Context and Color Vision / Color Induction


The perceived color of a spot of light on the retina depends on more than the wavelength, intensity and purity of the light. Color constancy is a good example of how the perception can remain the same (constant) even when the physical stimulus varies dramatically. The wavelengths reflected from a blue shirt in bright sunlight (which have a high intensity and are biased toward the long wavelength end of the spectrum) are very different from the wavelengths reflected from the same shirt in dim moonlight (much lower intensity and biased toward the short wavelength end of the spectrum). Yet in both cases, the shirt is perceived as the same color. This is color constancy.

Simultaneous color contrast is another example of how context influences the perception of color. In simultaneous color contrast, identical colors are perceived differently depending on the color that is surrounding it. A light green dot on a dark green background will appear brighter than the same green dot on a lighter lavendar background. Simultaneous color contrast is a type of color induction in which the color perception of the target object is shifted away the surrounding object -- the light green appears lighter than the dark green.

A second type of color induction exists -- assimilation. Assimilation occurs when the color perception of the target object is shifted toward the surrounding.

When these two types of color induction are shown in the same stimulus, the perceived color of the target is shifted in two different directions and can make the same physical color appear very different perceptually. The first activity is a strong demonstration of the opposite effects of assimilation and simultaneous contrast.

The Activity:

Compare the colors of the words "color" and "vision" and the words "is" and "amazing" in the stimulus below. Most people will perceive "color" and "vision" as very different colors -- "color" appears blue and "vision" appears green. Similarly, "is" and "amazing" appear to be very different colors -- "is" appears as magenta and "amazing" as orange. In reality, "color" and "vision" are the same color and "is" and "amazing" are the same color too.

Remove the inducing agents by changing the opacity of the stripes and the opacity of the background. When the inducers are gone, you can see that the pairs of words are identical in color. Which inducer is causing the simultaneous contrast -- the stripes or the background or both? Which inducer is causing the assimilation -- the stripes or the background or both? To help answer the question try setting the stripe opacity to 0 and the background opacity to 1. Then set the stripe opacity to 1 and the background opacity to 0. What happens to the perceived color of the words in each condition?

Stripe opacity: 0 1
Background opacity: 0 1

Based on

Monnier, P. & Shevell, S. K. (2004). Chromatic induction from S-cone patterns. Vision Research, 44, 849-856.

This is another example of the effects of context on color perception. The two rings in the stimuli are physically the same color. If you change the color of the gradient (the blue blob in the center) toward 0, you can easily see that the rings are identical.

Gradient opacity: 0 1

If you still don't believe it, capture the screen image (alt-Print Scrn in Windows), open an image editor (e.g. Paint in Windows) and paste the screen image. Save the image in a lossless image format (e.g. BMP or PNG in Windows.) Visit the image colors page, click the "Choose File" button, select the image you just saved, and click the "Load Image" button. Mouse over the various parts of the image and note the representations of the color of the pixel under the tip of the mouse.