Review Article

Why testing vergence is so complicated

The subjective and the objective fixation disparity

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1Work Research Institute of the Technical University of Dortmund (IfADo)
Binocular vision
fixation disparity


This review article describes recent findings of eye movement research of ocular vergence and provides an interpretation 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 stimulus, 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”.


The correlation between subjective and objective measures of fixation disparity tends to be low. Subjective measures are typically much smaller than objective measures.


A precise terminology for the results of these measurements allows one to describe the physiological differences 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 trackers, it is now possible to conduct clinically-oriented optometric research of subjective and objective fixation disparity

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