An inability to perceive colors that is caused by damage to the central nervous system.
spectral power distribution
The physical energy in a light as a function of wavelength.
The tendency of a surface to appear the same color under a fairly wide range of illuminants.
Referring to any color that lacks a chromatic (hue) component. Black, white, or gray.
A color, such as brown or gray, that is seen only in relation to other colors. For example, a “gray” patch in complete darkness appears white.
A stimulus whose removal produces a change in visual perception or sensitivity.
A color that can be experienced in isolation.
The light that illuminates a surface.
problem of univariance
The fact that an infinite set of different wavelength-intensity combinations can elicit exactly the same response from a single type of photoreceptor. One photoreceptor type cannot make color discriminations based on wavelength.
Any of four colors that can be described with only a single color term: red, yellow, green, blue. Other colors (e.g., purple or orange) can be described as compounds (reddish blue, reddish yellow).
spectral reflectance function
The percentage of a particular wavelength that is reflected from a surface.
A cell type—found in the retina, lateral geniculate nucleus, and visual cortex—that, in effect, subtracts one type of cone input from another.
Referring to light intensities that are bright enough to stimulate the cone receptors and bright enough to “saturate” the rod receptors (that is, drive them to their maximum responses).
The perceptual consequence of the physical intensity of a light.
A failure to recognize objects in spite of the ability to see them, typically due to brain damage.
An individual who suffers from color blindness that is due to the absence of L-cones.
Referring to light intensities that are bright enough to stimulate the rod receptors but too dim to stimulate the cone receptors.
An individual who suffers from color blindness that is due to the absence of S-cones.
An individual with only one cone type. Individual is truly color-blind.
A cone that is preferentially sensitive to middle wavelengths; colloquially (but not entirely accurately) known as a “green cone.”
subtractive color mixture
A mixture of pigments. If pigments A and B mix, some of the light shining on the surface will be absorbed by A, and some by B. Only the remainder contributes to the perception of color.
A cone that is preferentially sensitive to long wavelengths; colloquially (but not entirely accurately) known as a “red cone.”
trichromatic theory of color vision (or trichromacy)
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.
The chromatic (colorful) aspect of color (red, blue, green, yellow, and so on).
The chromatic strength of a hue. White has zero saturation, pink is more saturated, and red is fully saturated.
The percentage of light hitting a surface that is reflected and not absorbed into the surface. Typically reflectance is given as a function of wavelength.
A visual image seen after the stimulus has been removed.
An inability to name objects in spite of the ability to see and recognize them (as shown by usage), typically due to brain damage.
additive color mixture
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.
An individual with no cones of any type. In addition to being truly color-blind, these individuals are badly visually impaired in bright light.
A cone that is preferentially sensitive to short wavelengths; colloquially (but not entirely accurately) known as a “blue cone.”
The three-dimensional space, established because color perception is based on the outputs of three cone types, that describes the set of all colors.
A color perception effect in which two colors bleed into each other, each taking on some of the chromatic quality of the other.
opponent color theory
The theory that perception of color is based on the output of three mechanisms, each of them resulting from an opponency between two colors: red-green, blue-yellow, and black-white.
Different mixtures of wavelengths that look identical. More generally, any pair of stimuli that are perceived as identical in spite of physical differences.
lateral geniculate nucleus (LGN)
A structure in the thalamus, part of the midbrain, that receives input from the retinal ganglion cells and has input and output connections to the visual cortex.
An individual who suffers from color blindness that is due to the absence of M-cones.
An afterimage whose polarity is the opposite of the original stimulus. Light stimuli produce dark negative afterimages. Colors are complementary; for example, red produces green, and yellow produces blue.
The point at which an opponent color mechanism is generating no signal. If red–green and blue–yellow mechanisms are at their neutral points, a stimulus will appear achromatic.
Another way to refer to cone-opponent cells.
A color perception effect in which the color of one region induces the opponent color in a neighboring region.
A cell type, found in the visual cortex, in which one region is excited by one cone type, combination of cones, or color and inhibited by the opponent cones or color (e.g., R+/G–). Another adjacent region would be inhibited by the first input and excited by the second (thus, in this example, R–/G+).
In sensation and perception, the idea that basic perceptual experiences (e.g., color perception) may be determined in part by the cultural environment.
A better term for what is usually called “color-blind.” Most “color-blind” individuals can still make discriminations based on wavelength, but the discriminations are different from the norm.