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Why do these two surfaces in the same shade of grey appear so different?
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...
... 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.
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?
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.
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 it we should be able to create a really strong
illusion by making the meaning of the two tiles very different indeed.
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 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).
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.
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Posted: 2010899@484.45
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stranger
I am truly amazed! 'Seeing is believing' is (or was) my foremost mantra. Now I
am confused, not to say somewhat worried. Am I only seeing what my brain tells
me is there - and not what is actually there?!
Chris Ferguson, Wisbech, England
I found the article very iintersting. About 50 years ago - I'm now 80 - I did
an entire exibit on opital illussions at the library in Wilton, Connecticut
inclcuding a wire cube that the viewer thougt was a square. There were also
depth perception pieces which where flat but seen as 3-dimensianal. In all
there were about 49 exhibits and lots of delighted visitors.
Prof Haji Rudin Salinger, Putrajaya, Malaysia
This suggests to me that a large part of the percieved world and therefore
constructed from experience - I wonder what the world really looks like? I
suppose we have all seen the world in this way when we were babies - but is it
possible to recreate this? How do we achieve "real seeing" as adults?
David Nixon, Hebden Bridge
Very interesting and informative - if you are colour blind how much would this
affect what you see
Barbara, Bamako, Mali
I actually Photoshopped the image of the cubes because I didn't believe it, but
yeah, same colour!
Michael , Belfast
There's a painting on my wall. Even though I know it's a flat canvas splodged
with pigment, it looks like a wonderful sea view with sand, water, birds and
the rest of it. Is this an optical illusion? No, because I'm not misjudging
anything! I'm correctly judging the image be a representation of the sea. I'm
inclined to say exactly the same thing about the "optical illusions" in this
article. For instance, we're not making a mistake when we judge the squares on
the Rubik's cube to be differently coloured. We are correctly taking the flat
image to represent a 3D cube on which the squares really are different colours.
Jonathan Birch, Cambridge, UK
A truly beautiful presentation, and lucidly explained. I admit that in a few
cases I took measurements to check if the images in the sets weren't "enhanced"
to magnify the illusions: they weren't.
Jaroslaw, Cairo, Egypt
Very interesting article but the last example with the two tables is a bad
drawing which is why the brain gets the wrong idea rather than being due to the
brains intrepretation of perspective. Assuming it's a table and therefore has
right angles at the legs then using the legs and the square floor tiles as a
measure, the green table is 5 tiles along it's longest side between the legs
and the red table is less than 4. The same kind of difference applies to the
widths. I agree that the table tops are the same width and length give or take
a bit but the brains interpretation is confused by where we see the distance
between the legs on the floor which are not drawn as they should be so it's a
trick drawing rather than the brains interpretation of perspective.
Paul Orrock, London, UK