Background: Most people attribute a higher weight to the input from one eye than to that from the other eye when they have to align stereodisparate objects in the same visual direction. This preference for visual directions has been termed ‘ocular prevalence’, according to the Latin praevalentia = superior power. Questions: (1) Is ocular prevalence of one eye (or its correlate, partial suppression of the other eye in the prevalence task) restricted to large stereodisparities, close to Panum's limit, or does it occur also at small stereodisparities, near the stereoscopic threshold? (2) Is ocular prevalence a handicap for stereoacuity?
Methods: Six non-strabismic observers with equal visual acuity of their two eyes were examined. To determine their ocular prevalence, they were presented with vertical vernier lines at stereodisparities ranging between 30 and 430 arcsec. They had to judge whether the lower, anterior line was located on the right- or left-hand side of the upper, posterior line. Their stereoscopic threshold was measured with an adaptive staircase procedure, using the Freiburg Stereoacuity Test.
Results: All six observers exhibited some ocular prevalence. It changed considerably on repeated measurements. In three observers, it even switched from one eye to the other. Ocular prevalence occurred not only at large stereodisparities, close to Panum's limit, but also at small stereodisparities. The stereoscopic threshold of the six observers ranged between 1.7 and 12.3 arcsec.
Conclusion: Ocular prevalence is common, intra-individually variable and occurs even at small stereodisparities close to the stereoscopic threshold. It is compatible with ‘optimal’ stereoacuity. Hence, ocular prevalence appears to be a harmless feature of normal binocular vision.