Chapter 5: Color Perception Flashcards
Perceiving Color
Color is perceptual experience evoked by wavelengths of light reaching the eyes
Color Vision
Ability to see difference between light and different wavelengths
- investigation of trichromacy and opponency, two psychological mechanisms that create our experience of color
Visible Spectrum
portion of electromagnetic spectrum in the range of 400-700 nm
- people with normal vision perceive differences in wavelength as differences in color
Spectral Power Distribution (SPD)
Intensity (power) of a light at each wavelength in the visible spectrum
- perceived color depends on SPD of light and how things reflect light
Heterochromatic Light
Light that consists of more than 1 wavelength
Monochromatic Light
Consists of only one wavelength
Achromatic Light (white light)
Light containing wavelengths from across visible spectrum with no really dominant wavelengths
- perceived as colorless
Spectral Reflectance
Proportion of light that a surface reflects at each wavelength
- refers to the amount of light that is reflected back to our eyes from the object’s surface
Perceptual counterparts of color consist of three independent dimensions
- Hue- quality usually referred to as color; perceptual characteristic most closely associated with wavelength of light
- Saturation- vividness (or purity or richness) of hue
- Brightness- perceived amount of light
Color Circle
2D depiction in which hue varies around circumference and saturation varies along any radius
Nonspectral Purples
Where red and blue ends of spectrum meet
- mixtures of shorted-wavelength violet and longest- wavelength red
- don’t exist on visual spectrum as single wavelength
Red Wavelength
650 nm
Yellow Wavelength
600 nm
Green Wavelength
550 nm
Cyan Wavelength
500 nm
Blue Wavelength
450 nm
Color Solid
3D depiction in which hue varies around circumference, saturation varies along any radius, and brightness varies vertically
Subtractive Color Mixture
Primaries for subtractive color mixing= red, yellow, and blue
Light reflected by mixture has certain wavelengths subtracted (absorbed) by each substance in mixture
Additive Color Mixture
Primaries for additive mixing= red, green, and blue
Compositions of mixture is result of adding together all wavelengths in all lights in mixture
Complementary Colors
Hues that are opposite each other on color wheel
- are pairs of colors that, when combined in equal proportion, are perceived as a shade of gray
Primary Color
Are any three color that can be combined in different proportions to produce a range of other colors
- cyan, magenta, yellow
Trichromatic Color Representation
Light evokes different response from three different types of cone photoreceptors in retina
- Young- Helmholtz trichromatic theory
Opponent Color Representation
Responses from cones are combined and processed in subset of RGCs and by color-selective neurons in brain
- Hering opponent- process theory
Trichromatic color representation experiments
Use of metameric matching provided early evidence that there were three types of color receptors
- Metamers- composed of different wavelengths of light but looking identical
Spectral Sensitivity of Photopigments
- evidence for trichromatic theory
- the spectral sensitivity function
- photopigments in each type of cone are most responsive to a particular range of wavelength
- spectral sensitivity functions may overlap considerably
*The principle of univariance means that means that color vision depends on crucially on the relative responses of multiple cone types
Spectral Sensitivity Function
Probability that cone’s photopigment will absorb photon of any given wavelength
Principle of Univariance
The charge in a cone cell’s firing rate does not depend on the wavelength of light
- The absorption of photon of light causes fixed response by cone regardless of photon’s wavelength
- also applies to rods and explains why night-vision is colorblind
Physiological evidence for trichromacy emerged from the work of […]
George Wald
Photocurrent measurements can be used as evidence for trichromacy
- directly measures an individual cone’s response to light
- produced three distinct patterns of sensitivity to light (spectral sensitivity curves of three different types of cones)
- trichromatic representation of wavelength can be thought of as form of data compression
Retinal Densitometry
High-resolution images of the retina
Opponent color representation
- at the level of the retina, most humans have three (some have four) types of cones
- color-sorting tasks, after images, and hue cancellation tasks all support the idea that there are four primaries
