Effects of a Head-Mounted Display on the Oculomotor System of Children

ORIGINAL ARTICLE

Effects of a Head-Mounted Display on theOculomotor System of Children

Peter Kozulin*, Shelly L. Ames†, and Neville A. McBrien‡

ABSTRACTPurpose. This study evaluated the effects of short term and extended viewing of virtual imagery using the BinocularViewer (new generation bi-ocular viewer) on the visual system of children, and compared these effects with that ofviewing a high definition television (HDTV) display.Methods. Sixty children aged 5 to 16 years viewed 30 min of virtual imagery using the Binocular Viewer and a HDTVdisplay on two occasions. Sixteen subjects, aged 13 to 16 years, completed a third session of extended viewing (80 min)with the Binocular Viewer. Oculomotor function and symptoms were assessed previewing, immediately postviewing, and10 min postviewing.Results. Thirty minutes of Binocular Viewer use resulted in symptom increases (p � 0.05) immediately postviewing(“feeling tired,” “feeling sleepy,” “difficulty concentrating,” and “sore/aching eyes”) however most symptoms haddissipated by 10–min postviewing. There were no significant symptom differences between viewing with the BinocularViewer and the HDTV display at either time point. An increase in symptoms (p � 0.05) immediately postviewing wasrecorded after 80 min of Binocular Viewer use (“feeling tired,” “feeling bored,” “feeling sleepy,” and “tired eyes”),however only “feeling tired” and “feeling bored” remained significantly increased (p � 0.05) 10-min postviewing. Nearunaided visual acuity demonstrated a significant and consistent reduction immediately (p � 0.01) and at 10 min (p �0.05) following 30 min of Binocular Viewer use and immediately following 80 min of use (p � 0.01). Near unaided VAwas also significantly reduced (p � 0.01) immediately after 30 min of HDTV display use.Conclusions. Virtual imagery viewing with the Binocular Viewer in children aged 5 to 16 years had few additionaladverse effects when compared to viewing a more conventional HDTV display. The Binocular Viewer was comfortableto wear for up to 80 min of viewing. The consistent reduction in near vision for both viewing durations with the BinocularViewer requires further investigation.(Optom Vis Sci 2009;86:845–856)

Key Words: head-mounted displays, virtual reality, symptoms, ergonomics, oculomotor system

Virtual reality (VR) has been described as an “interactivecomputer-generated environment.”1 A VR set-up usuallyconsists of a computer system to generate virtual imagery,

a visual display and a variety of interactive devices. Visual displays,however, can be used on their own to view non-computer gener-ated, non-interactive imagery. A popular device for viewing virtualimagery today is the head-mounted display (HMD). Since theirintroduction in the 1960s,2 HMDs have advanced technologically,improving in display quality and ergonomics. Their applicationsinclude medical training,3–5 engineering,6 military aviation,7,8 and

an expanding entertainment market.9 However, there have beenreports of adverse side effects on humans after viewing virtualimagery with HMDs. These examples include ocular and non-ocular symptoms,10–13 physiological changes,14 and oculomotorchanges.11,15–17 Inconsistencies in the findings of previous studiesinvestigating the effects of HMD use can be attributed in large partto differences in the optical and ergonomic characteristics of theindividual HMDs assessed, the test batteries administered, theviewing times, and the presence or absence of an interactive com-ponent to the imagery viewed.

HMDs can be categorized as being either monocular, biocular(non-stereoscopic), or binocular (stereoscopic). Monocular HMDspresent imagery to only one eye and are commonly used in militaryaviation.7,8 Biocular HMDs present identical imagery to both eyes.The lack of disparity in the imagery results in constant accommo-

*BOptom†PhD, BSc(Optom)‡PhD, MCOptom, FAAODepartment of Optometry and Vision Sciences, University of Melbourne,

Melbourne, Victoria, Australia.

1040-5488/09/8607-0845/0 VOL. 86, NO. 7, PP. 845–856OPTOMETRY AND VISION SCIENCECopyright © 2009 American Academy of Optometry

Optometry and Vision Science, Vol. 86, No. 7, July 2009

dative and vergence demands, but these are not always matched. Inbinocular HMDs, different images corresponding to the left andright eye views, are relayed to the appropriate eye through separatechannels.9 The images are focused at a fixed plane providing aconstant accommodative demand. However, vergence demandschange as disparity cues in the imagery change. The majority ofcommercially available HMDs are either binocular or biocularwith most studies having used the binocular type. Few studies haveinvestigated the effects of viewing non-stereoscopic imagery withHMDs and the results from these studies have been inconsis-tent.11,17–24 Although the conflicting accommodative and ver-gence demands present in stereoscopic displays have been pro-posed as a cause of adverse oculomotor or symptom responses,16

several attributes of biocular HMDs may also affect user comfort.Such attributes include the prismatic effect of high powered lensesused to focus the imagery,25 poor display quality,8 screen proxim-ity,26 and poor ergonomics.14 In addition, the presence of monoc-ular depth cues in the non-stereoscopic imagery of biocular HMDsmay lead to accommodation or vergence-related changes followingviewing.

There is also a lack of knowledge on the effects of viewing virtualimagery with HMDs on the visual and oculomotor systems ofchildren. It is known that before the ages of 6 to 8 years, the visualsystem undergoes a “critical period” during which time it is plasticand vulnerable to change imposed by abnormal visual input.27–29

The oculomotor system is also amenable to change during child-hood, however, it is unclear if plasticity is greater in children thanin adults.30–32 This is of particular concern as children are a grow-ing market for HMD use. The reducing costs of production willmake HMDs more ubiquitous and accessible to the general public,including children, and improvements in the display quality andergonomics of HMDs will also increase their appeal. In addition, agrowing application of HMD use is for entertainment, where chil-dren are a likely target. The HMD investigated in this study, theBinocular Viewer (MicroOptical, Westwood MA), is a new gener-ation HMD that incorporates many improvements on the featuresof earlier generation HMDs, including improved ergonomics anddisplay quality. Despite the presence of these upgrades, the lack ofknowledge on the effect of viewing with biocular HMDs on thehuman oculomotor system, especially the developing visual sys-tems of children, warrants investigation.

