Color contrast: a contributory mechanism to color constancy

Improving RGB illuminant estimation exploiting spectral average radiance

We introduce a method that enhances RGB color constancy accuracy by combining neural network and k-means clustering techniques. Our approach stands out from previous works because we combine multispectral and color information together to estimate illuminants. Furthermore, we investigate the combination of the illuminant estimation in the RGB color and in the spectral domains, as a strategy to provide a refined estimation in the RGB color domain. Our investigation can be divided into three main points: (1) identify the spatial resolution for sampling the input image in terms of RGB color and spectral information that brings the highest performance; (2) determine whether it is more effective to predict the illuminant in the spectral or in the RGB color domain, and finally, (3) assuming that the illuminant is in fact predicted in the spectral domain, investigate if it is better to have a loss function defined in the RGB color or spectral domain. Experimental results are carried out on NUS: a standard dataset of multispectral radiance images with an annotated spectral global illuminant. Among the several considered options, the best results are obtained with a model trained to predict the illuminant in the spectral domain using an RGB color loss function. In terms of comparison with the state of the art, this solution improves the recovery angular error metric by 66% compared to the best tested spectral method, and by 41% compared to the best tested RGB method.

Effect of surrounding objects in the adapting scene on chromatic adaptation

Most of the existing chromatic adaptation transforms (CATs) were developed for flat uniform stimuli presented in a uniform background, which substantially simplifies the complexity of the real scene by excluding surrounding objects from the viewing field. The impact of the background complexity, in terms of the spatial properties of the objects surrounding the stimulus, on chromatic adaptation is ignored in most CATs. This study systematically investigated how the background complexity and color distribution affect the adaptation state. Achromatic matching experiments were conducted in an immersive lighting booth, with the illumination varying in chromaticity and the adapting scene varying in surrounding objects. Results show that compared to the uniform adapting field, increasing the scene complexity can significantly improve the degree of adaptation for the Planckian illuminations with low CCT levels. In addition, the achromatic matching points are substantially biased by the color of the surrounding object, implying the interactive effect of the illumination color and the dominant scene color on the adapting white point.

Influence of Stimulus Size on Simultaneous Chromatic Induction

Chromatic induction is a major contextual effect of color appearance. Patterned backgrounds are known to induce strong chromatic induction effects. However, it has not been clarified whether the spatial extent of the chromatic surrounding induces a chromatic contrast or assimilation effects. In this study, we examined the influence of the width of a center line and its flanking white contour on the color appearance when the line was surrounded by chromatic backgrounds. A strong color shift was observed when the center line was flanked by white contours with the L/M- and S-cone chromatic backgrounds. There was a difference between the optimal widths of the center line and the contour for the shift in color appearance for the L/M-cone chromaticity (0.9 and 1.1–1.7 min, respectively) and the S-cone chromaticity (8.2–17.5 and 0.9–2.5 min, respectively). The optimal width of the center line for the L/M-cone was finer than the resolution-limit width of the chromatic contrast sensitivity and coarser than that of the luminance contrast sensitivity. Thus, the color appearance of the center line could be obtained by integrating broad chromatic information and fine luminance details. Due to blurring and chromatic aberrations, the simulated artifact was large for the darker center line and S-cone background, thus suggesting that the artifact could explain the luminance dependency of the induction along the S-cone chromaticity. Moreover, the findings of this study reveal that the dominant factor of the color shift is neural instead of optical.

The effect of scene articulation on transparent layer constancy

In this article, we examine the influence of scene articulation on transparent layer constancy. We argue that the term articulation may be understood as an aspect of the more general concept naturalness of a stimulus that relates to the degree of enrichment compared with a minimal stimulus and to the extent to which a stimulus contains regularities that are typically found in natural scenes. We conducted two matching experiments, in which we used strongly reduced scenes and operationalized articulation by the number of background reflectances (numerosity). The results of the first experiment show that higher numerosity actually leads to an increase in transparent layer constancy when reflectances are randomly drawn from a fixed population. However, this advantage disappears if the spatial mean and the variation of the subset colors are controlled as in our second experiment. Furthermore, our results suggest that the mechanism underlying transparent layer constancy leads to a rather stable compromise between two matching criteria, namely, proximal identity and constant filter properties according to our perceptual model. For filters with an additive component, which appear more or less hazy, we observed improved recovered filter properties and correspondingly higher degrees of transparent layer constancy, suggesting an additional mechanism in this type of filter.

