Sustained convergence induced changes in phoria and divergence dynamics

Purpose: The purpose of this study was to investigate how sustained convergence induces phoria adaptation and changes divergence dynamics. Methods: Near dissociated phoria and divergence step responses were recorded using an infrared eye movement monitor on four binocularly normal subjects. We tested three different adapting vergence positions (16°, 6°, and 0.5°) and measured 4° divergence step responses for two different initial vergence positions (16° and 4.5°). Dynamics were quantified by measuring peak velocities of the divergence responses. Results: Phoria was significantly adapted after subjects fixated on a sustained convergent target at near (16°) and at far (0.5°). As a result of sustained convergence, divergence peak velocity from a 4° step stimulus significantly changed. A regression analysis showed that when the phoria became more esophoric (near adapted) the peak velocity for the divergence steps with an initial position of 16° decreased (R = 0.54, p = 0.04). A trend was observed between the change vergence velocity and the change in phoria. Change was defined as the post-adapted data minus the pre-adapted data for vergence steps with an initial position of 16° (R = 0.65) and 4.5° (R = 0.66). Furthermore, the modification of divergence dynamics was dependent on the initial position of those divergence steps (initial position of 16° versus 4.5°). Conclusion: As a result of sustained convergence, phoria and divergence dynamics changed in a correlated manner. Such correlated changes in phoria and divergence dynamics are not explained by current models of disparity vergence eye movements.