Sustained convergence induced changes in phoria and divergence dynamics

The purpose of this study was to investigate how sustained convergence induces phoria adaptation and changes divergence dynamics.

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 degrees , 6 degrees , and 0.5 degrees ) and measured 4 degrees divergence step responses for two different initial vergence positions (16 degrees and 4.5 degrees ). Dynamics were quantified by measuring peak velocities of the divergence responses.

Phoria was significantly adapted after subjects fixated on a sustained convergent target at near (16 degrees ) and at far (0.5 degrees ). As a result of sustained convergence, divergence peak velocity from a 4 degrees 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 degrees 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 degrees (R=0.65) and 4.5 degrees (R=0.66). Furthermore, the modification of divergence dynamics was dependent on the initial position of those divergence steps (initial position of 16 degrees versus 4.5 degrees ).

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.