Comparison of the monocular occlusion and a direct method for objective measurement of fixation disparity

Abstract

Purpose: Previous studies have measured objective fixation disparity using a monocular occlusion method, by which the monocular components of misalignment are revealed by measuring the shift in the position of the fixating eye after occlusion of the other eye.

Methods: We assessed the use of the monocular occlusion method by comparing it to direct measurements of binocular vergence.

Results: The direct method required precise calibration, but was found to be reliable. Several problems were discovered with the monocular occlusion method: (1) when using a central fusion stimulus, fixation was difficult to control accurately enough to measure the small monocular shifts in eye position expected at lower vergence demands; (2) with a peripheral fusion target, objective fixation disparity could not be measured directly; and (3) upon occlusion, a variable saccade occurred in the fixating eye.

Conclusion: Objective measurement of fixation disparity with the direct method is quicker and more reliable than with the monocular occlusion method.