Contrast thresholds and fixation disparity during 5-Hz sinusoidal single- and dual-axis (vertical and lateral) whole-body vibration

The present study assumed that whole-body vibration, transmitted through th e seat, impairs spatial retinal resolution and oculomotor alignment paralle l to the vibration axis. More specifically, that the decrement increases gr adually from single-axis lateral via single-axis vertical and dual-axis lin ear to dual-axis circular motion. Twenty participants (19-26 years of age) with good vision volunteered for the experiment where in three sessions one of the following three conditions, contrast threshold, nonius bias or fixa tion disparity, for vertically and horizontally oriented test patterns was determined during five experimental conditions. The latter comprised a cont rol (a(z) = a(y) = 0) and four conditions where 5-Hz sinusoidal motion of 1 .2 ms(-2) rms were applied separately, either in the vertical or in the lat eral direction, or simultaneously in both directions, once without and once with a phase shift of 90 degrees, thus causing dual-axis linear or circula r motion. Contrast thresholds for horizontal gratings and the variability o f Vertical fixation disparity increased significantly whenever the particip ants were exposed to vertical motion (alone or combined with lateral motion ). These effects may result in an increased difficulty in properly recogniz ing characters and graphic patterns containing horizontal lines and in the development of asthenopic complaints.