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J.S. McLellan, A.Y. Leonova, S.A. Burns; Do Interactions Between Defocus and the Stiles–Crawford Effect Produce Stereo Disparity . Invest. Ophthalmol. Vis. Sci. 2005;46(13):5640.
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Purpose: It has been suggested that the Stiles–Crawford effect (SC) can provide an accommodative cue to distinguish between myopic and hyperopic defocus (Fincham, 1951; Kruger, et al., 2001). Because the SC is decentered in the pupil of most eyes, it will cause an apparent displacement of the retinal image in opposite directions with opposite signs of defocus. Furthermore, because the SC is inhomogeneous across the central retina, it can also distort the shape of the retinal image. This displacement and distortion could produce stereo disparity and therefore might provide another cue to depth. We have studied this possibility with computational simulations. Methods: Point spread functions (PSFs) for both myopic and hyperopic defocus were calculated for a 5 mm pupil with the SC centered 1 mm nasal in the pupil. To simulate the inhomogeneity of the SC, rho values varied from 0.02 in the foveal center to 0.08 at 1° eccentricity. These PSFs were convolved with images of a cross pattern with line width of 1 arcmin to model the effects of the SC on the retinal image. Results: Hyperopic and myopic defocus shift the center of the cross image in opposite directions; ± 0.5 D produces a shift of about 1.5 arcmin. The change in rho with eccentricity causes opposite curvatures in vertical features for different signs of defocus. The apparent curvature at 1° is about 15 mm. Conclusions: The inhomogeneity of the Stiles–Crawford effect across the central retina and its decentration in the pupil combine to produce opposite image distortions for myopically and hyperopically defocused stimuli. These distortions could produce detectable curvature in defocused images. Ocular aberrations may attenuate or enhance this cue depending upon sign of defocus and aberration magnitudes.
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