The science of optical illusions

In the left image, the black and white spaces in the central pin-wheel seem to blend more than in the right image - though the two are the same

Optical illusions are more than just a bit of fun. Scientist Beau Lotto is finding out what tricking the brain reveals about how our minds work. Here he explains his findings.

Sight, hearing, taste, touch, smell. We believe what our senses tell us but most of all we trust our eyes.

But our brains are extraordinarily powerful organs.

Without us realising it, they are instantly processing the information they receive to make sense of the world around us.

And that has been crucial to our evolution.

Jungle scene

Take colour. Why do we need to see in colour? The next two images will show you why.

Here you see a black-and-white version of a jungle scene. Try to find the predator that's about to jump out at you. If it takes you more than a second, you are dead. Why is it so difficult to find? Because you are only seeing the surfaces according to the amount of light they reflect. Now click to the next image...

BACK {current} of {total} NEXT

... this time in colour. Now you'll probably see the panther immediately (in the lower right corner).

Why is it so easy this time? The reason is because the image shows the surfaces according to the quality of light they reflect (not just their intensity).

In other words, your brain has a lot more information to go on in making its decisions.

Please turn on JavaScript. Media requires JavaScript to play.

So colour enables us to see a greater number of similarities and differences between objects, which is necessary for survival.

What is amazing about what you have just done (finding the panther in the coloured image) is that it is so easy for you.

But while it seems so easy, our best computers are hopeless at doing what you have just done.

Understanding how we see is one of the main aims of brain science (called neuroscience). And illusions hold the key to answering this question.

Brightness illusion

Below we have two physically identical squares. Not surprisingly, they also look the same. Explaining vision would be easy if all we had to do is see the image that falls onto the back of the eye (called the retinal image). But we don't.

In fact we never see what our eyes see. That's because the eyes have very little to do with what we see. This is good news: an image of the world is very different from the world itself.

For instance, the retinal image has only two dimensions, whereas the world of course has three. The retinal image is upside-down, but we see the world right-side-up. So what happens if I change the context surrounding the squares, but not the two squares themselves?

BACK {current} of {total} NEXT

Toggle the image and the two identical squares now look different.

And yet all we've done is put them on different backgrounds. As a result, the small square on the dark background looks lighter than the one on the light background.

This is called the "brightness contrast illusion", which proves that context is everything when it comes to what we see, even when seeing the simplest qualities of the world, namely lightness.

But why is context everything?

Table and shadow illusion

Here we have two, smaller versions of an identical brightness contrast illusion - one on the right and the other on the left.

In both cases the tiles on the dark backgrounds look lighter than the tiles on the light backgrounds. So far so good. Now let's see if we can change the strength of these two illusions by changing the overall scene.

BACK {current} of {total} NEXT

Notice that the illusion on the left is now much stronger. In other words, the tile in shadow under the table looks much brighter because the brain thinks it is in shade. The tile to the right looks as if it is under bright light so the brain assumes it is darker and tells us so.

On the other hand the illusion on the right is now much weaker. In other words, the two tiles, one on a black stripe the other on a light stripe, look nearly identical because the brain is interpreting them as two similarly reflective tiles under a shared light source.

This shows that we see illusions because the brain doesn't actually want to see the image on your eye, but to see the meaning of that image and here it finds that in the context of the table and the light from the window. And that meaning - and this is really important - is created from experience.

Cube illusion

Here we have two tiles that are identical in their colour. But what happens if we change their context in a specific way? Now if we're right that what we see is the meaning of an image, then we should be able to create a really strong illusion by making the meaning of the two tiles very different indeed.

BACK {current} of {total} NEXT

In their new context, the two physically identical tiles do indeed now look very different.

Why? The information in the image strongly suggests that the dark brown tile on the top now means a poorly reflective surface under bright light, whereas the bright orange one at the side means a highly reflective surface in shadow.

So you see them differently because your brain thinks they have a different meaning - given the rest of the information in the scene.

Table illusion

What's true for seeing colour is also true for seeing form and shape. In fact it's true about everything we see. When you look at this image, you are aware of two very differently sized tables.

The one of the left seems a lot longer and thinner than the one on the right. What if I tell you that the red table is simply the green table on its side, in other words that the dimensions of the two table tops is identical.

It is actually true. The only real difference between the tables is the angles at their corners (other than their colour, which is irrelevant in this case).

BACK {current} of {total} NEXT

The two red and green lines are the same length. The length of the red table is the same as the width of the green table and vice versa.

So why do they look so different? Because your brain takes the image on the retina and creates what it sees according to what the information would have meant in the brain's past experience of interacting with the world.

In this case the angles suggest depth and perspective and the brain believes the green table is longer than it is while the red table appears squarer.

The beautiful thing about illusions is they make us realise things are never what they seem, and that our experiences of the world shape our understanding of it.

R Beau Lotto is a lecturer in neuroscience at University College London. All images supplied by R Beau Lotto. See more at the Lotto Lab website

His illusions feature in Horizon: Is seeing believing? on BBC Two at 9pm on Monday 18 October or afterwards via BBC iPlayer.

You can see more illusions from the programme at the BBC Science website.

Response to the above comment from Beau Lotto: There are a number of 'cues' being used in this image of the tables. The idea that it's a 'bad' drawing is not accurate (unless you're just referring to aesthetics, in which case you might of course be right). Actually, the image - or more accurately the combination of information presented in the image - is designed to make the projections of the two tables statistically consistent with your past experience of two tables of different spatial dimensions. If this weren't the case (ie, the drawing was 'bad' in the sense of conflicting information), then - as the argument for why we see what we do goes - the illusion would decrease, not increase - see for instance the lightness illusion above with depicted shadows). You are right, however, that decreasing spatial frequency of the floor tiles is an important bit of information about depth (though of course there is no depth in this image, just one's perception of it). A much stronger cue, however, is the angle at each corner of the table tops. You can appreciate this by simply removing the background all together (see image below). When you do this, the illusion persists (though is weaker - again consistent with the argument being made).