A model for vestibular function in altered gravitational states.

During evolution, the vestibular organ was made to serve mainly two purposes: 1) to guide eye movements during sharp turns, so that the point of fixation in the visual field can be kept steady, a function accomplished by the semicircular canal system and 2) to indicate the terrestrial vertical, so that upright posture and gait can be maintained even in the dark. The otolith system serves the latter purpose. Since the function of the semicircular canal system does not depend on gravity, it is not grossly disturbed by gravitational levels different from 1 g. The proper function of the otolith system depends entirely on the presence of a gravitational force vector of 9.8/m/sec2 directed towards the center of the earth. This system therefore malfunctions when the amplitude of the combined gravito-inertial load is different from 1 g and also when the direction of the sensed gravitational pull is "contaminated" by additional inertial reactive forces as during horizontal acceleration. The effect of such inertial stimulations is probably even stronger in a weightless environment, in which case the background stimulation of terrestrial gravity is absent. Moreover, minor mass differences between the otolithic membranes of the left and right inner ear, even if well compensated on the ground, might lead to malcompensation in weightlessness as well as in hypergravity. A hypothetical model is developed to describe in the central nervous system compensating mechanisms in hypo- and hypergravitational states. The "space-sled" is introduced as a new research tool and recommendations are made for a prophylactic training regimen to reduce or prevent space sickness.