Simulator sickness

Simulator sickness is a subset of motion sickness that is typically experienced while playing video games from first-person perspective. It was discovered in the context of aircraft pilots who undergo training for extended periods of time in flight simulators. Due to the spatial limitations imposed on these simulators, perceived discrepancies between the motion of the simulator and that of the vehicle can occur and lead to simulator sickness. It is similar to motion sickness in many ways, but occurs in simulated environments and can be induced without actual motion. Symptoms of simulator sickness include discomfort, apathy, drowsiness, disorientation, fatigue, and nausea. These symptoms 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 and hence can reduce the number of potential users. It can also compromise training in two safety-critical ways:

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 created a helicopter simulator for the Navy during the 1950s, and it was found "that a large number of observers (mostly helicopter pilots) experienced some degree of vertigo during these demonstrations". Navy psychologists performed further study on the pilots who participated in these simulator exercises, and found that 28 out of 36 respondents to their evaluations experienced sickness. Additionally, psychologists found that experienced 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."

Two main theories exist about the causes of simulator sickness. The first is sensory conflict theory. Optical flow patterns generated in virtual environments typically induce 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 notes that situations producing simulator sickness are denoted by their unfamiliarity to the participant more than the degree of sensory conflict; for example, sea sickness is, for many, a transient problem that is solved with experience to being on a ship. 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 otoliths and semicircular canals of the inner ear. It is unclear whether or not these stimuli are encountered in significant amounts in a simulator to induce sickness in the expert pilots. 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.

While anyone can experience simulator sickness, studies in flight simulators have found a correlation between the appearance of symptoms and the flight experience of the pilot. Studies conducted independently by the US Navy, US Coast Guard, and US Army during the 1980s all came to the same conclusion: the greater experience of the pilot, the higher the likelihood of sickness symptoms during simulation training exercises.^{[citation needed]}

In 1989, the US Army released a report detailing the results of a study examining simulator sickness in UH-60 Blackhawk flight simulators, confirming the above hypothesis. The report also found that longer periods between sessions of flight simulation training resulted in greater probability of detrimental symptoms appearing increased.