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Chapter 5&7

PSYB51 Chapter 5 & 7 Definitions

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Department
Psychology
Course
PSYB51H3
Professor
Matthias Niemeier
Semester
Fall

Description
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 wavelength • 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– Helmholtz theory. • 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 together. • 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 cortex. • 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 sensitivity. • 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 nervous system. • deuteranope: An individual who suffers from color blindness that is due to the absence of M-cones. • protanope: An individual who suffers from color blindness that is due to the absence of L- cones. • tritanope: An individual who suffers from color blindness that is due to the absence of S- cones. • 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 truly color-blind. • 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 environment. • 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
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