Color Vision System Research Articles

Guinea pigs reared in a monochromatic environment exhibit changes in cone density and opsin expression

This study aimed to determine if a monochromatic environment will affect the development of cones in a guinea pig model. Thirty 3-day-old guinea pigs were randomized into three groups and exposed to green, violet, and white light (control) for 8 weeks. The animals were sacrificed and the density of middle-wavelength cones (M cones) and short-wavelength sensitive (S cones) and expression of M-opsin and S-opsin were determined. The density of M cones was increased in the green light group as compared to the control group, and decreased in the violet light group as compared to the control group (both, p < 0.05). There was no significant difference in the density of the S cones among the groups (all, p > 0.05). The density of coexpressing cones in the middle retina was significantly increased in the green light group in comparison to the violet light group (p < 0.01). In addition, there was a significant increase in the level of M-opsin as determined by Western blotting and M-opsin mRNA expression as determined by PCR analysis in the green light group as compared to the control group and a significant decrease in violet light group as compared to the control group (all, p < 0.05). No significant difference in S-opsin level or S-opsin mRNA expression was noted among the groups. We concluded that monochromatic lighting affected the density of cones and expression of opsins in a guinea pig model, and this indicates that the retinal color visual system of the guinea pig possess developmental plasticity.

On the relationship between wavelength and perceived hue

AbstractUnlike in the case of sound where there is a compressed but straightforward relationship between frequency of sound waves and the perceived pitch, in the case of color the relationship is highly nonlinear, as first pointed out by Ostwald in 1916. In this article, the relationship between the perceptual Munsell hue scale and the related dominant wavelengths of the color chips involved is investigated. A hypothesis is proposed according to which the immediate cause of the nonlinearity can be found in the relative activation of the kinds of chromatically opponent functions shown to exist in neurons of the lateral geniculate nuclei. However, at this time, the relationship is only correlational, given the lack of deeper understanding of the relationship between the physiological operation of the color vision system and conscious experiences of color. © 2011 Wiley Periodicals, Inc. Col Res Appl, 2011;

Beyond pollinator attraction: extra-floral displays deter herbivores in a Mediterranean annual plant

The evolution of several floral traits is thought to be driven by multiple selective agents, including pollinators and herbivores. Similar combinations of selection pressures may have shaped extra-floral traits. The conspicuous purple tufts of leaves (“flags”), which often terminate vertical inflorescences in the Mediterranean annual Salvia viridis, were shown to attract insect pollinators to the flowering patch. Here we test whether they also function as anti-herbivore signals. We determined the aposematic potential of S. viridis flags on three levels: concentrations of anthocyanins, suggested to function as aposematic visual signals, in leaves and flags; spectrometry to estimate whether the color-vision system of two common Mediterranean generalist herbivores (locusts and goats) can discriminate flags from leaves; and choice experiments to determine food preferences of the same herbivores. Anthocyanin concentrations in flags were >10-fold higher than in leaves. Flags exhibited peak reflectance at 450 and 700 nm wavelengths, while leaves reflected maximally at 550 nm. According to the Vorobyev-Osorio color vision model, these differences in color reflection are likely to allow visual discrimination by herbivores. Goats preferred feeding on clipped inflorescences over control inflorescences. Locusts preferred leaves over flags. To test whether this was due to deterrence from the flags’ coloration, we also offered them choice between leaves and a rare, white morph, of the flags. The locusts chose both equally immediately after presentation, but leaves attracted more individuals after 5 min of feeding. The locusts also preferred green cabbage over anthocyanin-rich red cabbage. These results support the function of colorful extra-floral displays as warning signals.

A Study of Metameric Blacks for the Representation of Spectral Images

Spectral images contain a large volume of data and can be efficiently compressed by low dimensional linear models. However, there is a trade-off between the accuracy of spectral and colorimetric representation. When a spectral image is reproduced by a low-dimensional linear model, spectral error and color difference are contrary to each other and minimizing the colour error is by no means equivalent to minimizing the spectral error. Although one aim of a spectral-image file format is to preserve and represent the spectral information, most users are likely to reproduce a spectral image on a trichromatic image-reproduction system and therefore it is important that the spectral information is not preserved at the expense of colorimetric accuracy. In this study a method for spectral encoding that provides an efficient representation of the spectral information whilst perfectly preserving the colorimetric information is analysed. The lossy compression technique that is considered in this work is based on a low-dimensional linear model of spectral reflectance, with the first three basis functions represent color information and the additional basis functions are metameric blacks which preserve spectral information.

