Color-normal Observers Research Articles

Sunglasses, Traffic Signals, and Color Vision Deficiencies

To determine (a) the effect of different sunglass tint colorations on traffic signal detection and recognition for color normal and color deficient observers, and (b) the adequacy of coloration requirements in current sunglass standards. Twenty color-normals and 49 color-deficient males performed a tracking task while wearing sunglasses of different colorations (clear, gray, green, yellow-green, yellow-brown, red-brown). At random intervals, simulated traffic light signals were presented against a white background at 5 degrees to the right or left and observers were instructed to identify signal color (red/yellow/green) by pressing a response button as quickly as possible; response times and response errors were recorded. Signal color and sunglass tint had significant effects on response times and error rates (p < 0.05), with significant between-color group differences and interaction effects. Response times for color deficient people were considerably slower than color normals for both red and yellow signals for all sunglass tints, but for green signals they were only noticeably slower with the green and yellow-green lenses. For most of the color deficient groups, there were recognition errors for yellow signals combined with the yellow-green and green tints. In addition, deuteranopes had problems for red signals combined with red-brown and yellow-brown tints, and protanopes had problems for green signals combined with the green tint and for red signals combined with the red-brown tint. Many sunglass tints currently permitted for drivers and riders cause a measurable decrement in the ability of color deficient observers to detect and recognize traffic signals. In general, combinations of signals and sunglasses of similar colors are of particular concern. This is prima facie evidence of a risk in the use of these tints for driving and cautions against the relaxation of coloration limits in sunglasses beyond those represented in the study.

Impulse response of an S-cone pathway in the aging visual system.

Age-related changes in the temporal properties of an S-cone pathway were characterized by the psychophysical impulse-response function (IRF). Participants included 49 color-normal observers ranging in age from 16.8 to 86.3 years. A double-pulse method was used to measure the IRF with S-cone modulation at constant luminance. Stimuli were presented as a Gaussian patch (+/-1SD = 2.3 degrees ) in one of four quadrants around a central fixation cross on a CRT screen. The test stimulus was modulated from the equal-energy white of the background toward the short-wave spectrum locus. Each of the two pulses (6.67 ms) was separated by an interstimulus interval (ISI) from 20 to 720 ms. Chromatic detection thresholds were determined by a four-alternative forced-choice method with staircases for each ISI in one session. IRFs were calculated from the threshold data using a model with four parameters of an exponentially damped sine wave. S-cone IRFs have only an excitatory phase and a much longer time course compared with IRFs for luminance modulation measured with the same apparatus. The results demonstrated significant age-related losses in IRF amplitude, but the latency (time to peak) of the IRF was stable with age.

A whiter shade of pale, a blacker shade of dark: Parameters of spatially induced blackness

The surface-mode property of "blackness" is induced by simultaneous contrast with an adjacent, more luminant surround. As numerous studies have shown, the degree of blackness induced within an achromatic test field is a function of the relative luminance of the adjacent chromatic inducing field, but not of its hue. But in the converse case of chromatic test fields, susceptibility to blackening has been reported to vary with wavelength. The present study investigates this possibility, that some wavelengths are more susceptible. We also questioned whether "white" and "black" sensory components function as opposites in blackness appearance. We recorded the appearance of a central monochromatic test field of constant luminance (10 cd/m2), with wavelength ranging across the visible spectrum, while a broadband white annulus was set to six luminance levels ranging across three log steps. Three color-normal observers followed a color-naming technique. All six opponent-hue names and their combinations were response options; blackness and whiteness in the test field could therefore be reported independently. Of primary interest were the achromatic responses. When represented within a multidimensional space, these revealed the "white-to-black" dimension but in addition a quality (dimension) of "desaturation." Compared against chromatic properties of the test field, the results provide evidence that blackness is a function of inducing field brightness (not luminance). This result is in accord with observations made by Shinomori et al. (1997) using a different procedure. We conclude that blackness induction occurs at a stage of visual processing subsequent to the origin of the brightness signal from a combination of opponent-process channels.

Unique Hue Stimulus Choice: A Constraint on Hue Category Formation

AbstractBerlin &amp; Kay hue-related basic color categories are compared with the ISCC-NBS system of object color categorization. Though independently derived, categories of the former form a small subset of the latter. A conjecture is proposed that explains the absence of yellow-green and blue-green basic hue categories and the potential for a violet category as the result of constraints on primitive hue category formation due to considerable variation in stimuli selected by color-normal observers as representing for them unique hues.

