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At least 217 records · Page 12Linked to original sources
PubMed2004

Chromatic assimilation: spread light or neural mechanism?

Chromatic assimilation is the shift in color appearance of a test field toward the appearance of nearby light. Possible explanations of chromatic assimilation include wavelength independent spread light, wavelength-dependent chromatic aberration and neural summation. This study evaluated these explanations by measuring chromatic assimilation from a concentric-ring pattern into an equal-energy-white background, as a function of the inducing rings' width, separation, chromaticity and luminance. The measurements showed, in the s direction, that assimilation was observed with different inducing-ring widths and separations when the inducing luminance was lower or higher than the test luminance. In general, the thinner the inducing rings and the smaller their separation, the stronger the assimilation in s. In the l direction, either assimilation or contrast was observed, depending on the ring width, separation and luminance. Overall, the measured assimilation could not be accounted for by the joint contributions from wavelength-independent spread light and wavelength-dependent chromatic aberration. Spatial averaging of neural signals explained the assimilation in s reasonably well, but there were clear deviations from neural spatial averaging for the l direction.

Adult↗
PubMed2002

Individual differences in chromatic (red/green) contrast sensitivity are constrained by the relative number of L- versus M-cones in the eye.

Many previous studies have shown that the relative number of long-wavelength-selective (L) versus medium-wavelength-selective (M) cones in the eye influences spectral sensitivity revealed perceptually. Here, we hypothesize that the L:M cone ratio should also influence red/green chromatic contrast sensitivity. To test this, in each subject we derived an estimate of L:M ratio based on her red/green equiluminance settings (obtained with heterochromatic flicker photometry), and measured both red/green chromatic and luminance contrast sensitivity at different spatial and temporal frequencies. Factor analysis was applied to the data in order to reveal covariance between conditions. As expected, chromatic and luminance contrast sensitivity were found to be independent of one another, and no relationship was observed between L:M ratio and luminance contrast sensitivity. However, a significant relationship was observed between L:M ratio and chromatic contrast sensitivity, wherein subjects possessing the most symmetrical L:M cone ratios (i.e., near 1:1) appear to possess the relatively greatest chromatic contrast sensitivity. This relationship can be accounted for by a simple model based on the notion of random L- and M-cone inputs to the center and surround receptive fields of chromatic (L-M) mechanisms.

Adolescent↗
PubMed2003

Chromatic induction and the layout of colours within a complex scene.

A target's apparent colour is influenced by the colours in its surrounding. If the surrounding consists of a single coloured surface, the influence is a shift 'away' from the surface's colour. If the surface is more than 1 degrees from the target area the shift is very small. If there are many surfaces, then not only the average luminance and chromaticity of the surfaces matters, but also the chromatic variability. It is not yet clear whether it makes any difference where the chromatic variability is within the scene, so we constructed stimuli in which the chromatic variability was restricted to certain regions. We found that it made very little difference where the chromatic variability was located. The extent to which the average colour of nearby surfaces influences the apparent colour of the target seems to depend on the average chromatic variability of the whole scene.

Color Perception↗
PubMed1998

On the binocular summation of chromatic contrast.

The binocular summation of chromatic contrast was investigated under a variety of stimulus conditions. Binocular and monocular contrast detection thresholds were measured using 0.5 cpd Gabor patches. It was found that, using stimuli which contained combinations of chromatic and luminance contrast, binocular detection could take place independently in luminance-contrast- and chromatic-contrast-sensitive mechanisms. It was also found that, with chromatic stimuli, levels of binocular summation were above those expected from probability summation between the eyes, and thus showed evidence for binocular neural summation within chromatic detection mechanisms. The implications of these results for (a) the binocularity of chromatic detection mechanisms, and (b) the suggested link between stereopsis and binocular neural summation, are discussed.

Color Perception↗
PubMed1998

Temporal analysis of the chromatic flash VEP--separate colour and luminance contrast components.