Optical filter for highlighting spectral features Part I: design and development of the filter for discrimination of human skin with and without an application of cosmetic foundation

Light reflected from an object's surface contains much information about its physical and chemical properties. Changes in the physical properties of an object are barely detectable in spectra. Conventional trichromatic systems, on the other hand, cannot detect most spectral features because spectral information is compressively represented as trichromatic signals forming a three-dimensional subspace. We propose a method for designing a filter that optically modulates a camera's spectral sensitivity to find an alternative subspace highlighting an object's spectral features more effectively than the original trichromatic space. We designed and developed a filter that detects cosmetic foundations on human face. Results confirmed that the filter can visualize and nondestructively inspect the foundation distribution.

Evolution of ultraviolet vision in the largest avian radiation - the passerines

BackgroundInterspecific variation in avian colour vision falls into two discrete classes: violet sensitive (VS) and ultraviolet sensitive (UVS). They are characterised by the spectral sensitivity of the most shortwave sensitive of the four single cones, the SWS1, which is seemingly under direct control of as little as one amino acid substitution in the cone opsin protein. Changes in spectral sensitivity of the SWS1 are ecologically important, as they affect the abilities of birds to accurately assess potential mates, find food and minimise visibility of social signals to predators. Still, available data have indicated that shifts between classes are rare, with only four to five independent acquisitions of UV sensitivity in avian evolution.ResultsWe have classified a large sample of passeriform species as VS or UVS from genomic DNA and mapped the evolution of this character on a passerine phylogeny inferred from published molecular sequence data. Sequencing a small gene fragment has allowed us to trace the trait changing from one stable state to another through the radiation of the passeriform birds. Their ancestor is hypothesised to be UVS. In the subsequent radiation, colour vision changed between UVS and VS at least eight times.ConclusionsThe phylogenetic distribution of SWS1 cone opsin types in Passeriformes reveals a much higher degree of complexity in avian colour vision evolution than what was previously indicated from the limited data available. Clades with variation in the colour vision system are nested among clades with a seemingly stable VS or UVS state, providing a rare opportunity to understand how an ecologically important trait under simple genetic control may co-evolve with, and be stabilised by, associated traits in a character complex.

Correlation and Cause When Inferring Attentional Guidance in the Rainforest and Beyond

