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PubMed2002

The natural center of chromaticity space is not always achromatic: a new look at color induction.

Although current theories of color vision differ in many respects, they all assume the existence of a uniquely defined neutral point in chromaticity space. It generally is assumed that this point satisfies several criteria simultaneously. One of these criteria is that it is perceived as achromatic. A further criterion shared by most theories is the structural assumption that lines in chromaticity space of constant hue converge on the neutral point. The basic assumption that these two criteria coincide is clearly true for isolated spots of light presented in darkness, and it usually is taken for granted that this coincidence generalizes to more complex visual stimuli. Here, we show that this is not the case. Our experiments with infields in chromatic surrounds revealed that the point in chromaticity space that appears gray is clearly different from the point on which lines of constant hue converge. A plausible interpretation of this apparently paradoxical finding in terms of color scission is proposed.

Adaptation, Physiological↗
PubMed2005

Spectrophotometric detection of iodide and chromic (III) in urine after oxidation to iodine and chromate (VI).

Tests for oxidizing adulterants in urine are a continuing challenge to the drug-testing program. Iodine was found to destroy morphine and 6-acetylmorphine almost immediately. The effects were less evident on 11 -nor-delta9-tetrahydrocannabinol-9-carboxylic acid (THC-acid). When the urine solution was tested for iodine by a chromogenic substrate, 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS), no iodine was detected. Masking drug and adulterant simultaneously made iodine a preferred oxidizing adulterant for drug abusers. In this study, the reduced iodide was oxidized by sodium nitrite to iodine. The excess nitrite was decomposed by sulfamic acid and the iodine was detected by ABTS. Linearity was 12.7 to 635 mg/L (0.1 to 5 mmol/L, y = 0.9966x + 0.0016, R2 = 1.0000). Precisions (coefficient of variation) were within +/- 4.1% and quantitative accuracies were within 97% of expected values (n=5). Chromate, iodate, periodate, and persulfate interfered with the method. To alleviate the problem, the positive specimens were tested again by an iodine-specific method. After oxidation, the samples were treated with sodium azide and ammonium thiocyanate. In presence of thiocyanate, the azide reduced iodine to iodide almost immediately, and the solutions showed negative response to ABTS. The results were compared with that of a control group tested without thiocyanate. When iodine was present, the ratios of thiocyanate to control were less than 6%. Chromate was also found to destroy THC-acid in urine, and during storage most of the chromate changed to chromic (III). In this study, chromic was oxidized to chromate by hydrogen peroxide and sodium hydroxide and detected by 1,5-diphenylcarbazide. Linearity was 5.2 to 156 mg/L (0.1 to 3.0 mmol/L, y = 1.0285x - 0.0034, R2 = 0.9998). Precisions were within +/- 8.5% and quantitative accuracies were within 92% of expected values (n=5). The test was not interfered by other oxidizing agents. Both iodide and chromic oxidation methods showed urine backgrounds less than 1.27 and 0.52 mg/L, respectively (< 0.01 mmol/L). It indicated that a response more than 10 times of the background could be considered as oxidant contamination or adulteration of urine specimens.

Chromium↗
PubMed1978

Effect of a yellow ocular filter on chromatic aberration: the fish eye as an example.

Reduction of chromatic aberration is one of the suggested functions of yellow ocular filters. This possibility was tested by (1) determining the effect of an artificial yellow filter on the chromatic aberration of the eye of two fish species having no obvious ocular filters and (2) comparing in vivo chromatic aberration with that of the excised lens in a species of fish having a yellow cornea. The results indicate that yellow filters reduce the measured chromatic aberration of the eye by more than one-third.

Animals↗
PubMed1988

Effect of chromatic dispersion of a lens on visual acuity.

Materials with a high refractive index have a considerable level of chromatic dispersion which, around the periphery of corrective lenses, may affect the wearer's visual acuity. By measuring visual acuity through prisms of increasing chromatic power we have established the relation between chromatic power and visual acuity. The maximum chromatic dispersion of materials useable in ophthalmic optics can be deduced from these results.

Adult↗
PubMed2001

Aging of the chromatic onset visual evoked potential.

