Horizon, Host: Ted Simons

November 23, 2010


Host: Ted Simons

Optical Illusions


  • Optical Illusions are fun to look at, but they can also give scientists insight into how our eyes and brains function. Researchers Stephen Macknik and Susana Martinez-Conde of Barrow Neurological Institute will explain what they’ve learned about human eyesight and brain function from studying how we see optical illusions.
Guests:
  • Gary Stuart - Senior Policy Advisor for ASU’s Sandra Day O’Connor College of Law
Category: Science   |   Keywords: optical illusions,

View Transcript
Ted Simons:
Optical illusions are fun to look at but there's a lot of science involved in why they work. Researchers at Barrows Neurological Institute in Phoenix are looking into optical illusions and how the brain perceives the world. I recently talked to Stephen Macknik and Susana Martinez-Conde of Barrows Neurological Institute about their research into optical illusions.

Susana Martinez-Conde:
So these illusions are one of the most important tools to understand how our brain constructs our visual experience, because they disassociate what's out there from the way that we see it. That gives us a handle into the brain mechanisms behind it.

Ted Simons:
And there is a difference between visual illusions and optical illusions?

Stephen Macknik:
That's right. The real world has physical things happening in it. Like, you can have smoke and mirrors, reflections, those are optical illusions. A piece of pencil in a glass of water looks like it's bent, that's refraction of light, an optical illusion. Visual illusions happen because of the way we process information in our brains. We're not saying the real world isn't out there, it really is. But we've never actually been there, none of us have. By studying these illusions we can understand better how it is that we interact with each other and the world.

Ted Simons:
How much do we know as far as how the brain perceives? What do we know and not know?

Susana Martinez-Conde:
The brain has over two dozen areas dedicated to processing visual information. We know more or less well what the first three stages do. The rest is mostly an open question.

Ted Simons:
Interesting.

Ted Simons:
We have a bunch of examples here. I want to get to these before we get too deeply into the conversation. The first is interesting in the sense that we've got square A and square B, and you're telling me they are the same shades of grays?

Stephen Macknik:
If you look at the soda can's shadow, that covers the square 13 labeled B. There is the same amount of light there, and you can convince yourself by putting a mask over this and looking at the two squares.


Ted Simons:
Why do they look different?

Stephen Macknik:
It depends on context for the brain, there's no absolutely black and white. Everything depends on what you compare it with. It doesn't access A and B individually, but places it in the context. Shadow placed by the soda can. It concludes that B actually must be lighter.

Ted Simons:
That plays a part in something as simple as reading because you've got a white background with black print?

Susana Martinez-Conde:
It does. If you look at a newspaper, and under artificial light, and take it outside, in both cases you see black letters on a white background. However, the reflections by the black letters outside is more than is reflected by the white paper inside, yet you see black letters on a white background in both cases.

Ted Simons:
Interesting. This next one looks a little bit like a Rubik's cube but it's different. You're telling me again these two squares -- which are the two you say are the same color?

Stephen Macknik:
So the middle square inside the shadow --

Ted Simons:
The bright one there.


Stephen Macknik:
That's right. And the middle square on the top surface.

Ted Simons:
Dark one.

Stephen Macknik:
That's right. Those two shades of brown are identical on your retina, but you see them as different because you're seeing them under different lighting conditions. These brightness and color illusions are incredibly important for us and allow us to recognize the same object as looking -- as being the same inside and outside, and in different lighting conditions.

Ted Simons:
Again, why is our brain -- I know mine is -- why are our brains going, those aren't the same colors?

Susana Martinez-Conde:
It wouldn't be very useful to know the exact color or wavelength of each object in the environment, whether there is shadow or direct light. That would mean that the same sweater that you wear inside and outside would change colors when you step in and out of the house. It would be very confusing to know what fruit is ripe, what isn't. You want to discount environment.

Ted Simons:
Yeah, interesting. All right.

Stephen Macknik:
Just a follow-up on that point. If you're inside the cave with your child, you walk outside in direct sunlight, physically that child looks very different, different colors and brightnesses of light. It looks like a different person. But you recognize it as your child so you don't eat it or something. So you know it's the same object, but it's an illusion because they are physically different.

Susana Martinez-Conde:
If I can suggest something, it used to be thought by most people in the field of vision research that illusions are errors of perception. In other words, the brain gets it wrong. But more and more we're seeing that illusions play an active value, and they are fundamental at all processes of our mechanisms that help us survive.

Ted Simons:
This next one reminds me of the Spirograph people had as kids. The deal is if you move your eyes around this, you're saying -- and I hope it translates on TV as well as it does in print. Everything's moving, but if you focus on one of the black Dots in the middle, everything slows down. Why?

Stephen Macknik:
Your motion neurons in your brain, they see when you move your eye around, they are seeing the different shades of gray. We're seeing it in color here. If you think about them in shades of gray, it goes from dark to light and dark to light. The motion neuron sees that as motion in one direction. When they see changes in the opposite direction, they see it as well in the opposite direction.

Ted Simons:
Why does the caveman, who just saved me from eating my kid, why is this?

Susana Martinez-Conde:
Not all illusions have an adaptive value, per se. But there are processes in your brain that do have an adaptive value to you. The neurons in your brain are wired up, it just has a side effect that sometimes you're going to see motion when there isn't. The fact is this is -- if you want kind of an artificial stimulus with the repetition of the little bits and pieces all in the same sequence, it would be very rare, almost impossible to see this in nature. You're not going to get deluded.



Ted Simons:
That's encouraging to hear. The next two are similar in the sense that they show things, it's the exact same -- this is the exact same photograph of the leaning tower of Pisa. Why does the one on the right look like it's angled more than the one on the left?

Stephen Macknik:
This has to do with the way we see depth in the real world. When we see things like two towers, we're standing at the base and looking up, we see two towers completely parallel going up. They would converge in the distance. Or looking at train tracks, you see they converge in the distance when they are parallel. What's happening here with two photographs, which we haven't evolved to see, those are two parallel things that don't actually converge. They fully are parallel, though they look like they are receding into the distance. Your brain interprets that as they must be diverging.

Susana Martinez-Conde:
It has to do with the mechanisms the brain has evolved to perceive distance and volume. The fact that we can see distance in a painting, a piece of art, and how the painters take advantage and develop the rules of perspective, this is because the same mechanisms that makes us see three dimensionality, this is how our brain operates.

Ted Simons:
You mentioned the railroad tracks. Again, you're looking at two photographs that are exactly the same, but the one on the left just looks so different.

Stephen Macknik:
That's right. It's the same concept. So you see the parallel train tracks in either one of the images converges into the distance. If you take these two identical photographs of the train tracks, they don't converge into the distance, they are parallel, physically, geometrically. That means to your brain they must be diverging. This is the kind of concept that scientists at McGill realized was critically important to how we see depths.



Ted Simons:
What kind of response are you getting to this research?

Susana Martinez-Conde:
Its visual illusions, they are not only from the mental to studying vision, but they are also important to understand visual disease. That's part of what we try to do at Barrows Neurological Institute, to apply the basic discoveries that we do in our laboratories, to translate them to disease and to the clinic.

Ted Simons:
Again, the response in what you're learning to this kind of research.

Stephen Macknik:
It's been tremendous. One of our articles on this stuff, our online articles with "Scientific American," we have a monthly column, was the most downloaded article in "Scientific American" history.

Ted Simons:
Thank you for joining us.

Susana Martinez-Conde:
Thank you for having us.

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