Humans shift their eyes to sample the most important parts of the visual world. How do we know if it is worth looking at a certain location before we look there? An intuitively appealing answer to this question is that the brain may use a shortcut based on early visual processing to determine “salient” parts of the scene that are likely to be worth fixating. In the last decade, research scrutinizing this proposal (and driven principally by the Saliency Map model as described by Itti and Koch, 2000) has given rise to something of a deadlock between demonstrations that fixation correlates with bottom-up saliency in free viewing (Parkhurst et al., 2002), and experiments showing that eye guidance is dominated by top-down, cognitive control (e.g., Einhauser et al., 2008). The recent studies by Frey et al. (2011) take a novel approach that begins to address two major issues with such research. The first issue concerns the causal role of certain features in eye guidance. There are systematic differences between fixated and non-fixated locations in natural images, with frequently looked-at regions tending to have higher luminance and chromaticity and to contain edges (e.g., Tatler et al., 2005). However, it does not necessarily follow that these features actually cause fixation. Instead, it might be that saccades are guided by top-down processes (e.g., a desire to look at objects), and that salient features merely coincide with such regions. In other words, saliency might be a useful shortcut for computational models while telling us nothing about what the brain is actually doing. It is necessary, therefore, to move away from correlations between fixations and features and take an experimental approach (e.g., by manipulating the saliency of key objects: Foulsham and Underwood, 2007). Frey et al. (2011) modified the color content of their stimuli and report changes in fixation behavior with some modifications (eliminating red–green information) but not others (removing the blue–yellow channel). This is stronger evidence regarding the salient color features that are causally related to fixation. Frey et al. (2011) also take a different and ingenious approach by examining the impact of the observer – in this case their ability to perceive color in the normal way. If color features actually cause fixation in normal observers then color-blind individuals should not fixate in the same locations. Conversely, if color features merely coincide with semantic properties of the scene (e.g., because objects are colorful) then deuteranopes may behave the same. The fact that Frey et al. (2011) show both similarities (in the color features at fixation) and differences (in the timing of fixations to colored regions) shows that this is a powerful technique for pinning down causal influences of color in attentional guidance. Moreover, this avenue of research complements other work investigating the attention of atypical observers with object recognition problems who have normal low-level vision but lack higher-level scene semantics (Foulsham et al., 2009). Frey et al. (2011) also begin to address a second issue, which might be called an ecological problem: In what sorts of stimuli should we study visual attention? While traditionally most research has used simple stimuli, advances in modeling feature extraction in complex images means that there is now a large literature on eye movements in complex, color scenes. One of the advantages of using natural images is that they are associated with semantic information—gist and expectations about the environment to which humans are accustomed. Furthermore, the stimuli used by Frey et al. (2011) are arguably taken from the very same environment in which our trichromatic visual system evolved to identify ripening fruit in the rainforest. This offers the tantalizing prospect of linking the fundamentals of visual attention to both proximate mechanisms in the brain and distal causes of phylogeny and function in our evolutionary history. Converging evidence from correlational and experimental studies, from typical and atypical individuals, and from simple and more ecologically complex stimuli can therefore provide a more complete explanation of attention – in the rainforest and beyond.

Diversity of Color Vision: Not All Australian Marsupials Are Trichromatic

Color vision in marsupials has recently emerged as a particularly interesting case among mammals. It appears that there are both dichromats and trichromats among closely related species. In contrast to primates, marsupials seem to have evolved a different type of trichromacy that is not linked to the X-chromosome. Based on microspectrophotometry and retinal whole-mount immunohistochemistry, four trichromatic marsupial species have been described: quokka, quenda, honey possum, and fat-tailed dunnart. It has, however, been impossible to identify the photopigment of the third cone type, and genetically, all evidence so far suggests that all marsupials are dichromatic. The tammar wallaby is the only Australian marsupial to date for which there is no evidence of a third cone type. To clarify whether the wallaby is indeed a dichromat or trichromatic like other Australian marsupials, we analyzed the number of cone types in the “dichromatic” wallaby and the “trichromatic” dunnart. Employing identical immunohistochemical protocols, we confirmed that the wallaby has only two cone types, whereas 20–25% of cones remained unlabeled by S- and LM-opsin antibodies in the dunnart retina. In addition, we found no evidence to support the hypothesis that the rod photopigment (rod opsin) is expressed in cones which would have explained the absence of a third cone opsin gene. Our study is the first comprehensive and quantitative account of color vision in Australian marsupials where we now know that an unexpected diversity of different color vision systems appears to have evolved.

Eyes with basic dorsal and specific ventral regions in the glacial Apollo, Parnassius glacialis (Papilionidae)

Recent studies on butterflies have indicated that their colour vision system is almost species specific. To address the question of how this remarkable diversity evolved, we investigated the eyes of the glacial Apollo, Parnassius glacialis, a living fossil species belonging to the family Papilionidae. We identified four opsins in the Parnassius eyes--an ultraviolet- (PgUV), a blue- (PgB), and two long wavelength (PgL2, PgL3)-absorbing types--and localized their mRNAs within the retina. We thus found ommatidial heterogeneity and a clear dorso-ventral regionalization of the eye. The dorsal region consists of three basic types of ommatidia that are similar to those found in other insects, indicating that this dorsal region retains the ancestral state. In the ventral region, we identified two novel phenomena: co-expression of the opsins of the UV- and B-absorbing type in a subset of photoreceptors, and subfunctionalization of long-wavelength receptors in the distal tier as a result of differential expression of the PgL2 and PgL3 mRNAs. Interestingly, butterflies from the closely related genus Papilio (Papilionidae) have at least three long-wavelength opsins, L1-L3. The present study indicates that the duplication of L2 and L3 occurred before the Papilio lineage diverged from the rest, whereas L1 was produced from L3 in the Papilio lineage.

