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James P. Comerford, Frank Thorn, Brad Bodkin; The chromatic Hermann grid illusion for stimuli equated in chroma. Journal of Vision 2005;5(8):53. doi: 10.1167/5.8.53.
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A Chromatic Hermann Grid evokes the perception of illusory colored spots at the crossings of grid lines. Using a restricted range of contrasts near isoluminance, we have found that the chromatic illusion may be weaker than the equivalent achromatic illusion for grids darker than the background. Here we explore the interaction of hue and contrast in the chromatic Hermann Grid illusion for a full range of negative contrasts using a constant luminance dark grid. Because stimuli that differ in hue may also differ in saturation, we equated our stimuli for chroma.
Method. We constructed grids with 21 intersections. Grid lines were 0.18 deg wide and presented at 200 cm. 5 colors were presented at 5 negative contrasts. Stimuli were equated for chroma using the CIE 1976 uniform color space. Subjects rated the magnitude of the illusion with the highest rating referenced to a standard achromatic grid presented adjacent to the comparison stimulus.
Results. The Hermann Grid Illusion increased with contrast for all hues (P < 0.0001). For some observers, the colored illusion was marginally visible even without luminance contrast. At high contrasts, the achromatic illusion was stronger than the chromatic illusion for red and blue backgrounds. The hue x contrast interaction was significant for red and blue backgrounds (P < 0.05) but not for green and yellow backgrounds. In all cases the illusory spots appeared as a dark version of the background hue.
Conclusions. The discrepancy between the strength of the Hermann Grid illusion for red and blue grids and for luminance equated achromatic grids is even larger at high contrast levels than we found in our previous work for low contrast levels; the discrepancy was not significant for green and yellow grids. This effect of hue is unlikely to be due to differences in saturation among our stimuli since we equated the stimuli for chroma. These results will be related to Oehler and Spillman's (1981) suggestion that this illusion is mediated by R and G cones.
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