The primary aim of this investigation was to evaluate symptomsand oculomotor changes after viewing with a new generation bi-ocular HMD for 30 min, in children aged 5 to 16 years, andcomparing these changes with those that occur after viewing witha high definition television (HDTV) display. It is envisaged thatviewing with the Binocular Viewer will not lead to any more dis-comfort or oculomotor effects than viewing with the HDTV. Thesecondary aim was to determine if the effect of an extended viewingtime of 80 min led to greater symptom or oculomotor changes thanthose measured after 30 min of viewing.

METHODS

The Binocular Viewer is a new generation biocular HMD. Itrepresents a significant improvement over past generations ofHMDs in ergonomics and display quality with a light weight of58 g and resolution of Snellen equivalent 6/11. Subjects viewed

imagery with the Binocular Viewer for 30 min. In the controlcondition, subjects viewed 30 min of different imagery projectedonto a wall in non-stereoscopic mode to simulate a HDTV. Sub-jects, in the 13 to 16 years age group, returned for a third viewingsession to determine the effects of viewing imagery with the HMDfor an extended period of 80 min.

Subject Profile

The subjects consisted of 60 children, aged from 5 to 16 years.There were 29 males and 31 females with a mean age of 11.0 years.To ensure an even distribution of ages, the 60 children were di-vided into three age ranges (5 to 8 years, 9 to 12 years, and 13 to 16years) with 20 subjects recruited into each of these age groups. Allsubjects had at least 6/9.5 unaided visual acuity (VA) in each eye,stereopsis �60 sec arc, normal binocular vision, no ocular pathol-ogy and were functionally emmetropic according to the followingcriteria: �0.25 to �0.75 dioptres (D) mean spherical equivalent,astigmatism �0.75 D, and anisometropia �0.75 D mean spheri-cal equivalent. None of the subjects suffered from migraines orepilepsy. This investigation was approved by the Institutional Hu-man Ethics Committee.

Experimental Design

A modified cross over experimental design was used and in-volved two variables: viewing condition and imagery presented.The subjects were first divided into two groups according to theviewing condition; one group viewed 30 min of imagery with theHMD at the first session and with the HDTV at the second ses-sion. The order was reversed for the other group. The two groupswere balanced for age, gender, and the preliminary oculomotormeasures as shown in Table 1. Thirty minutes of viewing virtualimagery was in keeping with previous studies where viewing timesof 20 min,10,12,22 25 min,33,34 and 30 min21 have been used. Inaddition, each viewing condition group was further divided intotwo subgroups according to the imagery presented, with fifteen ineach subgroup. The first subgroup in each group viewed the twosets of imagery at the two viewing sessions in a particular order.This order was reversed for the second subgroup to reduce theeffect of treatment-period interactions. The sets of imagery usedwere animated and were similar in length and differed only in the

TABLE 1.Age, gender, and oculomotor status of subjects

Subjects’ feature Group 1 Group 2

No. subjects (n) 30 30Age (yr) 11.03 10.98Gender 15 M, 15 F 14 M, 16 FDistance vision (logMAR) �0.09 �0.05Stereo-acuity (sec arc) 29 28Near heterophoria at 50 cm (�) 2.2 XOP 1.4 XOPRefractive error as BVS (D) �0.33 �0.35

Group 1, subjects viewed Binocular Viewer first; Group 2,subjects viewed projector first; M, male; F, female; sec arc, sec-onds of arc; XOP, exophoria; BVS, best vision sphere.

846 Head-Mounted Display Effect on Children’s Oculomotor System—Kozulin et al.


story (see “Imagery” section). The two sessions were administeredat least 1 week apart.

The 13- to 16-year-old subjects participated in a third viewingsession of 80 min duration using the HMD to determine if anextended viewing session resulted in any greater symptom or ocu-lomotor changes. The duration of many animated movies is ap-proximately 80 min and this represents a typical length of timewhich children may use the HMD. Subjects younger than 13 yearsof age were not used in the extended viewing session as it wasenvisaged that fewer subject withdrawals would occur with olderchildren. Thus, a total of two sessions were completed by subjectsaged 5 to 12 years, and three sessions for subjects aged 13 to 16years. The third session was administered at least 1 week after thesecond session.

An oculomotor test battery and symptom questionnaire wereadministered to each subject at three time points in each session:immediately before viewing, immediately after viewing, and 10min after viewing, as previous studies have shown a rapid dissipa-tion of oculomotor measures and symptoms after viewing virtualimagery with a HMD.11,35,36 The oculomotor test battery andsymptom questionnaire were designed to take no longer than 10min to administer.

Viewing ConditionsExperimental Condition: Binocular Viewer

Subjects viewed non-stereoscopic virtual imagery with a Binoc-ular Viewer which had a field of view of 12° (diagonal), resolutionof 320 � 240 pixels (Snellen equivalent of 6/11), refresh rate of 60Hz and weight of 58 g. The inter-ocular distance (IOD) of thelenses used to focus the virtual imagery had three fixed settings andcould not be adjusted. The smallest size HMD (IOD 53 mm) wasused for subjects with a inter-pupillary distance (IPD) of less then58 mm, the medium size (IOD 58 mm) was used for subjects withan IPD from 58 to 62 mm, and the largest size (IOD 63 mm) wasused for subjects with a distance IPD �62 mm. The image focusplane of the HMD (all sizes) was 1 m and could not be adjusted.The mean luminance level at the subjects’ eyes was 61.0 � 48.0cd/m2 and ranged from 5 to 208 cd/m2 according to the imageryviewed. Unlike previous VR studies10,37 head-tracking was notused in the current investigation, thus the Binocular Viewer waslikened to a “personal viewing system.”18

Control Condition: HDTV

Subjects viewed imagery projected onto a featureless white wallusing an Eiki LC-7000 multimedia projector (Eiki Industrial,Osaka, Japan). The projected imagery was non-stereoscopic, witha refresh rate of 60 Hz, resolution of 860 lines and a diagonal sizeof 68 cm which simulated commercially available HDTVs at thetime of the study. The projected imagery was viewed at 3.2 m(diagonal field of view of 12° and Snellen equivalent of 6/4.2). Theluminance level at the subjects’ eyes was 30.0 � 29.8 cd/m2 andranged from 1 to 105 cd/m2 according to the imagery viewed.

