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of the two eyes (prism base-out) and divergence (prism base-in) was tested. Using a Risley prism (an adjustable prism), prism vergence was incremented until ...
Physiological Correlates of Spatial Perceptual Discordance in a Virtual Environment Larry Baitch, Randall C. Smith General Motors Research & Development Center Virtual Environments Laboratory [email protected]

1. Introduction The sense of vision overrides all other sensory modalities in an immersive virtual environment. This phenomenon, called visual capture, demands that conflicts between real and virtual visual sensation be minimized or eliminated. Regardless of other sensory modalities which are brought into the environment (sound, tactile feedback, etc.), breakdowns in size or distance perception, or irregularities in binocular three-dimensional visual space will result in cognitive discord and failure of the virtual experience. At the GM R&D Center we use a CAVE Automatic Virtual Environment (CAVE) for rendering automobile or truck interiors. Because of the familiarity people have with automobile interiors, this stimulus requires a particularly accurate sensation of visual space and image quality. For most individuals the CAVE provides a compelling sensation of a vehicle interior with steering wheel, controls and other features within appropriate reach, with accurate texture and three-dimensional contour. The distance view outside the windshield and side windows appears properly distinct from the interior. However, for a smaller number of individuals the interior fails to evoke a realistic perception. Some features are seen to be inappropriately large or small, they may seem to appear at the wrong distance, the three-dimensional space inside the vehicle may appear distorted, subjects may have difficulty with double vision, or may complain of image blur. These differences occur to persons with normal stereoacuity as well as to those with previously identified binocular vision disorders. In this study, we attempted to understand the sources of intersubject perceptual differences in the CAVE by correlating perceptual differences with established parameters of visual function identified by clinical assessment. In our attempt to address these intersubject differences we took a physiological approach, performing a parametric study of visual function. This approach explored differences between subjects based on the various properties of the visual system (visual acuity,

convergence/accommodation relationships, refractive status and depth perception) and correlated those measurements with differential perceptions in the CAVE. The purposes of this investigation were therefore: • to determine the effect of different accommodative/convergence demands on perceptions of size and distance of virtual objects within the CAVE; • to assess intersubject differences between those measurements; • to relate those findings to intersubject differences both in visual function (as determined by the eye examination) and in Virtual Reality (VR) size and distance perceptions. Because our goal is to create a virtual environment with wide usage and versatile application, these intersubject differences are seen as particularly problematic to the project. The long-term goal of this work is to establish the most effective means for normalizing anomalous VR perceptions across the population.

2. Background: CAVE Automatic Virtual Environment The CAVE was originally developed at the Electronic Visualization Laboratory at the University of Illinois at Chicago. GM’s R&D CAVE is basically an 8 foot cube (see Fig. 1a) where high-speed graphics computers (Silicon Graphics Onyx2) render images which are backprojected onto the forward screen and two lateral sides. Projectors mounted above the CAVE project a fourth image onto the floor. The back side of the CAVE is open. Stereopsis is achieved by presenting independent disparate images to the two eyes using time-synchronized shuttering LCD glasses worn by the subjects. Images from the two eyes are alternated at 96 Hz, providing 48 Hz/eye and a minimum of interocular flicker perception. The subject can interact with the CAVE by controlling a wand, which feeds back to the graphics computer. The CAVE’s open viewing situation has many advantages

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over head-mounted VR displays (HMDs). The most salient advantages include: • Greater field of view • Lack of an awkward and often heavy helmet • Preclusion of “in-instrument” effects on vision induced by unnatural visual conditions • Ability for multiple individuals to simultaneously interact with the environment • Ability to combine real and virtual features within the environment For viewing vehicle interiors in the GM R&D CAVE the subject sits in a real automobile seat at a 0.75 m (29.8 inches) distance from the front screen. The forward view is projected on the front screen which consists of the outward view through the windshield in uncrossed binocular disparity, the instrument panel slightly inside the plane of the forward screen, and the steering wheel located in space between the screen and the subject in crossed disparity. The images are seamless from the forward to side and floor screens. In Figure 1b, the image drawn on the screen results from the viewer’s eye location and the location of the mathematical model being projected (also shown for reference). Imagery can be presented in two modes: 1) Head-tracking mode: The head position is monitored by magnetic sensors and projected images are rendered relative to head position in the CAVE. All viewing angles, perspectives and object sizes are compensated and preserved as the subject moves his/her head and looks about. For example, as the subject moves closer to the forward screen, the steering wheel and instruments enlarge proportionally, making them appear closer and larger as they would if one were to move forward in a real interior. As head movement translates laterally, so to do the interior objects with appropriate parallax. This allows subjects to see any forward, lateral and downward feature in the interior -- subjects can even dip their head down and look up under the instrument panel.

the graphics to megapixel resolution (1280 X 1024), resulting in pixels which each subtend 10.8’ X 8.7’ arc. This translates to a Snellen equivalent slightly worse than 20/200, which is further degraded by diffusion of the image through the screen.

