Roller coasters typically do not expose the occupants to much more than about 3 g. An untrained individual not used to the g-straining maneuver can black out between 4 and 6 g, particularly if this is pulled suddenly. The g thresholds at which these effects occur depend on the training, age and fitness of the individual. Dreams are reported to follow g-LOC which are brief and vivid. This is doubly dangerous because, on recovery as g is reduced, a period of several seconds of disorientation occurs, during which the aircraft can dive into the ground. This is because when the body accelerates up at such high rates the blood rushes from the brain which causes loss of vision.Ī further increase in g-forces will cause g-LOC where consciousness is lost.

Humans can withstand a positive acceleration forward at higher g-forces than they can withstand a positive acceleration upwards. The human body has different tolerances for g-forces depending on the acceleration direction. The reverse effect is experienced in advanced aerobatic maneuvers under negative g-forces, where excess blood moves towards the brain and eyes (' redout'). These effects are due to a reduction of blood flow to the eyes before blood flow to the brain is lost, because the extra pressure within the eye ( intraocular pressure) counters the blood pressure. If g-forces increase further, complete loss of vision will occur, while consciousness remains. As g-forces increase, visual effects include loss of colour vision (' greyout'), followed by tunnel vision (where peripheral vision is lost, retaining only the centre vision).

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