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H. Radhakrishnan, R.I. Calver, S. Pardhan, D.J. O'Leary; Can Ocular Spherical Aberrations Predict the Differences in Contrast Sensitivity with Defocus in Myopes? . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2125.
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Purpose: Previous studies have shown that ocular aberrations are higher in myopes when compared to emmetropes. We measured contrast sensitivity (CS) at two levels of defocus (+2.00 and -2.00) in a group of myopes and emmetropes. We also measured third order ocular spherical aberration in the two groups in order to examine any possible correlations between predicted and measured CS. Methods: Eight myopes and eight non-myopes took part. CS was measured under cycloplegia with +2.00D and -2.00D defocus for up to 20 spatial frequencies. Ocular aberrations were also measured under cycloplegia using a Shack-Hartmann aberrometer. The measured third order ocular spherical aberration was used to calculate the modulation transfer functions (MTF) for the two levels of defocus on both groups of subjects. Results: In emmetropes the measured CS showed a symmetrical loss to both positive and negative defocus (p=0.474). In myopes the loss of CS with defocus was assymmetrical, with CS loss being significantly higher with positive defocus compared to negative defocus (p<0.001). The obtained MTF's were used to predict CS with defocus for a given spatial frequency using the equation: CS (defocus) = CS (defocus) = CS (infocus) X (|MT (defocus) |/ |MT (infocus) |). Although the above equation slightly overestimates the measured CS, Chi square analysis showed a significant association between the measured CS and the predicted CS values. Conclusions: Differences in the effects of blur on CS exist between myopes and non-myopes. Third order ocular spherical aberration can predict the difference in contrast sensitivity between the two groups.
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