- there is evidence from the level of RGCs and beyond (LGN, visual cortex) that the visual system is organized in pairs of opposites
Color Blindness
Dichromacy
- two cones work properly, one doesn’t
- could be S, M, or L cones
Hue Cancellation
Experimental technique in which the person cancels out any perception of a particular color in a test light by adding light of the complementary color
- first physiological evidence for opponency: measurements of neurons in LGN of monkeys
Color Afterimages
Result from photopigment bleaching known as chromatic adaptation
Ex. If you start at green square, photopigment molecules in M-cones will bleach more than those in L-cones
- M-cones will become less sensitive than L-cones
Photopigment Bleaching
Lack of ability to absorption after phototransduction
Chromatic Adaptation
There’s a lot of one color that a visual system needs to adapt to
- kind of photopigment bleaching that results from exposure to relatively intense light consisting of narrow range of wavelengths
- discounting the Illuminant- estimate illuminance and reflectance
- adjusts cones to account for imbalance of wavelengths
Hering (color afterimages)
- Human visual system operates as if there were four basic colors
- 4 colors can be divided into 2 pairs of colors: red-green and blue-yellow
Physiological evidence for opponency
- Until 1950s: support was psychophysical; based on hue cancellation experiments
- Later research: existence of neural circuits underlying opponent color representations was confirmed
- circuits supporting color vision with four basic colors groups in two pairs of opposites - explains color categorization, color afterimages, and results of hue-cancellation
S cones
Bluish- greenish
M cones
Greenish- yellowish
L cones
Yellowish- reddish
+S-ML
Responds to short wavelength light and below baseline rate in response to medium and long- wavelength light
+ML-S
M and L cones respond above baseline and S cones respond below baseline
+L-M
L cones respond above baseline and M cones respond below baseline
+M-L
M cones respond above baseline and L cones respond below baseline
Color-opponent neurons in the visual pathway
- color-opponent RGCs
- other color-selective neurons in the visual pathway- including RGCs, LGN cells, and cortical cells
- double-opponent neurons: contributes of high acuity of color processing
Color Contrast
Perception of surrounded color as shifted toward complement of surrounding color
Color Assimilation
Perception of surrounded color as shifted toward non complementary surrounded color
aka “spreading effect”
Single-Opponent Neuron
Provide info about wavelength of light within uniformly colored regions, but don’t have much info about visual edges
Double-Opponent Neuron
Provide info about color edges
Suppressed response doesn’t go all the way down
Color Constancy
- tendency to see surface as having same color under illumination by lights with different SPD
- color is perceived as constant under different types of light as long as enough wavelengths are available
- visual system tends to achieve color constancy through chromatic adaptation
- SPD of reflected lights is determined by multiplying intensity of illuminating light at each wavelength by reflectance of paper at each wavelength
- wanting to see color how we know it
Lightness Constancy
- tendency to see surface as having same lightness under illumination by very different amounts of lights
- suggests relative lightness (or darkness) of an object is perceived as consistent under different intensities of light
Lightness
Perceived reflectance of a surface (proportion of illumination that surface appears to be reflecting)
Ratio Principle
Perceived lightness of region is not based on absolute amount of light reflected from region, but on relative amounts reflected from region and its surround
Color Blindness
Inherited deficiencies of color vision
- genetic
- sex-linked
- other causes of color deficiency include diabetes, glaucoma, certain medications, and chemical exposure
Rod Monochromacy
Only rods
Extremely sensitive to light
Low visual acuity
Cone Monochromacy
Only one type of cone
Can process light with some acuity
Dichromacy
Has two functional cones
- limited form of color vision
Types of dichromacy
- Protanopia: person lacks L-cones
- Deuteranopia- person lacks M-cones
- Tritanopia: person lacks S-cones
Ishihara Color Vision Test
Symbols can be seen by people with normal color vision but not by people with particular color vision deficiencies
Color Center in Brain
Area V4
- ventral pathway
Cortical Achromatopsia
Color blindness from brain damage
- rarer than color deficiencies and occurs when there’s damage to visual cortex
