Reduced Discrimination in the Tritanopic Confusion Line for Congenital Color Deficiency Adults

Abstract

In congenital color blindness the red-green discrimination is impaired resulting in an increased confusion between those colors with yellow. Our post-receptoral physiological mechanisms are organized in two pathways for color perception, a red-green (protanopic and deuteranopic) and a blue-yellow (tritanopic). We argue that the discrimination losses in the yellow area in congenital color vision deficiency subjects could generate a subtle loss of discriminability in the tritanopic channel considering discrepancies with yellow perception. We measured color discrimination thresholds for blue and yellow of tritanopic channel in congenital color deficiency subjects. Chromaticity thresholds were measured around a white background (0.1977 u', 0.4689 v' in the CIE 1976) consisting of a blue-white and white-yellow thresholds in a tritanopic color confusion line of 21 congenital colorblindness subjects (mean age = 27.7; SD = 5.6 years; 14 deuteranomalous and 7 protanomalous) and of 82 (mean age = 25.1; SD = 3.7 years) normal color vision subjects. Significant increase in the whole tritanopic axis was found for both deuteranomalous and protanomalous subjects compared to controls for the blue-white (F 2,100 = 18.80; p < 0.0001) and white-yellow (F 2,100 = 22.10; p < 0.0001) thresholds. A Principal Component Analysis (PCA) found a weighting toward to the yellow thresholds induced by deuteranomalous subjects. In conclusion, the discrimination in the tritanopic color confusion axis is significantly reduced in congenital color vision deficiency compared to normal subjects. Since yellow discrimination was impaired the balance of the blue-yellow channels is impaired justifying the increased thresholds found for blue-white discrimination. The weighting toward the yellow region of the color space with the deuteranomalous contributing to that perceptual distortion is discussed in terms of physiological mechanisms.

Keywords: anomalus trichromacy; chromatic thresholds; chromaticity discrimination; color vision; tritanopic color confusion line; visual psychophysics.

FIGURE 1

(A) Graphic representation adapted from the CIE 1976 L^∗u^∗v^∗ Color Space and u′, v′ Uniform Chromaticity Scale Diagram (www.cie.co.at/ index.php/Publications/Standards). This spatial representation of colors is a non-linear transformation from the CIE 1931 L^∗x^∗y^∗ color space to attempt a perceptual uniformity. The black lines represent the three – protanopic (L-cone), deuteranopic (M-cone), and tritanopic (S-cone) – color confusion axis in which colors of both sides along each of those lines could be mixed to be confused with the white center. (B) Schematic view of the S, M, and L cones contribution to the chromatic information of the koniocellular pathway, based on the physiological recordings of cone inputs to the lateral geniculate nucleus (LGN) of Macaca fascicularis performed by Tailby et al. (2008). Cone input configuration of the S+ cells in which the S-cone signal is opposed to the sum of the L- and M-cones. (C) Cone input of the S- cells in which the S-cone signal is summed with the M- cone and opposed to the L- cone.

FIGURE 2

(A) The target – a Landolt “C” – that differed in chromaticity from the single neutral background [coordinates 0.1977, 0.4689 of the (CIE) u′v′ 1976 color space]. The target and background composed of small patches of varying sizes and luminance levels randomly distributed in the display. (B) The CIE 1976 u′ v′ Chromaticity Diagram used to draw the MacAdam ellipses or the non-discriminable area. The letters B, G, and R represent the spatial position of the blue, green, and red primaries, respectivelly. The bright central area (coordenates 0.1977 u′, 0.4689 v′) was the stimulus background and the “Xs” represents the thresholds measured in the each of the eight color confusion vectors. The ellipses drawn is regarding to one of our deuteranomalous subjects based on the method of the least squares. (C) Magnification of the Chromaticity Diagram in which we present a sample of thresholds measured in the tritanopic axes to the yellow (upper cross) and blue (lower cross) directions. The central square represents the backgroud chromaticity coordinates.

FIGURE 3

Box Plot showing the median and interqurtile ranges with brackets showing the 95% of blue–yellow discrimination diameter. Normal color vision subjects had significant better discrimination than deuteranomalous or protanomalous subjects. Control’s are statistically significant better than others (^∗p > 0.001).

FIGURE 4

Differences between thresholds obtained at 90 and 270° axes in order to analyses the yellow–blue symmetry. Normal color vision subjects (△) had thresholds spread symmetrically between the orthogonal axes with a mean blue–yellow index around zero. Both protanomalous and deuteranomalous subjects (▲) had a asymmetrical shift toward the yellow direction (positive values) showing worse discrimination to that direction.

FIGURE 5

Graphical analysis of the two factors computed by the PCA. Factor 1 represent the subject groups (controls and congenital color deficiency) and factor 2 the thresholds to the yellow and blue regions of the tritanopic axes. It is interesting that the deuteranomalous subjects (labeled 3) had longer distances from the central than protanomalous (labeled 2) and controls (labeled 1), which means that they were the more affected group (more to the left side) and they showed the worse yellow color discrimination (more to upper side).