PURPOSE: To examine changes in the chromatic onset visual evoked potential (VEP) as a function of aging. METHODS: VEP's were measured in response to chromatic sinusoidal gratings (1.0 and 0.5 cpd), selectively chosen to modulate the L-M channel and S - (L+M) channel and presented in onset-offset mode. Responses to achromatic gratings presented in a reversal mode were also measured. Twenty subjects were tested, ranging in age from 21 to 93 years. RESULTS: Unlike changes observed earlier in life, the general shape of the chromatic onset wave-form changed little with age; however, latencies increased significantly as a function of age. Amplitude changes revealed a decreasing trend that was not statistically significant. There was little change in the achromatic responses with age. CONCLUSIONS: Our results demonstrate a systematic slowing of the chromatic onset VEP with age. The gradual nature of the latency changes and the lack of dramatic and complex wave-form shape changes may allow development of age-based normative data for use in clinical settings.

Adult↗
PubMed1993

The tuning of human photopigments may minimize red-green chromatic signals in natural conditions.

Humans and other Old World primates (Catarrhini) share very similar L (long-wavelength, 'red') and M (medium-wavelength, 'green') cone photopigment spectral sensitivities, with peaks at around 563 nm and 535 nm, respectively. Changes of single amino acid residues at critical sites in photopigment opsins can alter this peak tuning. Moreover, the photopigment alleles and spectral sensitivities of human populations are polymorphic, so there is potential for adaptive change or genetic drift. The manifest lack of variability suggests that the tuning of the L and M photopigments has adaptive significance, but the reason for this conservatism is unclear. To assess how natural spectral reflectances may have influenced pigment tuning, we have measured the chromatic (i.e. difference) signals available in natural scenes, and estimated how these signals would vary if spectral sensitivities of the pigments moved to longer or to shorter wavelengths. The size of the chromatic signal is, predictably, dependent principally on the spectral separation of the photopigments, but in addition we find that for a fixed separation there is a marked dependence on the specific peak tuning of the photopigments. Indeed, the naturally occurring L and M cone peaks may be set at a pair of points on the spectrum that on average minimizes the 'L-M' (i.e. red-green) chromatic signal. This somewhat paradoxical observation supports the view that red-green vision has evolved for a specific task, such as finding fruit, whilst minimizing interference by the chromatic signal in luminance vision to which both L and M cones contribute.

Adaptation, Physiological↗
PubMed1981

The photoregulated expression of multiple phycocyanin species. A general mechanism for the control of phycocyanin synthesis in chromatically adapting cyanobacteria.

The regulation of phycocyanin synthesis in response to growth in chromatic illumination was studied in 69 strains of cyanobacteria. Cyanobacteria (24 of 31 strains examined), which chromatically adapt by modulating the synthesis of both phycocyanin and phycoerythrin, controlled phycocyanin synthesis through the differential, photoregulated expression of two phycocyanin species (two alpha-type and two beta-type subunits). For these strains the expression of one pair of phycocyanin subunits was constitutive (i.e. irrespective of the light wavelength in which the cells were grown); the expression of the second pair of phycocyanin subunits occurred specifically during growth in red light. Two facultatively heterotrophic cyanobacteria, Calothrix strains 7101 and 7601, synthesized both the constitutive and the inducible pairs of phycocyanin subunits when grown heterotrophically in the dark after transfer from either red or green light. No evidence for the existence of multiple and/or photoregulated phycocyanin species was found for cyanobacteria (25 strains) incapable of chromatic adaptation, nor for cyanobacteria (13 strains) which chromatically adapt by modulating the synthesis of phycoerythrin alone.

Acclimatization↗
PubMed2006

Retinotopic distribution of chromatic responses in human primary visual cortex.

