Illusions Across Species: Do Animals See Optical Illusions?

If illusions were simply quirks of human wiring, other animals wouldn't see them. Many do. Researchers have found that primates, birds and fish can be fooled by illusions very like the ones that fool us, and sometimes in surprising ways that differ from us. Those similarities and differences are some of the best evidence for what illusions reveal about vision in general.

How do you ask an animal?

You can't ask a monkey or a fish which circle looks bigger, so researchers train animals to make a choice, such as picking the larger of two shapes for a food reward. Once the animal reliably picks the larger one, it's shown an illusion in which two identical shapes differ only in their surroundings. If its choices are biased in the same direction as a person's reports, it's probably seeing the illusion too (Feng et al., 2017).

Primates

Monkeys and apes are the obvious comparison. Fujita compared rhesus monkeys, chimpanzees and humans on the Ponzo illusion and found that all three were affected, though not identically (Fujita, 1997), which suggests that the depth-based size scaling behind it has roots older than our species.

But not every illusion carries over. Baboons tested on the Ebbinghaus illusion were far less fooled than people, and Parron and Fagot suggested that baboons attend more to the parts of a scene than to how they group together (Parron & Fagot, 2007). That echoes the reduced Ebbinghaus effect found in the Himba and in young children, covered in do optical illusions work the same for everyone and do children see optical illusions.

Birds

Bird eyes differ from ours in several ways; many birds have four types of colour receptor rather than three and can see ultraviolet light. Pigeons, which are easy to train to peck at targets, have been tested on the Müller-Lyer illusion alongside humans (Nakamura et al., 2006). The Ebbinghaus illusion gives one of the most striking results. Nakamura and colleagues found that bantam chickens see it reversed (Nakamura, Watanabe & Fujita, 2014): a circle surrounded by large circles looks larger to them, not smaller, as if the surrounding circles pull the middle one towards their own size rather than making it look different by contrast. Results like these help researchers separate the parts of an illusion that are general to visual systems from the parts that depend on how a particular species processes scenes.

Fish

Perhaps the most surprising findings come from fish, whose brains have no layered visual cortex like ours. Sovrano and colleagues trained redtail splitfin fish to pick the larger or smaller of two discs, then showed them identical discs surrounded by large or small circles. The fish chose as if they saw the Ebbinghaus illusion, in the same direction as people (Sovrano, Albertazzi & Rosa Salva, 2015). Judging size against the surroundings seems to be such a useful strategy that very different brains have arrived at it.

What comparing species tells us

When an illusion fools animals as distantly related as primates and fish, the mechanism behind it is probably a basic solution to a basic problem of vision, not an accident of human anatomy. When an illusion fails to carry over, or even reverses, the difference helps pin down which part of the mechanism depends on a particular species' brain or experience (Kelley & Kelley, 2014). Across the animal kingdom, illusions are evidence of how any visual system, built from very different hardware, tends to solve the same problem: getting a useful picture of the world from limited and ambiguous information.

Sources and further reading

1. Fujita, K. (1997). Perception of the Ponzo illusion by rhesus monkeys, chimpanzees, and humans: Similarity and difference in the three primate species. Perception & Psychophysics, 59(2), 284–292. Find this reference on Google Scholar
2. Nakamura, N., Fujita, K., Ushitani, T., & Miyata, H. (2006). Perception of the standard and the reversed Müller-Lyer figures in pigeons (Columba livia) and humans (Homo sapiens). Journal of Comparative Psychology, 120(3), 252–261. Find this reference on Google Scholar
3. Sovrano, V. A., Albertazzi, L., & Rosa Salva, O. (2015). The Ebbinghaus illusion in a fish (Xenotoca eiseni). Animal Cognition, 18(2), 533–542. Find this reference on Google Scholar
4. Kelley, L. A., & Kelley, J. L. (2014). Animal visual illusion and confusion: The importance of a perceptual perspective. Behavioral Ecology, 25(3), 450–463. Find this reference on Google Scholar
5. Feng, L. C., Chouinard, P. A., Howell, T. J., & Bennett, P. C. (2017). Why do animals differ in their susceptibility to geometrical illusions?. Psychonomic Bulletin & Review, 24(2), 262–276. Find this reference on Google Scholar
6. Parron, C., & Fagot, J. (2007). Comparison of grouping abilities in humans (Homo sapiens) and baboons (Papio papio) with the Ebbinghaus illusion. Journal of Comparative Psychology, 121(4), 405–411. Find this reference on Google Scholar
7. Nakamura, N., Watanabe, S., & Fujita, K. (2014). A reversed Ebbinghaus–Titchener illusion in bantams (Gallus gallus domesticus). Animal Cognition. Find this reference on Google Scholar