Blue-yellow Mechanisms Research Articles

Visual perception alterations in COVID-19: a preliminary study.

To compare the visual perception (color and chromatic-achromatic contrast vision) of a small cohort of COVID-19 patients at the time of infection and after 6mo with that of a healthy population matched for sex and age. A total of 25 patients (9 females, 16 males, mean age: 54±10y) with COVID-19 hospitalized in the COVID-19 Unit of the University Clinical Hospital of Valladolid were recruited for this preliminary study. Visual perception, as determined by monocular measurement of contrast sensitivity function (CSF) and color vision was assessed in each patient using the Optopad test. The results obtained were then compared with those of a sample of 16 age- and sex-matched healthy controls (5 females, 11 males, mean age: 50±6y) in which the same measurement procedure was repeated. Statistically significant differences between groups were assessed using the Mann-Whitney U test. Measurements were repeated after a minimum follow-up period of 6mo and statistically significant differences between the two time points in each group were assessed using the Wilcoxon signed rank test. Discrimination thresholds (color and chromatic-achromatic contrast vision) and their corresponding sensitivity, calculated as the inverse of the discrimination threshold, were evaluated. Analysis of the data revealed higher contrast threshold results (i.e., worse contrast sensitivity) in the COVID-19 group than in the control group for all spatial frequencies studied in the Optopad-CSF achromatic test and most of the spatial frequencies studied in the Optopad-CSF chromatic test for the red-green and blue-yellow mechanisms. In addition, color threshold results in the COVID-19 group were also significantly higher (i.e., worse color sensitivity) for almost all color mechanisms studied in the Optopad-Color test. At 6mo, most of the differences found between the groups were maintained despite COVID-19 recovery. The present results provide preliminary evidence that visual perception may be impaired in COVID-19, even when the infection has passed. Although further research is needed to determine the precise causes of this finding, analysis of CSF and color vision could provide valuable information on the visual impact of COVID-19.

Blue-yellow deficiencies in young moderate smokers

PurposeTo evaluate whether tobacco affects color vision in young moderate smokers. MethodsChromatic mechanisms of 13 moderate smokers (10–20 cigarettes/day and at least 5 years smoking) and 17 non-smokers in the 18–35 age range were assessed with the Farnsworth-Munsell 100-hue (FM100h) test and short wavelength automated perimetry (SWAP). ResultsFM100h Total Error Scores (TES) were higher for smokers, and although differences were not significant (p = 0.14), a linear model with principal component analysis was able to explain 95% of the variance in TES and red-green partial error scores, though not in blue-yellow partial error scores (p = 0.07), using the number of years as smokers and the number of cigarettes/day as predictors. SWAP sensitivity values were globally worse (p = 0.002) for smokers (25.7 + 6.2 dB) than for non-smokers (26.7 + 6.2 dB). In the upper visual hemifield sensitivity, total deviation and pattern deviation values were worse for smokers (p < 0.001). Differences in mean defect and pattern standard deviation were not significant (p > 0.05 in both cases). The number of out-of-limits points in the total difference and pattern difference map were significantly larger for smokers. After correcting for multiple comparisons, only the differences in the upper visual hemifield were significant. ConclusionsThis pilot study suggests that even young moderate smokers show small sensitivity loss in the blue-yellow mechanism, it is statistically significant, restricted to the upper visual hemifield. This corresponds to a retinal region where literature reports a lower density of retinal ganglion cells and where, therefore, the chromatic mechanisms would be more fragile.

Red-green vs. blue-yellow spatio-temporal contrast sensitivity across the visual field

We measured contrast sensitivity (CS) to sinusoidal spatio-temporal patterns isolating the red-green and blue-yellow mechanisms, at 21 locations in the visual field (including the fovea). These measurements complete the available data for the red-green mechanism at fovea and for both mechanisms outside fovea with non-stationary patterns. Chromatic detection surfaces are low-pass at fovea and CS decreases with eccentricity at a rate that depends on the spatial and temporal frequencies. Our results confirm that, in general, sensitivities decrease with eccentricity at different rates for stationary red-green and blue-yellow patterns at each point of the spatio-temporal domain and that the chromatic detection surfaces are still low-pass at each spatial location. The results suggest some superiority of the temporal visual field for both mechanisms. Surprisingly the spatial region with maximum sensitivity for blue-yellow patterns for stimuli in the low-spatial–high temporal corner of the spatio-temporal domain is not the fovea, as happens always for red-green patterns, but shifted towards the upper-nasal quadrant of the retina.

