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Day and Jory (1980) and Day and Kasperczyk (1983a, 1983b) have proposed that illusory contours, apparent edges in physically uniform regions (Ehrenstein, 1942; Gregory, 1972; Kanizsa, 1955; Schumann, 1904), are due primarily to the spread of induced contrast to partially delineated borders. Brightness (or darkness) is enhanced by simultaneous and line-end contrast and reduced by assimilation of brightness. The spread of one or more of these forms of contrast to borders delineated by features such as fragments of real edge, ends of lines, and dots give rise to illusory contours coincident with the borders. In these terms, it can be expected that the strength of illusory contours will be greater when enhanced brightness (or darkness) spreads to one side of a partially delineated border and reduced brightness (or darkness) to the other. This is probably so with the well-known Kanizsa triangle (Kanizsa, 1955, 1976, 1979), in which simultaneous and line-end contrasts spread inside the partially delineated triangle and assimilation of brightness outside it (Day & Jory, 1978). Day and Kasperczyk (1983a) have also argued that apparent depth due to overlay or other stimulus features correlated with depth serve to render partially delineated borders more perceptually salient and the coincident illusory contours consequently stronger. An illusory color effect, "neon color spreading," has been described by van Tuijl (1975). The effect occurs, for example, when, in a lattice pattern of black 5-mm squares, sections of the black lines are replaced by red lines, which together form an embedded figure. The red coloration then appears to spread out around the colored elements into the adjacent squares to form a larger desaturated red figure. If, for example, the figure is roughly circular, the observer has a strong impression of a lattice with a desaturated "misty" red circle that appears to be projected onto it or is floating above its surface. The effect has been investigated in detail by van Tuijl and his associates (van Tuijl & de Weert, 1979; van Tuijl & Leeuwenberg, 1979)and by Redies and Spillman (1981). The latter generated the effect in the Ehrenstein radiating-line figure (Ehrenstein,

Neon color spreading, partially delineated borders, and the formation of illusory contours