Oculomotor changes within virtual environments

Applied Ergonomics 30 (1999) 59—67

Oculomotor changes within virtual environments

Peter Alan Howarth*Visual Ergonomics Research Group (VISERG), Department of Human Sciences, Loughborough University, Loughborough,

Leicestershire LE11 3TU, UK

Received 16 September 1996; accepted 6 August 1998

Abstract

This paper discusses the oculomotor changes which might be expected to occur during immersion in a virtual environment whilstwearing a Head mounted display (HMD). To do so, it first examines the stimulus presented to the eyes, and then considers how thisstimulus could affect the visual system. Theoretical analysis and empirical results from the use of three different HMDs point towardsthe same conclusion, that in this context a mismatch between the instrument inter-ocular distance (IOD) and the user’s inter-pupillarydistance is of little concern, unlike the mismatch between the instrument IOD and the inter-screen distance. ( 1998 Published byElsevier Science Ltd. All rights reserved.

Keywords: HMDs; Oculomotor system; Vision; Heterophoria; VR adaptation

1. Introduction

The increasing availability of computing systemswhich generate artificial, virtual, environments hasbrought about an increased awareness that the equip-ment used to view these environments may not be prob-lem free. As well as potential psychological problemsassociated with immersion in computer-generated envi-ronments, and changes in adverse symptoms, physio-logical changes have been reported by some people usingthe head mounted displays which are part of the system.Many of these changes are associated with the visualsystem, and this paper aims to identify changes thatmight be expected to occur to the oculomotor system.

Concern about potential visual problems was initiallyraised by a report which indicated US military helicopterpilots failed stereoscopic depth perception tests followingthe prolonged use of night vision goggles (Sheehy andWilkinson, 1989). The goggles used were of a similardesign to virtual reality (VR) HMDs. Subsequent re-search using commercially available HMDs showed thatphysiological changes in the visual system occur afterperiods of exposure of the order of 20 min (Mon-Will-iams et al., 1993; Miyake et al., 1994; Rushton et al., 1994;Howarth and Costello, 1996). Subjects have also

*Tel.: 00 44 01509 223040; fax: 00 44 01509 223940; e-mail: [email protected]; URL: http://www.lboro.ac.uk/departments/hu/groups/viserg/viserg1.htm

reported symptomatic changes, both anecdotally andusing more structured methods such as questionnaires(Regan and Price, 1993a,b; Dobson, 1993; Regan, 1995;Kennedy et al., 1995; Cobb et al., 1995; Howarthand Costello, 1996; Howarth, 1998). Symptoms thathave been reported in the various studies include head-aches, tired eyes, blurred vision, nausea and doublevision.

To date, concern about changes within the visual sys-tem has largely focused on potential causal factors ofvirtual simulation sickness (Wilson, 1996, 1997;Howarth, 1998). However, physiological changes couldoccur independently of these symptom changes. Thiscould happen when, for example, sensory conflict gaverise to feelings of nausea whilst at the same time anoptical misalignment gave rise to changes in oculomotorbalance, which, as described below, refers to the positionof the two eyes. The ocular changes could have short-term asthenopic (eyestrain and headaches) consequences,but, depending upon the immersion conditions, couldalso have long-term adaptational consequences. This isnot necessarily detrimental to the visual system, and, infact, it has been suggested (Howarth and Winn, 1994)that orthoptic visual training could be accomplished byusing HMDs, with virtual environments providing excit-ing visual conditions. Although immersion for a single,short, period is unlikely to be permanently detrimental tothe visual system, the effects of both long-term exposureand repeated exposure are, as yet, unknown.

0003-6870/99/$ — see front matter ( 1998 Published by Elsevier Science Ltd. All rights reserved.PII: S 0 0 0 3 - 6 8 7 0 ( 9 8 ) 0 0 0 4 3 - X

This paper outlines the physiological oculomotor cha-nges which might be expected to occur during immersionin a virtual environment. To do so, it first examines thestimulus to the eyes presented by an HMD, and con-siders how this stimulus could affect the oculomotorsystem. Results from a recent study into the use of VRequipment are then presented and are shown to fit thesetheoretical expectations.

