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Original articles
Clinical science
Ophthalmological factors influencing visual asthenopia as a result of viewing 3D displays
  1. Sung wook Wee1,
  2. Nam Ju Moon1,
  3. Won Ki Lee2,
  4. Sohee Jeon2
  1. 1Department of Ophthalmology, Chung-Ang University Hospital, College of Medicine, Chung-Ang University, Seoul, Korea
  2. 2Department of Ophthalmology, Seoul St Mary's Hospital, College of Medicine, The Catholic University of Korea, Seoul, Korea
  1. Correspondence to Dr Nam Ju Moon, Department of Ophthalmology, Chung-Ang University Hospital, College of Medicine, Chung-Ang University, #224-1, Heukseok-Dong, Dongjak-Gu, Seoul 156-755, Korea; njmoon{at}chol.com

Abstract

Aims To identify ophthalmological factors influencing asthenopia as a result of viewing three-dimensional (3D) displays.

Methods Thirty adult subjects without ophthalmological abnormality watched the same 3D displays for 30 min. Each subject's near point of accommodation (NPA) and convergence (NPC), amplitude of fusional convergence and divergence, stereopsis, tear break-up time and temperature of ocular surface, and angle of phoric deviation were measured before and after viewing the 3D displays. In addition, a survey for subjective symptoms was conducted immediately following the viewing of the 3D displays. The above mentioned experiments were performed equally with two-dimensional (2D) displays in the same 30 subjects for detection of innate influence of 3D displays.

Results The NPA and NPC in the subjects were significantly altered after watching the 3D displays (p<0.05) as compared with 2D displays. In addition, all of the 10 subjective symptoms measured were significantly increased after watching 3D displays (p<0.05).

Conclusions Accommodation and binocular vergence are predominant ophthalmological factors that may influence asthenopia significantly following the viewing of 3D displays. Subjective visual discomfort also significantly increased following the viewing of 3D displays. And there is the need for more detailed evaluation for detecting the practically related factors with asthenopia.

  • Optics and Refraction

https://doi.org/10.1136/bjophthalmol-2012-301690

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    Introduction

    With the advances in three-dimensional (3D) display technology, 3D images are now available for viewing with various multimedia tools. Unfortunately, the viewing of stereoscopic images on these 3D displays may induce visual discomfort and fatigue. The discomfort and fatigue are defined as asthenopia. Although many studies have made mention of abnormal visual fatigue with close visual tasks such as video display terminals,1 very few studies on 3D have been carried out, and no definite conclusions have been reached.

    It is widely thought that conflict between accommodation and vergence is a possible factor of visual fatigue.2 Several studies have examined the accommodation–vergence conflict and its impact on visual fatigue under unnatural conditions, including 3D displays. Hoffman et al3 reported decreased fatigue and less discomfort with their newly developed 3D displays which present focus cues that are correct or nearly correct for the depicted scene. The other factors that may influence visual asthenopia, such as a reduction of accommodation velocity,4 transient myopia5 and the reduction of pupillary movement in the near reflex,6 ,7 were also clarified with previous studies.

    However, there are few studies that examine which specific ophthalmological factors may be directly related to asthenopia. In addition, no standard method of objective measurement and subjective questionnaire has been identified for the evaluation of asthenopia with 3D displays. In this study, we evaluated the change of objective ophthalmological factors and subjective symptoms after watching 3D and two-dimensional (2D) displays and compared results to baseline data before watching any 3D or 2D displays.

    Our study was designed as follows:

    1. Evaluate the change in ophthalmological factors after watching 3D displays for 30 min compared with baseline data.

    2. Evaluate difference in subjective symptoms after watching 3D displays for 30 min compared with baseline data.

    3. The above mentioned experiments on 3D displays are identically conducted with 2D displays.

    4. Evaluate ophthalmological factors at least 1 week before watching any of displays, and this measured value is considered the baseline data.

    5. Comparative analysis of ophthalmological factors and subjective symptoms is performed between the baseline data and the results after watching 3D and 2D displays. Correlation analysis is also performed between the changes identified in both ophthalmological factors and subjective symptoms after watching 3D displays.

    Methods

    Subjects

    Young healthy volunteers were recruited through advertisement at the Chung-Ang University Hospital, Seoul, Korea. There was no other ocular disease or surgical history in each of the included volunteers in the study. All cases with tropic deviation, disorder of accommodation, vergence, stereoacuity and reporting visual discomfort before viewing any displays were excluded. But cases with refractive errors and phoric deviation were included. The protocol devised was in accordance with the Declaration of Helsinki and was approved by the institutional review board of Chung-Ang University Hospital, Seoul, Korea. All participants granted informed consent forms after a detailed explanation of the study design, ancillary investigations for scientific purposes and related imaging procedures.