Visual search and the conspicuity of coloured targets for colour vision normal and colour vision deficient observers

Background: Colour is known to facilitate visual search although its role as a determinant of target conspicuity is not so clear. People with abnormal colour vision have problems recognising and differentiating colours but the extent to which they may be at a disadvantage in visual search when redundant colour is a target attribute has not been investigated previously.Method: Six colour normal and 29 colour deficient subjects (7 P, 6 D, 7 PA, 9 DA) were instructed to find a diamond‐shaped target embedded in a background of 138 differently shaped distracters and to report its position as quickly as possible. The target was redundantly colour‐coded red, yellow, green, blue or white and the distracters had the same colours in various combinations. The target was uniquely colour‐coded in some presentations, partially redundant in others and in some presentations the target and distracters had the same colour. In experiment 1 the displays were presented for 20 seconds and search time was measured; in experiment 2 the exposure time was 500 ms and the proportion of targets located correctly was taken as a measure of the target's conspicuity.Results: Unique colour coding reduced search times significantly and greatly enhanced conspicuity but had no benefit when some distracters had the same colour as the target. Observers with colour vision deficiency had longer search times and the coloured targets were less conspicuous to them compared with colour normal observers. However, unique colour coding assisted their search, especially when the targets were red, blue or white.Conclusions: Observers with colour vision deficiency are less efficient than colour normal observers at visual search when the target is marked out by colour and coloured targets are less conspicuous for them.

Olive green or chestnut brown?

AbstractReflectance and spectral power functions are poor predictors of color experiences. Only in completely relativized conditions (single observer, non-metameric set of stimuli, and single set of viewing conditions) is the relationship close. Variation in reflectance of Munsell chips experienced by color-normal observers as having a unique green hue encompasses approximately sixty percent of the complete range of hues falling under the category “green”; and in recent determinations of unique hues, ranges of yellow and green as well as green and blue unique hues overlap.

Detection and Identification of Light-Emitting Diode Traffic Signals by Protan Observers

Protan observers have lower spectral sensitivity than color-normal observers at long wavelengths of the visible spectrum. Responses of protan and color-normal subjects to light emitting diode (LED) and incandescent traffic signals of red and yellow nominal color during simulated daytime viewing conditions are described. Reaction times, missed signal percentages, and color-identification accuracy were measured. The results indicate that for protans, detection performance to red LED signals was enhanced when the dominant wavelength was moved toward shorter wavelengths, but this shift also appeared to decrease their ability to correctly identify the signal color. Although the yellow LED signal used in this study provided similar detection performance for protans as a yellow incandescent signal, its dominant wavelength was sufficiently long for it to sometimes be confused with red. In general, the results of this study are consistent with the 1994 recommendations of the Commission Internationale de l’Éclairage for signal colors to be seen by color-normal and protan observers. Nonetheless, neither detection nor color identification for protans approached that of color-normal observers.

Color-naming of M-cone incremental flashes

To study which hues are associated with brief excitations of foveal middle-wavelength (M) cones, two highly practiced color-normal observers (the authors) gave basic color names to 500 and 530 nm increments. The test spots were presented to the fovea and foveola in conditions that included M-cone isolation. 1° and 3.6 min arc tests were flashed for 200 ms on steady 8.6′, 1°, or 10° monochromatic adapting fields (481, 530, 610, and 630 nm), or on mixtures of these fields, or on a dark field. Tests were flashed at 1, 2, 4, and 8 × thresholds. Field hue had little overall effect. Yellow, green, blue-green, and blue color-names were elicited, both for the foveal 1° tests and for 3.6 min tests confined to the S-cone free foveola. These data add to previous research by showing a contribution of M-cones to blueness on monochromatic fields, as well as on achromatic fields. Because the 500 and 530 nm tests appear green when presented steadily on these same fields, the M-cone contribution to blue may well be transitory. © 2001 John Wiley & Sons, Inc. Col Res Appl, 26, 132–140, 2001

The spatial arrangement of L and M cones in the peripheral human retina

The spatial arrangement of L and M cones in the human peripheral retina was estimated from red–green color naming of small test flashes (0.86 min of arc, 555 nm, constant intensity) presented at different locations (grid with 1.5 min of arc steps) centered at 17° temporal eccentricity. Simulated red–green color naming ratings were generated by a model based on an ideal observer for all possible patterns of placement and relative numerosities of L and M cones, constrained by the anatomical data on the statistics of cone spacing at this retinal location. The best matching simulated performance as compared to the human observer’s data determined the cone array most likely to produce that observer’s color naming results. The mosaics for two color normal observers showed L and M cones randomly arrayed over this retinal region. Consequences of random cone placements for spectral sampling and color opponency are discussed.