Temporal analysis of the chromatic flash visual evoked potential (VEP) was studied in human subjects with normal and anomalous colour vision using a deterministic pseudo-random binary stimulus (VERIS). Five experiments were carried out on four normal subjects investigating heterochromatic red-green exchange and single colour/achromatic (either red/grey or green/grey) exchange over a wide range of luminance ratios for the two stimuli, the effects of lowered mean luminance on the chromatic VEP and the effects of colour desaturation at constant mean luminance and constant luminance contrast. Finally, the performance of three dichromats, a protanope and two deuteranopes, on heterochromatic exchange VEP and on colour desaturation were investigated. In contrast to the chromatic electroretinogram, which shows great symmetry with respect to luminance ratio on opposite sides of the isoluminant point, the chromatic VEP demonstrated a distinct asymmetry when the colours exchanged included red. On the red side of isoluminance (red more luminant than green), a wave with longer latency and altered waveform became dominant. The effects of green stimulation were indistinguishable from those of achromatic stimulation at the same luminance contrast over the whole range of chromatic contrast and for all levels of desaturation studied. Desaturation of red with constant luminance contrast (desaturated red/grey stimulation) resulted in a systematic alteration in the evoked waveform. Subtraction of the achromatic first- and second-order responses from responses recorded in the red desaturation series resulted in remarkably uniform waveforms, with peak amplitudes growing linearly with saturation. The absence of interaction between achromatic and coloured components for all (including the most intense colour) stimulus parameters used suggests that the generators of these components are separate. Recordings from the dichromats showed that the contrast response minimum shifted from the point of photopic isoluminance to the point of zero cone contrast (at the silent substitution point) for the remaining cone type. The waveforms recorded with a series of luminance ratios were much simpler than those recorded from trichromats and symmetrical with respect to their isoluminant points. Despite the indication of the presence of L cones of apparently normal spectral sensitivity in the deuteranopes (on the basis of flicker photometry), there was no evidence for a red-sensitive component in the desaturation or heterochromatic stimulation series. The results are discussed in terms of the possibility of separate generation of chromatic and achromatic contributions to the VEP.

Color Perception↗
PubMed1999

The effect of spatial frequency on chromatic and achromatic steady-state visual evoked potentials.

OBJECTIVE: Little is known about the physiological properties of the major components of steady-state visual evoked potentials (VEPs). Based on the hypothesis that isoluminant color and high contrast pattern differentially activate the parvo- and magnocellular pathways, we studied difference in spatial frequency function between chromatic and achromatic VEPs to sinusoidal gratings. METHODS: Steady-state VEPs to isoluminant chromatic (red-green) and high contrast (90%) achromatic (black-white) sinusoidal gratings with nine spatial frequencies (0.5 to 8.0 cycles/degrees (cpd)) at 4 Hz (8 reversals/s) were recorded in 13 normal subjects. VEPs were Fourier analyzed to obtain phase and amplitude of the second (2F) and fourth (4F) harmonic responses. RESULTS: The 2F amplitude of chromatic VEPs decreased above 4.0 cpd in a low-pass function while that of achromatic VEPs showed a band-pass function with a peak at 4.0 cpd. The 4F amplitude of chromatic VEPs was not affected significantly by spatial frequency whereas that of achromatic VEPs exhibited a high-pass function. The phases of 2F and 4F showed a non-monotonic function of spatial frequency in both chromatic and achromatic stimuli with peaks at middle spatial frequencies. CONCLUSION: Chromatic and achromatic visual stimuli differently affected 2F and 4F components, which thus suggests that 2F and 4F components are generated from different neuronal subgroups largely in the parvocellular pathway.

Adolescent↗
PubMedDate not supplied

Pattern-reversal electroretinogram in response to chromatic stimuli: I. Humans.

We have studied the steady-state PERG in human subjects in response to red-green plaid patterns modulated either in luminance or in chromaticity or both. By varying the relative luminance of the red and green components, a value could be obtained at which the PERG amplitude was either minimum or locally maximum. This always occurred at equiluminance, as measured by standard psychophysical techniques. PERG amplitude and phase were measured as a function of spatial and temporal frequency of sinusoidal contrast reversal. In both space and time, the response to chromatic patterns was low-pass, while that to luminance was band-pass, and extended to higher spatial and temporal frequencies. The phase of the PERG to chromatic stimuli was systematically lagged compared with that to luminance stimuli, by an amount corresponding to about 20 ms under our experimental conditions. The variation of phase with temporal frequency suggested an apparent latency of about 67 ms for color contrast compared with 47 ms for luminance. These estimates were confirmed with separate measurements of transient PERGs to abrupt contrast reversal. For both luminance and chromatic stimuli, the amplitude of PERGs increases with increasing stimulus contrast. By summing vectorially the responses to appropriate luminance and chromatic contrasts, we were able to predict with accuracy the response as a function of color ratio (ratio of red to total luminance). The above findings all agree with those reported in the accompanying paper for the monkey PERG (Morrone et al., 1994), and indicate that the differences in response latency and integration time of luminance and chromatic stimuli observed by psychophysical and VEP techniques may arise at least in part from the properties of retinal mechanisms.

