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At least 1,729 records · Page 96Linked to original sources
PubMed1978

Effect of chromatic uncertainty on detectability of a visual stimulus.

The detectability of a brief color shift towards red or towards green of a foveally viewed yellow target is less by 25% if the observer is uncertain as to the direction of the color shift. This result matches a prediction of the theory of signal detectability: When signal parameters become uncertain, detectability declines.

Color Perception↗
PubMed1981

Flicker photometric study of chromatic adaption: selective suppression of cone inputs by colored backgrounds.

Flicker photometric equivalence is both additive and transitive when the test and standard are alternated upon a relatively more intense colored background. When the balance of red versus green cone excitation from the background is unequal, the contribution of one cone type to flicker photometric spectral sensitivity may be depressed in relation to that of the other by at least 1 order of magnitude more than Weber's law predicts. The resultant spectral sensitivity is determined predominantly by only one class of cone. The cone spectral sensitivities of normals are then seen to be the same as those of dichromats, although there is some individual variation. A model is developed to explain this surprising phenomenon.

Adaptation, Ocular↗
PubMed1995

Color matches in diseased eyes with good acuity: detection of deficits in cone optical density and in chromatic discrimination.

Reduced foveal cone optical density in diseased eyes with normal acuity can affect color matches. Using field diameters of 1 degree, 2 degrees, 4 degrees, and 8 degrees, we measured mean color-match midpoints and match widths in patients who had good acuity and who exhibited three categories of eye disease: hereditary macular degeneration (n = 12), retinitis pigmentosa (n = 19), and glaucoma (n = 18). Results were compared with those for normal observers of comparable ages. Mean color-match midpoints were abnormal only for the population with hereditary macular degeneration, indicating a reduction in cone optical density in the central 4 degrees. Mean color-match widths were enlarged for both hereditary macular degeneration and retinitis pigmentosa, a result consistent with a reduction in the number of foveal cones.

Adult↗
PubMed1998

Adaptation to textured chromatic fields.

Probe-flash threshold curves were used to show that adaptation to textured fields consists not only of adaptation to the steady local constituents but also of a process that is similar to habituation to prolonged temporal modulation, which in this case could be caused by miniature eye movements across element boundaries. The response curves derived from probe-flash thresholds are compressive on both sides of the adaptation level after adaptation to spatially uniform fields but have an accelerating form when they are measured after adaptation to textured backgrounds. This change is suggestive of a response equalization process, which modifies the response function of each mechanism to match the cumulative frequency distribution of its inputs.

Adaptation, Physiological↗
PubMed2000

Cone pigment gene expression in individual photoreceptors and the chromatic topography of the retina.

Human trichromatic vision is based on three classes of cones: L, M, and S (long-, middle-, and short-wavelength sensitive, respectively). Individuals can have more than one M and/or more than one L pigment gene on the X chromosome along with an S pigment gene on chromosome 7. In some people the X-linked pigment gene array can include polymorphic variants that encode multiple, spectrally distinct cone photopigment subtypes. A single-cell, polymerase chain reaction approach was used to examine visual pigment gene expression in individual human cone cells and identify them as L or M. The ratio of L:M pigment gene expression was assayed in homogenized retinal tissues taken from the same eyes. Results indicate that there is a close correspondence between the cone ratio determined from counting single cells and the L:M pigment mRNA ratio estimated from homogenized pieces of retina. The results also show that the different pigment genes in one array are often expressed at very different levels, giving rise to unequal numbers of L and M cones. Expression of only one photopigment gene was detected in each cone cell. However, individual males can have more than the classically described three spectrally distinct cone types in their retinas.

Adult↗
PubMed2001

Blur tolerance for luminance and chromatic stimuli.

We investigated the blur tolerance of human observers for stimuli modulated along the isoluminant red-green, the isoluminant yellow-blue, and the luminance (black-white) direction in color space. We report the following results: (i) Blur difference thresholds for red-green and luminance stimuli (of equal cone contrast) are very similar and as low as 0.5 min of visual angle; for yellow-blue the lowest blur thresholds are much higher (1.5 min of visual angle). (ii) The smallest blur thresholds are found for slightly blurred square waves (reference blur of 1 arc min) and not for sharp edges. (iii) Blur thresholds for red-green and black-white follow a Weber law for reference (pedestal) blurs greater than the optimum blur. (iv) Using the model proposed by Watt and Morgan [Vision Res. 24, 1387 (1984)] we estimated the internal blur of the visual system for the black-white and the red-green color directions and arrived at the following estimates: 1.2 arc min for black-white stimuli at 10% contrast and 0.9 arc min for red-green stimuli at 10% cone contrast. Blur tolerance for yellow-blue is independent of external blur and cannot be predicted by the model. (v) The contrast dependence of blur sensitivity is similar for red-green and luminance modulations (slopes of -0.15 and -0.16 in log-log coordinates, respectively) and slightly stronger for yellow-blue (slope = -0.75). Blur discrimination thresholds are not predicted by the contrast sensitivity function of the visual system. Our findings are useful for predicting blur tolerance for complex images and provide a spatial frequency cutoff point when Gaussian low-pass filters are used for noise removal in colored images. They are also useful as a baseline for the study of visual disorders such as amblyopia.

