Effect of vertical prism imbalance on fixation performance

It is well known that induced vertical imbalance will bring about deviation from central fixation in the vertical meridian. This study seeks to determine if vertical imbalance will affect the fixation performance in the horizontal meridian as well. For this purpose, a technique previously developed for monitoring fixation eccentricity was used. The technique, which is based on the effect of fixation eccentricity on border enhancement, is unaffected by adaptive changes in perceived visual direction. The results show a rapid increase in horizontal fixation misalignment with the first increments of vertical prism difference, followed by a slower increase for additional prism differences. When horizontal forced vergence was applied in addition, fixation misalignment increased throughout the range of vertical vergences. Similarly, vertical fusion misalignment was increased by horizontal fusion stress. It was concluded that forced vergence presented in one meridian will amplify any fusion misalignment that might exist in other meridians. This calls for a reevaluation of existing methods of assessing binocular fixation performance.