Delboeuf illusion: Displacement versus diameter, arc deletions, and brightness contrast.

Abstract

Determined the magnitude of the Delboeuf (concentric circle) illusion by the up-and-down (staircase) method. Variables were (1) diameter ratio between concentric circles, (2) circumference remaining after dotting 1 of them, and (3) reflectance contrast, the reflectance of the circumference relative to the white background. 40, 520, and 60 adults were used in 3 experiments, respectively. Previous evidence is supported: assimilation occurred, the test (judged) circle being displaced toward the inducing circle; the distance paradox was evident with maximum displacement near diameter ratios (test: inducing circle) of 2:3 and, less consistently, 3:2. Dotting or reducing the reflectance contrast of the test circle increased illusory effects. Dotting or reducing the reflectance contrast of the inducing circle diminished them. Reducing equally the reflectance contrast of both circles had little effect on the size of the illusion. (16 ref.) (PsycINFO Database Record (c) 2006 APA, all rights reserved)