A multiplicity of color-responsive cortical mechanisms revealed by the dynamics of cVEPs

Our results connect higher-order color mechanisms deduced from psychophysics with the known diversity of populations of double-opponent, color-responsive cells in V1. We used the chromatic visual evoked potential, the cVEP, to study responses in human visual cortex to equiluminant color patterns. Stimuli were modulated along three directions in color space: the cardinal directions, L-M and S, and along the line in color space from the white point to the color of the Red LED in the display screen (the Red direction). The Red direction is roughly intermediate between L-M and S in DKL space in cone-contrast coordinates. While cVEP response amplitude, latency, and width--and their dependences on cone contrast-- were similar in the L-M and Red directions, the Transientness of the Red response was significantly greater than for responses to stimuli in the L-M direction and in the S direction. This difference in response dynamics supports the concept that there are multiple, distinct neuronal populations, so-called higher- order color mechanisms, for color perception within human V1 cortex.

Large enhancement of simultaneous color contrast by white flanking contours

Simultaneous color contrast and assimilation are mutually opposing effects on color appearance, and their magnitude depends on spatial context. The Monnier-Shevell illusion induces a large color shift by a synergy of simultaneous assimilation and contrast using the alternating color of proximal and distant surrounds. The illusion induces a prominent effect along the blue-yellow color axis, but a subtle effect along the orthogonal color axis. In this study, we report an illusion generated by an extremely thin gray line on a cyan background that appears reddish when the line is flanked by thin white contours. We quantified the color appearance of the gray line in a color matching experiment and found that the color shift of the gray line with white contours induced large color shifts. It is also known that luminance contrast between a center and its surrounds affects the magnitude of simultaneous color contrast. However, our color contrast effects were larger for a dark line rather than for a pale line. In contrast, the perceived color shift of the line without the contours increased as the luminance of the gray line increased, supporting the known effect of Kirschmann's third law. These results indicate that Kirschmann's third law fails to explain the perceived color shift of our illusion, even after accounting for optical factors like aberrations. Observed color shifts could be explained by an augmented synergy theory based on intensity space, rather than chromaticity.

Tooth color and whitening - digital technologies

OBJECTIVES

To review the key concepts of color in the dental domain with specific reference to the use of digital technology to measure color and color appearance.

MATERIALS AND METHODS

The literature on color assessment in dentistry is considered and methods for assessing whiteness, yellowness and color appearance are collated and described.

RESULTS AND CONCLUSION

A variety of methods for assessing color have been shown to exist and be viable including digital imaging. Equations to predict whiteness are identified; there is evidence that they are effective but further evaluation may be needed.

Colour Constancy Beyond the Classical Receptive Field

The problem of removing illuminant variations to preserve the colours of objects (colour constancy) has already been solved by the human brain using mechanisms that rely largely on centre-surround computations of local contrast. In this paper we adopt some of these biological solutions described by long known physiological findings into a simple, fully automatic, functional model (termed Adaptive Surround Modulation or ASM). In ASM, the size of a visual neuron's receptive field (RF) as well as the relationship with its surround varies according to the local contrast within the stimulus, which in turn determines the nature of the centre-surround normalisation of cortical neurons higher up in the processing chain. We modelled colour constancy by means of two overlapping asymmetric Gaussian kernels whose sizes are adapted based on the contrast of the surround pixels, resembling the change of RF size. We simulated the contrast-dependent surround modulation by weighting the contribution of each Gaussian according to the centre-surround contrast. In the end, we obtained an estimation of the illuminant from the set of the most activated RFs' outputs. Our results on three single-illuminant and one multi-illuminant benchmark datasets show that ASM is highly competitive against the state-of-the-art and it even outperforms learning-based algorithms in one case. Moreover, the robustness of our model is more tangible if we consider that our results were obtained using the same parameters for all datasets, that is, mimicking how the human visual system operates. These results suggest a dynamical adaptation mechanisms contribute to achieving higher accuracy in computational colour constancy.

