Chapter 9: Perceiving Color

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.

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What Colors Do We Perceive? 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 - adding white to a color results in less saturated color

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Color and Wavelength 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

Color and Wavelength - continued 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

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Color and Wavelength - continued Selective transmission: –Transparent objects, such as liquids, selectively allow wavelengths to pass through Simultaneous color contrast - background of object can affect color perception

Color and Wavelength - continued 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

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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 in a comparison field to match a test field of one wavelength.

Behavior Evidence of the Theory Results showed that: –It is possible to perform the matching task –Observers with normal color vision need at least three wavelengths to make the matches. –Observers with color deficiencies can match colors by using only two wavelengths.

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Physiological Evidence for the Theory Researchers measured absorption spectra of visual pigments in receptors (1960s). –They found pigments that responded maximally to: Short wavelengths (419nm) Medium wavelengths (531nm) Long wavelengths (558nm) Later researchers found genetic differences for coding proteins for the three pigments (1980s).

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Cone Responding 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.

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Are Three Receptor Mechanisms Necessary for Color Vision? One receptor type cannot lead to color vision because: –absorption of a photon causes the same effect, no matter what the wavelength is. –any two wavelengths can cause the same response by changing the intensity. Two receptor types (dichromats) solve this problem but three types (trichromats) allow for perception of more colors.

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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

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Monochromatism 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

Dichromatism There are three 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

Dichromatism - 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

Dichromatism - 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

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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.

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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 positively to red and negatively to green –These responses were believed to be the result of chemical reactions in the retina.

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Physiology Evidence for the Theory 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

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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

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Color in the Cortex There is no single module for color perception –Cortical cells in V1, 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

Types of Opponent Neurons in the Cortex Single-opponent neurons Double-opponent neurons

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Color Constancy 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

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Color Constancy - continued Chromatic adaptation - prolonged exposure to chromatic color leads to receptors: – “ Adapting ” when the stimulus color selectively bleaches a specific cone pigment –Decreasing in sensitivity to the color Adaptation occurs to light sources leading to color constancy

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Color Constancy - continued Experiment by Uchikawa et al. –Observers shown sheets of colored paper in three conditions: Baseline - paper and observer in white light Observer not adapted - paper illuminated by red light; observer by white Observer adapted - paper and observer in red light

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Color Constancy - continued Experiment by Uchikawa et al. results showed that: –Baseline - green paper is seen as green –Observer not adapted - perception of green paper is shifted toward red –Observer adapted - perception of green paper is slightly shifted toward red Partial color constancy was shown in this condition

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

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Experiment by Hansen et al. Observers saw photographs of fruits with characteristic colors against a gray background. –They adjusted the color of the fruit and a spot of light. –When the spot was adjusted to physically match the background, the spot appeared gray. –But when this done for the fruits, they were still perceived as being slightly colored.

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

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Lightness Constancy - continued 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.

Lightness Perception Under Uneven Illumination Lightness perception under uneven illumination –Perceptual system must distinguish between: Reflectance edges - edges where the amount of light reflected changes between two surfaces Illumination edges - edges where lighting of two surfaces changes

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Lightness Perception Under Uneven Illumination - continued Sources of information about illumination: –Information in shadows - system must determine that edge of a shadow is an illumination edge System takes into account the meaningfulness of objects. Penumbra of shadows signals an illumination edge.

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Color Is a Construction of the Nervous System Physical energy in the environment does not have perceptual qualities. –Light waves are not “ colored. ” Different nervous systems experience different perceptions. Honeybees perceive color which is outside human perception. –We cannot tell what color the bee actually “ sees. ”

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Infant Color Vision It is a complex problem to know what an infant really “ sees ” –Chromatic color –Brightness Bornstein et al (1976) –Habituation –Young infants have color vision

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