The Perception of Color - Comprehensive Notes

The Perception of Color

Basic Principles of Color Perception

• Color is not a physical property but a psychophysical property. As Steven Shevell (2003) stated, “There is no red in a 700 nm light, just as there is no pain in the hooves of a kicking horse.”
• Most of the light we see is reflected from light sources like the sun, light bulbs, or fire.
• We only see the part of the electromagnetic spectrum between 400 and 700 nm.
• Three steps to color perception:
  1. Detection: Wavelengths of light must be detected.
  2. Discrimination: We must be able to tell the difference between one wavelength (or mixture of wavelengths) and another.
  3. Appearance: We assign perceived colors to lights and surfaces and want these colors to be stable over time, regardless of lighting conditions.

Step 1: Color Detection

• Three types of cone photoreceptors:
  • S-cones: Detect short wavelengths (blue range).
  • M-cones: Detect medium wavelengths (green range).
  • L-cones: Detect long wavelengths (red range).
• It is more accurate to refer to the three cones as “short,” “medium,” and “long” rather than “blue,” “green,” and “red,” since they each respond to a variety of wavelengths.
  • The L-cone’s peak sensitivity is 565 nm, which corresponds to yellow, not red!
• Photopic: Light intensities that are bright enough to stimulate the cone receptors and bright enough to “saturate” the rod receptors to their maximum responses.
  • Sunlight and bright indoor lighting are both photopic lighting conditions.
• Scotopic: Light intensities that are bright enough to stimulate the rod receptors but too dim to stimulate the cone receptors.
  • Moonlight and extremely dim indoor lighting are both scotopic lighting conditions.

Step 2: Color Discrimination

• The principle of univariance: An infinite set of different wavelength and intensity combinations can elicit exactly the same response from a single type of photoreceptor.
  • Therefore, one type of photoreceptor cannot make color discriminations based on wavelength.
• Rods are sensitive to scotopic light levels.
  • All rods contain the same photopigment molecule: rhodopsin.
  • Therefore, all rods have the same sensitivity to different wavelengths of light.
  • Consequently, rods obey the principle of univariance and cannot sense differences in color.
  • Under scotopic conditions, only rods are active, so that is why the world seems drained of color.
• With three cone types, we can tell the difference between lights of different wavelengths.
  • Under photopic conditions, the S-, M-, and L-cones are all active.
• Trichromacy (trichromatic theory of color vision):
  • The theory that the color of any light is defined in our visual system by the relationships of three numbers, the outputs of three receptor types now known to be the three cones.
  • Also known as the Young-Helmholtz theory.
• Metamers: Different mixtures of wavelengths that look identical; more generally, any pair of stimuli that are perceived as identical in spite of physical differences.
• History of color vision:
  • Thomas Young (1773–1829) and Hermann von Helmholtz (1821–1894) independently discovered the trichromatic nature of color perception. This is why trichromatic theory is called the “Young-Helmholtz theory.”
  • James Maxwell (1831–1879) developed a color-matching technique that is still being used today.
• Additive color mixing: A mixture of lights
  • If light A and light B are both reflected from a surface to the eye, in the perception of color, the effects of those two lights add together.
• Subtractive color mixing: A mixture of pigments.
  • If pigment A and B mix, some of the light shining on the surface will be subtracted by A and some by B. Only the remainder contributes to the perception of color.
• Lateral geniculate nucleus (LGN) has cells that are maximally stimulated by spots of light.
  • Visual pathway stops in LGN on the way from retina to visual cortex.
  • LGN cells have receptive fields with center-surround organization.
  • Cone-opponent cell: A neuron whose output is based on a difference between sets of cones.
    • In LGN there are cone-opponent cells with center-surround organization.

Step 3: Color Appearance

• Color space: A three-dimensional space that describes all colors. There are several possible color spaces.
  • RGB color space: Defined by the outputs of long, medium, and short wavelength lights (i.e., red, green, and blue).
  • HSB color space: Defined by hue, saturation, and brightness.
    • Hue: The chromatic (color) aspect of light.
    • Saturation: The chromatic strength of a hue.
    • Brightness: The distance from black in color space.
• The Limits of the Rainbow
  • Nonspectral colors: Some colors that we see do not correspond to a single wavelength of light.
    • Purple and magenta are only perceived when both S- and L-cones are stimulated but M-cones are not.
• Opponent color theory: The theory that perception of color depends on the output of three mechanisms, each of them based on an opponency between two colors: red–green, blue–yellow, and black–white.
  • Some LGN cells are excited by L-cone activation in center, inhibited by M-cone activation in their surround (and vice versa).
    • Red versus green
  • Other cells are excited by S-cone activation in center, inhibited by (L + M)-cone activation in their surround (and vice versa).
    • Blue versus yellow
• Ewald Hering (1834–1918) noticed that some color combinations are “legal” while others are “illegal.”
  • We can have bluish green (cyan), reddish yellow (orange), or bluish red (purple).
  • We can not have reddish green or bluish yellow.
• Hue cancellation experiments
  • Start with a color, such as bluish green.
  • The goal is to end up with pure green with no hints of blue or yellow.
  • Shine some yellow light to cancel out the blue light.
    • Adjust the intensity of the yellow light until there is no sign of either blue or yellow in the green patch.
• We can use the hue cancellation paradigm to determine the wavelengths of unique hues.
  • Unique hue: Any of four colors that can be described with only a single color term: red, yellow, green, blue.
    • For instance, unique blue is a blue that has no red or green tint.
• The three steps of color perception, revisited
  • Step 1: Detection
    • S-, M-, and L-cones detect light.
    • Each cone responds to a different range of wavelengths of light.
  • Step 2: Discrimination
    • Cone-opponent mechanisms discriminate wavelengths.
    • [L – M] and [M – L] compute something like red vs. green.
    • [L + M] – S and S – [L + M] compute something like blue vs. yellow.
  • Step 3: Appearance
    • Further transformations of the signals create final color-opponent appearance.
• Color in the Visual Cortex
  • Is there a particular place in the cortex specialized for color processing?
    • Not clear: V1, V2, and V4 all involved in color perception, but not exclusively.
  • Achromatopsia: Loss of color vision from brain damage.

