Achromatic (color) A neutral color such as black, white or grey (see chromatic).
Achromatic response function (See chromatic response function)
Achromatopsia (See central achromatopsia)
Additive mixture A mixture in which the light from each of the components reaches the eye in an unmodified state. Lights superimposed on a projection screen are an example of an additive mixture. (See subtractive mixture)
Agraphia An impairment in the ability to write.
Alexia An impairment in the ability to comprehend written words or to read aloud.
Anomaloscope An instrument used for detecting anomalies of color vision. The test subject adjusts the ratio of two monochromatic lights to form a match with a third monochromatic light. The most common form of this procedure involves a Rayleigh match: a match between a mixture of monochromatic green and red lights, and a monochromatic yellow light. Normal subjects will choose a matching ratio of red to green light that falls within a fairly narrow range of values. Subjects with anomalous color vision will choose a ratio of red to green that falls outside this range, and red-green dichromats will accept any ratio of red to green as forming a match.
Anomalous trichromacy Condition of having three functioning cone types where one or more of the cones has abnormal spectral sensitivity. Anomalous trichromats, like normal subjects, require three primaries for color matching but will accept matches that normal subjects will reject.
Area 17 (See primary visual cortex)
Assimilation A perceptual phenomenon in which the color of an area is perceived as closer to the color of the surround than it would if viewed in isolation. Assimilation occurs with stimuli with fine spatial structure, e.g. a thin stripe of color between two black bars will look darker than it would between two white bars. Also known as the Von Bezold spreading effect.
Basic color term A color word (a) that is monolexemic (unlike "reddish-yellow"); (b) whose extension is not included in that of any other color term (unlike "scarlet", whose extension is included in "red"); (c) whose application is not restricted to a narrow class of objects (unlike "roan"); and (d) that is psychologically salient (unlike "puce"). A basic color term names a basic color category.
Bezold-Brücke hue shift A shift in apparent color of a stimulus towards yellow or blue with increasing intensity. If a pair of long wavelength lights differing only in intensity are compared, the higher intensity stimulus will look more yellow and less red than the lower intensity light. For shorter wavelengths higher intensity lights look more blue and less green than lower intensity lights. There are three invariant points in the spectrum: monochromatic lights whose color appearance does not change with intensity. (See invariant hue.)
Binary hue A hue that is perceptually mixed, as orange appears to be a mixture of red and yellow. All binary hues are mixtures of two of the unique hues.
Bipolar cell Retinal interneurons [neurons who communicate only with other neurons in the same part of the central nervous system] that connect the photoreceptors with the ganglion cells.
Blobs Regions of primary visual cortex that contain a relatively high proportion of neurons that respond more strongly to some wavelengths than to others (wavelength selective cells) and a relatively low proportion of orientation selective cells. They receive their main input from the parvocellular system. Layers 2 and 3 of the primary visual cortex form a mosaic of blobs when stained for the mitochondrial enzyme cytochrome oxidase. The regions between the blobs (interblobs) contain fewer wavelength selective cells and many orientation selective cells. The blobs are thought to be an important part of the pathway underlying color vision.
Brightness Attribute of a visual sensation according to which an area appears to emit more or less light. (CIE 45-25-210.) The perceived amount of light coming from an area. "Brightness" is often restricted to apply only to lights and "lightness" is used for the corresponding dimension of the colors of surfaces. One of the three standard elements of color appearance (the other two are hue and saturation). Its colorimetric equivalent is luminance.
Central achromatopsia A defect of color vision due to damage to the brain, typically to areas of the visual cortex. Central achromatopsia is distinct from the more common forms of color defect which are the result of photoreceptor loss or abnormality.
Chroma Attribute of a visual sensation which permits a judgment to be made of the amounts of pure chromatic color present, irrespective of the amount of achromatic color. (CIE 45-25-235.) For colors of the same hue and brightness, chroma and saturation are equivalent. Chroma will increase with brightness, however, even if saturation is held constant. Chroma is used instead of saturation in the Munsell color system.
Chromatic (color) A hue. All colors other than the neutral colors white, black, and the pure greys, are chromatic. The word "color" in ordinary language is often used to refer exclusively to chromatic colors, e.g., color vs. black-and-white television.
