A popular optical illusion can fool some animals, but not others, according to a recent study – and it may shed new light on how perception shaped their survival and evolution.
The optical illusion used in this study is called the Ebbinghaus illusion, where two identical circles appear to be different sizes, because one is surrounded by large circles and the other is surrounded by small circles.
When we look at the illusion as a whole, the outer circles trick our brains into believing that the inner circles are different sizes. But, if we focus solely on the inner circles, we may realise that they are identical.
Whether or not we fall for this illusion depends on how our brains process what we see – as a whole picture, or detail by detail.
So, to understand how different animals perceive the world, scientists at the University of Vienna, in Austria, reconstructed the Ebbinghaus illusion using food – placing circles of differing sizes around central feeding pools.
Then, they tested the reactions of two species: guppy fish (Poecilia reticulata) and ring dove birds (Streptopelia risoria).
The scientists found that the guppies consistently fell for the optical illusion. Much like humans, they chose to eat the fish flakes that were surrounded by smaller circles, as if it was a larger portion of food.
But, as a group, the ring doves did not. Some birds chose millet seeds surrounded by smaller circles, others chose food surrounded by larger circles, and many mixed up their choices.
The scientists concluded that these choices indicated how each species was uniquely adapted to navigate their own environments and cope with their own challenges.
For instance, guppies live in shallow tropical streams. Their world is full of flickering lights, dense plants, fast-moving predators, and other guppies.
So, the scientists supposed, their survival depends on making quick decisions about which fish are safe in a cluttered visual landscape. If this is true, making judgements quickly would a useful to a guppy.
But ring doves, on the other hand, spend their lives on the ground, pecking seeds. For them, it might be more helpful to spot little details rather than analyse an entire scene.
That said, the scientists noticed that some of the doves seemed to respond as if they had been fooled by the illusion – so, there could be variation among the species, just as there is among humans.
In this way, the team concluded, optical illusions can serve as a way to reveal all the different ways in which the world can be perceived.
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