Illusions: How Our Brain Can Trick Us
It feels like we have direct access to reality but, in order to produce that feeling, our brain is stitching together all the different information from our senses and filling in the gaps using its best guess as to what is out there in the world. We notice the things that our brain thinks are important and can process very complex information quickly because our brain sometimes takes shortcuts. Normally we don’t even notice these shortcuts but visual illusions have helped us understand lots of the shortcuts used by the brain. Many illusions are based on your expectations – things that you have encountered in the past that have become wired into your brain. This is knowledge that influences the way we interpret and experience the world. This is because we assume that the world is fairly reliable based on our previous encounters so we assume that it is predictable.
The remarkable thing about illusions is that even when you know things are not what they seem, you still cannot force your mind to see it any other way. This is because your conscious mind is a product of your brain, which is feeding you the information.
Seeing patterns where there aren't any [All Ages]
Is it possible to see patterns where there aren’t any?
Why does the brain try so hard to find patterns?
One shortcut our brain makes is to always look for patterns. We even see patterns in random noise such as a cloud or an ink stain, or in scattered coffee beans.
Scatter coffee beans (or something similar) onto a flat surface and show the audience. Ask them to pick out any patterns or pictures in the distribution of the beans.
Q: Why do you see patterns in the coffee beans?
A: You see patterns in the coffee beans because the brain is always looking for patterns. Even when presented with something random like spilled coffee beans, the brain is drawing lines in between the coffee beans to make shapes according to where there are light bits and dark bits. This is probably how astrology – seeing patterns in the stars – came about.
Q: Why is the brain always looking for patterns?
A: The brain is always looking for patterns because one of the most important jobs the brain does is making sense of the stream of information that is coming in through all the senses. The brain filters the information so that you are consciously aware only of what it thinks are the most important bits. This information might be incomplete (for instance if you blinked while you were looking at something) so the brain looks for information to fill in the gaps in the most sensible way. The brain sees patterns even when there aren’t any because it is constantly looking for patterns to make sense of the world.
The Ponzo 2-dimensional Perspective Illusion [All Ages]
Does the context of a picture effect how you see it?
One common short-cut the brain makes is to assume that under certain circumstances, things that are further away are smaller. Here we have what looks like a set of train tracks with a couple of yellow bars across them. Ask the audience to put their hands up if they think that the upper yellow line is longer than the lower yellow line.
When you have got their response, tell the audience that the lines are actually the same size. Use a ruler to prove to them that this is the case.
Q: Why does the lower line look shorter than the upper line?
A: The lower line looks shorter than the upper line because the railway tracks give the impression that they are going into the distance. Our brain knows that parallel lines seem to get closer together as they go into the distance so it assumes that the yellow line will also get smaller in the distance. The fact that it spills over the railway tracks at the top and not the bottom suggests to the brain that the line on top is longer.
Q: Why does the brain assume the tracks are going into the distance?
A: Things often look smaller or closer together in the distance, this is just a natural way that eyes work. Because the brain knows this it makes the assumption that the two vertical lines are parallel (like train tracks) but that they are receding into the distance. Note that it doesn’t assume that the lines are slanting in towards each other (as is actually the case). This is a trick that has been used by artists for hundreds of years to make 2-dimensional pictures look more 3-dimensional.
The Ames Room 3-Dimensional Perspective Illusion [All Ages]
Does the Ponzo Illusion (above) work in 3-dimensional scenes as well?
Are all visual illusions 2-dimensional?
Play the audience the following video. It shows two volunteers in the Ames Room, one appearing to be much bigger than the other. Pause at 1:23.
The Ames Room:
Explain that this room has been cunningly painted to make all the lines and perspective look right but actually it is a very odd room. Because the brain assumes that the room is normal, everything in the room looks very odd in comparison. This is explained in the rest of the video, which you can play the audience now. This part of the video also explains how the Ponzo Illusion (above) was used to create the Ames Room Illusion.
Q: Why does one person look so big and one person look so small?
A: The room is crooked so that one side is much lower than the other but because the floor and walls are painted to make it look like they are straight your brain assumes that it must be the people in the room that are odd.
Q: Why does the brain make assumptions about the Ames Room?
A: Visual information arrives at the eyes as a continuous 2-dimensional stream of information that the brain has to then process to add things like depth and perspective and give us our conscious perception of a scene. To do this, the brain makes use of certain rules such as believing that straight lines moving into the distance will look like they are leaning towards each-other. By knowing that the brain makes these assumptions, artists and illusion-makers can trick it into seeing things that aren’t there – like giants and midgets!
The Kaniza Square: Seeing 3D in 2D [Key Stages 2-4]
Is it possible to see things that aren’t there?
Tell the audience that the brain uses all sorts of rules that shape how we experience the world. Take this very simple visual illusion that is just made up of 4 circles with a corner cut out of them.
