Impact of interocular differences in corneal asphericity on binocular summation.

PURPOSE To evaluate the impact that interocular differences in corneal asphericity (Q) exert on binocular summation measured as the contrast-sensitivity function. DESIGN Interventional case series. METHODS A total of 92 emmetropic subjects took part in the experiment, classified according to the interocular differences in corneal asphericity (deltaQ) measured with an EyeSys-2000 corneal topographer. Fifty-four subjects had deltaQ < 0.1; 21 subjects had 0.1 < or = deltaQ < or = 0.2; and 17 had deltaQ > 0.2. The contrast-sensitivity function (CSF) was measured monocularly (for each eye) and binocularly with a B VAT II device. The spatial frequencies used were as follows: 2.4, 3.7, 6.0, 9.2, 12, 15, 20, and 24 cycles per degree. RESULTS Although the binocular CSF for the three groups studied was greater than the monocular in all the spatial frequencies studied, there were significant differences in binocular summation. The average binocular summation (for all the spatial frequencies) for the group with deltaQ < 0.1 was 1.46, significantly higher than the group with 0.1 < or = deltaQ < or = 0.2, in which the average binocular summation was 1.39 (P = .035), which was also significantly higher than the group deltaQ > 0.2, for which the average binocular summation was 1.26 (P < .0001). In this last group, the summation decreased to the level of the probability summation. CONCLUSIONS Differences in corneal asphericity may affect the binocular visual function by diminishing the binocular contrast-sensitivity function. This result may have important implications in refractive surgery given that, although the subject becomes emmetropic, if interocular differences are induced in corneal asphericity, it could reduce binocular visual performance.