Color Perception↗
PubMedDate not supplied

Chromatic assimilation unaffected by perceived depth of inducing light.

Chromatic assimilation is a shift toward the color of nearby light. Several studies conclude that a neural process contributes to assimilation but the neural locus remains in question. Some studies posit a peripheral process, such as retinal receptive-field organization, while others claim the neural mechanism follows depth perception, figure/ground segregation, or perceptual grouping. The experiments here tested whether assimilation depends on a neural process that follows stereoscopic depth perception. By introducing binocular disparity, the test field judged in color was made to appear in a different depth plane than the light that induced assimilation. The chromaticity and spatial frequency of the inducing light, and the chromaticity of the test light, were varied. Chromatic assimilation was found with all inducing-light sizes and chromaticities, but the magnitude of assimilation did not depend on the perceived relative depth planes of the test and inducing fields. We found no evidence to support the view that chromatic assimilation depends on a neural process that follows binocular combination of the two eyes' signals.

Calibration↗
PubMed1979

Chromatic information processing.

The investigation of visual processing mediated solely by chromatic information requires conditions preventing a subject's use of the luminance differences normally accompanying a chromatic change. In Experiment 1, which involved a discriminative reaction time (RT) task, chromatic and white stimuli of the same luminance were presented on a dimmer achromatic background. Subjects were instructed to respond only to the chromatic stimuli. RT was slowest at 570 nm and somewhat faster to short wavelengths than to long wavelengths. In Experiment 2, which compared two discriminative RT tasks, RT was faster when subjects responded to color than when they responded to white. Experiments 3 and 4 demonstrated that a brighter white surround decreased the perceived brightness of chromatic stimuli as well as their perceptual similarity to white, but did not affect RT. The results are discussed in terms of the response strength of the chromatic processing channel.

Color Perception↗
PubMed1998

Separate colour-opponent mechanisms underlie the detection and discrimination of moving chromatic targets.

Current opinion holds that human colour vision is mediated primarily via a colour-opponent pathway that carries information about both wavelength and luminance contrast (type I). However, some authors argue that chromatic sensitivity may be limited by a different geniculostriate pathway, which carries information about wavelength alone (type II). We provide psychophysical evidence that both pathways may contribute to the perception of moving, chromatic targets in humans, depending on the nature of the visual discrimination. In experiment 1, we show that adaptation to drifting, red-green stimuli causes reductions in contrast sensitivity for both the detection and direction discrimination of moving chromatic targets. Importantly, the effects of adaptation are not directionally specific. In experiment 2, we show that adaptation to luminance gratings results in reduced sensitivity for the direction discrimination, but not the detection of moving chromatic targets. We suggest that sensitivity for the direction discrimination of chromatic targets is limited by a colour-opponent pathway that also conveys luminance-contrast information, whereas the detection of such targets is limited by a pathway with access to colour information alone. The properties of these pathways are consistent with the known properties of type-I and type-II neurons of the primate parvocellular lateral geniculate nucleus and their cortical projections. These findings may explain the known differences between detection and direction discrimination thresholds for chromatic targets moving at low to moderate velocities.

Adaptation, Physiological↗
PubMed2005

Alternative use of chromatic and achromatic cues in a hawkmoth.

The diurnal hummingbird hawkmoth Macroglossum stellatarum can learn the achromatic (intensity-related) and the chromatic (wavelength-related) aspect of a spectral colour. Free-flying moths learn to discriminate two colours differing in the chromatic aspect of colour fast and with high precision. In contrast, they learn the discrimination of two stimuli differing in the achromatic aspect more slowly and less reliably. When trained to use the chromatic aspect, they disregard the achromatic aspect, and when trained to use the achromatic aspect, they disregard the chromatic aspect, at least to some degree. In a conflicting situation, hummingbird hawkmoths clearly rely on the chromatic aspect of colour. Generally, the moths pay attention to the most reliable cue that allows them to discriminate colours in the learning situation. This is usually the chromatic aspect of the colour but they can learn to attend to the achromatic aspect instead. There is no evidence for relative colour learning, i.e. moths do not learn to choose the longer or shorter of two wavelengths, but it is possible that they learn to choose the darker or brighter shade of a colour, and thereby its relative intensities.

Animals↗
PubMed2004

A new class of chromatic filters for color image processing. Theory and applications.

