Directional asymmetries of human optokinetic nystagmus

Optokinetic nystagmus in the four principal directions was investigated on the occurrence of directional asymmetries in 7 normal human subjects. Instructions were aimed at obtaining a 'stare' type of OKN. The movement of both eyes was recorded simultaneously with a scleral sensor-coil method. Subjects viewed a full-field random dot pattern rotating at velocities of 9 to 57 deg/s binocularly, as well as monocularly with either eye. Gain was always less than 0.85 and decreased when the pattern velocity increased. Horizontal and vertical nystagmus differed in a number of respects. (1) We found no evidence for an overall asymmetry for rightward or leftward motion. However, human OKN showed a clear preference for upward stimulus motion. Mean gain was ca. 0.15 larger for upward than for downward motion. (2) The decrease of the gain of OKN as a function of increasing stimulus velocity was steeper for vertical than for the horizontal direction. (3) The eyes moved nearly perfectly yoked for vertical pattern movement, irrespective of the viewing conditions. In contrast, during horizontal OKN the gain of the eye tracking in the nasal direction was higher (by about 4%) than the gain of the other eye moving simultaneously in the temporal direction. This difference persisted irrespective of the viewing conditions and appears to be motor, not sensory in origin. In addition, for any direction of the pattern motion a statistically significant increase of the gain occurred when the pattern motion was seen binocularly instead of monocularly with either eye.