    Displays

    We used commercial Blu-ray disc IMAX® Space Station 3D (Warner Bros., California, USA) as a display for both the 3D and 2D versions of the experiment. A 3D display-compatible self-emitting plasma display panel (Neovix, Paoview, Seoul, Korea) was used. The display size was 23′′ with full high vision pixels (1920×1080). The subjects watched these 3D displays for 30 min with film patterned retarder glasses (FPG-2000, LG, Seoul, Korea). To exclude other effects on asthenopia besides the stereoscopic stimuli caused by watching 3D displays, the subjects watched the same display in 2D for 30 min without film patterned retarder glasses 1 week following the initial 3D experiment.

    The examiner was blind to whether the subject was watching 3D or 2D display and was unaware of the viewing order of 3D and 2D displays.

    Objective ophthalmological evaluations

    Following a comprehensive evaluation to rule out concurrent ocular disease, the monocular near point of accommodation (NPA) was obtained using Donder's push-up method. A 20/30 single letter on a fixation stick, approximately 50 cm from the subject, served as the target and was moved progressively closer to the subject about 5.0 cm/s, until the subject noticed the target starting to blur. This was considered the endpoint.

    The near point of convergence (NPC) was obtained. The fixation target, starting point of examination and the moving velocity of fixation target were same as those previously described for the NPA measurement. The first point that the corneal reflex of the subjects began to extend outward was considered the endpoint.

    The fusional convergence break point was obtained with the bar prism at first. With the subject watching the 20/30 sized fixation target at 40 cm, we take the base-out bar prism on the subject's right eye. While gradually increasing the extent of the prism, the first point that the subject noticed diplopia was defined as the break point. As the extent of the prism gradually decreased, the first point that the subject recovered single vision was considered the recovery point. Then, the fusional divergence break point and recovery point were also obtained with the base-in bar prism. The procedure was identical to the fusional convergence break point and recovery point. We used the data of break point of fusional convergence and divergence as parameters of this experiment, based on the previously published article by Sharma et al.8

    Near stereopsis was measured by Random dot, Randot Stereotest (Stereo Optical Co, Chicago, Illinois, USA) with subjects wearing Polaroid spectacles. The test stereogram was held at a distance of 40 cm from the subject during testing. Patients with refractive errors wore their spectacles under their Polaroid lenses. Patients were asked to determine which circle in each successive group appeared to ‘pop out of the page’. This procedure was repeated until two mistakes were made successively. Threshold stereopsis level was recorded in seconds of arc.

    Tear break-up time was measured with a Fluorescein strip (Haag-Streit International, Köniz-Bern, Switzerland) coated with one drop of balanced salt solution (BSS, Alcon laboratories, Fort Worth, Texas, USA) based on the literature.9 After applying the strip to the inferior conjunctival fornix, the subject kept his normal blinking frequency for several seconds. After the fluorescein solution spread equally onto the corneal surface, the subject was required to keep the eye open until the first defect of tear film occurred. The time at which the first defect of tear film occurred was considered the tear break-up time. The slit lamp examination was done with 10× magnification.

    The ocular surface temperature was measured on the corneal surface using a non-contact infrared thermometer (Raynger ST: Raytek, Berlin, Germany). Caution was taken during the procedure to prevent the laser beam from crossing into the pupil. The examiner targeted the laser beam to 2 mm inside the corneal limbus, and set the arm with thermometer on fixed table. The distance between the pointer and the corneal surface was 15 cm. Measurements were taken in a room with constant temperature (24°C), and constant humidity (40%). After 15 min of rest with a normal blinking rate, the temperature was measured by the same instrument operator.

    The angle of phoric deviation was measured using a 20/30 distant fixation target at 5 m. A standard set of loose plastic prisms was used for all measurements. The individual prisms increased in power from 1 prism dioptre (pd) to 10 pd in single pd increments, from 10 to 20 pd in 2 pd increments. As in common clinical practice, deviations were recorded as prism diopters which were the closest value neutralising the misalignment. All measurements were repeated three times for each eye tested, with results reported as the mean value.

    All measurements that require single eye exam were performed on the right eye only. And all subjects were examined by a single examiner after the full correction of refractive error with glasses.

