Mirror-image confusions: Implications for representation and processing of object orientation

Perceiving the orientation of objects is important for interacting with the world, yet little is known about the mental representation or processing of object orientation information. The tendency of humans and other species to confuse mirror images provides a potential clue. However, the appropriate characterization of this phenomenon is not entirely clear, in part because the stimuli used in most previous studies were not adequate for distinguishing various forms of mirror-image and non-mirror-image error. In the present study we explore the nature of mirror-image confusion and what the phenomenon can reveal about object-orientation representations. We report several experiments in which participants reported the orientations of pictures. In all of the experiments mirror-reflection errors were more frequent than other orientation errors. However, whereas mirror-image confusion has previously been described as a tendency to confuse stimuli that are related by reflection across an extrinsic (usually vertical) axis, the vast majority of mirror-image errors in our experiments were reflections across an object axis. This finding calls into question several hypotheses proposed to explain mirror-image confusion. We describe a coordinate-system orientation representation (COR) hypothesis that can account for our results (McCloskey, Valtonen, & Sherman, 2006). COR assumes that orientation representations map an object-centered reference frame onto a reference frame extrinsic to the object, with this mapping specified by several parameters. According to COR, mirror-image confusions and other orientation errors arise from failures in representing or processing specific parameters. Considered in light of COR, our results suggest that orientation representations are compositional, and that object-centered reference frames play a central role in orientation representation.