It’s one of the most well known optical illusions around, the Müller-Lyer illusion. Two lines, bounded by arrows. Simple. Through the trickery of human visual perception, lines of equal length look different when arrows facing different directions cap off their ends. For more than a century, says PopSci, the success of the illusion held unshaken:

ision researchers assumed that the illusion told us something fundamental about human vision. When they showed the illusion to people with normal vision, they were convinced that the line with the inward-pointing arrows would seem longer than the line with outward-pointing arrows.

But then, in the 1960s, the idea that cultural experience might come into to play arose. Up until that point, says PopSci, in an excerpt from a recent book by New York University marketing and psychology professor Adam Alter, “almost everyone who had seen the illusion was WEIRD—an acronym that cultural psychologists have coined for people from Western, Educated, Industrialized, Rich, and Democratic societies.”

Taking the test worldwide, the persistence of the illusion fell apart. In the U.S. and for European descendants in South Africa, the illusion worked.

Then the researchers journeyed farther afield, testing people from several African tribes. Bushmen from southern Africa failed to show the illusion at all, perceiving the lines as almost identical in length. Small samples of Suku tribespeople from northern Angola and Bete tribespeople from the Ivory Coast also failed to show the illusion, or saw Line B as only very slightly longer than Line A. Müller-Lyer’s eponymous illusion had deceived thousands of people from WEIRD societies for decades, but it wasn’t universal.

The biological basis of how these different groups of people saw the illusion is identical, but the response was totally different. The success or failure of the illusion is a cultural effect. But what is driving that difference has been a matter of ongoing debate.

In his book, Alter proposes the theory that western societies, used to seeing straight lines and geometric forms in buildings and houses, grow accustomed to looking at lines as three-dimension representations of space—the out-turned arrows of the “longer” line and the inward arrows of the “short” line invoke this spatial reasoning and underpin the illusion.

These intuitions are bound up in cultural experience, and the Bushpeople, Suku, and Bete didn’t share those intuitions because they had rarely been exposed to the same geometric configurations.

But recent research, says Charles Q. Choi for LiveScience, suggests that this “geometric experience” argument may fall flat.

In the research, scientists led by Macquarie University’s Astrid Zeman found that a computer trained to mimic the perceptions of the human eye was also susceptible to the Müller-Lyer Illusion.

“In the past,” writes Choi, “scientists had speculated this illusion was caused by human brains misinterpreting arrowheads and arrow tails as depth cues — in modern-day environments, rooms, buildings and roads present boxy scenes with many edges, and so might lead people to unknowingly make predictions regarding depth whenever they run across angles and corners. However, since this computer model was not trained with 3D images, these findings may rule out that idea.”

“Recently, many computer models have tried to imitate how the brain processes visual information because it is so good at it,” Zeman said. “We are able to handle all sorts of changes in lighting and background, and we still recognize objects when they have been moved, rotated or deformed. I was curious to see whether copying all of the good aspects of object recognition also has the potential to copy aspects of visual processing that could produce misjudgments.” The scientists discovered these artificial mimics of the brain could get duped by the illusion.

A computer failing the Müller-Lyer test doesn’t eliminate the cultural aspects of the perception of the illusion reported in the previous studies, but it does open the question as to what drives the differences.

All in all, these findings suggest the illusion doesn’t necessarily depend on the environment or any rules people learn about the world. Rather, it may result from an inherent property of how the visual system processes information that requires further elucidation.

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