A Model of human dichromat color vision that may help to explain the evolution of trichromacy

A Model of human dichromat color vision that may help to explain the evolution of trichromacy

Images Perceived After Chromatic or Achromatic Contrast Sensitivity Losses

We simulate how subjects with losses in chromatic and achromatic contrast sensitivity perceive colored images by using the spatiochromatic corresponding pair algorithm. This is a generalized version of the algorithm by Capilla et al. (J Opt Soc Am (A) 2004;21:176-186) for simulating color perception of color deviant subjects, which incorporates a simple spatial vision model, consisting of a linear filtering stage, with a band-pass achromatic filter and two low-pass chromatic ones, for the red-green and blue-yellow mechanisms. These filters, except for the global scaling, are the subject's contrast sensitivity functions measured along the cardinal directions of the color space. In its present form, the algorithm would serve to simulate alterations both in the spectral sensitivities and in the contrast sensitivities of the visual mechanisms. After a preliminary theoretical study on the effect of frequency selective and overall reductions in the contrast sensitivity function of a single mechanism, we present cases of real subjects with glaucoma and diabetes, suffering alterations of different magnitude in the three mechanisms. The simulations allow us to learn about the different types of distortions that can be experienced by a subject with impaired contrast sensitivities (blur, haloes, color shifts, local or global contrast, brightness and colorfulness reductions, etc.) and highlight the difficulties arising when trying to predict the quality of the final image from the losses in the individual mechanisms.

Chromatic parameters derived from increment spectral sensitivity functions

We propose a mathematical model to derive the chromatic parameters from increment spectral sensitivity functions. This model was applied to determine the effective red, green, blue, and yellow mechanism contribution to the detection of the spectral stimuli of five normal trichromatic subjects. Detection thresholds were measured for a 300 ms, 1.2 degrees circular test flash presented on a 100 cd/m2 white background for spectral wavelengths between 410 and 660 nm. The model analysis confirmed that in the red-green wavelength area, the detection of our chosen stimuli was mediated by two distinct (L-M) antagonistic mechanisms: a red-green and a yellow, from the blue-yellow system. We inferred that the red-green mechanism receptive fields consisted of a single L- or M-cone center with a homogeneous or heterogeneous surround devoid of S-cone projections. For the receptive fields of the yellow half of the blue-yellow mechanism, we propose a similar configuration but with S-cone projections present in the surround. This proposal is not concordant with what is currently understood regarding retinal physiology. However, two L-M antagonistic mechanisms in the red-green wavelengths as proposed by our results predict what would appear as an intuitive yellow mechanism with a maximal sensitivity at the 578 nm wavelength, where the red-green mechanism sensitivity is null.

Effect of a yellow filter on brightness evaluated by asymmetric matching: measurements and predictions

We found that yellow filters reduce the mean brightness of a set of coloured samples significantly less with blue surrounds than with black, achromatic or yellow ones, in comparison with the naked eye and less than a yellow surround in comparison with a luminance-matched grey filter. The predictions of Hunt's model and Guth's ATD95 agree with these results. The yellow filter is actually capable of significantly increasing the brightness with the blue surround both in comparison with the naked eye (although only for our younger observer) and with the neutral filter. We show that in some samples the yellow filter causes global response increments in the chromatic mechanisms that compensate the response reduction in the achromatic mechanism, resulting in increased brightness. Although global chromatic response increments may arise from any chromatic mechanism, the blue–yellow mechanism determines in most cases the final result, even when the response of the red–green mechanism decreases.

Orientation selectivity in luminance and color vision assessed using 2-d band-pass filtered spatial noise

We evaluated orientation discrimination in color and luminance vision using an external noise paradigm. Stimuli were spatiotemporal patches of 2D orientation noise isolating the achromatic, red–green and blue–yellow mechanisms, and matched in multiples of contrast detection threshold. We found a monotonic increase of orientation discrimination thresholds with the stimuli orientation bandwidths that is similar for both color and luminance contrasts. This dependence was fitted with two suitable models. A variance summation model suggests that internal orientation noise is significantly greater for the chromatic than for the achromatic mechanisms, while the efficiencies are similar. A gain control model of orientation tuning suggests that both chromatic and achromatic mechanisms are characterized by broadly tuned orientation detectors and that the relative chromatic deficit in orientation discrimination may only result from a slightly broader orientation tuning for the chromatic mechanisms. The moderate deficiency in chromatic orientation discrimination may account for the small differences found in shape perception between color and luminance vision.

Red–green and blue–yellow mechanisms are matched in sensitivity for temporal and spatial modulation

The spatial and temporal properties of human colour vision are examined using isoluminant, red–green and blue–yellow tritanopic gratings. Chromatic sensitivity is found to be low-pass as a function of both spatial and temporal frequency along all the chromatic axes investigated, including the tritanopic confusion lines employed to examine the properties of the S-cone driven mechanism. Comparison of sensitivity to on-off and contrast reversing stimuli indicates that transient mechanisms contribute to the detection of red–green patterns but that the detection of S-cone specific patterns is governed by sustained mechanisms. By compensating for transient contributions to red–green sensitivity, it is shown that sensitivity of chromatic mechanisms dominated by L- and M-cone input are closely matched to those with S-cone input.

Evidence for the contribution of S cones to the detection of flicker brightness and red-green.