    Evaluation of subjective symptoms

    Ten symptoms were included in the questionnaire. The questionnaire was based on a study previously published by Sheedy et al.10 The symptom sensation questionnaire contained six identical analogue scales (0=none and 6=too severe to stand) on which the subject recorded the magnitude of each of the symptoms. At the end of watching the 3D display, the subjects answered the questionnaire. Also, the same procedures were performed with the 2D display.

    As the subjects did not report any symptoms of visual discomfort before viewing any of the displays, we regarded that the baseline data of 10 symptoms were zero.

    Statistical analysis

    Statistical analyses were performed using the statistical software (SPSS for Windows, V.16.0 SPSS Science, Chicago, Illinois, USA). For all tests, significance was set at p<0.05. Differences in the ophthalmological factors were compared using a paired t test. Changes in subjective symptoms were also compared using a paired t test. Correlations between each ophthalmological factor and subjective symptom were also analysed. Pearson's correlation test was engaged to analyse correlations.

    Results

    Overall, 19 male subjects and 11 female subjects with the mean age of 25.33±3.91 years were enrolled. There were six subjects with phoric deviation, and all subjects had refractive errors. Table 1 summarises the changes observed in ophthalmological factors after watching 3D and 2D displays. Baseline NPA and NPC increased significantly following viewing of the 3D displays (p=0.016 and 0.013, respectively). The amplitude of fusional convergence and stereopsis showed a tendency to increase although these results were not significant in 3D displays (p=0.068 and 0.063, respectively). No ophthalmological factors demonstrated significant differences after viewing 2D displays.

    View this table:
    Table 1

    Comparison between changes of ophthalmological factors after watching 3D and 2D displays

    Table 2 summarises the comparison between subjective symptoms after watching 3D and 2D displays. All symptoms showed significant differences between watching 3D and 2D displays (p<0.05). Table 3 summarises correlations between the ophthalmological factors after watching 3D displays. In the experiment using 3D displays, the NPA difference between baseline and after watching the 3D display showed a positive correlation with the NPC change (r=0.403, p=0.027). The NPC change showed a negative correlation with the change of temperature on the ocular surface (r=−0.439, p=0.015). The change of amplitude of fusional convergence exhibited a negative correlation with the change of angle of phoric deviation (r=−0.393, p=0.032). Stereopsis change demonstrated a negative correlation with the change of amplitude of fusional divergence (r=−0.409, p=0.025).

    View this table:
    Table 2

    Comparison between parameters representing subjective symptoms after watching 3D and 2D displays

    View this table:
    Table 3

    Significant correlations between ophthalmological factors after watching 3D display

    In the correlation analysis between the changes of both ophthalmological factors and subjective symptoms, there was no significant correlation between the ophthalmological factors and the subjective symptoms.

    Discussion

    We evaluated several ophthalmological factors before and after watching 3D and 2D displays. The NPA and NPC were significantly increased after watching 3D displays. We measured these two factors as a unit of centimetre; therefore, the increase measured after watching 3D displays suggests a decrease of the subject's ability of accommodation and convergence. It implies that accommodation and vergence conflict within the subjects viewing of 3D displays due to an increase in near visual tasks led to eventual deterioration of the capability to accommodate and converge. To evaluate the actual time of that deterioration, a more detailed experimental design needs to be conducted. For example, increasing the number of timepoints for the evaluation of ophthalmological factors to include 10, 20 and 30 min would allow for the identification of the actual time in which this occurs.

    In addition, the NPA and NPC had correlation when they increased after watching 3D displays. It means that the accommodation and vergence response occurred as a coupled response according to the 3D stimulus. In general, binocular disparity produces the near vision complex that includes accommodation, vergence and miosis. We evaluated two factors that represent accommodation and vergence. But there are some limitations in that our study conducted non-simultaneous experiments and the NPA and NPC were evaluated before and after watching displays. Therefore, utilisation of an eye-tracking instrument with the ability to measure real time ocular movement with pupillary changes could be included in future studies.

    Fusional vergence is a disjunctive ocular movement responsible for maintaining normal ocular alignment and the control of deviation in exotropia. And stereopsis is an indicator of sensory status and fusional vergence is concerned with motor alignment of binocular function.

    The factors previously mentioned are related to binocular function. Fusional convergence and phoric deviation have interdependence on each other, and an abnormal change in any of these factors may affect the patient's binocular function with resulting in decline in stereoacuity. In our experiment, fusional convergence and stereopsis had a tendency to increase without significance but demonstrated a partial correlation with subjective asthenopia with 3D displays. Also, there were negative correlations with significance between amplitude of fusional convergence and angle of phoric deviation, and between amplitude of fusional divergence and stereopsis with viewing 3D displays. Although there are few studies evaluating vergence with 3D displays, previous studies have demonstrated that most patients with intermittent exotropia have grossly reduced stereopsis.8 ,11 ,12 Therefore, to identify the correlation between factors affecting binocular function with watching 3D displays, the tropic deviation including intermittent exotropia and binocular function abnormality such as reduced stereopsis should be included in future studies.