Exploring the functional nature of synaesthetic colour: Dissociations from colour perception and imagery

Individuals with grapheme-colour synaesthesia experience anomalous colours when reading achromatic text. These unusual experiences have been said to resemble 'normal' colour perception or colour imagery, but studying the nature of synaesthesia remains difficult. In the present study, we report novel evidence that synaesthetic colour impacts conscious vision in a way that is different from both colour perception and imagery. Presenting 'normal' colour prior to binocular rivalry induces a location-dependent suppressive bias reflecting local habituation. By contrast, a grapheme that evokes synaesthetic colour induces a facilitatory bias reflecting priming that is not constrained to the inducing grapheme's location. This priming does not occur in non-synaesthetes and does not result from response bias. It is sensitive to diversion of visual attention away from the grapheme, but resistant to sensory perturbation, reflecting a reliance on cognitive rather than sensory mechanisms. Whereas colour imagery in non-synaesthetes causes local priming that relies on the locus of imagined colour, imagery in synaesthetes caused global priming not dependent on the locus of imagery. These data suggest a unique psychophysical profile of high-level colour processing in synaesthetes. Our novel findings and method will be critical to testing theories of synaesthesia and visual awareness.

Nonlinear dynamics of cortical responses to color in the human cVEP

The main finding of this paper is that the human visual cortex responds in a very nonlinear manner to the color contrast of pure color patterns. We examined human cortical responses to color checkerboard patterns at many color contrasts, measuring the chromatic visual evoked potential (cVEP) with a dense electrode array. Cortical topography of the cVEPs showed that they were localized near the posterior electrode at position Oz, indicating that the primary cortex (V1) was the major source of responses. The choice of fine spatial patterns as stimuli caused the cVEP response to be driven by double-opponent neurons in V1. The cVEP waveform revealed nonlinear color signal processing in the V1 cortex. The cVEP time-to-peak decreased and the waveform's shape was markedly narrower with increasing cone contrast. Comparison of the linear dynamics of retinal and lateral geniculate nucleus responses with the nonlinear dynamics of the cortical cVEP indicated that the nonlinear dynamics originated in the V1 cortex. The nature of the nonlinearity is a kind of automatic gain control that adjusts cortical dynamics to be faster when color contrast is greater.

#TheDress: Categorical perception of an ambiguous color image

We present a full analysis of data from our preliminary report (Lafer-Sousa, Hermann, & Conway, 2015) and test whether #TheDress image is multistable. A multistable image must give rise to more than one mutually exclusive percept, typically within single individuals. Clustering algorithms of color-matching data showed that the dress was seen categorically, as white/gold (W/G) or blue/black (B/K), with a blue/brown transition state. Multinomial regression predicted categorical labels. Consistent with our prior hypothesis, W/G observers inferred a cool illuminant, whereas B/K observers inferred a warm illuminant; moreover, subjects could use skin color alone to infer the illuminant. The data provide some, albeit weak, support for our hypothesis that day larks see the dress as W/G and night owls see it as B/K. About half of observers who were previously familiar with the image reported switching categories at least once. Switching probability increased with professional art experience. Priming with an image that disambiguated the dress as B/K biased reports toward B/K (priming with W/G had negligible impact); furthermore, knowledge of the dress's true colors and any prior exposure to the image shifted the population toward B/K. These results show that some people have switched their perception of the dress. Finally, consistent with a role of attention and local image statistics in determining how multistable images are seen, we found that observers tended to discount as achromatic the dress component that they did not attend to: B/K reporters focused on a blue region, whereas W/G reporters focused on a golden region.

Adjusting to a sudden “aging” of the lens

Color perception is known to remain largely stable across the lifespan despite the pronounced changes in sensitivity from factors such as the progressive brunescence of the lens. However, the mechanisms and timescales controlling these compensatory adjustments are still poorly understood. In a series of experiments, we tracked adaptation in observers after introducing a sudden change in lens density by having observers wear glasses with yellow filters that approximated the average spectral transmittance of a 70-year-old lens. Individuals were young adults and wore the glasses for 5 days for 8 h per day while engaged in their normal activities. Achromatic settings were measured on a CRT before and after each daily exposure with the lenses on and off, and were preceded by 5 min of dark adaptation to control for short-term chromatic adaptation. During each day, there was a large shift in the white settings consistent with a partial compensation for the added lens density. However, there was little to no evidence of an afterimage at the end of each daily session, and participants’ perceptual nulls were roughly aligned with the nulls for short-term chromatic adaptation, suggesting a rapid renormalization when the lenses were removed. The long-term drift was also extinguished by brief exposure to a white adapting field. The results point to distinct timescales and potentially distinct mechanisms compensating for changes in the chromatic sensitivity of the observer.