We were interested in the question of how cones contribute to the detection of brightness, red-green, and blue-yellow. The linear combination of cone signals contributing to flicker detection was determined by fitting a plane to 64 points (colors) of equal heterochromatic flicker brightness. A small S-cone contribution to flicker brightness of similar amplitude in all five subjects was identified. The ratio of L- to M-cone contribution was found to vary considerably among subjects (1.7-4.1). Chromatic detection thresholds were determined for small patches in the isoluminant plane defined by flicker brightness. These stimuli were presented at an eccentricity of 40 arc min. By using color naming at the detection threshold, one can attribute different segments of the resulting detection ellipses to different chromatic mechanisms. Linear approximation of these segments provided an estimate for the contribution of the different cone types to the detection of red-green and blue-yellow. The results are consistent with the hypothesis that S cones contribute to the red-green mechanism with the same sign as that of the contribution from L cones. The blue-yellow mechanism very probably subtracts S-cone contrast from luminance contrast. The detection ellipse can be mapped into a circle in cone difference space. The base of this canonical transformation is a set of three cone fundamentals that differs from previously published estimates. Projecting the circle onto the three cone difference axes produces sinusoidal changes within the respective excitations. We propose that simultaneous sinusoidal changes of equal increment in the three cone difference excitations generate stimuli differing by equal saliency.

Contour integration in color vision: a common process for the blue–yellow, red–green and luminance mechanisms?

We compare the performance of the red–green, blue–yellow and luminance postreceptoral mechanisms on a contour integration task requiring the linking of oriented Gabor elements across space to extract a winding ‘path’ or contour. We first establish that for all three mechanisms curvature and contrast are independent; losses in performance due to one cannot be compensated by changes in the other. We then compare contour integration by the three mechanisms using a method that controls for their differences in cone contrast thresholds. Our results show that despite the poor orientation discrimination thresholds and poor spatial sampling found for the blue–yellow mechanism, all three mechanisms perform similarly on contour integration over a wide range of curvatures. Furthermore, all three mechanisms have the same dependence on path curvature. We also investigate the effects of adding external orientation noise. Our results imply that the internal orientation noise for extracting ‘aligned’ path elements is similar in the three mechanisms and for all path curvatures, and the relative efficiencies are also similar for the three mechanisms. To account for our results, we propose that the three postreceptoral mechanisms use a common contour integration process. This linking process, however, cannot be color-blind; our last experiment shows that linking between different chromatic mechanisms or between opposite spatial phases disrupts contour integration. We thus propose that the common integration process remains sensitive to the color contrast and phase of its inputs.

Estimation of the L-, M-, and S-cone weights of the postreceptoral detection mechanisms

We have obtained two- and three-dimensional detection threshold contours in cone contrast space for sinusoidal gratings for three subjects at three spatiotemporal conditions (1 cycle/degree (c/deg), 0 Hz; 0.125 c/deg, 0 Hz; 1 c/deg, 24 Hz). These conditions were chosen to favor the response of each of the three postreceptoral mechanisms in turn. Contours were obtained from measurements in as many as 60 axes in (L, M, S) cone contrast space and were fitted by superellipses. Our technique permitted us to improve on earlier estimates of the cone weightings to the mechanisms. We found that the red–green mechanism has an input cone weighting of L−M with a 2% S-cone input; the luminance mechanism has a weighting of kL + M, where k varies between 3 and 5 at the high-temporal condition, with a 5% S-cone input in opposition to L- and M-cones; and the blue–yellow mechanism consists of S inputs in closely balanced opposition to L and M inputs. These cone weights were found to be consistent among our three subjects.

The absence of the Ives effect in a deuteranope.

Ives found that when monochromatic stimuli are matched to white by flicker photometry, they are not equal in brightness to the white by direct comparison, and the discrepancy is minimal for yellow but is increased for longer and shorter wavelengths. On the two sides of yellow, the colors are more saturated, and Ives postulated that brightness involves the sum of a chromatic component and an achromatic component and that the chromatic component varies with the saturation. In the case of a deuteranope, one would expect a vigorous chromatic response for yellow and blue stimuli but a poor response for the neutral part of the spectrum. The Ives effect is virtually nonexistent for subject SR, who is a deuteranope. In terms of the zone theory of color vision, this would mean that the blue-yellow chromatic channel contributes little or nothing to brightness. In a normal observer, the blue-yellow mechanism can be isolated by using blues and yellows depurified with white, but in this case the Ives effect is found to exist.

Dichromatism and the mechanisms subserving color vision.

In previous papers the writer has formulated a theory of color vision in which chromaticness is subserved by independent red-green and blue-yellow mechanisms. Each mechanism involves two photosensitive substances. The relative rates of response of these substances for the various wavelengths can be determined from color mixture and adaptation data for normal observers, and this has been done. The present paper makes use of certain assumptions about the color mixture data of the tritanope and the luminosity curve of the normal observer to derive the response curves for the four photosensitive substances subserving color vision.