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