Look at the drawing below. Which square is darker, square A or square B?
Incredibly, the answer is that squares A and B are the same color, but your brain perceives them to be different based on surrounding color and shadow information. What's amazing about this illusion, is that even after you learn that the 2 squares are the same color, your brain continues to perceive square A as significantly darker than square B. This proof drawing helps your brain believe what your eyes aren't seeing:
Why can't we see the colors as they actually are, even after we've been told, and shown, that they're the same? It largely has to do with our brain's ability to interpret a 2-D drawing as a 3-D representation. We expect the cylinder to cast a shadow on the checkerboard- and because we are used to compensating for shadows in "real life" we perceive and compensate for the shadow we assume has been created by the cylinder. However, the creator of the image has manipulated the colors and the shadow to give the illusion of the shadow–without there actually being a realistically rendered or predictable shadow. In addition, because we interpret a checkerboard pattern in an expected way (alternating light and dark squares), due to the respective positions of squares A and B, our brains automatically categorize square A as a dark square and square B as a light square, despite their actual color.
Here is another variation on Adelson's illusion, which is also called the Checker Shadow illusion.
In this drawing, the bottom circle appears to be light orange, while the top circle appears to be brown. Of course, they are actually identical in color, just like the gray checkerboard squares.
Can you see the three-dimensional image in the picture below?
It is easier to view a bigger image. Click the image to enlarge.
If you can't find the 3D image, you're not alone. This picture is an example of a visual illusion called an autostereogram, and it can take practice to train your eyes to see them. Once you learn how to "unfocus" your eyes, a 3D image will appear to pop out of the "flat" pattern behind it.
If you still can't see it after reading the instructions below, scroll down to the bottom of this page to reveal the hidden image.
How Human Vision Works
Humans have binocular vision, meaning both eyes work together to see. Visual information enters each eye and travels through the optic nerve to the visual cortex located in the brain's occipital lobe at the back of the head. The optic nerve has two branches—one coming from each eye—that feature a half crossover at the optic chiasma. This means that the left visual cortex receives signals from the left side of both eyes, and vice versa: The right visual cortex receives information from the right side of both eyes. The brain takes these signals and blends them into a single image.
Since the eyes are set apart in the head, each eye sees a slightly different image. You can test this out by looking at a nearby object. Close your right eye and view it only with your left, and then switch so your left eye is closed and you're looking at the object only with your right. If you switch back and forth between the eyes quickly, you'll notice a slight shift in the object's location.
How Autostereograms Work
The brain uses the differences between the images from each eye (along with other cues) to perceive depth. When you look at something that features a repeating pattern—like an autostereogram—the brain can have a hard time matching the images from the eyes accurately. The 3D image in an autostereogram will pop out when you successfully match parts of the image that it wouldn't pair together when viewing it normally. When this happens the brain can be tricked into seeing an image that appears to be in front or behind the real image.
How to View Autostereograms
There are many different methods you can use to view autosterograms, and different techniques will work for different people. The most important thing to remember, however, is that you must let your eyes relax and "unfocus." A good way to teach yourself how to do this is to draw two large dots on a piece of white paper about two inches apart. Hold the paper close to your eyes and look "past" or "through" the paper; that is, don't cross your eyes to try and keep the dots in focus. Let the dots get blurry.
Once the dots are blurry, slowly move the paper away from your face. Concentrate on keeping your eyes the way they were when the paper was near your eyes. Eventually, you should see three dots instead of two, the middle dot being an illusion made up by your brain.
Remember how you saw the "third dot" above and try the same technique with the autostereogram: Hold it close to your eyes, let them relax, and don't focus on the pattern. Slowly move the picture away from your face. The pattern should "blend" together and the 3D image should emerge.
Click the question mark to reveal the image hidden in the autostereogram.