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With the Oculus Rift and Project Morpheus somewhere on the horizon for consumers, and the work that Valve is doing, virtual reality (VR) is once again a hot issue for games with all the interest, hype, and business hypothesising that comes along with it.
One often mentioned issue with VR is that some people can feel sick when using it. Indeed, when Sony showed off the Morpheus at GDC this year they warned people if they started to feel sick to let the attendants know and stop playing. In academia this is often referred to as Simulator Sickness (or sometimes “Cybersickness”, if you want to go all Gibsonian).
Simulator sickness is a real problem for some people when using any simulator, although VR is particularly notorious, likely because of the sensory immersion, latency issues, and the added weight of a headset. Simulator sickness is also an issue that is of particular interest to me given my background working with driving simulators. As such, this article aims to lay out what the current science, that I am aware of, has to say about simulation sickness, what it is, why it occurs, and what developers and players can do about it.
What is it?
Simulation Sickness is a syndrome, which can result in eyestrain, headaches, problems standing up (postural instability), sweating, disorientation, vertigo, loss of colour to the skin, nausea, and - the most famous effect - vomiting. It is similar in effects to motion sickness, although technically a different thing. Simulation sickness can occur during simulator or VR equipment use and can sometimes persist for hours afterwards.
Furthermore, it is possible to use a simulator/VR set without simulation sickness only to get sick once you return to reality, particularly after extended use. Sailors called this “getting your land legs back” in relation to sea sickness, but perhaps we should take the cyberpunk approach and call it “Reality Sickness”. Also, people who use a simulator or VR for extended periods of time have been reported by some to have simulation sickness “flashbacks” later. Both of the above factors led the US military, according to some reports, to have a 24 hour ban on flight for any pilots after using a simulator.
One of the big issues with simulation sickness is that it is understudied (and some studies that exist are old, have questionable methodology, or rely on anecdotes), and that there is no universally agreed explanation for why it occurs. As such, the aforementioned ban by the US military may be an over-reaction or it may not.
Why does it occur?
There is no firm agreement on why, but there are a few theories. I will go briefly into them below, but if you are not interested in theorising, and just want to read tips on how to reduce simulator sickness, please do skip to the next section:
Cue Conflict Theory
This is probably the most dominant theory and simply put says that simulation sickness is caused by a mismatch between what we expect to see and feel and what we are actually seeing and feeling. Basically, this theory says that our body has an expected sensory input (based on past experiences and biology) and if that sensory input is different, for example you are running around in a VR first person game but in real life are sitting on your couch, we can experience simulation sickness.
The supporting evidence for this theory is:
- People who are more experienced at an activity in reality are more likely to experience simulation sickness than novices. According to this theory this is because experts in the activity have a stronger/more sensitive sensory expectation and therefore virtual reality is more likely to provide the wrong input.
- That adding motion to a simulation can help. However, it must be accurate otherwise it just adds even more to the sensory conflict. So, there is still an argument in the scientific community about the merit of adding movement to simulators in terms of a cost-benefit trade off.
- That extended exposure can reduce symptoms. The argument being that you form new expectations related to the simulation - this then also explains possible sickness upon returning to reality where your expectations have changed.
Evidence against this theory:
- It is a theory that has low reliable predictive value. In that it cannot be used to reliably predict who will or will not get motion sickness. Basically, people don’t get sick every time they experience different sensory input than they expect, just sometimes.
- The theory doesn’t explain why sensory expectations not matching input would cause such an extreme reaction. Throwing up and falling over are not usually very evolutionary useful reactions, so why feel that way when the only issue is that your eyes say you are moving but the rest of your body says you are not?
Postural Instability Theory
The next theory is focused on the falling down and feeling dizzy part of simulation sickness and is promoted as an ecological theory that doesn’t rely on fluffy cognitive things like “expectations”. This theory says that our body is constantly making small adjustments to not fall over and relies on accurate sensory input to do so. When the sensory input is off the small adjustments also get thrown off resulting in simulator sickness.
Evidence for this theory:
- Balance problems and dizziness can precede other simulation sickness symptoms.
- Helps explain why simulation sickness goes away over time, basically your body adjusts. It also explains why people who are more experienced with a real life activity are more likely to get simulator sick (they have more to “train away”).
