Symmetry cues for matching mirrored objects

The study explores cues for matching pairs of objects. These objects were arranged into mirror-symmetrical displays and the task was to judge whether a pre-specified 180 degree rotation around the X, Y or Z axis carries one object into the mirror object. For some rotations, the object's mirror symmetry (M) and, for other rotations, the object's point-symmetry (P) could serve as a cue. The matching results suggest that M is a better cue than P, say M > P. Various attempts are made to explain this effect. The most promising one focuses on simplest structural object representations as these capture M and not P. It is furthermore plausible that M captured by reference frames at high hierarchical representation levels, say M1, serves as a better cue than M captured at low levels, say M2. The prediction M1 > M2 > P merely applies to the open surface objects in the experiment. For the closed solid objects in the experiment the expectation is M1 = M2 = P. Both predictions roughly agree with the accuracy and reaction-time data. The results suggest the perceptual relevance of representation cues and, aditionally, that cues stemming from reference frames at higher hierarchical representation levels are more effective than those from lower levels.