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Color Perception

Demonstrations (Direct Links)

Demonstration 7.1 Predicting Additive Mixtures
Demonstration 7.2 Two Mixture Techniques
Demonstration 7.3 Additive Mixtures and Color Television
Demonstration 7.4 Simultaneous Color Contrast
Demonstration 7.5 Successive Color Contrast
Demonstration 7.6 Benham's Top
Demonstration 7.7 Purkinje Shift


Before You Start

• Think about how different your world would appear if you didn't see in color. Compare the figures below as a starting point. Suppose that you were dependent on edible berries (not these!) for sustenance. Would you be at a disadvantage if you couldn't see in color?

color tree tree b&w
Photo by H. Foley

• Although other topics in perception would work equally well, color perception provides a good opportunity to consider the subjective experience of perception. How would you explain the color "red" to a person who is blind? How confident are you that your experience of a color stimulus is identical to another's experience of that same stimulus?

• Color perception also provides an opportunity to distinguish between the physical stimulus and one's perception of that stimulus. Will one's perception of color be influenced by context? By prior (e.g., linguistic) experience? Could one perceive a color when none is present?

 


Introduction to Color Perception

At this point, you're familiar with the nature of the visual system, including the perception of basic visual phenomena (such as lightness) and also shape and depth. Although we've talked a bit about how the brain processes color information, we will now explore this topic in greater detail.

Link - Webvision on Color Vision (Gouras) - The focus of this introduction is on the physiology underlying color vision.

Link - Webvision on Perception of Color - A good brief introduction to many topics (e.g., matching, mixing, theories) in the perception of color.

Link - DIY Calculator on Color Vision - Another overview of color vision, including useful links.

 


Nature of Color
In this section, we turn again to the distinction between characteristics of the physical stimulus and the nature of the perceptual phenomena that are associated with the stimulus. The varying wavelengths of the light stimulus are depicted below, and these wavelengths are closely associated with the perceptual phenomenon of hue (the color names seen below the wavelengths). However, as you'll note, the association is often a loose one.

spectrum

Link - BYK Gardner has developed a short paper that provides an overview of color perception and color measurement.


Color Mixing

Subtractive Mixtures

 

Additive Mixtures

Demonstration 7.2 Two Mixture Techniques First, try making an additive mixture.  Take small pieces of blue and yellow paper.  Cut the blue paper into narrow strips and then cut the strips into tiny squares.  Arrange the squares in a random order on part of the yellow paper so that they cover about half the surface.  Tape the squares in place and hang the paper on the wall at the end of a long hallway.  Walk backward down the hallway, looking at the paper, until you can no longer see the individual squares.  The combination of yellow and blue should look gray.

Now, to make a subtractive mixture, you might try mixing blue food dye with yellow dye.  You could also try blue and yellow paint, or even coloring a white piece of paper with both blue and yellow crayons.  In each case, the resulting mixture should be green.

Demonstration 7.3 Additive Mixtures and Color Television The screen of a color television is like a miniature patchwork quilt consisting of many tiny dots.  Typically, it has about 1 million of these pixels of three types.  When irradiated from behind the screen by a special beam, one type glows blue, one green, and another glows red (Boynton, 1979).
How can a patch on the screen look yellow?  Take a magnifying glass and notice that a yellow patch is actually tiny green and red dots of a carefully chosen hue.  The dots are too small to be seen with the unaided eye when you sit a normal distance from the television, but they are combined by additive mixture.  With the magnifying glass, notice other blends of colored dots.  See whether you can predict what they would look like from the normal viewing distance.

Link - Wolfram Demonstrations illustrates CIE Color Space.


Anatomy and Physiology of Color Vision

The Cone Mosaic and Trichromatic Theory

[Scientific American]: What Birds See by Timothy H. Goldsmith
Evolution has endowed birds with a system of color vision that surpasses that of all mammals, including humans. The typical human has three cone photopigments, while birds typically have four cone photopigments. In addition, birds' retinas contain some cones with tiny oil droplets (red, yellow, and colorless) that filter the light hitting the cones, thereby allowing the birds to distinguish more colors. Birds can even see into the near ultraviolet portion of the spectrum! Thus, birds that may be largely indistinguishable to humans probably look quite different to other birds, which probably plays an important role in mate selection.

Opponent-Process Theory

Update [Scientific American 2/2010]: Seeing Forbidden Colors by Vincent A. Billock and Brian H. Tsou
Opponent-Process Theory grew, in part, from a belief that one couldn't see reddish green or yellowish blue. Crane and Piatanida (1983) published a paper in Science that illustrated how one might do so. Billock and Tsou illustrate the special circumstances under which people can see these "forbidden" colors, suggesting that color opponency may not be completely hardwired in the brain.

