Effects of head positions on horizontal optokinetic afternystagmus in humans

Abstract

In order to study the space orientation of optokinetic afternystagmus in humans, I examined optokinetic afternystagmus in the yaw axis (horizontal OKAN, HOKAN) in both sitting and the 90 degree lateral tilted positions in 29 normal volunteers. Optokinetic stimuli of 80 degree/s were given for 120 seconds using a projection type stimulator. Eye movements were recorded in two dimensions, horizontal and vertical, to the head fixed coordinate axis using conventional electronystagmography. Five parameters of HOKAN (maximum slow phase velocity, time constant of slow phase velocity, total nystagmus number, frequency and duration) in the sitting position were compared with those in the 90 degree lateral positions tilted to the right and to the left. The following results were obtained. 1) OKAN were recorded from all subjects in the sitting position. The mean and the standard deviation of maximal slow phase velocity, duration and the time constant were 11.6 +/- 4.0/s, 35.7 +/- 15.6 seconds and 15.1 +/- 8.5 seconds, respectively. 2) In the lateral positions, the rate of appearance of OKAN decreased to 72%. In the lateral positions, the slow phase velocity decreased to 68% of that in the sitting position. 3) The rate of appearance and the slow phase velocity of OKAN were more active in the direction horizontal to the ground than in the perpendicular direction. These results suggest that otolith input modifies HOKAN in humans.