Saccade Characteristics during Fusional Vergence Tests as a Function of Vergence Demand

This study investigated saccadic behaviour during the fusional vergence test as a function of disparity vergence demand. Participants' divergence and convergence fusional vergence ranges were measured in an haploscopic setup. Disparity of a column of letters increased to 45 prism dioptres (PD) at 1 PD/s for both divergence and convergence. Eye movements were recorded using an Eyelink 1000 Plus, and a velocity-threshold-based algorithm was used to detect saccades. Saccades followed the main sequence, and their characteristics varied significantly with the direction of the underlying vergence movement. Saccades' amplitude while fusion was maintained was smaller than during diplopia. Participants experienced diplopia for a longer time during divergence evaluation, which resulted in a higher prevalence of horizontal saccades than during convergence. This study highlights the impact of binocular conditions on saccade characteristics during fusional vergence tests, thus contributing to the understanding of how the visual system adapts to different vergence demands.