Trichromatic Research Articles

Cone pigment gene expression in individual photoreceptors and the chromatic topography of the retina.

Human trichromatic vision is based on three classes of cones: L, M, and S (long-, middle-, and short-wavelength sensitive, respectively). Individuals can have more than one M and/or more than one L pigment gene on the X chromosome along with an S pigment gene on chromosome 7. In some people the X-linked pigment gene array can include polymorphic variants that encode multiple, spectrally distinct cone photopigment subtypes. A single-cell, polymerase chain reaction approach was used to examine visual pigment gene expression in individual human cone cells and identify them as L or M. The ratio of L:M pigment gene expression was assayed in homogenized retinal tissues taken from the same eyes. Results indicate that there is a close correspondence between the cone ratio determined from counting single cells and the L:M pigment mRNA ratio estimated from homogenized pieces of retina. The results also show that the different pigment genes in one array are often expressed at very different levels, giving rise to unequal numbers of L and M cones. Expression of only one photopigment gene was detected in each cone cell. However, individual males can have more than the classically described three spectrally distinct cone types in their retinas.

Three-dimensional interpretation of the color system of Aguilonius/Rubens 1613

The two-dimensional color system in the textbook on optics, written by F. Aguilonius (1613) and illustrated by PP Rubens,1 is equivalent to a three-dimensional color solid, which can also be constructed by applying the trichromatic theory of color vision on a color circle. © 2000 John Wiley & Sons, Inc. Col Res Appl, 26, S17–S19, 2001

Subliminal perception of colour by cattle

In humans subliminal perception is more evident in vision than other senses (Dixon, 1987) but it has not been reported in animals. The presence of subliminal visual perception might be suspected in cattle because of their low level of perceptual discrimination ability of visual cues relative to humans (e.g. Phillips and Weiguo, 1991), despite their sensory apparatus being similar in many respects. Experiments were therefore conducted to determine the extent of cattle colour perception and examine whether the effects of colour on cattle behaviour transcend that their perceptual abilities. We sought to a) confirm that cattle are dichromats, taking account of stimulus brightness, which has not always been the case in previous experiments investigating cattle colour vision, and b) investigate whether cattle exhibit differences in behaviour in isoluminant primary colours for trichromatic vision. Confirmation that cattle are dichromats, together with demonstrations of differences in behaviour in the three primary colours would suggest the existence of subliminal perception, and would question the validity of determining animal welfare requirements solely on psychophysical testing of supraliminal perception.

Dichromatism in macaque monkeys.

Old World primates have trichromatic vision because they have three types of cone photoreceptor, each of which is maximally sensitive to short, middle or long wavelengths of light1. Although a proportion of human males (about 8% of caucasians, for example) have X-chromosome-linked colour-vision abnormalities2, no non-human Old World primates have been found to be colour-vision defective3,4. We have tested 3,153 macaque monkeys but found only three dichromats, a frequency that is much lower than in humans.

Uniformity of colour vision in Old World monkeys.

It is often assumed that all Old World monkeys share the same trichromatic colour vision, but the evidence in support of this conclusion is sparse as only a small fraction of all Old World monkey species have been tested. To address this issue, spectral sensitivity functions were measured in animals from eight species of Old World monkey (five cercopithecine species and three colobine species) using a non-invasive electrophysiological technique. Each of the 25 animals examined had spectrally well-separated middle- and long-wavelength cone pigments. Cone pigments maximally sensitive to short wavelengths were also detected, implying the presence of trichromatic colour vision. Direct comparisons of the spectral sensitivity functions of Old World monkeys suggest there are no significant variations in the spectral positions of the cone pigments underlying the trichromatic colour vision of Old World monkeys.

Trichromatic color vision with only two spectrally distinct photopigments.

