Ch. 9 Notes PDF

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Chapter 9: Perceiving Color 

Cerebral Achromatopsia- color blindness caused by a cortical injury after a lifetime of experiencing color Functions of Color Perception

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Evolutionary; helps us detect objects in their environments/scenes Color and Light

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Colors on the spectrum are associated with different wavelengths of light

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Reflections and Transmission o Colors of objects depend on how wavelengths of light reflect off of them onto our eyes o Chromatic Colors- blue, green, red; occur when some wavelengths are reflected more than others, a process called selective reflection o Achromatic Colors- white, gray, black; occur when light is reflected equally across the spectrum o Reflectance Curves- plot the percentage of light reflected from lettuce and tomatoes at each wavelength in the visible spectrum

o Selective Transmission- only some wavelengths pass through the object o Transmission Curves- plots of the percentage of light transmitted at each wavelength 

Color Mixing o Subtractive Color Mixture- aka mixing paints

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o Mixing Paints 

When mixing two colors: both paints still absorbed the same wavelengths they absorbed when alone, so the only wavelengths reflected are those that are reflected by both paints in common

o Mixing Lights 

All of the light that is reflected from the surface by each light when alone is also reflected when the lights are superimposed

o Additive Color Mixture- mixing lights; mixing lights involves adding up the wavelengths of each light in the mixture o Summary: 

Colors of light are associated with wavelengths in the visible spectrum

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Colors of objects are associated with which wavelengths are reflected (for opaque objects) or transmitted (for transparent objects)

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The colors that occur when we mix colors are also associated with which wavelengths are reflected into the eye 

Mixing paints causes fewer wavelengths to be reflected (each paint subtracts wavelengths from the mixture)

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Mixing lights causes more wavelengths to be reflected (each light adds wavelengths to the mixture) Perceptual Dimensions of Color

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Spectral Colors- colors on the spectrum

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Nonspectral Colors- colors that do not appear in the spectrum because they are mixtures of other colors, such as magenta

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Hues- aka chromatic colors

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Saturation- determined by the amount of white that has been added to a particular hue o Desaturated Colors- faded or washed out in appearance

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Value- refers to the light-to-dark dimension of color

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Color Solid- 3D color space in which colors are systematically arranged The Triarchic Theory of Color Vision

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Trichromatic Theory of Vision- aka Young-Helmholtz Theory; states that color vison depends on the activity of 3 different receptor mechanisms

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Color-Matching Evidence for Trichromatic Theory o Color Matching- a procedure in which observers are asked to match the color in one field by mixing two or more lights in another field

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Physiological Evidence for Trichromatic Theory o Cone Pigments 

There are 3 different cone pigments: 

The short wavelength pigment (S), with max absorption at 419 nm

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The middle wavelength pigment (M), with max absorption at 531 nm

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The long wavelength pigment (L), with max absorption at 558 nm

o Cone Responding and Color Perception 

Wavelengths cause certain patterns of receptor responding

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Metamerism- a situation in which 2 physically different stimuli are perceptually identical

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Metamers- 2 identical fields in a color-matching experiment

o Are 3 Receptor Mechanisms Necessary for Color Vision? 

Vision with One Receptor Type 

Principle of Univariance- once a photon of light is absorbed by a visual pigment molecule, the identity of a light’s wavelength is lost

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The receptor does not know the wavelength of the light is has absorbed, only the total amount it has absorbed

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Vision with 2 Receptor Types 

The ratios of responses of the two pigments to wavelengths help indicate differences between 2 different wavelengths

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Monochromats- people with 1 type of pigment; see different shades of gray

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Dichromats- people with 2 types of cone pigments; see chromatic colors (can confuse some colors)

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Trichromats- people with 3 visual pigments; full color vision; 3rd pigment provides additional ratios that allow additional discriminations of wavelengths across the visual spectrum Opponent Process Theory of Color Vision

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Hering’s Phenomenological Evidence for Opponent-Process Theory

