<P> A grey - out may be experienced by aircraft pilots pulling high positive g - forces as when pulling up into a loop or a tight turn forcing blood to the lower extremities of the body and lowering blood pressure in the brain . This is the reverse of a redout, or a reddening of the vision, which is the result of negative g - forces caused by performing an outside loop, that is by pushing the nose of the aircraft down . Redouts are potentially dangerous and can cause retinal damage and hemorrhagic stroke . Pilots of high performance aircraft can increase their resistance to greyout by using a g - suit, which controls the pooling of blood in the lower limbs but there is no suit yet capable of controlling a redout . In both cases symptoms may be remedied immediately by easing pressure on the flight controls . Continued, or heavy g - force will rapidly progress to g - LOC (g - force induced Loss of Consciousness). </P> <P> Surprisingly, even during a heavy grey - out, where the visual system is severely impaired, pilots can still hear, feel, and speak . In other words, complete grey - out and loss of consciousness are separate events . </P> <P> Another common occurrence of greyouts is in roller coaster riders . Many roller coasters put riders through positive g - forces, particularly in vertical loops and helixes . Roller coasters rarely have high enough negative g - forces to induce redouts, as most low - g elements are designed to simulate weightlessness . </P>

Why do my eyes go black on roller coasters