Chromatic response function A function from the wavelength of a stimulus to an aspect of its perceived color. For example, the yellow-blue process of opponent process theory is characterized by a function from the wavelength of the stimulus to the amount of perceived yellowness or blueness. The red-green process and the black-white process are similarly characterized (the latter by an achromatic response function).
Chromaticity coordinates Coordinates that specify position in a chromaticity diagram. The chromaticity coordinates of a stimulus are derived from its tristimulus values by taking the ratio of each of the tristimulus values to their sum; i.e. x=X/X+Y+Z, y=Y/X+Y+Z, where x and y are the chromaticity coordinates and X,Y, and Z the tristimulus values.
Chromaticity diagram A diagram that represents the unit plane (the plane defined by the equation X+Y+Z=1) in a tristimulus space. The location of a stimulus with a particular set of tristimulus values on a chromaticity diagram represents its direction from the origin of the space ignoring its distance. The chromaticity diagram is often used as a convenient approximation to a constant brightness plane in the tristimulus space.
Chromaticity Location in a chromaticity diagram. Chromaticity is often used as a convenient approximation to chromaticness, which is the hue and saturation of a color ignoring brightness.
Chromophore The light absorbing part of a photopigment. The photopigments contained in photoreceptors consist of two components: an opsin bound to a chromophore. The chromophore in human (and all mammalian) photopigments is retinal (a form of vitamin A). Upon absorbing a photon, retinal changes its conformation which results in its separation from the opsin group. This chemical change initiates the visual response. The differences in spectral sensitivity among the photopigments found in the different types of human photoreceptors are due to differences in their opsins.
CIE Commission Internationale de l'Eclairage. An international organization that recommends standards and procedures for light and lighting, including colorimetry.
CIE illuminant C or illuminant C A standard illuminant that is an approximation to average daylight. Like all illuminants, illuminant C is specified by its spectral power distribution.
CIE Standard Observer A set of tri-stimulus values for spectral lights that is designed to allow a standard and objective way of describing the color matching properties of different lights. Although very few people will accept all the color matches of the Standard Observer, lights that are matches for the Standard Observer will be near matches for most people with normal color vision. Since the matching properties of stimuli vary with size and other features of the viewing conditions there is more than one Standard Observer.
Color anomia An inability to name colors despite intact color vision as demonstrated by non-verbal tests.
Color constancy Stability in the perceived color of a surface across changes in illumination and the consequent changes in the light reaching the eye.
Color space A system for ordering colors that respects the relationships of similarity among them. There are variety of different color spaces, but they are all three dimensional.
Color temperature The temperature of the perfect black body radiator whose chromaticity is closest to that of the light under consideration. A useful measure of the quality of a light with a particular spectral power distribution when used as an illuminant. Color temperature is most useful when applied to illuminants with broad and smoothly changing spectral power distributions.
Colorimetry The science of measuring color and color appearance. Classical colorimetry deals primarily with color matches rather than with color appearance as such. The main focus of colorimetry has been the development of methods for predicting perceptual matches on the basis of physical measurements.
Complementary colors Two color stimuli that can be additively mixed to produce an achromatic color (see additive mixture).
Cone One of the two main classes of photoreceptor found in the vertebrate eye. Cones produce usable outputs only at relatively high light levels and provide the main inputs for color vision. There are three types of cones in the human eye, each type having a different spectral sensitivity.
Confusion line A line on a chromaticity diagram representing colors that are not discriminable by a subject with a particular defect of color vision. The protan confusion line represents colors that are indistinguishable by a protanope (see protanopia), and similarly for the other varieties of color blindness.
Depolarization A decrease in the potential difference across the cell membrane of a neuron. Most neurons depolarize in response to stimulation.
Deuteranopia One of the two varieties of red-green color blindness (also known as green-dichromacy). Deuteranopia results from the loss of function of the M-cones. Deuteranopes, in contrast to protanopes (see protanopia), display essentially normal visual sensitivity throughout the spectrum.
Dichoptic matching A matching experiment in which stimuli are presented separately to the two eyes. The match is thus between a stimulus seen using one eye and a stimulus seen using the other eye.
