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ISSN : 1229-6457(Print)
ISSN : 2466-040X(Online)
The Korean Journal of Vision Science Vol.26 No.1 pp.1-9
DOI : https://doi.org/10.17337/JMBI.2024.26.1.1

The Relationship between Binocular Visual Function and Cybersickness by Vection in Virtual Reality

In-Sun Park1,2), Jung Un Jang2)
1)Dept. of Optometry, Graduate School, Eulji University, Student, Uijeongbu
2)Dept. of Optometry, Eulji University, Professor, Seongnam
* Address reprint requests to Jung Un Jang (https://orcid.org/0000-0002-2475-315X) Dept. of Optometry, Eulji University, Seongnam
TEL: +82-31-740-7491, E-mail: jju@eulji.ac.kr
December 15, 2023 March 22, 2024 March 22, 2024

Abstract


Purpose : This study aims to identify the correlation between binocular visual function and Cybersickness induced by vection in HMD VR content.



Methods : The subjects were 52 adults aged 20 to 40 with no issues in near visual acuity. Binocular visual function and phoria, near point accommodation, near point convergence, binocular accommodation facility, stereo acuity, relative accommodation teat was assessed before the use of VR contents, and the subjects used three different VR contents categorized by optic flow. Cybersickness was measured with a virtual reality sickness questionnaire (VRSQ). A Pearson’s correlation coefficient was calculated to identify the linear relationship between the binocular visual function and the cybersickness using an alpha level of 0.50.



Results : Content T and R showed a significant increase in cybersickness (p<0.001) for all items after using VR content. There was no significant difference in oculomotor discomfort for content B. Correlation analysis showed that near phoria was significantly correlated with disorientation (p<0.050) and total VRSQ (p<0.050) in Content T, and oculomotor discomfort (p<0.050) in Content R. The correlation analysis showed that oculomotor discomfort was significantly correlated with disorientation (p<0.050). The binocular accommodative facility was significantly correlated with the overall VRSQ of content T (p<0.050) No other binocular visual functions demonstrated a significant correlation with cybersickness components.



Conclusion : As a result of cyber motion sickness, it is thought that the optical flow element of the content implementing 3D has a higher effect than the user's personal characteristics or the effect of the binocular function itself on cyber motion sickness. In future studies, it is thought that there is a need to consider the visual element and visual perception element of VR content.


Binocular disparity , Binocular visual function , Cybersickness , Optic flow , Virtual reality

가상현실에서 양안시 기능과 Vection에 의한 사이버 멀미 상관관계

박인선1,2), 장정운2)
1)을지대학교 대학원 안경광학과, 학생, 의정부
2)을지대학교 안경광학과, 교수, 성남

    Ⅰ. Introduction

    Virtual reality (VR) refers to a technology that enables a user to experience a virtualized environment as if it were real.1)

    As for VR hardware, HMD VR, which is worn on the head for the presence of the user to immerse himself/herself in the real world and for the realization of 3D images of VR, is most commonly used.2) In order to use the equipment after wearing it on the head for a long time, it is important to minimize the volume and weight, so the display is usually placed at a distance of 3 to 7 cm from the eye, and perspective is constructed using the principle of binocular disparity.3) once our eyes can feel the stereoscopic effect simply by the difference in binocular vision without a clear object structure, the HMD VR display also has a structure similar to the stereoscopic mirror by utilizing this point. Stereopsis is described as the final step of the binocular vision function.4,5) In order for binocular vision to be established, both eyes must be able to fuse through sensory and motor processes, so lack of fusion and adjustment can affect stereoscopic vision.6-9)

    In addition to physical factors such as binocular disparity to provide perspective, VR content uses stereoscopic monocular depth cues to create optic flow in the video, which creates immersion.10) However, because the stereoscopic perception of virtual reality is literally a virtual representation of the real world, it can lead to visual fatigue. Our eyes respond to accommodation and convergence according to the distance they watch to see the image output on the display. No matter what you're looking at, you're actually looking at the display at the same distance, so the amount of accommodation required to see the image clearly is always constant.11) This causes an accommodation-convergence mismatch when using VR, which hinders image fusion.12) This is believed to contribute to the visual discomfort experienced while using VR. In the real world, accommodation-convergence mutually influences binocular vision to maintain near vision.13,14) so visual function should be evaluated together to monitor HMD VR that creates stereoscopic sensation based on the principle of binocular disparity. Previous studies have analyzed various types of motion sickness triggering factors together, focusing on the correlation between cybersickness and hardware factors and user characteristics.14-18) Therefore, this study aimed to analyze the association between binocular vision function and cybersickness based on the optical flow factor of the content.

