Disambiguating the Stream/Bounce Illusion With Inference

The ‘stream/bounce’ illusion refers to the perception of an ambiguous visual display in which two discs approach each other on a collision course. The display can be seen as two discs streaming through each other or bouncing off each other. Which perception dominates, may be influenced by a brief transient, usually a sound, presented around the time of simulated contact. Several theories have been proposed to account for the switching in dominance based on sensory processing, attention and cognitive inference, but a universally applicable, parsimonious explanation has not emerged. We hypothesized that only cognitive inference would be influenced by the perceptual history of the display. We rendered the display technically unambiguous by vertically offsetting the targets’ trajectories and manipulated their history by allowing the objects to switch from one trajectory to the other up to four times before the potential collision point. As the number of switches increased, the number of ‘bounce’ responses also increased. These observations show that expectancy is a critical factor in determining whether a bounce or streaming is perceived and may form the basis for a universal explanation of instances of the stream/bounce illusion.