Dichromacy Condition of possessing two independent channels for conveying color information. Color matching for dichromats requires only two primaries. Most "color blind" humans are dichromats and have lost the function of one of the cone types and consequently one of the opponent processes. Dichromats are not strictly color blind in the sense that their vision is sensitive to a single chromatic dimension as well as to brightness.
dLGN (See Dorsal lateral geniculate nucleus)
Dominant wavelength (of a stimulus) The wavelength of the monochromatic light that when mixed with a specified achromatic light in suitable proportion yields a match to the stimulus under consideration. Since there is no unique best choice of achromatic light, the dominant wavelength of a stimulus is relative to the choice of a particular achromatic light. Illuminant C is a popular and convenient choice of achromatic reference. The colorimetric equivalent of hue.
Dorsal lateral geniculate nucleus (dLGN) An area of the thalamus that serves as an important relay station for visual information on its way to the visual cortex. The dLGN (or LGN) is the primary target for outputs from retinal ganglion cells and receives its main input from them. Its main output is to primary visual cortex via the optic radiations.
Double opponent cell A neuron that exhibits opposite spectral opponency in the center and surround of its receptive field. For example, a double opponent cell might be stimulated by the L-cones and inhibited by the M-cones in the center of its receptive field, and vice versa in the surround. Double opponent cells respond best to stimuli with contrasting colors and are not found in the visual pathway prior to primary visual cortex.
Electron-volt The amount of energy required to move an electron across a potential difference of one volt. A common unit of energy in physics.
Equiluminant Equal in luminance. Two lights may be equally bright although chromatically different (which implies they have different spectral power distributions). One consequence of the possession of color vision is the ability to discriminate some equiluminant stimuli.
Extrastriate cortex Those areas of visual cortex lying outside primary visual cortex.
Farnsworth-Munsell 100-hue test A standard test for deficiencies of color vision in which the subject is asked to arrange a set of 100 colored chips in circle. Subjects with normal color vision will arrange the chips in a specific order with very few deviations. Subjects with abnormal color vision will deviate from the normal arrangement in ways that provide information about the nature of their defect.
Fluorescence The absorption of light at one wavelength and its re-emission at a longer wavelength. Fluorescence plays an important role in the perceived color of many objects: the unnatural brightness of "day-glo" paints is due to fluorescence.
Focal color A paradigm example of a member of a basic color category (see basic color term).
Fovea The central area of the retina. The fovea contains the densest concentration of photoreceptors (cones only) and displays other adaptations for high resolution vision.
Ganglion cells The output cells of the retina. They indirectly receive their inputs from the photoreceptors and send their outputs to the brain. There are several varieties of ganglion cells which differ in which photoreceptor types they draw their inputs from, the spatial organization of their inputs, and other factors. The axons of the ganglion cells form the optic nerve.
Grassman's laws Laws governing the results of additive color matching experiments (see additive mixture). Grassman's laws assert the existence of exactly three dimensions of variation in perceived color, e.g. hue, brightness, and saturation. They also imply the linear additivity of color matches, a fact of importance to colorimetry. Thus if stimulus a matches stimulus b, and stimulus c matches stimulus d, then the additive mixture of a and c will match the mixture of b and d irrespective of the stimuli's spectral composition.
Grating (chromatic and achromatic) A stimulus pattern consisting of alternating stripes. The stripes can differ in color (chromatic grating) or only in brightness (achromatic grating).
Helson-Judd effect The tendency of lighter achromatic surfaces to take on the hue of the illuminant under which they are viewed and darker achromatic surfaces to take on the complementary hue.
Hemianopia The loss of visual capacity in one half of the visual field, typically due to damage to primary visual cortex. (Strictly speaking this type of visual field defect is a homonymous hemianopia.) Because each half of the visual field is represented by the contralateral side of the brain, damage to one side of the brain will result in the loss of visual capacity in the opposite side of the visual field.
Horizontal cell Neurons found in the retina that make widespread synaptic contacts with other retinal neurons and that are thought to play a role in generating the receptive field properties of other retinal neurons, including ganglion cells.
Hue Attribute of visual sensation which has given rise to color names such as: blue, green, yellow, red, purple, etc. (CIE 45-25-215.) Hue differences depend primarily on variations in the wavelength of light reaching the eye. One of the three standard elements of color appearance (the other two are brightness and saturation). Its colorimetric equivalent is dominant wavelength.
