1 CHANGE OF PHORIA BETWEEN PLAYING VIRTUAL REALITY AND
SMARTPHONE GAMES ON EMETROPIA Mega Wulan Purnama Sari, Ine Renata Musa, Syumarti
Department of Ophthalmology, Faculty of Medicine Universitas Padjadjaran Cicendo Eye Hospital, National Eye Center
Abstract
Background : Playing smartphone-based virtual reality (VR) games with a three- dimensional (3D) display is reported to cause more eye fatigue than playing games with smartphones alone. When viewing 3D images there is an imbalance between accommodation and vergence (accommodation-vergence conflict). In stereoscopic 3D, accommodation will remain focused on the screen, while the eye will unify the image (binocular fusion) by changing the vergence. This change will decrease the ability of vergence for binocular fusion and the occurrence of phoria.
Purpose : to compare change of phoria in individuals who perform activities playing VR games and smartphone games.
Methods : This research is a cross sectional analytic study with a cross-study design. Subjects were divided into two groups. The first group played virtual reality games first continued with playing smartphone games and the second group vice versa. Measurements of phoria were taken with the prism alternate cover test before and after the activity of playing smartphone games and virtual reality for 30 minutes with a washout between them for 30 minutes.
Result : The total number of subjects in this study were 32 people with a mean age of 21 years with a range of 19-32 years. The number of male subjects was 17 people (53.1%) and women were 15 people (46.9%). The most common phoria pattern was exophoria of 20 people (62.5%) followed by orthophoria of 12 people (37.5%).
Using the wilcoxon test, a significant difference was found between changes in near phoria after playing smartphone and VR games, where changes in near phoria were greater after playing VR games (p <0.000).
Conclusion : The change in near phoria after playing games in virtual reality is greater than after playing games in smartphone on emetropia.
Keywords : phoria, virtual reality, smartphone.
INTRODUCTION
The development of digital technology is rapidly increasing, not only in communication, information, education but also in the world of gaming. At least one person in 60% of the American household population plays games regularly, minimal 3 hours during the week, and at least nearly 65% of the population has one device to play video games. In Southeast Asia in 2014, it was
estimated that almost 126 million people played games out of a total population of 626 million. While in the same year in Indonesia, the percentage of playing games reached 13.4% with total income from the gaming world reaching 181 million US dollars. From the technology of two-dimensional (2D) games with simple image quality, there are now
2 many three-dimensional (3D)
games.1-3
The development of the 3D game besides using smartphone, consoles, active-shutter glasses and joysticks, one of the latest technologies is the use of head-mounted virtual reality (HM-VR). The type of head-mounted virtual reality (HM-VR) that is quite widely used today is smartphone- based virtual reality (mobile- rendered HMD). It is easy to use, the price is cheaper, and the resulting image quality is quite adequate.4-6 One of the advantages in the use of VR is the 3D effect with the perception of space, thus providing a more lively, real and interesting image due to the stereoscopic vision.
Stereoscopic 3D is a popular form of entertainment, and is quickly becoming a big industry. However, many people find stereoscopic 3D uncomfortable. One effect that has been discussed for quite some time that are related to 3D stereoscopic are symptoms of asthenopia. These complaint can be caused by external factors due to reduced of blink reflex that is causing dry eye, watering and red eye and also due to internal factors such as accommodation - convergence conflict, long and excessive use of accommodation, binocular parallax, head motion, and visual-vestibular conflicts and motion sickness.7-13
In normal vision, accommodation and vergence aligned with each other.
In 3D stereoscopic displays, there is an imbalance in accommodation and vergence. The accommodation distance will be fixed on the screen, while the vergence distance will vary depending on the simulated object.
Visual fatigue and discomfort occur
when the eye tries to adjust the vergence and accommodation appropriately, resulting in an accommodation-vergence conflict.
This conflict will cause several problems including distortion of perception, difficulty in fusion, and the occurrence of phoria due to varying of vergence.14-8
Changes in phoria patterns are associated with decreased of convergence abilities. When viewing 3D stereoscopic at close range, regardless of the existing accommodation-verification conflict, the eye will converge to achieve binocular fusion. Continuous muscle contraction to converge will result in fatigue in the extraocular muscles.
