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Ocular prevalence is defined as an unequal weighting of the eyes in the directional perception of stereo objects. Opinions differ as to the cause and relevance of ocular prevalence. Hans-Joachim Haase suggested that ocular prevalence is due to fixation disparity, brought about by incomplete compensation of heterophoria. He further suggested that prismatic spectacles determined by his "measuring and correcting methodology" (MKH) could restore bicentral fixation and thus establish a perceptual balance between both eyes.We examined 10 non-strabismic subjects with a visual acuity ofor = 1.0 in both eyes. It turned out that all 10 had a "fixation disparity type II", characterised according to Haase by a "disparate retinal correspondence". All subjects underwent the automatic Freiburg Ocular Prevalence Test, without and with MKH prisms. In addition we examined ocular prevalence under forced vergence and compared ocular prevalence with stereoacuity.Spontaneous ocular prevalence ranged between 1 and 69 %. Averaged over all 10 subjects, ocular prevalence without and with the MKH prisms were not significantly different. Statistical evaluation of single subjects revealed only in one of the 10 a significant difference (Bonferroni-corrected p = 0.001). In the subgroup of 5 subjects who underwent forced vergence, ocular prevalence remained unaltered between 0 and 18 Delta base out. The stereoscopic threshold of all 10 subjects ranged between 1.5 and 14.5 arcsec. There was no correlation between ocular prevalence and stereoscopic threshold (r = - 0.2, p = 0.5).Our results indicate that ocular prevalence is largely independent of phoria correction and vergence stress. The excellent stereoacuity of all subjects suggests that ocular prevalence is abandoned for the sake of optimal resolution when very small differences in depth have to be judged.

The "Measuring and Correcting Methodology" after H.-J. Haase (MKH) aims at converting "fixation disparity" into bicentral fixation, using prismatic spectacles. In the context of the MKH, fixation disparity is diagnosed by a series of subjective tests. According to H.-J. Haase, a long-standing fixation disparity can lead to "disparate correspondence" between the central areas of both retinae, which consolidates the fixation disparity and gradually converts a "young" into an "old fixation disparity". In "old fixation disparity" it is thought that bicentral fixation does not occur anymore, so that stereoacuity is impaired. However, prismatic spectacles can, according to H.-J. Haase, restitute bicentral fixation and consequently improve stereoacuity, even in some cases of "old fixation disparity".Ten non-strabismic subjects with a visual acuity of/= 1.0 in both eyes were examined. It turned out that all ten had, according to MKH, a "disparate correspondence", 5 subjects with a "young" and 5 with an "old fixation disparity". According to the MKH, a correcting prism was determined. All 10 subjects underwent the automatic Freiburg Stereoacuity Test, without and with the MKH-prism.Without the MKH-prism, the stereoscopic threshold ranged between 1.5 and 14.5 arcsec. With the MKH-prism, the values were not significantly different.Stereoacuity ranged between good and excellent in the 5 subjects with "young" as well as in the 5 subjects with "old fixation disparity". The MKH-prism did not improve the stereoacuity in any of the subjects. These results cast doubt on Haase's assertion that an "old fixation disparity" implies a reduced stereoacuity. Hence, the premise for a benefit of the MKH-prism with respect of stereoacuity is not substantiated. In the 5 subjects with a "young fixation disparity", the good stereoacuity is consistent with Haase's theory, so that a benefit of the MKH-prism for stereoacuity was not expected. In previous studies, stereoacuity was found to be better with the MKH-prism than without it. These studies are questionable since learning with repeated testing was not taken into account. We conclude that there is no sound evidence for the assumption that the MKH-prism can improve stereoacuity.