Background: For prism correction of associated phoria (at 5-6 m viewing distance), Hans-Joachim Haase developed--among other tests--the prevalence test, which consists of a central fixation target and two triangles that appear stereoscopically about 1.5 m in front of or behind the fixation target; the vertex of each triangle is directed towards the centre of the fixation target. Only when both eyes contribute equally to the perception of visual directions do the triangles appear centred (equivalence), while any perceived horizontal offset between the triangles and fixation target indicates a prevalence of one eye. Provided that monocular vision is equal in both eyes, Haase interpreted ocular prevalence as being due to a small vergence error (fixation disparity with a shift of retinal correspondence). This vergence error indicates, according to Haase, a (not yet fully corrected) heterophoria. From practical experience, Haase developed rules for transferring ocular prevalence into equivalence with prisms (MKH). If equivalence has been reached, Haase assumed that the heterophoria was fully corrected.
Method: In 19 subjects we examined whether spherical and prismatic corrections reduce ocular prevalence. The perception of the subjects was ascertained with three methods: they were asked 1. to describe their perception orally--as proposed by H.-J. Haase--, 2. to make a drawing of their perception, and 3. to align the position of the stereo images to the central fusion target with a computer-controlled device.
Results: Based on the group mean, the three methods did not reveal a consistent reduction of prevalence. Only in two individuals did the drawing and the computer-controlled alignment showed reductions of prevalence that were confirmed statistically on the individual level. However, it has to be noted that these two individuals not only received prisms but also a correction of their hyperopia and anisometropia.
Conclusion: The prevalence of one eye is a common phenomenon in normal binocular vision (with the amount of prevalence often being different for triangles presented in front of or behind the fixation target). In only a few subjects can prevalence be reduced with prisms. Equivalence as reported by oral description (method 1) was often not confirmed by drawing (method 2) or by the computer-controlled alignment (method 3). As an explanation for the differences between methods 1, 2 and 3 we assume that the oral description of the test perception was influenced by imponderables such as the expectation on the side of the experimenter and/or the desire to please on the side of the subject. The present results do not support the prevalence test--as developed by H.-J. Haase--for the correction of associated phoria.