Unilateral colour vision defects and the dimensions of dichromat experience

Temporal and spatial analysis of event-related potentials in response to color saliency differences among various color vision types

Introduction

Human color vision exhibits significant diversity that cannot be fully explained by categorical classifications. Understanding how individuals with different color vision phenotypes perceive, recognize, and react to the same physical stimuli provides valuable insights into sensory characteristics. This study aimed to identify behavioral and neural differences between different color visions, primarily classified as typical trichromats and anomalous trichromats, in response to two chromatic stimuli, blue-green and red, during an attention-demanding oddball task.

Methods

We analyzed the P3 component of event-related potentials (ERPs), associated with attention, and conducted a broad spatiotemporal exploration of neural differences. Behavioral responses were also analyzed to complement neural data. Participants included typical trichromats (n = 13) and anomalous trichromats (n = 5), and the chromatic stimuli were presented in an oddball paradigm.

Results

Typical trichromats exhibited faster potentiation from the occipital to parietal regions in response to the more salient red stimulus, particularly in the area overlapping with the P3 component. In contrast, anomalous trichromats revealed faster potentiation to the expected more salient blue-green stimulus in the occipital to parietal regions, with no other significant neural differences between stimuli. Comparisons between the color vision types showed no significant overall neural differences.

Discussion

The large variability in red-green sensitivity among anomalous trichromats, along with neural variability not fully explained by this sensitivity, likely contributed to the absence of clear neural distinctions based on color saliency. While reaction times were influenced by red-green sensitivity, neural signals showed ambiguity regarding saliency differences. These findings suggest that factors beyond red-green sensitivity influenced neural activity related to color perception and cognition in minority color vision phenotypes. Further research with larger sample sizes is needed to more comprehensively explore these neural dynamics and their broader implications.

How the orientation of the color gamut of natural scenes influences color discrimination in red-green dichromacy

In natural scenes, visual discrimination of colored surfaces by individuals with X-linked dichromacy is known to be only a little poorer than in normal trichromacy. This surprising result may be related to the properties of the colors of these scenes, like the shape and orientation of the color gamut, uneven frequency, and a considerable variation in lightness. It is unclear, however, how much each of these factors contributes to the small impairment in discrimination, in particular, what is the contribution of the orientation of the gamut. We measured the discrimination of colors from natural scenes by six normal trichromats and six dichromats. Colors were drawn either from the original color gamut of the scenes or from gamut-rotated versions of the scenes. Pairs of colors were randomly drawn from hyperspectral images of one rural and one urban environment and presented on a screen. As expected, dichromats were only a little poorer than normal trichromats at discrimination but the disadvantage varied systematically with the orientation of the color gamut by a factor of about three with a minimum around a yellow-green axis. Dichromats also took longer to respond, and the response times were modulated with the orientation of the color gamut in a similar way as the loss in discrimination. For the scenes tested here, these results imply an important impact of the orientation of the gamut on discrimination. They also indicate that the predominantly yellow-blue orientation of the gamut of natural scene might not be optimal for discrimination in dichromacy.

Do You See What I See? Diversity in Human Color Perception

In our tendency to discuss the objective properties of the external world, we may fail to notice that our subjective perceptions of those properties differ between individuals. Variability at all levels of the color vision system creates diversity in color perception, from discrimination to color matching, appearance, and subjective experience, such that each of us lives in a unique perceptual world. In this review, I discuss what is known about individual differences in color perception and its determinants, particularly considering genetically mediated variability in cone photopigments and the paradoxical effects of visual environments in both contributing to and counteracting individual differences. I make the case that, as well as being of interest in their own right and crucial for a complete account of color vision, individual differences can be used as a methodological tool in color science for the insights that they offer about the underlying mechanisms of perception. Expected final online publication date for the Annual Review of Vision Science, Volume 8 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

On the dichromatic object-colour palette

The object- and light-colour palettes prove to be different for both trichromats and dichromats. This explains why there is no consensus on what colours dichromats see, since, until now, studies of dichromatic vision have mainly focused on the light-colour palette. By contrast, this study concentrates on the dichromatic object-colour palette, assuming that it is as much determined by optimal reflectances as the trichromatic palette. In this case, the dichromatic object-colour palette is simply part of the trichromatic object-colour palette. This is a consequence of the fact that the dichromatic optimal reflectances bring about identical perceptions in both dichromats and trichromats. Since the optimal reflectances cannot be physically implemented, a set of Munsell chips was selected that was close enough to the dichromatic optimal reflectances. By examining these chips, trichromats can get an idea of what the dichromatic object-colour palette looks like. These chips clearly contain red, green and blue component hues. As to green, it was tinged with such a considerable amount of white that it was hard to judge its presence even for trichromatic observers. By hue scaling, the amount of component hues (Y, B, R, G, W and Bk) that trichromats see in these chips was evaluated. Although the amount of green was found to be low, its presence for some chips was statistically significant. Thus, dichromats should see all six component hues. Also, the opponency of black and white was confirmed, which contradicts the generally accepted view that grey is a mixture of black and white.

