Virtual reality sickness
Virtual reality sickness occurs when exposure to a virtual environment causes symptoms that are similar to motion sickness symptoms. The most common symptoms are general discomfort, headache, stomach awareness, nausea, vomiting, pallor, sweating, fatigue, drowsiness, disorientation, and apathy. Other symptoms include postural instability and retching. Virtual reality sickness is different from motion sickness in that it can be caused by the visually-induced perception of self-motion; real self-motion is not needed. It is also different from simulator sickness; non-virtual reality simulator sickness tends to be characterized by oculomotor disturbances, whereas virtual reality sickness tends to be characterized by disorientation.
Virtual reality sickness may have undesirable consequences beyond the sickness itself. For example, Crowley (1987) argued that flight simulator sickness could discourage pilots from using flight simulators, reduce the efficiency of training through distraction and the encouragement of adaptive behaviors that are unfavorable for performance, compromise ground safety or flight safety when sick and disoriented pilots leave the simulator. Similar consequences could be expected for virtual reality systems. Although the evidence for performance decrements due to virtual reality sickness is limited, research does suggest that virtual reality sickness is a major barrier to using virtual reality, indicating that virtual reality sickness may be a barrier to the effective use of training tools and rehabilitation tools in virtual reality. Estimates of the multi-study incidence and main symptoms of virtual reality sickness (also called cybersickness) have been made.
Currently, there are very few theories as to why virtual reality sickness in particular occurs; however, it is closely related to simulator and motion sickness. Sensory conflict theory provides a framework for understanding motion sickness; however, it can be applied to virtual reality sickness to better understand how it can occur, and is commonly used for that purpose. Sensory conflict theory posits that sickness will occur when a user's perception of self-motion is based on incongruent sensory inputs from the visual system, vestibular system, and non-vestibular proprioceptors, and particularly so when these inputs are at odds with the user's expectation based on prior experience. Applying this theory to virtual reality, sickness can be minimized when the sensory inputs inducing self-motion are in agreement with one another.
The physiology behind VR sickness is not currently clearly understood. Fortunately, research has uncovered some clear indications of certain conditions that cause VR sickness. It seems that the images projected from virtual reality have a major impact on sickness. The refresh rate of on-screen images is often not high enough when VR sickness occurs. Because the refresh rate is slower than what the brain processes, it causes a discord between the processing rate and the refresh rate, which causes the user to perceive glitches on the screen. When these two components do not match up, it can cause the user to experience the same feelings as simulator and motion sickness which is mentioned below.
The resolution on animation can also cause users to experience this phenomenon. When animations are poor, it causes another type of discord between what is expected and what is actually happening on the screen. When onscreen graphics do not keep the pace with the users' head movements, it can trigger a form of motion sickness.
Another trigger of virtual reality sickness is when there is disparity in apparent motion between the visual and vestibular stimuli. This disparity occurs if there is a disagreement between what the stimuli from the eyes and inner ear are sending to the brain. This is a fundamental cause of both simulator and motion sickness as well. In virtual reality, the eyes transmit that the person is running and jumping through a dimension, however, the ears transmit that no movement is occurring and that the body is sitting still. Since there is this discord between the eyes and the ears, a form of motion sickness can occur.
Not all scientists agree with sensory conflict theory. A second theory of motion sickness, which has also been used to explain virtual reality sickness, is the theory of postural instability. This theory holds that motion sickness and related sicknesses occur because of poor postural adaptations in response to unusual coupling between visual stimuli and motor coordination. Characteristic markers of postural instability occur prior to appearance of symptoms and predict the later development of symptoms. This theory can explain some otherwise surprising situations in which motion sickness did not occur in the presence of sensory conflict.
