Contents lists available at ScienceDirect Applied Ergonomics journal homepage: www.elsevier.com/locate/apergo Using biomechanics to investigate the effect of VR on eye vergence system Julie Iskander * , Mohammed Hossny, Saeid Nahavandi Institute for Intelligent Systems Research and Innovation (IISRI), Deakin University, Australia ARTICLE INFO Keywords: Virtual reality Eye vergence movement Eye tracking Biomechanical simulation Extraocular muscles ABSTRACT Vergence-accommodation conflict (VAC) is the main contributor to visual fatigue during immersion in virtual environments. Many studies have investigated the effects of VAC using 3D displays and expensive complex apparatus and setup to create natural and conflicting viewing conditions. However, a limited number of studies targeted virtual environments simulated using modern consumer-grade VR headsets. Our main objective, in this work, is to test how the modern VR headsets (VR simulated depth) could affect our vergence system, in addition to investigating the effect of the simulated depth on the eye-gaze performance. The virtual scenario used in- cluded a common virtual object (a cube) in a simple virtual environment with no constraints placed on the head and neck movement of the subjects. We used ocular biomechanics and eye tracking to compare between ver- gence angles in matching (ideal) and conflicting (real) viewing conditions. Real vergence angle during im- mersion was significantly higher than ideal vergence angle and exhibited higher variability which leads to incorrect depth cues that affects depth perception and also leads to visual fatigue for prolonged virtual ex- periences. Additionally, we found that as the simulated depth increases, the ability of users to manipulate virtual objects with their eyes decreases, thus, decreasing the possibilities of interaction through eye gaze. The bio- mechanics model used here can be further extended to study muscular activity of eye muscles during immersion. It presents an efficient and flexible assessment tool for virtual environments. 1. Introduction Virtual reality (VR) headsets have become more affordable and accessible to a broader population that includes young adults and children. In a few years, it turned from expensive devices that needed extensive setup and expertise into an affordable, easy at-home setup headset which has a fast growing market of technologies and applica- tions (Iskander et al., 2018b; Statista, 2016). Multiple VR applications have been developed in various fields. VR has been used in health care and rehabilitation (Rose et al., 2018) for patients suffering from stroke (Jack et al., 2001), Parkinson's disease (Mirelman et al., 2011) and even mental disorders (Freeman et al., 2017; Maples-Keller et al., 2017; Le and Beidel, 2017) among others (Hsu et al., 2017; McComas et al., 1998). Multiple studies have been conducted on the effects of VR headsets or head mounted displays (HMD) on the visual system (Mon-Williams et al., 1995, 1996, 1998; 1993; Stanney et al., 1998; Turnbull and Phillips, 2017), since it's conception by Sutherland (1968) in 1968. Moreover, the prolonged immersion in virtual environments (VEs) is still associated with visual fatigue symptoms like eye-strain, nausea, dizziness, headache and double vision (Kuze and Ukai, 2008; Brunnström et al., 2017; Ohno and Ukai, 2000; Iskander et al., 2018b; Hua, 2017), despite of the rapidly developing technology. However, there is a limited number of recent studies investigating the effects of the modern consumer-grade VR headsets. In (Mai et al., 2017), visual discomfort was estimated using electroencephalography (EEG). In ad- dition, a questionnaire (VRSQ) was developed to assess motion sickness in VR (Kim et al., 2018) which includes two components, the oculo- motor and the disorientation component. VRSQ is based on the simu- lator sickness questionnaire (SSQ) (Kennedy et al., 1993). Ocular effect of VR was studied in (Turnbull and Phillips, 2017), they found that VR has no effect on the binocular vision status. In addition, there is evi- dence that VR headsets may not present a myopia-inducing stimulus. However, as per our knowledge, no studies targeted VAC in a con- sumer-grade VR headset. Our vision system presents the world to us in a 3D layout where depth is perceived through monocular and binocular cues (Julesz, 1960). Time needed to perceive depth could take from a few milli- seconds to a few minutes depending on the depth cues provided; smaller area size, and large parallax shift increase time required (Julesz, 1964; Emoto et al., 2004). When our fixation point changes from an object in one depth plane to another, our eyes moves in opposite https://doi.org/10.1016/j.apergo.2019.102883 Received 13 August 2018; Received in revised form 5 February 2019; Accepted 23 June 2019 * Corresponding author. E-mail address: j.iskanderistafanos@deakin.edu.au (J. Iskander). Applied Ergonomics 81 (2019) 102883 0003-6870/ © 2019 Elsevier Ltd. All rights reserved. T