1.1. The optical system of an HMD

Conventionally, the design of most HMDs consists ofa pair of screens with a combination of lenses or mirrorsplaced in front. The optical system allows the eyes tofocus on the screens, which may be physically only50—70 mm away. The purpose of using two screens is toenable, if the software allows, each eye to see a slightlydifferent view of the displayed scene. This gives rise to theimpression of apparent depth due to the stereoscopicfusion of the images.

The basic elements of an HMD optical system areshown in Fig. 1. The system is mounted within a helmetor headset which can be worn by the user, althoughdesigns such as the Fakespace binocular omni-orienta-tion monitor (BOOM) (Bolas, 1994) do not need to beworn on the head. The display screens most commonlyused are liquid crystal displays (LCD), however, moreexpensive systems may use cathode ray tube (CRT) tech-nology as this provides greater resolution.

The arrows on Fig. 1 show the potentially adjustablefeatures of an HMD. The two most important in thecurrent context are the inter-ocular distance (IOD) andthe inter-screen distance (ISD). The former is determinedphysically by the separation of the HMD optical systems,the latter is a function of both the physical separation ofthe screens and the software placement of the images onthe screen. If HMDs are not custom fitted to the indi-vidual, difficulties might well arise because differentpeople have different eye separations. At first sight, themain problem which might be expected to accompanythese differences is a prismatic adaptational effect on theoculomotor system, leading to changes in oculomotorbalance (heterophoria). The next section considers thetheoretical background to this expectation, and shows itto be false.

1.2. Heterophoria

Most people who have two normal eyes generally seethe world singly. This is because the brain combines thesignals from the two eyes. For this synthesis to happen,the eyes needs to be coordinated so that they point in thesame direction. There are a variety of mechanisms whichprovide input into the binocular motor coordinationsystem, of which accommodation is one, but the ‘finetuning’ which ultimately determines eye position is

Fig. 1. Key optical factors in HMD design. Arrows show the potentialfor adjustment of the equipment to suit an individual, including theinter-screen distance (ISD) and the inter-ocular distance (IOD). Thedistance between the eyes (the inter-pupillary distance, IPD) varies fromperson to person. In the configuration shown, part of the Image(shaded) is seen binocularly (and hence stereopsis is possible) and partof it is seen monocularly (unshaded). (Adapted from Howarth, 1994).

provided by visual feedback (see Harris, 1994; Rushtonand Riddell, 1999). For the normal visual system, thisfeedback acts to prevent the diplopia (double vision)which would be present if the eyes were not aligned. Ifyou cover one eye, by doing so removing the sensoryfeedback, then this eye will take up a position of rest (a‘passive’ position). If this position of rest differs in anyway from the normal eye position (when feedback isallowed, the ‘active’ position) the person is said to havea heterophoria. An outward (away from the nose) move-ment of the covered eye is termed an exophoria, and aninward movement is termed an esophoria.

Heterophoria is a condition of motor imbalance of theeyes where the passive, fusion-free, position of the eyesdeviates from the active position. Normal subjects exhi-biting heterophoria can generally obtain binocular fix-ation but must exert a fusional effort to do so. For somepeople, the effort required to do this may give rise tostrain and visual discomfort (Sheedy and Saladin, 1978).Heterophoria is measured in prism dioptres (*) where 1*represents an angle with a tangent of 1/100. (If aneye moves between two points on a wall which are a

60 P.A. Howarth / Applied Ergonomics 30 (1999) 59—67

centimetre apart, then the eye movement is 1* when thewall is 1 m away.) Heterophoria will vary dependingupon the inputs to the motor control system, and suchthings as different accommodation demands will changeit. It can also be changed by wearing prisms (Maddox,1893; Carter, 1963, 1965; Sethi, 1986; Schor, 1986; Sethiand North, 1987), or by using misaligned binocular op-tical instruments, and this is the issue which will beaddressed here.

