María José Luque

Corresponding-pair procedure: a new approach to simulation of dichromatic color perception

The dichromatic color appearance of a chromatic stimulus T can be described if a stimulus S is found that verifies that a normal observer experiences the same sensation viewing S as a dichromat viewing T. If dichromatic and normal versions of the same color vision model are available, S can be computed by applying the inverse of the normal model to the descriptors of T obtained with the dichromatic model. We give analytical form to this algorithm, which we call the corresponding-pair procedure. The analytical form highlights the requisites that a color vision model must verify for this procedure to be used. To show the capabilities of the method, we apply the algorithm to different color vision models that verify such requisites. This algorithm avoids the need to introduce empirical information alien to the color model used, as was the case with previous methods. The relative simplicity of the procedure and its generality makes the prediction of dichromatic color appearance an additional test of the validity of color vision models.

Multichannel functional testing in normal subjects, glaucoma suspects, and glaucoma patients.

PURPOSE To evaluate visual function with a novel multichannel functional test named the ATD Multichannel Functional Test. METHODS This multicenter study had a prospective and cross-sectional design. A total of 186 eyes were included: 42 with glaucoma, 14 glaucoma suspects due to optic nerve characteristics, 25 ocular hypertensives, and 105 normal eyes. All patients performed standard visual fields (Humphrey 24-2) and ATD with eight stimuli configurations: four achromatic (A), two red-green (T), and two blue-yellow (D). To derive main outcome measures, mean sensitivity, mean defect (MD), and pattern standard deviation (PSD) were calculated and compared among groups and types of stimuli with the Kruskal-Wallis test. The percentage of cases outside normal limits (ONL) was calculated. RESULTS MD and PSD were significantly different in glaucoma eyes than in normal subjects for all types of stimuli except D-0.5 cycles per degree (cpd)/12Hz. PSD was also lower for normals than for all pathologic groups with A-4cpd/2Hz, A-4cpd/12Hz, D-0.5cpd/2Hz, and T-0.5cpd/2Hz. The highest percentage of ONL cases was obtained with the two low-spatial-frequency chromatic stimuli, with D-0.5cpd/2Hz and T-0.5cpd/2Hz using PSD, which classified as ONL 81.6% and 86.7% of glaucoma eyes, 51.8% and 44.5% of hypertensives, and 72.2% and 41.2% of optic disc suspects, respectively. CONCLUSIONS ATD assessed different aspects of visual function, and the most sensitive tests to detect glaucomatous damage were the low-temporal-frequency chromatic tests.

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.

Application of the S-CIELAB color model to processed and calibrated images with a colorimetric dithering method

This work uses the S-CIELAB color model to compare images that have been calibrated and processed using a colorimetric dithering method which simulates increments in viewing distance. Firstly, we obtain XYZ calibrated images by applying the appropriate color transformations to the original images. These transformations depend on whether the image is viewed on a display device or encoded by a capture device, for example. Secondly, we use a colorimetric dithering method consisting of a partitive additive mixing of XYZ tristimulus values. The number of dithered pixels depends on simulated viewing distance. The dithered tristimulus values are transformed to digital data to observe the dithering effects in the image. Finally, we predict color differences using the S-CIELAB model as color appearance model for images. Moreover, this paper proposes some applications of this method to artistic and industrial problems where one must compare two images that appear different at close viewing distance, but match when they are seen from afar.

Implementations of a novel algorithm for colour constancy

In agreement with the principles of the relativistic model proposed by Creutzfeldt et al., with the photometric rule (lightness anchoring rule) and with the influence of simultaneous contrast in the appearance of a visual scene, we propose a first-stage mechanism yielding substantial colour constancy. We have defined a set of first-stage colour descriptors, and to test their utility, we have performed a simulation using a Machine Vision System (MVS). The statistical stability of the descriptors for Munsell samples under different illuminants is good.

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 Hunts model and Guths 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.

On the intrinsic two-dimensionality of the CSF and its measurement

Classically the visual system sensitivity has been studied separately as a function of spatial frequency and orientation, without explicitly pointing out the relation between both parameters. As the visual system processes two-dimensional signals, its Contrast Sensitivity Function (CSF) is intrinsically two-dimensional too, and therefore, the dependence of the 1-D CSF on orientation is obtained by considering different radial profiles of the 2-D CSF. In the present work, we study the simplest and most straightforward method to obtain the maximum amount of information about the 2-D CSF with a minimum of measurements. It follows from the results that, for emmetropic observers, it is enough to measure two perpendicular radial profiles of the CSF to obtain enough information about the whole 2-D frequency domain, where as for the most common astigmatism cases, its necessary to include another intermediate measure. Assuming that the Modulation Transfer Function (MTF) is proportional to the CSF, its possible to compute the visual system Point Spread Function (PSF), optical and neural parts, from the function experimentally obtained. To estimate the accuracy of the method, the 2-D PSF is calculated and, we simulate, by computer, the perception of a test using the 2-D CSF measured for emmetropic and astigmatic observers.

Simulating images seen by patients with inhomogeneous sensitivity losses.

