Interactions between real and subjective contours in the Bourdon illusion

Abstract

The Bourdon illusion is a contour-attraction effect in which two colinear test edges of a plane figure appear displaced toward a bent induction edge located nearby. This illusion and related effects are stronger in subjective contours than in real contours. In Bourdon figures combining both varieties of contours, real induction contours produced a strong illusion in subjective test contours, but subjective induction contours produced no illusion in real test contours. Thus, interactions between real and subjective contours were asymmetrical. Luminance contrast between real and subjective contours weakened the Bourdon illusion.