Cybersickness and Nauseation
Understanding why it occurs and how to mitigate (Written by Avi Bar-Zeev)
Summary
Nauseation remains a challenge for all XR devices. A significant percentage of participants (up to 75%, by survey) have felt nauseated under some circumstances in XR experiences, with wide variance of severity and duration. There also appears to be a gender bias against women by a significant amount which we feel needs to be better understood.
Definitions
Nauseation is a feeling of disorientation, dizziness, discomfort, leading to an urge to empty one’s upper digestive tract. The feeling gets worse with prolonged negative stimuli, and often requires extensive rest (from a few hours to days) to recover.
Cybersickness is a form of nauseation caused by the use of XR devices.
Examples
All XR headsets can cause nauseation for most people under some circumstances. We expect the most nauseation to occur in VR headsets with very wide fields of view and with content that simulates fast vehicular motion without the corresponding physical forces. The more dramatic the motion (including being closer to large fast-moving objects), the worse the effect. Putting people in unusual orientations, like looking down on the planet from orbit can also contribute to the negative stimuli.
As we experience more AR-style (e.g., optical or video pass-through) headsets, the amount of content from a person’s natural environment increases, and therefore artificial motion makes less sense. In the case of full video or optical pass-through, the only motion is that of the participant’s own body, which matches closely between visual and vestibular cues. However, there are still cases where nauseation can be triggered in full-AR headsets, such as with large amounts of motion blur or high update latencies from video-pass-through (e.g., above 12 ms is known to be problematic), where the virtual and physical content can be out of sync. This is also often called 'registration error.'
Concerns
The best understood aspect of nauseation observes a mismatch between vestibular sensation (from the human inner ear, generally used for balance and orientation) and the visual system, which includes multiple strategies to make sense of our 3D environment. This mismatch can happen when XR visuals indicate motion but the inner ear senses no change. It can also happen when a person moves but the visuals don't change in the same way, as can occur with 3DOF content (e.g., spherical panoramas).
One theory as to why this mismatch results in an impulse to vomit vs. other sensations is that the human body apparently has no direct sensory organs to detect most kinds of food poisoning. It evolved to recognise a mismatch in senses as a symptom of poisoning. Excessive alcohol consumption has a similar result, both by blurring our senses while intoxicated. Changes to the density of inner ear fluids may occur during hangovers, requiring re-hydration.
Researchers are deeply concerned that there seems to be a gender bias in terms of more women feeling nausea from HMD use. One theory, not currently supported by empirical data, is that because women have narrower inter-pupillary distances and not all headsets support them, they may see the virtual content less clearly (on average) than men who have larger IPDs. There may be subtle differences in how men and women tend to process 3D visual information involved as well.
Known Mitigations
In the most immersive VR situations, narrowing the field of view during simulated motion can help lower symptoms of nauseation. Avoiding all artificial motion as a top-level design constraint can also help significantly. For example, jumping between fixed portals can significantly reduce the negative effects of traveling vs. simulated walking or flying. It’s generally best to match the participant's natural motions 1:1 in the virtual world, while avoiding physical furniture and walls.
Across AR & VR type experiences, keeping the frame-rate as high as possible, lowering motion-to-photon latency, reducing screen-door and optical distortion artifacts, and avoiding motion-blur effects all can help. In fact, no one has yet reported a maximum framerate above which no improvement in nauseation can be detected. Better optics inside HMDs do indeed seem to help reduce nauseation. We also know that undue heat and weight on the face are contributory factors to this kind of discomfort.
At the more AR end of the spectrum, avoiding all artificial motion, or the motion of many large proximal virtual objects, or mismatches in registration between virtual and natural content can help. Reducing the amount of virtual content entirely can help reduce nausea. Seeing oneself move in the physical environment 1:1 with AR graphics ordinarily delivers the correct sensation to the inner ear to avoid most nausea in this modality. However, it’s possible that differences in the way different people perceive 3D cues may be worse in optical-see-through AR systems, e.g., most OST devices can not perfectly render shadows or light-to-dark lighting variations without resulting in unwanted transparency.
As far as direct hardware mitigations, some researchers have experimented with adding forces on the inner ear to simulate those missing vestibular sensations that would arise from motion. Magnets or electrical signals may be able to induce artificial sensations of motion. Injecting ferrous fluids into the inner ear doesn’t seem like a practical solution for most people. Anti-nausea medicines may help, but may not be practical to take on a long-term basis.
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