Protanomaly is a common, X-linked abnormality of color vision. Like people with normal color vision, protanomalous observers are trichromatic, but their ability to discriminate colors in the red-green part of the spectrum is reduced because the photopigments that mediate discrimination in this range are abnormally similar. Whereas normal subjects have pigments whose wavelengths of peak sensitivity differ by about 30 nm, the peak wavelengths for protanomalous observers are thought to differ by only a few nanometers. We found, however, that although this difference occurred in some protanomalous subjects, others had pigments whose peak wavelengths were identical. Genetic and psychophysical results from the latter class indicated that limited red-green discrimination can be achieved with pigments that have the same peak wavelength sensitivity and that differ only in optical density. A single amino acid substitution was correlated with trichromacy in these subjects, suggesting that differences in pigment sequence may regulate the optical density of the cone.

The Evolution of Trichromatic Color Vision by Opsin Gene Duplication in New World and Old World Primates

Trichromacy in all Old World primates is dependent on separate X-linked MW and LW opsin genes that are organized into a head-to-tail tandem array flanked on the upstream side by a locus control region (LCR). The 5' regions of these two genes show homology for only the first 236 bp, although within this region, the differences are conserved in humans, chimpanzees, and two species of cercopithecoid monkeys. In contrast, most New World primates have only a single polymorphic X-linked opsin gene; all males are dichromats and trichromacy is achieved only in those females that possess a different form of this gene on each X chromosome. By sequencing the upstream region of this gene in a New World monkey, the marmoset, we have been able to demonstrate the presence of an LCR in an equivalent position to that in Old World primates. Moreover, the marmoset sequence shows extensive homology from the coding region to the LCR with the upstream sequence of the human LW gene, a distance of >3 kb, whereas homology with the human MW gene is again limited to the first 236 bp, indicating that the divergent MW sequence identifies the site of insertion of the duplicated gene. This is further supported by the presence of an incomplete Alu element on the upstream side of this insertion point in the MW gene of both humans and a cercopithecoid monkey, with additional Alu elements present further upstream. Therefore, these Alu elements may have been involved in the initial gene duplication and may also be responsible for the high frequency of gene loss and gene duplication within the opsin gene array. Full trichromacy is present in one species of New World monkey, the howler monkey, in which separate MW and LW genes are again present. In contrast to the separate genes in humans, however, the upstream sequences of the two howler genes show homology with the marmoset for at least 600 bp, which is well beyond the point of divergence of the human MW and LW genes, and each sequence is associated with a different LCR, indicating that the duplication in the howler monkey involved the entire upstream region. [The sequence data described in this paper have been submitted to GenBank under accession nos. AF155218, AF156715, and AF156716.]

Prospects for trichromatic color vision in male Cebus monkeys

Polymorphic color vision is characteristic of many species of New World monkey. A fundamental feature of the polymorphism is that male monkeys are routinely dichromatic. A recent paper describes an experiment in which Cebus monkeys were required to discriminate between pairs of Munsell color chips (Pessoa VF, Tavares MCH, Aguiar L, Gomes UR, Tomaz C. Color vision discrimination in the capuchin monkey Cebus apella: evidence for trichromaticity. Behav Brain Res 1997;89:285–288). The results were interpreted as demonstrating trichromatic color vision in male Cebus monkeys. An examination of the literature on Cebus monkey photopigments and results from a replication of the discrimination experiment conducted with dichromatic human subjects cast doubt on this claim.

Analysis of the short wavelength-sensitive ("blue") cone mosaic in the primate retina: comparison of New World and Old World monkeys.

The distribution of short wavelength-sensitive (SWS or "blue") cone photoreceptors was compared in primates with dichromatic ("red-green colour blind") and trichromatic colour vision. We compared a New World species, the marmoset (Callithrix jacchus), with an Old World species, the macaque monkey (Macaca nemestrina). The SWS cones were identified by their immunoreactivity to an antiserum against the human SWS cone opsin. A single retina from a male capuchin monkey (Cebus apella) also was studied. The SWS cones make up less than 10% of all cone photoreceptors throughout the retina of all animals studied. In marmoset, the peak spatial density of SWS cones is close to 10,000/mm2 at the foveola. In macaque, the peak spatial density of SWS cones, close to 6,000/mm2, is at the fovea, but SWS cones are absent within 50 microm of the centre of the foveola. In both species, the density of SWS cones is higher on the nasal retinal axis than at corresponding eccentricities on the other retinal axes. The SWS cones in macaque are arranged in a semiregular array, but they are distributed randomly in marmoset. There is no difference in the spatial density or local arrangement of SWS cones between dichromatic and trichromatic marmosets. The results suggest that the SWS cone photoreceptor system is subject to different developmental and evolutionary constraints than those that have led to the formation of the red-green photoreceptor systems in primate vision.