Color Coding Beyond the Photoreceptors

Link - The physiology of color vision (Peter Gouras at WebVision).


Individual Differences in Color Vision

Link - Wikipedia on Color Blindness

Color Vision Deficiencies

• Color vision deficiencies typically arise from differences in cone photopigments and lead to differences in color experience. For dichromats, such as protanopes and deuteranopes, either the L- cone system or the M-cones system is missing, which leads these people to have difficulty discriminating reds and greens. For the other type of dichromat -- the tritanope -- the S-cone system is missing, which leads these people to have difficulty discriminating yellows and blues.

Link - To get a sense of the visual experience of people with color vision deficiencies, try out the Vischeck site.
Link - Here's another site that illustrates the impact of color vision deficiencies.
Link - Jay Neitz has provided an interesting illustration of the processing of color information in the retina, including the impact of color vision deficiencies.

• Color vision deficiencies (e.g., protanopia and deuteranopia) lead to difficulties distinguishing red and green. As you might imagine, that distinction is particularly important when one encounters a stoplight when driving. Most stoplights have been standardized such that the top light is red and the bottom light is green. That way, even a person who might have difficulty distinguishing red and green knows to stop when the top light is lit. The placement of the lights on stoplights was not always consistent. In fact, you can still find at least one stoplight with green on top. In the Tipperary Hill area of Syracuse, NY (an area that was once predominantly Irish) one can find the stoplight seen below. Although some people may be happy that the Irish green color is on top, the light may be a bit confusing to a person with color-vision deficiencies used to stopping when the top light is lit.

Tipperary Light

Photo by H. Foley

Diagnosing Color Deficiences

Link - Try out a simulation of the Farnsworth color arrangement test, along with useful links for color vision deficiencies. (As the author notes, this exercise is purely a simulation because the colors cannot be easily calibrated from monitor to monitor.)

Link - You can try out a web-based color vision test developed at City University London.

More than Three Cone Systems

Aging and Color Perception

 


Color Phenomena

Color Constancy

 

Simultaneous and Successive Color Contrast

Demonstration 7.4 Simultaneous Color Contrast Look at the image below.  Does the gray circle appear to be the same color on both sides of the plate?  Against the blue background the gray circle takes on a yellowish tint.  Against the yellow background, the gray circle takes on a bluish tinge.  You can convince yourself that the gray is uniform throughout the circle by covering over the yellow and blue patches.

Color Contrast

Link - WebExhibits example of simultaneous color contrast.

Demonstration 7.5 – Successive Color Contrast Turn to Color Plate 8 inside the back cover of your textbook and place it under a bright light. Place a white piece of paper and an object (ideally another sheet of paper) that is a basic color (e.g., blue) next to the image. Stare at the black dot in the center of the plate for 2-3 minutes. It will most likely get boring, but tough it out and don’t look away too soon. After enough time has passed, quickly transfer your focus to the white piece of paper.  The complement of each color should now appear, making the afterimage appear to be somewhat like a mirror image version of the original. Now look at the colored object. Note the impact of the afterimage on your perception of the basic color. Of course, the underlying principle is the same as that for negative color afterimages, such as the example below. Stare at the oddly colored flag for a while, then look at the white space below it. What do you see?

 

Inverted American Flag

 

 

 

 

 

 

 

 

 

 

Subjective Colors

Demonstration 7.6 Benham’s Top (Subjective Colors) Print out a copy of the image below. Cut it out and glue it to a piece of cardboard. Punch a hole in the center (at the +) and place it on the end of a pencil.  Hold the pencil with one hand and spin the circle with the other. When the speed is just right – not too fast nor too slow – you should see pastel colors. If the top is turning clockwise, the bands should look somewhat blue, green yellow and red (going from the outside to inside bands). If the top is turning counter clockwise, the bands should seem in reverse order.

Benham's Top

If you'd like to avoid constructing your own Benham's Top, you can play with a web version (Flash) created by the folks at Project LITE: Link - Benham's Disk at Project LITE (Boston University)

Purkinje Shift

Demonstration 7.7 Purkinje Shift Under brightly lit conditions, select an example of a red object and a blue object that look equally bright to you. Now go to a darkened room and wait about 10 minutes until you are mostly dark adapted. Now compare the two objects to see which appears lighter.

red vs. blue
Photo by R. Oldmixon

Memory Color

Color Names and Color Perception

Link - Steven Derose on Eskimo words for snow.


Test Yourself

1. Try to think of some evolutionary advantages that color vision afforded our early ancestors. For example, what advantages might come to a predator who could see colors? What advantages might come to a herbivore that could see colors? Why might a trichromat be at an adaptive advantage to a dichromat?


