Determining transformation distance in similarity: Considerations for assessing representational changes a priori

The representational distortion (RD) approach to similarity (e.g., Hahn, Chater, & Richardson, 2003 Hahn, U., Chater, N. and Richardson, L. B. 2003. Similarity as transformation. Cognition, 87: 1–32.  [Google Scholar]) proposes that similarity is computed using the transformation distance between two entities. We argue that researchers who adopt this approach need to be concerned with how representational transformations can be determined a priori. We discuss several roadblocks to using this approach. Specifically we demonstrate the difficulties inherent in determining what transformations are psychologically salient and the importance of considering the directionality of transformations.