There are various technical aspects of virtual reality that can induce sickness, such as mismatched motion, field of view, motion parallax, and viewing angle. Additionally, the amount of time spent in virtual reality can increase the presence of symptoms. Mismatched motion can be defined as a discrepancy between the motion of the simulation and the motion that the user expects. It is possible to induce motion sickness in virtual reality when the frequencies of mismatched motion are similar to those for motion sickness in reality, such as seasickness. These frequencies can be experimentally manipulated, but also have the propensity to arise from system errors. Generally, increasing the field of view increases incidence of simulator sickness symptoms. This relationship has been shown to be curvilinear, with symptoms approaching an asymptote for fields of view above 140°. Altering motion parallax distances to those less than the distance between the human eyes in large multiple-screen simulation setups can induce oculomotor distress, such as headaches, eyestrain, and blurred vision. There are fewer reports of oculomotor distress on smaller screens; however, most simulation setups with motion parallax effects can still induce eyestrain, fatigue, and general discomfort over time. Viewing angle has been shown to increase a user's sickness symptoms, especially at extreme angles. One example of such an extreme angle would be when a user must look downwards a short distance in front of their virtual feet. As opposed to a forward viewing angle, an extreme downward angle such as this has been shown to markedly increase sickness in virtual environments. Time spent immersed in a virtual environment contributes to sickness symptom presence due to the increasing effects of fatigue on the user. Oculomotor symptoms are the most common to occur due to immersion time, but the nature of the user's movements (e.g., whole-body vs. head-only) is suggested to be the primary cause of nausea or physical sickness.
Techniques for reducing VR sickness
According to several studies, introducing a static frame of reference (independent visual background) may reduce simulation sickness. A technique called Nasum Virtualis shows a virtual nose as a fixed frame of reference for VR headsets.
Other techniques for reducing nausea involve simulating ways of displacement that don't create or reduce discrepancies between the visual aspects and body movement, such as reducing rotational motions during navigation, dynamically reducing the field of view, teleportation, and movement in zero gravity.
In January 2020, the French start-up Boarding Ring, known for their glasses against motion sickness, released an add-on device against virtual reality sickness. Using two small screens in the user's peripheral field of view, the device displays visual information consistent with vestibular inputs, avoiding the sensory conflict.
Galvanic vestibular stimulation, which creates the illusion of motion by electric stimulation of the vestibular system, is another technique being explored for its potential to mitigate or eliminate the visual-vestibular mismatch.
With the integration of virtual reality into the more commercial mainstream, issues have begun to arise in relation to VR sickness in head-mounted gaming devices. While research on head-mounted VR for gaming dates back to the early 1990s, the potential for mass usability has only become recently realized.
While certain features are known to moderate VR sickness in head-mounted displays, such as playing from a seated position rather than standing, it has also been found that this merely puts off the onset of sickness, rather than completely preventing it. This inherently presents an issue, in that this type of interactive VR often involves standing or walking for a fully immersive experience. Gaming VR specialists argue that this unique brand of VR sickness is only a minor issue, claiming that it disappears with time spent (multiple days) using the head-mounted displays, relating it to "getting your sea legs". However, getting users interested in sickness for multiple days with the promise of "probably getting over it" is a struggle for developers of head-mounted gaming tech. Surveys have shown that a large percentage of people won't develop their "VR legs," in particular women. These same developers also argue that it has more to do with the individual game being played, and that certain gaming aspects are more likely to create issues, such as change in speed, walking up stairs, and jumping, which are all, unfortunately, fairly normal game functions in predominant genres.
Individual differences in susceptibility
- Age: Susceptibility to motion sickness is highest between the ages of 2 and 12. It then decreases rapidly until about age 21, and continues to decrease more slowly after that. It has been suggested that virtual reality sickness might follow a similar pattern, but more recent research has suggested that adults over the age of 50 are more susceptible than younger adults to virtual reality sickness.
- Postural stability: Postural instability has been found to increase susceptibility to visually-induced motion sickness. It is also associated with increased susceptibility to nausea and disorientation symptoms of virtual reality sickness.
- Flicker fusion frequency threshold: Because flicker in the display has been associated with increased risk of virtual reality sickness, people with a low threshold for detecting flicker may be more susceptible to virtual reality sickness.
- Ethnicity: Asian people may be more susceptible to virtual reality sickness. Chinese women appear to be more susceptible to virtual reality sickness than European-American and African-American women; research suggests that they are more susceptible to vision-based motion sickness. Tibetans and Northeast Indians also appear to be more susceptible to motion sickness than Caucasian people, suggesting that they would also be more susceptible to virtual reality sickness, since susceptibility to motion sickness predicts susceptibility to a wide range of motion-sickness related disturbances.
