PSYC4365 - Chapter 5

Ewald Hering

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

Three steps to color perception

1. *Detection:* Wavelengths of light must be detected in the first place 2. *Discrimination*: We must be able to tell the difference between one wavelength (or mixture of wavelengths) and another 3. *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

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, that is seen only in relation to other colors - A "gray" 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 If we shine "blue" and "yellow" lights on the same patch of paper, the wavelengths will add, producing an additive color mixture Georges Seurat's painting La Parade (1887-88) illustrates the effect of additive color mixture with paints

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

Color space:

A three-dimensional space that describes all colors. There are several possible color spaces *RGB color space:* Defined by the outputs of long, medium, and short wavelength lights *HSB color space:* Defined by hue, saturation, and brightness - *Hue: *The chromatic (color) aspect of light - *Saturation: *The chromatic strength of a hue - *Brightness:* The distance from black in color space

Afterimages:

A visual image seen after a stimulus has been removed

Does everyone see colors the same way?—No

About 8% of male population, 0.5% of female population has some form of color vision deficiency: Color blindness

Animals provide insight into color perception in humans

Advertisements for bees to trade food for sex (for pollination) Colorful patterns on tropical fish and toucans provide sexual signals Primates - trichromatic Dogs - dichromatic Chickens - quadchromatic

Rods are sensitive to scotopic light levels.

All rods contain the same photopigment molecule: Rhodopsin All rods have the same sensitivity to various wavelengths of light Therefore, rods suffer from the problem of univariance and cannot sense differences in color - but this isn't as big of a problem because rods don't need to detect light Under scotopic conditions, only rods are active, so that is why the world seems drained of color

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

Achromatopsia:

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

Step 2: Color Discrimination - Univariance

An infinite set of different wavelength-intensity combinations can elicit exactly the same response from a single type of photoreceptor Therefore, one type of photoreceptor cannot make color discriminations based on wavelength Lights of 450 and 625 nm each elicit the same response from the photoreceptor whose responses are shown here and in Figure 5.2

Basic Principles of Color Perception

Color is not a physical property but a psychophysical property - "There is no red in a 700 nm light, just as there is no pain in the hooves of a kicking horse." - 700nm = 700nm ≠ red Most of the light we see is reflected - Typical light sources: Sun, light bulb, fire

Colors in Conjunction

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

Several types of color-blind people:

Deuteranope Protanope Tritanope Color-anomalous Cone monochromat Rod monochromat

Metamers

Different mixtures of wavelengths that look identical. More generally, any pair of stimuli that are perceived as identical in spite of physical differences - Red + green wavelengths of light can produce yellow - Yellow wavelengths of light also produce yellow

Protanope

Due to absence of L-cones

Deuteranope

Due to absence of M-cones

Tritanope

Due to absence of S-cones

Does everyone see colors the same way?—Yes

General agreement on colors People with limited color vocabulary can still differentiate between colors Some variation due to age (lens turns yellow)

Rod monochromat

Have no cones of any type; truly color-blind and badly visually impaired in bright light

Cone monochromat

Have only one cone type; truly color-blind

Color-anomalous

Have two types of cones (typically L- and M-cones) which are so similar that they can't make discriminations based on them

Physical constraints make constancy possible:

Intelligent guesses about the illuminant Assumptions about light sources Assumptions about surfaces

LGN and Color

Lateral geniculate nucleus (LGN) 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 *Cone-opponent cell:* A neuron whose output is based on a difference between sets of cones - In LGN there are cone-opponent cells with center-surround organization

Photopic:

Light intensities that are bright enough to stimulate the cone receptors and bright enough to "saturate" the rod receptors - Sunlight and bright indoor lighting are both photopic lighting conditions

Scotopic:

Light intensities that are bright enough to stimulate the rod receptors but too dim to stimulate the cone receptors - Moonlight and extremely dim indoor lighting are both scotopic lighting conditions - Color gets washed out in scotopic conditions

The retina contains four types of photoreceptors

Rods Cones - Short (blue) - Medium (green) - Long (red)

Opponent Color Theory in the LGN

Some LGN cells are excited by L-cone onset in center, inhibited by M-cone onsets in their surround (and vice-versa) - Red versus green Other cells are excited by S-cone onset in center, inhibited by (L + M)-cone onsets in their surround (and vice-versa) - Blue versus yellow

Color in the Visual Cortex

Some cells in LGN are cone-opponent cells - These respond to RED-center/GREEN-surround and vice-versa In primary visual cortex, double-opponent color cells are found for the first time - These are more complicated, combining the properties of two color opponent cells from LGN

Hue cancellation experiments

Start with a color, such as yellowish green The goal is to end up with pure green Shine some blue light to cancel out the yellow light Adjust the intensity of the blue light until there is no sign of either yellow or blue in the green patch

The three steps of color perception, revisited

Step 1: Detection. S, M, and L cones detect light Step 2: Discrimination. Cone opponent mechanisms discriminate wavelengths - [L - M] and [M - L] compute red vs. green - [L + M] - S and S - [L + M] compute yellow vs. blue Step 3: Appearance. Further recombination of the signals creates final color-opponent appearance

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 - To achieve color constancy, we must discount the illuminant and determine what the true color of a surface is regardless of how it appears

Opponent color theory:

The theory that perception of color is based on the output of three mechanisms, each of them based on an opponency between two colors: Red-green, blue-yellow, and black-white

Tricromacy

The theory that the color of any light is defined in our visual system by the relationships of three numbers, the outputs of three receptor types now known to be the three cones - Also known as the Young-Helmholtz theory Only with information from all 3 cones can we figure out color Example: - M cone fires at 50% so we know the color is orange or blue - If S cone is also firing at 75% we know it is blue - If L cone is also firing at 75% we know it is orange

History of color vision

Thomas Young (1773-1829) and Hermann von Helmholtz (1821-1894) independently discovered the trichromatic nature of color perception James Maxwell (1831-1879) developed a color-matching technique that is still being used today

Step 1: Color Detection

Three types of cone photoreceptors - S-cones detect short wavelengths (blue) - M-cones detect medium wavelengths (green) - L-cones detect long wavelengths (red) More accurate to refer to them as "short," "medium," and "long" rather than "blue," "green," and "red" since they each respond to a variety of wavelengths The L-cone's peak sensitivity is 565 nm, which corresponds to yellow, not red!

Does everyone see colors the same way?—Maybe

Various cultures describe color differently *Cultural relativism: *In sensation and perception, the idea that basic perceptual experiences (e.g., color perception) may be determined in part by the cultural environment

Unique Hues

We can use the hue cancellation paradigm to determine the wavelengths of unique hues *Unique hue:* Any of four colors that can be described with only a single color term: Red, yellow, green, blue - For instance, unique blue is a blue that has no red or green tint

Using all three cones

With three cone types, we can tell the difference between lights of different wavelengths Under photopic conditions, the S-, M-, and L-cones are all active The two wavelengths that produce the same response from one type of cone (M) produce different patterns of responses across the three types of cones (S, M, and L)