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B.C. Hendricks, J.P. Comerford, F. Thorn; The Effect of +1 Diopter of Blur on Contrast Matching and Contrast Gain . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4093.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: High spatial frequencies in the retinal image are severely degraded relative to low spatial frequencies. This degradation is due to defocus and optical aberrations, which result in a sharp decrease in contrast sensitivity at high frequencies. Surprisingly, Georgeson and Sullivan (1976) found that observers were often not affected in their contrast matching by a blurry retinal image; high contrast images in the environment subjectively appeared to have high contrast despite the effects of retinal blur. They call this effect contrast constancy. As part of a systematic exploration of the effect of blur on visual perception in patients with refractive error and with ophthalmic disease, we wish to further explore the adaptation processes which underlie this effect by evaluating the effect of +1D of blur on contrast matching and on contrast gain control. Methods: 6 observers were presented with a 5 cpd grating at contrasts of 0.03, 0.1, 0.3, and 0.8. The perceived contrast of these gratings was matched with gratings of 2.2, 5, 10.2, 15, and 23.6 cpd. Contrast sensitivity was also determined for the same spatial frequencies. After adapting for 15 minutes to +1 D of blur, observers were retested while wearing the +1 D lenses. Results: There was a significant effect of optical blur on contrast sensitivity above 2.2 cpd (p<.0001). Contrast matching functions were relatively unaffected, except when image degradation brought a stimulus close to threshold (i.e. high spatial frequencies). Relative contrast gain functions were constructed by plotting stimulus contrast vs. apparent contrast for a 5cpd grating. Gain functions were derived with the assumption that stimuli had equal perceived contrast at threshold. Conclusions: Observers demonstrate constrast constancy except when object constrast levels are near threshold; for this reason the least degree of constancy is found for low contrast stimuli. Optical blur of +1 D hardly affects the perception of high contrast stimuli of any spatial frequency tested. Contrast gain functions varied with spatial frequency; the larger the discrepancy between perceived contrast and retinal image contrast, the steeper the slope. Supported by the New England College of Optometry Research Fund
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