Motion sickness in virtual reality: not a simple conflict between the senses

Probably everybody is familiar with the train illusion: the feeling that your train is departing when, in fact, you see the train at the neighboring platform leave the station. This powerful illusion shows that motion of the visual surroundings can cause a compelling sense of self-motion in stationary observers. This phenomenon is exploited in many virtual reality environments and vehicle simulators. A negative side effect, however, is that prolonged exposure to a visually induced sense of self-motion can give rise to motion sickness: you may have a headache, feel dizzy, or feel nauseated. Why does this happen?

Our scientist Suzanne Nooij (incl. Paolo Pretto and Heinrich H. Bülthoff) in the Motion Perception and Simulation research group published a new study in PLOS one, in collaboration with Heiko Hecht and Daniel Oberfeld from the Johannes Gutenberg University in Mainz.

1. What was your study all about? 
In this study, we investigated the contribution of different factors (the illusion of self-motion, eye movements and head movements) in visually induced motion sickness. We chose a situation in which participants looked at a scene rotating around them, as if they were in a merry-go-round. Different theories give different predictions for this case. The most well-known theory, the Sensory Conflict theory, proposes that sickness is caused by a conflict between, on the one hand, what you actually see and feel, and on the other hand, what you expect based on previous experience. According to this theory, experiencing the illusion of self-motion is a prerequisite for visually induced motion sickness and, more importantly, it suggests that especially changes in perceived velocity are provocative. Another theory states that, even when you feel this motion illusion, you will not get sick as long as you keep your head still, and a third theory proposes that the reflexive eye movements cause the symptoms of motion sickness. To test the contribution of each factor, participants came to our panoramic lab multiple times and viewed the moving visual scene under different conditions for a duration of 20 minutes. During each test, we measured the level of motion sickness, the strength of the motion illusion and the participant’s eye and head movements. The results clearly show that of all these factors, the strength of the motion illusion is by far the most important one: More sickness is experienced in conditions that give a stronger motion illusion. By varying and comparing the motion illusion strength within the same participants, this is the first study demonstrating this effect so clearly.

2. Why were you interested in this topic?
It is quite surprising that there still is so little consensus about the actual causes of motion sickness, a problem that exists for centuries! I am very sensitive to motion sickness myself and interested in how this negative side-effect relates to our entire sense of spatial orientation. In that sense, investigating contributing factors helps to understand the causal mechanisms of motion sickness and to shape and define the theoretical background. Only then we can look for cures. 

3. What should the average person take away from your study?
The main message of this study is that the experience of the illusion of self-motion, and not a moving scene per se, is a prerequisite for visually induced motion sickness to occur. The level of the symptoms depends on the strength of the illusion and on their individual motion sickness susceptibility.

4. Are there any major caveats? What questions still need to be addressed?
Still puzzling was our finding that even perceived motion at constant velocity can result in motion sickness, contrary to what the Sensory Conflict theory predicts. This result calls for a refinement of the most widely adopted motion sickness theory. We are currently testing the possibility that it is not so much a conflict between the senses that causes motion sickness, but a conflict in what we experience as being moving (ourselves) and what is stationary (the surroundings).

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