Purpose. We aim to simulate how colored images are perceived by subjects with local achromatic and chromatic contrast sensitivity losses in the visual field (VF). Methods. The spatiochromatic corresponding pair algorithm, introduced in a previous article (J Opt Soc Am (A) 2004;21:176–186), has been implemented with a linear model of the visual system. Spatial information is processed separately by the chromatic and achromatic mechanisms by means of a multiscale model, with sensors selective to frequency, orientation, and spatial position, whose mechanism-dependent relative weights change with the spatial location of the image. These weights have been obtained from perimetric data from a patient with Lebers Hereditary Optic Neuropathy and an age-matched sample of normal subjects, using achromatic, red-green, and blue-yellow gratings of different spatial frequencies. Distortion contours for each mechanism have been derived from the images simulating the perception of these subjects at different locations in the VF. Results. The images simulating the perception of normal subjects at different locations of the VF show a fast decrease in image quality with eccentricity. The same analysis carried out with the Lebers Hereditary Optic Neuropathy patient reveals worse overall image quality throughout the VF, plus a color vision defect resembling red-green dichromacy at fovea and trichromatic anomaly in the rest of the VF. Conclusions. In the present article, we show that implementing the algorithm with a spatial vision model that considers the changes in contrast sensitivity with spatial location of the stimulus may reveal the local effects that are suffered, in general, by pathological subjects, and that are ignored by simpler spatial vision models.

Chromatic and achromatic visual fields in relation to choroidal thickness in patients with high myopia: A pilot study

PURPOSE To analyse the relationship between the choroidal thickness and the visual perception of patients with high myopia but without retinal damage. METHODS All patients underwent ophthalmic evaluation including a slit lamp examination and dilated ophthalmoscopy, subjective refraction, best corrected visual acuity, axial length, optical coherence tomography, contrast sensitivity function and sensitivity of the visual pathways. RESULTS We included eleven eyes of subjects with high myopia. There are statistical correlations between choroidal thickness and almost all the contrast sensitivity values. The sensitivity of magnocellular and koniocellular pathways is the most affected, and the homogeneity of the sensibility of the magnocellular pathway depends on the choroidal thickness; when the thickness decreases, the sensitivity impairment extends from the center to the periphery of the visual field. CONCLUSIONS Patients with high myopia without any fundus changes have visual impairments. We have found that choroidal thickness correlates with perceptual parameters such as contrast sensitivity or mean defect and pattern standard deviation of the visual fields of some visual pathways. Our study shows that the magnocellular and koniocellular pathways are the most affected, so that these patients have impairment in motion perception and blue-yellow contrast perception.

Design and testing of a perimeter of increment threshold by projection

espanolEn el presente estudio se ha disenado y testeado un campimetro de deteccion de anomalias en la percepcion cromatica utilizando un proyector de video (Multicampimetro). Para ello se ha caracterizado colorimetricamente un proyector de video, pixel a pixel, intentando modelizar las inhomogeneidades de la proyeccion. Se han determinado los valores triestimulo de los primarios del proyector, en funcion del nivel digital, en 49 localizaciones de la superficie de proyeccion y, a partir de ellas, se ha llegado a un modelo de caracterizacion del dispositivo que minimiza las diferencias de color entre el color predicho y el reproducido, en comparacion con el metodo tradicional, que generaliza la caracterizacion del centro a toda la pantalla. Con este perimetro, hemos medido los umbrales de deteccion de estimulos que favorecen la respuesta de los mecanismos cromaticos (rojo-verde y azul-amarillo) del sistema visual humano, utilizando dos configuraciones de tamano: tamano constante y tamano variable con la excentricidad y adaptado a los campos receptivos del cortex visual. El metodo psicofisico de medida utilizado es el MOBS (Modified Binary Search). Se han estudiado los parametros que pueden influir en la distorsion de la medida, asi como la repetitividad, la especificidad y la sensibilidad del dispositivo, resultando favorables estos tres ultimos parametros. Se ha creado una base de datos para la configuracion de estimulo a tamano constante y se ha comprobado que los resultados de la sensibilidad de ambos mecanismos reproducen las caracteristicas ya descritas en la bibliografia. Al comparar ambas configuraciones de tamano, se encuentra mayor sensibilidad periferica y mejor repetitividad con el estimulo de tamano adaptado a la excentricidad. EnglishIn the present study, we have designed and tested a perimeter for the detection of damage in the chromatic mechanisms using a video projector. To this purpose, we have characterized pixel to pixel a video projector, to account for the inhomogeneities in the projection. We have measured the tristimulus values of the projector primaries as a function of digital level, at 49 locations of the projection screen and, from them, we have arrived to a characterization model which reduces the color differences between predicted and reproduced color, in comparison with the traditional method, which generalizes the characterization of the screen center to all the screen. With the perimeter, we measured the detection thresholds of stimuli favouring the response of the red-green and blue-yellow chromatic mechanisms of the human visual system. Stimulus size could either be constant or change with eccentricity with the size of the receptive fields of the neurons in the visual cortex. The psychophysical method of measurement used is the MOBS (Modified Binary Search). We have studied the factors that can distort the measurements, as well as the repeatability, the specificity and the sensitivity of the device, with good results. A database has been created for the configuration of constant size stimulus. The trends shown by the data are consistent with the literature. When comparing both size configurations, the peripheral sensitivity is increased and repeatability is better with the stimulus size adapted to the eccentricity.