Color classification using color vision models

According to recent physiological research results, there are many individual differences already at the detection level of our color vision system. It is not completely clear yet, how the other levels of color vision system compensate the detection differences. Instead of detecting and analyzing colors exactly in the same way, we all just have learned to classify colors in a certain way, which seems to lead almost always to the same result independent of the individual differences in the color vision system. In this article, we experiment with four models developed for replicating certain properties of human color vision. We examine the color classification abilities of these models and show the differences and similarities in their behavior. © 2010 Wiley Periodicals, Inc. Col Res Appl, 2010.

Evolution of anatomical and physiological specialization in the compound eyes of stomatopod crustaceans

Stomatopod crustaceans have complex and diverse visual systems. Among their many unique features are a specialized ommatidial region (the midband) that enables the eye to have multiple overlapping visual fields, as well as sets of spectral filters that are intercalated at two levels between tiers of photoreceptors involved in polychromatic color vision. Although the physiology and visual function of stomatopod eyes have been studied for many years, how these unique visual features originated and diversified is still an open question. In order to investigate how stomatopods have attained the current complexity in visual function, we have combined physiological and morphological information (e.g. number of midband rows, number of filters in the retina, and the spectral properties of filters) with new phylogenetic analyses of relationships among species based on nucleotide sequence data from two nuclear (18S and 28S rDNA) and two mitochondrial [16S and cytochrome oxidase I (COI)] genes. Based on our recovered phylogenetic relationships among species, we propose two new superfamilies within the Stomatopoda: Hemisquilloidea and Pseudosquillodea. Maximum likelihood ancestral state reconstructions indicate that ancestral stomatopod eyes contained six midband rows and four intrarhabdomal filters, illustrating that the visual physiological complexity originated early in stomatopod evolutionary history. While the two distal filters contain conservative sets of filter pigments, the proximal filters show more spectral diversity in filter types, particularly in midband row 2, and are involved in tuning the color vision system to the photic environment. In particular, a set of related gonodactyloid families (Gonodactylidae, Protosquillidae, Takuidae) inhabiting shallow, brightly lit coral reef waters contain the largest diversity of filter pigments, which are spectrally placed relative to the underlying photoreceptors to take advantage of the broad spectrum of light available in the environment.

Are Gaussian spectra a viable perceptual assumption in color appearance?

Natural illuminant and reflectance spectra can be roughly approximated by a linear model with as few as three basis functions, and this has suggested that the visual system might construct a linear representation of the spectra by estimating the weights of these functions. However, such models do not accommodate nonlinearities in color appearance, such as the Abney effect. Previously, we found that these nonlinearities are qualitatively consistent with a perceptual inference that stimulus spectra are instead roughly Gaussian, with the hue tied to the inferred centroid of the spectrum [J. Vision 6(9), 12 (2006)]. Here, we examined to what extent a Gaussian inference provides a sufficient approximation of natural color signals. Reflectance and illuminant spectra from a wide set of databases were analyzed to test how well the curves could be fit by either a simple Gaussian with three parameters (amplitude, peak wavelength, and standard deviation) versus the first three principal component analysis components of standard linear models. The resulting Gaussian fits were comparable to linear models with the same degrees of freedom, suggesting that the Gaussian model could provide a plausible perceptual assumption about stimulus spectra for a trichromatic visual system.

