Quantitative evaluation of the function of looking at visual target in optokinetic nystagmus

A new method is presented to qualitatively evaluate the function of OKN which certainly looks at a visual target. Optokinetic stimulation was given to a test subject by projecting stripes with a 5 degrees visual angle width at 30 degrees intervals, which was accelerated by 1 degree/s2 up to 100 degrees/s. The correlation between slow phase and fast phase was computed with our program. Two correlations were mixed in a normal response, one indicated low correlation and the other high correlation. As normal subjects look at wide interval visual targets one by one regardless of the target velocity, low correlation type OKN beats are considered to be those which certainly look at the visual target. In order to quantitatively analyze this type of OKN, approximate regression lines of the low correlation type, [y = ax + b (x: slow phase velocity, y: fast phase amplitude or velocity)], were computed by the minimum square method. In normal subjects, value a indicates low and value b high velocity and their normal limits are determined. The results of cases with central disequilibrium indicate that this analysis provides information of pathological OKN different from that of such conventional parameters as slow phase velocity and number of nystagmus beats.