Towards a relative motion processing account of direction and speed perception

Spatial context influences the perceived velocity of moving objects. A number of theories have been proposed to account for a variety of visual motion phenomena that arise from suchcontextual effects. One theory proposes that the perceived direction and speed of a moving object are determined by an interaction of referenced (object-relative) and unreferenced (non-object-relative) motion components within the larger visual field. Presented here is an attempt to formalise a model based on this theory. The model dictates that the velocity of an object is perceptually decomposed into its constituent object-relative and non-object-relative component velocities. The two component velocities are differentially processed by the visual system such that the non-object-relative component velocity is underestimated with respect to the object-relative component velocity as a constant ratio. The research presented here investigates whether such differential processing does occur and whether it can explain various instances in which the velocity (direction and speed) of an object is misperceived. To test the model and to further explore the nature of the mechanisms involved, two well-known direction illusions are investigated: Duncker-type induced motion and the direction illusion arising in bidirectional transparent motion. In addition, a new illusion called the statically induced direction illusion is introduced. It is suggested that with some further development the proposed model will potentially account for a diverse range of visual motion illusions.