Reorientation of Poststimulus Nystagmus in Tilted Humans 1

Recent studies indicate that the direction of postrotatory nystagmus and optokinetic afternystagmus reorients toward earth-horizontal in tilted subjects. To further examine this phenomenon in humans, we studied 8 adults (4M, 4F) with no history of neurologic or otologic disease. Vestibular stimulation consisted of yaw off-vertical axis rotation (OVAR) trapezoids with eyes open in the dark at 60°/s constant velocity with a tilt angle of 30° and a deceleration at 100°/s² to a stop in either the right-ear-down or the left-ear-down position. The optokinetic stimulus consisted of 5° wide black and white stripes projected against a cylindrical visual surround 1 m in diameter rotated at a constant velocity of 30°/s. The projector, visual surround, and subject were all tilted by 30°; subjects were placed either in the right-ear-down or left-ear-down position. Eye position was measured using the magnetic scleral search coil technique. Each trial was scrutinized for the presence of reorientation of nystagmus in the subject's roll plane by looking for vertical nystagmus in the appropriate direction. Results indicated that reorientation of postrotatory nystagmus following OVAR was variable but often present. Reorientation of optokinetic afternystagmus was neither as consistent nor as robust as that seen following OVAR. These findings confirm that humans exhibit a predilection for eye rotations in the earth-horizontal plane. The presence of reorientation of postrotatory nystagmus (and to a lesser extent of optokinetic afternystagmus) suggests that static otolith inputs, which are known to alter velocity storage dynamics, also appear to alter the relative influence of velocity storage in different directions in humans.