Visual illusions are fun: we know with our rational mind that, for example, these lines are parallel to each other, yet they don’t appear that way.

Similarly, I could swear that squares A and B are different colours.

But they are not. This becomes clearer when a connecting block is drawn between the two squares (see the image below).

Illusions aren’t just fun tricks for us to play with, they can also tell us something about our minds. Things in the world look to us a certain way, but that doesn’t mean that they are that way in reality. Rather, our brain represents the world to us in a particular way; one that has been selected over evolutionary time. Having such a system means that, for example, we can see some animals running but not others; we couldn’t see a mouse moving from a mile away like a hawk could. This is because there hasn’t been the evolutionary selective pressures on our visual system to be able to do such a thing, whereas there has on the hawk’s. We can also see a range of wavelengths of light, represented as particular colours in our brain, while not being able to see other wavelengths (that, for example, bees and birds can see). Having a system limited by what evolution has given us means that there are many things we are essentially blind to (and wouldn’t know about if it weren’t for technology). It also means that sometimes our brain misrepresents physical properties of the external world in a way that can be confusing once our rational mind realises it.

Of course, all animals have their own representation of the world. How a dog visually perceives the world will be different to how we perceive it. But how can we know how other animals perceive the world? What is their reality? One way we can try to get this is through visual illusions. If an animal experiences the same visual illusion that we experience then it is likely that their brain is integrating information from the world and representing it in the visual system in a way that is similar to how our brains do it. If we then have this information from enough different species we can even gain an understanding of how visual perception systems evolve, in other words, how our reality came to be the way it is.

A recent study by Sovrano and colleagues at the University of Trento in Italy looked to see whether a particular fish, the redtail splitfin, experienced the Ebbinghaus illusion. This is a well-known illusion where two identically-sized cirlces appear to be different sizes because of the presence of other circles around them.

The Ebbinghaus Illusion

How do you go about asking a fish whether it perceives two circles as different sizes? The researchers started by training some fish to go to a larger orange circle to get a food reward, and some to go to a smaller orange circle. Once a fish had learned this, it was then tested by being given the Ebbinhaus illusion (two circles that are actually the same size but appear to be different sizes to us), and seeing where it would go. If the fish see the illusion the way we see it, then you would expect that the ones that had been trained to go to larger circles would go to the one that appears larger to us, and the ones that had been trained to go to the smaller circles would go to the one that appears smaller to us.

The redtail splitfin

This is exactly what they did. As the apparent perception of this illusion is also shared by birds, it could be that an ancestor that we all share also had a similar way of perceiving such visual stimuli.



Picture and Photo Credits

‘Café Wall’ by Fibonacci

‘Checker Shadow Illusion’ by Edward H. Adelson

Ebbinghaus illusion - Hermann Ebbinghaus

redtail splitfin Xenotoca eiseni: Auteur: Marie France Janelle



Sovrano, V. A., Albertazzi, L., & Salva, O. R. (2015). The Ebbinghaus illusion in a fish (Xenotoca eiseni). Animal cognition, 18:533–542