If you arrange them like this you will see a ghostly square that is not really there because it disappears when you remove each circle. This is called the Kaniza illusion and it is an example of your brain filling in missing information to make sense of what you see. The reason that you see the white square sitting above four black circles is because your brain assumes that the only way each circle can have a piece missing and be lined up like this is because there must be a white square on top of them.
Play the following video where Bruce explains this. Pause at 1:22.
Even when you KNOW that the lines of the square don’t meet up, you still see this display as 4 whole circles with a white square lying over the top of them. You just can’t help yourself. And the visual system of your brain is telling you that’s what you’re seeing even though another part of your brain knows it isn’t true. The activation in your visual system is exactly the same when looking at this illusory white square as it would be if you were looking at a real white square.
The illusion even works when the display is moving, as if your brain is treating the illusory square as a real object. Play the rest of the video.
This is a great example of the brain taking a shortcut – it assumes that the likelihood of the angles lining up so perfectly by chance is so low that it is better to assume that there is an square there than spend the time trying to work it out.
Q: Why do you see the white square?
A: You see the white square because the brain saves time by paying attention to regularity in the world. The chances of the 4 circles lining up in this manner by chance is very low while the chance of a white square obscuring part of our view of the circles is relatively high. Therefore your brain concludes that the square option is more likely and feeds that assumption to your conscious awareness.
Q: How real is the illusion to your brain?
A: Brain scientists have measured the activation of the visual part of the brain and shown that there is no difference for the visual part of your brain between seeing this illusory square and seeing a real white square. When looking at the Kaniza illusion, the same neural networks fire as when you look at a real white square, even though you know it isn’t true!
The Necker Cube [Key Stages 2-4]
Is it possible for illusions to change in front of our eyes?
Show the audience the following image of a Necker Cube and ask them to look at it for a long time. After a few moments, ask if it seems to have flipped sideways for anyone. If they are having difficulty it might help to tell them to switch their centre of attention between the two middle corners.
The Necker Cube illusion also works when it is moving. Play the following video to the audience and ask them if they can see the cube flipping from one orientation to another. If they are having difficulty it can help if they blink while watching it.
Q: Why does changing where you look on the Necker Cube help us to experience it flip from one side to the other?
A: Focusing on different parts of the cube make different corners seem to be closer and the rest of the cube therefore ‘flips’ to accommodate that new perspective.
Q: Most people see the cube with the lower-left front face before the other perspective. Why do you think this might be?
A: Nobody is sure – it might be because we are more used to looking down on objects than looking up at them. Alternatively it may be because we tend to process visual information )in the West at least) from left to right rather than the reverse.
Q: Why is the Necker Cube Illusion interesting when thinking about how the brain makes sense of the visual world?
A: Because it shows that it is very difficult for us to see the Necker Cube for what it really is – a collection of 12 2-dimensional lines. Instead, the bain seems to impose a 3-dimensional structure onto it. It also seems to be very difficult, if not impossible, for the brain to perceive both perspectives of the cube at the same time.
The McGurk Effect [All Ages]
How does the brain put different types of sensory information together?
Are there any illusions that use senses other than vision?
Tell the audience that we are so dependent on our vision that it can shape not only where we hear a sound coming from but also what we hear. For example, when we learn language, we watch people as they are speaking and remember the shape their mouths make when they make different sounds. We remember these patterns of sights and sounds and they become combined in our brains.
The McGurk Effect:
Ask the audience to watch this video then ask them to put their hands up if they think the man is saying “DA_DA_DA_DA_DA”. Now ask them to close their eyes and play the video again. Ask how many people think the man is now saying “BA_BA_BA_BA_BA”?
Explain that actually the man is saying “BA_BA_BA_BA_BA”. Because your brain sees the mouth making the sound for “GA_GA_GA_GA_GA” but hears the noise “BA_BA_BA_BA_BA” it tried to map the two onto each-other and the solution it comes up with is something in between the two – “DA_DA_DA_DA_DA”. This is evidence again that the brain is constantly processing information that comes in rather than delivering it directly to your conscious experience.
Q: Why did it look like the man was saying ‘DA-DA-DA’?
A: It looked like the man was saying ‘DA-DA-DA’ because the mouth shape did not match up with the sound he was really making. As a compromise, the brain chose a sound that was somewhere between that being produced (BA-BA-BA) and that which the mouth was making (GA-GA-GA).
Q: What does this illusion tell us about the brain?
A: This illusion shows us that the brain is constantly recreating the world for our conscious experience, taking the information that is coming in and reformulating it into a coherent and sensible story. Our past experiences of our world are so powerful in shaping our minds that even when we know something to be true, an illusion can still force us to experience it in one way and not another.