Color Perception

ewald hering

(1834-1918) noticed that some color combinations are "legal" while others are "illegal." We can have bluish green (cyan), reddish yellow (orange), or bluish red (purple). We cannot have reddish green or bluish yellow.

s cones

(420 nm): Cones that are preferentially sensitive to short wavelengths ("blue" cones)

m cones

(535 nm): Cones that are preferentially sensitive to middle wavelengths ("green" cones)

l cones

(565 nm): Cones that are preferentially sensitive to long wavelengths ("red" cones)

dichromats

(M 2.4%, F 0.03%) Protanope: No L-cones (M 1.3%, F 0.02%) Deuteranope: No M-cones (M 1.2%, F 0.01%) Tritanope: No S-cones (M 0.001%, F 0.03%)

anomalous trichromats

(M 6.3%, F 0.37%) Protonomalous - L-cone (M 1.3%, F 0.02%) Deuteranomalous - M-cone (M 5.0%, F 0.35%) Tritanomalous - S-cone defect (M/F 0.0001%)

univariance

- Rods experience problem of ... - All use rhodopsin - Same sensitivity - In scotopic light only rods are active - No color differentiation

basic principals of color perception

- color detection - color discrimination - color appearance

lateral geniculate nucleus

... has cells that are maximally stimulated by spots of light. Visual pathway stops in LGN on the way from retina to visual cortex. LGN cells have receptive fields with center-surround organization.

physical constraints

... make constancy possible. Intelligent guesses about the illuminant Assumptions about light sources Assumptions about surfaces

tetrachromancy

4 cones. found in less than 2% of women and no men

color contrast

A color perception effect in which the color of one region induces the opponent color in a neighboring region.

color assimilation

A color perception effect in which two colors bleed into each other, each taking on some of the chromatic quality of the other

unrelated color

A color that can be experienced in isolation.

related color

A color, such as brown or gray, which is seen only in relation to other colors. a grey patch in complete darkness appears white

additive color mixing

A mixture of lights If light A and light B are both reflected from a surface to the eye, in the perception of color, the effects of those two lights add together.

subtractive color mixing

A mixture of pigments If pigment A and B mix, some of the light shining on the surface will be subtracted by A and some by B. Only the remainder contributes to the perception of color.

cone opponent cell (or color opponent cell)

A neuron whose output is based on a difference between sets of cones

cone opponent cell

A neuron whose output is based on a difference between sets of cones. In LGN there are ... with center surround organization

color anomalous

A term for what is usually called "color blindness." Most "color-blind" individuals can still make discriminations based on wavelength. Those discriminations are just different from the norm

color space

A three-dimensional space that describes all colors. There are several possible color spaces

afterimages

A visual image seen after a stimulus has been removed.

color blindness

About 8% of male population and 0.5% of female population has some form of color vision deficiency (anomaly)

negative afterimage

An afterimage whose polarity is the opposite of the original stimulus. - Light stimuli produce dark negative afterimages. - Colors are complementary. Red produces green afterimages and blue produces yellow afterimages (and vice versa). - This is a way to see opponent colors in action. - There is no way of explaining these after-images with Trichromacy alone.

achromatopsia

An inability to perceive colors that is caused by damage to the central nervous system.

achromatopsia

An inability to perceive colors; caused by damage to the central nervous system

problem of univariance

An infinite set of different wavelength-intensity combinations can elicit exactly the same response from a single type of photoreceptor. one type of photoreceptor cannot make color discriminations based on wavelength.

isolation, influence each other

Colors very rarely appear in ... Usually, many colors are present in a scene. When many colors are present, they can ...

hsb color space

Defined by hue, saturation, and brightness

rbg color space

Defined by the outputs of long, medium, and short wavelength lights (i.e., red, green, and blue).

metamers

Different mixtures of wavelengths that look identical (more generally: any pair of stimuli that are perceived as identical in spite of physical differences)

subtractive

Filters are ... because they absorb light Different filters absorb different wavelengths

cultural relativism

In S&P, the idea that basic perceptual experiences (e.g., color perception) may be determined in part by the cultural environment.