Hue coefficient The ratio between the response in one of the chromatic channels to the total chromatic response. A stimulus that is seen as equally bluish and reddish would for example have a hue coefficient of .5 for both red and blue while the unique hues all have coefficients of 1.0 for one channel and zero for the others.
Hyperpolarization An increase in the potential difference across the cell membrane of a neuron. Retinal photoreceptors differ from most other neurons in hyperpolarizing in response to stimuli.
Illuminant point (of an illuminant) The illuminant's location on the chromaticity diagram.
Intensity The power of a light, often weighted by the spectral sensitivity of the eye. Since the eye is much more sensitive to light of some wavelengths than it is to others, two monochromatic lights can have equal power, while one appears dim and the other appears bright. Visual scientists often use a system of units (photometric units) that scale the physical power (given in radiometric units) to the sensitivity of the eye at each wavelength.
Interblob (See blobs)
Invariant hue The perceived hues of monochromatic lights that do not change with intensity. There are three invariant hues: blue, green, and yellow. (See Bezold-Brücke hue shift.)
Ishihara test A test for color blindness using a set of pseudoisochromatic plates.
Isoluminant (See equiluminant)
Isomerization The change in conformation upon absorption of a photon that the chromophores in photopigments undergo.
L-cone One of three cone types that contribute to human color vision. The L-cones have their peak spectral sensitivity at a longer wavelength than the other two cone types, the M-cones and S-cones.
L-M cell A spectrally opponent type of ganglion cell that is excited by inputs from L-cones and inhibited by inputs from M-cones. These cells (together with M-L cells) are of interest because of their possible connection with the red-green opponent channel (see opponent process theory).
Lateral geniculate nucleus (LGN) (See dorsal lateral geniculate nucleus)
Lightness (See brightness)
Luminance The intensity (power weighted by the overallspectral sensitivity of the eye) per unit area of a light source.
Luminosity (See brightness)
M-cone One of the three cone types that contribute to human color vision. The peak spectral sensitivity of the M-cones is between the peak sensitivity of the other two cone types, the L-cones and S-cones.
M-L cell A spectrally opponent type of ganglion cell that is excited by inputs from M-cones and inhibited by inputs from L-cones. These cells (together with L-M cells) are of interest because of their possible connection with the red-green opponent channel (see opponent process theory).
Magnocellular pathway One of the two main neural pathways from the retina to the primary visual cortex (the other is the parvocellular pathway). The magnocellular pathway is not thought to play an important role in color vision.
Metameric match A color match between physically different stimuli, i.e. a match between stimuli with different reflectances or spectral power distributions. Such stimuli that match (for an observer and a viewing condition) are metamers (with respect to that observer and that viewing condition).
Microspectrophotometry A technique for obtaining measurements of the spectral absorption of a single photoreceptor cell. It is especially useful as a means for investigating non-human color vision without behavioral tests.
Midget Bipolar Cell Bipolar cells that are thought to feed the Pb ganglion cells which are the origin of the parvocellular pathway.
Monochromacy The condition of possessing only a single channel for conveying information about color. Monochromats are strictly color blind and perceive only variations in brightness.
Monochromatic Consisting of a single wavelength or narrow range of wavelengths. (See spectral light.)
MRI Magnetic resonance imaging. A non-invasive technique for imaging soft tissues including the brain. MRI has been mostly used for detecting and localizing damage to the brain but recently versions of the technique have been developed that allow imaging of local differences in brain activity.
Munsell color system A widely used system for describing the color appearance of samples. The Munsell system uses matching against a set of samples and interpolation between them to arrive at a designator for the appearance of a given test sample. Color appearance in the Munsell system is characterized using sets of three symbols, for example 2.5 YR 5/10. 2.5 YR is the hue, 5/ the value (= lightness), and /10 the chroma.
Nanometer One billionth (10-9) of a meter. The most common unit used for characterizing the wavelength of light in visual science.
Neutral point All dichromats will accept a match between some spectral light and a white light. The wavelength of this spectral light is the neutral point for that dichromat. No trichromatic subject, including anomalous trichromats, will accept a match between a white light and any spectral light.
