Domestic chicks perceive stereokinetic illusions

Stereokinetic illusions occur when certain 2-D patterns are set in slow rotation in a plane perpendicular to the line of sight. Such phenomena have never been investigated in animal species other than our own. We used the domestic chick (Gallus gallus) to check whether these illusions are experienced by non-human species, taking advantage of filial imprinting. Newly hatched visually naive chicks were individually exposed for 4 h to 2-D stimuli producing, to a human observer, the perception of a stereokinetic cone (experiment 1) or of a stereokinetic cylinder (experiment 2). Thereafter, each chick underwent a free-choice test between a solid 3-D cone and a solid 3-D cylinder. A control group of newly hatched but not imprinted chicks underwent the same testing procedure, to check for the presence of any spontaneous preference for one or other of the two solid objects. Imprinted chicks approached the 3-D stimulus closely resembling the stimulus they had been exposed to during imprinting (the cone in experiment 1 and the cylinder in experiment 2). Non-imprinted chicks did not show any preference. These results suggest that domestic chicks experience stereokinetic illusions.