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B Pinna, L Spillmann, JS Werner; Anomalous Lightness Induced by Radial Lines . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4711.
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Purpose: To study a new illusion called Anomalous Lightness Induction. When a light blue annulus surrounds the central gap of an Ehrenstein figure, so as to connect the inner ends of the radial lines, a striking new lightness effect emerges: The disk inside the colored annulus appears unusually white as though a solid white paste has been added to it, and is different from the lightness enhancement in the classical Ehrenstein illusion. Methods: Patterns of 4 x 4 modified Ehrenstein figures served as stimuli, and the strength of induced lightness inside the annular regions was determined using magnitude estimation. Figures having light blue annuli but no radial lines, and figures having black instead of blue annuli were used as controls. The following parameters were systematically varied: number, length, width, and contrast of the radial lines as well as width, contrast, and color of the ring. Fourteen naive subjects participated in the experiments. Results: The presence of the radial lines was found to be crucial for inducing anomalous lightness. The variation of the lines and rings affect the rated lightness of both anomalous induction (colored annulus) and simultaneous contrast (black annulus) in the same manner. However, simultaneous contrast is much weaker than the intense lightness and whiteness characteristic of anomalous lightness induction. Conclusion: It is known that a black ring connecting the inner ends of the radial lines can cancel the Ehrenstein illusion. The observation that a colored ring produces anomalous lightness induction is therefore puzzling; it also seems to run counter to an explanation in terms of end-stopped receptive fields. The anomalous lightness induction may be based on a (still unknown) interaction between illusory lightness enhancement (radial lines) and simultaneous contrast (annulus).
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