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Bo Cao, Arash Yazdanbakhsh, Ennio Mingolla; The effect of contrast intensity and polarity in achromatic watercolor effect. Journal of Vision 2010;10(7):427. doi: 10.1167/10.7.427.
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© ARVO (1962-2015); The Authors (2016-present)
The watercolor effect (WCE) is a filling-in phenomenon on a surface surrounded by two thin abutting lines with the chromaticity of the interior line. We developed a series of achromatic WCE stimuli and a method to quantitatively compare the lightness of the filling-in region surrounded by lines of various luminances. We define the interior line as “the inducer”, the luminance of which is fixed, while the exterior line “the suppressor”, the luminance of which varies across different stimuli. The results of a psychophysical experiment for seven subjects (five naive) show that the achromatic WCE exists. Moreover, we found that suppressors with both high and low luminance can induce the WCE with an inducer with a moderate luminance as long as the contrast difference between the inducer and the suppressor passes a certain threshold. All the subjects show a single peak of the effect strength, which is never at the extreme contrast difference, though there are individual differences in the location of the peak. That is, the effect is never the strongest when the suppressor is black or white. Most subjects show an inverted-U curve for suppressors with both higher and lower contrast than the inducer. For most subjects, the suppressor with an opposite contrast polarity to that of the inducer, generates a stronger effect than the suppressor with the same contrast polarity as that of the inducer. These results suggest that the contrast difference affects the existence and the strength of the WCE, but not in a linear way. Moreover, as in the Craik-O'Brien Cornsweet Effect, besides the contrast intensity, the contrast polarity also plays a role in the WCE.
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