Transformation direction influences shape-similarity judgments.

Three experiments provide evidence that the perceived similarity between two images is systematically affected by the inherent direction of a transformation that links the two. Participants were shown short animations morphing one object into another from the same basic category. They were then asked to make directional similarity judgments ("How similar is object A to object B?") for two stationary images drawn from the morph continuum. Across three experiments, similarity ratings for identical comparisons were higher when the reference object, B, had appeared before the comparison object, A, in the preceding morph sequence. This response to dynamic transformational sequences is in accordance with the view that similarity depends on the ease of transformation between object representations and that transformations between objects in categorization and object recognition are psychologically real.