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o Opponent Process Theory of Color Vision- color vision is caused by opposing physiological responses generated by blue and yellow, red and green, and black and white o Hering’s Primary Colors- red, yellow, green, blue 

Hurvich and Jameson’s Psychophysical Measurements of the Opponent Mechanisms o Complimentary Afterimages- afterimage is the color on the opposite side of the color circle o Hue Cancelation- shows how blue opposes yellow and green opposes red 

Add yellow to blue until blue is no longer perceived; add green to red until red is no longer perceived

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Physiological Evidence for Opponent-Process Theory o Opponent Neurons- respond with an excitatory response to light from one part of the spectrum and with an inhibitory response to light from another part

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How Opponent Responding Can Be Created by Three Types of Receptors o Trichromatic Theory describes processes that take place in the receptors in the retina, whereas Opponent-Process Theory describes processing that takes place within opponent neurons within the LGN o First, trichromatic “ratio” info, then opponent “difference information”, then info is sent to cortex Color in the Cortex

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Is There a Single Color Center in the Cortex?

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o Color processing is distributed across a number of different cortical areas that process information about color and about other types of information as well 

Types of Opponent Neurons in the Cortex o Single Opponent Neurons- neurons that increase firing to long wavelengths presented to the center of the receptive field and decrease firing to short wavelengths presented to the surround (or vice-versa)

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center-surround receptive field; important for perceiving colors within a region

o Double Opponent Neurons- neurons with receptive fields in which stimulation of one part of the receptive field causes an excitatory response to wavelengths in one area of the spectrum and an inhibitory response to wavelengths in another area, and stimulation of an adjacent part of the receptive field causes the opposite response 

side-by-side receptive field; important for perceiving boundaries between different colors Color Deficiency

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Monochromatism- rare form of color blindness that is hereditary; no functioning cones, so their vision has the characteristics of rod vison in both dim and bright lights (white, gray, black)

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Dichromatism- condition in which people are missing one of the three cone pigments and hence experience some colors; they can’t, however, distinguish as many colors as can trichromats

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Unilateral Dichromat- a person with trichromatic vision in one eye and dichromatic vison in the other

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Anomalous Trichromatism- mixes wavelengths in different proportions than a trichromat 6

Color in a Dynamic World 

Color Constancy- we perceive colors of objects as being relatively constant even under changing illumination

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Chromatic Adaptation- prolonged exposure to chromatic color;

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Partial Constancy- the perception of the object is shifted after adaptation, but not as much as when there was no adaptation; this means that the eye can adjust its sensitivity to different wavelengths to keep color perception approximately constant as illumination changes

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The Effect of the Surroundings o Color constancy breaks down when an object is seen in isolation o The visual system uses information, provided by the way objects in a scene are illuminated, to estimate characteristics of the illumination and to make appropriate corrections.

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Memory and Color o Memory Color- the effect on perception of prior knowledge of typical colors of objects

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Lightness Constancy o Lightness- shades of white, gray and black o Lightness Constancy- we see lights, grays and blacks as staying about the same shade under different illuminations o The intensity of light reaching the eye from an object depends on 2 things: 

The illumination – the amount of light that is striking the object’s surface 7

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The objects reflectance – the proportion of this light that the object reflects into our eyes

o The Ratio Principle 

Ratio Principle- as long as the ratio of reflectance remains the same, the perceived lightness will remain the same

o Lightness Perception Under Uneven Illumination 

Reflectance Edge- an edge where the reflectance of two surfaces changes

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Illumination Edge- an edge where the lighting changes

o The Information in Shadows 

Penumbra- fuzzy border at the edge of a shadow; helps viewer identify that it is a shadow, not a darker object. Lightness constancy occurs when the penumbra is present but does not occur when it is masked

o The Orientation of Surfaces 

The visual system needs adequate information about the conditions of illumination; without this info lightness constancy can break down and a shadow can be seen as a darkly pigmented area

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