Twisted grid. The basic white-on-black (Hermann) grid has illusory gray blotches at its intersections. Adding nodes gives the Scintillating grid illusion. Do these illusions work if grid lines are broken (try adding a gap to the line)? Do they require that the lines form a square grid (try adding a skew)? What if the lines are curved (add a twist)? Can you estimate the size of a center-surround receptive field in the fovea by altering line width in the basic Hermann grid?
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Sizes in visual angle: ° line width; ° line gap; ° block side; ° node diameter; ° blur.