The relationship between cybersickness, vection, and postural instability induced by optimized Fraser Wilcox illusions in virtual reality (VR)

Cybersickness is often experienced when viewing virtual environments through head-mounted displays. Motion-in-depth compared to lateral motion is believed to result in increased visual discomfort. This discomfort is related to the accommodation–vergence conflict and the conflict between the visual and vestibular systems (e.g., Li et al., 2014). Dominance of visual information can lead to postural instability, the experience of vection (illusory self-motion), and consequently cybersickness. In four experiments (E1–E4), we aimed to understand the relationship between these concepts by presenting rotating and expanding optimized Fraser–Wilcox illusions (E1–E2: artworks by Kitaoka, E3–E4: controlled stimuli) (Kitaoka, 2006) in VR, allowing us to compare motion-in-depth and lateral motion while minimizing accommodation–vergence conflict. Results showed that rotating motion illusions are generally perceived as stronger than expanding motion illusions (E1), which complicates interpreting the effect of motion direction on experienced discomfort. In order to match both illusions for illusion strength, the spatial frequencies were varied (E3 and E4). Contradictory to expectations, discomfort seems to be related to strength rather than direction of the illusion, with stronger illusions causing more discomfort (E4). Vection magnitude is affected by illusion strength and experienced discomfort, particularly the amount of dizziness observers experience while viewing the illusions (E2). Observers that experienced more dizziness also showed more head movements in the anterior–posterior direction. Both postural stability and vection are affected by the amount of dizziness an observer experiences while viewing an illusion, indicating that dizziness is the component of cybersickness that mainly relates to postural instability and vection.