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A. Demberg, L.S. Gray; Dependency of Contour Interaction Upon the Contrast of the Crowding Bars . Invest. Ophthalmol. Vis. Sci. 2005;46(13):5648.
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Purpose: Previous work has shown significant reductions of contour interaction for low contrast letter targets. In previous studies the contrast of both the letters and the crowding bars was kept the same. The aim of this study was to examine whether the contrast dependency of contour interaction is determined by the contrast of the letter or the contrast of the crowding bars. Methods: 25 visually normal subjects (mean age 22.4 ± 6.4 years) participated with informed consent in the study. Subjects wore their full spectacle correction and had VA of 6/6 (20/20) or better. A computer based vision testing program (TestChart2000, Thomson Software) was employed to present single optotypes at six levels of contrast (90, 40, 30, 20, 10 and 5%). Letters were presented on a standard SVGA monitor at a viewing distance of 6m. The letters were presented with and without crowding bars, and the contrast of the crowding bars was either the same as the letter or fixed at 90% for all levels of letter contrast. Crowded and uncrowded acuity thresholds were determined psychometrically at each contrast level using a forced choice paradigm. Results: With the crowding bars fixed at 90% contrast, the mean uncrowded VA (–0.14 ± 0.08 logMAR) for the 90% contrast letters was significantly (p=0.0001) higher than the mean crowded VA (–0.02 ± 0.06 logMAR) for that contrast. Similarly the mean uncrowded VA (0.16 ± 0.09 logMAR) for the 5% contrast letters was significantly (p=0.0001) higher than the mean crowded VA (0.27 ± 0.04 logMAR) for that contrast. When the contrast of the crowding bars was the same as the letters, the mean uncrowded VA was not significantly different from the mean crowded VA at all levels of contrast. Conclusions: Contour interaction is absent at low letter contrasts when the crowding bars have the same contrast as the letters. Contour interaction is present at all levels of letter contrast when the crowding bars are fixed high contrast. This suggests that the presence of contour interaction is determined by the contrast of the inhibitory surrounding contours.
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