Representation of Color Surfaces in V1: Edge Enhancement and Unfilled Holes

The neuronal mechanism underlying the representation of color surfaces in primary visual cortex (V1) is not well understood. We tested on color surfaces the previously proposed hypothesis that visual perception of uniform surfaces is mediated by an isomorphic, filled-in representation in V1. We used voltage-sensitive-dye imaging in fixating macaque monkeys to measure V1 population responses to spatially uniform chromatic (red, green, or blue) and achromatic (black or white) squares of different sizes (0.5°-8°) presented for 300 ms. Responses to both color and luminance squares early after stimulus onset were similarly edge-enhanced: for squares 1° and larger, regions corresponding to edges were activated much more than those corresponding to the center. At later times after stimulus onset, responses to achromatic squares' centers increased, partially "filling-in" the V1 representation of the center. The rising phase of the center response was slower for larger squares. Surprisingly, the responses to color squares behaved differently. For color squares of all sizes, responses remained edge-enhanced throughout the stimulus. There was no filling-in of the center. Our results imply that uniform filled-in representations of surfaces in V1 are not required for the perception of uniform surfaces and that chromatic and achromatic squares are represented differently in V1.

SIGNIFICANCE STATEMENT

We used voltage-sensitive dye imaging from V1 of behaving monkeys to test the hypothesis that visual perception of uniform surfaces is mediated by an isomorphic, filled-in representation. We found that the early population responses to chromatic and achromatic surfaces are edge enhanced, emphasizing the importance of edges in surface processing. Next, we show for color surfaces that responses remained edge-enhanced throughout the stimulus presentation whereas response to luminance surfaces showed a slow neuronal 'filling-in' of the center. Our results suggest that isomorphic representation is not a general code for uniform surfaces in V1.

Colour contrasting between tissues predicts the resection in 5-aminolevulinic acid-guided surgery of malignant gliomas

Due to the various intensities of 5-aminolevulinic acid (5-ALA) fluorescence, neurosurgeons tend to be uncertain about which tissues to resect. This study aimed to reveal the shortcomings of the human visual perception of fluorescence, particularly the factors guiding the tissue removal and the correlation of fluorescence with contrast enhancement (CE) on magnetic resonance imaging (MRI). Various colour features [CIE L*a*b* colour space, colour difference described by ΔE and contrast ratio (CR)] of total 206 noticed fluorescent areas and their surroundings were measured from the video recordings of 21 primary high grade glioma (HGG) surgeries. The position of a fluorescent region was related to the corecorded navigational image. Following early postoperative MRI, 17 additional regions of corresponding to CE remnants were identified, their colour features were compared to the resected CEs. The targeted video post-processing method was designed, based on the results. There were no complications attributed to 5-ALA use and the median survival was <10 months. 82.5 % of recognised fluorescent areas were removed. Colour spaces of the resected regions and their backgrounds did not overlap. Opposite to the separate colour components (p > 0.05), the distant background colour (p < 0.05) and higher CR and ΔE (p < 0.01) determined the resection of a fluorescent region. Noneloquent location and CR both independently increased the resection rate in logistic regression. However, greater area under the receiver operating characteristic curve (AUC) in case of CR (AUC = 0.78; 95 % CI 0.71-0.83) determined its dominant role in neurosurgeon's fluorescence perception. CE regions presented with a significantly more saturated shade of violet (consistently higher a* and b*) than other tumour parts (p < 0.05). Regions corresponding to tumour remnants had a significantly lower a* component value (p = 0.02) as well as a lower ΔE than the matched background (AUC = 0.73; 95 % CI 0.65-0.80). In order to increase the resection rate, ΔE > 60 was needed. These results directed essential improvements in the 5-ALA fluorescence visualisation toward enhanced resection rate. The conventional filtering, unadjusted to the 5-ALA colour space converted some background shades to colours resembling relevant fluorescence. This is one of the first studies to demonstrate that perceived colours, their contrasting and CR are of significance in the decision-making during HGG 5-ALA fluorescence-guided surgery. Irrespective of the shortcomings of conventional video filtering, further development of a tailored post-processed contrast stretching will allow to achieve safe and radical tumour resection.

