Why testing vergence is so complicated – The subjective and the objective fixation disparity

Abstract

Purpose. This review article describes recent findings of eye movement research of ocular vergence and provides an in- terpretation regarding those aspects of binocular vision that are assessed by clinical nonius tests. Material and Methods. In the clinical practice of binocular testing, different types of nonius tests are traditionally used for the vergence position of the eyes: near to a fusion stim- ulus, the test includes monocularly presented nonius lines; the perceived nonius offset is understood as a measure of “subjective fixation disparity”. However, in basic research precise eye movement recording systems are applied since the 1980s for measuring the “objective fixation disparity”. Results. The correlation between subjective and objective measures of fixation disparity tends to be low. Subjective measures are typically much smaller than objective measures. Conclusion. A precise terminology for the results of these measurements allows one to describe the physiological dif- ferences between the two aspects of fixation disparity. Recent research findings may be the basis for reconsidering the mechanisms of motor and sensory fusion and interpreting the clinical findings accordingly. With the help of video eye track- ers, it is now possible to conduct clinically-oriented optom- etric research of subjective and objective fixation disparity. Keywords Binocular vision, fixation disparity, vergence, fusion