- Experience with the system: Users seem to become less likely to develop virtual reality sickness as they develop familiarity with a virtual reality system. Adaptation may occur as quickly as the second exposure to the virtual reality system.
- Gender: Women are more susceptible than men to virtual reality sickness. This may be due to hormonal differences, it may be because women have a wider field of view than men, or gender differences in depth cue recognition. Women are most susceptible to virtual reality sickness during ovulation and a wider field of view is also associated with an increase in virtual reality sickness. In more recent research, there is some disagreement as to whether gender or sex is a clear factor in susceptibility to virtual reality sickness.  
- Health: Susceptibility to virtual reality sickness appears to increase in people who are not at their usual level of health, suggesting that virtual reality may not be appropriate for people who are in ill health. This includes people who are fatigued; have not had enough sleep; are nauseated; or have an upper respiratory illness, ear trouble, or influenza.
- Mental rotation ability: Better mental rotation ability appears to reduce susceptibility to virtual reality sickness, suggesting that training users in mental rotation may reduce the incidence of virtual reality sickness.
- Field dependence/independence: Field dependence/independence is a measure of perceptual style. Those with strong field dependence exhibit a strong influence of surrounding environment on their perception of an object, whereas people with strong field independence show a smaller influence of surrounding environment on their perception of the object. While the relationship between field dependence/independence and virtual reality sickness is complex, it appears that, in general, people without a strong tendency towards one extreme or the other are most susceptible to virtual reality sickness.
- Motion sickness sensitivity: Those who are more sensitive to motion sickness in reality are also more sensitive to virtual reality sickness.
- LaViola, J. J. Jr (2000). "A discussion of cybersickness in virtual environments". ACM SIGCHI Bulletin. 32: 47–56. CiteSeerX 10.1.1.544.8306. doi:10.1145/333329.333344.
- Kolasinski, E. M. "Simulator sickness in virtual environments (ARI 1027)". www.dtic.mil. U.S. Army Research Institute for the Behavioral and Social Sciences. Retrieved 22 July 2014.
- Stanney, K. M.; Kennedy, R. S.; Drexler, J. M. (1997). "Cybersickness is not simulator sickness". Proceedings of the Human Factors and Ergonomics Society Annual Meeting. 41 (2): 1138–1142. doi:10.1177/107118139704100292.
- Crowley, J. S. (1987). "Simulator sickness: A problem for Army Aviation". Aviation, Space, and Environmental Medicine. 58 (4): 355–357. PMID 3579825.
- Barrett, J. (2004). "Side effects of virtual environments: A review of the literature (DSTO-TR-1419)" (PDF). Edinburgh, Australia: Defense Sciences and Technology Organisation Information Sciences Laboratory. Cite journal requires
- Brooks, J. O.; Goodenough, R. R.; Crisler, M. C.; Klein, N. D.; Alley, R. L.; Koon, B. L.; ...; Wills, R. F. (2010). "Simulator sickness during driving simulation studies". Accident Analysis & Prevention. 42 (3): 788–796. doi:10.1016/j.aap.2009.04.013. PMID 20380904.CS1 maint: numeric names: authors list (link)
- Lawson, B. D. (2014). "Motion sickness symptomatology and origins". Handbook of Virtual Environments: Design, Implementation, and Applications. pp. 531–599.
- Johnson, D. (April 2005). "Introduction to and Review of Simulator Sickness Research (Research Report 1832)". U.S. Army Research Institute for the Behavioral and Social Sciences. Cite journal requires
- Reason, J. T.; Brand, J. J. (1975). Motion sickness. London: Academic Press.
- Stoffregen, T. A.; Riccio, G. E. (1988). "An ecological theory of orientation and the vestibular system". Psychological Review. 95 (1): 3–14. doi:10.1037/0033-295x.95.1.3. PMID 3281178.
- Smart, L. J.; Stoffregen, T. A. & Bardy, B. G. (2002). "Visually induced motion sickness predicted by postural instability". Human Factors. 44 (3): 451–465. doi:10.1518/0018720024497745. PMID 12502162.
- Riccio, G. E.; Martin, E. J.; Stoffregen, T. A. (1992). "The role of balance dynamics in the active perception of orientation". Journal of Experimental Psychology: Human Perception and Performance. 18 (3): 624–644. doi:10.1037/0096-1518.104.22.1684.