This paper advances a new framework for chromatic filtering of color images. The chromatic content of a color image is encoded in the CIE u'v' chromaticity coordinates whereas the achromatic content is encoded as CIE Y tristimulus value. Within the u'v' chromaticity diagram, colors are added according to the well-known center of gravity law of additive color mixtures, which is generalized here into a nonlinear filtering scheme for processing the two chromatic signals u' and v'. The achromatic channel Y can be processed with traditional filtering schemes, either linear or nonlinear, depending on the specific task at hand. The most interesting characteristics of the new filtering scheme are: 1) the elimination of color smearing effects along edges between bright and dark areas; 2) the possibility of processing chromatic components in a noniterative fashion through linear convolution operations; and 3) the consequent amenability to computationally efficient implementations with fast Fourier transform. The paper includes several examples with both synthetic and real images where the performance of the new filtering method is compared with that of other color image processing algorithms.

Algorithms↗
PubMed1991

Chromatic Discrimination in a Cortically Colour Blind Observer.

We tested the ability of a subject with cerebral achromatopsia to discriminate between colours and to detect chromatic borders. He was unable to identify colours or to arrange them in an orderly series or choose the odd colour out of an array or even to pick out a colour embedded in an array of greys. Nevertheless, he could select the odd colour when the colours were contiguous, even when they were isoluminant, and could discriminate an ordered from a disordered chromatic series as long as the colours in each row abutted one other. His verbal replies showed that he did so by detecting an edge between two stimuli that were, to him, perceptually identical. Introducing a narrow isoluminant grey stripe between adjacent colours abolished or greatly impaired this ability. As long as isoluminant colours were contiguous the patient could identify the orientation of the chromatic borders. Photopic spectral sensitivity showed evidence both for activity of three cone channels and for chromatic opponent processing, indicating that postreceptoral chromatic processing is occurring despite the absence of any conscious awareness of colour. The results indicate that both parvocellular colour opponent and magnocellular broad-band channels are active and that the cortical brain damage has selectively disrupted the appreciation of colour but not the ability to detect even isoluminant chromatic borders, which would be invisible to a retinal achromat. The subject's performance on non-colour tasks involving the discrimination of shape, texture, greyness and position was excellent. His disorder is therefore not like that of macaque monkeys in which cortical area V4 has been removed, and which are much more severely impaired at discriminating shape than colour.

Journal Article↗
PubMed1984

Chromatic mechanisms in lateral geniculate nucleus of macaque.

This paper introduces a new technique for the analysis of the chromatic properties of neurones, and applies it to cells in the lateral geniculate nucleus (l.g.n.) of macaque. The method exploits the fact that for any cell that combines linearly the signals from cones there is a restricted set of lights to which it is equally sensitive, and whose members can be exchanged for one another without evoking a response. Stimuli are represented in a three-dimensional space defined by an axis along which only luminance varies, without change in chromaticity, a 'constant B' axis along which chromaticity varies without changing the excitation of blue-sensitive (B) cones, a 'constant R & G' axis along which chromaticity varies without change in the excitation of red-sensitive (R) or green-sensitive (G) cones. The orthogonal axes intersect at a white point. The isoluminant plane defined by the intersection of the 'constant B' and 'constant R & G' axes contains lights that vary only in chromaticity. In polar coordinates the constant B axis is assigned the azimuth 0-180 deg, and the constant R & G axis the azimuth 90-270 deg. Luminance is expressed as elevation above or below the isoluminant plane (-90 to +90 deg). For any cell that combines cone signals linearly, there is one plane in this space, passing through the white point, that contains all lights that can be exchanged silently. The position of this 'null plane' provides the 'signature' of the cell, and is specified by its azimuth (the direction in which it intersects the isoluminant plane of the stimulus space) and its elevation (its angle of inclination to the isoluminant plane). A colour television receiver was used to produce sinusoidal gratings whose chromaticity and luminance could be modulated along any vector passing through the white point in the space described. The spatial and temporal frequencies of modulation could be varied over a large range. When stimulated by patterns of low spatial and low temporal frequency, two groups of cells in the parvocellular laminae of the l.g.n. were distinguished by the locations of their null planes. The null planes of the larger group were narrowly distributed about an azimuth of 92.6 deg and more broadly about an elevation of 51.5 deg, which suggests that they receive opposed, but not equally balanced, inputs from only R and G cones. These we call R-G cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1988

Anisotropy in the chromatic channel: a horizontal-vertical effect.

