By Dr. Becker
Monkeys and humans may see the world in ways that are more alike than they are different, according to research published in the Journal of Experimental Psychology: Animal Learning and Cognition.1
Studying visual illusions can help further understanding of typical visual perception, and the researchers used the Delboeuf illusion to determine whether humans and monkeys share similarities in their perceptual systems.
The Delboeuf illusion includes two dots, one surrounded by a large ring and the other surrounded by a small ring. Although the two dots are the same size, humans tend to perceive the dot surrounded by the large ring as smaller than the dot surrounded by a small ring.
The researchers wanted to find out whether monkeys (capuchin monkeys and rhesus monkeys) would perceive the illusion in the same way.
Monkeys and Humans See Visual Illusion Similarly
A series of two computer studies were used to investigate the Delboeuf illusion in humans and monkeys. In the first study, they had to choose the larger of two central dots that were sometimes encircled by concentric rings.
While humans showed evidence of the Delboeuf illusion, overestimating the size of the dots when they were encircled by small rings and underestimating the size of the dots when large rings surrounded them, the monkeys did not show evidence of the illusion.
The researchers believed in this case the monkeys may have been using the outer circles to gauge size, so they conducted a second experiment. In the second test, monkeys and humans had to classify one central dot as small or large.
Sometimes the dot was surrounded by a ring of varying sizes. In this case, both humans and monkeys showed evidence of the Delboeuf illusion. The researchers explained:
“ … [W]e found evidence of the Delboeuf illusion in all three species. Humans and monkeys underestimated central dot size to a progressively greater degree with progressively larger rings.”
A previous study that demonstrated the illusion using food items for “dots” and plates for the outer circles, showed similar results in chimpanzees and humans.2 The findings of this and the featured study suggest humans and monkeys share similarities in the way we perceive and misperceive the world.
Both Chimps and Humans Misperceive Food Portions Based on Plate Size
For humans, the type of plate on which you serve your food may fool you into thinking there’s less or more of it than there really is. Generally, people may over-serve and over-consume food presented on large plates while under-serving, and eating less of, food served on small plates.
However, this isn’t a uniquely human misperception. In a study published in Animal Cognition, chimpanzees chose between two amounts of food presented on either plates of the same or different sizes.3
When the plates were the same size, they correctly chose the larger food portion, but this became more difficult when the food was presented on small and large plates.
And when the same food portions were presented on small or large plates, chimps tended to choose the smaller plate (suggesting they, like humans, perceived the portion on the small plate to be larger).
The study shows another case where humans and chimpanzees share similar behaviors.
And in the case of humans, researcher Audrey Parrish of Georgia State University explained in Phys.org that the results “suggest that it might be harder to convince the stomach to ignore the eyes than we would hope to be true, as this illusion seems to occur across species.”4
Monkeys Can Count Surprisingly Well and Have Impulse Control Similar to Young Children
Another intriguing set of experiments was published in Nature Communications that determined to show whether long-tailed macaque monkeys understand the concept of relative quantity.
The first test involved presenting monkeys with two plates of raisins (one with more than the other). The monkeys were supposed to choose the plate with more raisins, which they were then allowed to eat, but most monkeys chose the plate with fewer raisins.
In the second test, the experiment was repeated with pebbles instead of raisins. In this case, the monkeys were successful at choosing the plate with the greater amount of pebbles.
In a third experiment, the plates with raisins were repeated, but the monkeys were rewarded with raisins hidden under the plates. The monkeys were successful in this experiment as well.
It turns out that, much like young human children, the monkeys’ desire to eat the raisins in the first experiment clouded their ability to count. In reality, monkeys can likely count much better than researchers realized. As explained by Phys.org:5
“ … [N]atural impulses in the monkeys and their desire to eat the raisins interfered with their judgment in the initial experiment … Similar to young children and the reverse reward paradigm, these monkeys were not able to see past their desire to eat the raisins …
In the reverse reward paradigm, young children are presented with two piles of candies of different sizes. The children will always point to the larger pile and then this pile is given to another child. Children have difficulty understanding that by choosing the smaller pile, they will receive the larger pile.
However, if this test is repeated with non-edible objects, the children are able to understand and perform the experiment correctly. Researchers in this study point to the same interference in judgment by the monkeys when presented with food.
Their desire to eat the food gets in the way of the task at hand. These researchers believe that previous studies performed on other primates using food as a test symbol may have impaired results and therefore underestimated the primate’s numeracy abilities.”
This study may also inadvertently reveal one reason why monkeys should not be kept as pets; they behave much like young children who will not grow out of the “terrible twos” (but unlike human children, monkeys have the strength and ability to harm their adult caregivers).
And once monkeys mature their behavior may become erratic or aggressive, and most owners are no longer able to safely care for them. So, please marvel at the many ways monkeys are similar to humans, but respect their need to live safely in the wild.