The effects of interocular differences in retinal illuminance on vision and binocularity

Background: It is known that binocular function is affected by interocular differences in retinal image size, shape, clarity, and illumination. The present study was performed to systematically examine the effects of interocular differences in retinal illuminance on monocular visual acuity, fusion, and stereopsis. Methods: Fifty adults with normal binocularity and a best-corrected visual acuity of 20/20 or better were enrolled. We examined best-corrected visual acuity, fusion, and stereoacuity as monocular retinal illuminance was gradually reduced by increasing neutral density filters from 0.2 neutral density (ND; 63% transmission) to 3.0 ND (0.1% transmission) in increments of 0.2 ND. We measured the smallest size of fusion slide by major amblyoscope, and evaluated stereoacuity with the Titmus test and the Lang test. Results: The best-corrected visual acuity began to decrease significantly when the value of the neutral density filter reached 2.0 ND (1% transmission; p < 0.01). The level of fusion decreased significantly when monocular retinal illuminance was reduced to 2.0 ND (p < 0.01). Stereoacuity began to decline significantly when the value of the neutral density filter was 1.4 ND (4% transmission) with the Titmus test and 1.6 ND (2.5% transmission) with the Lang test. Conclusions: A binocularly conducted Titmus test was the most sensitive to the presence of monocularly induced retinal illuminance deterioration.