The perception of the Ebbinghaus illusion in two distant vertebrates: domestic chicks and redtail splitfins

In the Ebbinghaus illusion, a central circle surrounded by small circles (inducers) appears bigger than an identical one surrounded by large inducers. Three previous studies failed to demonstrate sensitivity to this illusion in bantams chickens, pigeons and baboons, suggesting that other animals might lack the neural substrate necessary to perceive the Ebbinghaus illusion in a human-like fashion. Using procedures that do not force the animals to look at stimuli from a close distance, allowing them to take in their global configuration, we were show that his illusion is perceived by 4-day-old domestic chicks (Gallus gallus domesticus) and by a fish of the Goodeidae family (redtail splitfin, Xenotoca eiseni). Animals were trained to recognize the reward location with either a big or a small orange circle (grey circles of different dimensions were present at both rewarded and unrewarded locations). When tested with the illusory configurations (two same-sized orange circles surrounded one by big gray inducers, the other by small gray inducers), animals reinforced on the small target choose the configuration with big inducers, in which the central target appears perceptually smaller; the opposite was true for subjects reinforced on the big target. Thus, animals chose the stimulus that, on the basis of a perception of the Ebbinghaus illusion, appeared deceptively larger or smaller, consistent with the condition of training. These results have important implications for the evolutionary history of the neural substrate involved in the perception of the Ebbinghaus illusion.