Figure 1a The distance view through the windshield is therefore degraded, as are the alphanumeric figures on the instrument panel (speedometer/odometer, radio/HVAC controls, etc.). This results in variable accommodative posture (plane of eye focus). 2) Convergence/accommodation conflicts: Although virtual objects appear in a great variety of stereoscopic depths, all stimuli are presented at the same accommodative plane -- the screen (Figure 1b). This creates conflicts between convergence and accommodation, which are linked by the synkinesis triad1.

Although the CAVE currently has a state-of-the-art highspeed rendering system, there still remains a temporal lag between head movements and the resulting images. 2) Non-tracking Mode: When Head tracking is disabled, still images are rendered on the screens from a fixed viewpoint. It is useful when evaluation only requires primary forward gaze position, but objects will not appear with correct size or shape when viewed from a different location.

3. Visual problems induced by the CAVE 1) Image quality: Images are back-projected and diffused through white 8’X8’ screens. The stereoscopic interocular flicker rate of 96 Hz limits

Figure 1b

1 The synkinesis triad is the combined reflex response which occurs when visual fixation is changed from near to far objects of regard: 1) accommodation: change in focus of the eyes from the near to far distance; 2) convergence: the eyes change the angle of fixation from parallel (far), converging toward the nose (near). The amount of convergence depends on the position of the object as well as the subject’s interpupillary distance (IPD). 3) pupillary constriction is the third element of the synkinesis triad, whose purpose is to increase the eye’s focal depth of field, reducing the burden of the eye’s ciliary focusing muscles.

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3) Size/distance perception: As previously discussed, many subjects find serious conflicts with the perceived size and distance of the objects within the image field. This occurs most significantly with the most conspicuous feature in the system, the steering wheel. Because the size and distance geometry are directly correlated, items seen as inappropriately distant are also too large (for the crossed disparity case—see Figure 1b). Early on in our pilot work, it was noted that presentation of forced horizontal prism vergence induced changes in perceived size and distance consistent with those changes noted in previous non-virtual studies2. That is, base-out prism forces convergence of the eyes and induces a concomitant perception of size decrease and distance change (“smaller and closer”); conversely, base-in prism forces an exo-posture, inducing an increase in size and distance perception.

5. Methodology Part I: Eye Examination: A limited eye examination was performed on each subject prior to experimental trials in the CAVE. Tests were made at three working distances: • • •

6 m (defined as optical infinity3) 0.75 m (the distance from the subject to the forward screen in the CAVE) 0.4 m (normal reading distance)

The following assessments were made: 1) Visual acuity: Measured at the three working distances4. 2) Eye muscle testing: Binocular eye teaming abilities, eye movements and tonic fusional binocular oculomotor status (normal/ phoria/ strabismus) were assessed. Testing was performed at the three working distances. 3) Refractive status: Determination was made of each subject’s optical requirements for all three working distances. A standard retinoscopic and phoroptric refraction was performed to assess whether subjects had myopia (nearsightedness), hyperopia (farsightedness), astigmatism (corneal optical distortion) or presbyopia (age-dependent inability to focus near objects), or a combination of those refractive errors (e.g. myopic astigmatism with presbyopia).

Figure 1c

4. Subject selection The 20 volunteer subjects recruited by in-house solicitation for this study were all employees of the General Motors Research and Development Center. Some subjects had prior knowledge of the CAVE’s existence and its potential utility for the Company. However, no subjects had prior experience in the CAVE, and all were naive as to the purposes and/or nature of the experiment itself. Subjects’ ages ranged from 25 to 62 years, with a mean of 42.3 years. Sixty-five percent of the subjects were myopes, twenty-one percent emmetropic and fourteen percent hyperopic. Seventy-seven percent were male. All subjects signed inform consent prior to participation.

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[see References]

4) Gradient AC/A: The ratio of accommodative convergence (AC) to accommodative demand (A) was determined in all subjects using the gradient method5. Measurements were made at the 0.4 m and 0.75 m working distance. 5) Interpupillary distance (IPD): The distance between the eyes is important, as the geometry of the GM CAVE is set for a “standard” IPD of 65 mm. IPD was measured with an electronic pupillometer. 3 Optical infinity is defined as the object distance at which rays of light coming from the object are essentially parallel. It is estimated at 2000 times the width of the aperture of the eye, the pupil. As a clinical standard, this has been established at 6 meters (20 feet). At this far distance, the visual axes of the eyes should be parallel to each other, with no convergence or divergence. 4 Visual acuity can be indexed many ways. However, for our purposes we will utilize Snellen acuity, which expresses the ratio between normal and expected visual resolution. Acuity of “20/20” means that at 20 feet the subject can resolve an 8.28mm letter “E”. Acuity of 20/40 therefore means that a letter must be twice as large to be seen; 20/200 ten times as large, etc. 5 The gradient AC/A is determined by measuring phoria at one near distance using two different accommodative demands: at the distance manifest and manifest +1.00D. The difference in the phorias is considered the gradient AC/A. For example, if the phoria through a -3.00D at 0.4 m is 3 exo and through a -2.00D 6 exo, then the resultant gradient AC/A is 3:1. Normal gradient AC/A in the adult population using this method varies from 3 to 7.

