Experimentally induced anisometropia thresholds for distance stereoacuity in adults: A psychophysical approach

Purpose: Stereopsis, a critical aspect of binocular vision, can be influenced by refractive errors. This study investigated how experimentally induced anisometropia affected distance stereopsis in adults and aimed to determine threshold levels for different types and amounts of experimentally induced anisometropia. Methods: Twenty healthy adults (aged 20–45 years) with normal visual acuity and baseline distance stereopsis were included. Distance stereopsis was assessed using the Distance Randot Stereotest at 3 m. Trial lenses inducing myopic, hyperopic and astigmatic anisometropia (axes: 45°, 90° and 180°) were applied over the dominant eye in 0.25 D increments. After each lens placement, a 30-s adaptation period preceded assessment of stereoacuity. At each disparity level (400, 200, 100 and 60 arc seconds), the minimum anisometropic lens power causing loss of stereoacuity was recorded. As per the test manual, each disparity level included plates A and B, but only B plate responses were analysed, as correct identification was required to proceed. Results: Experimentally induced myopic anisometropia led to the earliest loss of stereopsis, with a mean threshold of 1.38 D at the 400 arc seconds level. Against-the-rule astigmatism exhibited the greatest tolerance, requiring higher refractive errors to degrade stereoacuity (p < 0.001). At 400 and 200 arc seconds, even low levels of experimentally induced myopic anisometropia (<0.50 D) impaired stereopsis significantly (p < 0.001), whereas against-the-rule astigmatism did not affect stereopsis significantly until higher anisometropia levels were induced (60 and 100 arc seconds). Conclusions: Even low levels of experimentally induced myopic anisometropia may disrupt distance stereopsis in visually normal adults. These findings may highlight consideration of a precise refractive correction in the presence of anisometropia to optimise distance stereopsis.