Three-Dimensional Display-Induced Transient Myopia and the Difference in Myopic Shift between Crossed and Uncrossed Disparities

Abstract

Purpose.: To investigate whether three-dimensional (3D) images cause nearwork-induced transient myopia (NITM) more than 2D images and whether there is any difference between 3D images with crossed and uncrossed disparities in the development of NITM.

Methods.: Twenty-five volunteers, enrolled in this study, watched 2D and 3D movies and read 3D texts with crossed and uncrossed disparities for 2 to 3 hours with spectacle correction. The viewing distance was 50 to 70 cm. The refractive error was measured before and after each visual task. If there was a myopic shift after a task, the refractive error was measured at 3-minute intervals until it was resolved. The changes in the refractive error and the amount of NITM were evaluated and compared.

Results.: The mean age of volunteers was 27.8 ± 2.87 years, and the mean refractive error before the visual tasks was −4.19 ± 2.87 diopters (D). Thirteen subjects (52%) showed NITM after watching a 2D movie, whereas 20 subjects (80%) had NITM after a 3D movie (P = 0.037). The mean extent of NITM was 0.36 ± 0.27 D after watching a 3D movie and 0.10 ± 0.28 D after a 2D movie (P = 0.002). The 3D text with crossed disparity significantly induced NITM (P < 0.001), but that with uncrossed disparity did not. There was a tendency for the NITM to persist longer after subjects watched a 3D movie than after a 2D movie.

Conclusions.: Viewing 3D images with crossed disparity induced a greater degree of NITM than 2D images. These results suggest that the greater NITM induced by 3D images may have a greater effect on the development and progression of permanent myopia.

Introduction

Three-dimensional (3D) display has received considerable attention as a next-generation technology. Since the 3D image contains depth information, we obtain more visual information from a 3D image than that from a 2D image. This new perception of depth is attractive to the public with regard to movie and television viewing. Recently, there has been a vast increase in consumer purchasing of 3D displays, resulting in a boom of the 3D industry. The 3D movies that previously could be seen only in special theaters are now shown in conventional theaters. Furthermore, the use of 3D television (TV) and 3D computers has extended to the home. Thus, 3D images can be experienced in our daily lives. 

However, these developments enable people to watch 3D images for extended periods of time without a break. Moreover, it can be problematic that children, who are visually immature, are exposed to 3D images for a long period of time. Viewing a 3D image reportedly induces more fatigue than viewing 2D images. ^1–4 It is not known if viewing 3D images for long periods of time causes visual problems; however, we believe that it would be prudent to establish safety protocols regarding prolonged exposure to 3D images to ensure the sound development of the 3D industry. 

Myopia, a leading ophthalmologic problem, has a high prevalence of up to 80% among Asian populations. ^5,6 There are numerous studies on the development and progression of myopia. ^7,8 Although many factors have been suggested as possible causes of myopia, the exact mechanism of myopic progression is still under investigation. Among these factors, nearwork-induced transient myopia (NITM) has been proposed. ^7,8 Nearwork induces accommodation, and the myopic shift of refraction is temporarily observed even after cessation of the work. It has been suggested that such transient myopia induced by nearwork may contribute to the development and progression of permanent myopia. ^7,8  

All 3D technology shows two different images on a display, one being visible only to the right eye and the other only to the left eye. Combining the separate images enables the perception of stereoscopic vision by fusional vergence, which may influence accommodation. If viewing a 3D image induces greater accommodation and NITM than viewing a 2D image, it may further induce myopic progression. In addition, viewing the 3D images with different disparities can affect NITM differently because the 3D images with uncrossed or nasal disparity induce fusional divergence, whereas those with crossed or temporal disparity induce fusional convergence. 

The purpose of this study was to investigate whether the extent of NITM induced by viewing 3D image differs from that of 2D images. We also examined whether 3D images with crossed or uncrossed disparity differently affect the development of NITM. 

Materials and Methods

Informed consent was obtained from all 25 Korean volunteers (20 to 35 years of age) enrolled in this study. The study protocol was reviewed and approved by the Korea University Guro Hospital Institutional Review Board and adhered to the tenets of the Declaration of Helsinki. Those volunteers with a history of ophthalmologic disease including strabismus, amblyopia, glaucoma, retinal disease, or low vision were excluded. 

