Asymmetry of vertical optokinetic after-nystagmus in squirrel monkeys.

Asymmetry of vertical optokinetic after-nystagmus (OKAN) was studied in 6 squirrel monkeys. The slow-phase eye velocity (SPEV) of upward OKAN first-phase (OKAN-I) increased with increasing stimulus velocity, whereas the SPEV of downward OKAN-I diminished. The time constant of OKAN-I was shortened with the increase in stimulus speed in both directions. With a downward stimulus, the short stimulus duration failed to produce OKAN second-phase (OKAN-II) (upward slow-phase); however, with an increase in stimulus duration, the percentage appearance increased. There was no change in percentage appearance, regardless of the duration of upward stimulus. The asymmetry of OKAN-I and that of OKAN-II differed to a certain degree.