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Illusions

Below are free to use, class-friendly printouts that provide an optical illusion and an explanation of their behavior. These were originally distributed by ARVO at the USA Science and Engineering Festival.

 
Blind spot
How do you test for blindspots?

Close your left eye and stare at the cross mark on the other side of this page with your right eye. Off to the right you should be able to see the spot. Don’t look directly at it; just notice that it is there, off to the right. If it’s not, move farther away. You should be able to to see the dot if you are a couple of feet away.

Now slowly move toward the piece of paper while still looking at the cross mark. When you’re approximately a foot away from the paper, the spot will disappear. As you move closer, it will reappear.

Why does this happen?

The point where your optic nerves converge to exit the eye and into the brain is known as the optic disc. This area of the eye has no light-sensitive cells to detect light rays. This results in a break in the visual field known as your “blind spot.”

When the cross mark disappears, you instead “see” a continuous white field. This is not actually the case. Here, you see something the brain is actually making up, sincethe eye isn’t actually sending any information back via the optic nerve.

Why does this matter?

Vision scientists study our blind spots and what effects they have on vision. Their research reveals the brain’s actions. Eye and vision scientists aim to understand whether the brain is actually filling in missing information or simply ignoring things about which is has no information.

Download the handout.

Negative afterimage
How do you test negative afterimage?

Stare at the dots located at the center of the woman’s face on the back of this page between 30–60 seconds, then turn your eyes immediately to the center x of the white image on the right. Now, blink quickly, several times. You are experiencing what is known as a negative after-image.

Why does this happen?

The photoreceptors—primarily the cone cells—in your eyes have become overstimulated and fatigued causing them to lose sensitivity. You normally don’t notice this because tiny movements of your eyes keep the cone cells located at the back of your eyes from becoming overstimulated.

As you shift your eyes to the white side of the image, the overstimulated cells continue to send out a weak signal, so the affected colors remain muted. However, the surrounding photoreceptors are rested, sending out stronger signals, as if we were looking at the opposite colors. The brain interprets these signals as the opposite colors, essentially creating a full-color image from a negative photo.
We consistently experience negative afterimages. For example, when we view a bright flash of light, briefly look at the sun, or are blinded by the headlights of an approaching car at night, we see both
positive and negative afterimages.

Why does this matter?

Vision scientists study why photoreceptors get fatigued and how they recover. These studies help us to understand how people see, and demonstrate how prolonged exposure to screens or reading materials can affect the eyes.

Download the handout.

Cone fatigue
How do you test for cone fatigue?

Stare at the image on the other side of this page for 20 seconds and then look at a blank white area. The after image you see has the opposite colors of the picture you just stared at due to cone fatigue.

Why does this happen?

The eye contains special cells called photoreceptors that detect light. There are two types of photo-receptors—rods and cones. Rods can detect light and dark, and cones are good at detecting colors like red, green, and blue. When you stare at one color for too long, the cells that detect that color will become fatigued. The after image is a result of photoreceptors not being balanced. As the photoreceptors become less tired, which takes between 10–30 seconds, the balance is restored and the after image disappears.

For example, when you stare at something red, your red cones will become tired. However, the cones that perceive red’s complementary color—cyan—are completely rested. Thus, when you look at a blank white area, the after image is cyan.

Why does this matter?

Vision scientists study why photoreceptors get fatigued, and how they recover. These studies help us to understand more about what happens to our eyes due to with prolonged exposure to colored screens or reading materials.

Download the handout.