Monkeys and Humans Perceive the World Similarly, New Study Shows

A test designed to examine how humans and monkeys perceive the world has shown that both species do so in very similar ways, reseachers have revealed.

Researchers from Georgia State University performed their study using the Delbouef illusion, a test that involves two equal-sized dots surrounded by different sized rings– an optical illusion that causes the dots to appear different in size.

For their first test, GSU researchers placed humans in front of a computer and asked them to select the larger of the two central dots that were sometimes surrounded by Delbouef rings (see illustration). They were rewarded with written feedback for every correct answer. Monkeys were asked to do the same (after being trained to use joysticks to express their choice). The monkeys were rewarded with banana-flavored pellets when they answered correctly. Although the test had the usual results for humans (misperceptions), monkeys had no problem selecting the larger of the two central dots.

So, researchers created another experiment.

For this test, monkeys and humans played a computer game where they were asked to classify the central dots as either "small" or "large." As usual, Delbouef rings varying in size surrounded the dots.

This time, the monkeys had been tricked. Capuchin and rhesus monkeys – as well as humans – all classified the dots inside larger rings as smaller than the dots of equal size that were surrounded by smaller rings. The study suggests that humans and monkeys have similar perceptual systems.

An earlier test of Chimpanzees – designed to substitute equal-sized portions of food on different sized plates – yielded similar results. Chimpanzees successfully selected the larger portion of food when the plates were the same size but misperceived food portions when different-sized plates were used.

"These results, along with others, show that humans and monkeys share similarities in their perceptual systems," Audrey Parrish, a graduate student at Georgia State who conducted the research as part of her doctoral dissertation, said in a statement. "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."

Their study was published in the Journal of Experimental Psychology: Animal Learning and Cognition.

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