The best-known version of the Delboeuf illusion - the two black circles are the same size, but the one on the left may appear larger or smaller.

Researchers test whether size illusions affect choice

Dogs are not fooled by plate size when it comes to choosing between portions of food, according to new research.

In the study, researchers from the University of Padova, Italy, assessed whether dogs perceive the Delboeuf illusion - a visual illusion of relative size perception (pictured).

In humans and primates the illusion creates a misperception of item size as a function of its surrounding context.

To examine whether dogs are affected by the illusion, the researchers adapted a spontaneous preference model that had been used with chimpanzees.

Subjects were offered two plates of food. In control trials, two different amounts of food were presented, multiple times, on two identical plates. Sometimes both plates were large and sometimes both plates were small. But dogs consistently chose the plate that contained the most food.

Next, the researchers gave the dogs equal portions of food on plates of differing sizes. If the dogs perceived the illusion like humans do, they would select the food on the smaller plate, even though the amount of food was the same.

But the researchers found that the dogs choices were no different than if they picked a plate at random. They were not more likely to choose the large plate or the small plate, suggesting that the Delboeuf illusion does not affect dogs in the same way it affects us.

'The fact that dogs do not seem to be susceptible to the Delboeuf illusion suggests a potential discontinuity in the perceptual biases affecting size judgments between primates and dogs,' the team concludes.

The study, 'Do domestic dogs (Canis lupus familiaris) perceive the Delboeuf illusion?' is published in Animal Cognition.

Image (C) Famousdog/Wikimedia Commons