- Postural instability doesn’t always precede simulation sickness.
- It, like cue conflict theory, has low predictive value.
- Why does this lead to such an extreme “blllarrrhggghh” response?
This theory mostly exists because of the last problem with conflict theory and postural instability theory in that the reaction to simulation sickness seems extreme. Poison Theory says that, evolutionarily speaking, one of the only times you will experience sensory input that is different than what you expect, or when you get all dizzy and have problems standing up, is when you have been poisoned. So what is evolutions best trick to get rid of poison? Be sick.
Evidence for this theory:
- People get sick and fall down…
Evidence against this theory:
- Low predictive value (notice a theme?). It is an evolutionary story that sounds good, but can’t really be reliably and accurately used to predict who will get sick under what conditions.
- Why are young children resistant to simulation sickness? They are more vulnerable to toxins, so shouldn’t this defence be stronger?
- Doesn’t stand by itself very well, so it is sometimes referenced just as an explanation for the pukey parts of the other two theories.
Other, lesser known, theories include:
- Eye movement theory: Suggests that certain stimuli cause eye movements that create tension in the eye muscles which stimulate the vagus nerve leading to sickness.
- Subjective vertical mismatch theory: An alteration of conflict theory that suggests the most important factor is the mismatch between a users sense of verticality vs the sensor input they are receiving about their verticality from the simulation.
- Negative Reinforcement Model: Like Poison theory, this is an evolutionary theory that argues that movement that would generate sickness, or mismatched sensory information, suggests weakness or doing something that could make you weak. Therefore, to stop you doing it you get sick.
What developers can do to minimise simulation sickness
So, there is no agreement on exactly what causes simulation sickness, and indeed the theories overlap quite a bit, so the likely answer is a mixture of these ideas (plus probably some new ones). But despite not understanding what causes it there are some techniques that research suggests developers can do in games that may reduce its occurrence. Please note, these are only guidelines and may have varying levels of influence depending on the game, the hardware, and the person playing.
- Maintain high frame rates and low latency: It isn’t just for game quality reasons that the folks at Oculus, Sony, and Valve have been stressing the need for low latency, responsiveness, and high frame rates. The low persistence tech Oculus is building into the Rift will help, but you still have to pull your weight. Basically, if there is lag between a player input and game output then the chance of simulation sickness increases. So, when it comes to VR if there is choice between better graphics vs low latency, the latency should win out. In terms of strict guidelines, it is hard to say. Some academics cite any latency above 46ms could start to get problematic, others cite 20ms, and indeed, Oculus targets under 20ms. Similarly, avoid motion blurring, which increases the sensation of movement.
A comparison of the low persistence offered by the new Oculus dev kit versus full persistence. Full persistence is much more likely to make you sick.
- Avoid flicker: Flicker and flashing should be avoided, particularly near the edges of vision, where it can be perceived by the brain as a signal for movement.
- Make use of appropriate movement: Basically, match sensory expectations. So if the player is a human character, don’t use unrealistic movement (e.g. strafing) and tie camera movement to the head (i.e. it works like a turret, or… a head…). This includes taking advantage of head tracking when offered. Particularly inappropriate movement that should be avoided includes rapid tilting, rolling, and sinusoidal movement (e.g. up and down, or left and right, wave like movement, particularly at frequencies between 0.05 and 0.8 Hz). One example of these types of movement in games could be certain types of head bob or going up and down stairs - both of which can be troublesome in VR. Another example could be gun sway that fills a significant amount of the player’s vision and could create the illusion of left to right swaying motion.
An example of headbob that could be problematic in VR (This GIF was generated from this video, which is simply a engine demo and is not showing a VR game)
- Limit uncontrolled movement: Related to above, basically if movement is happening without player control, particularly tilting, spinning, or flipping movement, then you are on your way to a vomit comet simulator. This idea of the advantages of being in control is similar to how drivers are less likely to get motion sick than passengers. Examples of uncontrolled movement could be as simple as a death animation where the character ragdolls or falls to the ground, tilting and shaking the camera, or a zoom transition between scenes.