We evaluated spectacle corrected visual acuity, near stereoacuity, and the presence of ocular misalignment. The near stereoacuity was obtained using a near stereopsis vision test (Stereo Fly SO-001 test; Stereo Optical Co., Chicago, IL), whereas ocular misalignment was measured with the prism and cover test at 6 m and 33 cm using an accommodative target. 

Subjects viewed 3D images on a 120-Hz liquid crystal display monitor (22 inches) using 3D vision shutter glasses (nVIDIA 3D Vision; NVIDIA Corp., Chicago, IL). The distance between the eyes and monitor was 50 to 70 cm. All subjects watched 2D and 3D movies and read 3D text with crossed and uncrossed disparities. The 2D and 3D movies were watched for 3 hours each. The 3D texts with different disparities were each read for 2 hours. The volunteers who had been wearing spectacles performed these visual tasks with spectacle correction. The 3D text was produced by scanning popular story books. The 3D text with uncrossed disparity was generated by placing the image intended for the right eye to the right of the image intended for the left eye, and the 3D text with crossed disparity was generated in the opposite fashion. The degree of disparity was 3000 seconds of arc, which was determined by the distance between the right and left images. When one sees 3D text with uncrossed disparity, it appears as though the text is located behind the monitor plane. When one sees 3D text with crossed disparity, it seems like the text is in front of the plane. Each subject watched the movies and read the text on 4 different days, and the order of viewing tasks was randomly assigned. 

The refractive error was obtained objectively with an autorefractor (RK-F1; Canon, Tokyo, Japan) before and after watching the movie or reading the text. For 18 eyes in 9 individuals, refractive errors were measured five times to estimate the repeatability of the autorefractor using a coefficient of variation (CV). The CV was 0.64%, indicating that the autorefractor was highly reliable. Spherical equivalent values were obtained from the measured refractive errors, and the axis of cylindrical component was not taken into consideration. We repeated the measurement of refractive errors until we obtained the same value of both sphere and cylinder in three consecutive measurements for each subject at each time point. In most cases, we could obtain the same values in the first three consecutive measurements. None of participants underwent measurements more than five times. The examiner was masked to the type of visual task the subject had performed. If there was a myopic shift after watching the movie or reading the text, the refractive error was measured every 3 minutes until it was resolved. We obtained a spherical equivalent from the measured refractive error. 

The differences between refractive errors before and after the visual tasks showed a deviation from the standard distribution in the Kolmogorov–Smirnov test. Therefore, we used nonparametric analysis in this study. The amount of myopic shift was compared using the Wilcoxon signed-rank test. Values of P for multiple comparisons were adjusted with the Bonferroni correction. The number of subjects who showed myopic progression was compared with the χ² test. The difference in myopic shift between the crossed and uncrossed images was compared using the Mann-Whitney U test. The correlation between the amount of myopic shift for individual subjects after watching the 2D movie and that after watching the 3D movie were assessed with the Spearman correlation test. 

Results

The mean age of the 25 volunteers was 27.8 ± 2.87 years. The mean refractive error was −4.19 ± 2.87 (range, −0.50 to −8.75) D. The spectacle-corrected visual acuity at 5 m was 20/20, and 22 subjects wore glasses. The near stereopsis was better than or equal to 50 seconds of arc in all subjects. The prism and alternate cover test revealed that 3 subjects had exophoria at distant fixation and 8 subjects at near fixation. The mean distant exophoric angle was 1.1 prism diopters (range, 0–10), and the near angle was 4.8 prism diopters (range, 0–18). 

Myopic shift (range, 0.12–0.75 D) was noted in 13 subjects (52%) after watching the 2D movie, whereas 20 subjects (80%) exhibited a myopic shift (range, 0.12–0.87 D) after watching the 3D movie. Figure 1 shows the distribution of the subjects according to the refractive changes after watching the 2D and 3D movies. Although 7 subjects showed a hyperopic shift after watching the 2D movie, no subjects showed it after watching the 3D movie. The refractive changes after watching the 2D and 3D movies are summarized in Table 1. After subjects watched the 2D movie, the degree of NITM was 0.10 D, an insignificant change compared with baseline. However, a significant change in the degree of NITM (0.36 D) compared with baseline was observed after the subjects watched the 3D movie (P < 0.001). The amount of NITM was significantly higher after subjects viewed the 3D movie than after subjects viewed the 2D movie (P = 0.002). 