Determination of unique hues using Munsell color chips

Forty observers determined their unique hues from arrays of Munsell chips in a standard surround under artificial daylight. There was some discrepancy in the results of males and females. Essentially no variability due to age of observer was found. The standard deviation around the total mean was less than 1 Munsell 40 hue step. Simple linear opponent-color a and b values were calculated. The ranges were found to straddle in all but the red color the system axes for the CIE 2°observer, but not for the 10°observer. NCS unique hues determined at similar chroma and lightness values fell in all cases within the ranges. The shifts in cross-over wavelengths of the color-matching functions necessary to match the extreme range values were determined to be between 6–11 nm. The results provide support for an opponent-color system based on subtractions of color-matching functions. They also point to significant variation in color normal observers. © 2001 John Wiley & Sons, Inc. Col Res Appl, 26, 61–66, 2001

Spatial color contrast matching: broad-bandpass functions and the flattening effect

The contrast matching function (CMF) is the reciprocal of test contrast that perceptually matches the contrast of standard pattern, measured as a function of test spatial frequency (SF). Achromatic CMFs usually flatten as the contrast of the standard is raised, and are broader than the achromatic, bandpass, contrast sensitivity function (CSF). This report investigates whether chromatic CMFs have similar characteristics. For this purpose, the red–green color channel was defined using minimum flicker and hue cancellation techniques. Spatially localized (D6), vertical, equiluminant patterns (SFs: 0.063–8 cpd; contrast: 3–80%) were used to measure the CSF and CMF of isoluminant patterns presented with a temporal Gaussian envelope. CMFs were measured using a randomized double-staircase procedure and the two-interval forced choice technique. Two color-normal observers, whose task was to select the interval that had higher color contrast, participated in experiments. Results show that: (a) the color CMFs are lowpass functions of SF at low standard contrasts (3–12.5%), broad-bandpass at intermediate contrasts (6.25–60%), and near-flat at high contrasts (80%); and (b) isoluminant CMFs have higher upper cut-off frequencies than isoluminant CSFs. It is concluded that: (i) color-contrast-constancy (CMF independent of SF) is partly achieved at high contrasts because color CMFs flatten as contrast increases; (ii) the information processing at suprathreshold levels is different from that at the threshold levels; and (iii) the model that explained achromatic CMFs using achromatic threshold mechanisms could not explain chromatic CMFs using chromatic threshold mechanisms.

Research note: a multidimensional scaling comparison of color metrics for response times and rated dissimilarities.

Individual-differences multidimensional scaling was applied to a set of proximity data for equiluminant lights (Paramei & Cavonius, 1999) to explore any differences between two data collection procedures (rated dissimilarities, and same/different response times [RTs]), as well as between color-normal and abnormal observers. Two conclusions emerged: (1) The pattern of similarities from observers with anomalous color vision can be understood in terms of a compressed color plane (the weighted Euclidean model of individual differences); and (2) there is evidence that the color "plane" is either curved or governed by a non-Euclidean distance function. When color-normal observers are examined in the weighted-Euclidean framework, minor differences emerge between RT and rating data. But the main distinguishing feature of RT data is a pattern of decreasing accuracy for larger color distances. This must be taken into account, since it can itself induce curvature.

Color spaces of color-normal and color-abnormal observers reconstructed from response times and dissimilarity ratings.