Opponent process theory The theory that color appearances are the result of a recoding of photoreceptor outputs into three processes or channels, two opponent chromatic channels (yellow-blue and red-green), and one achromatic channel (white-black). The new channels are created by taking sums and differences of the outputs of the three cone types. The two opponent chromatic channels can take on positive, negative, or zero values. When (for example) the yellow-blue channel is positive (negative), the result is an appearance of yellowness (blueness). Thus the incompatibility of yellowness and blueness is explained by the fact that the yellow-blue channel never simultaneously has positive and negative values. The theory predicts (correctly) that the blueness of a stimuli may be canceled by adding to it a stimulus that produces an appearance of yellowness.
Opsin The protein component of a photopigment. Different opsins give rise to photopigments with different spectral absorption characteristics and consequently account for the differences in spectral sensitivity of the various photoreceptor types. (See chromophore.)
Optic radiations The bundles of axons projecting from the lateral geniculate nucleus to primary visual cortex.
Pa cell Ganglion cells, found in the primate retina, that are thought to be the origin of the magnocellular pathway. Also referred to as M cells, parasol cells, or A cells. They are analogous to the well-studied ganglion cells of the cat retina known as Y cells or a cells.
Parvocellular pathway One of the two main pathways from the retina to primary visual cortex (the other is the magnocellular pathway). The parvocellular stream primarily carries information about luminance contrast at high spatial resolution, but is also the main carrier of information about stimulus wavelength.
Pb cell Ganglion cells, found in the primate retina, that are thought to be the origin of the parvocellular pathway. Also referred to as P cells, midget cells, or B cells. They are analogous to the well-studied ganglion cells of the cat retina known as X cells or b cells.
Photophore Light emitting (bioluminescent) organ.
Photopic vision Vision under relatively high light levels when the visual response is primarily controlled by the cones.
Photopigment Pigments in photoreceptors that change their conformation when they absorb a photon. The change in conformation of the photopigments in response to light is the first step in the process that leads to the photoreceptor producing a neural output. Different photopigments have different probabilities of absorbing a photon of a given wavelength and it is these differences that give rise to the different spectral sensitivity characteristics of different types of photoreceptors.
Photoreceptor A light sensitive neuron. Photoreceptors interact with light which produces changes in their electrical properties which are communicated to other neurons. They constitute the first stage in the physiological process which underlies vision. The human retina, like the retina of most vertebrates, contains two broad classes of photo-receptors, rods and cones.
Polarization The spatial orientation of the electromagnetic waves that make up a beam of light. Light in which the spatial orientation of the waves is not random is said to be polarized. Although most light sources, such as the sun, emit essentially unpolarized waves many physical processes such as scattering in the atmosphere and reflection off glossy surfaces impose a preferred polarization on light.
Polymorphism A trait of an organism that is found in more than one state in a population. Also used for the existence of different forms of a gene in a population. For example, one well known polymorphism of color vision is the existence of both red-green dichromats and normal trichromats in the human population.
Porphyropsin Photopigment utilizing 3,4 dehydroretinal instead of retinal as its chromophore. This substitution has the effect of shifting peak absorption to longer wavelengths.
Positron emission tomography (PET) A method for imaging cerebral blood flow and, indirectly, brain activity making use of tracers that emit positrons. The tracer is introduced into the subject's blood and then its concentration is measured using the emitted positrons. Since local cerebral blood flow appears to be correlated with neuronal activity PET scans can be used to monitor local brain activity.
Power Energy per unit time (see intensity).
Prestriate cortex (See extrastriate cortex)
Primary A member of a minimal set of stimuli (usually lights), mixtures of the elements of which are capable of matching all colors. For normal human beings three primaries are sufficient to provide a perceptual match. There are indefinitely many sets of three primaries, mixtures of which are capable of matching all colors. For colorimetric purposes the concept of a primary has been extended to include "imaginary" primaries-ones that are not physically realizable. Thus the CIE standard observer makes use of imaginary primaries. The results of mixing imaginary primaries are derived from data using real lights.
Primary visual cortex An area of the occipital lobe that performs the first stage of cortical visual processing. It receives inputs from the retina via the dLGN and sends outputs to other areas of the visual cortex. Also referred to as V1, striate cortex, and area 17.
Prosopagnosia An impairment in the ability to visually recognize individual faces.
Protanopia One of the two varieties of red-green color blindness (also known as red-dichromacy). Protanopia is thought to result from the loss of function of the L-cones. Protanopes display a marked loss of sensitivity to light at the long wavelength end of the spectrum. (See deuteranopia.)