1.3. HMDs and eye position

In the adult population at large there is considerablevariation in the distance between the eyes, usually mea-sured as the interpupillary distance (IPD). IPDs rangefrom roughly 53—73 mm with an average IPD for theadult population of approximately 63 mm (North, 1993).Ideally, the IPD measured for the user should coincidewith the distance between the centres of the lenses of theHMD, which should then be placed directly in front ofthe HMD screen. If this is not the case, the user will viewthe virtual image through an off-centre portion of eachlens. If an HMD is correctly designed and used, then theprismatic distortion arising from this mismatch is nota serious concern even if (as is usually the case) the imageis slightly within optical infinity. There is a caveat, how-ever, which is that the distortion may be a problem forametropes, and also for emmetropes when the virtualimage is not close to optical infinity. Both situations aresimilar to the situation of a person wearing incorrectlypositioned spectacle lenses. A fuller consideration of theissue is provided by Peli (1995, 1996a).

Optometrists and opticians go to great length, parti-cularly for those patients who have high-power prescrip-tions, to ensure that the optical centre of spectacle lensesare positioned directly in front of each eye. In some cases,such as varifocals, this is because the particular form ofthe lens requires it to be precisely positioned so thatcomplicated asymmetric distortions are avoided. In othercases, it is because an incorrectly centred lens will intro-duce simple prismatic distortions as will be shown. Thisdistortion will be manifest as a differential shift in appar-ent position of the images seen by each eye, and it isaccepted clinically that this shift is of concern because itcan lead to headaches and ‘eyestrain’ for some patients(Sheedy and Saladin, 1978).

To illustrate the problem, consider a person who ismyopic (short-sighted). This person has a ‘far point’,when the eye is relaxed and unaccommodated, which iscloser than infinity. Anything further away thanthis looks blurred to the person, unless they are wearingan optical correction. A myope is corrected with a nega-tive lens, which acts to diverge light. Considering Fig. 2,an object which is optically at infinity (e.g. a star)will send parallel light rays to the lens. This lens thendiverges the light, and for a person looking through it

Fig. 2. Correction of myopia by a negative lens.

the light rays now act as if they were coming from animage closer than infinity. If the negative lens properlycorrects an individual’s myopia, then the virtual image ofthe star is positioned at the person’s far point. Althoughperceptually the person sees the star as being a long wayaway, optically they are actually looking at something(the image of the star) which is only feet away from them.In this situation, if the person’s eye is directly behind theoptical centre of the lens, A, the direction of the image isthe same as that of the object. However, if the eye is notcentred behind the lens, B, then it needs to turn if it is tolook towards the image.

If the figure were an illustration of a left eye, such thatwhen the eye is at position B the optical centre of thespectacle lens is displaced to the left of it, then the personwould need to turn this eye outwards to look at the star.Similarly, if a negative spectacle lens in front of the righteye were displaced to the right, then the right eye alsowould need to turn outwards. This situation, with eachlens displaced outwards, would occur if the distancebetween the spectacle lens centres was larger than thedistance between the person’s eyes. It is not hard toimagine that having both eyes pointing outwards couldlead to eyestrain.

Exactly the same problem arises with positive lenses,but here an outward displacement of the lenses causes theeyes to turn in, and an inward displacement of the lensesturns the eyes out. The higher the lens power, the greaterthe effect of any mismatch between the spectacle lensinterocular distance (IOD) and the patient’s interpupil-lary distance (IPD).

Consider now the situation where a normally sightedperson is viewing an HMD screen through a high-pow-ered positive lens. If the screen is placed at the focal pointof the lens, as in Fig. 3, then the light rays emerging fromthe lens are collimated, or, in other words, they emergefrom the lens parallel as if they had come from an objectat infinity. Here, if the eye moves from position A toposition B, it maintains its direction, and does not need toturn outwards or inwards, unlike the situation with thespectacle lens. The reason for the difference between the

P.A. Howarth / Applied Ergonomics 30 (1999) 59—67 61

Fig. 3. Eye position when viewing a screen: when the screen is at the focal point of the lens, the emerging light is collimated.

two situations is that in one the person is looking at animage which is very close, even though the object is a longway away whereas in the other they are looking at animage which is a long way away, even though the object isvery close. The practical consequence for HMD design isthat if the inter-screen distance (ISD) equals the inter-ocular distance (IOD) the two eyes will be parallel, irre-spective of the individual’s IPD, as long as the image is atinfinity.