Probing the functions of contextual modulation by adapting images rather than observers

Countless visual aftereffects have illustrated how visual sensitivity and perception can be biased by adaptation to the recent temporal context. This contextual modulation has been proposed to serve a variety of functions, but the actual benefits of adaptation remain uncertain. We describe an approach we have recently developed for exploring these benefits by adapting images instead of observers, to simulate how images should appear under theoretically optimal states of adaptation. This allows the long-term consequences of adaptation to be evaluated in ways that are difficult to probe by adapting observers, and provides a common framework for understanding how visual coding changes when the environment or the observer changes, or for evaluating how the effects of temporal context depend on different models of visual coding or the adaptation processes. The approach is illustrated for the specific case of adaptation to color, for which the initial neural coding and adaptation processes are relatively well understood, but can in principle be applied to examine the consequences of adaptation for any stimulus dimension. A simple calibration that adjusts each neuron's sensitivity according to the stimulus level it is exposed to is sufficient to normalize visual coding and generate a host of benefits, from increased efficiency to perceptual constancy to enhanced discrimination. This temporal normalization may also provide an important precursor for the effective operation of contextual mechanisms operating across space or feature dimensions. To the extent that the effects of adaptation can be predicted, images from new environments could be "pre-adapted" to match them to the observer, eliminating the need for observers to adapt.

Chromatic induction from surrounding stimuli under perceptual suppression

The appearance of colors can be affected by their spatiotemporal context. The shift in color appearance according to the surrounding colors is called color induction or chromatic induction; in particular, the shift in opponent color of the surround is called chromatic contrast. To investigate whether chromatic induction occurs even when the chromatic surround is imperceptible, we measured chromatic induction during interocular suppression. A multicolor or uniform color field was presented as the surround stimulus, and a colored continuous flash suppression (CFS) stimulus was presented to the dominant eye of each subject. The subjects were asked to report the appearance of the test field only when the stationary surround stimulus is invisible by interocular suppression with CFS. The resulting shifts in color appearance due to chromatic induction were significant even under the conditions of interocular suppression for all surround stimuli. The magnitude of chromatic induction differed with the surround conditions, and this difference was preserved regardless of the viewing conditions. The chromatic induction effect was reduced by CFS, in proportion to the magnitude of chromatic induction under natural (i.e., no-CFS) viewing conditions. According to an analysis with linear model fitting, we revealed the presence of at least two kinds of subprocesses for chromatic induction that reside at higher and lower levels than the site of interocular suppression. One mechanism yields different degrees of chromatic induction based on the complexity of the surround, which is unaffected by interocular suppression, while the other mechanism changes its output with interocular suppression acting as a gain control. Our results imply that the total chromatic induction effect is achieved via a linear summation of outputs from mechanisms that reside at different levels of visual processing.

Color and polarization vision in foraging Papilio

This paper gives an overview of behavioral studies on the color and polarization vision of the Japanese yellow swallowtail butterfly, Papilio xuthus. We focus on indoor experiments on foraging individuals. Butterflies trained to visit a disk of certain color correctly select that color among various other colors and/or shades of gray. Correct selection persists under colored illumination, but is systematically shifted by background colors, indicating color constancy and simultaneous color contrast. While their eyes contain six classes of spectral receptors, their wavelength discrimination performance indicates that their color vision is tetrachromatic. P. xuthus innately prefers brighter targets, but can be trained to select dimmer ones under certain conditions. Butterflies trained to a dark red stimulus select an orange disk presented on a bright gray background over one on dark gray. The former probably appears darker to them, indicating brightness contrast. P. xuthus has a strong innate preference for vertically polarized light, but the selection of polarized light changes depending on the intensity of simultaneously presented unpolarized light. Discrimination of polarization also depends on background intensity. Similarities between brightness and polarization vision suggest that P. xuthus perceive polarization angle as brightness, such that vertical polarization appears brighter than horizontal polarization.

Short-term memory affects color perception in context

Color-based object selection - for instance, looking for ripe tomatoes in the market - places demands on both perceptual and memory processes: it is necessary to form a stable perceptual estimate of surface color from a variable visual signal, as well as to retain multiple perceptual estimates in memory while comparing objects. Nevertheless, perceptual and memory processes in the color domain are generally studied in separate research programs with the assumption that they are independent. Here, we demonstrate a strong failure of independence between color perception and memory: the effect of context on color appearance is substantially weakened by a short retention interval between a reference and test stimulus. This somewhat counterintuitive result is consistent with Bayesian estimation: as the precision of the representation of the reference surface and its context decays in memory, prior information gains more weight, causing the retained percepts to be drawn toward prior information about surface and context color. This interaction implies that to fully understand information processing in real-world color tasks, perception and memory need to be considered jointly.

