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PubMed2006

Postreceptoral chromatic-adaptation mechanisms in the red-green and blue-yellow systems using simple reaction times.

Simple visual-reaction times (VRT) were measured for a variety of stimuli selected along red-green (L-M axis) and blue-yellow [S-(L + M) axis] directions in the isoluminant plane under different adaptation stimuli. Data were plotted in terms of the RMS cone contrast in contrast-threshold units. For each opponent system, a modified Piéron function was fitted in each experimental configuration and on all adaptation stimuli. A single function did not account for all the data, confirming the existence of separate postreceptoral adaptation mechanisms in each opponent system under suprathreshold conditions. The analysis of the VRT-hazard functions suggested that both color-opponent mechanisms present a well-defined, transient-sustained structure at marked suprathreshold conditions. The influence of signal polarity and chromatic adaptation on each color axis proves the existence of asymmetries in the integrated hazard functions, suggesting separate detection mechanisms for each pole (red, green, blue, and yellow detectors).

Adaptation, Physiological↗
PubMed2006

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.

Journal Article↗
PubMed1986

Method for computing the scene-illuminant chromaticity from specular highlights.

The perception of an unchanging surface color under different illuminations requires the computation of the scene-illuminant color either directly or indirectly. A possible source for the computation is the specular highlight of the surface reflection. Some issues related to color constancy are discussed, and a theory for computing the scene-illuminant chromaticity from specular highlight is described. An interesting result of the theory is that in an ideal situation, two surfaces of different colors will be sufficient for the computation.

Color Perception↗
PubMed1986

Principal-component analysis of macaque lateral geniculate nucleus chromatic data.

Principal-component analysis was used to find a set of orthonormal basis vectors that could span the chromatic space of neurons in the parvocellular layers of the macaque monkey lateral geniculate nucleus (LGN). A total of 441 spectral curves measured on 147 cells from a well-known study [R. L. De Valois, I. Abramov, and G. H. Jacobs, J. Opt. Soc. Am. 56, 966 (1966)] were used as the input data. The principal-component technique, in a completely objective manner, showed that 93% of the variance in the data could be accounted for by only three eigenvectors. An orthogonal Varimax rotation of the first two eigenvectors matched the mean spectra of the previously identified GR and YB opponent cell groups. A different rotation gave a photopic luminosity curve.

Animals↗
PubMed1986

Temporal integration of chromatic double pulses for detection of equal-luminance wavelength changes.

Detection probabilities for wavelength changes were measured as functions of stimulus onset asynchrony (SOA) with the chromatic double-pulse method. Two test stimuli of a wavelength lambda t were successively exchanged with a reference stimulus of a wavelength lambda r in equal luminance for a short duration with a SOA. Durations were 5 and 50 msec, and the SOA varied between 5 or 50 and 2000 msec. Lambda r's were 571 and 518 nm, chosen from wavelengths near unique yellow and unique green so that wavelength difference delta lambda = lambda t - lambda r was perceived mainly as redness and greeness for lambda r = 571 nm and yellowness and blueness for lambda r = 518 nm. The results showed that temporal integration characteristics for these equal-luminance wavelength changes were quite consistent: complete integration with SOA up to 20 msec, partial integration with SOA between 20 and 200 msec, and probability integration with SOA greater than 200 msec. They did not show any inhibitory integration that was found for luminance changes.

Adult↗
PubMed1988

Color-prediction discrepancies and differential chromaticity thresholds with photopigment bleaching.

The differential chromaticity thresholds of three color stimuli have been determined from metameric matches carried out during the cone-plateau period after photopigment bleaching. The results are compared with some recently published ones [E. Hita, L. Jiménez del Barco, and J. Romero, J. Opt. Soc. Am. A 3, 1203 (1986)] obtained under similar experimental conditions but without prior photopigment bleaching. The possible effects of rod intrusion on color-prediction discrepancies are discussed in the light of both sets of results. Under our experimental conditions, with foveal fields of 3 deg, luminance within the low photopic level, and sets of primary colors produced by filters of differing spectral bandwidth, the rod intrusion effect can be ruled out as a possible cause of color-prediction discrepancies, although it does appear to produce variations in the sizes of the discrimination ellipses and in their orientation.

Acclimatization↗
PubMed1990

Visual artifacts in chromatically subsampled images.