Decreased fusion vergence can cause symptoms of visual fatigue such as blurred vision, diplopia and headaches. The aim of this study is to compare change of phoria in individuals who perform activities playing VR games and smartphone games.19-21
SUBJECT AND METHOD
This research is a cross-sectional analytic study with a cross-study design. The sample selection is done by purposive sampling. The inclusion criteria in this study were individuals aged between 18 to 33 years, orthotropic eyeball position when seeing near and far, good eyeball movement in all directions, visual acuity of each eye 1.0 without correction for distant vision and 1M / N8 for near vision, stereoacuity TNO
≤60 degree arc and has no history of cataract surgery. Exclusion criteria include the presence of ocular abnormalities that can interfere with clarity of refractive media such as
3 corneal cicatrical, cataracts, having
myasthenia gravis and a history of using drugs that can affect the accommodation process such as the use of cyclopegic drops.
The number of samples is determined by using a sample size formula to test the differences in the two average paired data and the minimum number of subjects samples are 32 people. The study was conducted at the National Eye Center of Cicendo Eye Hospital in Bandung from February to March 2020 after obtaining approval from the Ethics Commission Research of the Faculty of Medicine, Padjadjaran University, Bandung.
The research subjects were divided into two groups based on randomization (simple random sampling), group A and group B.
Group A played virtual reality games for 30 minutes then continued with smartphone for 30 minutes after washingout for 30 minutes. Group B is done the opposite. Phoria measurement was performed using the prism alternate cover test before and after playing games. The games played in this study are Smash Hit arcade games. This game has the same features both smartphone and VR versions. In both versions of this game, the player will play as a first person shooter by throwing the ball to destroy the crystal object and the pyramid in front.22,23
The result data are processed and presented analytically. Data normality test using Shapiro-Wilk test. Numerical data were analyzed using paired t tests if the data were normally distributed and Wilcoxon if the data were not normally distributed. Categorical data were
tested using the Mc Nemar test. The test results are said to be meaningful based on p values <0.05. The data obtained were recorded in Microsoft Excel 2010 and then processed using the SPSS program version 24.0 for Windows.
RESULT
Table 4.1 shows the characteristics of the subjects, the majority of them were male (53.1%) with the most phoria pattern being exophoria (62.5%). Tables 4.2 and 4.3 show the characteristics of changes in phoria patterns, amount of deviation changes in phoria and comparison of changes in phoria before and after playing virtual reality (3D) and smartphone (2D) games.
Table 4.1 Characteristics of Subject
Variable N=32
Age
Mean±Std 23.16±3.819
Median 21.00
Range (min-max) 19.00-32.00 Sex
Man 17(53.1%)
Woman 15(46.9%)
Phoria Exophoria Orthophoria
20(62,5%) 12(37,5%)
For categorical data presented with the number / frequency and percentage while numerical data are presented with the mean, median, standard deviation and range
Table 4.2 shows that the phoria pattern changes towards more exophoria both in the group after playing smartphone games or after playing virtual reality games. In the comparison amount of deviation
4 Tabel 4.2 Comparison between changes in characteristics of phoria pattern and changes in phoria deviation before and after playing smartphone (2D) and VR (3D) games.
Characteristic
Pre Post 2D Post 3D P value
N=32 N=32 N=32
Pre- Post 2D
Pre- Post 3D
Post 2D- Post 3D
Phoria 0.031* 0.002* 0.125
Exophoria 20(62.5%) 26(81.3%) 30(93.8%) Orthophoria 12(37.5%) 6(18.8%) 2(6.3%) Phoria Dekat (PD)
Mean±Std 2.56±2.793 3.19±2.989 5.34±3.288 0.002* 0.000* 0.000*
PD (prism dioptre), the sign * indicates a value of p <0.05 meaning that it is significant or statistically significant. For categorical data presented with the number / frequency and percentage while numerical data are presented with the mean, median, standard deviation and range
changes in near phoria before and after playing smartphone games and before and after playing VR games, there is a significant change in deviation with a p value, p<0.002 and p<0.000.
Table 4.3 Comparison between change of near phoria in smartphone (2D) and VR (3D) groups.
r Near Phoria
(PD)
Group
p value Smartphone
(2D)
VR (3D) N=32 N=32
Mean±
Std
0,75±
0,950
2,78±
1,453
0,000*
PD (prism dioptre); the sign * indicates a value of p<0.05 meaning that it is significant or statistically significant.
In the smartphone group (2D), for the average (r) change in near phoria is 0.75 ± 0.950 PD. In the virtual reality (3D) group, the average (r) change in near phoria is 2.78 ± 1.453 PD. Comparison of the two groups
obtained meaningful results with a p value p <0.000.
