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An optical illusion your horse can see

One common visual illusion that tricks the human brain also seems to fool horses as well.

An optical illusion that commonly tricks the human brain also seems to fool horses, according to research from Italy.

Optical illusions occur when the brain misinterprets visual information, causing a viewer’s perception to differ from reality. Researchers believe that if animals perceive illusions as people do, the mechanisms that enable them to discern the size, color and shape of objects may be similar, too.

Illustration of Muller-Lyer illusion, with multiple lines with arrow heads.

Horses are susceptible to the Muller-Lyer Illusion, in which two identical lines appear to be of different lengths based on the orientation of arrowheads at their ends.

To evaluate the equine capacity to perceive the size of objects, researchers devised a study to test whether horses were susceptible to the Muller-Lyer Illusion, in which two identical lines appear to be of different lengths based on the orientation of arrowheads at their ends. Lines with the arrowheads pointing inward appear longer to the human viewer, while those with arrowheads pointing outward seem shorter. Previous studies have shown that other primates are also fooled by the illusion, but dogs are not.

For the study, 10 horses were first tested to see if they would consistently select the longer of two otherwise identical carrots. In these trials, pieces of white plastic that would later form arrowheads were placed alongside the carrots to acclimate the horses to their presence. Once the horses selected one carrot, the other was removed. The horses, as expected, consistently chose the longer carrots.

In the second part of the study, two carrots of equal length were placed on the same boards, but white arrowheads were attached to them to create the Muller-Lyer Illusion. Tracking of which carrots the horses chose revealed a strong preference for the one that appeared longer because of the arrowheads. (Repeated trials with different combinations of arrowheads controlled for the possibility that the horses were choosing based on the actual length of arrowhead and carrot together.) These results, the researchers note, suggest that horses are susceptible to the Muller-Lyer illusion.

Noting that there are many differences between equine and human vision, the researchers speculate that this shared susceptibility to the Muller-Lyer Illusion may be an evolutionary adaptation or it may have been inherited from a common ancestor. “Our results of a similarity in response to the visual size illusion of horses and primates are particularly interesting in view of the large differences in both their ecological adaptations and their visual system,” the researchers write. “Clearly, data from many more species need to be collected to unravel these questions, especially selecting organisms from a variety of habitats and with different visual adaptations.”

REFERENCE: “Susceptibility to size visual illusions in a non-primate mammal (Equus caballus),” Animals, September 2020

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