Perception of Color
Spectral reflection function (2)
1. Brain measures actual distributions of wavelengths entering eye 2. Considers surrounding environment to take into account color of the illuminant
McCann, McKee, Taylor experiment (6)
1. Collection of color patches under white light 2. Gray patch excites S, M, L equally 3. Gray looks gray and green looks green 4. Change illuminant to reddish light 5. Green patch excites S, M, L equally 6. Gray still looks gray and green still looks green
Three steps to color vision
1. Detect light (cones in retina) 2. Discriminate colors: tell difference between lights of different wavelengths 3. Perceive color: appearance of color is interpretation of sensory input
Yes, everyone sees color the same way (2)
1. General agreement on colors 2. Some variation due to age
No, everyone doesn't see color the same way
8% of males and 0.5% of females have color blindness
Best and worst stimulus for single opponent cell
Best: Red inside (L + red) Worst: Green inside
Young-Helmholtz Theory
Can tell difference between lights of different wavelengths w 3 cone types
Easier to remember which of two colors you have seen if choices are ___
Categorically different
LGN cells have ___
Center-surround receptive fields
Color contrast
Central square takes on chromatic attributes of that are opposite surround
In LGN and cortex there are color-opponent cells with center-surround receptive fields based on ___
Color
Trichromacy
Color is defined by relationship of 3 numbers, which are outputs of 3 cones
Ewald Hering
Color vision comes from two opponent processes
Color assimilation
Colors blend locally
Metamers
Different mixtures of wavelengths that look identical
Nervous system extracts color info by comparing outputs of ____
Different sets of cones
Spectral reflection function
Graph of how object reflects light across wavelengths
Difference between L+M and S
How blue image is
Difference between L-M cones
How red image is
Problem of Univariance (complex definition)
Infinite set of different wavelength-intensity combos can elicit same response from single type of photoreceptor
Visual pathway stops in ___ on the way from retina to visual cortex
LGN
Sum of L-M cones
Light intensity
Color vision begins with ___
Light reaching our eyes
L-type
Long wavelengths. Red
Double opponent cells
Look for specific color oppositions
M-type
Middle wavelengths. Green
Additive color mixing
Mixture of lights. Light A and light B both reflected from surface onto eye = effects of two lights add together
Subtractive color mixing
Mixture of pigments. Pigment A and pigment B mix = some light will be absorbed by A and some by B. Only remainder is perceived
Example of problem of univariance
Moonlit world appears drained of color because only one type of rod photoreceptor transducing light
Color opponent cell
Neuron whose output is based on difference between sets of cones
Protanope
No L cones
Deuteranope
No M cones
Tritanope
No S cones
Rod cone type (truly color blind and impaired in bright light)
No cones
Luminance change and hue change
No shadow
Problem of Univariance (simple definition)
One type of photoreceptor can't make color discriminations based off wavelength
Cone monochromat (truly color blind)
Only one cone type
Cultural relativism (maybe)
Perceptual experiences affected by cultural environment
___ make color constancy possible
Physical constraints
Color is not a physical property but a ____ property
Psychophysical
Best stimulus for double opponent cells
Red inside, green outside (L+/M- red, L-/M+ green)
Channels in Ewald Herring theory (2)
Red-green, blue-yellow
Three types of cone photoreceptors
S-type, M-type, L-type
Luminance change without hue change
Shadow
Visual system knows that brightness changes across ___ but ___ does not
Shadow boundary, hue
S-type
Short wavelengths. Blue
Illuminant
Source of light
Color constancy
Tendency of surface to appear same color under wide range of illuminants
Color anomalous
Two cones (L and M) are so similar they can't make discriminations
Example of subtractive color mixing
White light passed through colored filters