- Groen, E.; Bos, J. (2008). "Simulator sickness depends on frequency of the simulator motion mismatch: An observation". Presence. 17 (6): 584–593. doi:10.1162/pres.17.6.584.
- Lin, J. J.; Duh, H. B. L.; Parker, D. E.; Abi-Rached, H.; Furness, T. A. (2002). "Effects of field of view on presence, enjoyment, memory, and simulator sickness in a virtual environment". Proceedings IEEE Virtual Reality 2002. 9. pp. 164–171. doi:10.1109/VR.2002.996519. ISBN 978-0-7695-1492-5.
- Jinjakam, C.; Kazuhiko, H. (2011). "Study on parallax affect on simulator sickness in one-screen and three-screen immersive virtual environment". 東海大学紀要情報通信学部. 4 (1): 34–39.
- Ruddle, R. A. (2004). "The effect of environment characteristics and user interaction on levels of virtual environment sickness". IEEE Virtual Reality 2004. 11. pp. 141–148. CiteSeerX 10.1.1.294.5953. doi:10.1109/VR.2004.1310067. ISBN 978-0-7803-8415-6.
- Lin, James Jeng-Weei; Abi-Rached, Habib; Kim, Do-Hoe; Parker, Donald E.; Furness, Thomas A. (2002-09-01). "A "Natural" Independent Visual Background Reduced Simulator Sickness". Proceedings of the Human Factors and Ergonomics Society Annual Meeting. 46 (26): 2124–2128. CiteSeerX 10.1.1.897.4716. doi:10.1177/154193120204602605. ISSN 1541-9312.
- Prothero, J. D.; Draper, M. H.; Furness, T. A.; Parker, D. E.; Wells, M. J. (March 1999). "The use of an independent visual background to reduce simulator side-effects". Aviation, Space, and Environmental Medicine. 70 (3 Pt 1): 277–283. ISSN 0095-6562. PMID 10102741.
- Duh, Henry Been-Lirn; Parker, Donald E.; Furness, Thomas A.; A, Henry B. L. Duh; B, Donald E. Parker; A, Thomas A. Furness (2001). Does a Peripheral Independent Visual Background Reduce Scene-Motion-Induced Balance Disturbance in an Immersive Environment?. CiteSeerX 10.1.1.29.3699.
- Whittinghill, D. M., Ziegler, B., Moore, J., & Case, T. (2015). Nasum Virtualis: A Simple Technique for Reducing Simulator Sickness in Head Mounted VR. In Game Developers Conference. San Francisco. 
- "How to Reduce VR Sickness? Just Add a Virtual Nose". WIRED. Retrieved 2017-10-11.
- Kemeny, Andras; George, Paul; Mérienne, Frédéric; Colombet, Florent (2017-01-29). "New VR Navigation Techniques to Reduce Cybersickness". Electronic Imaging. 2017 (3): 48–53. doi:10.2352/ISSN.2470-1173.2017.3.ERVR-097.
- Fernandes, A. S.; Feiner, S. K. (March 2016). "Combating VR sickness through subtle dynamic field-of-view modification". 2016 IEEE Symposium on 3D User Interfaces (3DUI). pp. 201–210. doi:10.1109/3DUI.2016.7460053. ISBN 978-1-5090-0842-1.
- "Combating VR Sickness: Debunking Myths And Learning What Really Works". ARVI Games.
- "How game designers find ways around VR motion sickness". The Verge. Retrieved 2017-10-11.
- "Could These Glasses Cure Your Motion Sickness? - smithsonianmag.com". smithsonianmag.com.
- "The Seenetic VR Sickness Solution - intotomorrow.com".
- "Mayo Clinic May Have Just Solved One Of Virtual Reality's Biggest Problems". Forbes. Retrieved 2019-03-08.
- Lang, B. (January 16, 2014). "First impressions of Valve's VR head mounted display prototype". Road to Virtual Reality. Retrieved 14 July 2014.
- Merhi, O.; Faugloire, E.; Flanagan, M.; Stoffregen, T. A. (2007). "Motion sickness, video games, and head-mounted displays". Human Factors. 49 (5): 920–934. doi:10.1518/001872007x230262. PMID 17915607.
