1. Chapter 7: Perceiving Color

2. Overview of Questions
Why do we perceive blue dots when a yellow flash bulb goes off?
What does someone who is “color-blind” see?
What colors does a honeybee perceive?

3. What Are Some Functions of Color Vision?
Color signals help us classify and identify objects
Color facilitates perceptual organization of elements into objects
Color vision may provide an evolutionary advantage in foraging for food
Instructor can talk about monkeys looking for fruit in foliage and the experience of color-blind individuals when they try to perform the seemingly simple task of picking fruit.

5. Familiarity and recognition
Instructor can explain the study that was performed using fruit with correct and incorrect colors. Objects with correct colors were identified more accurately and more quickly than those with incorrect colors.
Familiarity and recognition

6. How Can We Describe Color Experience?
Basic colors are red, yellow, green, and blue
Color circle shows perceptual relationship among colors
Colors can be changed by:
Intensity which changes perceived brightness
Saturation which adds white to a color resulting in less saturated color
The differences between intensity and saturation are not intuitive to most people. It is helpful to perform simple demonstrations of the two principles by using the color adjustments on a computer.

7. Color Wheel – Known for centuries by artists

8. What Is the Relationship Between Wavelength and Color Perception?
Color perception is related to the wavelength of light:
400 to 450nm appears violet
450 to 490nm appears blue
500 to 575nm appears green
575 to 590nm appears yellow
590 to 620nm appears orange
620 to 700nm appears red

9. Colors of Objects
Colors of objects are determined by the wavelengths that are reflected
Reflectance curves - plots of percentage of light reflected for specific wavelengths
Chromatic colors or hues - objects that preferentially reflect some wavelengths
Called selective reflectance
Achromatic colors - contain no hues
White, black, and gray tones

11. Table 7.1 Relationship between predominant wavelengths reflected and color perceived

12. Color of Objects - continued
Selective transmission:
Transparent objects, such as liquids selectively allow wavelengths to pass through
Simultaneous color contrast - background of object can affect color perception
Simultaneous color contrast is discussed later as well, but here it is useful to provide an example such as red apples against the green foliage of a tree.

13. Trichromatic Theory of Color Vision
Proposed by Young and Helmholtz (1800s)
Three different receptor mechanisms are responsible for color vision
Behavioral evidence:
Color-matching experiments
Observers adjusted amounts of three wavelengths to match a comparison field to a test field
Note that the test field is produced by using ONE wavelength while the comparison field is composed of the three wavelengths that are adjusted. This can be seen on the next slide

15. Color Matching Experiments
Results showed that:
It is possible to perform the matching task without three colors in the S, M and L wavelengths
Observers with normal color vision need at least 3 wavelengths to make the matches
Observers with color deficiencies can match colors by using only 2 wavelengths
They think it looks OK!!
The instructor can note that for people with normal color vision, any three wavelengths can be used as long as any one of them can’t be matched by mixing the other two.

16. Physiological Evidence for the Trichromatic Theory
Researchers measured absorption spectra of visual pigments in receptors (1960s)
They found pigments that responded maximally to:
Short wavelengths (419nm)
Medium wavelengths (551nm)
Long wavelengths (558nm)
Later researchers found genetic differences for coding proteins for the three pigments (1980s)

17. Figure 7. 8 Absorption spectra of the three cone pigments
Figure 7.8 Absorption spectra of the three cone pigments. (From Dartnall, Bowmaker, and Mollon, 1983.)

18. Response of Cones and Color Perception
Color perception is based on the response of the three different types of cones
Responses vary depending on the wavelengths available
Combinations of the responses across all three cone types lead to perception of all colors
Color matching experiments show that colors that are perceptually similar (metamers) can be caused by different physical wavelengths

19. Color perception is based on the response of the three different types of cones

20. Figure 7.12 The proportions of 530- and 620-nm lights in the field on the left have been adjusted so that the mixture appears identical to the 580-nm light in the field on the right. The numbers indicate the responses of the short-, medium-, and long-wavelength receptors. There is no difference in the responses of the two sets of receptors so that two fields are perceptually indistinguishable.

21. Additive color mixture: Mixing lights of different wavelengths
Color Mixing
Additive color mixture:
Mixing lights of different wavelengths
All wavelengths are available for the observer to see
Superimposing blue and yellow lights leads to white
Subtractive color mixture:
Mixing paints with different pigments
Additional pigments reflect fewer wavelengths
Mixing blue and yellow leads to green
Students have trouble with these two concepts. It is helpful to have available lights of different colors (you can use flashlights with filters) and some paints you can mix, although the paint mixing is an activity that they are more likely to have had experience with in the past.
Subtractive color mixture can also be explained by saying that what color is seen is made up of the wavelengths that are reflected in common by the mixed pigments.

23. Figure 7.11 Mixing blue paint and yellow paint creates a paint that appears green. This is subtractive color mixture.

25. Are Three Receptor Mechanisms Necessary for Color Perception?
One receptor type cannot lead to color vision because:
Absorption of a photon causes the same effect no matter what the wavelength is - called the principle of univariance
Any two wavelengths can cause the same response by changing the intensity
Two receptor types (dichromats) solves this problem but 3 types (trichromats) allows for perception of more colors
Try This!