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wheel on a black background. Texture on the wheel was a 2-D grid of dark lines overlaid on the wheel, which was rendered with a white color. The steering wheel geometry had a 23cm diameter (0.6x actual size), positioned at a distance of 37cm from the forward screen. The wheel was reduced in size so it fit in the fixed field of view.

6) Stereopsis: Measured using the Titmus Stereofly test. Gross stereopsis, fine line stereopsis and existence of binocular suppression was assessed: •



Gross stereopsis: Subjects wore polarizing glasses and viewed a vectographic 3D housefly. The fly has a stereodisparity of 800 arc seconds. To the normal observer this large, hairy fly appears to stand up in depth on the vectogram. Subjects were asked to touch the tip of the fly’s wing; the distance from the wing tip to the vectogram surface was measured . Fine stereopsis: The Wirt dot test was used: Four rings are shown, one of which is supposed to appear in depth. Stereodisparity decreases with each subsequent group of rings; the smallest disparity is 48 seconds.

7) Binocular vergence: The ability to control convergence of the two eyes (prism base-out) and divergence (prism base-in) was tested. Using a Risley prism (an adjustable prism), prism vergence was incremented until first blur was induced; it was further incremented until splitting of the image occurred (double vision or diplopia). Prism was then decremented until recovery (single image) occurred. All pertinent prism positions were recorded. 8) Accommodative status: Focusing amplitude was measured and degree of presbyopia (loss of focusing ability with age) was assessed by measuring the additional plus (+) lens power required to see near 20/20 text clearly. This was performed at the 0.4 m and 0.75 m working distances. Part II: Influence of Prism on CAVE Size and Distance Judgments Following the ocular assessment, subjects were seated in the CAVE in an automobile bucket seat as described above. Eye height and proper distances from the forward and side screens were calibrated (see Figure 1c). In order to control for the multitude of cognitive and physiological influences which might affect size and distance judgments using the fully detailed automobile interior, we limited the visual stimulus and the assigned experimental task in the following ways: •

Stimulus Complexity: In this study an effort was made to limit the richness of the visual environment. The purpose of this was to preclude associative cognitive influences on perceived size and distance. Therefore, instead of a complete automobile interior with a view to the virtual “outside world” through the windshield, the imagery was limited to a 3-D steering



Head tracking mode: Head tracking mode was disabled for this experiment in order to stabilize and isolate the stimulus imagery. Although this created a potential risk for inaccurate geometry if head movements were to occur, the goal in this part of the study was to isolate the head in a static position, thus eliminating the need for dynamic rendering to compensate for changes in head and eye positions. Additionally, disabling the head tracking mode excluded temporal and motion influences in the size and distance judgments.

6. Measurement of size and distance Subjects were seated in the CAVE at the 0.75 m distance already described. Each subject wore his/her habitual spectacle prescription. A pair of wooden probe sticks was placed in subjects’ hands; the sticks were of differing length between the hands. The purpose of these probe sticks was to limit the influence of hand-position proprioception on the measurements. The forward screen was momentarily blacked out, so that there was no stimulus ahead of the subject. With each trial, a goggle was placed over the subject’s eyes. One of four goggles was used in each trial. Each of the four goggles had one of four pairs of prisms mounted over the goggle apertures, creating prism vergence demands of 12 P.D. base-out, 8 P.D. base-out, plano (no prism power) or 8 P.D. base-in. After the goggles were in place, the steering wheel was projected on the forward screen. The subject’s task was to place the points of the probes precisely on the outer edge of the steering wheel at the 3:00 and 9:00 o’clock positions. When the subject’s probes had stabilized at their perceived positions the distance between the probe tips was measured, indicating the perceived size of the virtual steering wheel, and the distance between the probe tips and the CAVE forward screen was measured, indicating perceived distance in space. Three measurements were recorded for each of the four prism powers, and goggle prism powers were interleaved. Thus, twelve measurements were recorded for each subject. The three measures for each condition were averaged, yielding a mean measure per condition. Clinical measures were plotted against experimental size and distance measures, and correlations were made using polynomial regression (with all statistical significance via linear fit).