The arrangement of the three cone classes in the living human eye.

Human colour vision depends on three classes of receptor, the short- (S), medium- (M), and long- (L) wavelength-sensitive cones. These cone classes are interleaved in a single mosaic so that, at each point in the retina, only a single class of cone samples the retinal image. As a consequence, observers with normal trichromatic colour vision are necessarily colour blind on a local spatial scale. The limits this places on vision depend on the relative numbers and arrangement of cones. Although the topography of human S cones is known, the human L- and M-cone submosaics have resisted analysis. Adaptive optics, a technique used to overcome blur in ground-based telescopes, can also overcome blur in the eye, allowing the sharpest images ever taken of the living retina. Here we combine adaptive optics and retinal densitometry to obtain what are, to our knowledge, the first images of the arrangement of S, M and L cones in the living human eye. The proportion of L to M cones is strikingly different in two male subjects, each of whom has normal colour vision. The mosaics of both subjects have large patches in which either M or L cones are missing. This arrangement reduces the eye's ability to recover colour variations of high spatial frequency in the environment but may improve the recovery of luminance variations of high spatial frequency.

Origins and antiquity of X-linked triallelic color vision systems in New World monkeys.

It is known that the squirrel monkey, marmoset, and other related New World (NW) monkeys possess three high-frequency alleles at the single X-linked photopigment locus, and that the spectral sensitivity peaks of these alleles are within those delimited by the human red and green pigment genes. The three alleles in the squirrel monkey and marmoset have been sequenced previously. In this study, the three alleles were found and sequenced in the saki monkey, capuchin, and tamarin. Although the capuchin and tamarin belong to the same family as the squirrel monkey and marmoset, the saki monkey belongs to a different family and is one of the species that is most divergent from the squirrel monkey and marmoset, suggesting the presence of the triallelic system in many NW monkeys. The nucleotide sequences of these alleles from the five species studied indicate that gene conversion occurs frequently and has partially or completely homogenized intronic and exonic regions of the alleles in each species, making it appear that a triallelic system arose independently in each of the five species studied. Nevertheless, a detailed analysis suggests that the triallelic system arose only once in the NW monkey lineage, from a middle wavelength (green) opsin gene, and that the amino acid differences at functionally critical sites among alleles have been maintained by natural selection in NW monkeys for >20 million years. Moreover, the two X-linked opsin genes of howler monkeys (a NW monkey genus) were evidently derived from the incorporation of a middle (green) and a long wavelength (red) allele into one chromosome; these two genes together with the (autosomal) blue opsin gene would immediately enable even a male monkey to have trichromatic vision.

Comparative retinal physiology in anthropoids

During the last decade it has become clear that colour vision in platyrrhines (New World monkeys) differs from the uniform trichromatic pattern normally found in catarrhines (Old World monkeys, apes and human). Colour vision in most platyrrhine species is polymorphic, with many dichromatic individuals. The comparison of response properties in retinal ganglion cells and lateral geniculate cells between catarrhines and platyrrhines elucidates how the evolution of trichromatic colour vision influenced the post-receptoral processing. We find that spatial and temporal processing is very similar in the platyrrhine and catarrhine retina, strongly suggesting that the retinal structure and function, found in living anthropoids, was already present in their common ancestor.

