The frequency of horizontal saccades in near and far symmetrical disparity vergence

In a natural environment, saccade and vergence eye movements shift gaze in different directions and distances. In a laboratory setting, targets can be positioned precisely to elicit symmetrical vergence movements; however, saccades occur during the vergence movement even though the stimulus should not stimulate a saccadic response. These saccades may facilitate the response when the kinematics of the vergence component are modest as indicated by reduced velocities. Hence, the purpose of this study is to assess whether the frequency of saccades within vergence responses are correlated with vergence peak velocity. Ten subjects with normal binocular vision participated in this study. Eye movements were quantified using a limbus tracking system. Stimuli included 4° symmetrical convergence and divergence steps with an initial vergence angle at far (2° and 6°, respectively) and near (12° and 16°, respectively) which are known to evoke different vergence peak velocities. A saccade detecting algorithm was utilized to compute the percentage of saccades present within all vergence responses. A repeated measures ANOVA confirmed with a post hoc Bonferroni test demonstrated that convergence steps at near were slower than convergence steps at far, whereas divergence steps at far were slower than divergence steps at near in all subjects (p<0.02). When the vergence peak velocity was slow, a greater number of saccades was observed. The average vergence peak velocities were inversely correlated to the number of saccades observed within the transient portion defined as after the latency to 400 ms of the movement (r=-0.41; p=0.008), between 400 ms and 1s of the response (r=-0.35; p=0.03) and within the steady-state period occurring between 1s and 3s of the response (r=-0.44; p=0.005). Peak velocity of vergence is dependent on the stimulus initial vergence angle. An increased prevalence of saccades was observed in vergence responses with reduced peak velocity, compared to responses with greater peak velocity. Prior research supports that saccades increase the peak velocity of vergence during combined vergence and saccadic tasks. This may in part explain the increased presence of saccades within vergence responses with reduced peak velocities. The recruitment of saccades may be utilized because of the longer period of diplopia resulting from slower vergence movements.