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Biomedical subjects

J Walraven

Publications and source records attributed to J Walraven.

15 recordsLinked to original sources
PubMed1996

Color constancy under natural and artificial illumination.

Color constancy was studied under conditions simulating either natural or extremely artificial illumination. Four test illuminants were used: two broadband phases of daylight (correlated color temperatures 4000 and 25,000 K) and two spectrally impoverished metamers of these lights, each consisting of only two wavelengths. A computer controlled color monitor was used for reproducing the chromaticities and luminances of an array of Munsell color samples rendered under these illuminants. An asymmetric haploscopic matching paradigm was used in which the same stimulus pattern, either illuminated by one of the test illuminants, or by a standard broadband daylight (D65), was alternately presented to the left and right eye. Subjects adjusted the RGB settings of the samples seen under D65 (match condition), to match the appearance of the color samples seen under the test illuminant. The results show the expected failure of color constancy under two-wavelengths illumination, and approximate color constancy under natural illumination. Quantitative predictions of the results were made on the basis of two different models, a computational model for recovering surface reflectance, and a model that assumes the color response to be determined by cone-specific contrast and absolute level of stimulation (Lucassen & Walraven, 1993). The latter model was found to provide somewhat more accurate predictions, under all illuminant conditions.

Color Perception↗
PubMed1993

TNO stereopsis test as an aid to the prevention of amblyopia.

In this investigation, carried out under a Youth Health Care postgraduate course in Nijmegen, the stereopsis of a school population of 730 children, aged 4-18 years, was recorded and clinically evaluated. Stereopsis was measured using the TNO test, a random-dot stereo test especially designed for the early detection of amblyopia. The main aims of this study were to establish the validity of the TNO test as a screening test for amblyopia, obtain information about the variability and age dependence of stereopsis, and to evaluate the efficacy of amblyopia prevention. The most important findings can be summarized as follows: 1. All amblyopes are detected by the TNO test's recommended referral criterion of 240 sec arc (binocular threshold parallax in sec arc). 2. The red-green anaglyphs used in the test do not pose problems for individuals with a colour vision deficiency. 3. The ability to discriminate depth improves by a factor of two over the age interval 4-12 years. 4. A stereoacuity of < or = 120 sec arc is a good predictor of normal or correctable normal vision, and may therefore help in evaluating the often incomplete results of eye tests of young children. 5. Where there is an increased perinatal risk, there is a greater chance of disturbed binocular vision. 6. It is estimated that 75% of amblyopes remain amblyopic, possibly because of delayed detection; 60% of the amblyopes in the population examined were not identified before the age of 5 years.

Adolescent↗
PubMed1991

The invariance of unique white; a possible implication for normalizing cone action spectra.

The locus of unique white was measured for two observers over a wide range of stimulus intensities, using both direct matching and absolute judgements. Chromaticity coordinates of each observer's unique white were found to be invariant over a range of about 4 log units. This result is discussed in the context of the class of models that explain the intensity-evoked change in perceived color (Bezold-Brücke effect) on the basis of response compression in color channels. Invariance of unique white may be interpreted, then, as evidence for its non-polarizing effect on color channels, including the three classes of cones. On the basis of that assumption we have used unique white as criterion for normalizing cone action spectra. Assuming an equal-energy spectrum for the standard observer's unique-white stimulus, as we found to be consistent with literature data, we calculated the associated coefficients of the conversion formulae between CIE space and cone space (both for XYZ to LMS and LMS to XYZ) of the Vos-Walraven (1971 Vision Research, 11, 799-818) and Smith-Pokorny (1975 Vision Research, 15, 161-171) receptor fundamentals. The action spectra that thus result, intersect at the spectral neutral (achromatic) points of the three classes of dichromats.

Color Perception↗
PubMed1982

Effect of chromatic adaptation on the achromatic locus: the role of contrast, luminance and background color.

