Chances are you’ve encountered some version of the “Ebbinghaus illusion,” in which a central circle appears to be smaller when encircled by larger circles and seems larger when surrounded by smaller circles. It’s an example of context-dependent size perception. But is this unique to humans or are some animals susceptible as well? According to a new paper published in the journal Frontiers in Psychology, it might depend on the specific sensory environment, since the illusion relies on contextual clues to be effective.
Prior research has produced mixed results on the question of animals and their susceptibility to optical illusions, per the authors. Dolphins, chicks, and redtail splitfins seem to be susceptible, for example, while pigeons, baboons, and gray bamboo snakes are not.
Perhaps the best-known example is cats’ undeniable love of boxes and squares—the “if it fits, I sits” phenomenon documented all over the Internet. This behavior is generally attributed to the fact that cats feel safer when squeezed into small spaces, but it also tells us something about feline visual perception. Both a 1988 study and a 2021 study concluded that cats are susceptible to the Kanizsa square illusion, suggesting that they perceive subjective contours much like humans.
The authors of this latest paper decided to test the Ebbinghaus illusion across two species that inhabit very different sensory environments: ring doves and guppies. The doves are terrestrial, pecking at small seeds scattered on the ground for food, so the authors reasoned that precision and attention to detail—rather than global processing to first analyze an entire scene, as humans do—would be more advantageous for the doves. Plus the doves have binocular vision and thus should be good at accurately judging size and distance.
Guppies, by contrast, live in the shallow waters of tropical streams. They must contend with dense vegetation, flickering light, and the unpredictability of predators. They must make rapid decisions in order to survive, and thus it would be advantageous for guppies to be able to judge relative size at a glance—a human-like global processing ability that is key to the Ebbinghaus illusion.