With multidimensional scaling analysis, color spaces were reconstructed from reaction times (RTs) required to make same-different judgements of pairs of 15 equiluminant colors and from dissimilarity ratings between them. In addition to normal trichromats, observers with red-green color deficiency were tested. Two main purposes were served by this study: (1) to compare spatial representations of colors derived from discriminative RTs with those derived from dissimilarity measures and (2) to examine whether the task may selectively affect the dimension reflecting, in color-abnormal spaces, the deficient red-green mechanism. Contradicting our hypothesis of lower dimensionality of RT spaces, as compared with rating spaces, no consistent differences in solution dimensionality were found. However, configurations derived from the two measures diverged. The rating procedure yielded the most logical results for recovering color space. The RT configuration revealed contraction in the tritanopic direction, indicating longer color processing when the short-wavelength mechanism is involved, and in addition, for color-abnormal observers, clustering in the protanopic and deuteranopic directions, indicating even longer processing by the deficient red-green mechanism. This finding implies that RTs are suitable for detecting temporal differences in color processing but, for that very same reason, rather ill-suited for reconstructing color spaces.

Aging and the saturation of colors. 1. Colorimetric purity discrimination.

Colorimetric purity (Pc) discrimination functions were measured for 21 color-normal observers (11 younger and 10 older observers with mean ages of 30 and 74 years, respectively). On each two-alternative-forced-choice trial, observers saw two flashes of light, a broadband white light [CIE(x, y) = (0.33, 0.35)] and a mixture of broadband and monochromatic light (420-680 nm). The observer's task was to choose the flash that had a chromatic component. Foveally viewed, circular, 1.2 degrees-diameter stimuli were presented as 1.5-s flashes with 3-s interstimulus intervals in Maxwellian view. Stimuli [250 trolands (td) and 10 td] were equated on the basis of individual heterochromatic flicker photometry functions. Measured Pc discrimination sensitivity was lower in the older group than in the younger group at both light levels, and the performance difference between the age groups was approximately constant across the spectrum. The difference between discrimination at 10 and 250 td was relatively small for the younger group but larger for the older group, indicating a selective performance decrement for older observers at low light levels. The data were modeled as a sum of differential responses from S-cone and L/M-cone chromatic channels. The model and the data indicate similar age-related losses of sensitivity in the two channels, perhaps secondary to receptorial sensitivity losses.

Aging and the saturation of colors. 2. Scaling of color appearance.

Saturation of two sets of stimuli was scaled by 21 color-normal observers (ten younger and 11 older observers, mean ages: 30 and 73 years). Circular fields, 1.2 degrees in diameter, were presented in Maxwellian view as 1.5-s flashes with 3-s interstimulus intervals. Stimuli were mixtures of broadband light [CIE(x, y) = (0.35, 0.39), 200 trolands (td)] and monochromatic light (420-700 nm, 50 td). Monochromatic lights were equated by the 1978 2 degrees fundamental observer's luminosity function in one set of stimuli [J. J. Vos, Color Res. Appl. 3, 125 (1978) and by each observer's heterochromatic flicker photometry function in the other set of stimuli. Comparing the two age groups reveals no sizable differences in saturation for either set of stimuli, neither supporting nor refuting neural compensation for age-related increases in ocular media density (OMD). Examining short-wavelength saturation as a continuous function of estimated OMD reveals a more complicated pattern of results, however, suggesting substantial compensation over a certain range of OMD values but incomplete compensation for observers with the highest OMD values.

Brightness-to-luminance ratio of colored light in the entire chromaticity diagram

Brightness-to-luminance (B/L) ratios based on the CIE 1924 V (λ) for 195 test stimuli equally sampled from the whole area of the CIE1976(u′, v′) chromaticity diagram were measured for four color normal observers. The results of two observers were similar to results in previous studies in that the B/L ratio increases as purity of the stimulus increases. However, the results of the other two observers showed very low B/L ratios, especially in the reddish region. The B/L ratios based on each observer's sensation luminance were also calculated. Although the contour lines of equal B/L ratio become less atypical for the latter two observers, they still showed low B/L values compared to typical results. Large individual differences of the B/L ratio in the whole area of the chromaticity diagram were indicated. © 1998 John Wiley & Sons, Inc. Col Res Appl, 23, 274–287, 1998

The contribution of color to visual memory in X-chromosome-linked dichromats.

