Chapter 5 & 7 Definitions
• 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
• scotopic: Light intensities that are bright enough to stimulate the rod receptors but too
dim to stimulate the cone receptors. Compare scotopic and mesopic.
• photopic: Light intensities that are bright enough to stimulate the cone receptors and
bright enough to “saturate” the rod receptors (i.e., drive them to their maximum
responses). Compare scotopic and mesopic.
• S-cone: A cone that is preferentially sensitive to short wavelengths; colloquially (but not
entirely accurately) known as a “blue cone.”
• M-cone: A cone that is preferentially sensitive to middle wavelengths; colloquially (but not
entirely accurately) known as a “green cone.”
• L-cone: A cone that is preferentially sensitive to long wavelengths; colloquially (but not
entirely accurately) known as a “red cone.”
• trichromatic theory of color vision (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–
• metamers: Different mixtures of wavelengths that look identical. More generally, any pair
of stimuli that are perceived as identical in spite of physical differences.
• 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
• subtractive color mixture: A mixture of pigments. If pigments 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.
• color space: The three-dimensional space, established because color perception is
based on the outputs of three cone types, that describes the set of all colors.
• hue: The chromatic (colorful) aspect of color (red, blue, green, yellow, and so on).
• saturation: The chromatic strength of a hue. White has zero saturation, pink is more
saturated, and red is fully saturated.
• brightness: The distance from black (zero brightness) in color space.
• 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.
• color-opponent cell: 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
• opponent color theory: The theory that perception of color is based on the output of
three mechanisms, each of them based on an opponency between two colors: red–
green, blue–yellow, and black–white. • unique blue: A blue that has no red or green tint.
• unique hue: 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).
• afterimage: A visual image seen after the stimulus has been removed.
• adapting stimulus: A stimulus whose removal produces a change in visual perception or
• negative afterimage: 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; yellow produces blue.
• neutral point: 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. (The black–white process has no neutral point.)
• achromatopsia: An inability to perceive colors that is caused by damage to the central
• deuteranope: An individual who suffers from color blindness that is due to the absence of
• protanope: An individual who suffers from color blindness that is due to the absence of L-
• tritanope: An individual who suffers from color blindness that is due to the absence of S-
• colour-anomalous: A better term for what is usually called “color-blind.” Most “color-blind”
individuals can still make discriminations based on wavelength. Those
discriminations are different from the normal—that is, anomalous.
• cone-monochromat: An individual with only one cone type. Cone monochromats are
• rod- monochromat: An individual with no cones of any type. In addition to being truly
color-blind, rod monochromats are badly visually impaired in bright light.
• agnosia: A failure to recognize objects in spite of the ability to see them. Agnosia is
typically due to brain damage.
• anomia: An inability to name objects in spite of the ability to see and recognize them (as
shown by usage). Anomia is typically due to brain damage.
• cultural relativism: In sensation and perception, the idea that basic perceptual
experiences (e.g., color perception) may be determined in part by the cultural
• unrelated color: A color that can be experienced in isolation.
• related color: A color, such as brown or gray, that is seen only in relation to other colors.
A “gray” patch in complete darkness appears white.
• illuminant: The light that illuminates a surface.
• spectral reflectance function: The function relating the wavelength of light to the
percentage of that wavelength that is reflected from a surface.
• spectral power distribution: The physical energy in a light as a function of wavelength.
• color constancy: The tendency of a surface to appear the same color under a fairly wide
range of illuminants. • reflectance: 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