Color↗
PubMed2001

Using color histogram normalization for recovering chromatic illumination-changed images.

We propose a novel image-recovery method using the covariance matrix of the red-green-blue (R-G-B) color histogram and tensor theories. The image-recovery method is called the color histogram normalization algorithm. It is known that the color histograms of an image taken under varied illuminations are related by a general affine transformation of the R-G-B coordinates when the illumination is changed. We propose a simplified affine model for application with illumination variation. This simplified affine model considers the effects of only three basic forms of distortion: translation, scaling, and rotation. According to this principle, we can estimate the affine transformation matrix necessary to recover images whose color distributions are varied as a result of illumination changes. We compare the normalized color histogram of the standard image with that of the tested image. By performing some operations of simple linear algebra, we can estimate the matrix of the affine transformation between two images under different illuminations. To demonstrate the performance of the proposed algorithm, we divide the experiments into two parts: computer-simulated images and real images corresponding to illumination changes. Simulation results show that the proposed algorithm is effective for both types of images. We also explain the noise-sensitive skew-rotation estimation that exists in the general affine model and demonstrate that the proposed simplified affine model without the use of skew rotation is better than the general affine model for such applications.

Journal Article↗
PubMed2005

Classification images for chromatic signal detection.

The number and nature of the mechanisms for the detection of colored stimuli are still unclear. We use the paradigm of classification images to investigate the detection of a signal of homogeneous color added to a noisy texture. Both signal and noise colors were chosen from the isoluminant plane of the Derrington-Krauskopf-Lennie (DKL) color space. The signal consisted of a square of homogeneous color that was chosen from either cardinal or noncardinal directions of the DKL color space. The noisy texture consisted of small squares of varying colors that were chosen randomly across the isoluminant plane. Classification images reveal that (1) the cardinal axes play no specific role; (2) the widths of the tuning curves vary between 30 and 90 deg, consistent with the variation of tuning widths of neurons at early cortical stages; and (3) detection is not based on the whole region covered by the signal but is influenced mostly by a small spot around the fixation point.

Color Perception↗
PubMed1990

Chromatic induction and brightness contrast: a relativistic color model.

It has been suggested that object colors in a colored environment are the result of combining in perception the (relative) brightness of each spectral component rather than of just mixing the spectral luminances. We tested this hypothesis with the following experiment: A pair of center-surround targets made of colored papers was illuminated with trichromatic white light. Two identical central color plates (test and match field, respectively) were surrounded by frames of different colors and thus looked different because of simultaneous color contrast. Observers were asked to match the colors by changing the illumination of the match field by means of a color-mixture projector (color match, CM). This color-matched reflectance was measured with a photometer, and its CIE coordinates were determined. We then illuminated the display with one of the three primaries that made up our trichromatic white light. The different reflectances of the different surrounds at each primary induced simultaneous brightness contrast. The brightnesses of the two central plates were therefore different. Observers were asked to change the intensity of the illumination of the match field at the respective primary so that it looked equally bright as the test field. This procedure was repeated for each primary (primary brightness match, PBM). Then the whole display except for the match field was illuminated with the trichromatic white as before, while the latter was illuminated with a trichromatic mixture consisting of the primaries at the intensities as set in the PBM experiment, and the CIE values were determined with the photometer. The CIE values of the match field after the CM and PBM procedures were nearly identical. This indicates that composite colors are composed in perception by combining the scaled (or relative) brightness of each spectral component and that this brightness scaling is largely restricted to interactions in the same spectral region. The results are compared with those of other models concerned with contrast colors as well as with neurophysiological data. Some limitations are mentioned.

Adult↗
PubMed2005

Optically tunable chromatic dispersion controller with coupled-cavity etalon structure.

We propose and demonstrate a novel optically tunable dynamic dispersion-compensation device with coupled-cavity etalon structure. Experimentally, it was achieved with an all-fiber coupled-cavity etalon made in an Er/Yb codoped fiber pumped by a 980-nm laser diode. The dispersion was tuned from -300 to +400 ps/nm in the experiment. The potential advantages of such optically tunable dispersion compensators include fast response time and remotely controllable operation.

Journal Article↗