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The electromagnetic Spectrum
Light is both a particle (photon) and a wave.
Reflected
Energy that is redirected when it strikes a surface, usually back to its point of origin. Most of the light we see is reflected.
- Typical light sources: Sun, lightbulb, fire
- We see only part of the electromagnetic spectrum, between 400 and 700 nm.
Color
does not exist in the physical world. is not a physical property but a psychophysical property. is how brains interpret and differentiate light information.
Dichromatic
two cones
Trichromatic
three cones (humans)
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.
Three types of cone photoreceptors
S-cones detect short wavelengths ("blue" range).
M-cones detect medium wavelengths ("green" range).
L-cones detect long wavelengths ("red" range). (Their spectral sensitivities overlap)
Humans to wavelengths
Most sensitive to mid-wavelength. We can differentiate the most different shades of GREEN.
Photopic
Light intensities that are bright enough to stimulate the cone receptors and bright enough to "saturate" the rod receptors to their maximum responses. (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. Cones do not stimulate during scotopic (at night/dim/etc) and instead rod receptors do.
(EX: Moonlight and extremely dim indoor lighting.)
Optic Disc
Experimental Consideration with seeing color in peripheral visions: We cannot see color in our peripheral vision.
Duplex Vision
the concept that the human eye functions with two distinct visual systems, one for bright light conditions (photopic vision using cones) and another for low light conditions (scotopic vision using rods)
The principle of univariance
An infinite set of different wavelength and 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.
Rod sensitivity
Rods are sensitive to scotopic light levels.
All rods contain the same photopigment molecule: rhodopsin.
Therefore, all rods have the same sensitivity to different wavelengths of light.
Consequently, rods obey the principle of univariance and cannot sense differences in color.
Trichromatic theory
is all colors can be made by matching three basic colors (red, green, blue)
Young-Helmholtz theory
credited for first proposing the trichromatic theory of color perception in humans
Metamers
Different mixtures of wavelengths that look identical; more generally, any pair of stimuli that are perceived as identical in spite of physical differences.
Maxwell's color matching theory
mixing red, green, and blue to get a blueish spotlight.
Additive color mixing
A mixture of lights.
Subtractive color mixing
A mixture of pigments.
Lateral geniculate nucleus (LGN)
has cells that are maximally stimulated by spots of light. The visual pathway stops in LGN on the way from the retina to the visual cortex.
Cone-opponent cell
A neuron whose output is based on a difference between sets of cones. In LGN with center-surround organization.
Color space
A three-dimensional space that describes all colors.
RGB color space
Defined by the outputs of long, medium, and short wavelength lights (i.e., red, green, and blue).
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.
Nonspectral colors
Some colors that we see do not correspond to a single wavelength of light. Purple is one. It is a mixture of a long and short wavelength.
Legal and illegal colors
We can have bluish green (cyan), reddish yellow (orange), or bluish red (purple). We can not have reddish 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.
Color opponent Ganglion cells
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). (EX; Blue versus yellow)
Hue cancellation experiments
Used 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.
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.
Step 1 Detection
S-, M-, and L-cones detect light. Each cone responds to a different range of wavelengths of light.
Step 2 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.
Step 3 Appearance
Further transformations of the signals create final color-opponent appearance.
Color in the visual cortex
V1, 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
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.
Lindsey and Brown (2014)
asked Americans to name color patches, everyone used the 11 "basic" color names. Other color terms were used by fewer and fewer people.
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.
Linguistic relativism
People have different names for the colors they see. Language can affect how we memorize and name colors. This is a color naming test designed to measure your personal blue-green boundary.
Genetic Differences in Color Perception
About 8% of males and 0.5% of females have some form of color vision. Deficiency: "color blindness.
Qualia
is a term that refers to the conscious experience (in this case of a color). We can talk about your _____—your experiences, but only you can actually experience them.
Tetrachromatic color vision
color vision based on four numbers per patch of light. In fact, some women can end up with four different cone pigments, and in very rare cases, that produces this.
Color-anomalous
A term for what is usually called "color blindness." Most "color-blind individuals can still discriminate based on wavelength. Those discriminations are just different from the norm.
Several types of color-anomalous people
- Deuteranope: Due to absence of M-cones.
- Protanope: Due to absence of L-cones.
- Tritanope: Due to absence of S-cones.
Koniocellular and parvocellular layers
the S-cone signals go through the koniocellular layers in the LGN, while the M- and L-cone opponent signals are mostly found in the parvocellular layers
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.
Synesthesia
When one stimulus evokes the experience of another stimulus that is not present.
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.
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.
The mantis shrimp
extraordinary eyes