From this analysis one might conclude that HMDswhich form an image at optical infinity will be unaffectedby any mismatch between IOD and IPD. However, thiswould be an over simplification because there is also theposition of the object to consider, and in an HMDcontaining two screens the ISD may not equal the IOD.

1.4. Mismatch between lens and screen

The inter-screen distance (ISD), see Fig. 1, can be seteither in hardware, by the physical separation of the twoscreens, or in software, by the positioning of the image onthe screens. Fig. 3 showed a (monocular) situation inwhich the lens and screen centres are aligned; considernow what happens when they are not (Fig. 4).

Here the eye, at position A, is directly behind the lenscentre, which corresponds to the situation in which theIOD and the IPD coincide. However, the screen is dis-placed, as it would be if the ISD differed from the IOD. Ifthis were the left eye and left optical channel of the HMD,then the eye would have to turn inwards in order to lookat the object O on the centre of the screen. Because thelight rays are leaving the lens collimated, or parallel, theamount the eye turns is independent of the eye position

Fig. 4. Eye position when viewing a screen: when the screen is dis-placed relative to the lens, the eyes are rotated.

(compare B with A), and so the turn required is the sameeven though the eye is not directly behind the lens centre.

Applying this analysis to the binocular case, if thecentres of the screens are not directly behind the opticalcentres of the lenses, i.e. if the ISD and the IOD do notcoincide, the eyes will each be forced outwards when theISD'IOD, and inwards when the ISD(IOD. As longas each screen is positioned at the focal point of the lens,these eye positions will be independent of the subject’sIPD and will depend solely upon the ISD/IOD mis-match. This mismatch will, in effect, produce prismaticdistortion in opposite directions for the two eyes.

The physiological effects of prismatic distortion arewell documented in optometric literature, and may lead


to either adaptation in the visual system or asthenopia(or both) depending upon the individual (see Sethi, 1986).In this context it is worth noting that stimulus conditionssuch as these are introduced by Orthoptists when theywish to alter the (plastic) oculomotor system of children.The consequences of this treatment are generally transi-ent discomfort for the patient, along with strengtheningof their oculomotor system.

Prismatic distortion is not just a theoretical concern:Regan (1995) has suggested that a mismatch between thesubject and the equipment leads to asthenopic symp-toms, and support for this idea has come from Kolasinski(1996). However, both authors report that subjectivesymptoms increase with increasing IPD, whereas thepreceding analysis suggests that any heterophoria changewhich occurs should be essentially independent of IPD.To examine this issue we have further analysed datacollected in a study recently carried out by VISERG(Costello and Howarth, 1996). This study was performedin conjunction with VIRART, at the University of Not-tingham, for the UK Health and Safety Executive.

2. Method

Subjects were immersed in three different virtual envi-ronments, using three different VR systems. Each systemused a different HMD, and these are detailed in Table 1.Distance heterophoria measurements were taken bothbefore and after immersion, and the metric used for theanalysis was the change over the immersion period. TheIPD of each subject was measured to allow determina-tion of its association with this heterophoria change.

2.1. Experiment

The first VR system used a Virtual i-glasses HMD witha 486 PC, which was used as being representative of

Table 1The three VR systems used

Virtual i-glasses Virtuality Division

3-D Scenario Yes Yes YesStereoscopic

display No-bi-ocular No-bi-ocular Yes, 46.4° overlapIOD 60 mm 70 mm 64 mmField of view

