Dynamic changes of optokinetic after-nystagmus (OKAN) caused by brief visual fixation periods in monkey and in man.

Optokinetic nystagmus (OKN) continues after the cessation of visual stimulation in complete darkness as primary optokinetic after-nystagmus (OKAN I). After variable periods of time, it is followed by secondary OKAN (OKAN II). Short periods of visual fixation during OKAN I in monkey and in man inhibit OKAN I, but enhance OKAN II. The enhanced OKAN II starts earlier, lasts longer, and often reaches higher slow-phase velocities than in control experiments. Therefore, OKAN II depends not on the ocurrence or strength of OKAN I, but mainly on parameters of the preceding optokinetic stimulus. Results suggest that OKAN I duration is partially determined by the development of OKAN II.