Introduction
Virtual Reality (VR) is not a new invention in the digital age. Stereoscopy technique has been used to create the illusion of depth and space since two hundred years ago\cite{Crary_2002}. During the development of virtual reality, the VR pioneers have not stopped trying to deliver the more realistic experience. Bringing people to where they will never get the chance to go has become the goal for nearly every VR company one can simply tell that by paying attention to the number of appearances of the phrase "being there" in the press about VR. Such mainstream visions of VR bring new questions into play: Is re-creating the reality the ultimate capability of digital media? In other words, the idea of "virtual realism" seemingly confine virtual reality solely to the verisimilitude based on our epistemology and limits its possibility of becoming a new medium beyond reproducing the reality. Quoting McLuhan's hackneyed aphorism "the medium is the message"\cite{Levine_1964}, the art movement going beyond the realism that liberates people from the sensing restriction and gives people a new perception of the reality could be found in any medium (e.g., the impressionism after the realism in painting) but not yet in VR. Because stereoscopy is still the fundamental basis of any current VR technology (and is going to maintain this way in the foreseeable future), one of the approaches to the unrealism VR could be related to binocular rivalry - the unique epistemic phenomenon occurring in dichoptic presentation (one image is presented to one eye and a very different image is presented to the other) of stereoscopy.
Because our eyes are horizontally separated, each eye has its own perspective of the world, and thus both eyes receive slightly different images. Stereopsis is the perception of depth that is constructed based on the difference between these two retinal images. The brain fuses the left and right images and, from retinal disparity, i.e., the distance between corresponding points in these images, extracts relative depth information. When it comes to the stereoscopic three dimensional (3D) displays, usually a pair of goggles is used to present one image one eye and a very slightly different image to the other, the brain can fuse these two images into a single perception and yield stereopsis. However, to perfectly create the 3D aspects of a virtual scene, the two images presented need to be precisely adjusted according to human's binocular vision, otherwise, our brain will extract the wrong depth information that contradicts to our epistemology \cite{Crick_1992}. When the pair of images is not set up correctly (the asymmetrical stereoscopy), dichoptically viewing those images pairs may produce competition in the form of binocular rivalry: perception alternates between different images presented to each eye\cite{Hohwy_2008}. Although the mechanisms behind binocular rivalry are still debated\cite{Brascamp_2015}, previous research\cite{Blake_1991,Kwanghyun_Lee_2015,GREGORY_1965} has proven that, asymmetrical stereopsis will lead to three different epistemic perception, depending on the different levels of visual stimuli of the images: (1) stereopsis, when the images are still similar enough for the brain to extract the depth information; (2) binocular rivalry, when the two images are too different fuse by the brain and they have similar sensory eye dominances; (3) binocular suppression (only one of the images is seen while the other is hidden) when one image leads to a significant eye dominance \cite{1962} than the other\cite{Blake_1980}.
Stereopsis & Binocular Suppression in Asymmetrical Stereoscopy
Other than the traditional symmetrical stereoscopy that is widely adopted in education and entertainment industry -3D TV, 3D movies, VR etc. - the stereopsis yielded by the asymmetrically viewing process has been researched in many aspects. Research has proven that the 2D visual will be fused into the 3D environment when the overlaying 2D information has similar eye dominance factor to the rest of the 3D scene\cite{Liu_1992,Zhaoping_2011}. Such cognitive phenomenon has been used in 3D content streaming where one image is transmitted in full resolution while the other is compressed. The stereopsis will still be achieved and the missing pixels in the compressed image will be filled by the corresponding ones from the full resolution image by the brain when viewing them stereoscopically\cite{Wang_2014}. This technique will preserve the 3D viewing quality while preventing the bandwidth to be doubled in 3D content compared to the 2D streaming. Another applicable situation of the asymmetrical stereopsis is the monocular, transparent (a.k.a "see-thru") head-mounted display that project 2D information to only one transparent goggle of the headset \cite{Laramee_2002}. Binocular opaque HMDs are useful for immersive virtual reality applications while monocular transparent displays are preferred when interacting with the world while looking at the display\cite{Feiner_1997}. Specific potential applications could be found mostly in Argumented Reality field, providing users with additional visual information or visual navigation aids\cite{Ockerman}. However, although the two images are shown to the eye asymmetrically, as long as the brain infuse them together into the single vision, there is no difference between the asymmetrical stereoscopy and the symmetrical one on the cognitive level. Therefore, the storytelling potential remains the same.
Binocular suppression happens when one image is extremely "interesting" to the brain and gets all of its attention in dichoptic viewing. As a result, the visual stimulus from the other image is completely blocked\cite{Wilson_2017}. The stereoscopically watching experience under such situation will be no different than watching a flat image with one eye closed.
Storytelling with Binocular Rivalry
Binocular rivalry is a stage in between stereopsis and binocular suppression dichoptic presentation. When the images are too different to be fused and, at the same time, are both similarly "interesting", the brain will find difficulty in choosing which image it should see and constantly switch focus between them. For example, when an image of a house is presented to one eye and an image of a face to the other, then subjective experience alternates between the house and the face. There have been many empirical studies of binocular rivalry but the data they produce are conflicting and it is very difficult to give them an unequivocal interpretation. A number of proposals have been made but the neurocognitive mechanism that explains this visual effect remains unresolved \cite{Tong_2006,N_K_2008,Sengpiel_2013}. Some research tried to look at this phenomenon from the cognitive perspective, holding the belief that binocular rivalry is an epistemic response from the brain to a seemingly incompatible stimulus condition where two distinct objects occupy the same spatiotemporal location.