Duration and direction of optokinetic after-nystagmus as a function of stimulus exposure time in the monkey.

1. Parameters of the optokinetic after-nystagmus (OKAN) outlasting optokinetic stimulation were studied in monkeys. With constant pattern velocity (60 degrees/s) exposure times were varied between 2 s and 15 min. 2. All monkeys showed a primary after-nystagmus moving in the same direction (OKAN I) as the preceding optokinetic nystagmus, under all conditions tested. A secondary after-nystagmus in the opposite direction (OKAN II), was only observed after exposure times of 30 s or longer. After a 15-min exposure, half of the monkeys showed an early onset of OKAN II in less than 1 min, whereas for the remaining half the transition to OKAN II occurred only after 4 min or not at all. 3. In monkeys showing an early onset of OKAN II, the duration of OKAN I decreased and the maximal slow phase velocity of OKAN II increased consistently with longer exposure times. In several instances OKAN III, moving in the same direction as OKAN I, was seen after OKAN II. 4. Monkeys in which OKAN II was late, or absent, often showed minima and additional maxima of slow phase velocity of OKAN. 5. The results are discussed in terms of two opposing mechanisms underlying the generation of OKAN and their connections to the vestibular system.