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The Three Traditional Theories of Simulator Sickness and their Implications for Scenario Design

 

Simulator sickness can complicate simulation-based driving research and negatively affect the validity of that research. When participants are uncomfortable or nauseous, they will be distracted from tasks, preoccupied with their discomfort. They may (consciously or unconsciously) avoid tasks or perform poorly to avoid further irritation.

Our Realtime Technologies lead engineer and general manager, Heather Stoner, has thought extensively about how to address simulator sickness. (In fact, she’s the primary author of the simulator sickness chapter in the Handbook of Driving Simulation for Engineering, Medicine, and Psychology.) She notes three widely recognized theories about simulation sickness and the design implications of each. Regardless of your driving simulation hardware, you can tweak your driving scenarios to minimize the likelihood and severity of simulator sickness.

The “Poison” Theory of Simulator Sickness

This theory, first proposed in the mid-1970s, explains simulator sickness and motion sickness from an evolutionary perspective. Treisman notes that the sensory experience of early simulators (e.g., blurry visuals, poorly coordinated visual/motion cues, low visual resolution, the rocking of a motion base, etc.) are similar to the symptoms of poisoning and intoxication. As a result, the body activates strategies used in nature when poison has been ingested: vomiting.

While this theory is an enduringly popular folk explanation for motion, simulator, and “game sickness,” it has notable shortfalls. For example, the poison theory fails to explain why some individuals (such as those with more experience with the real-world driving task or pregnant women) would be more susceptible to simulator sickness. It also fails to explain why individuals are more susceptible to simulation sickness on initial exposure and become desensitized over time (no one becomes “desensitized” to food poisoning, for example). Additionally, we’ve found that at times higher-fidelity simulators can perversely increase the likelihood of simulation sickness——which runs counter to what this theory might predict.

Although the “poison theory” offers a reasonable explanation of why simulator sickness includes nausea, it isn’t a useful way for trainers and researchers to think about sim sickness.

 

Cue Conflict Theory

This theory arises from a somewhat earlier theory of motion sickness, outlined by J.T. Reason and J.J Brand in 1975 and is considered one of the leading theories It suggests that simulation sickness occurs from mismatches between what sensory systems expect and what they perceive. The main sensory systems involved are the visual system and the vestibular system (which relies on a set of structures inside the ear to allow your body to track how it is oriented in and moves through space).

Cue conflicts are broken into two broad categories. The categories are intermodal (conflicts between the visual and vestibular systems) and intramodal (conflicts between cues from a single system). Most intramodal conflicts that cause driving simulator-related discomfort arise within the vestibular systems, where the otolith organs (which give us a sense of direction, linear acceleration, and head orientation) may disagree with the semicircular canals (which help us determine the direction and speed of angular acceleration). These are key considerations when tuning motion bases.

In either case, the conflict comes down to either:

  1. signals being sent by one system and not the other (e.g., a fixed-base simulator provides visual cues that you are turning, but the inner ear reports that you are sitting still)
  2. simultaneous disagreeing signals (e.g., a simulator with a motion base gives visual cues indicating a sharp turn, but the motion base can only emulate a gentle turn)

This understanding of simulator sickness implies that reducing sensory input will tend to lessen discomfort——which has shown to be the case.

 

Scenario Design Tips

  1. Reduce the expected inputs to minimize conflict or build-up to those events (e.g., design scenarios with few sharp turns or quick accelerations at the start of the experiment, as these are cues are harder to emulate in a simulator with a one-to-one match with the real world, that tend to lead to cue mismatches between the visual and vestibular systems. These scenarios should be introduced at a later state where the body can adapt to the cue conflict)
  2. Reduce optic input or incorrect optic input (e.g., reduce the flow of visual information that might conflict with other sensory input by reducing visual elements——like signs and lamp posts——and pushing landscape features further from the simulated roadway. Be sure that the eye point is drawn relative to the driver. Any offset from the driver’s eye point will impact the optic flow to the driver and can cause discomfort)

 

Postural Instability Theory

In 1991, Gary Riccio and Thomas Stoffregen suggested an alternate explanation for motion sickness that has applications in reducing the impact of simulator sickness. This theory combined with the Cue Conflict is the most widely accepted theory. In the Postural instability, theory notes that our bodies are constantly making small adjustments to maintain a stable posture in our environments. Discomfort (and subsequently simulator/motion sickness) arises when the body has not yet developed efficient strategies for maintaining postural stability in a new environment.

This theory explains why, for example, children are more prone to car sickness than adults, or why an experienced driver is more likely to develop simulator sickness than someone entirely new to driving. It also explains why passengers—who have less control over the movement (or apparent movement) of their environment or is not centered around that passenger—are more prone to motion/sim sickness than drivers.

Even more interestingly, Riccio and Stoffregen noted that postural instability doesn’t simply proceed simulator or motion sickness; it’s a necessary pre-condition to motion sickness (which is why, for example, a sleeping passenger does not get car sick).

This theory correctly explains why individuals with more driving experience are more prone to simulation sickness in driving simulators and also why people, in general, can acclimate to simulation sickness.

 

Scenario Design Tips

  1. Begin every session with one or more short “acclimation drives” (e.g., an uneventful five-minute drive in which drivers follow a lead vehicle, followed by a brief break to “stretch your legs”)
  2. Monitor for postural instability (such as burping, swallowing hard, touching the face, squinting or straining to see things, or even reaching to turn on the fan, these are all signs the participant is feeling discomfort) and intervene before it advances further into simulation sickness; offer a break to “stretch your legs” and get a drink of water