Protein moiety (See Opsin)
Pseudoisochromatic plates Common tests for color blindness that use patterns of colored dots. A normal subject will see a certain figure in the pattern, while a color blind individual will see a different figure or no figure at all.
Purity The ratio of two lengths on a chromaticity diagram, the first length being the distance between the point representing the specified achromatic stimulus and that representing the color stimulus being considered, and the second being the length of the line connecting the achromatic point to the border of the chromaticity diagram along the same direction as the first line. The colorimetric equivalent of saturation.
Quantum catch The number of photons absorbed by the photopigment molecules of a photoreceptor. The rate of quantum catch in a photoreceptor determines its output.
Rayleigh scattering Scattering of light off small uniform particles. Rayleigh scattering is inversely proportional to the fourth power of the wavelength consequently short wave light is much more strongly scattered than long wave light. Sunsets appear red because direct sunlight is depleted of short wavelengths on passage through the atmosphere and the sky appears blue because it is seen by scattered light which is correspondingly enriched in short wavelengths.
Receptive field (of a neuron) The region of the visual field in which stimuli will affect the neuron's level of activity.
Reflectance (of a surface) The proportion of incident light the surface reflects. (See spectral reflectance).
Retina The inside layer of the back of the eye that contains the photoreceptors and associated neurons. The earliest stages of visual processing take place in the neurons of the retina.
Retinal 1) In or having to do with the retina; 2) The chromophore contained in human photopigments.
Rhodopsin The rod photopigment.
Rod One of the two main classes of photoreceptor found in the vertebrate eye. Rods are very sensitive to light but fail to produce a usable signal at high light levels. They mediate night vision and have little effect on color vision in daylight.
S-cone One of the three cone types that contribute to human color vision. The peak spectral sensitivity of the S-cones is at a shorter wavelength than that of the other two cone types, the L-cones and M-cones.
Saturation Attribute of a visual sensation which permits a judgment to be made of the proportion of pure chromatic color in the total sensation. (CIE 45-25-225.) Pink and red differ in saturation with the red being the more saturated. The spectral colors are all maximally saturated examples of their hues and differ in this respect from pastels which are desaturated. One of the three standard elements of color appearance (the other two are hue and brightness). Its colorimetric equivalent is purity.
Scotopic vision Vision under relatively low light levels when the visual response is primarily controlled by the rods.
Simultaneous contrast The phenomenon in which the perceived color of an area of a scene tends to take on a hue opposite to that of the surrounding area. Thus a grey square on a red background will take on a greenish tint.
Snellen acuity Visual acuity as measured by the ability to discriminate high contrast shapes. The familiar Snellen eye chart which contains rows of letters each smaller than the one above provides one way of measuring Snellen acuity.
Spatial contrast A difference in some visible property between one area of a scene and adjacent areas. Black letters on a white background display high spatial contrast.
Spatial frequency A measure of how rapidly a property changes in space. A commonly used form of visual stimulus consists of vertical bars where the lightness varies according to a sinusoidal function. In this simple case the spatial frequency of the stimulus is just the frequency of the sinusoid used to generate the pattern. In general stimuli with fine detail including sharp edges have high spatial frequency while those where the stimulus properties change more slowly in space have low spatial frequency.
Spatially opponent (neuron) A neuron that is excited (or inhibited) by stimuli in the center of its receptive field and inhibited (or excited) by stimuli in the periphery of its receptive field.
Spectral color The color of a monochromatic or nearly monochromatic light, i.e. a color to be found in the spectrum. Other colors are extraspectral.
Spectral light A monochromatic or nearly monochromatic light.
Spectral power distribution (of a light) At each wavelength in the visible spectrum, the power of the light at that wavelength as a proportion of its total power over the visible spectrum.
Spectral reflectance (of a surface) At each wavelength in the visible spectrum, the proportion of incident light the surface reflects at that wavelength.
Spectral sensitivity (of a photoreceptor) The sensitivity of a photoreceptor at each wavelength in the visible spectrum. For a given wavelength, this is equal to the ratio of the photoreceptor response at that wavelength to its response at the most sensitive wavelength. Different photoreceptor types have different spectral sensitivities.
