Can Monkeys See Optical Illusions?

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A big part of being a skeptic is having a healthy (or some would say unhealthy) interest in optical illusions. After all, a lot of skepticism is just recognizing when we are being fooled, and optical illusions are all about your own brain fooling itself.

One of the simplest and most common optical illusions is pareidolia, in which an inanimate object looks like something else. This pops up in religious iconography all the time, like when Jesus appears in your toast, or the word “Allah” is written in your flatbread, or Mother Theresa in a cinnamon bun, or when Jesus also appears in your dog’s butthole.

More commonly, it’s just a face: a face on the moon, on mars, in your coffee, on an eggplant, or on your mailbox. That’s common because your brain is primed to pick out other human faces, so you’re really good at it. Too good, in fact, because you end up with some false positives. Basically, any two dots with a line under it will remind you of a human face. While you may occasionally think a “face” looks like a dog or a bird or some other animal, it’s far more common for you to be reminded of a human face, because that’s what your brain is primed for.

All of that brings me to a really fun recent study that shows for the first time ever that another species also experiences pareidolia. Neuropsychologists gathered photos that are examples of pareidolia for humans, and then showed them to rhesus monkeys alongside control photos of either an actual monkey’s face or the same object in the pareidolia photo but arranged in a way that it doesn’t look like a face at all.

They then studied how long the monkeys looked at each photo. Previous research suggests that both humans and other primates tend to spend more time looking at photos of faces than photos of anything else, so if the monkeys spent more time looking at the pareidolia pics compared to the regular object pics, that’s evidence that they’re seeing a face there. And sure enough, that’s exactly what happened. The monkeys spent more time looking at the pareidolia pics. In fact, they spent more time looking at the optical illusions than they spent looking at the photos of actual monkeys’ faces. The researchers point out that this could be because the monkeys are disinclined to look at their own species’ faces too long, but it could be that they are interested because they get that they’re looking at an optical illusion.

Finally, the researchers recorded where the monkeys spent the most time looking. For both the pics of monkeys and the pics of pareidolia, they spent the most time looking at the “eyes” and “mouth.”

It’s a simple study but I find it really interesting, because optical illusions are fairly normal to us humans, but that doesn’t mean other animals get the same experience. This study shows that monkeys are capable of that kind of processing, and so the evolutionary roots of our ability to find faces in everyday objects go deeper than we may have thought.

Next step: let’s get those monkeys to tell us whether that stupid dress is white and gold or black and blue.

Rebecca Watson

Rebecca is a writer, speaker, YouTube personality, and unrepentant science nerd. In addition to founding and continuing to run Skepchick, she hosts Quiz-o-Tron, a monthly science-themed quiz show and podcast that pits comedians against nerds. There is an asteroid named in her honor. Twitter @rebeccawatson Mastodon Instagram @actuallyrebeccawatson TikTok @actuallyrebeccawatson YouTube @rebeccawatson BlueSky

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