Electrophysiological assessment of spectral sensitivity in adult Nile tilapia Oreochromis niloticus: evidence for violet sensitivity

The cichlid fish radiations of the African Great Lakes are an important model for evolutionary biology. Cichlids have diverse colour vision systems and predominately express three cone visual pigments. However, rare populations of spectrally distinct cones have been found in a number of species, but it is not known whether they contribute to spectral sensitivity. Adult Nile tilapia, Oreochromis niloticus, an ancestral outgroup to the cichlid radiations in the Great Lakes, have three cone types: short-wavelength sensitive (SWS), medium-wavelength sensitive (MWS) and long-wavelength sensitive (LWS) cones, but evidence from microspectrophotometry and cone opsin gene expression suggests they may also have violet-sensitive (VS) cones. We used electrophysiology to assess spectral sensitivity in this species and found evidence of four sensitivity peaks in the ranges 380-420, 440-480, 500-600 and 600-680 nm, with maximal sensitivity at longer wavelengths. The continued presence of a 380-420 nm peak under long-wavelength chromatic adapting backgrounds indicates that this is due to a VS cone mechanism not the beta-band of the LWS cone mechanism. Differences in spectral sensitivity curves recorded at different times of year revealed evidence of A1/A2 shifts. The presence of notches in the sensitivity curves and a multiple-mechanisms model used to assess cone contributions indicated that the curves are the result of four cone mechanisms (VS, SWS, MWS and LWS cones) and that chromatically opponent processes occur between mechanisms. The spectral transmittance of the lens steeply declines between 410-380 nm, limiting the short-wavelength limb of the VS cone. As adults, Nile tilapia appear to possess the necessary retinal mechanisms for colour vision. While maximal sensitivity to longer wavelengths is an adaptation to the wavelengths of light predominantly available in their natural habitats, their broad sensitivity range suggests that Nile tilapia possess a flexible, generalised visual system able to adapt to changes in visual environment in their highly variable natural habitat.

The cone inputs to colour appearance mechanisms

What computation does the human brain perform when we experience ‘red’, ‘green’, ‘yellow’, or ‘blue’? How does the human visual system combine the retinal cone signals (L, M, S cones) to yield these colour sensations? To address this question we ask observers to select the colour which appears ‘neither red nor green’ (yielding unique yellow and unique blue) or ‘neither yellow nor blue’ (yielding unique red and unique green). Since we obtain these unique hue settings at different luminance and saturation levels, we have numerous points in 3-dimensional (LMS) cone space that correspond to a particular hue. This allows us to determine quantitatively how the cone signals are related to a particular perceived hue. Our results show that the human visual system employs as least three chromatic mechanisms to yield the four unique hues and, to a first approximation, these mechanisms combine the (L,M,S) cone signals linearly. Furthermore, the variability between observers is relatively small when expressed in terms of preceptual errors. Our results add further weight to the idea that the colour vision system in adult human is able to recalibrate itself based on the prior visual experience and that our colour appearance mechanisms may be a consequence of environmental contraints.

Flower color phenology in European grassland and woodland habitats, through the eyes of pollinators

Some studies have claimed that flowers in bloom at particular times of year are more likely to be of particular colors to better attract pollinating insects. To test this, we analyzed a data set collected from five field sites near Strausberg, Germany, which included information on flower color and months of blooming. However, we chose to consider flower color as perceived by bee as well as human visual systems, as well as independent of any color vision system, to reveal whether trends, if present, have any ecological relevance. Using randomization analyses, we were able to consider whether blooming time interacts with flower color, and how this interaction depends upon other factors. Our results show that there is an association between the months of flowering and the colors of flowers — but only when flowers are considered according to human color categories. Further analysis showed that this is merely a consequence of flowers from the same family being more likely to flower at the same time and have similar colors. All these effects disappeared when flowers were considered using bee color categories, and in the analyses of physical spectral reflectances.

Sensitivity analysis of Hopfield neural network in classifying natural RGB color space

This paper presents a study of the sensitivity analysis of the artificial Hopfield Neural Network (HNN) when segmenting natural color images. The color distinction or vision system relies on two st...