Imagery

The imagery viewed in both viewing conditions was animated,dynamic, and age-appropriate (rated “G” for general exhibition).

To ensure the imagery was interesting for subjects to view, differ-ent imagery was shown to the three age groups (5 to 8 years, 9 to 12years, and 13 to 16 years) for each viewing condition. The imageryexisted on digital versatile disc (DVD) format and was commer-cially available at the time of the study. The DVDs ranged inlength from 29.1 to 31.9 min. The DVD viewed by the 13- to 16-year-old subjects in the third, extended viewing session was “Mon-sters Inc.” This DVD was a full movie length. The DVDs wereplayed on a Pioneer DV-355-S DVD player.

Oculomotor Test Battery

The test battery consisted of eight oculomotor tests that wereadministered in the following order: distance unaided VA, nearunaided VA, stereo-acuity, near point of convergence, distanceheterophoria, near heterophoria, response accommodative conver-gence/accommodation (AC/A) ratio, and the convergence accom-modation/convergence (CA/C) ratio.

Vision Assessment

Lea symbols were used to assess the vision of subjects aged from5 to 8 years. Lea symbols consist of four picture optotypes (circle,square, apple, and house) that are readily recognized by preschooland young children and are well standardized.38–40 Sloan letterswere used for subjects aged from 9 to 16 years and were of a 5 � 5non-serif format.41 Both Lea symbols and Sloan letters were inlogMAR format, i.e., symbol/letter sizes progressed with a constantratio of 0.1 log units, and both distance and near unaided VA wereassessed binocularly. The Lea and Sloan distance charts were inter-nally illuminated, high contrast (�85%) and positioned at 3 and4 m, respectively. Both Lea and Sloan near vision charts were heldat 40 cm. The 9- to 16-year-old subjects responded verballywhereas the 5- to 8-year-old subjects had a choice of respondingverbally or by pointing to one of the four optotypes presented on anearby card. Vision was recorded in logMAR format.

Stereo-acuity

Stereo-acuity was assessed with the Randot Stereotest (StereoOptical, Chicago, IL). The Randot Stereotest (test distance 40 cm)consisted of 10 groups of three circles with one circle in each grouphaving crossed disparity. The amount of crossed disparity differedbetween groups and ranged from 400 to 20 sec of arc. Each subjectviewed the targets while wearing cross-polarized filters and wasasked to identify the circle with crossed disparity. A cardboardsheet with a rectangular hole allowed each group of circles to bepresented individually while occluding all other groups of circleson the test page. The groups of circles were presented randomly toprevent recall of correct answers based on order of presentation.

Near Point of Convergence

The near point of convergence was assessed using an objectivepush-up convergence test. The subject was asked to maintain fix-ation on a small target while it was moved along the midline oftheir head toward their ocular plane. The target was a picturelocated on an apparatus known as a “budgie stick” (a double-sided

Head-Mounted Display Effect on Children’s Oculomotor System—Kozulin et al. 847


paddle containing letter optotypes, text and pictures including thatof a budgie). To maintain the interest and fixation of the youngersubjects as the budgie stick was moved closer, a question was askedabout the target such as “what color is the budgie’s head?” As thetarget was moved closer, an objective assessment of the break inbifixation was made. The distance at which a break in convergencewas detected was measured using a tape measure held to the bridgeof the nose and attached to the budgie stick. This was recorded asthe near point of convergence.

Heterophoria

Distance and near heterophoria were assessed using the Modi-fied Thorington technique. A red Maddox rod, which removed thestimulus for binocular fusion, was placed in front of one eye. Theother eye viewed a tangent scale, with a scale resolution of a halfprism dioptre. A red light emitting diode was positioned at thecenter of the tangent scale and appeared through the Maddox rodas a vertical line orthogonal to the tangent scale. For distanceheterophoria, the Maddox rod was placed in front of the right eyeand the tangent scale placed at 6 m (and internally illuminated).For near heterophoria, the Maddox rod was placed in front of theleft eye with the scale at 50 cm. The eye with the Maddox rod wasoccluded and subjects were asked to focus on the numbers on thetangent scale. Once the numbers appeared clear, the occlusion wasbriefly removed and the subject asked to immediately report theposition of the red line along the tangent scale.

Response Accommodative Convergence/Accommodation Ratio

The AC/A ratio quantifies the gain of the accommodation-vergence cross-link and is equal to the change in convergence perunit change in accommodation.42 The vergence feedback loop wasopened by placing a Maddox rod in front of the left eye so that onlystimulation from the accommodative system, via the accommodative-convergence cross-link, could induce convergence. The Shin-Nippon SRW-5000 (Shin-Nippon Commerce, Tokyo, Japan)infrared autorefractor was used to measure the accommodationresponses to different accommodative stimuli and was interfacedwith a computer to record measurements online. The SRW-5000has been shown to be an accurate and reliable autorefractor whentested on adults43 and children aged between 4 and 8 years.44 Theaccommodative stimulus was altered using five lenses ranging from�1.50 to �1.50 D in increments of 0.75 D, and three measure-ments of the accommodative response were taken for each stimuluslevel. The order of presentation of the lenses was randomized. TheModified Thorington technique (tangent scale at 50 cm) was usedto measure the change in vergence for each lens.