The role of ternary projections in colour displays for geochemical maps and in economic mineralogy and petrology

Ternary (triangular) diagrams show the compositional variation of three end-members, recalculated to 100%, and represent the projection into two dimensions of three or more components (since an end-member may represent the sum of more than one original component), and hence multiple space. During the present century they have been of particular importance in chemistry, the earth sciences, colour-mixing and the studies of colour that underlie the development of geochemical colour maps. Such concepts can be traced back to Newton's Opticks although he did not use an explicit ternary theory of colour mixing. Development of the trichromatic theory of colour deficiency began with Mariotte in the 17th century and was definitively established by Maxwell in the 1850s. These colour theorems underlie the development in recent years of the colour maps now in widespread use in geochemistry. The progress of regional geochemical mapping in the British Isles has gone hand-in-hand with the development of computer software and hardware. The use of multi-element colour-combined geochemical maps can be traced from early work at Imperial College by Webb and his co-workers (1964–79) to its later application in the British Geological Survey's Geochemical Atlas series (from 1983 onwards). Between 1955 and 1975 the use of ternary diagrams came to be commonly used in studies of mineralogical composition, including economic minerals, petrology, classification, and phase relationships. In igneous petrology, the quartz–alkali-feldspar–plagioclase-feldspathoid double-triangle has achieved increasing international acceptance. We illustrate these applications with examples pertinent to the Northern Highland Caledonides, with particular emphasis on the lamprophyres and associated rocks. Two distinct multivariate trends of variation are shown to be present which may warrant further investigation to elucidate their bearing on Caledonian mineralisation.

The distribution of calcium-binding proteins in the lateral geniculate nucleus and visual cortex of a New World monkey, the marmoset, Callithrix jacchus.

Antibodies directed against the calcium-binding proteins, parvalbumin and calbindin, can be used to label distinct neuronal subgroups in the primate visual pathway. We analyzed parvalbumin immunoreactivity (P-IR) and calbindin immunoreactivity (C-IR) in the lateral geniculate nucleus (LGN) and visual cortex of the marmoset, Callithrix jacchus. We compared marmosets which were identified as having dichromatic or trichromatic color vision. Within the LGN, the density of P-IR neurones is highest in the parvocellular and magnocellular laminae, but C-IR neurones are found mainly in the koniocellular division of the LGN, that is, the interlaminar zones and S laminae. Not all interlaminar zone cells are C-IR. In the visual cortex, P-IR neurones are present in all laminae except lamina 1, in areas V1 and V2. Neurones which are strongly C-IR are mainly located in laminae 2 and 3 in V1 and V2. Lightly C-IR neurones are concentrated in lamina 4, and are more numerous in V1 than in V2. Quantitative analysis showed no differences in the density or distribution of IR neurones in either LGN or visual cortex when dichromat and trichromat animals were compared. We conclude that this functional difference is not associated with differences in the neurochemistry of calcium-binding proteins in the primary visual pathways.

Uniform-scale chromaticity diagrams: Opponent-chromatic responses as logarithms of the cone-activation ratios

Chromatic-response functions and a uniform-scale chromaticity diagram are derived by assuming that normal trichromatic vision is obtained by combining tritanopic and deuteranopic vision. Two different opponent-chromatic mechanisms can be related to these two dichromatic visions. The chromatic-response functions obtained are the logarithms of proper cone-activation ratios (CAR). Each opponent-chromatic mechanism consists of two parts separated by a properly defined neutral point that depends on the surround chromaticity. The Abney-hue shift, the wavelength discrimination for trichomats, deuteranopes, and tritanopes, and the threshold purity are well reproduced. © 1998 John Wiley & Sons, Inc. Col Res Appl, 23, 27–38, 1998

The Craik-O’Brien-Cornsweet Illusion in Honeybees

Humans perceive a low-contrast Craik-O’Brien-Cornsweet (COC) grating as being similar to, or even indiscriminable from, a squarewave grating (Fig. l), despite the fact that the spatial intensity profiles of the two patterns are very different [3]. This similarity has been attributed to processing at early stages of the visual pathway which selectively transmits edge information, followed by cortical mechanisms which “fill in” the appropriate intensities between the edges [9]. Here we find that bees behave as though they experience a similar illusion, suggesting that certain principles of visual processing are shared by insects and humans. While the visual systems of insects and humans display several differences an obvious one being that insects possess compound (multi-faceted) eyes whilst humans possess simple (single lens) eyes recent work is beginning to reveal several parallels with regard to the ways in which the two systems process visual information. For example, bees, like humans, possess trichromatic vision [ 161, are “colour-blind” with respect to the detection of motion [ 12, 131, appear to analyse pattern orientation by using orientation-tuned channels [20, 211 and even exhibit “top-down” processing with regard to the ability to detect camouflaged objects [22]. Indeed, insects even seem to experi-