Flower colours along an alpine altitude gradient, seen through the eyes of fly and bee pollinators

Alpine flowers face multiple challenges in terms of abiotic and biotic factors, some of which may result in selection for certain colours at increasing altitude, in particular the changing pollinator species composition, which tends to move from bee-dominated at lower elevations to fly-dominated in high-alpine regions. To evaluate whether growing at altitude—and the associated change in the dominant pollinator groups present—has an effect on the colour of flowers, we analysed data collected from the Dovrefjell National Park in Norway. Unlike previous studies, however, we considered the flower colours according to ecologically relevant models of bee and fly colour vision and also their physical spectral properties independently of any colour vision system, rather than merely looking at human colour categories. The shift from bee to fly pollination with elevation might, according to the pollination syndrome hypothesis, lead to the prediction that flower colours should shift from more bee-blue and UV-blue flowers (blue/violet to humans, i.e. colours traditionally associated with large bee pollinators) at low elevations to more bee-blue-green and green (yellow and white to humans—colours often linked to fly pollination) flowers at higher altitude. However, although there was a slight increase in bee-blue-green flowers and a decrease in bee-blue flowers with increasing elevation, there were no statistically significant effects of altitude on flower colour as seen either by bees or by flies. Although flower colour is known to be constrained by evolutionary history, in this sample we also did not find evidence that phylogeny and elevation interact to determine flower colours in alpine areas.

The Green-absorbing Drosophila Rh6 Visual Pigment Contains a Blue-shifting Amino Acid Substitution That Is Conserved in Vertebrates

The molecular mechanisms that regulate invertebrate visual pigment absorption are poorly understood. Through sequence analysis and functional investigation of vertebrate visual pigments, numerous amino acid substitutions important for this adaptive process have been identified. Here we describe a serine/alanine (S/A) substitution in long wavelength-absorbing Drosophila visual pigments that occurs at a site corresponding to Ala-292 in bovine rhodopsin. This S/A substitution accounts for a 10-17-nm absorption shift in visual pigments of this class. Additionally, we demonstrate that substitution of a cysteine at the same site, as occurs in the blue-absorbing Rh5 pigment, accounts for a 4-nm shift. Substitutions at this site are the first spectrally significant amino acid changes to be identified for invertebrate pigments sensitive to visible light and are the first evidence of a conserved tuning mechanism in vertebrate and invertebrate pigments of this class.

Fenton and photo‐Fenton oxidation of a model mixture of dyes – overall kinetic analysis

The decolorisation and mineralisation of a model mixture of reactive dyes, under Fenton and photo‐Fenton conditions, have been investigated. A kinetic study was performed and rate constants and half‐life times determined from batch experimental data according to a pseudo‐first‐order degradation kinetic model. Moreover, the use of natural or artificial light, as well as the effect of temperature, was evaluated with the aim of finding the best Fenton process to promote the efficient degradation of the standard trichromatic system. The experimental results showed that the Fenton process run under solar light was the most effective. It is suggested that Fenton‐ and photo‐Fenton‐type reactions are viable techniques for the treatment of these types of dye mixtures, according to the high levels of colour, chemical oxygen demand and total organic carbon removal.

Early evolution of multifocal optics for well-focused colour vision in vertebrates

Jawless fishes (Agnatha; lampreys and hagfishes) most closely resemble the earliest stage in vertebrate evolution and lamprey-like animals already existed in the Lower Cambrian [about 540 million years ago (MYA)]. Agnathans are thought to have separated from the main vertebrate lineage at least 500 MYA. Hagfishes have primitive eyes, but the eyes of adult lampreys are well-developed. The southern hemisphere lamprey, Geotria australis, possesses five types of opsin genes, three of which are clearly orthologous to the opsin genes of jawed vertebrates. This suggests that the last common ancestor of all vertebrate lineages possessed a complex colour vision system. In the eyes of many bony fishes and tetrapods, well-focused colour images are created by multifocal crystalline lenses that compensate for longitudinal chromatic aberration. To trace the evolutionary origins of multifocal lenses, we studied the optical properties of the lenses in four species of lamprey (Geotria australis, Mordacia praecox, Lampetra fluviatilis and Petromyzon marinus), with representatives from all three of the extant lamprey families. Multifocal lenses are present in all lampreys studied. This suggests that the ability to create well-focused colour images with multifocal optical systems also evolved very early.