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Sangeetha Metlapally, Vinay K Nilagiri, Tiffanie T Yu, Shrikant R Bharadwaj, Clifton M Schor; A Cross-correlation Model of the Effect of Higher and Lower-order Aberrations on Stereopsis in Keratoconus. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):6021. doi: https://doi.org/.
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Inter-ocular differences and averages of higher-order aberrations (HOAs) both co-vary with stereo-acuity in keratoconus (Sarkar et al., IERG, 2014). This study modeled the effects of inter-ocular differences in phase noise and contrast induced by these wavefront errors on stereo-acuity in keratoconus.
Lower-order aberrations (LOAs) and HOAs were measured in 9 keratoconus cases and 5 control subjects, post-cycloplegia with 6 mm diameter pupils, either unaided or through RGP contact lenses (CLs), using a Shack-Hartmann irx3™ wavefront aberrometer. Refractive data and stereo-depth thresholds were measured through spectacles and CLs under the same conditions to compare low and high magnitudes of HOAs. Phase effects were modeled from the horizontal cross-correlation functions of random dot patterns convolved with point-spread functions of right and left eyes. Contrast effects were modeled with retinal image quality (IQ) metrics computed for each eye using aberrations scaled to 4 mm diameter pupils. Analyses of variance were performed to investigate the contribution of inter-ocular differences in phase noise and contrast to the empirical stereo-depth thresholds.
Stereo-thresholds ranged from 56 - 872 arc sec in cases and from 29 - 134 arc sec in controls. Some of the variance seen in the empirical stereo-thresholds in keratoconus obtained with spectacles (i.e. with HOAs uncorrected) was explained by the signal to noise ratio derived from the cross-correlation functions (r = -0.59; R2 = 0.36; p < 0.05). The width of the cross-correlation function, estimated from the standard deviation, was moderately correlated with stereo-thresholds obtained with spectacles (r = 0.43; R2 = 0.18; p = 0.13). Inter-ocular differences in IQ metrics chosen to highlight contrast differences due to HOAs (VSOTF, VSMTF), showed little or no relationship to the stereo-thresholds.
Inter-ocular HOA differences in keratoconus subjects introduce phase noise in the disparity domain that affect the fidelity of disparity signals for stereo-depth perception. The contribution of contrast differences from HOAs is small in comparison. However, small residual LOAs could have a larger contrast-difference effect on stereopsis.
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