    Ⅱ. Subjects and Methods

    1. Subjects

    Fifty-two healthy young adults, aged 20 to 40 years, who had no problems with near vision, were selected for the study. The number of participants in this study was calculated using the program G*Power 3.1.9.4. Analyses of the measures were performed using the statistical program SPSS version 21.0 (SPSS Inc., Chicago, IL, USA). All testing procedures and protocols were approved by the Institutional Review Board (IRB), approval number: EU2022-038, and the subjects were fully informed orally and in writing about the purpose of the study and the testing methods, and consent was obtained. Patients were excluded if there was no problem with near vision through questionnaires and objective refraction, anisometropia with a difference in refractive error of 2.00 D or more between the two eyes, and alcohol consumption, which may affect the results.

    2. Method

    1) Refractive test and Binocular test

    The objective refraction was performed using an autorefractor (HRD-8100, Huvitz, Korea), and the average value was used after three repeated measurements for accuracy. The subjective refraction was performed at distance (3 m) and near (0.4 m) using a phoropter (OS-1000SK, Rodek, Korea) and an electronic chart (ACP-8, Topcon, Japan). For binocular visual function, lateral phoria was measured using the Von Graefe method, near point accommodation and near point convergence, binocular accommodation facility (BAF) was measured using a 12 Δ BO and 6 Δ BI prism flipper, and stereo acuity, negative relative accommodation, and positive relative accommodation were measured (Table 4).

    2) Measurement Tools

    To measure cyber sickness, we used Kim's20) Virtual reality sickness questionnaire (VRSQ). The most widely used questionnaire is the Simulator Sickness Questionnaire (SSQ) by Kennedy,21) but the SSQ was developed to measure sickness symptoms when using simulators based on visual stimuli, and contains items that are not suitable for the VR environment, so the VRSQ was reorganized to measure sickness due to VR use. Compared to the SSQ, the VRSQ used in the Boone study consists of two components, oculomotor disturbance and disorientation, with four questions for oculomotor disturbance (1,2,4,5) and five questions for disorientation (3,6,7,8,9). The scale of the VRSQ measures the severity of symptoms on a 4-point scale from 0 to 3 to derive a total score, and weights the items corresponding to each subscale variable (Table 1).

    3) HMD VR and VR contents

    The HMD VR headset used was VIVE PRO (VIVE PRO, HTC Corporation, Taiwan), and a single device was used to unify the hardware factor of interpupillary distance (PD).

    In this study, we selected three types of immersive VR content that are commercially available in the market: RPG, Simulation, and Animation, and used Beat Saber, Thief Simulator VR, and Epic Roller Coasters. The three contents were categorized based on the level of optical flow among the visual elements of the content (Table 2). Beat Saber takes place in a fixed location and makes good use of monocular cues such as graininess and linear perspective to create a sense of depth. It was designed to be more immersive through binocular cues, such as binocular parallax and runaway, and interacting with notes that approach you. The familiarity of the VR orientation is high, voluntary movement is possible, and predictability is very high, which fully satisfies the psychological fidelity factor. Thief Simulator VR uses monocular cues, has a high degree of orientation familiarity, and partially allows for spontaneous movement. However, there is a tendency for inconsistency in control and runaway behavior, and predictability is low. Epic Roller Coasters is a 360° video content that can be viewed seated or standing and relies heavily on monocular cues for depth perception. There are enough binocular cues, but orientation familiarity and low, spontaneous movement are not possible. Running along a set rail provides some predictability, but is challenging at higher speeds.

    Ⅲ. Results

    1. General characteristics

    The average age of the study subjects was 24.96±3.68 years, the distance between pupils was 63.59±3.41 mm, and the average value of isometric refractive power was –3.43±2.85 D, respectively (Table 3).

    2. Binocular function

    The subject had a distance phoria of 3.16±2.76 Δ, a near phoria of 7.69±5.45 Δ, and a binocular accommodation of 9.82±2.24 Δ. Ans also showed near point convergence 7.37±2.58 cm, 10.03±4.12 cpm for binocular accommodative facility, stereo acuity 67.5±73.43s, negative relative accommodation 2.39±0.50 D, and positive relative accommodation –2.63±-0.97 D (Table 4).

    3. Cyber sickness among VR content users

    Cyber sickness, as measured by the VRSQ questionnaire, increased across the board after using VR content, with significant differences (Table 5). All items showed a significant effect (p<.001) except for Content B, which had the lowest motion sickness score due to the lack of a strong optical flow element in the content. There was no significant difference in oculomotor discomfort in Content B.