- Think about your camera height: The closer a camera is to the ground, the more the ground fills a player's vision and the faster what they are seeing on the ground will change. This increases the sensation of movement.
A proximity flying wingsuit simulator may be out of the question for those who are vulnerable to simulation sickness. Although, the folks who made Aaaaaaaaa! for the Rift, a base jumping game where surfaces do wizz by, say their game has overcome simulation sickness problems.
- Limit movement through a scene: This is not to say remove control (see above) but rather use a fixed position where the player just has turret-like control of the camera. This of course limits game design, so it is a trade off.
- Limit rapid changes in acceleration (including deceleration) and rapid shifts of perspective: This speaks for itself. But interestingly this also has an application when zooming vision, say with a scope on a gun in a game. Not only could it be a bad idea to have a majority of the screen rapidly zooming in or out, but being zoomed in can also increase the sensory input discrepancy. This is because when you are zoomed in your cone of vision moves faster than expected. To try this out yourself look through binoculars and then try moving around, it can get sickening quickly. Also avoid to sudden death transitions or rewinding/fast forwarding time.
- Use the world to support the players sensory system: If a player looks down, show them an avatar body and if you have a camera then player body tracking could help with this too, assuming it is responsive. If the player is flying a spaceship, place them in a cockpit with controls. Basically try and create a logical world that the players can focus on and anchor themselves in. If possible, UI elements should also be anchored into this world, rather than floating around in front of players. This also refers to creating an environment with a clear stable horizon, with stable reference points that players can focus on, rather than a world with a listing, uneven, or moving background.
- Create an unexperienced experience: A bit weird to be suggesting this after saying to use the world to support the player, but as mentioned when talking about cue conflict theory, people who are experienced in a real world activity tend to be more likely to get simulator sick when that activity is simulated. Therefore, if you use VR for an abstract puzzler that has nothing to do with reality, or putting a player in a spaceship cockpit rather than having them walking around (walking being something we are all familiar with, flying less so), then it is possible that this may lessen the chance of simulator sickness..
- Limit player field of view: Field of view seems to be related to simulator sickness because the lower the field of view, the lower the amount of stimulation in edges of the visual system and therefore the lower the feeling of movement. A lower than 30 degrees field of view has been suggested by some academics as dramatically reducing simulation sickness. The big downside of limiting field of view is that it is strongly related to immersion and presence, which are very important not only in games but also to VR specifically. But, field of view can be limited for in-game reasons such as being in a cockpit looking out windows.
Eve Valkyrie is an example of how a cockpit may be able to somewhat break up the field of view and place the player in a supportive environment.
- Playtest: Hopefully playtesting your games is an obvious step in development, however, it is also important if you want to minimise simulation sickness. Another important point, which has been made well by Oculus, is to playtest with people who are unfamiliar with VR, as familiarity with a device can reduce simulation sickness. So, as a developer of a VR game yourself be aware that you make a poor playtester.
- Support Short Play Sessions: The longer someone who is prone to simulator sickness plays, the more likely they may get sick. Furthermore, if someone starts getting sick and abandons your game they may want to come back later and try again. So, support short play sessions if possible, either via plentiful checkpoints or a save anywhere system.
- Seek out experts: Oculus, Sony, and Valve have people much smarter than me working for them. They are running their own experiments, reading all the literature, and giving talks about what works and what does not. Seek them out, stay a while, and listen. Indeed, in the final stage of preparing this post I found that Oculus has a significant simulation sickness section of their best practice guide, which provides much of the same advice that I have. This not only shows that they are clued up on the literature, and that my googling skills need some work, but also that if we can both reach the same conclusions independently, then that is a good sign that this advice could be worth listening to.
What players can do to minimise simulations sickness
While developers and the VR companies have the most capability to prevent and reduce simulation sickness, there are a few things players who suffer simulation sickness can do to potentially help.
- Be young: If you happen to be young, then good news! You’re more resistant to simulation sickness. Interestingly, some of the literature suggests that older people may be resistant too, however, I think this is just a result of poor application of old motion sickness data. Rather, within the simulation community it is (anecdotally) known that working with older adults means be prepared for a lot of participant loss due to simulation sickness.
