[Fixation disparity in peripheral and central binocular vision under conditions of prismatic heterophoria (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.

[Decreasing local stereopsis with increasing horizontal disparity, indicating decreasing density of binocular cortical projection towards the limits of Panum's area (author's transl)]

Measuring local stereopsis by means of random dot stereograms with increasing horizontal disparity, the relative density of binocular cortical projection has been determined at a distance of 75 min of arc from the fovea. The accuracy of stereoscopic perception decreased with increasing horizontal disparity. The reciprocal value of the error frequency was used to measure the density of binocular cortical projection. Panum's area at the above mentioned distance from the fovea can be constructed by plotting the error frequency against the horizontal disparity. We observed a decrease of the relative density of binocular cortical projection towards the limits of Panum's area without a sharp break. The results are in agreement with those reported by Bishop and Henry and Joshua in the cat (receptive field disparity). The theory of stereopsis as proposed by Bishop explains our findings well and may be transferred to binocular vision in man.

The relationship between fixation disparity and heterophoria

Twenty-five asymptomatic subjects with normal binocular vision underwent determination of the magnitude and direction of their heterophoria and their fixation disparity. Fixation disparity was measured at a 2-m fixation distance with the phase-difference haploscope under conditions simulating those of casual seeing. When combined with careful attention to symmetrical control of accommodation, this method eliminates several known technical artifacts. Examples of the normal fluctuation of the magnitude and direction of fixation disparity for individual subjects were demonstrated. Whereas only a slight preponderance of subjects were esophoric, many subjects had convergent fixation disparity (esodisparity). Linear regression analysis of the data revealed a coefficient of correlation between fixation disparity and heterophoria at 6 m of 0.61, and at 33 cm of 0.50. The results suggest that normal, small magnitude heterophoria does not necessarily produce fixation disparity and that fixation disparity does not necessarily sustain heterophoria.

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

Measurements of fixation disparity and Panum areas were performed with a special device of the phase-difference haploscope in normal persons. There was a reproduceable influence on fixation disparity and size of Panum areas, when the conditions of examination were changed (position of the binocular fusion-pattern, brightness and contrast of the targets for sensory fusion and time of prism wearing). There found we found only under comparable conditions a quantitative relationship between the two phenomenons.