photopic vision

Light intensities bright enough to stimulate the cone receptors and "saturate" the rod receptors. - day or bright light - increased color perception and sharpness - requires central vision - cones stimulated and rods saturated

scotopic vision

Light intensities that are bright enough to stimulate the rod receptors but too dim to stimulate the cone receptors - nighttime or dim light - decreased visual acuity - no color perception - requires peripheral vision - rods are stimulated but cones are not

mescopic vision

Light intensities that stimulate cone and rod receptors (roughly .001-3 cd/m²) - twilight or transitional light - mixed color perception - both rods and cones stimulated

blue versus yellow

Other cells are excited by S-cone onset in center, inhibited by (L + M)-cone onsets in their surround (and vice-versa)

qualia

Private conscious experiences of sensation and perception. The question, "Is my perception of blue the same as your perception of blue?" is a question about ....

reflectance curve

Proportion of light at different wavelengths that is reflected from a pigment.

monochromats

Rod only - No cones (M/F 0.00001%) Cone only - One type of cone (M/F 0.00000001%)

basic color terms

Single words that describe colors and have meanings that are agreed upon by speakers of a language

red versus green

Some LGN cells are excited by L-cone onset in center, inhibited by M-cone onsets in their surround (and vice-versa)

hue cancellation experiment

Start with a color, such as bluish green. Goal is = pure blue. Shine some red light to cancel out the green light. Adjust the intensity of the red light until there is no sign of either green or red in the blue patch.

pigments

Substances that absorb light at some wave lengths and reflect light at others.

saturation

The chromatic strength of a hue

brightness

The distance from black in color space

sex linked

The genes that produce photopigments are carried on the X chromosome

distribution

The light coming from an object is composed of a ... of different wavelengths

illuminant

The light that illuminates a surface

color constancy

The tendency of a surface to appear the same color under a fairly wide range of illuminants. Must discount the illuminant and determine what the true color of a surface is regardless of how it appears

trichromancy

Theory that color of any light is defined in our visual system by the relationships of outputs of three receptor types now known to be the three cones. Thomas Young (1773-1829) and Hermann von Helmholtz (1821-1894) independently discovered the ... nature of color perception.

detection

Wavelengths of light must be detected in the first place. S, M, and L cones ... light

unique hue

We can use the hue cancellation paradigm to determine the wavelengths of ... Any of four colors that can be described with only a single color term: red, yellow, green, blue.

discrimination

We must be able to tell the difference between one wavelength (or mixture of wavelengths) and another. Cone opponent mechanisms discriminate wavelengths

surrounding context

We often base perception of color on .... Perceived color can disappear when contextual color is removed

400 and 700

We see only part of the electromagnetic spectrum, between .... nm.

appearance

We want to assign perceived colors to lights and surfaces in the world and have those perceived colors be stable over time, regardless of different lighting conditions. Further recombination of the signals creates final color-opponent appearance

different wavelengths

With three cone types, we can tell the difference between lights of ... Under photopic conditions, the S-, M-, and L-cones are all active. ... (chromatic light) elicit different responses to each cone type

color agnosia

can see but not know what it is

psychophysical

color is not a physical property, but a ... property

three steps to color perception

detection discrimination appearance

double opponent

in primary visual cortex ... color cells are found for the first time these are more complicated, combining the properties of two color opponent cells from LGN

anomia

inability to name

physical wavelength

mixing wavelengths does not change the ... ADDING a wavelength of 500 to one of 600 does not Create a wavelength of 550!! Nor the sum 1100. It produces a change in our PSYCHOPHYSICAL REALITY not in the PHYSICS of the light.

reflected

most of the light we see is ...

opponent color theory

perception of color depends on the output of three mechanisms, each of them based on an opponency between two colors: red/green blue/yellow black/white

cone opponent cells

some cells in the LGN are .... these respond to red center/green surround and vice versa

pointilism

style of painting in which additive color mixtures are achieved by visually by placing dots of different colors in close proximity to each other, rather than the subtractive mixtures that are obtained when pigments are mixed together in the same location.

hue

the chromatic (color) aspect of light

general agreement

there is a ... on colors. some variation due to age as the lens turns yellow