Unit 11 – Color Perception

▪ Wavelengths of Light and Color

humans see wavelengths of light that vary between 400 and 700 nm

Visual spectrum

the range of wavelengths that our eyes can detect (400-700)

o Heterochromatic light

white light, consisting of many wavelengths

o Monochromatic light

light consisting of one wavelength

o Spectral reflectance

the ratio of light reflected by an object at each wavelength

▪ Achromatic Lightness

when objects reflect all light wavelengths equally

• white (90%) to gray (50%) to Black (10%)

▪ Hue

the color quality of light, corresponding to the color names we use, such as orange, green, indigo, and cyan

• hue is the quality of color

▪ Saturation

• the purity of light

• the more saturated the stimulus is, the stronger the color experience

• the less saturated the stimulus, the more it appears white or gray or black (achromatic)

▪ Lightness

the psychological experience of the amount of light that gets reflected by a surface

▪ Brightness

the perceived intensity of the light present

▪ Additive color mixing

the creation of a new color by a process that adds one set of wavelengths to another set of wavelengths

▪ Subtractive color mixing

color mixing in which a new color is made by the removal of wavelengths from a light with a broad spectrum of wavelengths

▪ Metamer

a psychophysical color match between two patches of light that have different sets of wavelengths

▪ Color-Matching Experiments

observers adjusted the amounts of three different wavelengths of light mixed together in a "comparison field" until the color of this mixture matched the color of a single wavelength in a "test field"

▪ The Retina and Color

S, M, and L cone

o S-cone

the cone with its peak sensitivity to short-wavelength light, around 420 nm (blue)

o M-cone

the cone with its peak sensitivity to medium-wavelength light, around 535 nm (green)

o L-cone

the cone with its peak sensitivity to long-wavelength light, around 565 nm (yellow)

o The response of cones to a 500-nm light

each cone system responds to this light but with a weaker or stronger response. Color is partially determined by this pattern of responses of each cone to any particular wavelength

▪ Univariance

the principle whereby any single cone system is colorblind, in the sense that different combinations of wavelength and intensity can result in the same response from the cone system

o Why More Than One Receptor is Necessary to See in Color?

we are unable to see color with only one cone, with the help of three cones that all respond in different levels of strength, we are able to perceive color and its intensities.

▪ The Trichromatic Theory of Color Vision

the theory that the color of any light is determined by the output of the three cone systems in our retinae

▪ The Opponent Theory of Color Perception

the theory that color perception arises from three opponent mechanisms, for red-green, blue-yellow, and black-white

o Hering’s model of opponent processes

all colors on the color circle can be represented by two pairs of opposing colors

▪ Findings That Support Opponent Theory

Perception of color combination (spontaneous) sorting into four groups across all cultures Afterimages

Sorting into four groups

green, red, yellow, and blue (western and non-western cultures support this)

o Perception of color combinations

non-primary colors look like combinations of two primary colors but our perception of color supports that red and green don't combine and blue and yellow don't combine (it it hard to imagine with color they would make)

o (Spontaneous) sorting into four groups, across all cultures

People across cultures tend to group objects in similar ways, showing that some perceptual organization is universal, not just learned

o Afterimages

visual images that are seen after an actual visual stimulus has been removed

o Simultaneous color contrast

a phenomenon that occurs when our perception of one color is affected by a color that surrounds it

o Cone-opponent cells in LGN

neurons that are excited by the input from one cone type in the center but inhibited by the input from another cone type in the surround

o Color-opponent cells in V1

neurons that are excited by one color in the center and inhibited by another color in the surround , or neurons that are inhibited by one color in the center and excited by another color in the surround

o Double-opponent cells

cells that have a center, which is excited by one color and inhibited by the other, in the surround, the pattern is reversed

▪ Color Processing in the Brain

Cells in V4 (not far from V1) respond to perceived color, not ot the wavelengths that stimulate in the eye, but we do not know yet how the color information is transformed in the brain.

▪ Why are our veins GREEN?

green light has a shorter wavelength and penetrates skin better, reflecting back to your eyes, especially on lighter skin tones

▪ Color Deficiency

the condition of individuals who are missing one or more of their cone systems

o Rod Monochromacy

a condition in which a person has no functioning cones and therefore can be described as truly color blind

o Cone Monochromacy

(extreme rare) they have one cone but not the other two

• s-cone monochromacy is more common in men, has similar side effects to rod monochromacy but less severe, also have poor acuity and high sensitivity to bright light

o Dichromacy

only two of the three cone photoreceptors in the eye function, limiting color perception to two primary dimensions

▪ A Description of Different Types of Color Vision

tritanopia, protanopia, and deuteranopia

o Tritanopia

a lack of s-cones, leading to blue-yellow color deficiency

• this trait is rare and is not sex-linked

o Protanopia

a lack of L-cones, leading to red-green deficiency

• this trait is sex linked and thus more common in men

o Deuteranopia

a lack of M-cones, leading to red-green deficiency

• this trait is sex linked and thus more common in men

▪ Cortical Achromatopsia

loss of color vision due to damage to the occipital lobe

▪ Light source

monochromatic light source, yellow light. Under these conditions, only the yellow-blue opponent channel can help us do color discriminations. You will experience the world as a color-deficient individual does.

▪ Let’s examine the errors we made

brown-blue, red-green, yellow-orange

▪ Brown-blue

look alike

▪ Red-green

difficult to tell them apart

▪ Yellow-orange

look alike as well

▪ Yellow surfaces

this color is reflected and coded along the yellow-blue channel.

▪ Red and green surfaces

these surfaces reflect a little bit of yellow light

• cannot be coded by the yellow channel.

▪ Blue surfaces

blue is the opponent color of yellow. So, the yellow light will remove or “kill” the blue (go back to the Hue Cancellation experiments). Blue surfaces reflect only very little light, so they will look very dark.

▪ Brown surfaces

these surfaces don’t reflect a lot of light in general, so they will look dark.

▪ Orange surfaces

what is orange? Yellow + red. Yellow is reflected, but red is not and cannot be coded by the yellow channel.

Hue cancellation

an experiment in which observers cancel out the perception of a particular color by adding light of the opponent color

Hue cancellation experiments

• Start with a color, such as bluish green.  - The goal is to end up with 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

▪ What is constancy?

the ability to perceive an object as the same under different conditions

▪ What is color constancy?

the ability to perceive the color of an object despite changes in the amount and nature of illumination

▪ What is lightness constancy?

the ability to perceive the relative reflectance of objects despite changes in illumination

▪ Color-Based Acuity

1) cones are clustered together around the fovea and are in the midget system (one photoreceptor - one ganglion cell) giving us spatial acuity. 2) Three different types of cones respond maximally to three different wavelengths (S, M and L cones).

▪ 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

o Cultural Relativism

not judging a culture but trying to understand it on its own terms

▪ Eleanor Rosh vs. Debi Roberson

Rosch says color perception is universal

• Roberson says it depends on language and culture

▪ Synesthesia

describing one kind of sensation in terms of another ("a loud color", "a sweet sound")

o Watercolor illusion in synesthesia

brighter chromatic contour on the inside and dark chromatic contour on the outside. The brighter color spreads into the entire enclosed area