Contrast Constancy with Refractive Blur

Abstract

Abstract: : Purpose: Georgeson and Sullivan (1976) have demonstrated that for gratings of a given spatial frequency the perception of blur may be less for high spatial frequencies than for low spatial frequencies They call this phenomenon Contrast Constancy, and they have quantified it for astigmats viewing oriented gratings using a contrast matching technique. We have used their approach to further understand perceptual compensation for blur in myopia and other refractive errors. Methods: Six observers participated in the initial phase of the experiment. Observers were presented with a 5 cpd grating at contrasts of .03, .1, .3 and .8. The perceived contrast of these gratings was matched for stimuli of 2.2, 5, 10.2, 15 and 23.6 cpd. Contrast sensitivity was also determined for the same spatial frequencies. In the second phase of this experiment, three of the observers removed their refractive correction and adapted to distance viewing for a minimum of 15 minutes, after which they were retested without their refractive correction. Stimuli were presented on an Apple monitor and controlled using Morphonome. Results: Although patients wearing their best refractive correction demonstrate a fall-off in contrast sensitivity at high spatial frequencies, their contrast matching as a function of spatial frequency is flat for all but the lowest contrast level, thus demonstrating contrast constancy. Without refractive correction, there is a dramatic decrease in contrast sensitivity. Low contrast stimuli and high spatial frequency stimuli may not be seen at all. For stimuli well above threshold contrast constancy still occurs. For stimuli near threshold, contrast constancy was compromised. Conclusion: Perceived blur may be a function of the relative contrast of the low, intermediate and high spatial frequency components of a scene. With defocus of one to three diopters, our subjects show contrast sensitivity loss at all spatial frequencies. They also maintain contrast constancy at all spatial frequencies. One possible explanation for this is perceptual adaptation leading to contrast amplification for blurred stimuli. Variations of the present technique may also be used to quantify visual quality in low vision patients.

Keywords: 368 contrast sensitivity • 542 refraction • 481 myopia