Perception of the Ebbinghaus illusion in four-day-old domestic chicks (Gallus gallus)

In the Ebbinghaus size illusion, a central circle surrounded by small circles (inducers) appears bigger than an identical one surrounded by large inducers. Previous studies have failed to demonstrate sensitivity to this illusion in pigeons and baboons, leading to the conclusion that avian species (possibly also nonhuman primates) might lack the neural substrate necessary to perceive the Ebbinghaus illusion in a human-like fashion. Such a substrate may have been only recently evolved in the primate lineage. Here, we show that this illusion is perceived by 4-day-old domestic chicks. During rearing, chicks learnt, according to an observational-learning paradigm, to find food in proximity either of a big or of a small circle. Subjects were then tested with Ebbinghaus stimuli: two identical circles, one surrounded by larger and the other by smaller inducers. The percentage of approaches to the perceptually bigger target in animals reinforced on the bigger circle (and vice versa for the other group) was computed. Over four experiments, we demonstrated that chicks are reliably affected by the illusory display. Subjects reinforced on the small target choose the configuration with big inducers, in which the central target appears perceptually smaller; the opposite is true for subjects reinforced on the big target. This result has important implications for the evolutionary history of the neural substrate involved in the perception of the Ebbinghaus illusion.