“It doesn’t seem to depend on some external peripheral aspect, like monocular (one-eyed) versus binocular (two-eyed) vision, but rather on the central organization of visual perception in the brain, which is similar (between horses and people),” said Christian Agrillo, PhD,of the Department of General Psychology at the University of Padova, in Italy.
Agrillo and his fellow researchers used carrot sticks to build optical illusions based on the well-known Muller-Lyer illusion, in which a line that ends in outward-facing arrows seems longer than a line of the same length ending in inward-facing arrows. They first tested the horses’ preferences for different lengths of simple carrot sticks and found the animals generally preferred the longer stick. Then, the scientists placed two carrot sticks of the same lengths, but with Muller-Lyer illusion arrows at the ends, on two test boards 50 centimeters (20 inches) from each horse. When the horse chose one of the boards, the researchers removed the other, so the horse could only have one.
Nearly every time, the 10 horses (averaging 11 years old) in their study chose the stick that looked longer due to the illusion—even though the sticks were exactly the same, said Agrillo.
The findings reveal that horses seem to process vision—at least illusions—in a similar way to humans, despite having eyes on the sides of their heads compared to the front of the head, like humans, Agrillo said. Interestingly, however, while humans and monkeys see the illusion, dogs—which also have frontal eyes—don’t. That might be because dogs rely more on their senses of smell and hearing, compared to humans and horses, than on vision, he said.
From a scientific viewpoint, the researchers are excited to discover that even animals that use only one eye (monocular vision)—as horses often do, especially when looking up close as in this study—can still see the illusion. “Monocular and binocular vision are certainly not the first criteria for seeing illusions,” said Agrillo.
While we commonly refer to being “fooled” by an optical illusion, it’s important to keep in mind that seeing optical illusions isn’t a question of intelligence, and it doesn’t mean horses—or humans, for that matter—aren’t smart. “This isn’t a cognitive test, and it has nothing to do with being ‘clever’ or not,” he said.
Knowing our horses see optical illusions the same way we do could help us anticipate their reactions to what’s around us, said Agrillo. Although his team didn’t study the practical effects, it’s possible this could influence the way horses view the fences they jump during training and competition, or other lines and images they see; if a design creates an optical illusion for the rider, it might cause the same illusion for the horse, he said <ok?>.
But there might be bonus aspect to this discovery, Agrillo said. Because humans often find it fun to discover optical illusions—especially the kind that seem to move all by themselves—it’s possible that horses could find “entertainment” in them, too. “We’ve seen already that optical illusions work like a kind of ‘cognitive game’ for lions in captivity, providing them enrichment in their enclosures,” he said. “I’d like to extend this mental activity to horses.”