# Checker shadow illusion (Same color illusion)

The same color illusion (also known as Adelson's checker shadow illusion, checker shadow illusion and checker shadow) is an optical illusion in which identical shades of gray appear to be different. This illusion is one of many illustrations of the complexity of our visual perception.(New World Encyclopedia)

## Checker shadow illusion (Grey square optical illusion)

The checker shadow illusion is an optical illusion published by Edward H. Adelson, Professor of Vision Science at MIT in 1995. The image depicts a checkerboard with light and dark squares. The optical illusion is that the area of the image labeled A appears to be a darker color than the area of the image labeled B on the 2D plane of the rendered 3D projection. However, they are actually exactly the same color on the 2D plane of the image file (but not necessarily in the 3D projection) which becomes especially obvious if the projected 3D scenery is rendered partially or entirely defective. (Wikipedia)

The squares A and B on the illusion are the same color (or shade), although they seem to be different.

Explanation:
That the two squares are of the same color on the 2D plane can be proven using the following methods:
Opening the illusion in an image editing program and using the eyedropper tool to verify that the colors are the same.
Cut out a cardboard mask. By viewing patches of the squares without the surrounding context, you can remove the effect of the illusion. A piece of cardboard with two circles removed will work as a mask for a computer screen or for a printed piece of paper. Connecting the squares with a rectangle of the same color, as seen below in the middle figure.
Using a photometer.
Print the image and cut out the squares. Cut out each square along the edges. Remove them. Hold them side by side.
Isolating the squares. Without the surrounding context, the effect of the illusion is dispelled. This can be done by using the eyedropper tool in image editing programs, such as Gimp to sample the values of A & B, and to color in the newly adjacent rectangles using the paint bucket tool. (Wikipedia)

The fact that the squares A and B on the illusion are the same color (or shade) can be proven by isolating the squares. Without the surrounding context, the effect of the illusion is dispelled.. Alternately, you can surround the two squares with the same shade or create a rectangle of the same color connecting the two squares.

Proof that the squares A and B on the illusion are the same color (or shade).

Additional images and explanation: web.mit.edu   www.archimedes-lab.org

# Chubb illusion

The Chubb illusion is an optical illusion wherein the apparent contrast of a patterned object varies dramatically, depending on the context of the presentation. (New world encyclopedia)

An object of low-contrast visual texture surrounded by a field of uniform visual texture appears to have higher contrast than when presented on a field of high-contrast texture. This illusion was observed by Charles Chubb and colleagues and published in 1989. The Chubb illusion is similar to another visual illusion, the contrast effect. (Wikipedia)

Low-contrast texture surrounded by a uniform field appears to have higher contrast than when it is surrounded by high-contrast texture.

Explanation:
Lotto and Purves (2001) demonstrated that the Chubb illusion can be explained "by the degree to which imperfect transmittance is likely to have affected the light that reaches the eye." www.mind.duke.edu. Indeed, these observations suggest a wholly empirical explanation of the Chubb illusion.

www.purveslab.net   www.journalofvision.org   www.illusionism.org

# Contrast effect

A contrast effect is the enhancement or diminishment, relative to normal, of perception, cognition or related performance as a result of successive (immediately previous) or simultaneous exposure to a stimulus of lesser or greater value in the same dimension. (Here, normal perception, cognition or performance is that which would be obtained in the absence of the comparison stimulus—i.e., one based on all previous experience.) (Wikipedia)

## General application of Contrast effect

Perception example: A neutral gray target will appear lighter or darker than it does in isolation when immediately preceded by, or simultaneously compared to, respectively, a dark gray or light gray target.

Cognition example: A person will appear more or less attractive than that person does in isolation when immediately preceded by, or simultaneously compared to, respectively, a less or more attractive person.

Performance example: A laboratory rat will work faster, or slower, during a stimulus predicting a given amount of reward when that stimulus and reward are immediately preceded by, or alternated with, respectively, different stimuli associated with either a lesser or greater amount of reward.

The contrast effect was noted by the 17th century philosopher John Locke, who observed that lukewarm water can feel hot or cold, depending on whether the hand touching it was previously in hot or cold water. In the early 20th century, Wilhelm Wundt identified contrast as a fundamental principle of perception, and since then the effect has been confirmed in many different areas. Contrast effects can shape not only visual qualities like color and brightness, but other kinds of perception, including the perception of weight.[4] One experiment found that thinking of the name "Hitler" led to subjects rating a person as more friendly. Whether a piece of music is perceived as good or bad can depend on whether the music heard before it was unpleasant or pleasant.[6] For the effect to work, the objects being compared need to be similar to each other: a television reporter can seem to shrink when interviewing a tall basketball player, but not when standing next to a tall building.
(Wikipedia)

## Simultaneous contrast

Simultaneous contrast, identified by Michel Eugène Chevreul, refers to the manner in which the colors of two different objects affect each other. The effect is more noticeable when shared between objects of complementary color. (Wikipedia)

In the image here, the two inner rectangles are exactly the same shade of grey, but the upper one appears to be a lighter grey than the lower one due to the background provided by the outer rectangles.

Animated images & information of simultaneous Contrast effect at web.mit.edu

## Successive Contrast effect

Successive contrast occurs when the perception of currently viewed stimuli is modulated by previously viewed stimuli. (Wikipedia)

When one stares at the dot in the centre of one of the two colored disks on the top row for a few seconds and then looks at the dot in the centre of the disk on the same side in the bottom row, the two lower disks, though identically colored, appear to have different colours for a few moments.

## Metacontrast and paracontrast effect

One type of contrast that involves both time and space is metacontrast and paracontrast.

When one half of a circle is lit for 10 milliseconds, it is at its maximum intensity. If the other half is displayed at the same time (but 20-50 ms later), there is a mutual inhibition: The left side is darkened by the right half (metacontrast), and the center may be completely obliterated. At the same time, there is a slight darkening of the right side due to the first stimulus; this is paracontrast.