Tests of a functional account of the Abney effect

Appearance of special colors in deuteranomalous trichromacy

Deuteranomalous color matching behavior is different from normal because the middle-wavelength sensitive cones contain an abnormal L' pigment instead of the M pigment of the normal observer. However, there is growing evidence that deuteranomalous color experience is not very different from that of normal trichromats. Here, normal and deuteranomalous observers chose monochromatic unique yellow lights. They also chose broadband lights, displayed on a computer monitor, that corresponded to eight special colors: the Hering unique hues (red, yellow, green, blue), and binary colors perceptually midway between them (orange, lime, cyan, purple). Deuteranomalous monochromatic unique yellow was shifted towards red, but all the broadband special color selections were physically similar for normal and deuteranomalous observers. Deuteranomalous special colors, including monochromatic unique yellow, were similar to those of normal observers when expressed in a color-opponent chromaticity diagram based on their own visual pigments, but only if (1) color-opponent responses were normalized to white, and (2) the deuteranomalous diagram was expanded along the r - g dimension to compensate for the reduced difference between deuteranomalous L- and L'-cone photopigments. Particularly, deuteranomalous observers did not choose binary colors with extra r - g impact to overcome their insensitivity along the r - g dimension. This result can only be compatible with the known abnormality of the deuteranomalous L' photopigment if deuteranomalous observers adjust their perceptual representation of colors to compensate for their color vision deficiency.

The Verriest Lecture: Adventures in blue and yellow

Conventional models of color vision assume that blue and yellow (along with red and green) are the fundamental building blocks of color appearance, yet how these hues are represented in the brain and whether and why they might be special are questions that remain shrouded in mystery. Many studies have explored the visual encoding of color categories, from the statistics of the environment to neural processing to perceptual experience. Blue and yellow are tied to salient features of the natural color world, and these features have likely shaped several important aspects of color vision. However, it remains less certain that these dimensions are encoded as primary or "unique" in the visual representation of color. There are also striking differences between blue and yellow percepts that may reflect high-level inferences about the world, specifically about the colors of light and surfaces. Moreover, while the stimuli labeled as blue or yellow or other basic categories show a remarkable degree of constancy within the observer, they all vary independently of one another across observers. This pattern of variation again suggests that blue and yellow and red and green are not a primary or unitary dimension of color appearance, and instead suggests a representation in which different hues reflect qualitatively different categories rather than quantitative differences within an underlying low-dimensional "color space."

Individual differences and their implications for color perception

Individual differences are a conspicuous feature of color vision and arise from many sources, in both the observer and the world. These differences have important practical implications for comparing and correcting perception and performance, and important theoretical implications for understanding the design principles underlying color coding. Color percepts within and between individuals often vary less than the variations in spectral sensitivity might predict. This stability is achieved by a variety of processes that compensate perception for the sensitivity limits of the eye and brain. Yet judgments of color between individuals can also vary widely, and in ways that are not readily explained by differences in sensitivity or the environment. These differences are uncorrelated across different color categories, and could reflect how these categories are learned or represented.

Gaussian-Based Hue Descriptors

A robust and accurate hue descriptor that is useful in modeling human color perception and for computer vision applications is explored. The hue descriptor is based on the peak wavelength of a Gaussian-like function (called a wraparound Gaussian) and is shown to correlate as well as CIECAM02 hue to the hue designators of papers from the Munsell and Natural Color System color atlases and to the hue names found in Moroney's Color Thesaurus. The new hue descriptor is also shown to be significantly more stable under a variety of illuminants than CIECAM02. The use of wraparound Gaussians as a hue model is similar in spirit to the use of subtractive Gaussians proposed by Mizokami et al., but overcomes many of their limitations.

All effects of psychophysical variables on color attributes: a classification system

This paper reports the research and structuring of a classification system for the effects of psychophysical variables on the color attributes. A basic role of color science is to psychophysically specify color appearance. An early stage is to specify the effects of the psychophysical variables (as singles, pairs, etc) on the color attributes (as singles, pairs, etc), for example to model color appearance. Current data on effects are often scarce or conflicting. Few effects are well understood, and the practice of naming effects after their discoverer(s) is inadequate and can be confusing. The number and types of possible effects have never been systematically analyzed and categorized. We propose a simple and rigorous system of classification including nomenclature. The total range of effects is computed from the possible combinations of three psychophysical variables (luminance, dominant wavelength, purity) and six color attributes (lightness, brightness, hue, chroma, colorfulness, saturation) in all modes of appearance. Omitting those effects that are normally impossible to perceive at any one time (such as four- or five-dimensional colors), the total number perceivable is 161 types of effects for all modes of appearance. The type of effect is named after the psychophysical stimulus (or stimuli) and the relevant color attribute(s), e.g., Luminance-on-hue effect (traditionally known as Bezold-Brucke effect). Each type of effect may include slightly different effects with infinite variations depending on experimental parameters.

Neurobiological hypothesis of color appearance and hue perception

De Valois and De Valois [Vis. Res.33, 1053 (1993)] showed that to explain hue appearance, S-cone signals have to be combined with M versus L opponent signals in two different ways to produce red-green and yellow-blue axes, respectively. Recently, it has been shown that color appearance is normal for individuals with genetic mutations that block S-cone input to blue-ON ganglion cells. This is inconsistent with the De Valois hypothesis in which S-opponent konio-geniculate signals are combined with L-M signals at a third processing stage in cortex. Instead, here we show that color appearance, including individual differences never explained before, are predicted by a model in which S-cone signals are combined with L versus M signals in the outer retina.

Filling in, filling out, or filtering out: processes stabilizing color appearance near the center of gaze

Spectral sensitivity varies markedly across the center of gaze, in part because of the rapid decline in the density of macular pigment outside the fovea. Yet despite these retinal inhomogeneities, the color appearance of large uniform fields remains very uniform. We explored some of the processes contributing to these stable color percepts by measuring the effects of field size and eccentricity on saturated purples, whose spectra should show the largest biases with macular pigment screening. Small purple fields at 0° and 8° eccentricities differ in appearance but by much less than predicted by the macular screening or by compensation for the average effects of this screening at the two loci. This shows that the compensation is already nearly complete because of local adjustments that filter out the sensitivity variation and confirms that this filtering includes adjustments beyond average gain changes in the cones. In large fields, the appearance is dominated by the local peripheral color. This bias persists when the field edge is fixated or when abrupt edges are removed in Gaussian spots, suggesting that the spreading is not strongly dependent on luminance edges.