VISUALLY INDUCED MOTION SICKNESS
Previously several studies reported different symptoms due to watching stereoscopy videos/games such as visual fatigue, visual discomfort, eye strain, blurred vision, headache, dizziness, confusion, and disorienta- tion.19-22 There are differences among people’s feedback regarding the symptoms; in other words, not all people reported the same symptoms. Therefore, an individual experiencing such symptoms is considered as suffering from VIMS. This situation is categorized as a type of Cinerama sickness in which visual signals are present but signals from the vestibular system are absent.23
There are four main mechanisms responsible for depth in the human visual system, i.e., binocular parallax, motion parallax, accommodation, and convergence.24 The 3D stereoscopic technology uses disparity between images to produce depth, which is one of these mechanisms. The disparity can be categorized as positive (uncrossed) disparity, in which objects are at the back of screen and negative (crossed) disparity where the objects are in front of the screen. It has been reported that vergence is more active in 3D movies than in 2D.25 The disparity present in the visual scene causes the vergence and henceforth the eye will go through accommodation. In the accommodation process, the lens adjusts itself to focus the light beams on the fovea; consequently if the eyes produce vergence, accommodation will spontaneously take place. It has been reported that the main reason for visual fatigue is a conflict in vergence and accommodation response. Vergence and accommodation have their own limits that cause the problems in vision.
In a real-world scene, the accommodation and vergence point coincides no matter where the viewer looks. The accommodation and vergence responses are coupled and accommodative changes evoke vergences and vice versa. The benefit of coupling is the increased speed of accommodation and vergence. However, this has not happened in the case of 3D stereoscopy. In 3D displays, the correlation between focal and vergence distance is disrupted, i.e., the focal distance is fixed at the display while the vergence distance changes depending on the simulated visual scene the observer fixates.
Such problems with 3D technology cause discomfort in viewing. Users of 3D technology who are intolerant to the previously mentioned discomforts may experience VIMS. In previous studies, visual stress and symptoms ofVIMS are reported during and after watching a 3D movie.26 It is also reported that children must be cautioned when exposed to 3D TV, because they may not be able to realize any physical problem even if it exists.27 Furthermore, 5% of viewers in 3D cinemas experience symptoms of nausea or disorientation.28