Convergence Accommodation/Convergence Ratio

The CA/C ratio quantifies the gain of the convergence-accommodation cross-link and is equal to the change in accommo-dation per unit change in convergence.45 The accommodativefeedback loop was opened to allow it to respond solely to inputfrom the vergence system through the convergence-accommoda-tion cross-link. The accommodative feedback loop was opened by

having subjects fixate on a spotlight that was created by mountinga 0.2 mm diameter laser-drilled precision pinhole (Ealing Optical,Watford, Herts: Catalogue no. 43-5297) in front of a white lightemitting diode. At 50 cm, a 0.2 mm diameter spotlight is a non-accommodative target.24,46 The pinhole was surrounded by a blackbackground and placed at 50 cm in front of the left eye. Thevergence stimulus was altered using five prisms that were intro-duced randomly (in front of the left eye) with powers that wereevenly distributed across the subject’s predetermined fusional ver-gence range. The fusional vergence ranges varied between 11 and26 prism dioptres (�). Once the pinhole of light had been fused,three measurements of the accommodative response of the righteye were taken for each vergence stimulus level using the Shin-Nippon SRW-5000 infrared autorefractor. The same range ofprism powers was used in both sessions for each subject.

Symptom Questionnaire

The symptom questionnaire was based on the VR SymptomQuestionnaire (VRSQ) developed by Ames et al.36 to evaluate thechange in symptoms in adult subjects after viewing with a binoc-ular HMD. The VRSQ was formulated from questionnaires usedin other investigations of the effects of viewing virtual imagery, andcontained symptoms that were most frequently reported afterviewing. The questionnaire consisted of 13 symptoms, 8 nonocu-lar, and 5 ocular, with an average administration time of 1 min.

The age range (and therefore the range in the level of compre-hension) of the subjects used in the current investigation was rel-atively large, and warranted the generation of questionnairescontaining age-appropriate vocabulary. Three variations of theVRSQ were developed and used for the three different age ranges:5 to 8 years, 9 to 12 years, and 13 to 16 years. The symptomquestions for all three age ranges are shown in Table 2. Word lists,rating the familiarity of words to children of different ages, wereused to help develop the age-appropriate questions. In addition,three descriptors were used to grade the responses of the 5- to8-year-old subjects (“no,” “a bit,” and “yes”), whereas four descrip-tors were used to grade the responses of the subjects in the twooldest age ranges (“no,” “a little bit,” “a fair bit,” and “a lot”). Thequestionnaire was administered verbally. The alternative responsedescriptors were presented to the 9- to 16-year-old subjects on acard, but were presented verbally (and in random order) to the 5-to 8-year-old subjects.

One additional question to the VRSQ was included in the currentinvestigation. The question assessed the subjects’ enjoyment of theimagery viewed and five descriptors were used to grade the response.The average time to administer both the postviewing oculomotor testbatteries and symptom questionnaires was 7.0 � 1.5 min.

Experimental Procedures

Prior to subject participation, a parent or guardian was asked toread the information sheet and sign the consent form after anexplanation of all experimental procedures and possible conse-quences. At the first session, a preliminary eye examination wasadministered by one of the authors (PK). If eligible, each subjectwas allocated to a viewing condition in their age group to bestbalance that group’s age, gender and oculomotor measures.



The oculomotor test battery and symptom questionnaire wereadministered before viewing. The subjects were then seated on afixed chair facing a wall at 3.2 m. The HMD was fitted to thesubject’s head and the room lights switched off. The duration ofthe viewing session was 30 min, during which the subject couldwithdraw at any time. Immediately after viewing and at 10-minpostviewing, the oculomotor test battery and symptom question-naire were again administered. The average time between the firstand second sessions was 14 � 2 days. However, because of theunavailability of one subject, this subject’s sessions were separatedby 5 days.

Sixteen of the 13- to 16-year-old subjects returned for an addi-tional viewing session using the HMD at least 6 days following thesecond session. The viewing time was 80 min and the oculomotortest battery and questionnaire were administered at the same timepoints as the previous two sessions. The average time between thesecond and third sessions was 55 � 11 d. There were no subjectwithdrawals from either the 30- or 80-min viewing sessions.

Data Analysis

The change in symptom rating and oculomotor measures frompre- to postviewing were analyzed using the Wilcoxon matched-pairs signed-rank test. Non-parametric statistical techniques wereused due to the categorical nature of the symptom data and thenon-normal distribution of the changes for several of the oculomo-tor measures. All normality tests were carried out using GraphPadPrism (Version 3.02; GraphPad Software, La Jolla, CA), whichuses the method of Kolmogorov and Smirnov to test for deviationsfrom normality and calculates a p value using the Dallal andWilkinson approximation to Lilliefors method.47 The four-pointsymptom rating scale used for the 9- to 16-year-old subjects al-

lowed for a maximum change in symptom rating of three, whereasthe maximum change in symptom rating for the 5- to 8-year-oldsubjects was two because they were given a three-point rating scale.The symptom data for the 5- to 8-year-old subjects were, therefore,analyzed separately from the symptom data of the oldest two agegroups.

The changes in symptom rating and oculomotor measures be-tween the viewing conditions and between the 30- and 80-minviewing sessions were also analyzed using the Wilcoxon matched-pairs signed-rank test. The non-parametric statistical tests wereperformed using Minitab Statistical Software (Release 13.1,Minitab Inc., State College, PA). The response AC/A ratios andthe CA/C ratios were calculated using linear regression analysis(GraphPad Prism, Version 3.02; GraphPad Software). Changes inthe slopes of the response AC/A and the CA/C ratios were thenanalyzed in the same manner as the other oculomotor changes,using non-parametric tests. Cross-over effects (treatment-periodinteractions and period effects) were measured using the Mann-Whitney test.