(h]v) 30°]23.6° 60°]46.8° 105°]41°IOD adjust No Possible (fixed in

this experiment)Software

configuredFocust adjust No, spectacles

OKYes No, some

spectacles OK

home computers at that time. The Virtual i-glasses head-set weighed 0.226 kg and had a fixed inter-ocular distance(IOD) of approximately 60 mm. The headset was capableof providing stereoscopic 3D images, but the softwareused in the experiment (the shareware version of the PCgame Heretic) generated identical bi-ocular, non-stereo-scopic images to the two eyes. The second system wasa Virtuality/IBM Elysium VR entertainment system, us-ing a Virtuality Visette II headset. Although adjustable,the IOD of the headset was left at a setting of 70 mm tomaintain consistency between subjects. Again, imageswere presented bi-ocularly, not stereoscopically. Thethird system was a Division Provision 100VPX whichused a dVISOR headset. The software was configured toallow a 46.4° stereo overlap in the centre of the visualfield. The IOD of this headset is fixed at approximately64 mm. The position of the screen images was softwareadjusted to give an ISD of 63.5 mm, and portions of someimages were presented with an added disparity to pro-duce stereopsis.

2.2. Subjects

In all, 41 subjects were used, although the subjectnumbers differed slightly between the conditions. Themean age of the subject pool was 27 years (range 19—56years) and the gender split was 9 female and 32 male. Allpotential participants were questioned to ensure the ex-clusion of those people with medical conditions, such asepilepsy, head and neck injuries; also those with abnor-malities of the visual system, such as high refractive error,constant strabismus, abnormal eye movements, and lessthan 6/6 (20/20) binocular visual acuity with any correc-tion worn, were excluded from the study.

For the Virtual i-glasses condition there were 31 sub-jects, 37 subjects used the Virtuality equipment, and 40took part in the Division condition. All but one of thesubjects who took part in the Virtual i-glasses conditionalso took part in the other two conditions. The subjects’IPD details are given in Table 2.

2.3. Measurements

The assessment of distance heterophoria was carriedout using a Maddox rod (a standard Optometric test)

Table 2Subjects’ interpupillary distances

IPD Mean (mm) Range (mm) S.D. (mm)n"41

All subjects 63 56—71 3.8Males 64 59—71 4Females 61 56—65 3.3


and a calibrated horizontal scale which contained a smallwhite light at its centre. The subject was asked to stand ata marker 4.5 m from the scale and hold the Maddox rodin front of their right eye. The illuminance within theroom was reduced, and the subject viewed the scale andlight with both eyes open. When seen through the Mad-dox rod, the light appeared as a vertical red streakperpendicular to, and passing through, the measurementscale. The red streak was all that the right eye could see,and because the left eye was looking at the scale there wasno binocular fusion stimulus present. The eye thereforetook up a fusion-free passive position, which was evalu-ated when the subject reported the number on the scalenearest to the point where the red streak appeared tocross it. The measurement was taken both before andafter each immersion.

Interpupillary distance was measured using a standardoptometric technique, in which a ruler was placed belowthe subject’s eyes. The subject looked in turn to theobserver’s two eyes (to avoid parallax problems) and,after the zero point had been aligned with the pupilmargin of one eye, the IPD was read directly from theruler by observing the pupil margin of the other eye.

3. Results

3.1. Comparison between the three systems

The first question to consider is whether there was anychange in distance heterophoria as a consequence ofwearing the HMD in any of the conditions. This questioncan be answered by averaging the change within eachgroup for each condition. The means and standard errorsare given in Table 3; clear significant changes were seenfor all three systems. The Virtual i-glasses and Divisionsystems both induced significant exophoric changes (eyesturning outwards) whilst the Virtuality system inducedsignificant esophoric changes (eyes turning inwards).Given that most subjects were present in all three groups,the difference between the conditions is unlikely to havebeen because of subject differences. Rather, the differ-ences are more likely to have been caused by the differentoptical configurations of the systems.

Table 3Summary of statistical calculations for distance heterophoria measure.A positive value indicates an esophoric shift

Condition Virtual i-glasses Virtuality Division

Group mean !0.40 0.62 !0.48SEM 0.16 0.15 0.16n 31 37 40t-value !2.58 4.13 !3.11Significance level p(0.01 p(0.005 p(0.005

Statistical evaluation was performed using t-tests andfor each condition the mean change was found to differsignificantly from zero.