Spectrally opponent (neurons) Neurons that are excited by one cone type and inhibited by another.
Spectrum or visible spectrum Band of electromagnetic radiation ranging from wavelengths of approximately 400 to approximately 700 nanometers, corresponding to the sensitivity of the human eye. Sensitivity does not drop to zero at the standard endpoints of the visible spectrum, but is so low that light outside these limits rarely has a significant effect on visual response. Many non-human animals respond significantly to light outside this range, especially to light of shorter wavelengths.
Striate cortex (See Primary visual cortex)
Subtractive mixture A color mixture in which the light from each component is modified by the others. Since pigments modify light by absorbing a portion of the incident light and thus each pigment will modify the light from the others, pigment mixtures are subtractive mixtures. (See additive mixture.)
Successive contrast The influence of the color of an area on the perceived color of an area viewed immediately afterward, e.g. afterimages.
Surface spectral reflectance (See spectral reflectance)
Thalamus A midbrain structure that plays a major role in relaying information from the various sensory receptors to other brain areas.
Tri-stimulus colorimetry A set of techniques for predicting color matches by equating a given stimulus with the amounts of three specified primaries that would be required to match it. The amounts of three primaries that would be required to match the stimulus are the tri-stimulus values of that stimulus for that set of primaries. Any stimuli with the same tri-stimulus values will match perceptually. Tri-stimulus colorimetry is useful because once the tri-stimulus values for spectral lights are determined empirically it is possible to compute the tri-stimulus values of any mixture of spectral lights. Whether two different mixtures of spectral lights will match or not can thus be determined without anyone actually looking at the stimuli. Tri-stimulus colorimeters are used to determine the tri-stimulus values of a stimulus with respect to a standard set of primaries.
Tri-stimulus values (See Tri-stimulus colorimetry)
Trichromacy Condition of possessing three independent channels for conveying color information, derived from the three different cone types. A single set of three appropriately chosen primaries is sufficient to match the color appearance of any stimulus for a trichromat. Normal human color vision is trichromatic, but some other organisms, e.g. pigeons, have tetrachromatic color vision: they require sets of four primaries to produce complete color matches. Other organisms and some color deficient human beings are dichromats: they require only sets of two primaries to produce complete color matches.
Tritanopia Yellow-blue color blindness. Tritanopia is thought to result from the loss of function of the S-cones. Tritanopia is much less common than either protanopia or deuteranopia.
Unilateral dichromat A person with normal color vision in one eye and some form of dichromacy in the other. Unilateral dichromats are very rare.
Unique hue A hue that is perceptually unmixed, containing no other chromatic component. The four unique hues (red, green, yellow, and blue) are thought to be basic components out of which all other (binary) hues are composed. According to opponent process theory an appearance of a unique hue is produced when one of the two chromatic opponent channels is in balance and the other is either positive or negative.
Univariance (principle of) The fact that the effect on a photopigment of absorbing a photon is independent of its wavelength. Thus the response of a photoreceptor is determined solely by the number of photons absorbed and is not sensitive to the wavelengths of those photons. This has the important consequence that there is no information in the response of a single photoreceptor about the wavelength of the light which affects it. A single photoreceptor with a single photopigment is incapable of distinguishing between an intense light at a wavelength to which it is relatively insensitive and a weak light at a wavelength to which it is relatively sensitive. Chromatic as opposed to black-and-white vision thus requires multiple photoreceptor types.
V1 (See primary visual cortex)
Visible spectrum (See spectrum)
Visual cortex Those cortical areas primarily concerned with the processing of visual information. The visual areas of the cortex are primarily located in the occipital lobe: the rear, lower part of the cortex.
Von Bezold spreading (See assimilation)
Wavelength discrimination function A function from wavelength to the smallest discriminable difference in wavelength. Human wavelength discrimination varies substantially across the spectrum and although performance varies considerably with testing conditions the resulting curve is typically quite complex. The wavelength discrimination function is an important source of information about the contributions of the different cone types to color vision.
Weber's law The thesis that perceptual systems respond in such a way so as to make the ratio between the minimum perceptible change in a physical magnitude and the absolute value of that magnitude a constant. For example, Weber's law predicts that smallest detectable intensity difference between two lights will be larger on a bright background than a dim one. This conclusion is approximately true for high light levels.