    Within two factors of fusional vergence, we tested convergence amplitudes first before divergence amplitudes. To date, it has not been established whether the order of measuring convergence and divergence amplitudes may affect the results. Therefore, the influence of the order of measuring convergence and divergence on results may be considered in further study.

    It is widely believed that dry eye syndrome and asthenopia are correlated. Toda et al reported that 51.4% of patients reporting asthenopia had dry eye syndrome, and 71.3% of patients with dry eye syndrome also resorted asthenopia.13 In addition, Seo et al14 reported that the tear break-up time decreased significantly after near visual tasks, and a significant relationship between asthenopia and dry eye syndrome was noted. However, there was no significant relationship in the tear break-up time between baseline data and the results after watching 3D and 2D displays in our study.

    The normal range of temperature on the ocular surface has been reported as 32.9–36°C, and the temperature of the ocular surface has been shown to change with ocular disease.15 ,16 There are several studies regarding the relationship between the temperature of the ocular surface and asthenopia. Seo et al14 reported that the temperature of the ocular surface increased significantly in proportion to an increase of asthenopia. However, there was no significant change of the temperature of the ocular surface in our experiment after watching 3D and 2D displays.

    To evaluate subjective visual symptoms, we modified the questionnaire from the previously published report of Sheedy et al.10 In our experiment, all subjective symptoms were significantly altered after watching 3D displays in comparison with 2D displays. However, no specific symptom was found to be more related to 3D asthenopia. Also, there was no significant correlation between the change of the ophthalmological factors and the subjective symptoms. In other studies with 3D displays,17 there was no significant change after watching 3D displays compared with baseline data. It is possible that 10 items representing ocular symptoms or six nominal scales of the questionnaire did not accurately reflect the subject's real symptoms during or after viewing 3D displays. Also, the discordance of evaluating scales between ophthalmological factors and subjective symptoms may influence on the results. Therefore, further consideration of items and scales is needed for accurate analyses.

    In our study, the examiner was blind but the subjects were aware of which display they watched. And the subjects might get trained when they watched 2D display at 1 week after watching 3D display. With current 3D technology, since the viewers are usually aware of which version of display they are watching, this point may be a source of bias. And it needs to be considered in further studies.

    We compared the data after 3D and 2D viewing with single baseline data that were measured at least 1 week before watching any displays. We thought the baseline data reflected the usual ophthalmological status. There may be concern that separately measuring pre-data in day of 3D viewing and 2D viewing is better for comparative analysis. However, we think this method may develop confounding effects because predata of 3D day could differ from those of 2D day.

    In conclusion, NPA and NPC reflecting the capability of accommodation and convergence were significantly altered after watching 3D displays as compared with baseline data. But no ophthalmologic factors demonstrated significant differences after viewing 2D displays. All subjective symptoms were increased significantly in subjects viewing 3D displays compared with 2D displays. Therefore, we can infer that accommodation and binocular vergence are predominant ophthalmological factors that may influence asthenopia significantly following the viewing of 3D displays. To investigate more detailed factors regarding the incidence of asthenopia due to 3D displays, subjects with abnormal binocular sensory and motor function should be included in future studies. In addition, standardisation of the questionnaire should be established for quantifying asthenopia more accurately.

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    Footnotes

    • Contributors NJM: Corresponding author and WKL: Coauthor (1) substantial contributions to conception and design, interpretation of data; (2) revising the article critically for important intellectual content; and (3) final approval of the version to be published. SWW and SJ: Coauthors (1) substantial contributions to acquisition of data, or analysis and interpretation of data; (2) substantial contributions to drafting the article and revising it critically for important intellectual content; and (3) final approval of the version to be published.

    • Funding This research is supported by Ministry of Culture, Sports and Tourism (MCST) and Korea Creative Content Agency (KOCCA) in the Culture Technology (CT) Research & Development Programme 2011.

    • Competing interests None.

    • Patient consent Obtained.

    • Ethics approval Eye. The study protocol was approved by the institutional review board of Chung-Ang University Hospital, Seoul, Korea.

    • Provenance and peer review Not commissioned; externally peer reviewed.