The scintillating grid illusion refers to the illusory perception of black spots on luminance patches at the intersections of a grey grid on a black background. We examined how spatial parameters of luminance patches modulated the strength of the illusion. In experiment 1, we controlled the size and shape of the luminance patches. For the largest-size conditions tested, we found a significant reduction in the strength of the illusion with squares when compared to circles or diamonds. In experiment 2, we controlled the orientation of quadrangle patches and confirmed a significantly larger reduction in the strength of the illusion when the edge orientations of quadrangle patches were vertical and horizontal (square) than when they were oblique (diamond). To explore the relationship between orientation processing and scintillating grid illusion, we controlled, in experiment 3, the global orientation of the display; the strength of the illusion with diamonds was significantly weaker when it was rotated by 458 than when it was not rotated. These results indicate that it is not only the difference of edge orientation of luminance patches, but also the orientation with respect to the grid that determines the strength of the illusion.
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