26. Color Deficiency
Monochromat - person who needs only one wavelength to match any color
Dichromat - person who needs only two wavelengths to match any color
Anomalous trichromat - needs three wavelengths in different proportions than normal trichromat
Unilateral dichromat - trichromatic vision in one eye and dichromatic in other

27. Figure 7.15 Ishihara plate for testing for color deficiency.

28. Color Experience for Monochromats
Monochromats have:
A very rare hereditary condition
Only rods and no functioning cones
Ability to perceive only in white, gray, and black tones
True color-blindness
Poor visual acuity
Very sensitive eyes to bright light
The instructor can make sure to note how the limitations for the monochromat are a by-product of having only rod vision.

29. Color Experience for Dichromats
There are 3 types of dichromatism:
Protanopia affects 1% of males and .02% of females
Individuals see short-wavelengths as blue
Neutral point occurs at 492nm
Above neutral point, they see yellow
They are missing the long-wavelength pigment

30. Color Experience for Dichromats - continued
Deuteranopia affects 1% of males and .01% of females
Individuals see short-wavelengths as blue
Neutral point occurs at 498nm
Above neutral point, they see yellow
They are missing the medium wavelength pigment

31. Color Experience for Dichromats - continued
Tritanopia affects .002% of males and .001% of females
Individuals see short wavelengths as blue
Neutral point occurs at 570nm
Above neutral point, they see red
They are most probably missing the short wavelength pigment

33. Opponent-Process Theory of Color Vision
Proposed by Hering (1800s)
Color vision is caused by opposing responses generated by blue and yellow and by green and red
Behavioral evidence:
Color afterimages and simultaneous color contrast show the opposing pairings
Types of color blindness are red/green and blue/yellow
Use the next slide to show the two effects.

34. Figure 7.17 Color matrix for afterimage and simultaneous contrast demonstrations.

36. Opponent-Process Theory of Color Vision - continued
Opponent-process mechanism proposed by Hering
Three mechanisms - red/green, blue/yellow, and white/black
The pairs respond in an opposing fashion, such as positive to red and negatively to green
These responses were believed to be the result of chemical reactions in the retina

37. Figure 7.19 The three opponent mechanisms proposed by Hering.

38. Physiology of Opponent-Process
Researchers performing single-cell recordings found opponent neurons (1950s)
Opponent neurons:
Are located in the retina and LGN
Respond in an excitatory manner to one end of the spectrum and an inhibitory manner to the other

39. Trichromatic and Opponent-Process Theories Combined
Each theory describes physiological mechanisms in the visual system
Trichromatic theory explains the responses of the cones in the retina
Opponent-process theory explains neural response for cells connected to the cones further in the brain

40. Figure 7.21 Our experience of color is shaped by physiological mechanisms, both in the receptors and in opponent neurons.

41. Figure 7.22 Neural circuit showing how the blue-yellow and red-green mechanisms can be created by excitatory and inhibitory inputs from the three types of cone receptors.

42. Color Processing in the Cortex
There is no single module for color perception
Cortical cells in V1, V2, and V4 respond to some wavelengths or have opponent responses
These cells usually also respond to forms and orientations
Cortical cells that respond to color may also respond to white

43. Perceiving Colors Under Changing Illumination
Color constancy - perception of colors as relatively constant in spite of changing light sources
Sunlight has approximately equal amounts of energy at all visible wavelengths
Tungsten lighting has more energy in the long-wavelengths
Objects reflect different wavelengths from these two sources

44. Figure 7.24 The reflectance curve of a sweater (green curve) and the wavelengths reflected from the sweater when it is illuminated by daylight (white) and by tungsten light (yellow).

45. Possible Causes of Color Constancy
Chromatic adaptation - prolonged exposure to chromatic color leads to:
Receptors “adapt” when the stimulus color selectively bleaches a specific cone pigment
Sensitivity to the color decreases
Adaptation occurs to light sources leading to color constancy

46. Possible Causes of Color Constancy - continued
Effect of surroundings
Color constancy works best when an object is surrounded by many colors
Memory and color
Past knowledge of an object’s color can have an impact on color perception
Memory for color is not exact, so we don’t notice slight changes caused by illumination changes

47. Lightness Constancy
Achromatic colors are perceived as remaining relatively constant
Perception of lightness:
Is not related to the amount of light reflected by object
Is related to the percentage of light reflected by object

48. Figure 7.27 A black-and-white checkerboard illuminated by tungsten light and by sunlight.

49. Possible Causes of Lightness Constancy
The ratio principle - two areas that reflect different amounts of light look the same if the ratios of their intensities are the same
This works when objects are evenly illuminated
Shadows cause problems
Reflectance edges - edge where the reflectance of two surfaces changes
Illumination edges - edge where illumination of two surfaces changes

50. Figure 7.28 This unevenly illuminated wall contains both reflectance edges and illumination edges. The perceptual system must distinguish between these two types of edges to accurately perceive the actual properties of the wall and other parts of the scene, as well.

51. Figure 7.29 The pattern created by shadows on a surface is usually interpreted as a change in the pattern of illumination, not as a change in the material making up the surface. The fact that we see all of the bricks on this wall as made of the same material, despite the illumination changes, is an example of lightness constancy.

52. Figure 7. 30 (a) A cup and its shadow
Figure 7.30 (a) A cup and its shadow. (b) The same cup and shadow with the penumbra covered by a black border.

53. Creating Color Experience
Light waves are not “colored”
Color is a creation of our physiology
Animals with different sensory apparatus, such as honey bees, experience something we cannot
All of our sensory experiences are created by our nervous system