2 Overview of QuestionsWhy 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 objectsColor facilitates perceptual organization of elements into objectsColor vision may provide an evolutionary advantage in foraging for foodInstructor 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 blueColor circle shows perceptual relationship among colorsColors can be changed by:Intensity which changes perceived brightnessSaturation which adds white to a color resulting in less saturated colorThe 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.
8 What Is the Relationship Between Wavelength and Color Perception? Color perception is related to the wavelength of light:400 to 450nm appears violet450 to 490nm appears blue500 to 575nm appears green575 to 590nm appears yellow590 to 620nm appears orange620 to 700nm appears red
9 Colors of ObjectsColors of objects are determined by the wavelengths that are reflectedReflectance curves - plots of percentage of light reflected for specific wavelengthsChromatic colors or hues - objects that preferentially reflect some wavelengthsCalled selective reflectanceAchromatic colors - contain no huesWhite, 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 throughSimultaneous color contrast - background of object can affect color perceptionSimultaneous 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 visionBehavioral evidence:Color-matching experimentsObservers adjusted amounts of three wavelengths to match a comparison field to a test fieldNote 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 wavelengthsObservers with normal color vision need at least 3 wavelengths to make the matchesObservers with color deficiencies can match colors by using only 2 wavelengthsThey 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 conesResponses vary depending on the wavelengths availableCombinations of the responses across all three cone types lead to perception of all colorsColor 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 MixingAdditive color mixture:Mixing lights of different wavelengthsAll wavelengths are available for the observer to seeSuperimposing blue and yellow lights leads to whiteSubtractive color mixture:Mixing paints with different pigmentsAdditional pigments reflect fewer wavelengthsMixing blue and yellow leads to greenStudents 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.
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 univarianceAny two wavelengths can cause the same response by changing the intensityTwo receptor types (dichromats) solves this problem but 3 types (trichromats) allows for perception of more colorsTry This!
26 Color DeficiencyMonochromat - person who needs only one wavelength to match any colorDichromat - person who needs only two wavelengths to match any colorAnomalous trichromat - needs three wavelengths in different proportions than normal trichromatUnilateral 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 conditionOnly rods and no functioning conesAbility to perceive only in white, gray, and black tonesTrue color-blindnessPoor visual acuityVery sensitive eyes to bright lightThe 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 femalesIndividuals see short-wavelengths as blueNeutral point occurs at 492nmAbove neutral point, they see yellowThey are missing the long-wavelength pigment
30 Color Experience for Dichromats - continued Deuteranopia affects 1% of males and .01% of femalesIndividuals see short-wavelengths as blueNeutral point occurs at 498nmAbove neutral point, they see yellowThey are missing the medium wavelength pigment
31 Color Experience for Dichromats - continued Tritanopia affects .002% of males and .001% of femalesIndividuals see short wavelengths as blueNeutral point occurs at 570nmAbove neutral point, they see redThey 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 redBehavioral evidence:Color afterimages and simultaneous color contrast show the opposing pairingsTypes of color blindness are red/green and blue/yellowUse 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 HeringThree mechanisms - red/green, blue/yellow, and white/blackThe pairs respond in an opposing fashion, such as positive to red and negatively to greenThese 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 LGNRespond 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 systemTrichromatic theory explains the responses of the cones in the retinaOpponent-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 perceptionCortical cells in V1, V2, and V4 respond to some wavelengths or have opponent responsesThese cells usually also respond to forms and orientationsCortical 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 sourcesSunlight has approximately equal amounts of energy at all visible wavelengthsTungsten lighting has more energy in the long-wavelengthsObjects 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 pigmentSensitivity to the color decreasesAdaptation occurs to light sources leading to color constancy
46 Possible Causes of Color Constancy - continued Effect of surroundingsColor constancy works best when an object is surrounded by many colorsMemory and colorPast knowledge of an object’s color can have an impact on color perceptionMemory for color is not exact, so we don’t notice slight changes caused by illumination changes
47 Lightness ConstancyAchromatic colors are perceived as remaining relatively constantPerception of lightness:Is not related to the amount of light reflected by objectIs 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 sameThis works when objects are evenly illuminatedShadows cause problemsReflectance edges - edge where the reflectance of two surfaces changesIllumination 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 physiologyAnimals with different sensory apparatus, such as honey bees, experience something we cannotAll of our sensory experiences are created by our nervous system