    4. Cyber sickness correlates with general characteristics and binocular vision function

    Among the general characteristics of the study subjects, age, spatial distance, and spherical equivalent were not significantly correlated with cybersickness (Table 6).

    Among the binocular vision test items, near phoria was significantly negatively correlated with the disorientation factor of Content T and the overall VRSQ factor.

    In Content R, it was significantly negatively correlated with the oculomotor discomfort factor. On the other hand, binocular accommodative facility in Content T was significantly positively correlated with the overall VRSQ factor (Table 7).

    Ⅳ. Discussion

    We examined the correlation between cybersickness and binocular visual function in HMD VR content, and found that the more optical flow elements in the content created the illusion of virtual space and player movement, the greater the cybersickness. Among the binocular visual functions, near phoria and binocular accommodative facility were correlated, but the correlations were generally weak and did not show significant correlations with general characteristics.

    This suggests that the optical flow elements of the content have a greater impact on cybersickness than the impact on the user's personal characteristics. VR studies have shown that media foregrounds with forced motion elements and linear and circular motion can cause severe cybersickness.22) These results are consistent with the findings of the present study, suggesting that optic flow components are involved in cybersickness. Previous studies have shown a decrease in visual function after using 3D media, including a tendency to increase exophoria and a slowing of pupillary responses, in addition to perceived eye fatigue.23-26) Since cybersickness caused users' stability fatigue, it was expected that binocular vision would affect cybersickness, but only a weak correlation was found. This may be because only adults in their 20s and 40s who had no difficulty with accommodation-convergence function were selected for this study, so it is limited in providing direct clues. It is necessary to expand the research on cybersickness and dysregulation to specifically confirm the causal relationship. Since this study used a self-perceived measurement method through a questionnaire, users have to report their subjective judgment of their physical state while using VR. This has the disadvantage that physical reactions are already out of the normal range before they realize their condition and are difficult to measure objectively and quantitatively, so it will be necessary to use tools that measure electrophysiological indicators of the body to overcome this limitation.27-29) In future studies, it is necessary to analyze cyber sickness subjectively and objectively, focusing on the visual elements of VR content.

    Ⅴ. Conclusion

    Binocular vision was significantly correlated with cybersickness, but the correlation was generally weak. This suggests that the impact of the optical flow element of 3D-enabled content is more important than the impact of the user's personal characteristics or binocular vision function itself on cybersickness. Based on the results of this study, we propose visual characteristics related to cybersickness among user characteristics and lay the foundation for further analysis of cybersickness.

    Acknowledgement

    This paper was supported by Eulji University in 2022

    Conflict of interest

    TThe authors conclude that they have no interest in the products associated with this study.

    Figure

    Table

    Computation of VRSQ scores

    O<sup>†</sup>: oculomotor disturbance, D<sup>‡</sup>: disorientation
    The red brackets were added to the original formula for clarification.

    VR contents classified into optical flow

    Characteristics of the subjects (N=52)

    SD<sup>*</sup>: standard deviation, CI<sup>†</sup>: confidence interval, OU<sup>‡</sup>: oculi uterque, SE<sup>¶</sup>: spherical equivalent

    Binocular visual function of subjects (N=52)

    SD<sup>*</sup>: standard deviation, CI<sup>†</sup>: confidence interval, NPA<sup>‡</sup>: near point of accommodation, NPC<sup>§</sup>: near point of convergence, BAF<sup>∥</sup>: binocular accommodative facility, NRA<sup>¶</sup>: negative relative accommodation, PRA<sup>**</sup>: positive relative accommodation, D: diopter, cpm: cycles per minute, cm: centimeter

    Mean and standard deviation of VRSQ and 95% CI, Paired sample t-test for pre and post VRSQ scores (N=52)

    SD<sup>*</sup>: standard deviation, CI<sup>†</sup>: confidence interval, VRSQ<sup>‡</sup>: virtual reality sickness questionnaire

    Correlation between subject’s characteristics and post VRSQ score (N=52)

    VRSQ<sup>*</sup>: virtual reality sickness questionnaire, PD<sup>†</sup>: pupillary distance, SE<sup>‡</sup>: spherical equivalent
    O: oculomotor disturbance, D: disorientation, T: total VRSQ

    Correlation between subject’s binocular visual function and VRSQ score on VR contents (N=52)

    D*: Distance, N<sup>†</sup>: Near, NPA<sup>‡</sup>: near point of accommodation, NPC<sup>§</sup>: near point of convergence, BAF<sup>∥</sup>: binocular accommodative facility, NRA<sup>¶</sup>: negative relative accommodation, PRA<sup>**</sup>: positive relative accommodation

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