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My Favourite Visual Illusions

I love illusions- the more surprising the better. Not only are they great fun, but they also tell us something about how our amazing brains work. One of my favourite shows to perform with Braintastic! Science is That’s Non-Sense, which explores the wonderful world of our senses, and the tricks you can play on them. We cover the 5 main senses, as well as several you might not have heard of, but in this blog post I wanted to focus on just one- vision. There are so many classic visual illusions in psychology I could have chosen from, but I have picked 5 of my favourites to share with you today. Why not try them out on your class – they’d make great science club activities. Does everyone see them the same way?

Vanishing colours (Troxler effect)

Stare at the cross in the centre of this gif, and keep your eyes as still as possible- what happens?

An illusion consisting of a circle of lilac dots surrounding a cross, animated so that each dot in turn disappears
Image credit: Toto Baggins

Hopefully, you will see the pink colours begin to fade. This happens because of a process called adaptation. There is always so much going on around us, that our brains can’t pay attention to everything. So our brains and senses adapt to things that aren’t changing, and stop telling us about them. While you focused on the cross, your eyes were still, so the same messages were being sent over and over again, down the same neurons, about those particular colours in those particular areas. That meant these neurons began to adapt and stop sending their messages, and you stopped seeing the colours. It’s the same reason you sometimes notice a smell when you first arrive somewhere, but then realise an hour later you can’t smell it any more- your brain has stopped telling you about the smell, because it isn’t changing.

Colour constancy

We like to think we see the world exactly as it is, but our brains are always making guesses, and changing what we see based on what they think we should see. For example, if you held a banana outside in bright sunlight, the wavelength of light reflected from it (which is what our eyes interpret as colour) will be very different to if you looked at it under fluorescent lighting, indoors. But we don’t think the banana has changed colour when we take it outside. This is because our brains cancel out the effects of the lighting, to give us the ‘true’ colour of the banana.

Two images of a banana, the left with a blue overlay, the right with a red overlay

In these pictures, our brain cancels out the bluish or reddish tinge to the picture, leaving the banana looking yellow, even though their pixels are greeny-blue on the left and orange on the right. The bar at the bottom shows the actual colour of the bananas- but I bet it looks different to the bananas in the picture!

We can also see this in action by looking at Adelson's checker shadow illusion. Square B looks much lighter than square A, but if you cover up the rest of the picture, they are actually the same colour. Square B is in shadow, so our brains ‘subtract’ the shadow and make the square look white. A is in light, so our brains do the opposite, making it look dark.

Adelson's checker shadow illusion - a checkerboard with a green cylinder on it casting a shadow. Two squares (one grey, one white) labelled 'A' and 'B'

Another famous example of this illusion with colours was the dress illusion: white and gold versus black and blue. Which you saw depends on what assumptions your brain made about the lighting.

Ebbinghaus illusion

Take a look at this picture. Which orange dot is smaller?

Ebbinghaus illusion: a pair of orange circles, the left surrounded by larger circles and the right surrounded by smaller circles. The left orange circle looks smaller than the right

They are actually identical, but most people think the one on the left looks smaller. This is because we don’t see them in isolation, but compared to the blue dots around them. So the one on the left looks small compared to the large blue dots while the one on the right looks big compared to the small ones.

Ponzo illusion

This illusion is based on our understanding of perspective. We all know that things look smaller when they are further away, so our brain compensates for this, by making things that we know are far away seem bigger. This is what causes the Ponzo illusion- both these cats are the same size, but our brain tells us that the top one must be huge, because it seems so far away.

Ponzo illusion: Two images of a cat on a road extending to the horizon. The cat further away looks  giant compared to the on in the foreground

Mueller Lyer illusion

Which line do you think is longer?

Mueller Lyer illusion: A pair of straight lines with arrows on both ends - one pointing out, the other pointing in

They are actually the same length, but many people think the bottom one looks longer. Scientists don’t know exactly why this is, but there are a few ideas.

One is that it is a variation on the Ponzo illusion. Because we live in an environment with lots of corners, we have learned that concave corners (like the far corner of a room) are usually further away than convex (like the front edge of a rug). Line A looks like a convex corner, and B looks like a concave corner, so our brains assume B is further away, and make it look longer than it is (like the giant cat).

Another idea is that our brains look at the overall length of the shape, and generalise, so because B is longer overall, we assume that the straight middle part is also longer. Yet another suggests that we subconsciously look at the distance between the tips of the arrows, instead of the line, so the top one, where the arrows end closer together, looks shorter.

There are lots of other explanations for the Mueller Lyer illusion - from how our eyes move, to the blurriness of our peripheral vision, but really, scientists don’t yet know exactly why or how this illusion occurs. This is one of the things I love about neuroscience- there is still so much we have to learn about our brains, and that’s what I think makes careers in psychology and neuroscience so interesting.

I hope you found those illusions as fascinating as I do, and enjoyed learning a bit more about how your amazing brain works, based on the tricks we can play on it. Why not share these illusions with your class, they make great STEM activities or ideas for science week. Do let us know which ones your students like best by tagging us on social media, or if your favourite illusion isn’t here, share it with #CuriosityCorner and we’ll explain the science behind it.



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