lit indoor environment to examine similar restrictions that are faced in high-qualityenvironments simulated with HMDs. Since underestimation of space has consistentlybeen found in virtual environments, we asked whether simply limiting the same informa-tion in the real-world would lead to similar underestimation of distance. Although thismethod does not allow claims about the importance of isolated cues for distance, it cansuggest whether cues are necessary given the combination of other cues present.With respectto FOV, we determined that seeing the ground under one's feet or a full horizontal andvertical FOV was not necessary for accurate scaling of relative distance information,given free rotation of the head. Furthermore, the hypothesis that binocular cues informingperception of near distances could be required to scale farther distances was not supported.

One might also consider the measure used to reveal perceived distance. In thepresent studies we used a visually directed walking task shown in a number of studiesto result in accurate indications of target distance (Loomis et al 1992; Rieser et al1990; Steenhuis and Goodale 1988; Thomson 1983). However, several studies have shownthat other measures, such as verbal reports and perceptual matching (Loomis et al 1992,1996), indicate compression of space that increases with farther distances. Some haveconsidered the possibility that different types of measures of perceived distance revealeither a visuomotor calibration relating perceived distance to action (Loomis et al1992) or different underlying representations for perception and visuomotor control(Creem and Proffitt 1998; Goodale and Milner 1992). Evidence against the calibrationaccount comes from visually directed action measures that do not involve directlywalking to a target. For example, researchers have found accurate performance usingmeasures that involve triangulation by walking, or triangulation by pointing, that requireobservers to begin walking on an oblique path and then, given a cue, turn to face thetarget (Fukusima et al 1997; Loomis et al 1992; Philbeck et al 1997; Thompson et al2004). Although there is support in a number of domains for a model of visualprocessing that defines separate functional pathways for perception and action, someresearch suggests that visually directed walking and verbal reports are both controlledby the same representation of perceived distance (Philbeck and Loomis 1997; Philbecket al 1997). These researchers found that manipulating a given stimulus cue had the sameinfluence on both verbal and walking measures of perceived distance. We used a singlemeasure of direct walking to targets without vision because of the established accuracyand small variability associated with this task under full-cue conditions. Although wemight predict similar results to those found in the present studies when using othermeasures of perceived distance in the real-world, this still remains an open question.

5.2 Distance underestimations in virtual environmentsOne goal of the present studies was to examine common accounts of compressionin HMDsöa limited field of view and imperfect stereoscopic viewing. Knapp (1999)

Restricted viewing conditions and egocentric distance perception 201

and Thompson et al (2004) both found greater than 50% underestimation of distancesranging from 1 to 20 m. Thompson et al found these results with a triangulated walkingtask, whereas Knapp compared verbal, walking, and a size-based measure. Our findingsthat a full FOV and binocular viewing are unnecessary for accurate distance scalingin real-world settings suggest that it is unlikely that these viewing conditions can fullyaccount for the large distance compression seen in virtual environments.

One way to further test these potential viewing factors would be to directlymanipulate them in the HMD. Willemsen et al (2005) took this approach to investigatecontributions of stereoscopic viewing to distance perception in HMDs. With theknowledge that there are imperfections in creating stereoscopic depth in HMDs, theycompared binocular, bi-ocular, and monocular viewing of targets in a virtual environ-ment. Observers viewed targets on the ground at a range of 5 to 15 m and performeda triangulated walking task to estimate distance. Their results indicated compressionof space in the HMD conditions compared to the real-world consistent with previousstudies, but there was no difference between viewing conditions.

Investigations of FOV have also been carried out with manipulations in virtualenvironments, although most of these studies have focused on spatial tasks other thandistance perception. Arthur (2000) examined performance on several spatial tasksusing an HMD with a maximum of 176 deg horizontal (47 deg vertical) FOV. Hecompared 48, 112, and 176 deg FOVs using the same HMD. FOV had a significanteffect on search and walking tasks; a wider FOV led to faster performance. OtherHMD studies have revealed similar effects on search and spatial navigation tasks(Cunningham et al 1996; Piantanida et al 1992). However, others have found that screencurvature may be more important than FOV for accurately perceiving ego-rotations(Schulte-Pelkum et al 2002, 2003). Schulte-Pelkum and colleagues had observers viewoptic flow information on a flat or curved display screen (86 deg664 deg) with orwithout blinders that reduced the FOV to 40 deg630 deg to be consistent with view-ing through an HMD. They found that turning performance was significantly worse inthe HMD condition, but that there was no difference between viewing a large screenwith or without restricting blinders. There was also improved performance with a curvedcompared to a flat screen.

Given the present results, the question of what is contributing to the distanceunderestimations found in virtual environments still remains unanswered. We havemade recent efforts to identify the factors that may be important to perceive accuratescale in indoor virtual environments. For example, Thompson et al (in press) examinedvariations of the realism of graphics presented in an HMD. Distance estimations,although near perfect in the real-world, were equally compressed in photorealistic andimpoverished graphics HMD environments. It is possible that some of the distinctionbetween distance estimations in the real-world and in HMDs might relate to theergonomics of acting with the HMD itself, rather than the nature of the FOV restriction.This possibility is being examined in ongoing studies (Willemsen et al 2004). Notably,the present viewing manipulations allow generalization only to indoor-environmentstimuli. Future research is needed to examine the influence of viewing restrictions onreal and virtual outdoor environments in which walls and ceilings are not present.

In all, the present studies demonstrate that human perception of absolute distancein a well-lit indoor environment remains accurate when the FOV or binocular viewingis restricted. Our results in the real-world suggest that these viewing restrictions are nota dominant cause of distance compression seen in indoor environments presented inHMDs, although it is still possible that these viewing restrictions could be more influentialin virtual environments when other information may be missing or distorted. Futureresearch should involve manipulations in both real and virtual environments to furtherunderstand the mechanisms involved in how the visual system recovers absolute scale.

202 S H Creem-Regehr, P Willemsen, A A Gooch, W B Thompson

Acknowledgments.We thank Jayson Neil, Cynthia Shepherd Sahm, Jason Wise, and Brody Youngfor help in conducting the experiments. This research was supported by National Science Foun-dation grants CDA-9623614 and IIS-0121084.

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