The shape of luminance increments at the intersection alters the magnitude of the scintillating grid illusion
The scintillating grid illusion refers to an illusory perception of black spots on the luminance increments at the intersections of gray grids on a black background. In this study, we examined how the shape of luminance increments modulated the strength of the illusion. In Experiment 1, we concurrently controlled the size and shape of luminance increments, and found significant reduction of the illusory strength on the square, compared with circle and diamond, in the largest size condition. In Experiment 2, we controlled overall orientation of squared luminance increments, and confirmed the significant reduction of the illusion when the relative edge orientation of luminance increments and the grids was larger than 30 deg. This indicates that not the categorical difference of the shape, but the orientation difference between the grids and the luminance increments determines the strength of the illusion. We discussed about the contribution of orientation processing to scintillating grid illusion.