The effect of background and illumination on color identification of real, 3D objects

For the surface reflectance of an object to be a useful cue to object identity, judgments of its color should remain stable across changes in the object's environment. In 2D scenes, there is general consensus that color judgments are much more stable across illumination changes than background changes. Here we investigate whether these findings generalize to real 3D objects. Observers made color matches to cubes as we independently varied both the illumination impinging on the cube and the 3D background of the cube. As in 2D scenes, we found relatively high but imperfect stability of color judgments under an illuminant shift. In contrast to 2D scenes, we found that background had little effect on average color judgments. In addition, variability of color judgments was increased by an illuminant shift and decreased by embedding the cube within a background. Taken together, these results suggest that in real 3D scenes with ample cues to object segregation, the addition of a background may improve stability of color identification.

Lateral interactions in the outer retina

Lateral interactions in the outer retina, particularly negative feedback from horizontal cells to cones and direct feed-forward input from horizontal cells to bipolar cells, play a number of important roles in early visual processing, such as generating center-surround receptive fields that enhance spatial discrimination. These circuits may also contribute to post-receptoral light adaptation and the generation of color opponency. In this review, we examine the contributions of horizontal cell feedback and feed-forward pathways to early visual processing. We begin by reviewing the properties of bipolar cell receptive fields, especially with respect to modulation of the bipolar receptive field surround by the ambient light level and to the contribution of horizontal cells to the surround. We then review evidence for and against three proposed mechanisms for negative feedback from horizontal cells to cones: 1) GABA release by horizontal cells, 2) ephaptic modulation of the cone pedicle membrane potential generated by currents flowing through hemigap junctions in horizontal cell dendrites, and 3) modulation of cone calcium currents (I(Ca)) by changes in synaptic cleft proton levels. We also consider evidence for the presence of direct horizontal cell feed-forward input to bipolar cells and discuss a possible role for GABA at this synapse. We summarize proposed functions of horizontal cell feedback and feed-forward pathways. Finally, we examine the mechanisms and functions of two other forms of lateral interaction in the outer retina: negative feedback from horizontal cells to rods and positive feedback from horizontal cells to cones.

Color in the cortex: single- and double-opponent cells

This is a review of the research during the past 25years on cortical processing of color signals. At the beginning of the period the modular view of cortical processing predominated. However, at present an alternative view, that color and form are linked inextricably in visual cortical processing, is more persuasive than it seemed in 1985. Also, the role of the primary visual cortex, V1, in color processing now seems much larger than it did in 1985. The re-evaluation of the important role of V1 in color vision was caused in part by investigations of human V1 responses to color, measured with functional magnetic resonance imaging, fMRI, and in part by the results of numerous studies of single-unit neurophysiology in non-human primates. The neurophysiological results have highlighted the importance of double-opponent cells in V1. Another new concept is population coding of hue, saturation, and brightness in cortical neuronal population activity.

Simultaneous color contrast in the foraging swallowtail butterfly, Papilio xuthus

This study demonstrates that the color vision of foraging Japanese yellow swallowtail butterflies, Papilio xuthus, involves simultaneous color contrast. We trained newly emerged Papilio to select a disk of pale green among a set of differently colored disks presented on a black background. When the same set of disks was presented on blue background, the pale green-trained butterflies selected blue-green. The difference in spectra between pale green and blue green was similar to the spectrum of yellow for human vision, suggesting that blue induces yellow. Similarly, the pale green-trained Papilio selected a more bluish spring green on yellow background. We also trained Papilio with orange disks and tested on a green and violet background. The results showed that green induced violet and vice versa. Taken together, we concluded that simultaneous color contrast of Papilio is similar to the effect of complementary colors in human color vision.

The orientation selectivity of color-responsive neurons in macaque V1

Form has a strong influence on color perception. We investigated the neural basis of the form-color link in macaque primary visual cortex (V1) by studying orientation selectivity of single V1 cells for pure color patterns. Neurons that responded to color were classified, based on cone inputs and spatial selectivity, into chromatically single-opponent and double-opponent groups. Single-opponent cells responded well to color but weakly to luminance contrast; they were not orientation selective for color patterns. Most double-opponent cells were orientation selective to pure color stimuli as well as to achromatic patterns. We also found non-opponent cells that responded weakly or not at all to pure color; most were orientation selective for luminance patterns. Double-opponent and non-opponent cells' orientation selectivities were not contrast invariant; selectivity usually increased with contrast. Double-opponent cells were approximately equally orientation selective for luminance and equiluminant color stimuli when stimuli were matched in average cone contrast. V1 double-opponent cells could be the neural basis of the influence of form on color perception. The combined activities of single- and double-opponent cells in V1 are needed for the full repertoire of color perception.

