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Monika A Formankiewicz, M Izzuddin Hairol, Sarah J Waugh; Effects of contrast on foveal acuity and contour interaction using luminance and contrast modulated Cs. Journal of Vision 2010;10(7):1333. doi: https://doi.org/10.1167/10.7.1333.
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Interactions between suprathreshold luminance-modulated and contrast-modulated stimuli have been found for letter contrast detection (Chung et al, 2007) and Gabor contrast matching (Ellemberg et al, 2004) however it is not known how they interact for the classical visual acuity task (Flom et al, 1963). Luminance-modulated and contrast-modulated square Cs and bars were constructed by adding or multiplying square-wave modulating signals to dynamic binary noise. C gap acuity thresholds were obtained using a 4AFC paradigm with the method of constant stimuli. Bar separations varied from abutting to two letter widths. Cs were either equated in their visibilities (∼3.5x above contrast thresholds) or presented at maximum producible modulations. Threshold versus separation data were fit with a Gaussian to objectively determine magnitude and extent of contour interaction. When the C and bars are of the same type, typical patterns of contour interaction are observed. Acuity threshold elevation for abutting bars is significantly larger for the contrast-modulated C (0.2 logMAR) than for the luminance-modulated C (0.1 logMAR); p<0.05; the lateral extent of interaction (2sd of Gaussian) is also larger (4.37±1.4 gap widths versus 3.25±0.4 gap widths) but not significantly different (p>0.1). When the luminance-modulated C is flanked by contrast-modulated bars (212), acuity is adversely affected in similar fashion to same-type contour interaction but with peak magnitude: 0.15 logMAR. Reduced bar visibility significantly reduces the effect (p<0.05). When contrast-modulated C is flanked by luminance-modulated bars (121), a similar pattern of results occurs with peak magnitude: 0.15 logMAR. Increasing bar visibility significantly increases contour interaction (p<0.05). The similar effects of flanks of different type on C visual acuity suggest that luminance and contrast modulations are not treated independently at the level of resolution processing.
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