Laboratory for Color Perception

As we've noted, species who have the ability to perceive colors are at some adaptive advantage. The above images indicate how difficult it would be to detect the red berries without color vision.

1. Color Contrast; Hue, Saturation, and Lightness

Here are a couple of links to sites that provide information about color contrast, hue, saturation, and lightness:Link 1, Link 2. Write yourself brief notes on your worksheet regarding these concepts. Janet Lynn Ford has developed a very helpful site that explains color representation (click on the color link at top of page) from an artist's or web-page developer's perspective.

2. Subtractive and Additive Color Mixing

If you think that mixing blue and yellow give you green, you are familiar with subtractive color mixing. Subtractive color mixing involves pigments/paints. Blue paint absorbs a range of wavelengths (e.g., those we would call yellow, orange, red), reflecting the shorter wavelengths (e.g., those we would call violet, indigo, blue, green, but primarily those around "blue"). Yellow paint absorbs a range of wavelengths (e.g. those we would call violet, indigo, blue), reflecting the longer wavelenghts (e.g., those we would call green, yellow, orange, red, but primarily those around "yellow"). When we mix those two paints, the wavelengths that they would each reflect (and not absorb) would be those that give rise to a sense of green.

Additive color mixing, on the other hand, involves lights rather than pigments. You can use the Exploratorium Mix-and-Match demonstration to determine the combinations of the three lights that are needed to produce matches to various colors. [Be sure that you've set your monitor to millions of colors! If you're using an Intel-based Mac, you may need to set your browser to run in Rosetta mode to allow the Shockwave plug-in to run.] Click on the three colored squares to set a new background color. Then adjust the three lights until you can match the background color (the inner circle will "disappear"). To check the accuracy of your match, click on the three colored triangles.

3. Negative Color Afterimages

The opponent-process theory of color vision explains a great deal about how color vision occurs, including simple color afterimages. Because of the opponent processes, blue gives rise to a yellow afterimage (and vice versa), red gives rise to a green afterimage (and vice versa), and black gives rise to a white afterimage (and vice versa). Go to the Illusionworks demonstration of color afterimages, or, if the image doesn't appear, go to the Exploratorium demonstration (Bird in a Cage).

Next, try the McCullough Effect demonstration. Celeste McCullough developed this effect in the 1960s and people continue to study and develop explanations for the effect. However, note that explanations cannot be based on simple "fatigue" of receptors. Can you explain why not?

4. Seeing Colors "Inaccurately"
Simultaneous color contrast tells us that colors are seen in context--and that context affects our color perception. Dale Purves and Beau Lotto have developed a number of clever demonstrations. In fact, we've seen some in class and in earlier labs. For now, look at the demonstration of a cube of colors (bottom of color demonstration choices). Click on the arrow below the figure to see the squares that appear as identical shades of gray when seen in isolation.

5. Seeing Colors that Aren't There: Color spread

In the two examples you see below, you should notice a number of different effects. First, in looking at the figure on the left, you should experience noticeable color spreading in some (but not all) of the examples. Describe your perceptual experience with the four examples. (Keep in mind that the figures all occur on a plain white background!) Then compare the effects seen in the figure on the right. Note that all that I've done is to remove some of the non-colored parts of some of the examples. Nonetheless, your experience of color spread should be greatly curtailed. Why?

In the two examples below, you should again notice greater color spreading in the figure on the left. Why?

As a potential aid in thinking about what might be happening, I've presented an illusory square on the left below. Then, on the right you can see a square with lots of color spreading.

In the figure on the left below, the color spreading should appear to be minimized. Then, in the figure on the right, even though it still has little circles added, the color spreading should be pronounced. Thus, the placement of the little circles is somehow important.

Now, take all of these examples together and see if you can weave an explanation for the principles that underlie color spreading.

[Most of these color-spreading figures were prompted by Donald D. Hoffman's excellent book Visual Intelligence: How We Create What We See. Hoffman, in turn, was compiling many of the examples from prior work by Hans Wallach, Christoph Redies, Lothar Spillmann, and others.]

5. Seeing Colors that Aren't There: Benham's Top

Try out the demonstration of Benham's top to see the faint illusory colors that can emerge just from rapid alteration of black and white stimuli. Mark Newbold has developed related demonstrations that gives credit to Fechner: Demo1 and Demo2.

6. Color Vision Deficiencies

Color vision deficiencies play an important role in supporting both Trichromatic Theory and Opponent-Process Theory. How might dichromats (people with only two cone systems) support Trichromatic Theory? Opponent-Process Theory?

To learn more about color vision deficiencies, read the article in Wikipedia (Color Blindness). To see other examples of how the world would look to people with various color vision deficiencies, check out this site. (Protanopes are missing the L-cone system, deuteranopes are missing the M-cone system, and tritanopes are missing the S-cone system.)

To give you a sense of how color vision deficiencies are detected, try out the Ishihara color plates (keep in mind that these images will vary across computer monitors, so they are not an accurate test of color vision).

7. The Stroop Effect

This demonstration really has less to do with color perception than it has to do with color naming and speech processes. On the other hand, if you've never experienced the Stroop effect before, it's well worth the time to go through the demonstration.

For the following screen, your task is to name (as quickly as you can) the color in which the words appear (not the words themselves). Go to the Stroop test!