Fusion lock diameter and the forced vergence fixation disparity curve.

The effect of the diameter of the fusion lock on the forced vergence fixation disparity (FD) curve was investigated because of ambiguity in the results of previous investigations. Data gathered from 20 asymptomatic subjects with normal binocularity showed that the fusion lock diameter had no noticeable effect on the shape of the curve. Further investigations using symptomatic subjects showing large FDs or steep forced vergence FD curves showed that fusion lock diameter did have an effect on the curves of these subjects. The results are interpreted for their clinical implications through a control systems model of the accommodation-vergence interaction.