Table 4.4 measures the additional visual characteristics change before and after playing smartphone and VR games related to abilities of accommodation and vergence.
DISCUSSION
This research was followed by 32 people with emetropia which was carried out in the Cicendo Eye Hospital in Bandung. The number of samples shows more males than females, 53% are male and 47% are female. The average age of subjects was 23.15 years with a range of 19-32 years. Based on statistical data in America in 2019, the age range of millennial gamers is between 18-34 years with a percentage of men 54%
more than women 46%. The number of emetropia samples more male accordance with the data of research conducted by Midelfart et al, which is 54,1% in 51,8% of the group
5 Tabel 4.4 Change of visual characteristics before and after playing
smartphone (2D) and VR (3D) games.
Characteristic
Pre Post 2D Post 3D P Value
(Mean±Std) N=32
(Mean±Std) N=32
(Mean±Std) N=32
Pre- Post 2D
Pre- Post 3D
Post 2D- Post 3D Far Phoria (PD) 0.31±1.030 0.50±1.437 0.56±1.366 0.180 0.046* 0.705 Near Divergent
Fusion (PD)
11.91±2.644 10.69±2.764 10.94±2.951 0.018* 0.113 0.557 Far Divergent
Fusion (PD)
8.50±1.344 7.81±1.713 7.75±1.741 0.008* 0.014* 0.808 Near Convergent
Fusion (PD)
21.09±5.642 19.06±5.741 18.63±6.030 0.005* 0.007* 0.492 Far Convergent
Fusion (PD)
16.44±5.180 13.97±5.498 13.84±5.513 0.001* 0.004* 1.000 Near Point of
Accomodation (cm)
10.19±1.635 10,81±2,086 11.13±1.897 0.014* 0.001* 0.256
Near Point of Convergence (cm)
10.25±1.666 10.88±2.324 11.00±2.436 0.035* 0.069 0.593 Amplitude of
Accomodation (D)
10.00±1.571 9.49±1.814 9.13±1.746 0.017* 0.001* 0.085 TNO ("of arc) 60.00 61.88±10.607 61.88±10.607 0.317 0.317 1.000
PD(prism dioptre); cm (centimetre); D (dioptre); TNO (The Netherland Observation).
the sign * indicates a value of p<0.05 meaning that it is significant or statistically significant.
of young adults (20-25 years) emetropia are men. The most common phoria pattern was exophoria, 62.5% followed by orthoporia 37.5%. The highest prevalence of heterophoria in young adults reported by Chen et al. 69%
was exophoria and also reported by Razavi ME et al. 51.4% was exophoria.2,24-6
Changes in phoria patterns in this study were found to be more exophoria, whether using a smartphone or VR. In the research conducted by Zhuo when comparing phoria after watching 3D TV there was a change in the pattern of phoria towards exophoria. Another research conducted by Karpicka and Howarth
changes in the pattern of phoria towards exophoria after playing 3D computer games. In a study conducted by Park et al there was a change in phoria towards exophoria after playing a smartphone for 20 minutes. Changes in phoria towards exophoria due to decreased convergence abilities. While using whether a smartphone or VR, there is a triad of near vision, accommodation, convergence and miosis. The need for continuous convergence results in fatigue in the extraocular muscles and exophoria.18-
21,27
Changes in near phoria occur either after the use of a smartphone or VR (p <0.002 and p <0.000). The
6 types of games played in this study
have the same features on both smartphones and VR. The player is throwing the ball with the perspective of the first person, where the object seen as a moving object. The eyes will try to maintain binocular fusion even the object were moving, make the extraocular muscles work harder. It caused a change in phoria in both smartphone and VR games.22,23,27,28
In the comparison of changes in near phoria (r) between smartphones and VR, change of phoria is greater in the use of VR (p<0,000) compared to smartphones.
When viewing stereoscopic 3D at close range, due to an imbalance between accommodation needs and vergence (accommodation-vergence conflict), where accommodation will be focused on one focus point while the vergence will vary depending on the object that being simulated, in front or behind the screen, so vergence needs are felt to be heavier.
