The Delboeuf illusion is an optical illusion of relative size perception. In the best-known version of the illusion, two circles of identical size have been placed near to each other and one is surrounded by an annulus; the surrounded circle then appears larger than the non-surrounded circle if the annulus is close, while appearing smaller than the non-surrounded circle if the annulus is distant. A 2005 study suggests it is caused by the same visual processes that cause the Ebbinghaus illusion.
According to Girgus, and Coren, the Delfbouf illusion uses both assimilation and contrast as elements in it visual distortion. Assimilation, is the predominate factor in the circle with the smaller outer ring (the example on the right in the image above). Girgus and Coren mentioned that this inner circle “tends to be overestimated” when compared to a regular circle without the additional concentric circle. As the two circles are so close, they are perceived as a pair and the inner circle is overestimated.
The circle on the right however, will often appear smaller when compared to a simple circle of the same size. This is attributed to the contrast effect. The distance between the circles causes them to be perceived as separate and contrasting. The larger circumferencing circle dwarfs the smaller central circle and causes it to be perceived as smaller.
After a few minutes of looking at this illusion, the illusory effects are not as pronounced. .
Studies regarding variations of the Delboeuf illusion found that when the outer circle is incomplete, the illusion is not as potent. When an additional circle was added surrounding the original two, the effect of the illusion was augmented.
Dieting and food perception
In 2012, Ittersum and Wansink published a study that hinted to the Delboeuf illusion as a factor in increased food servings. The study tested three different bowl diameters and measured how individuals served themselves differently depending on the bowl's diameter. The results showed that consumers poured 9.9% more soup in larger bowls, and 8.2% less in smaller bowls, as compared to the control bowls. It was mentioned that this reaction could be driven by the Delboeuf illusion.
This illusion in connection to food however appears to be nuanced. A study conducted by Tzvi and Zitron-Emnual in 2018 highlighted how effects of the Delboeuf illusion when related to food items are less potent when the participants are experiencing mild hunger. These researchers suggest that these findings are potential grounds for mitigating the use of the Delboeuf illusion as a dieting aid.
Use in animal cognition
The Delboeuf illusion (often in connection with the Ebbinghaus illusion) has been used with great frequency in testing animal perception. The ability to discern size is relevant for many aspects of survival. The perception of the Delboeuf illusion differs greatly depending on the species.
In 2014, Parrish and Beran found that chimpanzees would regularly select food platters that contained more food. Further testing showed that when chimpanzees were offered food on small and large plates, as they often picked the food on the smaller plate, even when the amounts were the same. This was discussed as a sign of the chimpanzee’s susceptibility to the Delboeuf illusion.
A later study showed that capuchin and rhesus monkeys however, were unaffected by the illusion when asked to discriminate between the two circles. In contrast, when the illusion was later presented to the monkeys as part of an absolute classification task (deciding if the circles were "big" or "small"), both species reacted to the illusion and made selections that were much like the selections made by humans and chimpanzees.
An attempt to run tests of portion discrimination in connection to the Delboulf illusion on ring-tailed lemurs was unsuccessful, because the lemurs' actions were not statistically influenced by increased food portions as a factor in meal selection, unless one option was nearly 40% larger.
Miletto Petrazzini, Bisazza and Agrillo, replicated the study conducted by Parrish and Beran in 2014, but used dogs as the participants instead of chimpanzees. In this study, conducted in 2016, the dogs were allowed to select whichever food portion appeared larger as presented on larger and smaller plates. The response however was reversed from what humans usually exhibit. Dogs selected the meal presented on the larger plate most often as the larger meal. The authors went on to discuss how this may hint towards the dogs' reactions to the Delboeuf illusion as a matter of assimilated learning.
Fish have been repeatedly studied as well to understand if they perceive the Delboeuf illusion. Fish trained to select larger center circles for reward, responded to the illusion differently depending on the species of fish. A 2008 study of damselfish illustrated that damselfish responded to variations of the Delboeuf illusion in a similar ways to humans and dolphins, while guppies responded in reverse, selecting the circles with the larger annulus. bamboo sharks did not generally make selections significantly higher than chance during the testing, and only showed preference to larger diagrams in general.
Bearded dragons and red-footed tortoise were both studied to understand if these species can perceive the Delboulf illusion. Bearded dragons showed action that suggests that they perceive the illusion in a way similar to humans. The tortoises however, showed no preference to larger portions (a similar problem found in the study of ring-tailed lemurs) and were thus not testable by the method that had been outlined by the test designers.
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