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7. Results decreases with base-out prism. However, there were concurrent changes in the variability of those measurements as indicated by the change in standard deviation (Figure 4b).

Clinical findings by themselves yielded no surprises. Relationships between age and accommodative amplitude were typical of the literature, and the spread of phorometric data approximated normal populations.

8. Discussion There was a significant variation in the size and distance measures among the subjects, as well as the degree of change in those measures as a factor of induced prism power. However, strong and appropriate correlations were seen between the size and distance measures, demonstrating that CAVE geometry was preserved in all cases. Thus, when steering wheel size was measured as inappropriately large, the distance from the screen was correspondingly reduced (Figure 2).

12 BO Wheel Width 8 BO Wheel Width Plano Wheel Width 8 BI Wheel Width

There were two goals of this study: 1) to relate clinical vision parameters to CAVE perceptions and 2) to determine whether changes in induced prism power affect perceived size and distances of virtual CAVE features. From a clinical standpoint the most salient relationship found was with positional measures in a test of gross stereopsis. This should not be surprising, as the virtual steering wheel in the CAVE can also be thought of as a

Gross Stereo .60

AC/A @ 30” .45

Phoria @ 16” .24

Age

IPD

AC/A @ 16”

Phoria @ 30”

.20

.08

.04

.08

.65

.42

.24

.10

.08

.13

.05

.63

.45

.24

.20

.12

.11

.14

.52

.43

.24

.10

.16

.03

.04

AC/A @ 16” AC/A @ 30”

Rx .32 .57

Age .31 .66

Prism

Wheel Width .99

Wheel Width S.D. .99

Included in the spectrum of responses to the CAVE stimulus were subjects who showed no depth at all. These subjects reached the full distance toward the screen, and actually came in contact with the forward wall of the CAVE. Likewise, the size of the perceived wheel was the full size of the screen projection. In most cases these subjects showed no sensitivity to changes in induced prism power. Figure 3 features regression curves between size and distance measures and their associated correlation coefficients. Tables 1-3 list those correlations. Figure 4a shows the mean effects of induced prism on the size and distance measurements. As can be seen, apparent size increases with base-in (diverging) prisms, and

feature requiring only gross stereopsis to perceive it in depth. However, this data suggests that a test similar to the Titmus Stereofly could be developed and employed as a fundamental screening method for predicting a subject’s performance in the CAVE. The second relationship was with AC/A as measured at the CAVE accommodative demand (30”). Although the correlation is lower, it should be noted that AC/A and subject age were correlated, but that age and perceived width were uncorrelated. This shows an independence of the measures on age, and the salience of the AC/Aperception relationship. It demonstrates that CAVE perceptions are affected, at least to a statistically significant extent, by habitual physiological parameters such as focusing and eye convergence.

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It is interesting that subjects indicated that although their perception of stimulus size and distance could be clearly changed by prism power induced over the stereo goggles, the measured changes were far less than would be predicted by those initial impressions. We propose that our methods of position measurement may have been contaminated. As the measurement probes were brought into the visual field for each measurement, it is likely that the probes were themselves rescaled in size and distance, thus “nulling out” the actual perceived changes. This is also suggested by a behavior that we noted, where subjects would initially place the measurement probes at one depth, followed by rescaling movements of the probes to a second and stable position. The next study will therefore take a diametrical strategy: using a modified Risley prism apparatus mounted over the stereo goggles, subjects will view the CAVE and adjust the prism power until the CAVE stimulus features appear at appropriate depths. Although this method is likely to show more variability, it keeps the measurement apparatus out of the visual field, and should answer the question about measurement nulling and rescaling. Finally, there were subjects in this study for whom the CAVE stimulus failed completely. These subjects located the stimulus at the plane of the forward wall no matter what modifications were induced. It is likely that there will indeed be subjects for whom virtual reality fails, and for whom the CAVE is ineffective for purposes such as engineering, design and marketing. In contrast, there are subjects with profound visual deficits such as severe amblyopia, strabismus or monocular blindness for which the CAVE stimulus provides an appropriate and usable environment. This strongly suggests that there is a significant cognitive component that should be addressed in future studies.

9. Acknowledgements The authors would like to acknowledge Richard R. Pawlicki for his help with the described experiment. The CAVE is a trademark of the Board of Trustees of the University of Illinois.

10. References Leibowitz, H and Moore, D: Role of changes in accommodation and convergence in the perception of size, J. Opt. Soc. Am. 56(8):1120-1123, 1966. Alexander, KR: The foundations of the SILO response, Optom. Weekly, 65(18):446-450, 1974.

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Figure 2. Size and distance correlation

Figure 3a

Figure 3b

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Figure 3c

Figure 3d

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Figure 4.

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