Optokinetic nystagmus (OKN) and optokinetic afternystagmus (OKAN) were investigated in Japanese monkeys (macaca fuscata). Although the OKN basically resembled that in the rhesus monkey, a lower adaptation limit of OKN velocity (about 40-60°/sec) and longer rise time to reach the maximum eye velocity after the onset of stimulation (about 30 seconds on average) were observed.
Turning off the lights induced the first and second phases of the OKAN, and occasionally the third or fourth phases were induced. While an increase in the stimulus speed changed patterns of OKAN decline from an almost straight to catenary-like or sinusoidal fashion, the total duration time of the first and second phases remained stationary (about 150-200 seconds). Various patterns of the OKAN may represent interactions between the different mechanisms inducing after-nystagmus in opposite directions.
