8.4 Mismatched Motion and Vection

Vection was mentioned in Section 2.3 as an illusion of self motion that is caused by varying visual stimuli. In other words, the brain is tricked into believing that the head is moving based on what is seen, even though no motion actually occurs. Figure 2.20 showed the haunted swing illusion, which convinced people that were swinging upside down; however, the room was moving while they were stationary. Vection is also commonly induced in VR by moving the user's viewpoint while there is no corresponding motion in the real world.

Vection is a prime example of mismatched cues, which were discussed in Section 6.4. Whereas the McGurk effect has no harmful side effects, vection unfortunately leads many people to experience sickness symptoms, such as dizziness, nausea, and occasionally even vomiting. Thus, it should be used very sparingly, if at all, for VR experiences. Furthermore, if it is used, attempts should be made to alter the content so that the side effects are minimized. Industry leaders often proclaim that their latest VR headset has beaten the VR sickness problem; however, this neglects the following counterintuitive behavior:

If a headset is better in terms of spatial resolution, frame rate, tracking accuracy, field of view, and latency, then the potential is higher for making people sick through vection and other mismatched cues.

Put simply and intuitively, if the headset more accurately mimics reality, then the sensory cues are stronger, and our perceptual systems become more confident about mismatched cues. It may even have the ability to emulate poorer headsets, resulting in a way to comparatively assess side effects of earlier VR systems. In some cases, the mismatch of cues may be harmless (although possibly leading to a decreased sense of presence). In other cases, the mismatches may lead to greater fatigue as the brain works harder to resolve minor conflicts. In the worst case, VR sickness emerges, with vection being the largest culprit based on VR experiences being made today. One of the worst cases is the straightforward adaptation of first-person shooter games to VR, in which the vection occurs almost all the time as the avatar explores the hostile environment.