Colors sampled in an array of pixels were converted into various luminance-chrominance representations, and the chrominance values were spatially subsampled to achieve a compression of the data. For viewing, the chrominance values were reinterpolated and then transformed back to RGB. For each of a range of image types and color spaces, we varied the chromatic sample spacing to determine how much compression would be possible before perceptual artifacts appeared, as determined by a panel of individual viewers. The spacing at which an image was mistaken half the time for a remembered original depended on image content and also on the color representation. Some color spaces, such as hue-lightness-saturation, a polar-coordinate color space, are inherently ill suited for chrominance averaging and produce noticeable artifacts on almost any image. Likewise, some images cannot be subsampled satisfactorily, regardless of the color space used. In general, however, images with 16-fold reduction in the amount of chrominance information (that is, a factor of 4 in each spatial direction) are still satisfactory in appearance.

Color↗
PubMed1992

Stereopsis at isoluminance in the absence of chromatic aberrations.

Stereo (front-back) discrimination thresholds were measured in a two-interval forced-choice paradigm for chromatic (red-green) random-dot stereograms that had all detectable longitudinal and transverse aberrations removed by low-pass filtering. The thresholds were measured as a function of the luminance ratio of the red and the green stereo elements. Although individual differences were apparent, three subjects were able to fuse all the stimuli, including those at isoluminance. A quantitative, ideal-observer analysis was used to determine the neural efficiency with which color and luminance information was used in this stereo task. For two subjects, efficiency was constant as a function of the red-to-green ratio; for the third subject, efficiency was less near isoluminance.

Color Perception↗
PubMed2003

Dispersion management with microstructured optical fibers: ultraflattened chromatic dispersion with low losses.

We numerically demonstrate ultraflattened chromatic dispersion with low losses in microstructured optical fibers (MOFs). We propose using two different MOF structures to get this result. Both structures are based on a subset of a triangular array of cylindrical air holes; the cross sections of these inclusions are circular, and a missing hole in the fiber's middle forms the core. In this MOF structure the diameters of the inclusions increase with distance from the fiber axis until the diameters reach a maximum. With this new design and with three different hole diameters, it requires only seven rings to reach the 0.2-dB/km level at lambda = 1.55 microm with a variation amplitude of dispersion below 3.0 x 10(-2) ps nm(-1) km(-1) of lambda = 1.5-1.6 microm. With the usual MOF (made from holes of identical diameter), we show that at least 18 hole rings are required for losses to decrease to < 1 dB/km at lambda = 1.55 microm.

Journal Article↗
PubMed2003

Distributed measurement of chromatic dispersion by four-wave mixing and Brillouin optical-time-domain analysis.

A new nondestructive method for measuring the spatial distribution of chromatic dispersion along an optical fiber is presented. It is based on using Brillouin optical time-domain analysis to probe the power distribution of the four-wave mixing generated by two continuous-wave lasers. The results obtained prove that this new method is capable of providing better performance than comparable techniques. Furthermore, sensing the variations of Brillouin gain maximum produces additional information about the fiber, such as presence of strain and concentration of GeO2.

Journal Article↗
PubMed2004

Chromatic dispersion characterization by direct instantaneous frequency measurement.

A new concept for the measurement of chromatic dispersion is presented. The group delay of the device under test is directly obtained from the instantaneous frequency of a short optical pulse after propagation in the device, itself obtained with temporal and spectral interferometry. Temporal and spectral fringe encoding provide excellent accuracy and resistance to noise, and a wide variety of dispersions can be characterized even at high insertion losses.

Journal Article↗
PubMed2004

Cascaded Raman generation in optical fibers: influence of chromatic dispersion and Rayleigh backscattering.

We study experimentally the influence of chromatic dispersion and Rayleigh backscattering on cascaded Raman generation in silica optical fibers. Effects ranging from enhanced spectral broadening of the Stokes orders to generation of higher Stokes order at unexpected wavelengths are observed. Additionally, we show that four-wave-mixing processes can quench the noisy Rayleigh lasing lines generated in power Raman amplifiers. Our observations are confirmed by numerical simulations.

Journal Article↗
PubMed2005

Analytical evaluation of chromatic dispersion in photonic crystal fibers.

We present a two-dimensional modal approach for the evaluation, in an analytical manner, of chromatic dispersion in any kind of optical fiber. It combines an iterative Fourier technique to compute the propagation constant at any fixed wavelength and an analytical procedure to calculate its derivatives. The proposed formulation takes into account the effective anisotropy of the interfaces and allows us to deal with microstructured fibers, in general, and specifically with realistic photonic crystal fibers (PCFs), including arbitrary spatial refractive-index distributions of dispersive and absorbing materials. This fast and accurate numerical technique is extremely useful for both analysis and design. We show some results of analysis of PCFs with high anisotropy, and we also describe PCFs with new dispersive properties.

Journal Article↗
PubMed2005

Specializations for chromatic and temporal signals in human visual cortex.

