Perceptual mislocalization of a single set of the Müller-Lyer wings.

In the present communication, a possible role of perceptual displacements of stimulus elements in the occurrence of visual illusions of extent has been considered. In psychophysical experiments with a single set of Müller-Lyer wings, subjects were asked to place an imaginary reference rectangle into a position that made the apex of the wings appear to be at the rectangle center. Three different stimulus parameters (the length, internal angle, or tilt angle of the wings) were used as independent variables in different series of experiments. It was demonstrated that the magnitude of perceptual displacements of stimulus terminator is commensurate with that of illusions of extent obtained in our previous studies of full versions of illusory figures. Good correspondence between the experimental data and the predictions of our computational model of automatic centroid extraction strongly supported the suggestion that the effects of centroid extraction are powerful enough to be considered as one of the main causes of illusions of extent of the Müller-Lyer type.