Optical Illusions: Why seeing Isn’t Always Believing

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We all like to think we can trust our own eyes. But if you’ve ever experienced an optical illusion, you know sometimes things aren’t always as they appear.

Take this image, for example:

duck rabbit optical illusion

Source: Wikimedia Commons

What animal do you see? Ask someone nearby what they see. There’s a chance you both had different answers — most people see either a rabbit or a duck. Neither answer is wrong. It’s a matter of perception, of how you individually experience the image. But why did you see what you did? Why do your eyes see one thing while your brain thinks another?

Take a look at some more famous illusions to find out how your brain works.

Adelson’s Checker-Shadow Illusion

checkers shading illusion

Source: http://web.mit.edu/persci/people/adelson/checkershadow_illusion.html

Your brain makes a lot of assumptions when it’s processing visual information. Take a look at the following image. Which square is lighter in color: A or B?

You probably said B, right?

Look again:

checkers shadow illusion explained

Source: http://web.mit.edu/persci/people/adelson/checkershadow_proof.html

They’re actually the same exact shade of grey. So why did one look lighter the first time?

Your brain looked at this two-dimensional picture and interpreted it as three-dimensional, with lighting and shadows to consider. There is no real shadow in the image, though. You see the green cylinder and your brain assumes that it’s casting one.

But your brain also knows shadows can be misleading, making colors look darker than they are, so it uses a few tricks to ignore shadows and judge the color of the image.

One is local contrast. Square A is a dark square surrounded by light squares. Square B is a light square surrounded by darker squares, so even in shadow it looks lighter than normal.

Also, your brain is aware that shadows often have fuzzy edges, so it ignores these and focuses on the sharp edges of the squares when determining their color.

The Hermann Grid

Look carefully at the following grid:

hermann illusion

Source: http://psylux.psych.tu-dresden.de/i1/kaw/diverses%20Material/www.illusionworks.com/html/hermann_grid.html

In the white space between the black squares, you probably noticed small grey dots flashing in the intersection. There are no grey dots in this grid, so why do you seem them?

Try to imagine the white space as being made up of two parts: The space in the very middle of each intersection where the corners of four squares meet and the bands of white between each pair of black squares that are larger and receive more light.

A physiological limitation called lateral inhibition causes areas with bright surroundings to appear darker, while areas with dark surroundings appear brighter.

You have receptors all over your retina, located in the back of your eye. If you shine a light on just one receptor, you get a strong response. Shine lights on multiple receptors, and that response gets weaker.

Since an intersection has light shining from four places around it, it looks dimmer. A band only has light shining from two other places around it, so it appears brighter.

Muller-Lyer Image

which line is longer illusion

Source: https://www.rit.edu/cla/gssp400/muller/muller.html

Which of these vertical lines is the longer one?

Trick question — they’re the same. The left one might look longer, but hold a ruler up to each line and you’ll see otherwise.

Nobody actually knows for sure how this optical illusion occurs, but there are several theories. For example, it may have to do with your depth perception, illustrated by the inside and outside corners of a building.

depthe perception illusion

Source: https://www.rit.edu/cla/gssp400/muller/muller.html

Other arguments say the line with outward slanting fins appears longer because its fins lead your eyes away from it, while the other line has fins that lead your eyes back in, causing it to appear shorter.


Bonus: A Real World Example

Now that you’ve witnessed how your eyes can play tricks on you, it’s time to test this out for yourself. Skoda wants to know if you can pay attention. Are you up for the challenge?

How well did you do?

If you lacked attention span, don’t worry. Cars are meant to be driven, not to be show pieces.

Just think, there are still countless other optical illusions to ponder and enjoy. Our eyes, and our brains, are still mysterious things, and we’re learning more about them all the time. What are some of your favorites?


nicholsMegan Ray Nichols loves discussing the latest scientific discoveries with others on her blog Schooled By Science. You can follow her on twitter @nicholsrmegan