Color constancy: phenomenal or projective?

Naive observers viewed a sequence of colored Mondrian patterns, simulated on a color monitor. Each pattern was presented twice in succession, first under one daylight illuminant with a correlated color temperature of either 16,000 or 4000 K and then under the other, to test for color constancy. The observers compared the central square of the pattern across illuminants, either rating it for sameness of material appearance or sameness of hue and saturation or judging an objective property-that is, whether its change of color originated from a change in material or only from a change in illumination. Average color constancy indices were high for material appearance ratings and binary judgments of origin and low for hue-saturation ratings. Individuals' performance varied, but judgments of material and of hue and saturation remained demarcated. Observers seem able to separate phenomenal percepts from their ontological projections of mental appearance onto physical phenomena; thus, even when a chromatic change alters perceived hue and saturation, observers can reliably infer the cause, the constancy of the underlying surface spectral reflectance.

The contribution of the outer retina to color constancy: a general model for color constancy synthesized from primate and fish data

Color constancy is one of the most impressive features of color vision systems. Although the phenomenon has been studied for decades, its underlying neuronal mechanism remains unresolved. Literature indicates an early, possibly retinal mechanism and a late, possibly cortical mechanism. The early mechanism seems to involve chromatic spatial integration and performs the critical calculations for color constancy. The late mechanism seems to make the color manifest. We briefly review the current evidence for each mechanism. We discuss in more detail a model for the early mechanism that is based on direct measurements of goldfish outer retinal processing and induces color constancy and color contrast. In this study we extrapolate this model to primate retina, illustrating that it is highly likely that a similar mechanism is also present in primates. The logical consequence of our experimental work in goldfish and our model is that the wiring of the cone/horizontal cell system sets the reference point for color vision (i.e., it sets the white point for that animal).

Spatial and temporal properties of cone signals in alert macaque primary visual cortex

Neurons in the lateral geniculate nucleus cannot perform the spatial color calculations necessary for color contrast and color constancy. Under neutral-adapting conditions, we mapped the cone inputs (L, M, and S) to 83 cone-opponent cells representing the central visual field of the next stage of visual processing, primary visual cortex (V1), to determine how the color signals are spatially transformed. Cone-opponent cells, constituting approximately 10% of V1 cells, formed two populations, red-green (L vs M; 66 of 83) and blue-yellow (S vs L+M; 17 of 83). Many cone-opponent cells (48 of 83) were double-opponent, with circular receptive-field centers and crescent-shaped surrounds (0.63 degree offset) that had opposite chromatic tuning to the centers and a time-to-peak 11 ms later than the centers. The remaining cone-opponent cells were either spatially opponent in only one cone system (20 of 83) or lacked spatial opponency (15 of 83). Cells lacking spatial opponency had smaller receptive fields (0.5-0.7 degrees) than spatial-opponent cell centers (approximately 1 degree). We found that red-green cells received S-cone input, which aligned with M input, and, unlike blue-yellow cells, red-green cells gave push-pull responses: receptive-field centers of red-ON cells were excited by both L increments (bright red) and M decrements (dark red) and were suppressed by both L decrements (dark green) and M increments (bright green). Excitatory responses to decrements were slightly larger than to increments, which may account for the lower detection and discrimination thresholds of decrements shown psychophysically. By virtue of their specialized receptive fields, the neurons described here spatially transform the cone signals and represent the first stage in the visual system at which spatially opponent color calculations are made.

Monge: The Verriest lecture, Lyon, July 2005

In 1789, when neither the physical basis of hue nor the retinal basis of color perception was established, the mathematician Gaspard Monte stated firmly that our color perceptions do not depend on the absolute value of the physical variable, but are influenced by the context and in particular by our estimate of the illuminant. He used this insight to explain color contrast effects and the Paradox of Monge (the desaturation of red objects seen through a red filter). He proposed that we can estimate the chromaticity of the illuminant in any scene because all surfaces reflect to us varying mixtures of (i) the body color and (ii) a specular component that represents the illuminant. He also realized that white objects have a special property: Provided that they are illuminated by a single illuminant, such objects exhibit no variation in chromaticity across their surface. Thus at least one of the unique hues exists as an external reference on which observers can agree. It is suggested that other unique hues may also have a basis in the external world.