RESULTSEffects of Viewing with the Binocular Viewer for30 min

Experimental results revealed very few differences in the oculo-motor or symptom changes between the three age ranges of sub-jects and any differences were inconsistent. Thus, the oculomotordata for all subjects was pooled. Symptom data for subjects agedfrom 9 to 12 years and 13 to 16 years were pooled together. How-ever, the symptom data for the 5- to 8-year-old subjects, whichused a different symptom rating scale, remained separate. There

TABLE 2.Symptom questions for each age group

5–8-year-old subjects 9–12-year-old subjects 13–16-year-old subjects

1. Are you feeling tired now? 1. Are you feeling tired now? 1. Are you feeling tired now?2. Are you feeling bored now? 2. Are you feeling bored now? 2. Are you feeling bored now?3. Are you feeling sleepy now? 3. Are you feeling sleepy now? 3. Are you feeling sleepy now?4. Do you have a headache now? 4. Do you have a headache now? 4. Do you have a headache now?5. Are you feeling dizzy or wobbly now? 5. Are you feeling dizzy now? 5. Are you feeling dizzy now?6. Are you feeling confused or

puzzled now?6. Are you having difficulty concentrating

or paying attention now?6. Are you having difficulty

concentrating now?7. Are you feeling sick now? 7. Are you feeling sick now? 7. Are you feeling sick now?8. Is anything making you feel

uncomfortable now?8. Are you feeling generally

uncomfortable now?8. Are you feeling generally

uncomfortable now?9. Are your eyes feeling tired now? 9. Are your eyes feeling tired now? 9. Are your eyes feeling tired now?

10. Are your eyes sore or aching now? 10. Are your eyes sore or aching now? 10. Are your eyes sore or aching now?11. Are your eyes feeling like they are

working too hard now?11. Are you feeling eyestrain now? 11. Are you feeling eye strain now?

12. Do the shapes on that line look fuzzy?a 12. Do the letters on that line look fuzzyor blurred?a

12. Do the letters on that line lookblurred?a

13. Do the shapes on this line look fuzzy?b 13. Do the letter on this line lookfuzzy or blurred?b

13. Do the letter on this line lookblurred?b

aEach subject was asked to look at the line directly above the line representing the subject’s best level of vision on the distance visualacuity chart.

bEach subject was asked to look at the line directly above the line representing the subject’s best level of vision on the near visualacuity chart.



were no consistent significant differences between the symptomdata of the three age groups.

Symptom Changes

There were no significant differences between the symptom in-creases following viewing with the HMD and the HDTV at eitherof the postviewing time points in both age ranges. In addition,there were no early withdrawals from viewing with the HMD;therefore symptoms experienced were not enough to induce anysubjects to withdraw from viewing.

Immediately after viewing with the HMD, four symptoms in-creased significantly above the previewing level in the 9- to 16-year-old subjects: “feeling tired” (p � 0.01), “feeling sleepy” (p �0.01), “difficulty concentrating” (p � 0.05), and “sore/achingeyes” (p � 0.05). The increases in the symptoms “feeling tired”and “feeling sleepy” were the most commonly reported symptomincreases and were reported by 43 and 40% of subjects, respec-tively. Increases in the symptoms “sore/aching eyes” and “difficultyconcentrating” were reported by fewer subjects (18 and 15% ofsubjects, respectively). At 10-min postviewing, no symptomswere significantly increased. In the 5- to 8-year-old subjects,there were no symptoms that were increased significantly im-mediately after viewing and only one symptom was increasedsignificantly above the previewing level at 10-min postviewing:“difficulty concentrating” (p � 0.05).

After viewing with the HDTV, two symptoms were increasedsignificantly above the previewing level in the 9- to 16-year-oldsubjects: “feeling tired” (p � 0.01) and “feeling sleepy” (p � 0.01).Increases in these symptoms were reported by 40 and 35% ofsubjects, respectively. At-10 min postviewing, “feeling tired” and“feeling sleepy” remained significantly increased and increaseswere reported by 38 and 25% of subjects, respectively. Two othersymptoms also increased significantly above the previewing level:“sore/aching eyes” (p � 0.05) and “eyestrain” (p � 0.05). Twenty-three percent of subjects reported an increase in both of thesesymptoms. There were no symptoms that increased significantlyafter viewing with the HDTV in the 5- to 8-year-old subjects.

None of the significant symptom increases were caused by cross-over effects (treatment-period interactions or period effects). Thesignificant symptom increases following viewing with the HMDand the HDTV are summarized in Table 3. For symptoms thatincreased significantly in the HMD viewing condition, the per-centages of 9- to 16-year-old subjects and 5- to 8-year-old subjects

that rated either no change or an increase in the symptoms arepresented as pie charts in Figs. 1 and 2, respectively.

Oculomotor Changes

The most commonly observed oculomotor change during view-ing with the HMD was a significant reduction in near unaided VA,which occurred immediately after viewing (p � 0.01) and at 10-min postviewing (p � 0.05). The reduction in near unaided VAimmediately after viewing occurred in 67% of subjects and thechange ranged from 0.14 logMAR increase to 0.18 logMAR de-crease. At 10-min postviewing, near unaided VA was reduced in47% of subjects. Near unaided VA was also significantly reduced(p � 0.01) immediately after viewing with the HDTV in 45% ofsubjects. The change ranged from 0.06 logMAR increase to 0.2logMAR decrease. Box-plots, showing the range of near unaidedVA changes for both viewing conditions, are shown in Fig. 3. Asignificant reduction in distance unaided VA (p � 0.05) also oc-curred immediately after viewing with the HMD. Distance un-aided VA was reduced in 42% of subjects, and the change rangedfrom 0.08 logMAR increase to 0.2 logMAR decrease. Unlike nearunaided VA, the change in distance unaided VA did not remainsignificantly reduced at 10 min after viewing and was not signifi-cantly reduced after viewing with the HDTV. The reductions innear and distance unaided VA did not significantly differ betweenthe two viewing conditions.

