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Monkeys perceive visual illusions the same way humans and great apes do, according to new research.

The study, funded by the National Institutes of Health and the Duane M. Rumbaugh Fellowship, found that monkeys and humans both perceive -– and misperceive -– the world in the same way.


In the first of two experiments, monkeys (Capuchin monkeys and rhesus monkeys) and humans were both asked to complete a computer task that required them to choose the larger of two central dots, sometimes surrounded by the Delboeuf rings.

The humans, using a PC and mouse to perform the experiment, perceived the Delboeuf illusion, overestimating central dots when small rings surrounded them and underestimating the size of central dots when large rings surrounded them. At first the monkeys, who used a joystick and were rewarded with banana-flavoured pellets for right answers, did not seem to see the illusion.

But in a second experiment, devised to check if the monkeys were just using the outer ring to judge the size, the monkeys and humans had to choose if a central dot were merely small or large. In this version of the experiment, the Capuchin and rhesus monkeys saw dots inside large rings as 'small' more often than when surrounded by small rings. The context of the outer ring created an illusion, which all of the species perceived.


Published in the Journal of Experimental Psychology: Animal Learning and Cognition, the study claims to have established a clear resemblance between monkey and human perceptual systems.

Georgia State graduate Audrey Parrish explained: "These results, along with others, show that humans and monkeys share similarities in their perceptual systems. They perceive and misperceive some types of physical stimuli in similar ways. Although these results do not mean that primates and nonhuman primates see their worlds identically, they do show that monkeys are an appropriate model for studying human perception and that contextual cues affect perception in ways that are shared across species."

The similarities between monkeys and humans are leading in a number of strange directions; back in June, WIRED reported that monkeys may be next in line to receive successful head transplants that enable them to survive "at least for a little while" -- possibly paving the way for similar surgery on humans.