Figure 1. 

 

Distribution of the subjects according to changes in refractive error after viewing 2D and 3D images. The subjects showed a greater myopic shift after viewing the 3D images (positive value: myopic shift; negative value: hyperopic shift).

Table 1. 

 

Change in Refractive Error before and after Watching the 2D and 3D Movies

	Before Watching*	After Watching*	Refractive Change* (Range)	P
2D	–4.21 ± 2.88	–4.30 ± 2.85	0.10 ± 0.28 (−0.50 to 0.75)†	0.09
3D	–4.16 ± 2.86	–4.52 ± 2.88	0.36 ± 0.27 (0–0.87)	<0.001
P			0.002

*  Diopters.

†  Positive value: myopic shift; negative value: hyperopic shift.

A myopic shift (range, 0.12–0.87 D) was noted in 11 participants (44%) after reading the 3D text with uncrossed disparity, whereas there were 21 participants (84%) with a myopic shift (range, 0.12–0.50 D) after reading the 3D text with crossed disparity. The refractive changes after reading the 3D text with different disparities are listed in Table 2. The text with uncrossed disparity did not induce NITM, but the one with crossed disparity induced a significant myopic progression (P < 0.001). The degree of NITM was higher for the text with crossed disparity than that with uncrossed disparity (P < 0.001). Figure 2 shows the changes in the mean value of myopic shift at each time interval after watching the 2D and the 3D movies. There was a tendency for the NITM to persist longer after the 3D movie because the initial degree of NITM in 3D images was greater than that of 2D images. 

Figure 2. 

 

Changes in the mean value of myopic shift at each time interval after watching the 2D and the 3D movies.

Table 2. 

 

Changes in Refractive Error before and after Viewing the 3D Texts with Crossed and Uncrossed Image Disparities

Image Disparity	Before Watching*	After Watching*	Refractive Change* (Range)	P
Crossed	–4.24 ± 2.91	–4.59 ± 2.90	0.35 ± 0.26 (0–0.87)†	<0.001
Uncrossed	–4.17 ± 2.81	–4.25 ± 2.85	0.08 ± 0.19 (−0.25 to 0.50)†	0.052
P			<0.001

*  Diopters.

†  Positive value: myopic shift; negative value: hyperopic shift.

The NITM after reading the 3D text with crossed disparity was similar to that after watching the 3D movie (P = 1.00), and it was higher than that after watching the 2D movie (P = 0.006). However, the myopic shift after reading the uncrossed disparity 3D text was not significantly different compared with the results after watching the 2D movie (P = 0.77). 

Table 3 shows the difference of myopic shift according to the baseline refractive error of the subjects. The amount of myopic shift in the subjects with myopia of more than 4.00 D did not significantly differ from that in the subjects with myopia of 4.00 D or less. 

Table 3. 

 

Changes in Refractive Error according to Baseline Myopia

Visual Task		Baseline Myopia		P
		≥ −4.00 Diopters	< −4.00 Diopters
Movie	2D	0.13 ± 0.29	0.06 ± 0.29	0.559
	3D	0.33 ± 0.24	0.42 ± 0.32	0.522
3D text	Crossed image disparity	0.04 ± 0.18	0.16 ± 0.20	0.121
	Uncrossed image disparity	0.35 ± 0.23	0.36 ± 0.32	0.890

 Positive value: myopic shift.

There was no correlation between the amount of myopic shift after watching the 2D movie and after watching the 3D movie for individual subjects (Spearman coefficient = 0.217, P = 0.297). 