- Play in moderation: I know this makes me sound like a nagging parent, but remember, even if you don’t get simulator sick there’s a chance that after long exposure to VR you may get sick or feel dizzy/weird after returning to reality. This may not happen of course, but be aware that it is a possibility and try not to play for extended time before immediately driving a car or operating heavy machinery.
- Play in a well ventilated, temperature regulated, safe space: Pretty self explanatory. Most of the symptoms of simulation sickness are aggravated by poor air and uncomfortable temperatures (particularly heat). As for safe, well this won’t help reduce simulator sickness but if you are going to get dizzy and potentially fall over then better make sure that your glass table/grandparent isn’t nearby.
- Play when healthy: If you are feeling sick already, hungover, or particularly drunk/on certain drugs then jumping into a VR experience may not be the way to go.
- Calibrate your device: If the VR device needs calibration, make sure it is done and done well (applies to Devs too! Support calibration, remind players about it).
- Try to act natural: Don’t hold yourself rigid (although, see below about being supported), go with the movements of the game (if safe), do what feels natural even if it makes you look silly. If the game has you seated, then sit. Avoid rapidly and unnecessarily shaking your head around.
- Focus on stable objects on the horizon: Assuming they have been provided it can be helpful if you start to feel sick that you focus on a stable object near the horizon in the game. This is a trick used by dancers to avoid getting dizzy when spinning, and by sufferers of motion sickness, and can have some success for simulator sickness too.
- Try again: For most people repeated exposure to a VR experience will reduce or remove simulator sickness. This does not mean just tough it out or use the VR equipment for long periods despite feeling sick. Rather, use it for a little while, then stop if you start feeling sick, then wait for a day or so (but not more than a week), and then try again. Take it easy. Unfortunately, this kind of adaptation may be game or hardware specific and it is also estimated that up to 5% of the population may be particularly prone to simulation sickness and unable to adapt (Bles & Wertheim, 2000). If this estimate is true, perhaps we end up with another cyberpunk trope - the sim sick underclass who can never join the VR future - but perhaps advancements in VR can lower or remove this figure completely.
- Be stable: If postural instability theory is right, then sitting in a stable, supported position, or even lying down should help as your body won’t have to work to maintain postural stability. However, this may induce more discrepancy between the movement in the game and movement in real life, so it may be useful for a limited number of games and hardware setups.
Lying down to play may, in some cases, help reduce simulation sickness (photo credit Kotaku)
- Research the games you are going to play: Does it look like it has a lot of bouncing rolling movement? Rapid accelerations and decelerations? Poor latency and frame rate? Is there a lot of uncontrolled movement? Then maybe if you are concerned about simulation sickness this game isn’t for you.
- Don’t expect to be sick: Easier said than done, especially after reading a long article telling you all about what the symptoms are and what causes them. However, some element of simulation sickness is due to the nocebo effect - i.e. if you are cued about the fact you may get sick, and expect to get sick, you may be more likely to be sick. This is not saying it’s the fault of those who get simulator sick, but rather to say try and relax, and don’t stress out.
Finally, if you happen to get simulator sick, please be careful. Give yourself time to recover and do not do any activities afterward that involve you having to show motor control or maintain your balance. Don’t operate heavy machinery or a vehicle for a while after getting sim sick. Just in case.
However, hopefully developers and VR hardware companies will succeed in creating simulation sickness free experiences that mean that players don’t need any of the above advice!
Below is a list of some of the academic reports I used in putting together this article. Sadly not all of them are open access and available to the public, but many of the summary articles are (e.g. Bles & Wertheim; Johnson; Kolasinski etal; & Mollenhauser).
- Brooks, J.O., Goodenough, R.R., Crisler, M.C., Klein, N.D., Alley, R.L., Koon, B.L., Logan Jr, W.C., Ogle, J.H., Tyrrell, R.A., & Wills, R.F (2010) Simulator sickness during driving simulation studies. Accident Analysis and Prevention, 42, 788-796.
- Merhi, O., Faugloire, E., Flanagan, M., & Stoffregen, T.A. (2007). Motion Sickness, Console Video Games, and Head-Mounted Displays. Human Factors, 49(5), 920-934. [Open Access]