3.2. Comparison between the subjects

The average results, however, say nothing about indi-vidual changes. Having found that using each of theHMDs induced heterophoric shifts, we may now askwhether these shifts are associated in any way with thesubjects’ IPD.

It was argued in Section 1.4 that the change in hetero-phoria would be independent of the subjects’ IPD. Thisquestion can be examined by plotting the heterophoriachange against subjects’ IPD. Figs. 5—7 show this analy-sis for the three systems. If IPD and heterophoria changewere independent we would expect to see no pattern inthese graphs, other than having the data points parallel-ing the x-axis.

Fig. 5. Change in distance heterophoria for different IPDs: Virtuali-glasses VR system. A negative change indicates an exophoric shift.Squares signify a single subject, open circles signify two subjects withthe same IPD and heterophoria change, filled triangles signify threesubjects with the same IPD and heterophoria change.

Fig. 6. Change in distance heterophoria for different IPDs: VirtualityVR system. A negative change indicates an exophoric shift. Symbols areas for Fig. 5, but in addition the open inverse triangle shows foursubjects with the same IPD and heterophoria change.


Fig. 7. Change in distance heterophoria for different IPDs: DivisionVR system. A negative change indicates an exophoric shift. Symbols areas for Fig. 6.

The distribution of points in Fig. 5 suggests that thereis little relationship between the measures, and althoughthere is a negative correlation between them it is weakand non-significant.

Fig 6 shows the same change for the Virtuality HMD.Again, the distribution of points suggests that there islittle relationship between the measures and althoughthere is a negative correlation between them, again it isweak and non-significant. It is worth noting here thatmost points lie above the axis, indicating that the changesare esophoric (inwards) in direction. This is the oppositedirection to that predicted by Regan (1995), given thatthe system IOD was 70 mm and all but two subjects hadan IPD less than this.

The pattern of the data points in Fig. 7 once againsuggests that there is little relationship between themeasures, and the negative correlation between them isnon-significant.

These results support the idea that induced hetero-phoria is essentially independent of subjects’ IPD. Al-though for all three conditions there is a very slightnegative slope to the regression line, none of the functionsare significantly different from zero. Furthermore, thedata sets are not independent of each other as manysubjects are present in all three.

4. Discussion

The relationship between changes in heterophoria andexposure to immersive virtual environments has pre-viously been investigated with a limited set of headsets(Mon-Williams et al., 1993; Dobson, 1993; Rushton et al.,1994; Regan and Price 1993a,b; Kolasinski 1996). Thecurrent study has shown that heterophoria does changeas a consequence of using VR headsets, but that thechange which occurs is idiosyncratic to the equipment. Itcan be deduced that this change is not a consequence ofeye position, because a change in IPD (i.e. change in eye

position) has a negligible effect on heterophoria. It isworth noting that the direction of change for the Virtuali-glasses (the only headset for which the information wasavailable) was as expected by the manufacturers on thebasis of the optical design (Peli, 1996b).

The changes found cannot be explained on the basis ofthe suggestion put forward by Wann’s group (e.g. Wannet al., 1995) to account for the heterophoria differencesbetween the studies of Mon-Williams et al. (1993) andRushton et al. (1994). Their idea is based on the fact thatin the former study the presentation was stereoscopic,whereas in the latter it was non-stereoscopic (bi-ocular).Although in the current study exophoric (outwards) chan-ges were seen after the use of the Division (stereoscopic)system, and esophoric (inward) changes were seen afterthe use of the Virtuality (bi-ocular) system, the exophoricchanges seen after the use of the bi-ocular Virtuali-glasses system mirror those of the Division system andnot those of the Virtuality system, as one would expectfrom their suggestion.