Discrimination of hue angle and discrimination of colorimetric purity assessed with a common metric

It has been suggested that thresholds for discriminating colorimetric purity are systematically higher than those for discriminating hue angle, a difference captured in Judd's phrase "the super-importance of hue." However, to compare the two types of discrimination, the measured thresholds must be expressed in the same units. An attractive test is offered by measurements along the horizontal lines in the chromaticity diagram of MacLeod and Boynton [ J. Opt. Soc. Am.69, 1183 (1979)JOSAAH0030-394110.1364/JOSA.69.001183], i.e., a chromaticity diagram. A horizontal line that extends radially from the white point represents a variation in colorimetric purity alone (and subjectively a variation that is primarily in saturation). In contrast, a horizontal line that runs along the $x$x axis of the diagram, close to the long-wave spectrum locus, corresponds predominantly to variation in hue angle. Yet, in both cases, only the ratio of the excitations of the long- and middle-wave cones is being modulated, and so the thresholds can be expressed in a common metric. Measuring forced-choice thresholds for 180 ms foveal targets presented on a steady field metameric to Illuminant D65, we do not find general support for Judd's working rule that thresholds for purity are systematically twice those for saturation. Thresholds for colorimetric purity were only a little higher than those for hue angle, and the advantage for hue was seen in only part of the ranges that were tested. However, in the upper-left quadrant of the MacLeod-Boynton diagram, where the excitation of short-wave cones is high and where both hue angle and colorimetric purity vary along any given horizontal line, thresholds were indeed sometimes half those observed for discrimination of purity alone.

The colors of natural scenes benefit dichromats

Dichromacy impairs color vision and impoverishes the discrimination of surface colors in natural scenes. Computational estimates based on hyperspectral imaging data from natural scenes suggest that dichromats can discriminate only about 10% of the colors discriminated by normal trichromats. These estimates, however, assume that the colors are equally frequent. Yet, pairs of colors confused by dichromats may be rare and thus have small impact on overall perceived chromatic diversity. This study estimated, empirically, how much dichromats are disadvantaged in discriminating surface colors drawn from natural scenes. The stimulus for the experiment was a scene made of real three-dimensional objects painted with matte white paint and illuminated by a spectrally tunable light source. In each trial the observers saw the scene illuminated by two spectra in two successive time intervals and had to indicate whether the colors perceived in the objects in the two intervals were the same or different. The spectra were drawn randomly from hyperspectral data of natural scenes and therefore represented natural spectral statistics. Four normal trichromats and four dichromats carried out the experiment. It was found that the number of pairs that could be discriminated by dichromats was almost 70% of those discriminated by normal trichromats, a proportion much higher than anticipated from estimates of discernible colors. Moreover, data from model simulations show that normal trichromats and dichromats use lightness differences for discrimination in about 40% and 50% of the discriminable pairs, respectively. Together these results suggest that the color distributions of natural scenes benefit the color vision of dichromats.

Orthogonal relations and color constancy in dichromatic colorblindness

This paper employs uniform color space to analyze relations in dichromacy (protanopia, deuteranopia, tritanopia). Fifty percent or less of dichromats represent the classical reduction form of trichromacy, where one of three cones is inoperative but normal trichromatic color mixture such as complementary colors (pairs that mix white) are accepted by the dichromat, whose data can thus be plotted to CIE chromaticity spaces. The remaining dichromats comprise many and varied more-complex gene arrays from mutations, recombinations, etc. Though perhaps a minority, the three reductionist types provide a simple standard, in genotype and phenotype, to which the more complex remainder may be compared. Here, previously published data on dichromacy are plotted and analyzed in CIELUV uniform color space to find spatial relations in terms of color appearance space (e.g., hue angle). Traditional residual (seen) hues for protanopia and deuteranopia (both red-green colorblindness) are yellow and blue, but analysis indicates the protanopic residual hues are more greenish yellow and reddish blue than in tradition. Results for three illuminants (D65, D50, B) imply four principles in the spatial structure of dichromacy: (1) complementarity of confusion hue pairs and of residual hue pairs; (2) orthogonality of confusion locus and residual hues locus at their intersection with the white point, in each dichromatic type; (3) orthogonality of protanopic and tritanopic confusion loci; and (4) inverse relations between protanopic and tritanopic systems generally, such that one's confusion hues are the other's residual hues. Two of the three dichromatic systems do not represent components of normal trichromatic vision as sometimes thought but are quite different. Wavelength shifts between illuminants demonstrate chromatic adaptation correlates exactly with that in trichromatic vision. In theory these results clarify relations in and between types of dichromacy. They also apply in Munsell and CIELAB color spaces but inexactly to the degree they employ inexact complementarity.