Allen L. Nagy

Large-field substitution Rayleigh matches of dichromats

A large-field substitution procedure for making Rayleigh matches was used to study the color matches of eight red-green dichromats and two extreme anomalous trichromats. Matches were made under three experimental conditions: (1) During the cone plateau period after a bleach; (2) after the rods had recovered from the bleach; and (3) on a blue background. Under conditions (1) and (3) rods were desensitized and appear to be unable to contribute to the color match. Seven of eight dichromats were still able to make unique matches with the rods desensitized. The matches of six of these seven dichromats were consistent with predicted matches for simple protanomalous and simple deuteranomalous trichromats. Matches made under condition (2) were typically more nearly consistent with matches predicted for the rods and the remaining normal cone mechanism, but there were also some individual differences in these matches. The matches of the two extreme anomalous trichromats were similar to the matches of the dichromats.

Chromaticity and luminance as coding dimensions in visual search

Visual search times were measured as a function of chromaticity and luminance differences between a target and distractor stimuli. Results showed that mean search time increased linearly with the number of distractors if the luminance difference between target and distractors was small but was roughly constant if the luminance difference was large. Similar results were previously found for chromaticity differences. With the number of distractor stimuli held constant, the mean search time decreased with increases in the difference between target and distractors, up to some critical difference. Further increases in target-distractor difference had little effect. Results were similar for targets defined by luminance and chromaticity. There was some advantage to combining luminance differences with chromaticity differences when the target was dimmer than the distractors. Generally there was no advantage for combining a chromaticity difference with a luminance difference when the target was brighter than the distractors.

Four cone pigments in women heterozygous for color deficiency

We find that additivity of trichromatic color matches holds, even in parafoveal vision, for most observers if rods are prevented from seeing the stimuli. Some heterozygous women are exceptions. The failure of the additivity law for these women implies that their eyes contain more than three types of cone with different spectral sensitivities.

Large-field color naming of dichromats with rods bleached

A color-naming method was used to examine the large-field red/green discriminations of dichromats screened with standard tests. The stimulus was a 12 degrees annular field with the central 4 degrees removed, flashed for 300 ms. Four wavelengths were equated in brightness for each observer at two retinal illuminance levels, approximately 10 and 100 trolands. The stimuli were then presented in random order and the observer was asked to name each, using one of four color terms. The entire experiment was done with the observers dark-adapted and also with the rods bleached. For all four deuteranopes and two of four protanopes, color names were very significantly related to both illuminance and wavelength in both adaptation conditions. The relationship between name and wavelength was similar to that of a normal trichromat, but the performance of a dichromat was very poor by comparison. Performance was generally somewhat better in the dark than with the rods bleached. However, the result in the bleached condition is consistent with recent evidence that at least some observers who are classified as dichromats with standard small-field screening procedures actually have a weak residual third cone mechanism.

Homogeneity of large-field color matches in congenital red–green color deficients

Rayleigh matches obtained from red-green color deficients with conventional methods show large individual differences within diagnostic categories. Similar matches obtained from the same observers with a large-field substitution method show much less variability and suggest that the differences observed among simple anomals, extreme anomals, and dichromats with conventional methods are probably not solely due to the visual pigments contained in the cones. A theory that attributes these differences to the relative number of abnormal cones present in the observers retina is described.

Similarity of Normalized Discrimination Ellipses in the Constant-Luminance Chromaticity Plane

Discrimination steps were measured for three subjects, along oblique axes passing through nine points in a 25 td constant-luminance chromaticity plane. When plotted in a normalized cone-excitation chromaticity diagram, the best-fitting discrimination ellipses for a given subject have approximately the same shape and orientation regardless of the reference chromaticity. Their orientation is consistent with the hypothesis that excitation of B-cones affects the red—green opponent balance, otherwise determined by R- and G-cone excitations, in a manner independent of initial cone-excitation levels. The CIELAB formula predicts an orientation for normalized ellipses in agreement with the data, but it also predicts systematic changes in the ratio of minor to major axes which are not observed experimentally.

Equilibrium hue judgements of dichromats

It is generally held that protanopes and deuteranopes see only regions of blues and yellows in the visible spectrum, with an achromatic point, called the neutral point, separating these regions. Considerations of a zone model of color vision for the dichromatic observer led us to predict that a reduced form of red/green discrimination would allow equilibrium blue judgements to be made by protanopes. We show that protanopes can make equilibrium blue determinations with as much reliability as they make neutral point settings. Our results indicate that protanopes but not deuteranopes are able to rely on a reduced form of red/green discrimination in the short wavelength part of the spectrum. Protanopes describe wavelengths longer than the neutral point as yellow. Between the neutral point and equilibrium blue, different wavelengths are described as having varying aspects of blue and green; and short of equilibrium blue they appear reddish blue. For dueteranopes, the spectrum longer than the neutral point appears yellow, and short of it, blue. The results of our experiments showing that the protanopic equilibrium blue is invariant with intensity variations, as it is in the trichromat, add support to the idea of a reduced form of red/green discrimination for protanopes. Our results also allow the evaluation of various models of protanopia.

Cone mechanisms Underlying the color discrimination of deutan color deficients

An alternation method of color matching was used to obtain a series of extended Rayleigh matches from several deutan color deficients with varying degrees of color discrimination. With large stimulus fields there were differences in the matches made by observers with good color discrimination and the matches made by observers with poor color discrimination. The matches made by observers with poor discrimination could not be modeled with normal cone action spectra. When the field size was reduced the matches of all observers were quite similar and could be modeled with two cone action spectra that were normal in shape and separated by approximately 5 nm. Results suggest that individual differences in ability to discriminate color among deutan observers are not solely related to differences in the cone action spectra.

Color discrimination and neural coding in color deficients

Rayleigh color match ranges obtained from color deficient observers varied considerably as a function of spatial and temporal parameters of stimulus presentation. The results suggest that color discrimination losses in color deficients result from abnormalities in the spatial and temporal properties of neural coding in addition to cone photopigment abnormalities.