Effects of heterophoria on stereopsis

The effects of horizontal heterophoria on Howard-Dolman stereopsis scores were investigated in a population of 1765 stereophotogrammetrists with no obvious impediment to binocular vision as determined in a vision screening. Exophoria over the 1 to 7 delta range showed no systematic and statistically valid effect on stereopsis but esophoria did show an effect. In a related population of 1339 stereophotogrammetrists, subjects with as little as 1 delta (+/- 0.5 delta) of vertical heterophoria exhibited a small but statistically significant decrease in stereopsis test scores compared to those with vertical orthophoria. Therefore, a heterophoria had different effects on stereoacuity depending on whether it was in the exo, eso, or vertical direction. An explanation for the difference is proposed using the known dependence of heterophoria and stereoacuity on fixation disparity.