Individual Differences in Color Perception

• Language and Color
  • General agreement on colors
    • Basic color terms: Single words that describe colors, are used with high frequency, and have meanings that are agreed upon by speakers of a language.
  • Various cultures describe color differently.
  • Cultural relativism: In sensation and perception, the idea that basic perceptual experiences (e.g., color perception) may be determined in part by the cultural environment.
• Genetic Differences in Color Perception
  • About 8% of males and 0.5% of females have some form of color vision deficiency: “color blindness.”
    • Color-anomalous: A term for what is usually called “color blindness.” Most “color-blind” individuals can still make discriminations based on wavelength. Those discriminations are just different from the norm.
  • Several types of color-blind/color-anamolous people
    • Deuteranope: Due to absence of M-cones.
    • Protanope: Due to absence of L-cones.
    • Tritanope: Due to absence of S-cones.
  • Cone monochromat: Has only one cone type; truly color -blind.
  • Rod monochromat: Has no cones of any type; truly color-blind and very visually impaired in bright light.
  • Anomia: Inability to name objects or colors in spite of the ability to see and recognize them. Typically due to brain damage.
• Synesthesia: When one stimulus evokes the experience of another stimulus that is not present.
  • Example: letters appearing to have colors (grapheme- color synesthesia) or sounds having tastes
  • About 4-5% of the population experiences synesthesia

From the Color of Lights to a World of Color

• Colors very rarely appear in isolation. Usually, many colors are present in a scene.
  • When many colors are present, they can influence each other.
  • Color contrast: A color perception effect in which the color of one region induces the opponent color in a neighboring region.
  • Color assimilation: A color perception effect in which two colors bleed into each other, each taking on some of the chromatic quality of the other.
• Unrelated color: A color that can be experienced in isolation.
• Related color: A color, such as brown or gray, which is seen only in relation to other colors.
  • A “gray” patch in complete darkness appears white.
• Afterimages: A visual image seen after a stimulus has been removed.
  • Negative afterimage: An afterimage whose polarity is the opposite of the original stimulus.
    • Light stimuli produce dark negative afterimages.
    • Colors are complementary. Red produces green afterimages and blue produces yellow afterimages (and vice versa).
    • This is a way to see opponent colors in action.
• Color constancy: The tendency of a surface to appear the same color under a fairly wide range of illuminants.
  • Illuminant: The light that illuminates a surface.
  • To achieve color constancy, we must estimate how the color of the illuminant changes an object’s color on our retina so that we can determine the true color of the surface out in the world.
• Physical constraints make constancy possible.
  • Intelligent guesses about the illuminant
    • Most illuminants are “broadband” and contain many different wavelengths
  • Assumptions about surfaces
    • Most surfaces are “broadband” and reflect many different wavelengths
  • How does the illuminant interact with the surface?
• “The Dress”
  • Caused controversy because people couldn’t agree on the colors of the dress Black & Blue or White & Gold?
  • People perceived the dress differently, depending on their assumptions about the color of the illuminant
    • Assume yellow illuminant = Black & Blue
    • Assume blue illuminant = White & Gold

What Is Color Vision Good For?

• Animals provide insight into color perception in humans
  • Food
    • It is easier to find berries and determine when they are ripe with color vision.
    • The perceived flavor of food can be affected by its color.
    • White wine dyed to look rosé tastes more like real rosé wine than white wine.
  • Sex
    • Flower colors are advertisements for bees to trade food for sex (for pollination).
    • Colorful patterns on tropical fish and birds provide sexual signals.
• Many animals have different color vision systems than humans
  • Dogs are dichromats (similar to deuteranopes)
  • Chickens are tetrachromats
  • Mantis shrimp have 12 types of cones
  • The silver spinyfin fish lives in the deep sea and has two cone types and 38 rod types!
• Humans and other mammals have color vision due to the different photopigments in our cones. Other animals have evolved a different system for color vision.
  • Birds and some reptiles have colored oils over each photoreceptor, which tunes them to different wavelengths.
• How well can you direct your attention to one color?
  • Can people direct their attention to a specific color of green?
    • When the dots vary in hue, people are accurate.
    • When the dots vary in saturation, people are not very accurate.
  • Conclusion: Attention can select a group of items based on hue, not saturation.