The need causes fatigue in the extraocular muscles so that the ability of convergence to maintain binocular fusion is reduced, the phoria occurs greater. The accommodation will remain while the of ability vergence decreases, resulting in a low convergence value. Whereas on 2D smartphone there is no accommodation-vergence
conflict.13,18-20,27,28
In this study also analyze changes in other visual characteristics related to accommodation and vergence capabilities such as far phoria, divergent and convergent fusion abilities, near point of accommodation, near point of convergence, accommodation amplitude and stereopsis. In the group
before and after playing smartphone games (2D), there is statistically significant on the decrease of near and far divergent fusion, near and far convergent fusion, whereas in comparison of the group before and after playing virtual reality (3D) only the decrease of far divergent fusion, and near and far convergent fusion.
When viewing stereoscopic 3D, convergence have a greater burden besides the need for near vision, so there is a decrease in near convergent fusion, not in near divergent fusion.19,28
In the research conducted by Wajuihian there is a significant inverse relationship between near phoria to convergent fusion.
Convergent fusion will decrease if there is a large phoria change. In the results of this study there was a decrease in convergent fusion, both convergent fusion near or far after playing smartphone or VR games, in addition to changes in phoria.
Fusional convergence eliminates the bitemporal differences of the retina and controls exophoria. Convergent fusion decreases due to the decreased of convergence ability caused by extraocular muscle fatigue.29-31 In a study by Mon-William et al in addition to a change in phoria, there was an increase in near point of convergence after the use of head- mounted displays. In a study conducted by Wee et al after watching 3D animation after 30 minutes there was an increase in the near point of accommodation and near point of convergence due to continuous accommodation-vergence conflict and near-vision need which resulted in a decrease in accommodation and convergence capabilities. In this
7 study there was a significant increase
in the near point of accommodation and near point of convergence after playing smartphone games accompanied by a decrease in accommodation amplitude due to changes from the point near accommodation, whereas after playing VR games there was no change in the point near convergence, only a change in point near accommodation accompanied by a decrease accommodation amplitude.
In a study conducted by Chang et al showed that near point of accomodation and near phoria were the characteristics of visual function that were most affected after watching 3D animation. Near point of accommodation represents the capability of binocular accommodation and near phoria represents extraocular muscle activity, indicating that the ciliary muscles and extraocular muscles bear the burden of 3-D animation simultaneously. This phenomenon also occurs in near vision, where accommodation and vergence needs are increasing. Accommodation and vergence are interrelated where if there is a change in accommodation will result in a change in the vergence (accommodative-vergence) which is assessed by AC / A ratio and conversely changes in the vergence result in changes in accommodation (vergence-accommodation) which is assessed with CA / C ratio. In this study, there was a change in the ability of accommodation both after playing smartphone or VR games, but the correlation of whether the decrease in accommodation ability affects the ability of vergence was not analyzed further.11,28,32,33
Stereopsis is an indicator of sensory status. Binocular fusion ability is obtained by combining motoric status and sensory status. In the use of smartphones and virtual reality there were no significant changes in stereopsis after 30 minutes of use, even though the ability of vergence decreased. The sensory function to assess the ability of binocular fusion can still be maintained even though there is a deviation in a small amount of ± 10 PD due to the presence of a horopter arc in the panum area of vision. In this study, despite a large increase in phoria deviation in each group, the deviation was still below ± 10 PD, in the smartphone group 3.19 ± 2.989 PD and the virtual reality group 5.34
± 3.288 PD so that the sensory status of the binocular fusion still well maintained and no significant stereopsis changes occur.11,13,34 Visual fatigue in the use of virtual reality, regardless of existing accommodation-vergence conflicts, can be caused by other factors. In 3D image display, it requires binocular disparity of different images on both screens so the 3D image can be seen in front of or behind the screen. The more 3D views created, more vergence capabilities required, more visual fatigue. This is depend on the type of games, the speed of games (frames per second), pixel images, image latency or the image display , whether it created more 3D displays and faster speed. Visual field also affect the comfort of using VR. The virtual reality field (field of view) to make immersive effect is still limited to ± 100 °, smaller than the normal field of view. It also requires the ability of a good motion sensor,
8 gyrometer and accelerometer from a
virtual reality device to make more comfortable in its use.9,35,36
One limitation of this study is that no AC/A ratio or CA/C ratio measurements were taken.
Accommodations and vergence are interconnected due to cross-links between the two.
CONCLUSION
From the results of the study it can be concluded that the change in near phoria after playing virtual reality games is greater than after playing smartphone games on emetropia.
Suggestions for further research should be carried out to see changes in AC/A ratio and CA/C ratio in the use of virtual reality to play games.
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