On the significance of heterophoria for stereoscopic vision

This study was undertaken to explain the possible correlation between heterophoria and stereopsis because, in the literature, there are different opinions. The subjects were 806 recruits. They were trained as rangefinder men, of whom a perfect stereopsis is required. The permitted maximal limits of heterophoria were 1 prism diopter (PrD) vertically, of esophoria 5 PrD, and of exophoria 6 PrD. Soldiers wit abnormal stereopsis, myopia, hyperopia more than 1.75, or astigmatism over 0.50 diopters, were omitted. The heterophoria was tested with Herschel's prism to 5 m distance. The stereoscopic vision was investigated using Pulfrich's device and the so-called three-needle test. No statistically significant correlation could be demonstrated between heterophoria and the degree of stereopsis. Surprisingly, it could be noticed that persons with esopheria finshed Pulfrich's test more quickly than those with exophoria. This difference was statistically highly significant. In the three-needle test, the results had the same trend. This might have some importance to rangefinder men and to pilots if it can be confirmed in the military practice.