Influence of gravity vector on eye movement elicited by linear acceleration.

When the body/head motion is sensed by otolith organs, they respond not only to the resultant acceleration of the motion but also to the gravitational force. We investigated the influence of the gravity vector on the otolithic-ocular reflex caused by motion in normal subjects. The sled type linear accelerator, moving back and forth with a frequency of 0.25 Hz and an amplitude of 2 m, generated right-left linear acceleration with a maximum magnitude of 0.5 g. We tested every subject under seven different postures: 1) 135 degrees forward tilted (F.T.), 2) 90 degrees F.T., 3) 45 degrees F.T., 4) upright sitting, 5) 45 degrees backward tilted (B.T.), 6) 90 degrees B.T., and 7) 135 degrees B.T. Horizontal eye movements with nystagmic pattern were elicited by these stimulations and were recorded by EOG. The eye movement data were analyzed by using a computer with the following procedure. After extraction of saccadic components from the data, the remaining parts were connected smoothly. The waves reconstructed in this manner, considered to be primary otolithic-ocular responses, were processed with the FFT method for calculating the amplitude of the component at 0.25 Hz. As a result, the responses were larger in the forward tilted postures than in the backward tilted postures.