[Fixation disparity under various test-conditions (author's transl)]

Stereoacuity versus fixation disparity as indicators for vergence accuracy under prismatic stress

BACKGROUND

Fixation disparity has been widely used as an indicator for vergence accuracy under prismatic stress. However, the targets used for measuring fixation disparity contain artificial features in that the fusional contours are thinned out. We considered that stereoacuity might be a preferable indicator of vergence accuracy, as stereo targets represent natural viewing conditions.

METHODS

We measured fixation disparity with a computer adaptation of Ogle's test and stereoacuity with the automatic Freiburg Stereoacuity Test. Eight subjects were examined under increasing base-in and base-out prisms.

RESULTS

The response of fixation disparity to prismatic stress revealed the curve types described by Ogle and Crone. All eight subjects reached a stereoscopic threshold below 10 arcsec. In seven subjects the stereoscopic threshold increased before double vision occurred.

CONCLUSION

Our data suggest that stereoacuity is suitable to assess the range of binocular vision under prismatic stress. As stereoacuity bears the advantage over fixation disparity in that it can be measured without introducing artificial viewing conditions, we suggest exploring whether stereoacuity under prismatic stress would be more meaningful in the work-up of asthenopic patients than is fixation disparity.

[Perimetry during monocular and binocular fixation with the phase-difference haploscope of Aulhorn]

For the first time it was possible to compare perimetry during monocular and binocualr fixation under homogeneous conditions by the aid of phase-difference haploscopy. A binocular perimetry was not practicable using common perimeters. The examination was based on the anatomical fixed papilla nervi optici, which appears in the visual field as the blind spot. Its extension corresponds more to the anatomical size during binocular examination than during monocular examination. The difference is highly significant. Likewise the binocular perimetry refers to the quality of reproduction of paracentral retinal diseases significantly improved compared with the monocular examination. Supposing that the use of a larger fixation target during monocular perimetry contributes to the stabilization of normal eye movements, all examinations were additionally made with two fixation targets of different sizes. We have shown that there is no significant difference between the measurement either concerning the extension of the blind spot or the paracentral retinal diseases, which means a larger fixation target represents no fixation aid.