Not all senses are taken over by VR. Thus, conflict will arise because of mismatch between the real and virtual worlds. As stated several times, the most problematic case of this is vection, which is a sickness-causing conflict between visual and vestibular cues arising from apparent self motion in VR while remaining stationary in the real world; see Section 8.4. As another example of mismatch, the user's body may sense that it is sitting in a chair, but the VR experience may involve walking. There would then be a height mismatch between the real and virtual worlds, in addition to mismatches based on proprioception and touch. In addition to mismatches among the senses, imperfections in the VR hardware, software, content, and interfaces cause inconsistencies in comparison with real-world experiences. The result is that incorrect or untended interpretations may arise. Even worse, such inconsistencies may increase fatigue as human neural structures use more energy to interpret the confusing combination. In light of the McGurk effect, it is easy to believe that many unintended interpretations or perceptions may arise from a VR system that does not provide perfectly consistent cues.
VR is also quite capable of generating new multistable perceptions. One example, which actually occurred in the VR industry, involved designing a popup menu. Suppose that users are placed into a dark environment and a large menu comes rushing up to them. A user may perceive one of two cases: 1) the menu approaches the user, or 2) the user is rushing up to the menu. The vestibular sense should be enough to resolve whether the user is moving, but the visual sense is overpowering. Prior knowledge about which is happening helps yield the correct perception. Unfortunately, if the wrong interpretation is made, then VR sickness in increased due to the sensory conflict. This, our perceptual system could by tricked into an interpretation that is worse for our health! Knowledge is one of many VR sickness factors discussed in Section 12.3.
Steven M LaValle 2020-01-06