Are mirror images really more difficult to discriminate than plane rotations? A study based on the COR hypothesis

Houbben, Marie [UCL] Vannuscorps, Gilles [UCL]

It is widely assumed that mirror images are more difficult to discriminate than plane rotations of the same object. However, this conclusion stems from studies in which arbitrary types of plane rotation (often 180 degrees) were compared to objects mirrored across both intrinsic (e.g., their axis of symmetry or elongation) and extrinsic (e.g., the gravitational vertical) axes. The goal of this study was to overcome these limitations. We used a “same-different” orientation judgement task with tilted asymmetrical 2D stimuli. The second stimulus of each pair could be the same as the first one or (1) have a different tilt direction (e.g., 20° CW vs. 20° CCW), (2) be rotated by 180°, (3) be mirrored across an axis intrinsic to object or (4) be mirrored across a vertical (extrinsic) axis. Like previous studies, we found that shapes tilted 180° were the easiest to discriminate. In addition, we found that shapes reflected across an intrinsic axis were not significantly more difficult to discriminate than those reflected across an extrinsic axis and that shapes differing in terms of tilt direction were the most difficult to discriminate. Thus, mirror images are not necessarily more difficult to discriminate than plane rotations. We discuss the implications of these findings for cognitive models of orientation representation.