Seeing is believing, right? Or is it? Look at Figure 1. To most people, the square region in the center looks brighter than the surrounding white area, but actually they're physically the same. To many, the square region appears to be a flat surface that sits above the rest of the figure and partially covers the lines. The illusory surface has clear illusory contours (edges) that delimit it. Figure 2 was created by adding lines that terminate at the same points as the lines in the first figure, one being added for each line in the first figure. But now the illusory surface and illusory contour are much weaker. Why? After all, the endpoints of the lines are still just as nicely lined up as the previous figure, there are still just as many points of contact to "guide" the illusory contour, and they are spaced apart in the same way as in the previous figure. In fact, since we have added more lines, we might reasonably expect the illusion to be strengthened instead of weakened.
by Marc K. Albert and Donald D. Hoffman