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The Effect of Myopia on Contrast Thresholds

From the Department of Physiology, Medical University Sofia, Sofia, Bulgaria.

PURPOSE. To examine the relationship between the degree of myopia and foveally measured contrast thresholds.

METHODS. Monocular thresholds for positive and negative contrast were obtained from 60 myopes (–1.0 to –8.0 D) and 20 emmetropes of similar age. The contrast thresholds were measured at different background luminance levels, photopic and mesopic, by using a visual stimulus generator and a staircase method. Against a gray background, light (dark) letters were used to study positive (negative) contrast. ANOVA was used to identify factors influencing the contrast sensitivity. For the myopes, the contrast thresholds were regressed against the spherical equivalent refractive error, accounting for different intercepts and slopes between positive and negative contrasts.

RESULTS. Myopes yielded lower sensitivity to contrast than did emmetropes and, in contrast to emmetropes, exhibited higher contrast thresholds for negative than for positive contrast in both photopic and mesopic conditions. At all background luminance levels, contrast thresholds of myopic subjects increased systematically with a higher spherical equivalent refractive error, and the rate of increase was higher for negative than for positive contrast. The regression models yielded adjusted coefficients of determination (R²_adj) of 0.735 and higher.

CONCLUSIONS. Despite having corrected visual acuity, myopes exhibited reduced sensitivity to contrast in comparison to emmetropes. Furthermore, the contrast sensitivity decreased with an increasing degree of myopia, and the rate of decline was higher for negative than for positive contrast.