There are two further possible causal factors of hetero-phoria change in users of VR headsets. The first is prismadaptation caused by an optical mismatch (as discussedearlier). The changes in heterophoria associated withprism adaptation in other circumstances are welldocumented (e.g. Maddox, 1893; Carter, 1963, 1965;Ogle, Martens, and Dyer, 1967; Sethi, 1986; Sethi andNorth, 1987). The other factor which may give rise toheterophoria changes is the potential of HMDs to inducetransient myopia. Transient myopia can be regarded asan accommodation spasm which leads to consequentialchanges in convergence, and hence heterophoria changes.

A study of immersive VR use for an assembly task byMiyake et al. (1994) reported that HMD wearers experi-enced a myopic shift in the order of !0.2 to !0.35dioptres after 36 min of VR work. A previous study ofVDU use (Yeow and Taylor, 1989) showed a small butsignificant myopic shift after continuous work. It is clin-ically established that uncorrected refractive errors canbe related to visual symptoms such as headache, blurredvision and eyestrain, and so some measure of changes inrefractive error is clearly important if one is assessing thevisual impact of immersive VR. For a review of this areasee Ong and Ciuffreda (1995).

If the equipment were correctly aligned, one mightexpect esophoric changes to be seen accompanyinginstrument myopia. However, if the equipment wereincorrectly aligned one would expect a change in hetero-phoria, but the direction of this change would dependupon the direction of misalignment between the screenand the HMD optical system (i.e. the difference betweenthe ISD and the IOD in Fig. 1). Although both explana-tions could account for changes in distance heterophoria,only optical misalignment can explain the different chan-ges seen with the different systems. Transient myopiawould induce an associated esophoria, not exophoria,


and two of the three systems produce an exophoricchange amongst the users. Of course, it is possible thatboth factors contributed to the observed changes in het-erophoria.

Turning now to the issue of individual differences inIPD, one would expect heterophoria change to be inde-pendent of IOD when the screen is imaged at infinity.However, this is not always the image distance chosen bythe hardware or software manufacturers. The Virtuali-glasses screen, for example, is imaged at a distance ofeleven feet (about 3.3 m). Peli has analysed the effect onthe visual stimulus, and states that ‘‘looking througha correcting lens not through the centre causes a pris-matic effect. The magnitude can be estimated from Pren-tice Rule2 the prismatic effect is equivalent to thatgenerated by a lens with a focal distance equal to thevirtual image distance. Thus, for an image distance of 2 mthe prismatic effect due to mismatch between the user’sIPD and the system IOD of 10 mm there will be a 0.5 *(prism dioptre) of prismatic effect, which is very small’’(Peli, 1996b). Applying this analysis to the Virtuali-glasses HMD, an image distance of 3.3 m would inducea 1/3 * effect for each eye for every 10 mm of mismatch.Given that, in the current experiment, the range of IPDswas 15 mm, the difference in prismatic effect between thesmallest and the largest would have been 0.5 *. Thisfigure, in the current context, is trivial, and is far smallerthan the range of changes seen in Figs. 5—7. Peli’s analysisactually predicts a very shallow positive slope to theregression line of the data shown in Fig. 5, rather than theslight negative slope found. However, the difference is sosmall that it is of no practical consequence, and is notunexpected given the subject numbers.

In summary, the use of HMDs in immersive VR canlead to changes within the oculomotor system. These donot necessarily correlate with changes in adverse symp-toms, but given the multitude of other causal factors ofvirtual simulation sickness (Howarth, 1998), this is notsurprising. However, this lack of correlation does notdiminish the importance of these changes, particularlygiven that the oculomotor and visual systems are knownto adapt to abnormal conditions. Theoretical analysisand empirical results point towards the conclusion that,in this context, a mismatch between the instrument IODand the user’s IPD is of little concern. However, a mis-match between the instrument IOD and ISD is, theoret-ically, of far greater concern; the effect of this mismatchon the visual and oculomotor systems remains to bedetermined.

Acknowledgements

This research was supported by the UK Health andSafety Executive, under research contact 3181/R53.133.I would like to acknowledge helpful disussions with Pat-

rick Costello, Eli Peli, Mathew Finch and John Wilsonwhich have taken place over the last two years.

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Oculomotor changes within virtual environments