[Low contrast weakens illusory contours but enhances the lightness filling-in of the Kanizsa square].

This study systematically investigated the relationship between the lightness filling-in of the test field and the clarity of illusory contours. The Munsell values of pacmen were N2.5, N3.5, N4.5, N5.5, N6.5 and N7.5, and those of the test field were N3.0, N4.0, N5.0, N6.0 and N7.0, respectively. Seventeen subjects (age: 20-28 yr.) observed 35 stimuli including five control stimuli. The lightness filling-in of the test field increased as the difference in brightness between the test field and pacmen decreased. On the contrary, the clarity of illusory contours decreased as the difference decreased. These results suggest that the lightness filling-in and the clarity of illusory contours have different roots.