Discussion

Fusional vergence is necessary to perceive a 3D image on a 2D display such as a TV or a computer monitor. When one sees a 3D image with crossed disparity, convergence occurs to achieve binocular fusion, and the image is perceived to be in front of the monitor. However, when the human eyes converge, accommodation and miosis should occur to achieve synkinetic eye movement. ⁹ Such accommodation is not necessary to appreciate 3D images because the practical distance between the eye and the display plane would not be affected whether the image is perceived in front of or behind the plane. Thus, theoretically more accommodation than is necessary occurs while viewing a 3D image with crossed disparity than when viewing a 2D image. This conflict between accommodation and vergence has been suggested as the cause of 3D image-induced eye fatigue. ^3,4 Although the exact mechanism of myopia and its progression has not been fully understood, the possibility that such unnecessary accommodation may induce and/or promote myopia cannot be excluded since nearwork and accommodation are reported as important environmentally based factors in the development and progression of myopia. ^7,10,11 NITM is one of many possible environmentally based factors contributing to nearwork-induced myopigenesis. ^7,12 Ciuffreda and Vasudevan ⁷ reported converging evidence from clinical, laboratory, and modeling studies that reveal the relationship between permanent myopia and NITM. They also mentioned that NITM might be a factor in the development of permanent myopia rather than simply reflecting an accommodative abnormality. ⁷  

Our results indicate that viewing a 3D image induces a greater degree of NITM than viewing a 2D image. Some subjects showed a hyperopic change after watching the 2D movie, whereas no one showed it after watching the 3D movie. Such hyperopic change after watching a 2D movie implies that viewing 2D images in a conventional manner at a viewing distance of 50 to 70 cm would not necessarily induce NITM. These findings concur that viewing a 3D image may induce greater NITM. If greater NITM is related to the development and progression of myopia, it can be postulated that viewing in 3D rather than in 2D may cause increased myopia. Moreover, there was also a tendency for the NITM to persist longer after viewing 3D images because the initial degree of NITM in 3D images was greater than that of 2D images. It was reported that prolonged NITM can affect the development and progression of myopia. ^7,13 However, the degree of NITM after watching the 3D movies differed according to the image disparity in the present study. Reading the 3D text with crossed disparity induced more NITM than that with uncrossed disparity. The results of reading the 3D text with uncrossed disparity were not different from the results of viewing the 2D image. When one sees a 3D image with uncrossed disparity, fusional divergence occurs instead of fusional convergence. Viewing a 3D image with uncrossed disparity does not induce a greater degree of NITM because fusional divergence does not accompany unnecessary accommodation. However, the results were taken at an intermediate viewing distance. The outcome with uncrossed disparity could be considerably different at a nearer (e.g., 33 cm) viewing distance. 

We also can postulate that, if viewing a 3D image with crossed disparity induces greater NITM and accommodation, then the conflict between accommodation and vergence would increase. This situation would subsequently cause more eye fatigue than viewing a 3D image with uncrossed disparity. It is conceivable that, when producing 3D movies, crossed disparity should be restricted to some degree. Images with uncrossed disparity would be preferred with regard to ocular comfort and myopia. For example, the 3D image with a background that appears to be located behind the screen would be better than the 3D image with characters that are perceived to be located in front of the screen. There needs to be an adequate period of visual rest between 3D image viewings to reduce prolonged unnecessary accommodation and to prevent NITM. These suggestions should increase the safety of 3D image and movie viewing. 

There are some limitations in our study. The number of enrolled subjects was small, and the age groups did not include the elderly and children. We cannot exclude the possibility that the elderly with presbyopia and naturally decreased accommodative power would respond differently to 3D vision. Including children as subjects would also have affected the responses. The ages of refractive development and myopia onset are younger than the ages of our participant age group. Another limitation was that our study did not include emmetropic and hyperopic populations. The degree of NITM might be different in these groups. A larger study including various age groups with a broad range of refractive error is necessary to expand our findings. We cannot conclude that watching 3D images induces myopia and its progression. However, before viewing in 3D becomes more popular, its relationship with myopia development needs to be evaluated further because prolonged exposure to 3D images may affect the development of the visual system in children. Our study merely provides an assumption that needs further study. A longitudinal, observational study of children who are in their emmetropization years will provide more informative data regarding the relationship between 3D viewing and myopic progression. 

In conclusion, viewing 3D images induces greater NITM than viewing 2D images. Viewing a 3D image with crossed disparity resulted in a greater degree of NITM than viewing a 3D image with uncrossed disparity. Further studies are warranted to elucidate whether NITM induced by viewing a 3D image is actually related to myopic progression. 

Footnotes

 Supported in part by the Basic Science Research Program through the National Research Foundation of Korea funded by Ministry of Education, Science and Technology Grant 2010‐0007817.

Footnotes

 Disclosure: Y.-W. Suh, None; J. Oh, None; H.-M. Kim, None; Y.A. Cho, None; J.-S. Song, None