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Michael Webster, Lucie Sawides, Sowmya Ravikumar, Larry Thibos, Arthur Bradley, Susana Marcos; Adapting to astigmatism. Journal of Vision 2009;9(8):986. doi: 10.1167/9.8.986.
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© ARVO (1962-2015); The Authors (2016-present)
Adapting to blurred or sharpened images alters the perceived focus of subsequently viewed images. We examined whether these adaptation effects could arise from actual sphero-cylindrical refractive errors, and whether they can be selective for specific aberrations by testing different axes of second-order astigmatism. Arrays of images were generated by titrating the magnitude of astigmatism over +0.3 microns, with varying defocus added to maintain constant blur strength (0.35 microns). The two ends of the series thus contained images blurred by only astigmatism, while the center image in the series was isotropically blurred with spherical defocus. Images subtended 4 deg, and were spatially jittered in time to avoid local light adaptation. Subjects adapted for 1 min to full astigmatic blur along either axis and then to 5-sec top-ups interleaved with test images. A 2AFC staircase was used to estimate the stimulus that appeared isotropic. Adaptation to horizontally blurred images caused a stimulus with no astigmatic blur to appear vertically biased and vice versa, shifting the perceived isotropic point to images that were thus more blurred along the adapting axis. Similar orientation-selective aftereffects were found for images of filtered noise or natural scenes. Because astigmatic blur distorts perceived shapes in the images (in addition to perceived “fuzziness”), the observed effects could reflect figural aftereffects for shape rather than adaptation to blur per se. These figural changes are larger in smaller images, since the angular distortion from blur is constant while the object angle varies with size or distance. In further measurements we compared aftereffects in different-sized images that have equivalent blur but different shapes or vice versa. Our results suggest that at least for lower-order aberrations, spatial vision can selectively adapt to the blur introduced by the eye's optics, and that this adaptation may include meridionally specific adjustments to perceived defocus and shape.
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