We used a recognition memory paradigm to assess the visual memory of X-chromosome-linked dichromats for color images of natural scenes. The performance of 17 protanopes and 14 deuteranopes, who lack the second (red-green opponent) subsystem of color vision, but retain the primordial (yellow-blue opponent) subsystem, was compared with that of 36 color normal observers. During the presentation phase, 48 images of natural scenes were displayed on a CRT for durations between 50 and 1000 msec. Each image was followed by a random noise mask. Half of the images were presented in color and half in black and white. In the subsequent query phase, the same 48 images were intermixed with 48 new images and the subjects had to indicate which of the images they had already seen during the presentation phase. We find that the performance of the color normal observers increases with exposure duration. However, they perform 5-10% better for colored than for black and white images, even at exposure durations as short as 50 msec. Surprisingly, performance is not impaired for the dichromats, whose recognition performance is also better for colored than for black and white images. We conclude either that X-chromosome-linked dichromats may be able to compensate for their reduced chromatic information range when viewing complex natural scenes or that the chromatic information in most natural scenes, for the durations tested, is sufficiently represented by the surviving primordial color subsystem.

Contributions of neural pathways to age-related losses in chromatic discrimination.

Chromatic-discrimination thresholds were measured for light mixtures lying along individually determined tritan axes and an axis of constant short-wavelength-sensitive- (S-) cone stimulation for 30 color-normal observers (age range 22-77 years). The stimulus was a foveally viewed 2 degrees, circular bipartite field consisting of a standard and a test light. Heterochromatic flicker photometry was used to equate the retinal illuminance of the stimuli at 120 Td for all observers. All stimuli were presented in Maxwellian view. Age-related losses in chromatic discrimination depended on the level of cone stimulation. At relatively lower levels of S- and long-wavelength-sensitive (L-) cone stimulation, discrimination thresholds were elevated for older relative to younger observers. As the level of simulation increased for these two cone types, thresholds converged, on average, for all observers. Application of a model of chromatic discrimination mediated by an S-cone pathway suggests that there is no significant age-related change in Weber fractions and that age-related losses in chromatic discrimination are due, at least in part, to spontaneous neural noise arising in the pathway and/or neural changes that multiplicatively scale all incident light.

Spectral efficiency across the life span: flicker photometry and brightness matching.

Spectral efficiency functions were measured for 50 color-normal observers (aged 19-85 years) by means of heterochromatic flicker photometry (HFP) and heterochromatic brightness matching (HBM). Foveally viewed, circular, 1.2 degrees-diameter stimuli were presented as 3-s flashes (50% duty cycle) in Maxwellian view. Monochromatic lights (420-700 nm; 16 wavelengths) were equated to a 100-Td (trolands), broadband white standard in both procedures. In both HFP and HBM, average sensitivity (specified at the cornea) decreased at short wavelengths with increasing age, consistent with age-related increases in the density of the ocular media. The short-wavelength HBM decline was of lower magnitude than the HFP decline. HFP data were modeled by a weighted additive combination of long- and middle-wavelength-cone inputs and density spectra of the ocular media and macular pigment. HBM data were analyzed with an upper envelope of additive and subtractive combinations of log-transformed cone absorption functions. These analyses indicated that HBM sensitivity, specified at the retina by correction for estimated lens and macular pigment density, increased with age in an approximately wavelength-independent manner over a broad range from 420 to 560 nm.

Serine/alanine amino acid polymorphism of the L-cone photopigment assessed by dual Rayleigh-type color matches

The dual Rayleigh-type color match is the ratio of 621 nm light to 550 nm light that in admixture matches 586 nm light, divided by the ratio of 667 nm light to 550 nm light that in admixture matches 586 nm light. Compared to the classical Rayleigh match, the dual-match procedure minimizes variation in color matching arising from differences in lens pigmentation and photopigment optical density, and thus amplifies individual differences due to shifts in L pigment λ max. We hypothesized that the dual matches would provide a clearer distinction between subjects with serine and subjects with alanine than would the classical Rayleigh match because individuals with serine express L pigments with a λ max shifted toward longer wavelengths than do those with alanine. Classical Rayleigh color matches were compared with dual Rayleigh-type color matches in 14 color-normal observers whose DNA had been analyzed previously for the presence of the amino acid serine or alanine at position 180 in the L opsin. The resulting distribution of dual-match measurements for the seven subjects with serine does not overlap the distribution of measurements for the seven subjects with alanine. The classical Rayleighmatch measurements for these two groups of subjects, on the other hand, overlap substantially. More than half of the subjects' classical Rayleigh matches are within the overlapping range. The dual Rayleigh-type matches, therefore, provide an improved psychophysical technique for assessing whether an individual observer has serine or alanine at position 180.