Reversible Image

What do you see in this diagram?

Ouchi Apparent Motion Illusion

If you look at the left (black/white) pattern you will probably notice that the vertical pattern in the center appears to move relative to the horizontal pattern in the surround.
This diagram was adapted from a design presented by a Japanese artist whose name is Ouchi.

If you see relative motion on the right pattern than you can conclude that the illusion also works with color patterns.

Perhaps not. It might be that if the lumiances of the red and green rectangles were equal then the illusion might not be as strong or prehaps would even disappear.

Necker Cube Illusion

Simultaneous Contrast

The round dots are all exactly the same color gray. I merely created the one on the extreme right and then copied it to each of the darker rectangles.
Clearly, the lightness of the background effects the way in which we perceive the center gray dot. This effect is called simultaneous contrast. Again, as can be seen in other demonstrations, the perception of this kind of display is a result of lateral interactions.

Mueller-Lyer Illusion

Which is longer, the red line or the blue line? Use a ruler to find out the answer. Select the answer to double check on your result

Kanizsa Illusion

This is called a Kanizsa figure after the person who invented it. If you look carefully you will probably see the edges of the entire triangle, even though the triangle is defined only by the notches in the red disks.

Horizontal-Vertical Illusion

Which line is longer, the red one or the green one? Use a ruler to find out. You will probably be surprised.**


**If you don't have a ruler handy, mark the length of one line on a piece of paper and compare it to the other line.

Hermann Grid Illusion

Look closely at this matrix of black squares. What do you see? While scanning over the matrix do you see something peculiar in the intersections of the white crosses formed by the black squares? If you see dark blobs, don't be surprised, that is what most people see.
This figure is called the Hermann grid after L. Herman (1870). The dark blobs can be explained by reference to receptive fields and lateral inhibition.

Just in case you think you are being fooled, try taking two pieces of paper and cover all but two vertical or horizontal rows of black squares. Do you still see the dark blobs when viewing only part of the Hermann grid?

There is a reasonable explanation for this phenomenon. If you haven't yet looked at the receptive field story you should look at it or else the explanation for the Hermann grid won't make a lot of sense to you.

Color Assimilation

Take two pieces of paper and place their edges as close as possible to both sides of one red diagonal. Note that the red square in the middle is the same as all the other squares on that diagonal. Now move your pieces of paper to expose mainly the red squares on the other diagonal. On this diagonal all the red squares, including the center one, are again the same color red. Because the center square is common to both diagonals this exercise proves that all the red squares in both diagonals are exactly the same red color. Yet, when the pattern is seen as a whole, the two red diagonals appear different from each other.

Step Tablet

Take two small pieces of paper and place them on the video screen so that you can see only one of the steps. You will probably notice that this step appears uniformly bright across its horizontal extent. However, when you view all the steps at once, each one appears lighter on the left and darker on the right.
The dashed red line represents the brightness function of the average physical luminance of each step. The solid line represents the apparent brightness of the steps.

Boynton Illusion

Examine these figures closely. Note the yellow areas on the left and on the right. Now step back from your computer screen about 6 or 8 feet and look at them again. Do they still appear as they did close up? Most people would probably see the shape of the right yellow area defined by the black squiggly line.
It is generally assumed that luminance is mediated primarily by the long and middle wavelength cones. There is little luminance contrast between the yellow and white areas. This implies that the excitations of L and M cones are approximately equal and the isoluminant contrast is mediated by the short wavelength cone system.

The filling-in phenomenon confirms that the spatial resolution of the short wavelength cone system is poor. Also, it demonstrates that areas of color, mediated by the y-b opponent system tend to be constrained by contours mediated by the long and middle wavelength sensitive cones.

Boynton predicted this illusion before he produced it for the first time.

Blind Spot (Optic Disk)

Close your right eye and look directly at the number 3. Can you see the yellow spot in your peripheral vision? Now slowly move towards or away from the screen. At some point, the yellow spot will disappear.
Note how far you are from the screen when the yellow spot vanishes. Repeat the experiment looking at a larger and then again at a smaller number. Did you notice the difference in distance from the video screen when the spot disappeared?

Afterimages

Stare at the plus sign on the left for about 30 seconds. As you do this you probably will see some colors around the blue and green circles. After about 30 seconds, shift your gaze to the plus sign on the right. What did you see?

You probably saw a yellow and desaturated reddish circle. Did you notice that they were not there before you tried this experiment? Furthermore, unlike the colored circles on the left, the ones you saw on the right moved around as your eyes moved, which proves that these perceptions are the result of afterimages in your visual system and not some trick of the stimulus presentation on the right.