The frequency of saccades correlates to peak velocity in symmetrical disparity vergence

A pure vergence stimulus requires the two eyes to turn equally inward or outward theoretically resulting in a pure symmetrical vergence response. However, saccades, a rapid conjugate eye movement, are frequently observed in vergence responses. This investigation sought to systematically quantify whether the occurrence of saccades within symmetrical vergence responses is correlated to vergence peak velocity. Eye movements are quantified using a limbus tracking system from three subjects. Symmetrical convergence and divergence 4° step responses with an initial position located at far or near which are known to evoke different peak velocities are analyzed. Data are quantified via peak velocity. A saccade detecting algorithm is utilized to quantify the frequency of saccades in the transient portion (first second) of vergence responses. Near convergence responses are slower than far convergence and far divergence responses are slower compared to near divergence movements. The occurrence of saccades is negatively correlated to vergence peak velocity. When the velocity is slower, the number of saccades increases. This study suggests that the brain may initiate a saccade to facilitate a slow vergence movement, potentially to allow object recognition before binocular fusion.