Abstract
Color vision assessment can be done using pseudoisochromatic stimuli, which has a luminance noise to eliminate brightness differences between the target and background of the stimulus. It is not clear the influence of the luminance noise on color discrimination. We investigated the effect of change in the luminance noise limits on color discrimination. Eighteen trichromats and ten congenital dichromats (eight protans, two deutans) had their color vision evaluated by the Cambridge Colour Test, and were genetically tested for diagnostic confirmation. The stimuli were composed of a mosaic of circles in a 5° circular field. A subset of the circles differed in chromaticity from the remaining field, forming a letter C. Color discrimination was estimated in stimulus conditions differing in luminance noise range: (i) 6–20 cd/m2; (ii) 8–18 cd/m2; (iii) 10–16 cd/m2; and (iv) 12–14 cd/m2. Six equidistant luminance values were used within the luminance noise limits with the mean stimulus luminance maintained constant under all conditions. A four-alternative, forced-choice method was applied to feed a staircase procedure to estimate color discrimination thresholds along eight chromatic axes. An ellipse model was adjusted to the eight color discrimination thresholds. The parameters of performance were threshold vector lengths and the ellipse area. Results were compared using the Kruskal-Wallis test with a significance level of 5%. The linear function model was applied to analyze the dependence of the discrimination parameters on the noise luminance limits. The first derivative of linear function was used as an indicator of the rate of change in color discrimination as a function of luminance noise changes. The rate of change of the ellipse area as a function of the luminance range in dichromats was higher than in trichromats (p < 0.05). Significant difference was also found for individual thresholds in half of the axes we tested. Luminance noise had a greater effect on color discrimination ability of dichromats than the trichromats, especially when the chromaticities were close to their protan and deutan color confusion lines.
Highlights
Several psychophysical protocols for assessment of color discrimination have been developed using pseudoisochromatic design, such as the Ishihara Test, the AO HRR pseudoisochromatic plates and the Cambridge Colour Test (Regan et al, 1994)
Linhares et al (2016) investigated the influence of the dynamic luminance contrast noise on the color discrimination thresholds of normal trichromats and anomalous trichromats. They observed that the presence of the dynamic noise had no effect on the color discrimination thresholds of normal trichromats but improved the color discrimination thresholds of the anomalous trichromats
For both trichromats and dichromats, there was statistically significant difference between the areas of the ellipses estimated by the luminance noise stimulus condition (H(7) = 37.96, p < 0.001)
Summary
Several psychophysical protocols for assessment of color discrimination have been developed using pseudoisochromatic design, such as the Ishihara Test, the AO HRR pseudoisochromatic plates and the Cambridge Colour Test (Regan et al, 1994) This approach allows elimination contour and luminance cues from the presentation of chromatic stimuli. They observed that the presence of the dynamic noise had no effect on the color discrimination thresholds of normal trichromats but improved the color discrimination thresholds of the anomalous trichromats These findings indicate the need to understand how several parameters of the pseudoisochromatic stimulus could influence color vision and to guide future protocol standardization that would permit comparisons between laboratories
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