Color constancy in natural scenes explained by global image statistics

To what extent do observers' judgments of surface color with natural scenes depend on global image statistics? To address this question, a psychophysical experiment was performed in which images of natural scenes under two successive daylights were presented on a computer-controlled high-resolution color monitor. Observers reported whether there was a change in reflectance of a test surface in the scene. The scenes were obtained with a hyperspectral imaging system and included variously trees, shrubs, grasses, ferns, flowers, rocks, and buildings. Discrimination performance, quantified on a scale of 0 to 1 with a color-constancy index, varied from 0.69 to 0.97 over 21 scenes and two illuminant changes, from a correlated color temperature of 25,000 K to 6700 K and from 4000 K to 6700 K. The best account of these effects was provided by receptor-based rather than colorimetric properties of the images. Thus, in a linear regression, 43% of the variance in constancy index was explained by the log of the mean relative deviation in spatial cone-excitation ratios evaluated globally across the two images of a scene. A further 20% was explained by including the mean chroma of the first image and its difference from that of the second image and a further 7% by the mean difference in hue. Together, all four global color properties accounted for 70% of the variance and provided a good fit to the effects of scene and of illuminant change on color constancy, and, additionally, of changing test-surface position. By contrast, a spatial-frequency analysis of the images showed that the gradient of the luminance amplitude spectrum accounted for only 5% of the variance.

Sensory, computational and cognitive components of human colour constancy

When the illumination on a scene changes, so do the visual signals elicited by that scene. In spite of these changes, the objects within a scene tend to remain constant in their apparent colour. We start this review by discussing the psychophysical procedures that have been used to quantify colour constancy. The transformation imposed on the visual signals by a change in illumination dictates what the visual system must 'undo' to achieve constancy. The problem is mathematically underdetermined, and can be solved only by exploiting regularities of the visual world. The last decade has seen a substantial increase in our knowledge of such regularities as technical advances have made it possible to make empirical measurements of large numbers of environmental scenes and illuminants. This review provides a taxonomy of models of human colour constancy based first on the assumptions they make about how the inverse transformation might be simplified, and second, on how the parameters of the inverse transformation might be set by elements of a complex scene. Candidate algorithms for human colour constancy are represented graphically and pictorially, and the availability and utility of an accurate estimate of the illuminant is discussed. Throughout this review, we consider both the information that is, in principle, available and empirical assessments of what information the visual system actually uses. In the final section we discuss where in our visual systems these computations might be implemented.

Information limits on neural identification of colored surfaces in natural scenes

If surfaces in a scene are to be distinguished by their color, their neural representation at some level should ideally vary little with the color of the illumination. Four possible neural codes were considered: von-Kries-scaled cone responses from single points in a scene, spatial ratios of cone responses produced by light reflected from pairs of points, and these quantities obtained with sharpened (opponent-cone) responses. The effectiveness of these codes in identifying surfaces was quantified by information-theoretic measures. Data were drawn from a sample of 25 rural and urban scenes imaged with a hyperspectral camera, which provided estimates of surface reflectance at 10-nm intervals at each of 1344 × 1024 pixels for each scene. In computer simulations, scenes were illuminated separately by daylights of correlated color temperatures 4000 K, 6500 K, and 25,000 K. Points were sampled randomly in each scene and identified according to each of the codes. It was found that the maximum information preserved under illuminant changes varied with the code, but for a particular code it was remarkably stable across the different scenes. The standard deviation over the 25 scenes was, on average, approximately 1 bit, suggesting that the neural coding of surface color can be optimized independent of location for any particular range of illuminants.

Colour constancy under simultaneous changes in surface position and illuminant

Two kinds of constancy underlie the everyday perception of surface colour: constancy under changes in illuminant and constancy under changes in surface position. Classically, these two constancies seem to place conflicting demands on the visual system: to both take into account the region surrounding a surface and also discount it. It is shown here, however, that the ability of observers to make surface-colour matches across simultaneous changes in test-surface position and illuminant in computer-generated 'Mondrian' patterns is almost as good as across changes in illuminant alone. Performance was no poorer when the surfaces surrounding the test surface were permuted, or when information from a potential comparison surface, the one with the highest luminance, was suppressed. Computer simulations of cone-photoreceptor activity showed that a reliable cue for making surface-colour matches in all experimental conditions was provided by the ratios of cone excitations between the test surfaces and a spatial average over the whole pattern.