Experimental study on directional asymmetry of vertical OKN and OKAN

Abstract

We studied the directional asymmetry of human vertical optokinetic nystagmus (OKN) and optokinetic after nystagmus (OKAN) based on two experiments conducted in 22 normal subjects. The results of the first experiment indicated that, during upward optokinetic stimulation OKN is produced by the pursuit system and the velocity storage system while during downward optokinetic stimulation the velocity storage system plays no significant role. Up-down asymmetry in vertical OKN seemed to be caused mainly by directional asymmetry of the velocity storage system. Furthermore, as the velocity storage function is very important for producing OKN at a higher stimulation velocity, domination of upward slow phase velocity becomes more apparent with a high stimulation velocity. In our second experiment the subjects were exposed to optokinetic stimulation with or without a stable fixation point. When subjects were exposed to optokinetic stimulation with fixation, they felt a rotating sensation within themselves or a moving sensation of the fixation point. When the stimulation disappeared, most experienced an illusion of small dots moving in the direction opposite the stimulation. In this condition some of the subjects showed after-nystagmus the slow phase of which was in the opposite direction. Subsequently the after-nystagmus became more distinct, with pursuit of the moving dot illusion. This result shows the existence of a mechanism which produces reverse after-nystagmus by mean of a retinal error input. We speculate that these phenomena play causative roles in reverse after-nystagmus (RAN), pursuit reverse after nystagmus (pRAN), and the RAN system.