Dimensions of figural identity and apparent motion.

Warren proposed that for apparent motion to be perceived between disparate shapes, the shapes must be identifiable as the same object. When disparate shapes are seen as different views of the same rigid form, they are called ecologically transformable. Ecological transformability seemed to be the crucial variable in Warren's results. However, Warren's findings conflict with those of Kolers and Pomerantz, who obtained frequent reports of apparent motion of disparate, non-transformable shapes. This discrepancy may be due to the response categories used in Warren's experiment. Warren asked subjects to report whether they saw one or two objects and did not directly assess the perception of motion. We replicated Warren's experiment using his figures but also asked subjects to rate the smoothness of motion and to describe the type of motion. We obtained frequent reports of motion of disparate, nontransformable figures. Surprisingly, after making this correction in Warren's experiment, we also found that transformability as well as "featural similarity" can affect the type and smoothness of the illusory motion.