Simulator sickness is a condition where a person exhibits symptoms similar to motion sickness (e.g. headache, drowsiness, nausea, dizziness, vomiting, sweating) caused by playing computer/simulation/video games. Researchers the University of Minnesota had students play the first person shooter ‘Halo’ for less than an hour, and found that up to 50 percent felt sick afterwards. The phenomenon was well known in popular culture before it was known as simulation sickness. In the 1983 comedy film ‘Joysticks,’ the manager of a local video arcade says, ‘The reason why I never play any of these games, well, they make me physically ill. I mean, every time I look in one of the screens, they make me dizzy.’
In 1995, the US Army Research Institute for the Behavioral and Social Sciences published the results of a study of 742 pilot exposures from 11 military flight simulators. Half of the pilots reported post-effects of some kind. Symptoms dissipated in under an hour for one third, after four hours for six percent, after six hours for four percent, and one percent reported cases of spontaneously occurring flashbacks.
Motion sickness due to virtual reality is very similar to simulation sickness and motion sickness due to films. In virtual reality, however, the effect is made more acute as all external reference points are blocked from vision, the simulated images are three-dimensional and in some cases stereo sound that may also give a sense of motion. The NADS-1, a simulator located at the National Advanced Driving Simulator, is capable of accurately stimulating the vestibular system (responsible for balance and spatial orientation) with a 360-degree horizontal field of view and 13 degree of freedom motion base. Studies have shown that exposure to rotational motions in a virtual environment can cause significant increases in nausea and other symptoms of motion sickness.
Simulator sickness is common among pilots who spend particularly long periods in simulators. Due to the spatial limitations imposed by a cramped simulator, perceived discrepancies between the motion of the simulator and that of the vehicle can occur and lead to sickness. It can reduce the effectiveness of simulators in flight training and result in systematic consequences such as decreased simulator use, compromised training, ground safety, and flight safety. Pilots are less likely to want to repeat the experience in a simulator if they have suffered from simulator sickness. It can also compromise training in two safety-critical ways: it can distract the pilot during training sessions, and it can cause the pilot to adopt certain counterproductive behaviors to prevent symptoms from occurring. Simulator sickness can also have post-training effects that can compromise safety after the simulator session, such as when the pilots drive away from the facility or fly while experiencing symptoms of simulator sickness.
Though human-piloted aviation has existed since the early 20th century, simulator sickness did not arise as an issue for pilots until much later when the first fixed-base simulators were created. Bell Aircraft Corporation created a helicopter simulator for the Navy during the 1950’s, and it was found ‘that a large number of observers (mostly helicopter pilots) experienced some degree of vertigo during these demonstrations.’ Navy psychologists investigating the effect found that veteran flight instructors seemed to be most susceptible. In fact, 60% of the instructors reported simulator sickness symptoms compared to only 12% of the students. ‘The SS usually occurred in the first ten minutes of a training session and frequently lasted for several hours afterward.’ Studies conducted independently by the US Navy, US Coast Guard, and US Army during the 1980’s all came to the same conclusion: the greater experience of the pilot, higher the likelihood of sickness symptoms during simulation training exercises.
In 1989, the US Army released a report detailing the results of a study examining simulator sickness in UH-60 Blackhawk flight simulators indicating that longer periods between sessions of flight simulation training increased the likelihood of detrimental symptoms. Research suggests that this is the body’s natural way of adjusting to these systems. The bodies of experienced pilots have adapted to different types of motion experienced during actual flight conditions. When placed into a flight simulator, visual and other stimuli cause their bodies to expect the same motions associated with actual flight conditions. But their bodies instead experience the imperfect motion of the simulator, resulting in sickness. A similar situation can arise for pilots who have long gaps between simulator uses. During simulation training, the body will eventually adapt to the environment to diminish the effects of simulator sickness. However, when long periods of time are spent outside of the simulator, the body is not able to adequately adapt and symptoms will reappear.
Often, adaptation is the single most effective solution to simulator sickness. For most individuals, adaptation can occur within only a few sessions, with only a minority of individuals (3-5 percent) never being able to adapt. This adaptation occurs within the psyche of the individual with repeated, controlled exposures, without any required alteration to the simulator. Through incremental exposures, dispersed regularly over a series of days, adaptation can occur faster than that of an abrupt all-encompassing exposure. However, following adaptation to the novel simulator motion environment, simulator sickness symptoms can reoccur with a return to the former environment. For this reason, simulator sickness is commonly referred to as a phenomenon of maladaptation sickness, due to incessant conflict between current and past environmental conditions.
Two main theories exist about the causes of simulator sickness. The first is sensory conflict theory. Optical flow patterns generated in virtual environments typically induces perception of self-motion (i.e., vection). Sensory conflict theory holds that, when this perception of self-motion is not corroborated by inertial forces transmitted through the vestibular system, simulator sickness is likely to occur. Thus, sensory conflict theory predicts that keeping the visual and vestibular inputs in agreement can reduce the likelihood of simulator sickness experienced by users. Additionally, according to this theory, people who do not have a functioning vestibular component of their nervous system should not show either simulator sickness or motion sickness.
The second theory for simulator sickness identifies postural instability as the determinant of simulator sickness. This theory emphasizes that situations producing simulator sickness are denoted by their unfamiliarity to the participant more than the degree of sensory conflict; for example, seasickness is, for many, a transient problem that is solved by time at sea. Thus, the novelty of the motion cues is hypothesized to lead to an inability to maintain postural control and this lack of control causes simulator sickness until the participant adapts. Key attributes here include the notation that the motions causing simulator sickness are in a nauseogenic low frequency range that overlaps with the frequency of motion within the human body as it maintains control over its posture. Experiments have measured markers of the onset of postural instability, and found that it precedes signs and symptoms of simulator sickness.
At present, it is accurate to say that both—and neither—of these theories are yet adequate to fully explain and predict simulator sickness. Although it is clear which types of pilots are affected by it, and both sensory conflict theory and postural instability theory relate its onset with certain physiological conflicts, neither theory suffices to predict why these specific conflicts (vision vs. vestibular on the one hand, posture vs. control on the other) elicit sickness in the subject. Additional possibilities for elicitation of motion sickness in general (including simulator sickness) include gaze destabilization, which is disrupted if the vestibuloocular reflex gain in the nervous system is altered, moving patterns of visual stimuli, and motions that stimulate the inner ear. However, since laboratory studies have shown the removal of the vestibular projection areas of the cerebellum (in laboratory animals) to result in insusceptibility to motion sickness, it is almost certainly probable that the first of these theories holds the most promise with regard to research into the direct physiological causes of the phenomenon.
The Daily Omnivore 
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