Optical Illusion: After Images

The phenomenon I studied is called After Images. I initially tested the optical illusion on my computer and it worked perfectly, so I printed it. After printing it, I mounted it on a small piece of cardboard and put a pencil/stick inside it so that it was easier and faster to flip. The illusion did not work; possibly because the printing and paper quality were not the best and did not make a strong enough impact on one’s eyes. I decided to print it on the school’s printer, and now it works a little better. The best result is still obtained when looking at the computer.

This is how it works: just stare at the dot for 30 seconds. Flip to the grey image of the picture, as your eyes adapt to the inverted image you were seeing first, you will see a color image for a few seconds and then it will change to black & white.
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I tested my friend Sunja and he was completely amazed, he said that optical illusions don’t usually work for him but he kept on saying that this was magical. He wanted to try it one more time, because the first time around he didn’t realize what had happened exactly; he just knew something was weird. So I let him try it again and this time he realized what it was. He said he had read about this phenomenon on the back of a cereal box! That was hilarious! He explained it to me as if I had showed him my report!

My roommate who was sitting next to us became interested in knowing what was going on. She is always very curious about what I am studying in Psychology class because she loves this subject. She came over and tested the illusion as well. She could not believe that I had created my own illusion and thought that it was “so cool”. I explained to her how it worked and she sort of remembered about the subject. She is an illustration major, so I told her I would show her how to make the illusion and that it was not difficult at all. She will probably do this to a good amount of her pictures-just to play around-.

I got this idea from a “Black and White Spanish Castle in Color” Illusion in John Sadowski’s website, he claims to be the original creator of this type of illusions. I goggled him, but nothing special came up. I proceeded to look for information of the scientific aspect of it.

Afterimages
An afterimage is an optical illusion of an image that continues to appear in one's vision after the exposure to the original image has stopped. One that we have all experienced without knowing it is the bright glow that floats before our eyes after staring at a light bulb for a few seconds.

Ewald Hering (Karl Ewald Konstantin Hering) was a German physiologist who not only described the Hering illusion, but explained how the brain sees afterimages, in terms of three pairs of primary colors. After-images appear once the original stimulus is removed. The colors revealed are usually the opposite (complementary) colors of the original picture. He explained the opponent process theory, which suggests that we receive information for three opponent color groups: red vs. green, blue vs. yellow, and black vs. white. This opponent process theory says that any visual receptor (cones and rods) that was turned off by one of these colors was excited by its coupled color. Therefore, a green image will produce a red afterimage.

There are two types of Afterimages: negative (inverted) and positive (retaining original color). The exact process behind positive afterimages is unknown but it is related to neural adaptation. Negative afterimages are better understood.


Negative afterimages
Negative afterimages are caused when the eye's photoreceptors, primarily the cones, adapt to the image by over stimulation and lose sensitivity. Normally the eye deals with this problem by rapidly moving, and the “flickering” is later "filtered out" by our brains so it is not noticeable. However, if the color image is large enough and the small movements are not enough to change the color under one area of the retina, those cones will eventually get tired or adapt and stop responding. This can also affect the rods.

When the eyes are diverted to a blank space, the adapted cones send out little signals and those colors remain muted. However, the surrounding cones that were not being excited by that color are still "fresh", and send out a strong signal. The brain interprets the signal as exactly the opposite color.

Positive afterimages
Positive afterimages appear the same color as the original image. They are brief, lasting usually less than half a second, and may not occur unless the stimulus is very bright. Although there is not much information on Positive Afterimages we know that activity in the visual system makes the retinal photoreceptor cells (cones and rods) send neural impulses to the occipital lobe revealing that the experience of a stimulus can vary with the intensity of the stimulus. Usually only very bright stimuli such as the sun produce positive afterimages, and a stimulus, which brings out a positive image, will usually trigger a negative afterimage quickly via the adaptation process. That is why my print out did not work the first time around; the intensity/saturation were not a strong enough stimulus.

So what is the difference between the positive and negative afterimage? A positive afterimage forms only when the backdrop used is the same external object and the negative afterimage forms only when the eyes are shifted away from the external object using a blank backdrop.
While doing this assignment I discovered a variety of optical illusions that blew my mind away. Choosing one was fairly difficult, but I am glad I chose this one. It was easy to make-easier than I though possible-and if you do it correctly it works really well. Even when you are not testing yourself if you are looking at it while showing someone, the result will also affect you.