The Effect of Monochromatic Aberrations of the Cornea on Contrast Sensitivity under Photopic and Mesopic Conditions

Purpose: Vision is one of the most important senses. The first stage of vision is the creation of the observed object’s image on the retina. The quality of the retinal image is affected by several factors, such as   diffraction, sampling on the retina, chromatic aberration, scattering and higher order aberrations. The measurement of the quality is achieved both with subjective (visual acuity, contrast sensitivity) and objective methods (PSF, MTF, Strehl ratio, RMS). The purpose of this project is the measurement of higher order aberrations of the anterior corneal surface (with a Placido corneal topographer) and of the contrast sensitivity (with a Pelli Robson optotype). Then, we will try to find if there is a correlation between them. Methods and Materials: 20 persons participated in this survey, divided in two groups of 10 persons each. The first group (group 1) included subjects up to 39 years old and the second group (group 2) from 40 years and up. The participants didn’t have any pathological problems, except lower order refractive errors. Both eyes of each individual were included in the procedure. First, the higher order aberrations of the anterior corneal surface were measured, with the implementation of a Placido corneal topographer. This was followed by a measurement of the contrast sensitivity. The whole procedure took place under two lighting conditions, both photopic and mesopic. The examination presentation of the higher order aberrations is performed with the use of Zernike polynomials. Results: The results of the measuring procedure showed that for the first group, under photopic conditions (luminance 32.70 cd/m2) the mean value (± standard deviation) for the higher order aberrations RMS and decimal logarithm contrast sensitivity was 0.073 ± 0.018μm and 1.54 ± 0,16 (contrast 2.88 ± 1.24%) respectively. For the second group the corresponding values were 0.080 ± 0.036μm and 1.59 ± 0.16 (contrast 2.57 ± 1.24%). Accordingly, under mesopic conditions (luminance 1.14 cd/m2) the values for the first group were 0.252 ± 0.064 μm and 1.27 ± 0.15 (contrast 5.37 ± 2.06%), while for the second were 0.253 ± 0.069μm and 1.32 ± 0.12 (contrast 4.79 ± 1.35%). Spherical aberration and coma (horizontal and vertical) were also measured for both groups under photopic and mesopic conditions. The results showed that for the first group, under photopic conditions, RMS for coma and spherical aberration is 0.033 ± 0.014 μm and 0.022 ± 0.011 μm respectively, while for the second group 0.041 ± 0.027 μm and 0.024 ± 0.008 μm. Under mesopic conditions, the relevant results are 0.139 ± 0.065 μm and 0.124 ± 0.035μm for the first group and 0.149 ± 0.066 μm and 0.107 ± 0.038 μm for the second group. Finally, we should mention here that the equivalent defocus error corresponding to the higher order aberration RMS was estimated. The results for all the participants (without age separation), are 0.23 ± 0.09D (photopic conditions) and 0.28 ± 0.07D (mesopic conditions). Conclusions: From the statistical analysis of the results we conclude that there exists a symmetry between left and right eyes regarding higher order aberrations. Furthermore, it seems that age is not a significant factor for differences on the magnitude of higher order aberrations both under photopic and mesopic conditions as well. Similar conclusions are also reached for the contrast sensitivity measurements. In addition, we have observed that a correlation exists between the higher order aberrations of the anterior corneal surface and the contrast sensitivity, for both photopic and mesopic conditions. Correlation also exists between coma and contrast only under photopic conditions. But there is no significant correlation between spherical aberration and contrast. The age difference of these two groups has not an effect on the results. Finally, the equivalent defocus is too small, both for photopic and mesopic conditions and between each   other, in order to be clinically significant.