The CA/C ratio was significantly increased (p � 0.01) imme-diately after viewing with the HMD, and remained significantlyincreased at 10 min after viewing (p � 0.05). The change in theCA/C ratio ranged from 0.028 D/� decrease to 0.050 D/� in-crease immediately after viewing and from 0.043 D/� decrease to0.069 D/� increase at 10 min after viewing. The repeatabilitycoefficient in the measurement of the CA/C ratio was 0.038 D/�.After viewing with the HDTV, the CA/C ratio was not signifi-cantly different to the previewing measure. Box-plots, showing therange of changes in the CA/C ratio for both viewing conditions, areshown in Fig. 3. Statistical outliers of the change in CA/C datawere removed from analysis. The changes in CA/C ratio did notsignificantly differ between the two viewing conditions.

Distance heterophoria was significantly shifted in the exophoricdirection immediately after viewing with the HDTV (p � 0.05). Thisexophoric shift occurred in 38% of subjects and was significantlygreater than the shift immediately after viewing with the HMD (p �0.01). The changes, however, ranged from 4� exophoric shift to 6�

TABLE 3.Symptom increases after viewing with the Binocular Viewer or the HDTV for 30 min

Viewing condition Post 0 min Post 10 min

Binocular Viewer Sore/aching eyes (p � 0.05) Sore/aching eyes (p � 0.05)Difficulty concentrating (p � 0.01) Difficulty concentrating (p � 0.05)Feeling tired (p � 0.01)Feeling sleepy (p � 0.01)

HDTV Feeling sleepy (p � 0.01) Feeling tired (p � 0.05)Sore/aching eyes (p � 0.05)

Binocular Viewer vs. HDTV NS NS

NS, no significant changes.



esophoric shift. After viewing with the HMD, distance heterophoriawas not significantly different to that measured at previewing. Box-plots, showing the range of changes in distance heterophoria for bothviewing conditions, are shown in Fig. 3.

Stereo-acuity was significantly reduced immediately after view-ing with the HMD (p � 0.01) and with the HDTV (p � 0.01). In

the HMD viewing condition, stereo-acuity was reduced in 30% ofsubjects and the change ranged from 10 sec arc increase to 50 secarc decrease. In the HDTV viewing condition, stereo-acuity wasreduced in 33% of subjects with the change ranging from a 10 secarc increase to a 100 sec arc decrease. Stereo-acuity was unchangedin 62 and 58% of subjects after viewing with the HMD and

FIGURE 1.Percentage of subjects aged from 9 to 16 years that rated either no increase or an increase in the symptoms “feeling tired,” “feeling sleepy,” “difficultyconcentrating” or “sore/aching eyes” after viewing with the Binocular Viewer or HDTV for 30 min. n � 40 for each viewing condition; BV � BinocularViewer; **p � 0.01; *p � 0.05.



HDTV, respectively. Stereo-acuity did not remain significantlyreduced at 10 min after viewing in either viewing condition. Thechanges in stereo-acuity did not significantly differ between thetwo viewing conditions.

Two oculomotor measures were significantly different from thepreviewing level, only at 10 min after viewing with the HMD: nearpoint of convergence was significantly reduced (p � 0.05) and theAC/A ratio was significantly increased (p � 0.05). The change inthe AC/A ratio ranged from 1.122 D/� decrease to 2.209 �/Dincrease at 10 min after viewing. The repeatability coefficient inthe measurement of the AC/A ratio was 1.715 �/D. Outliers ofthe change in AC/A data were removed from analysis. The

changes in near point of convergence and the AC/A ratio didnot significantly differ between the two viewing conditions.The p-values for the oculomotor changes in both viewing con-ditions are shown in Table 4.

Effects of Viewing for 80 min with the Head-Mounted Display

Symptom Changes

Immediately after viewing with the HMD, three non-ocularsymptoms increased significantly above the previewing level: “feel-ing tired” (p � 0.01), “feeling bored” (p � 0.05), and “feelingsleepy” (p � 0.01). The increases in “feeling tired,” “feelingbored,” and “feeling sleepy” were reported by 69, 37, and 69% ofsubjects, respectively. One ocular symptom, “tired eyes,” also in-creased significantly immediately after viewing and was reportedby 37% of subjects. At 10-min postviewing, the symptoms “feelingtired” and “feeling bored” remained significantly increased (p �0.05) and was reported by 56 and 37% of subjects, respectively.

There were no early withdrawals from viewing with the HMD for80 min. Thus, viewing for a period of time similar to the duration ofmany animated movies, did not lead to symptoms that were sufficientto induce any subjects to withdraw from viewing. The significantsymptom increases after viewing with the HMD for 80 min are sum-marized in Table 5. For the symptoms that increased significantly, thepercentages of subjects that rated either no change or an increase in thesymptoms are presented as pie charts in Fig. 4.

Oculomotor Changes

Similar to the 30-min viewing session, the most commonly ob-served oculomotor change measured immediately after viewing withthe HMD for 80 min was a significant reduction in near unaided VA(p � 0.01). The reduction in near unaided VA occurred in 75% ofsubjects and the change ranged from 0.02 logMAR increase to 0.22logMAR decrease. Near unaided VA did not remain significantly re-duced at 10 min postviewing. A significant reduction in near point ofconvergence (p � 0.05) also occurred immediately after viewing,

FIGURE 2.Percentage of subjects aged from 5 to 8 years that rated either no increaseor an increase in the symptom “difficulty concentrating” after viewingwith the Binocular Viewer or HDTV for 30 min. n � 20 for each viewingcondition; BV � Binocular Viewer; *p � 0.05.

