The importance of the point of fixation in optokinetic nystagmus.

Abstract

The relationship between visual line and target in optokinetic nystagmus was studied by means of the eye-mark camera and film-motion analyser. The eye-mark carmera provides a series of pictures in which the visual field and fixation point of the subject overlap. The visual line is observed as a spot of light on the film. Accurate analysis of films with the analyser produces detailed information on the calibration and movement of the target and visual line. Comparison with the ENG, taken simultaneously, indicates the adequate sensitivity and accuracy of this method. There are five types of relationship between visual line and target in optokinetic nystagmus. In type I, the visual line coincides with the target in the slow component of nystagmus. This type agrees with the conventional considerationation of the slow component of nstagmus. Type II indicates the existence of some servo-mechanism in the target catching action of the eye. In type III, there are three kinds of slow components, one chincides with one edge of the target, one with the other edge and another fixes on some point between the two and follows it. The velocities of these three are equal to that of the target. In type IV the visual line gradually lags behind the target, but the rhythm of the nystagmus coincides with that of the stimulation. In type V, dysrhythmic nystagmus develops and the visual line often jumps over several targets without fixation.