Vertical skew due to changes in gravitoinertial force: a possible consequence of otolith asymmetry.

During the alternating enhanced and reduced "gravity" levels of parabolic flight, subjects noted that a horizontally eccentric point target viewed binocularly in darkness seemed to split into two targets with vertical divergence. The amount of perceived divergence seemed to depend on instantaneous g level. This perceptual observation suggested a vertical misalignment of the eyes (vertical skew), dependent on the magnitude of g level sensed by the otoliths. While performing a different experiment during parabolic flight we recorded binocular eye position and analyzed these data to corroborate the behavioral observations. Records were obtained from seven trials in five subjects, containing eye alignment in both 0 g and 1.8 g. The recordings confirm the presence of vertical skew that changed with g level. "Skew-differential" was quantified for each trial as the difference in vertical skew from 0 g to 1.8 g. Mean skew-differential was 1.37 degrees; the largest was 2.57 degrees. Statistical significance was assessed using the student t-test and the more correct and stringent Generalized Estimating Equations (GEE). The observed skew is equivalent to a mild strabismus, which is known to result in diplopia. The underlying mechanism may be an asymmetry between the otolith organs on either side, which is centrally compensated in 1 g, but is inappropriately compensated in other gravitational fields. If equivalent amounts of vertical skew were to occur during changes in g level during dynamic phases of flight in high-performance aircraft or space shuttle reentry, the impact on visual fixation might be detrimental to piloting performance.