The role of mirror symmetry in the perception of chromatically homogeneous figures

A chromatically homogeneous surface is not always perceived as a single figure. It can sometimes be perceived as the result of superimposing two or more figures; in such a case illusory contours are seen bounding the figure in the foreground (Petter, 1956 Rivista di Psicologia 50 213^227). Local factors, such as relatability, have been proposed in order to explain perception of two or more figures (Kellman and Shipley, 1991 Cognitive Psychology 23 141 ^ 221). However, even when these factors are at work, there are conditions favouring the perception of a single figure which have not been explored so far. Here we propose that one such condition is the mirror symmetry of the surface. We have run three experiments in order to study several aspects of the problem. The main results show that: (a) mirror symmetry enhances perception of a single figure; (b) effectiveness of symmetry increases with the number of axes of symmetry; (c) vertical and horizontal axes are more effective than a 458-oriented axis. We conclude that the global factor of mirror symmetry plays an important role in the perception of chromatically homogeneous surfaces, along with local factors, such as relatability and, more generally, good continuation.