There are two types of optokinetic nystagmus (OKN) fast phase. One is the eye movement looking at a visual target passing in front of the test subject (type A), and the other is a reflex which resets the eye position after deviation by the slow phase (type B). We developed a new method of quantitatively analyzing these two types of OKN. Optokinetic stimulation is given to a test subject by projecting stripes with a 5 degrees visual angle width on a screen at 30 degree intervals. The visual target is accelerated at 1 deg/sec/sec up to 100 deg/sec. The OKN is recorded by ENG and analyzed with a PDPll/34 computer. Slow phase velocities, and fast phase amplitudes and velocities of each OKN are plotted on a correlogram. There are two types of OKN fast phase on the correlogram, one with a low correlation with slow phase velocity and the other with a high correlation. It is considered that the low correlation type OKN corres-ponds to type A and the high to type B.
In order to quantitatively analyze these two types, approximate regression lines of each type, {y=ax+b (x : slow phase velocity, y : fast phase velocity or amplitude) }, were computed by the minimum square method.
The normal limits of "a" and "b" were determined statistically from the responses of 30 normal subjects. In normal subjects the differences between the values of "a" and "b" in the low correlation type OKN and those in the high correlation type were quite significant. On the other hand, of those with normal responses of OKN elicited by a random dot visual target and caloric nystagmus, few had nystagmus of the low correlation type. In patients with ver-tebro-basilar insufficiency and cerebello-pontine angle lesions, the low correlation type of OKN was significantly reduced.
