The experiment of Dresp and Bonnet [(1991) Vision Research, 31, 1813-1817] was replicated and extended to include measurements of both luminance increment and decrement detection across high luminance illusory contours. The results reveal that illusory contours can influence the detectability of both luminance decrements and increments and, in addition, that the magnitude and even the direction of the effect which illusory contours produce on visual thresholds may vary considerably across individual observers. Finally, in contrast to the monophasic pattern of threshold variation across low luminance illusory contours, the pattern across high luminance contours is oscillatory.