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Fooling People But Not Dogs
A study of the Delboeuf illusion

Visual illusions reveal the inner working of the eyes and of the brain, and when used in comparative studies, they can teach us a lot about the differences and similarities in vision and neurological processing between species. A common research approach involves using illusions that affect perception of size and investigating whether the illusions affect choice. Allowing research subjects to choose between various options can elucidate the illusions’ effects on members of various species.

One such illusion is the Delboeuf illusion, which causes identically sized objects to appear different in size depending on what surrounds them. In the image below with dark circles of identical size, humans (and other primates) tend to overestimate the size of the circle on the left, which is surrounded by a ring that is smaller than the ring around the circle on the right.

In practice, this is the reason that people seeking to eat smaller portions of food are advised to use a smaller plate. That makes it appear as though there is more food on that plate than when the same portion is served on a larger plate. Can dogs who are watching their figures make use of this same tactic? In other words, are dogs also susceptible to the Delboeuf illusion? The answer is no, according to a study in Animal Cognition called “Do domestic dogs (Canis lupus familiaris) perceive the Delboeuf illusion?”

In the study, one set of trials tested whether dogs could correctly choose larger portions of food over smaller ones. Dogs were given a choice between two piles of biscuits on plates—one pile of biscuits weighed 18 grams and the other weighed 32 grams. Once dogs chose to go for one plate, the other one was picked up and no longer available. Sometimes both portions of food were on small plates, and sometimes both were on big plates. Each dog was offered this choice multiple times. Pooling the date into one big analysis, dogs consistently chose the bigger pile of biscuits.

In another series of trials, dogs were offered a choice between equal portions of food that were presented on different size plates. The dogs had to choose between 32 grams of food on a large plate and 32 grams of food on a small plate. If dogs are susceptible to the Delboeuf illusion, the expectation is that they would choose the smaller plate even though the quantity of food was identical on both plates. Instead, dogs’ choices were no different than if they picked a plate at random with no reference to its size. They were not significantly more likely to choose the large plate or the small plate, providing evidence that the Delboeuf illusion does not affect dogs the way that it affects humans. Dogs are not fooled by the size of the plate.

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Karen B. London, PhD, is a Certified Applied Animal Behaviorist and Certified Professional Dog Trainer who specializes in working with dogs with serious behavioral problems, including aggression. She is the author of five books on canine training and behavior.

photo by NatalieMaynor/Flickr

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