Uniform‐scale chromaticity diagrams: Opponent‐chromatic responses as logarithms of the cone‐activation ratios

Chromatic-response functions and a uniform-scale chromaticity diagram are derived by assuming that normal trichromatic vision is obtained by combining tritanopic and deuteranopic vision. Two different opponent-chromatic mechanisms can be related to these two dichromatic visions. The chromatic-response functions obtained are the logarithms of proper cone-activation ratios (CAR). Each opponent-chromatic mechanism consists of two parts separated by a properly defined neutral point that depends on the surround chromaticity. The Abney-hue shift, the wavelength discrimination for trichomats, deuteranopes, and tritanopes, and the threshold purity are well reproduced. © 1998 John Wiley & Sons, Inc. Col Res Appl, 23, 27–38, 1998

Color vision of ancestral organisms of higher primates.

The color vision of mammals is controlled by photosensitive proteins called opsins. Most mammals have dichromatic color vision, but hominoids and Old World (OW) monkeys enjoy trichromatic vision, having the blue-, green-, and red-sensitive opsin genes. Most New World (NW) monkeys are either dichromatic or trichromatic, depending on the sex and genotype. Trichromacy in higher primates is believed to have evolved to facilitate the detection of yellow and red fruits against dappled foliage, but the process of evolutionary change from dichromacy to trichromacy is not well understood. Using the parsimony and the newly developed Bayesian methods, we inferred the amino acid sequences of opsins of ancestral organisms of higher primates. The results suggest that the ancestors of OW and NW monkeys lacked the green gene and that the green gene later evolved from the red gene. The fact that the red/green opsin gene has survived the long nocturnal stage of mammalian evolution and that it is under strong purifying selection in organisms that live in dark environments suggests that this gene has another important function in addition to color vision, probably the control of circadian rhythms.

BEE COLOR VISION IS OPTIMAL FOR CODING FLOWER COLOR, BUT FLOWER COLORS ARE NOT OPTIMAL FOR BEING CODED—WHY?

Model calculations are used to determine an optimal color coding system for identifying flower colors, and to see whether flower colors are well suited for being encoded. It is shown that the trichromatic color vision of bees comprises UV, blue, and green receptors whose wavelength positions are optimal for identifying flower colors. But did flower colors actually drive the evolution of bee color vision? A phylogenetic analysis reveals that UV, blue, and green receptors were probably present in the ancestors of crustaceans and insects 570 million years ago, and thus predate the evolution of flower color by at least 400 million years. In what ways did flower colors adapt to insect color vision? The variability of flower color is subject to constraint. Flowers are clustered in the bee color space (probably because of biochemical constraints), and different plant families differ strongly in their variation of color (which points to phylogenetic constraint). However, flower colors occupy areas of color space that are significantly different from those occupied by common background materials, such as green foliage. Finally, models are developed to test whether the colors of flowers of sympatric and simultaneously blooming species diverge or converge to a higher degree than expected by chance. Such effects are indeed found in some habitats.

Trichromatic colour vision in New World monkeys.

Trichromatic colour vision depends on the presence of three types of cone photopigment. Trichromacy is the norm for all Old World monkeys, apes and humans, but in several genera of New World monkeys, colour vision is strikingly polymorphic. The difference in colour vision between these New and Old World primates results form differing arrangements of the pigment genes on the X chromosome. In Old World primates the three photopigments required for routine trichromatic colour vision are encoded by two or more X-chromosome pigment genes and an autosomal pigment gene. New World monkeys typically have only one X-chromosome pigment gene; multiple alleles allow different types of dichromatic colour vision and, in female heterozygous at this locus, variant forms of trichromatic colour vision. Here we report that multiple X-chromosome pigment genes and trichromatic colour vision are the norm for one genus of platyrrhine monkey, the howler monkey, Alouatta.