Perception of rotating spiral patterns by pigeons

Abstract

The ability of pigeons to discriminate indepth moving stimuli was studied with the rotating spiral illusion. Trained with tightly wound spirals, the birds were able to distinguish apparently approaching from apparently retreating spirals. Discrimination also persisted with loosely wound spirals, even though these did not induce an equivalent illusion in humans. Analysis of the optic flow created by the spirals indicates that the relevant cues were local divergent/convergent motion patterns. Global flow patterns, similar to those arising with approaching/retreating scenes, were only generated by tightly wound spirals. An unidimensional parameter τ could be derived that typified each and all the stimuli used. It is equivalent to the τ that has been used to characterize the optic flow of really approaching objects, indicating the time to collision. With a stationary rotating logarithmic spiral, τ is a joint function of winding tightness and rotation velocity. The τs associated with the rotation speeds yielding threshold discrimination gauged the effectiveness of spirals with different winding inclinations. Threshold τs were high with tight spirals and decreased with loose spirals. This indicates that both local and global kinetic cues must contribute to the detection of in-depth movement by pigeons. Even though the cue efficiency of local flow patterns alone is less than that of global flow patterns the former may be of value when they are dealing with scene elements looming at different rates or with looming objects that are partially occluded.