The Delbouef Illusion is well known. It absolutely works on humans, but scientists wanted to see if it also works on monkeys. That’s harder to establish than it looks. How do you resist having illusions about your optical illusion test?

Let’s start with an optical illusion of our own. Which of the dots below is bigger? Anyone reading this blog has probably seen this illusion before, and so knows from experience that both dots are the same size, even if their eyes tell them that the dot surrounded by the small ring is bigger. It’s tempting to think that other animals, especially those closely related to us, see the world the way we do, but the initial results from a recent study proved that this wasn’t the case.


Capuchin and rhesus monkeys were given a joystick and a look at a color computer screen. They attempted to judge which dot on the screen was bigger. If they got the answer right, they got a banana-flavored food pellet. (Humans were also tested. They were only rewarded with kind words, which is a rip-off, scientists.) The humans were consistently fooled by the Delbouef Illusion, but the monkeys were not. Capuchins and rhesus monkeys alike were much more likely to choose the bigger dot, even if it was surrounded by a bigger circle. It looked like the monkeys were seeing the world more clearly than the humans.

Then someone wondered, what if the monkeys just knew to go for the bigger circle, not the bigger dot?


This called for another experiment. This time neither primate got to compare two dots, they only got to classify a dot, surrounded by a ring, as small or large. Suddenly, the results matched up. Both humans and monkeys had a pronounced tendency to classify a dot as small when it was surrounded by a big ring even when they’d previously classified it as large when surrounded by a small ring. They see the world, or at least the Delbouef Illusion, the same way we do.

You just have to test them the right way.

[Source: Do You See What I See?]

Top Image: Hung Do