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Flight simulators cause a form of motion sickness, admits Hughes Electronics
of Los Angeles in EP 847 027. Some people lose the ability to perceive depth for
a time after they have left the simulator, and later see flashbacks and feel
sick. This happens because the focus of the pilot’s eyes drifts away from the
screen when aircraft appear to be moving towards or away from them. To cure
this, Hughes generates an image of another aircraft moving in the opposite
direction. The competing movements keep the pilot’s eyes at the correct
convergence angle, preventing the false perceptions of depth that lead to
sickness.