- "The Promise and Challenges of Head-Mounted Virtual Reality Displays - Tested.com". Tested.com. Retrieved 14 July 2014.
- "VR Motion Sickness Statistics - Age, Gender, Experience and More. VRHeaven.io". VRHeaven.io. Retrieved 17 July 2020.
- Smart, L. J., Jr.; Stoffregen, T. A.; Bardy, B. G. (2002). "Visually induced motion sickness predicted by postural instability". Human Factors. 44 (3): 451–465. doi:10.1518/0018720024497745. PMID 12502162.
- Kolasinski, E. M.; Jones, S. A.; Kennedy, R. S.; Gilson, R. D. (January 1994). "Postural stability and its relation to simulator sickness". Poster Presented at the 38th Annual Meeting of the Human Factors and Ergonomics Society. 38 (15): 980. doi:10.1177/154193129403801571.
- Stern, R. M.; Hu, S.; LeBlanc, R.; Koch, K. L. (1993). "Chinese hyper-susceptibility to vection-induced motion sickness". Aviation, Space, and Environmental Medicine. 64 (9 Pt 1): 827–830. PMID 8216144.
- Sharma, K; Aparna (1997). "Prevalence and correlates of susceptibility to motion sickness". Acta Geneticae Medicae et Gemellologiae. 46 (2): 105–121. doi:10.1017/S0001566000000660. PMID 9492893.
- Uliano, K. C.; Lambert, E. Y.; Kennedy, R. S.; Sheppard, D. J. "The effects of asynchronous visual delays on simulator flight performance and the development of simulator sickness symptomatology (NAVTRASYSCEN 85-D-0026-1)". Orlando, FL: Naval Training Systems Center. Cite journal requires
- Kennedy, R. S.; Frank, L. H. (September 1983). "A review of motion sickness with special reference to simulator sickness". Paper Presented at the National Academy of Sciences/National Research Council Committee on Human Factors. Monterey, CA.
- Park, G. D.; Allen, R. W.; Fiorentino, D.; Rosenthal, T. J.; Cook, M. L. (2006). "Simulator sickness scores according to symptom susceptibility, age, and gender for an older driver assessment study". Proceedings of the Human Factors and Ergonomics Society Annual Meeting. 50 (26): 2702–2706. doi:10.1177/154193120605002607.
- Kennedy, R. S.; Lilienthal, M. G.; Berbaum, K. S.; Baltzley, D. R.; McCauley, M. E. (1989). "Simulator sickness in U.S. Navy flight simulators". Aviation, Space, and Environmental Medicine. 60 (1): 10–16. PMID 2923588.
- Boyd, Danah (2001). Depth Cues in Virtual Reality and the Real World: Understanding Differences in Depth Perception by Studying Shape-from-shading and Motion Parallax (PDF) (Undergraduate honors thesis). Brown University. Retrieved 8 January 2016.
- Clemes, S. A.; Howarth, P. A. (2005). "The menstrual cycle and susceptibility to virtual simulator sickness". Journal of Biological Rhythms. 20 (1): 71–82. doi:10.1177/0748730404272567. PMID 15654072.
- Kennedy, R. S.; Berbaum, K. S.; Lilienthal, M. G.; Dunlap, W. P.; Mulligan, B. F.; Funaro, J. F. (1987). "Guidelines for alleviation of simulator sickness symptomatology (NAVTRASYSCEN TR-87007)". Orlando, FL: Naval Training Systems Center. Cite journal requires
- Lawson, Ben (2014-08-13), "Motion Sickness Symptomatology and Origins", Handbook of Virtual Environments, CRC Press, pp. 531–600, ISBN 978-1-4665-1184-2, retrieved 2020-10-11
- Saredakis, Dimitrios; Szpak, Ancret; Birckhead, Brandon; Keage, Hannah AD; Rizzo, Albert; Loetscher, Tobias (2019-12-13). "Factors associated with virtual reality sickness in head-mounted displays: a systematic review and meta-analysis". dx.doi.org. Retrieved 2020-10-11.
- Parker, D. E.; Harm, D. L. (1992). "Mental rotation: A key to mitigation of motion sickness in the virtual environment?". Presence. 1 (3): 329–333. doi:10.1162/pres.1922.214.171.1249. PMID 11538019.