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Chapter 5: The Perception of Color
Review
Light is both a particle(photon) and a wave
Reflected: energy that is redirected when it strikes a surface, usually back to its points of origin
Most of the light is reflected
typically white reflects almost all the light
Color
Color DOES NOT exist in the physical world
Color is not a physical property but a psychophysical property
not being able to see color would impair survival
Color is how brains interpret and differentiate light information
color only exists in our brain
Most mammals have 2 cone types, humans have 3
Color Perception
“The perception of color is generated Internally”
Detections: The wavelength of light must be detected in the first place
There are 3 types of cones with different sensitivities to any given wavelength of light given
S-cones detect short wavelengths (“blue” range).
M-cones detect medium wavelengths (“green” range).
most sensitive to mid-wavelength
acquiring most information from this region of the visible spectrum
We can differentiate different shades of green
L-cones detect long wavelengths (“red” range). peak!!
Photopic: light intensities that are bright enough to stimulate the cone receptors and enough to “saturate” the rods to their maximum response
Ex: sunlight and bright indoor lighting
Scotopic: light intensities that are bright enough to stimulate the rod receptors but too dim to stimulate the cone receptors
Ex: Moonlight and dim indoor lighting
4 types of photoreceptors give 3 color discrimination
rods do not contribute to color perception
Duplex Vision System: the idea that our visual system can operate in fundamentally different ways, depending upon the conditions in the environment
Scopic deals with light and core
Discrimination: we must be able to tell the difference between one wavelength ( or mixture of wavelengths_ and another
Important!! The principle of univariance is an infinite set of different wavelength and intensity combinations that can elicit exactly the same response from a single type of photoreceptor
Therefore, one type of photoreceptor cannot make color discriminations based on wavelength.
they are eliciting the same firing rate
receptor response is fixed whereas wavelength intensity can elicit different responses
Rods are sensitive to scotopic light levels
all rods contain the same photopigment molecules: rhodopsin
Therefore, all rods have the same sensitivity to different wavelengths of light
There is only one type of Rod
every rod will react in the same type of way
.– Consequently, rods obey the principle of univariance and cannot sense color differences.
There are individual differences in Cones and rod distribution which could affect periphery visions
lighting conditions
distance
spatial frequency
mid level SF is where we see best because it lines up best with our ganglion cells
Rate of Movement
Color Discrimination
Important!! Trichromacy is the theory that the color of any light is defined in our visual system by the relationships of three numbers, the outputs of 3 receptor types now known to be 3 cones.
Thomas Young and Hermann von Helmholtz proposed the trichromatic theory of color perception in humans aka “Young-Helmholtz Theory”
Trichromatic Theory is all colors can be made by matching 3 basic colors (red, green, blue)
we have 3 basic cone types are sensitive to corresponding wavelengths
degree of activation dictates color
ex. S-cones moat Activated, see blue
Metamers a different mixtures of wavelengths that look identical; generally, any pair of stimuli that are perceived as identical in spite of physical differences
James Maxel developed a color-matching technique used today still
There are 2 types of Color-Mixing
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 2 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
subtractive color Mixing of pigments
Lateral Geniculate Nucleus (LGN) has cells that are maximally stimulated by spots of light.
visual pathways stop in LGN on the way from the retina to the 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-srround organization.
There are color-opponent ganglion cells that are antagonist red-green or blue-yellow centre surround
Color Appearance
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 Space: a 3-dimensional space that describes all color
RGB color space: defined by the outputs of long, medium, and short wavelengths lights (ie. red, green, and blue)
HBS color space: defined by hue, saturation, and brightness !!Important!!
hue: the chromatic (color) aspect of light
saturation: the chromatic strength of hue
Brightness: the distance from black in color space.
Nonspectral colors: some colors that do not correspond to a single wavelength of light Ex. Purple_magneta
no single wavelength on the spectrum will have the hue of this mixture of long and short wavelength
Ewald Hering noticed that some color combinations are “legal” while others are “illegal”
The concept of legal and illegal colors comes from opponent process theory and the way our visual system processes color information.
It refers to color combinations that are either perceptually possible (legal colors) or impossible to see (illegal colors) due to the way our opponent neurons encode color information.
we can have bluish green (cyan, reddish-yellow(orange), or bluish red (purple)
but we cannot have green or bluish-yellow
Opponent color theory: The theory that perception of color depends on the output of three mechanisms, each of them based on an opponency between two colors: red–green, blue–yellow, and black–white.
Some LGN cells are excited by L-cone activation in center, inhibited by M-cone activation in their surround (and vice versa) Ex.Red versus green
Other cells are excited by S-cone activation in center, inhibited by(L + M)-cone activation in their surround (and vice versa).
Hue Cancellation experiments are used to determine the wavelength of unique hues
Unique hue: any of 4 colors that can be described with only a single color term: red, yellow, green, blue.
Afterimages: A visual image seen after a stimulus has been removed.
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.69
Steps in Color Perception
Detection
S-, M-, and L-cones detect light.
each cone responds to a different range of wavelength light.
Discrimination
Cone-opponent mechanisms discriminate wavelengths.
[L – M] and [M – L] compute something like red vs. green.
[L + M] – S and S – [L + M] compute something like blue vs. yellow.
Appearance
Further transformations of the signals create final color-opponent appearance.73
Color in The Visual Cortex
V!, V2, and V4 all involved in color perception, but not exclusively
no particular place in the cortex specialized for color
Achromatopsia: loss of color vision from brain damage
Language and Color
General agreement on colors
Basic color terms: Single words that describe colors, are used with high frequency and have meanings that are agreed upon by speakers of a language.
Cultural relativism: in sensation and perception, the idea that basic perceptual
Genetic Differences in Color Perception
About 8% of males and 0.5% of females have some form of color vision deficiency: “color blindness.”
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.
Several types of color-anomalous people
Deuteranope: Due to the absence of M-cones.
Protanope: Due to the absence of L-cones.
Tritanope: Due to the absence of S-cones.
Cone monochromat: Has only one cone type; truly color-blind.
Rod monochromat: Has no cones of any type; truly color-blind and very visually impaired in bright light.
Anomia: Inability to name objects or colors in spite of the ability to see and recognize them. Typically due to brain damage.
Color anomia: Can see color but cannot name it
!!Important!!Color constancy: The tendency of a surface to appear the same color under a fairly wide range of illuminants.
Illuminant: The light that illuminates a surface.
