Redout
A redout occurs when the body experiences a negative g-force sufficient to cause a blood flow from the lower parts of the body to the head. It is the inverse effect of a greyout, where blood flows away from the head to the lower parts of the body. Usually, a redout will only ever be experienced by pilots, as planes are the most common devices that allow such negative g-forces to be exerted. Redouts are potentially dangerous and can cause retinal damage and hemorrhagic stroke.
The predominant theory on the red appearing in the visual field is not due to the actual blood flow to the eye. It is most likely due to the blood laden lower eyelid coming into the visual field due to the pull of negative-Gs.
