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James P. Comerford, Brad Bodkin, Frank Thorn; Chromatic and achromatic processing in the Hermann Grid illusion. Journal of Vision 2004;4(8):328. doi: 10.1167/4.8.328.
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© ARVO (1962-2015); The Authors (2016-present)
When observing a Chromatic Hermann Grid, illusory dots with a color similar to the background are often perceived at the intersection of horizontal and vertical bars seen against a colored background. In this study we wish to follow up on our previous work suggesting that color may inhibit the Hermann Grid illusion. Circular Hermann Grids composed of 5 vertical and 5 horizontal bars were presented at 3m. Each bar subtended 0.14 deg. 7 color combinations and 18 variations of contrast were shown to 7 subjects. Magnitude estimation was used to rate the strength of the illusion. All subjects perceived the Chromatic Hermann Grid illusion except when the bars and background approached isoluminance, at which point no illusion was seen. The strength of both the achromatic and the chromatic aspects of the illusion increased with contrast. Under some contrast conditions, the chromatic Hermann grid illusion appeared weaker than the achromatic version. 6 of 7 subjects typically reported the color of the illusion as being the same as the color of the background. However, when yellow bars were presented against a blue background, 5 subjects reported green illusory dots. The achromatic illusion has been explained by center-surround interaction within concentric receptive fields, while the chromatic illusion has been associated with the function of double opponent cells. The present results will be discussed relative to predictions based on a quantitative theory of color processing.
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