If you think that the circle on the right (the one surrounded by the smaller blue circles) is larger, then you are either a human or a dolphin, but not a pigeon. As it turns out, both orange circles are exactly the same size – but your visual system is influenced by the presence of the surrounding blue circles, resulting in an optical illusion whereby you perceive the orange circle on the right as larger. Pigeons, on the other hand, are influenced by the blue circles in exactly the opposite way, perceiving the circle on the left as larger.
This illusion, called the Ebbinghaus illusion, was first created by the German psychologist Hermann Ebbinghaus in 1901. Recently,researchers tested dolphins for the first time to see if they might be sensitive to the Ebbinghaus illusion. After first teaching the dolphins to reliably choose between larger and smaller shapes (see image below), they were shown a a version of the Ebbinhaus illusion, and, just like humans, chose the circle surrounded by the smaller “inducer” circles as being larger in 84% of trials.
It is difficult to say what being susceptible to this optical illusion might mean in terms of how complex an animal’s understanding of objects might be. Studies with other animals have led scientists to reach polar opposite conclusions. One study found that baboons did not perceive the illusion, which the authors suggested might mean that humans, as opposed to baboons, adopt a more “global mode” of object perception, resulting from cognitive skills that evolved recently in humans but not other primates. Another study found that four day old chickens were sensitive to the illusion, which the authors suggest might point to the ancient origins of a visual perceptual system generating this illusion in a vast number of species. In any event, we can now add dolphins to the list of species that are sensitive to the Ebbinghaus illusion.
Murayama, T., Usui, A., Takeda, E., Kato, K., & Maejima, K. (2012). Relative Size Discrimination and Perception of the Ebbinghaus Illusion in a Bottlenose Dolphin (Tursiops truncatus) Aquatic Mammals, 38 (4), 333-342 DOI: 10.1578/AM.38.4.2012.333