Everything You Wanted to Know About Simulator SicknessAugust 28, 2014
Motion sickness has been a pretty common theme in the Oculus Rift forums. The Best Practices Guide is full of tactics you should use in your application to help users avoid motion sickness. People discuss which gaming rigs and graphics cards help get the highest frame rates, the better to avoid motion sickness.
That said, there are a ton of questions people about motion sickness. How common is it? What are the most common symptoms? What makes people more prone to sickness?
Licensed under Public domain via Wikimedia Commons.
Fortunately there’s an organization that has been doing research on this for decades - the United States Navy. The Navy realized that doing training missions in flight simulators was much cheaper (and less risky) than in its aircraft, so starting in 1960, they began training their pilots in ever-more-advanced simulators. Unfortunately, some of their pilots would get sick, and they worried about how simulator training would carry over to the actual planes, so they commissioned lots of studies to determine what causes motion sickness, how common it is, and whether simulators would help Navy pilots fly planes better. Today we’re going to take a look at the “Introduction to and Review of Simulator Sickness Research”, written by David M. Johnson in 2005.
Can research into Navy simulators actually tell us anything about virtual reality headsets?
This is a legitimate question. Navy simulators use hydraulics to change the orientation of the simulator when you do things like turns, and Oculus et al do not. Navy pilots also tend to be young adults, mostly men, and headset users will make up a wider proportion of the population.
That said, the simulators used by the Navy have a very wide horizontal field of view, and the image being simulated is a flight during a cockpit (one of the most common VR demo types to date). From reading the document, it sounds like a lot of the things discovered by the Navy have also been discovered by the VR community.
Let’s get into it!!
General Facts about Simulator Sickness
Generally, simulator sickness decreases with repeated exposure. The link is more established for motion sickness than simulator sickness, though it exists in both cases. 95 to 97 percent of people eventually adapt to an environment that causes motion sickness; the rest will be forever sick.
There are differences in how quickly different people adapt to simulator sickness.
Adaptation is highly environment specific. As an example, pilots who adapted to one simulator became sick again when they began using a new simulator. This implies that getting your “VR Legs” may be difficult, and require a new adaptation period for each game.
Active movement in the environment will make you sicker initially, but also make you adapt more quickly than being passive.
In multiple studies, performance is not affected by motion/simulator sickness. Researchers tested users who were motion sick and users who were not on a wide range of tasks - 600 yard dash, throwing darts, shooting rifles, drawing lines in a mirror, and walking in a straight line. In each case, the performance of queasy people matched that of those who felt perfectly healthy. What does change is your motivation to perform - when you feel motion/simulator sickness, you don’t feel like doing much of anything, even if you do perform well when you do it.
There have been no documented cases of someone getting sick/disoriented in a simulator, then later crashing an aircraft or an automobile due to disorientation.
The longer the duration, the more likely it is for people to become sick.
How common is it?
This tends to be extremely difficult to measure, and varies widely across individuals and simulator tasks. Depending on the task and the criteria used for measurement, incidence ranged from 10% to 90% of users who got motion sick. Here are the incidences of sickness from five different studies of five different simulators:
- eyestrain (37%) and fatigue (27%)
- fatigue (43%), sweating (30%), eyestrain (29%)
- disorientation (24%), difficulty focusing (24%)
- fatigue (35%), eyestrain (34%)
- fatigue (34%), eyestrain (29%), headache (17%)
In one study, 61% of those sick had their symptoms persist between 15 minutes and six hours. Symptoms very rarely persisted the morning after a simulator session.
What things make you more prone to motion sickness?
Gender. Women are generally more prone to simulator sickness than men, though the difference is not large.
Age. Children between 2 and 12 are the most prone to sickness. This effect decreases rapidly between 12 and 21, then more slowly to age 50 or so. One study indicated that people above age 50 very rarely got sick; another had this group getting sick more often, possibly due to older pilots having more experience flying real planes. More research is needed into the effects of simulator sickness among users over 50 years old.
Alcohol/Drugs. These affect your inner ear, which affects your ability to balance and tell up from down. Not surprisingly, this means that you are more likely to get sick while under the influence.
Illness. People with illnesses such as hangover, flu, head cold, ear infection, or upset stomach all were more likely to get sick. The Navy generally would not let pilots fly if they were not at full health.
History of motion sickness. People who reported that they had previously gotten sick in cars or amusement park rides were more likely to become sick in a simulator.
Why do we get simulator sickness?
There are a few theories. Most pinpoint the difference between the motion perceived by your eyes and the motion reported by your inner ear. One theory says your brain interprets the sensory mismatch as evidence of having been poisoned, leading to nausea/vomiting.
Animals also get motion sickness.
Motion sickness is generally more correlated with gastrointestinal distress - symptoms like burping, nausea, and vomiting. Simulator sickness is more correlated with visual distress - eye strain, difficulty focusing, disorientation, and headache.
How were these tests conducted?
Generally, by asking pilots after they took turns in the simulator, and then aggregating data about them, or running correlations based on other facts about the people that got sick (age, etc). Some of the other studies about motion sickness date from World War II when a much wider range of people were being transported across the high seas.
Because symptoms vary so widely from person to person, a very large number of pilots are needed to establish statistical significance. This made testing in controlled environments difficult. In studies that had large numbers of subjects, the researchers had less control over the environment or the test conditions. The same testing difficulties will likely apply for user experience researchers. A wide number of subjects will need to be tested to draw conclusions about a given stimulus.
What other things should I know about?
When it came to simulator content, the Navy found out a lot of things that would make a lot of sense to people that have done research into VR usability:
Instructors had a button that let them freeze the simulation, or to reset it to the beginning. This button was a leading cause of motion sickness. The lesson is that changing the user’s camera angle without warning will lead to sickness.
Moving forward/backward in time, or flying backwards (a motion not controlled by the user) caused sickness.
Pilots report less discomfort than passengers. This is likely because pilots are controlling the motion of the plane and can anticipate turns and flips. When designing content, try to ensure all camera motion is controlled by the user.
Head movement increases susceptibility - consider designing your application so the user doesn’t have to move their head often, or keep it fixed away from dead center for a long period of time.
In one study, 67% of pilots closed their eyes to avoid getting sick at points where the simulation moved rapidly.
To adapt people to a new simulator, the Navy recommended brief tours in the simulator (less than one hour), followed by 24 hours of rest. The Navy also recommended putting the most intense experiences at the end of a session. Worth keeping in mind for people designing experiences - keep interactions short and in chunks, so people can adapt to the game.
It takes an average of 6-10 sessions to adapt to sickness in a new simulator.
Drugs like dramamine were effective at helping with simulator sickness.
Susceptibility increased the closer the simulator was flying to the ground; the further away the ground, the less sick the pilots got. This means good things for space-based cockpit games like Elite: Dangerous and less good things for ground-based flying games like Faceted Flight.
Almost everyone experiences sickness of some kind, depending on the environment, and almost everyone adapts. Because people adapt to simulator conditions, and because symptoms vary widely, you’ll need to test a large number of people to draw conclusions about whether your application causes sickness.
The most common symptoms are dizziness, eye strain, disorientation, and nausea. Actual vomiting is rare.
The Navy had strict conditions about when pilots were allowed in the simulator (as well as a smaller age range than the general public). People are going to try to play your game while drunk, people will play with head colds or the flu, children are going to try your application, even though they are all more susceptible to sickness, and you won’t be able to control it. You can add settings to your game to allow the user to control the intensity of the experience.
When users are just starting with your game, keep sessions short, try to avoid forcing the user to move their head, and put more intense experiences at the end of a session.