FIGURE 3.Box-plots of the changes in near unaided VA (left), the CA/C ratio (middle), and distance heterophoria (right) after viewing with the Binocular Vieweror HDTV for 30 min. A change in the positive direction indicates a reduction in near unaided VA or an esophoric shift in distance heterophoria anda change in the negative direction indicates an increase in near unaided VA or an exophoric shift in distance heterophoria. Asterisks (*) indicatesignificant changes from the previewing level and cross-hatches, (#) indicate significant differences in the changes between the two viewing conditions;n � 60 in each viewing condition for near unaided VA and distance heterophoria; n � 54 in each viewing condition for the CA/C ratio; BV � BinocularViewer; **p � 0.01; *p � 0.05; ##p � 0.01.



however, the change ranged from a decrease in length by 1 cm to anincrease in length by 1.5 cm.

Distance heterophoria was significantly shifted in the esophoricdirection (p � 0.01) immediately after viewing. This esophoric shiftoccurred in 56% of subjects and ranged from 0 to 1.5� esophoric. At10 min after viewing, distance heterophoria remained significantlyshifted in the esophoric direction (p � 0.05) in 50% of subjects, andranged from 0.5� exophoric to 1� esophoric shift.

The significant oculomotor changes measured after viewingwith the HMD for 80 min are summarized in Table 5. Box-plots,showing the range of changes in near unaided VA, near point ofconvergence, and distance heterophoria are shown in Fig. 5.

Comparison of Symptom and Oculomotor ChangesBetween the 30 min and 80-min Viewing Sessionswith the Binocular Viewer

Only one symptom, “feeling sleepy,” increased significantly more(p � 0.05) immediately after viewing with the HMD for 80 min thanfor 30 min of viewing. There were no significantly different symptomincreases between the two viewing durations at 10 min after viewing.In addition, there were no significant differences in the oculomotorchanges after viewing with the HMD for the two viewing durations.

DISCUSSION

The Binocular Viewer was found to be a safe HMD for children,and as comfortable to view with as the simulated HDTV. This

HMD’s light weight is likely to account for the few non-ocularsymptom increases during viewing. Only a few ocular symptomsincreased during viewing. This is most likely due to the high imagequality presented by the HMD. The symptoms reported in thecurrent investigation are an improvement on the severity of symp-toms reported in previous studies that used older generation bin-ocular10,35 or biocular18 HMDs. In addition, the most commonlyreported symptom increases were not significantly different fromthose in the control HDTV condition.

Despite symptoms being experienced during the 30-min view-ing sessions, these symptoms were mild and reported by �50% ofsubjects. In addition, the symptom increases were transient in na-ture, dissipating by 10-min postviewing. In a study by Ames,24

subjects aged from 10 to 15 years were able to view 30 min ofstereoscopic imagery with a VR6 HMD (Virtual Research, SantaClara), but when subjected to an extended viewing session, 50% ofthe subjects could not view for more than 1 h. Ames attributed thewithdrawals to the heavy weight of the HMD (821 g). In thecurrent investigation, a similar small number of significant symp-tom increases occurred in the 80-min viewing session as in the30-min viewing session, and there were no withdrawals fromeither session. Given that the average time between the 30 and80 min sessions was 55 � 11 d, and different imagery wasviewed between the sessions, it is unlikely that an increasedfamiliarity of the use of the HMD was responsible for thisobservation. Thus, the experience of adverse symptoms may notbe cumulative with time. Overall, use of the HMD as a personal

TABLE 4.p-values for the oculomotor changes in the Binocular Viewer and the HDTV viewing conditions (30 min viewing)

Binocular Viewer HDTV Binocular Viewer vs. HDTV

Post 0 min Post 10 min Post 0 min Post 10 min Post 0 min Post 10 min

Distance vision �0.05 0.197 0.554 0.492 0.275 0.835Near unaided visual acuity �0.01 �0.05 �0.01 0.712 0.136 0.090Stereo-acuity �0.01 0.076 �0.01 0.265 0.958 0.918NPC 0.101 �0.05 0.093 0.534 0.592 0.502Distance heterophoria 0.199 0.814 �0.05 0.140 �0.01 0.253Near heterophoria 0.727 0.890 0.167 0.474 0.489 0.623Response AC/A ratio 0.512 0.050 0.211 0.396 0.783 0.306CA/C ratio �0.01 0.050 0.332 0.775 0.176 0.106

TABLE 5.Symptom increases and oculomotor changes after viewing with the Binocular Viewer for 80 min

Post 0 min Post 10 min80 vs. 30 min of viewing with

the Binocular Viewer

Symptom increases Feeling tired (p � 0.01)Feeling bored (p � 0.05)Feeling sleepy (p � 0.01)Tired eyes (p � 0.05)

Feeling tired (p � 0.05)Feeling bored (p � 0.05)

Feeling sleepy increased moreafter 80 min of viewing atpost 0 min (p � 0.05)

Oculomotor changes Distance esophoric heterophoriashift (p � 0.01)

Distance esophoric heterophoriashift (p � 0.05)

NS

Decreased near unaided visualacuity (p � 0.01)

Decreased NPC (p � 0.05)

NS, no significant changes.



display by children for entertainment purposes was not hin-dered by user discomfort.

Except for the reduction in near unaided VA, which was consistentacross both viewing durations the profile of oculomotor changes after30 and 80 min of viewing were different. In addition, the magnitudeof the oculomotor changes was small and in many cases close to theresolution limit of the particular oculo-motor measures.

