Simultaneous objective and subjective measurements of fixation disparity

Abstract

Fixation disparity is generally used to describe the inexactness of binocular fixation. Typically fixation disparity is measured subjectively by the extent of binocularly perceived misalignment of dichoptically (one to each eye) presented Nonius lines. It is customary to obtain fixation disparity measurements as a function of stimulus disparity resulting in a fixation disparity curve. These curves (that purport to measure vergence error or, equivalently, the magnitude of sensory compensation to assure fusion) are used to characterize binocular disorders as well as prognostic indicators for orthoptic therapy. In previous experiments we found significant differences between subjective and objective measurements of cyclo-fusional response.1 We have, therefore, decided to perform simultaneous objective (eye movement measurements during binocular viewing and cover–uncover test) and subjective (Nonius line displacement) measurements of horizontal and vertical fixation disparities as a function of stimulus disparities. All our stimuli contained a fixation point. We found significant differences between the objective and subjective measurements. These differences render the Nonius line method an unreliable indicator of fusional performance or sensory status.