Chapter 26. Opponent Processes in Vision

Learning Objectives

cones
photoreceptor cells in the eye specialized for detailed color vision in bright light
rods
photoreceptor cells in the eye specialized for monochrome vision in dim light
opponent-process theory
theory proposed by Hering to explain color vision as the output of special channels containing opposing color receptors (red vs. green, yellow vs. blue, and black vs. white)
trichromatic theory
color vision theory proposed by Young and Helmholtz; it claimed that there are three different types of color receptors in the retina, each most sensitive to a different wavelength of light
photoreceptors
special cells in the retina that detect light
wavelength
the width of a light wave, measured as the distance between peaks; related to the hue (perceived color) of a light
retina
light-sensitive inner surface of the eye, containing rods and cones
Positive and Negative Correlations
true
true
true
a color pallette of the three opponent-process channels with two rows of color squares: top row (left to right) white, green, yellow; bottom row (left to right) black, red, blue
Learning Objectives:

Distinguish the opponent-process theory of color vision from the trichromatic theory of color vision.

Apply the opponent-process theory to predict the perceived color of an afterimage.

Review

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On the left side of the screen is a picture of three cone cells. These cells are cone shape and labeled for their respective colors: R for red, G for green, and B for blue. On the right is a graph with three bell-shaped curves. The horizontal X axis shows Wavelength in nanometers ranging from 350 to 650 in increments of 50. The vertical Y axis shows the relative response of the cones to the wavelengths. A blue curve representing short-wavelength (blue) cones starts low on the relative response axis at around 375 nanometers in wavelength, increases to 430 nanometers at a high relative response rate, and then decreases back down. A green curve representing medium-wavelength green cones starts low on the relative response axis at around 450 nanometers in wavelength, increases to 530 nanometers at a high relative response rate, and then decreases back down. A red curve representing long-wavelength red cones starts low on the relative response axis at around 475 nanometers in wavelength, increases to 570 nanometers at a high relative response rate, and then decreases back down. The red curve overlaps the green curve on the right side.

1. As the Young-Helmholtz trichromatic theory predicted, the human retina has three types of cones that respond differently to various wavelengths of light.

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There are two images of a flag. The first flag has a red vertical stripe, a white vertical stripe with a red maple leaf in the middle, and another red vertical stripe. This is the correct Candian flag. An arrow points from this flag to the second flag. The second flag has identical shapes but the colors have been changed to green in place of red and black in place of white

2. But the trichromatic theory can't explain why the afterimage of an object is the negative, or complement, of the color of the original object. If you stare at a Canadian flag for 30 seconds and then look away, you won’t see a white and red afterimage, but, rather, a pale black and green afterimage. Instead, you will see a pale black and green image.

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There are three pairs of colors. Each pair has colors on two lines that merge together. The first pair is red and green. The second pair is blue and yellow, and the third pair is white and black

3. To explain afterimages, Ewald Hering proposed the opponent-process theory, claiming that the visual system is organized in opposing channels: red versus green, blue versus yellow, and black versus white (or dark versus light). The channels are constructed by wiring together different types of photoreceptors (cones and rods).

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A gray circle appears on the left. Two horizontal lines extend from the gray circle to the right. The top line has a red bar, while the bottom line has a green bar. The lines merge and extend to another gray circle firther to the right

4. According to the opponent-process theory, when you stare at a neutral gray object, within each red-green channel the red cone and green cone are balanced, and you perceive the gray object accurately.

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A red circle appears on the left. Two horizontal lines extend from the red circle to the right. The top line has a red bar, while the bottom line has a green bar. The lines merge and extend to another red circle further to the right

5. But when you stare at a red object, the red cone is strongly activated, while the green cone is resting.

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Two horizontal lines extend from left to right. The top line has a transparent red and white striped bar, while the bottom line has a green bar. The lines merge and extend to a transparent green circle further to the right

6. This strong activation causes the red cone to become fatigued. Then, when the red object is removed, for a moment the output of the green cone is higher than the output of the red cone, so you perceive a greenish afterimage of the object.

Practice 1: Demonstrating Afterimages

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Practice 1: Demonstrating Afterimages

When you stare at a patch of color for several seconds and then look away, an afterimage appears. This afterimage is generally not the same color as the original patch, but instead is a negative image, as predicted by the opponent-process theory.


Let’s try it. After you read these instructions, stare at the X in the center of the circles, trying not to move your eyes at all. After 20 seconds, the colored circles will disappear, allowing you to examine the afterimage produced by each circle. (Blinking your eyes once or twice might intensify the afterimages.) When you are ready to begin, select the START TIMER button to start the 20-second timer, and immediately focus your eyes on the X.

Red Green × Yellow Blue

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Practice 2: Predicting Afterimage Colors

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Practice 2: Predicting Afterimage Colors

Hering's opponent-process theory proposed that the visual system is organized in opposing channels: red versus green, blue versus yellow, and black versus white, as this illustration shows.

Given what you now know about opponent processes, can you predict what colors you would perceive in the afterimage of this flag? When you are ready to begin, select the Start Timer button to start the 20-second timer, and immediately focus your eyes on the white X at the center of the flag.

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Practice 3: Altering Color Perception

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Practice 3: Altering Color Perception

When you finish reading these instructions, select the START TIMER button and spend the next 20 seconds staring at this unusual photo of the Taj Mahal in India. Try to keep your eyes on the entrance door at the center of the photo. When the timer reaches zero, the colors in the photo should look more normal.

Opponent-process channels can create afterimages of objects that don’t exist, and they can also influence the color of real objects seen after the viewer has experienced strong color stimulation.
Let’s try to demonstrate that now.

A picture of the Taj Mahal in India appears with the building in dark gray and black, the sky is orange, the small trees lining the rectangular pond that leads to the building are purple, and the sidewalks appear blue-green. A start timer button appears below the image. once 20 second timer has expired an aferimage of taj mahal picture over a black and white image version same there to be color on with green bushes, yellow grass, red-orange brick sidewalks, blue sky.
Mlenny Photography/Getty Images

Quiz 1

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Quiz 1

For each of the original color circles at left, drag the appropriate simulated afterimage color from the box at right and drop it on the gray area next to the original circle. When all the circles have been placed, select the CHECK ANSWER button.

A box on the left side of the screen has eight circles. On the left of the box is the label original colors. The circles on the left of the box are, from top to bottom: blue, green, yellow, and red. From each circle is an arrow pointing to a gray circle. Next to the gray circles is the label afterimages. A box on the right side of the screen has four circles that simulate afterimage colors: blue, green, red, and yellow. These colors are light and transparent. This page contains a matching activity. You need to match original colors with an appropriate simulated afterimage color. Choose the afterimage color using TAB button. Then enter one of the following numbers to select the corresponding original color.
Select the NEXT button and move to Quiz 2.
Perhaps you should review the opponent-process theory and channels that can create afterimages of objects.

Note: The colors that you actually experience in afterimages may vary depending on your monitor’s color settings—and on your eyes!

blue
green
red
yellow

Quiz 2

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Quiz 2

Answer the question above. Then, select the CHECK ANSWER button.

Select the NEXT button and move to the Conclusion.
Try to answer the question again.
According to Hering’s opponent-process theory:
a blue square will generate a green afterimage
the visual system is organized in opposing color channels
staring at a red circle causes the green channel to become fatigued

Conclusion

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