Given that near unaided VA was the most commonly observedoculomotor change during viewing with the HMD, one can spec-ulate that it may have been caused by an inappropriate accommo-

dative response (either over- or under-accommodation), relative tothe virtual imagery focal plane. However, current evidence doesnot support this proposition. It has been previously reported that amyopic shift (increase) in accommodation can occur when usingoptical instruments such as microscopes, a phenomenon known as“instrument myopia.”48 In addition, Kotulak and Morse8 ob-served that accommodation and vergence increased in response tothe “perceived nearness” of symbology presented with an aviatorHMD. An increase in accommodation great enough to reduce nearunaided VA would certainly reduce distance unaided VA and pos-sibly increase the CA/C ratio. The esophoric shift in distance het-erophoria, during 80 min of viewing, could also be attributed to anincrease in accommodation, since a positive vergence adaptation isunlikely to occur with a reduced near point of convergence. How-ever, the observed significant reduction in near unaided VA andincrease in CA/C ratio (10 min following the 30-min viewingsession), was not accompanied by a reduction in distance unaidedVA. A increased reduction of distance rather than near unaided VAwould be expected if accommodation had increased.

In contrast, a reduction in accommodation (in particular a re-duction in the accommodative response gradient), during viewingwith a biocular HMD has been reported.19 It was speculated thatthis reduction was caused by sympathetic over-stimulation, be-cause the reduction only occurred in subjects that exhibited symp-toms associated with sympathetic stimulation.22 In the currentinvestigation, however, the accompanying oculomotor changes donot support the notion of a reduction in accommodation. A reduc-tion in distance unaided VA would be unlikely, and the directionsof the changes in the CA/C ratio in the 30-min viewing session anddistance heterophoria in the 80-min viewing session are contradic-tory to those expected with a decreased accommodation.

It is unlikely that the accommodative response changed duringviewing with the Binocular Viewer for several reasons. First, theimagery had a high resolution (Snellen equivalent of 6/11) and,therefore, provided a good accommodative stimulus. Second, thefocal plane of the virtual imagery was approximately equal to thelevel of tonic accommodation in children.30,32,49 Third, the HMDwas used as a personal viewing display and was not linked to aninteractive VR system. Given that cognitive demand can influenceaccommodation,50 the lack of any interactivity between the userand the VR system in the current investigation is likely to havecontributed to a stable accommodative response during viewing.Previous studies on biocular HMDs17,22 involved a level of inter-activity, subjects not only viewed imagery but also performed a tasksuch as playing a game or navigating through a virtual environ-ment. The cognitive processing required to perform these tasksmay have contributed to the oculomotor changes observed in thosestudies. In addition, the lack of interactivity removes the need forhead tracking during use of the HMD. This removes the possibilityof visual-inertial sensory conflict that may occur with display up-date lag during head movement.51

It is also unlikely that any adaptation of the vergence systemoccurred during viewing with the HMD. There were no changes indistance or near heterophoria during viewing for 30 min. Althougha distance esophoric shift occurred during the 80-min viewingsession, it was not related to any possible prismatic effect inducedby the discrepancy between the subject’s IPD and the IOD of thelenses of the HMD. Consistent with previous reports,17,52 no cor-

FIGURE 4.Percentage of subjects aged from 13 to 16 years that rated either noincrease or an increase in the symptoms “feeling tired,” “feeling bored,”“feeling sleepy” or “tired eyes” after viewing with the Binocular Viewer for80 min. n � 16; **p � 0.01; *p � 0.05.



relation was found between the IPD-IOD mismatch and thechange in distance heterophoria. Although the IOD of the HMDwas fixed and could not be adjusted, the three HMD sizes weresufficient to cater for children between 5 and 16 years of age.Because heterophoria is partly determined by an accommodativeinput, changes in accommodation may also affect heterophoria.However, a change in the accommodative response was unlikely tohave occurred in the 80-min session given that no changes in nearheterophoria, the AC/A ratio or the CA/C ratio were observed.Also, no correlation was found between the changes in distanceand near heterophoria, which suggests that accommodative adap-tation is unlikely. Thus, the cause of the distance esophoric shift isnot clear and may simply be associated with the noise in the mea-surement of the distance heterophoria. This is a feasible proposi-tion given that noise is the likely reason for other oculomotorchanges such as the changes in AC/A ratio and the near point ofconvergence which were observed only at 10 min after viewing inthe 30-min viewing session.

In summary, the Binocular Viewer has been demonstrated to bea good HMD display for use by children. Viewing for 30 min wasrelatively asymptomatic, resulting in few symptom increases, andthese were transient in nature and reported by �50% of subjects.The HMD was able to be used comfortably for up to 80 min andis, therefore, ideal for such applications as watching movies andplaying video games. The high image quality and ergonomics ofthe HMD (particularly its light weight), resulted in few oculomo-tor changes, most of which were no greater than that observed afterviewing with a HDTV. The reduction in near unaided VA was theonly oculomotor measure to have consistently changed in both the30- and 80-min viewing sessions and was the most commonlyobserved oculomotor change in both these sessions. Further inves-tigation into the cause of the reduction in near unaided VA iswarranted.

ACKNOWLEDGMENTS

We thank the support of Essilor International PLC.This work was supported by an Ernst and Grace Matthaei postgraduate

scholarship (to PK).Received July 17, 2008; accepted December 10, 2008.

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FIGURE 5.Box-plots of the changes in near unaided VA (left), the Near Point of Convergence (middle), and distance heterophoria (right) in subjects aged from 13to 16 years after viewing with the Binocular Viewer for 80 min. A change in the positive direction indicates a reduction in near unaided VA or anesophoric shift in distance heterophoria and a change in the negative direction indicates an increase in near unaided VA or an exophoric shift in distanceheterophoria. Asterisks indicate significant changes from the previewing level; n � 16; **p � 0.01; *p � 0.05.



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Neville A. McBrienDepartment of Optometry and Vision Sciences

The University of MelbourneMelbourne, Victoria 3010, Australia

e-mail: [email protected]



Effects of a Head-Mounted Display on the Oculomotor System of Children

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