Two superposed annular test lights of complementary spectral composition were presented as 60-90' incremental test flashes on 480' steady backgrounds. Two observers adjusted the ratio of the two test lights to maintain an achromatic appearance under conditions of adaptation that varied with respect to background luminance, chromaticity and stimulus contrast. The shift in chromaticity of the achromatic point was in the direction of the chromaticity of the background, while the magnitude of the shift increased as an increasing function of background luminance and as a decreasing function of contrast. These data confirm and extend a model of chromatic adaptation that has the following properties: (1) non-additivity of transient test and steady background fields, in the sense that the background, although physically adding to the test flash, only affects its hue by way of altering the gain of cone pathways; (2) Vos-Walraven cone spectral sensitivities; and (3) adaptation sites in the cone pathways having the same action spectra as Stiles' pi 5, pi 4 and (modified) pi 1 mechanisms, and which generate receptor-specific attenuation factors (von Kries Coefficients) according to Stiles' generalized threshold vs intensity function, zeta (x).

Adaptation, Ocular↗
PubMed1980

The derivation of nerve signals from contrast flash data. A re-analysis.

This paper presents a new analysis of the contrast flash data of Alpern et al. (1970a--d). It was prompted by the criticism of Wandell (1976) who pointed out that Alpern et al., main conclusion, i.e, that the inhibitory signal N (phi) elicited by the contrast flash (phi) takes the form (formula: see text), would imply an unrealistic excitatory photo response. The present analysis shows the data to be consistent with an inhibitory signal of the form (formula: see text).

Animals↗
PubMed1975

Amblyopia screening with random-dot stereograms.

A stereoscopic test was developed for the visual screening of preschool children. The TNO test for stereoscopic vision utilized the same principle as the Julesz random-dot stereogram, and provided a simple and unequivocal test criterion understood even by young children. The results of a comprative evaluation of the TNO and Titmus stereoscopic tests proved the TNO test to be the more reliable of the two, particularly in the 2- to 4-year-old age range. Failure to pass this test at the 240 seconds of arc disparity level yielded an excellent screening criterion, as attested by the results of a validation experiment employing 81 patients (2 to 7 years old) with known visual health records. Furthermore, the screening results obtained from 129 preschool children (2 to 5 years old) tested in the classroom by a nonprofessional examiner, suggest that, under these more realistic conditions, the TNO test yields at least 60% less overreferrals than the Titmus test.

Amblyopia↗
PubMed1967

Monocular and binocular aftereffects of chromatic adaptation.

Supersaturated greens seen after long-wavelength adaptation depend upon contrast from the continuing afterdischarge of bleached "red" receptors in the surround, rather than merely upon inactivation from bleaching of "red" receptors in the test spot area. When test spot and bleach field coincide spatially, supersaturated greens are not seen. Since color mixing but not contrast was found binocularly, color contrast must be a retinal phenomenon.

Adaptation, Ocular↗
PubMedDate not supplied

Quantifying color constancy: evidence for nonlinear processing of cone-specific contrast.

Color constancy was studied by the method of comparing color samples under two different illuminants using a CRT color monitor. In addition to the classical approach in which one of the illuminants is a (standard) white, we performed experiments in which the range of differential illumination was extended by using pairs of lights that were both colored. The stimulus pattern consisted of an array of thirty-five color samples (including five neutral samples) on a white background. A trichromatic illuminant-object interaction was simulated analogous to that resulting from illumination by three monochromatic lights. The test samples, as seen under "test" and "match" illumination, were successively presented to the left and right eye (haploscopic matching). The data show systematic deviations from predictions on the basis of cone-specific normalization procedures like those incorporated in the Retinex algorithm and the von Kries transformation. The results can be described by a nonlinear response transformation that depends on two factors, receptor-specific sample/background contrast and the extent to which the illuminant stimulates the receptor system in question. The latter factor explains the deviations. These are mainly caused by the short-wave-sensitive system, as a consequence of the fact that this system can be more selectively stimulated than the other, spectrally less separated, cone systems.

Color Perception↗