We compared the chromatic contrast thresholds of drifting (2Hz) red-green sine-wave gratings of horizontal, vertical, and two oblique orientations at three spatial frequencies (2, 4, 8 cpd). Luminance contrast thresholds for yellow-black gratings were also obtained. The classic oblique effect was found for high spatial frequency luminance and chromatic stimuli. For chromatic thresholds, a significant difference was found between the horizontal and vertical thresholds of all observers. One observer was retested with her head tilted 45 deg and demonstrated that the anisotropy was specific to retinal coordinates. These results give evidence for orientation selectivity in the chromatic channel which is at least partially independent of that in the luminance channel. We estimated the degree of lateral chromatic aberration in our observers' eyes and discuss the possible contribution of this aberration to the horizontal-vertical difference in the chromatic channel.

Analysis of Variance↗
PubMed2002

Development, maturation, and aging of chromatic visual pathways: VEP results.

It has been argued that the development and aging of the different achromatic and chromatic visual pathways may proceed independently. We review here the evidence for such independent changes with particular emphasis on electrophysiological results. Changes in chromatic and achromatic visual processing throughout the life span were studied using visual evoked potentials (VEPs). VEPs were recorded in response to the presentation of patterns designed to preferentially stimulate achromatic and S-(L+M) and (L-M) chromatic mechanisms. Recordings were made in subjects aged 1 week to 90+ years. Longitudinal measurements were obtained from several infants and cross-sectional measurements were obtained from infants and older subjects. Responses to achromatic reversing patterns at low spatial frequencies appeared early and changed rapidly. Latencies of the achromatic reversal response decreased to mature values within the first 12-15 weeks of life. Responses to chromatic pattern onsets, however, appeared later (L-M: 4 weeks; S: 6-8 weeks) and changed continuously throughout the first year of life. Chromatic waveforms from 1 year to puberty appeared inverted relative to the adult waveform. The waveforms did not appear adultlike until about 12-14 years of age. The latencies of the major negative component of the adult response reached a minimum around 17-18 years of age. Throughout the remainder of the life span, VEP latencies steadily increased and amplitudes slightly decreased. Latencies of responses to chromatic pattern onsets increased more rapidly than latencies to moderate contrast achromatic pattern reversals.

Adolescent↗
PubMed2006

The chromodomain protein, Chromator, interacts with JIL-1 kinase and regulates the structure of Drosophila polytene chromosomes.

In this study we have generated two new hypomorphic Chro alleles and analyzed the consequences of reduced Chromator protein function on polytene chromosome structure. We show that in Chro(71)/Chro(612) mutants the polytene chromosome arms were coiled and compacted with a disruption and misalignment of band and interband regions and with numerous ectopic contacts connecting non-homologous regions. Furthermore, we demonstrate that Chromator co-localizes with the JIL-1 kinase at polytene interband regions and that the two proteins interact within the same protein complex. That both proteins are necessary and may function together is supported by the finding that a concomitant reduction in JIL-1 and Chromator function synergistically reduces viability during development. Overlay assays and deletion construct analysis suggested that the interaction between JIL-1 and Chromator is direct and that it is mediated by sequences in the C-terminal domain of Chromator and by the acidic region within the C-terminal domain of JIL-1. Taken together these findings indicate that Chromator and JIL-1 interact in an interband-specific complex that functions to establish or maintain polytene chromosome structure in Drosophila.

Alleles↗
PubMed2003

Effects of various storage conditions and alterations of antioxidant contents on chromatic aberration of hydroquinone ointment.

Ointments of the skin depigmentation agent hydroquinone (HQ) have been prepared by extemporaneous nonsterile compounding in our hospital. The HQ ointments were highly effective in the treatment of various types of skin pigmentations; however, various problems have emerged including chromatic aberration of the ointments, a relatively large variability of efficacy, and mild side effects. Chromatic aberration is expected to induce non-compliance, and this may be the reason for the relatively large variability in efficacy. In this paper, the effects of various storage conditions on the chromatic aberration and HQ content of HQ ointments were evaluated, and it was suggested that the chromatic aberration was accelerated by exposure to high temperature, air and light, although these had no effect on the HQ content. In addition, various types of HQ ointments were prepared to find a formulation to minimize chromatic aberration, and it was found that the concentrations of antioxidants, Na(2)SO(3) and L(+)-ascorbic acid (AsA), seemed to be too high, and that the protective effect of AsA on chromatic aberration was mainly due to its acidifying effect.

Antioxidants↗