Models of illusory pausing and sticking.

When identical dots moving at equal velocities but in opposite directions become coincident, a striking illusion occurs. The dots appear to pause momentarily, despite no objective change in velocity. Three models for the illusion were examined: A vector averaging model best accounts for the data. According to this model a motion system maps visual space and assigns motion vectors to each position on the map. When identical objects traveling at equal velocities but in opposite directions become coincident, they lose their phenomenal identity and a form system detects a single object. The motion system, however, has two vectors mapped onto the same position occupied by a now-single object. Since an object cannot move in two directions at the same time, the motion system averages the vector and assigns the resultant to the object, thus resolving the ambiguity. With equal but opposite vectors, averaging yields a zero resultant (null vector), and thus pausing is perceived. With unequal vectors, averaging yields a nonzero resultant and something other than pausing is perceived. Velocity and number of collisions strongly affect the magnitude of the illusion, but in all cases in which the illusion is perceived, the objects appear to stick together and move at a velocity near the average of the component velocities.