2. Each of the following is an example of a color mixture; specify whether it represents a subtractive or an additive mixture: (a) you are winding strands of purple and blue yarn together to knit a sweater; (b) you paint a layer of light green paint over a wall painted dark blue; (c) you mix red food coloring into yellow egg yolks; (d) you cover one flashlight with green cellophane and another with red cellophane and then shine them both on a white sheet of paper. Predict the color of the mixtures in as many of the combinations as possible. What happens when you make an additive mixture of complementary hues?


3. Summarize the trichromatic theory, as developed by Young and Helmholtz, and the original form of the opponent-process theory, as developed by Hering. Why were the two initially incompatible, and how was the controversy resolved?


4. To be sure that you understand the function of the six color opponent-process receptive fields, describe the activity of a ganglion cell with a –M center and a +L surround receptive field. What would happen if you illuminated all the cones contributing to the receptive field with short-wavelength light (i.e., what we’d call “blue”)? What would happen if you illuminated all the cones contributing to the receptive field with long-wavelength light (i.e., what we’d call “red”)? What would happen if you illuminated all the cones contributing to the receptive field with sunlight (i.e., what we’d call “white”)?


5. Imagine that a red light flashes in your left visual field. Trace the processing of that light as far as you can from the retina to areas of the visual cortex.


6. How does a normal trichromat differ from the three types of dichromats in color perception and in acuity? What everyday color discriminations would each type of dichromat find difficult? How would a rod monochromat or a person with cerebral achromatopsia differ in color perception from a normal trichromat?


7. Suppose that someone who didn’t know much about color vision asked the following questions about phenomena he or she had noticed. What explanations would you provide? (a) Why does a blue bird look gray when it’s far away? (b) Why does a white shirt look a little green when worn with a red vest? (c) Why does the world look slightly yellow after you take off blue-tinted dark glasses? (d) Why does a red stop sign look darker at twilight than a nearby blue car?


8. Throughout the text, you’ve encountered a number of different constancies. Describe color constancy in terms of the proximal and distal stimuli. Why might color constancy be easier to establish using naturally occurring stimuli under normal viewing conditions?


9. If you were interested in the impact of language on color perception, why would you study infants and people from other cultures? Regarding color names, would you consider yourself a universalist or a relativist? Why? Describe the evidence to support these two approaches.


10. Theme 4 stresses the role of cognitive factors (e.g., top-down processes) in perception. Use as many examples as you can from the chapter to support the premise that color perception involves much more than the wavelengths of light falling on the retina.


Teaching Materials

Lafayette Instrument Co. is one resource for color demonstrations. For example, they offer a color vision deficiency test (Hardy, Rand, and Ritter (HRR) Pseudoisochromatic Subset Book), an additive color mixing box, a stand perimeter, etc. You can use their Illusionator to produce subjective colors using the Benham's Top disk.

Link - Mark Newbold has produced some online demonstrations that illustrate subjective colors (Fechner colors).

Thomson Higher Education has published two very useful CD-ROMs. John Baro (Polyhedron Learning Media) has developed Insight: A Media Lab in Experimental Psychology [see Color Arrangement Test and Color Mixing] and Colin Ryan (James Cook University) has developed Exploring Perception [see Module 3].

Link - Eugene Vishnevsky developed an Introduction to Color to help explain color perception and various color spaces, color mixing, etc.

Link - Dale Purves (Duke) has provided a number of compelling color demonstrations.

Link - David Brainard's Lab (University of Pennsylvania)

Link - A Glossary of Color Science (Byrne and Hilbert)

Link - Donald Kline (University of Calgary) has a number of useful tutorials on his web pages, including a section on color perception and color vision deficiencies.

Link - Hans Irtel has produced an amazing teaching device for perception called PXLab (including Vision Demonstrations). You'll find a number of relevant demonstrations for color perception.

Link - Paul Kay's site contains many of his publications, as well as a link to the World Color Survey.

Link - David H. Foster (The University of Manchester) has made many contributions to the study of color. His web pages contain his papers and stimuli.

Link - Serendip's Color Links


Recommended Readings

Gegenfurtner, K. R. & Sharpe, L. T. (Eds.) (1999). Color vision: From genes to perception. New York: Cambridge.

Lammens, J. M. G. (1994). A computational model of color perception and color naming. (Dissertation)

Mausfeld, R. & Heyer, D. (Eds.) (2003). Colour perception: Mind and the physical world. New York: Oxford.

Mollon, J. D., Pokorny, J., & Knoblauch, K.(Eds.) (2003). Normal and defective colour vision. New York: Oxford.

Purves, D. & Lotto, R. B. (2003). Why we see what we do: An empirical theory of vision. Sinauer.

Shevell, S. K. (Ed.) (2003). The science of color, 2nd Ed.. New York: Elsevier.