Illusory contours: On the efficacy of their need for expression

When an illusory-figure pattern is presented against a physical lightness gradient, the illusory figure’s lightness relative to the surround is often the reverse of the usual effect, a fact that suggests that, rather than the figure’s depending upon illusory lightness, real lightness differences can be utilized in its expression. A second demonstration suggests that the apparent strength of the lightness effect may be greater where there is relatively less physical expression of the contour. Again, the establishment of the contour seems to involve more than merely “capturing” illusory lightness.