Neurological case studies and qualitative measurements suggest that regions within human extrastriate cortex are specialized for different perceptual functions, including color. However, there are few quantitative measurements of human extrastriate color specializations. We studied the chromatic and temporal responses in several different clusters of human visual field maps using functional magnetic resonance imaging. Contrast response functions were measured for luminance [(L + M)-cone], red-green [(L - M)-cone] and blue-yellow (S-cone) modulations at various temporal frequencies. In primary visual cortex (V1), temporal responsivities to luminance and red-green modulations are approximately constant up to 10 Hz, but responsivities to blue-yellow modulations decrease significantly. In ventral occipital cortex (VO), all colors elicit strong responses, and, for each color, low temporal frequency modulations are more effective than high temporal frequency modulations. Hence, VO represents the full range of color information but does not respond well to rapid modulations. Conversely, in human motion-selective cortex (MT+) and V3A, blue-yellow modulations elicit very weak responses, whereas luminance and red-green high temporal frequency modulations are equally or more effective than low temporal frequency modulations. Hence, these dorsal occipital regions respond well to rapid modulations, but not all color information is represented. Similar to human motion perception, MT+ and V3A respond powerfully to all temporal frequencies but only to some colors. Similar to human color perception, VO responds powerfully to all colors but only to relatively low temporal frequencies.

Brain Mapping↗
PubMed2002

Triatoma rubrovaria (Blanchard, 1843) (Hemiptera: Reduviidae) I: isoenzymatic and chromatic patterns of five populations from the State of Rio Grande do Sul, Brazil.

Triatoma rubrovaria has become the most frequently captured triatomine species since the control of T. infestans in the State of Rio Grande do Sul (RS), Brazil. In order to evaluate the genetic variability of this species, field collections were performed in four municipalities where it has been reported and distant from 75 to 322 km. Specimens were analyzed by color pattern and isoenzymes. Nine enzymatic loci were interpreted from nine enzymatic systems. The Santiago population was isolated from the others with chromatic monomorphism and diagnostic alleles at Idh and Pgm loci. The study shows the existence of, at least, two distinct populations of T. rubrovaria in RS with different phenotypic and genetic pattern.

Animals↗
PubMed2003

Diffuse reflectance spectroscopic determination of phosphate with applications of chromaticity coordinates and color temperature.

A simple and sensitive method has been developed for the determination of phosphate in the ng ml(-1) concentration range. The method utilizes the quantitative extraction of the Molybdenum Blue species of phosphate by polyurethane foam (PUF) discs prior to a diffuse reflectance spectrometric (DRS) measurement of the colored foam. A preconcentration factor of 150 has been achieved for a 100 ml sample volume. The effect of the pH of the solution on the rate of color development of the phosphomolybdenum blue has been carefully examined. A calibration graph has been established based on the Kubelka-Munk function (F(R)695), which was found to favorably correlate with the phosphate concentration. The lower detection limit (3sigma) was found to be 5 ng ml(-1), with a working analytical range of 5-70 ng ml(-1). At the same time, the chromaticity coordinates y (CIE 1931) and v' of the CIELUV (1976) were found to show good correlation with the concentration of phosphate in water. On the other hand, the calculated color temperature has been found to excellently correlate with the phosphate concentration. The different experimental parameters were thoroughly investigated.

Journal Article↗
PubMedDate not supplied

Effects of organic amendments on the reduction and phytoavailability of chromate in mineral soil.

In this study, seven organic amendments (biosolid compost, farm yard manure, fish manure, horse manure, spent mushroom, pig manure, and poultry manure) were investigated for their effects on the reduction of hexavalent chromium [chromate, Cr(VI)] in a mineral soil (Manawatu sandy soil) low in organic matter content. Addition of organic amendments enhanced the rate of reduction of Cr(VI) to Cr(III) in the soil. At the same level of total organic carbon addition, there was a significant difference in the extent of Cr(VI) reduction among the soils treated with organic amendments. There was, however, a significant positive linear relationship between the extent of Cr(VI) reduction and the amount of dissolved organic carbon in the soil. The effect of biosolid compost on the uptake of Cr(VI) from the soil, treated with various levels of Cr(VI) (0-1200 mg Cr kg(-1) soil), was examined with mustard (Brassica juncea L.) plants. Increasing addition of Cr(VI) increased Cr concentration in plants, resulting in decreased plant growth (i.e., phytotoxicity). Addition of the biosolid compost was effective in reducing the phytotoxicity of Cr(VI). The redistribution of Cr(VI) in various soil components was evaluated by a sequential fractionation scheme. In the unamended soil, the concentration of Cr was higher in the organic-bound, oxide-bound, and residual fractions than in the soluble and exchangeable fractions. Addition of organic amendments also decreased the concentration of the soluble and exchangeable fractions but especially increased the organic-bound fraction in soil.

Agaricales↗