In non-human primates at least three anatomically and functionally distinct channels convey signals from the retina to the primary visual cortex (V1). Two of these channels, the parvocellular and the koniocellular, are sensitive to chromatic contrasts and form the basis of color vision. In humans, common phylogenetic history with other primates and psychophysical experiments suggest identical retinocortical mechanisms but separate evaluation of the distinct anatomical channels has been difficult because signals are already combined in V1. We studied the spatial distribution of activation to chromatic stimuli along the two opponent chromatic axes in human V1 with multifocal functional magnetic resonance imaging. The signal strength was quantified from three experiments with stimuli up to 20 degrees eccentricity. The hypothesis was that, although the parvo- and koniocellular signals are mixed in V1, distinct distributions of signal strength would be evident. We found that whereas different conditions activated the same areas of cortex, indicating that they have identical magnification factors, the responses to red/green stimulation were stronger close to the fovea whereas the blue/yellow responses were much less diminished with increasing eccentricity. Both chromatic axes showed saturating contrast response functions. Our measure directly from human V1 is in line with earlier psychophysical studies suggesting relatively stronger parvocellular channel representation close to the fovea, and more uniform distribution of the koniocellular and achromatic channels. In addition, our study presents a way to rapidly quantify retinotopic signal transmission in distinct retinocortical pathways of individual subjects.

Adult↗
PubMed1999

Dynamics of primate P retinal ganglion cells: responses to chromatic and achromatic stimuli.

1. The majority of primate retinal ganglion cells (RGCs) project to the parvocellular layers of the lateral geniculate nucleus (LGN). These P cells play a central role in early visual processing. 2. An improved method of systems analysis has allowed us to explore the dynamics of the colour-opponent subregions of P-cell receptive fields with a single chromatic stimulus. The data show that the centre and surround subregions of the P-cell receptive field have similar temporal responses, but the surround is slightly delayed. The centre and surround demonstrate a large degree of chromatic selectivity. 3. The responses of the centre and surround subregions were fitted with a linear model and the model was used to predict the responses of P cells to new chromatic and achromatic stimuli. Although linear models predict the chromatic responses well, simple linear combinations of centre and surround responses fail to predict P-cell responses to achromatic stimuli. 4. The temporal responses of the different subpopulations of P cells, such as ON/OFF or L-centre/M-centre were not significantly different.

Animals↗
PubMed1988

The effect of optical defocus on the accommodative accuracy for chromatic displays.

Blur, probably arising from focusing inaccuracies, has been cited in ergonomic and vision literature as one of the causes of visual stress and decreased task efficiency in operation of video display units (VDUs). With the advent of colour coding in electro-optical displays, the need for a detailed quantification of focusing responses to chromatic stimuli is particularly important because of the influence of the chromatic aberration present in ocular optics on the focusing response of the eye. In this regard, we used a computer-aided laser speckle optometer system to measure the accommodative responses of 20 visually normal subjects, to brightness-matched monochromatic and multichromatic stimuli displayed on a high-resolution RGB monitor. We also investigated the effect of target size, target clarity, and viewing distance on the response accuracy of the focusing system of the eye. Our results show that while the accommodative responses are systematically influenced by target chromaticity, they are relatively independent of target size. There was no evidence of any anomalous focusing responses resulting from either target chromaticity or defocus that could account for the asthenopia frequently reported by VDU users. Furthermore, moderate levels of optical defocus did not drive accommodation into any visible hunting pattern for optimal focus or towards its tonic resting position. Implications of these findings in display designs are presented.

Accommodation, Ocular↗
PubMed1992

Binocular chromatic rivalry and single vision.

Depth perception is known to be impaired for chromatic equiluminant patterns. To investigate this phenomenon I have compared the effects of binocularly presented stimuli in the form of stripes, which contain only luminance information with similarly presented stimuli which contain only chromatic information. Observations of the reported percepts for the two conditions demonstrate that mechanisms of colour vision can impede stereopsis based on binocular fusion when the chromatic stripes are at, or even near, equiluminance, provided that their saturation is high. This observation is consistent with inhibitory interactions within the chromatic-sensitive neuronal groupings in the visual cortex.

Color Perception↗
PubMed2005

Chromatic and achromatic transient VEPs in adults with Down syndrome.

Oculo-visual abnormalities such as strabismus and high refractive error are common in people with Down syndrome, and account in large part for reduced visual function in this group. In the absence of such abnormalities, however, some spatial vision deficits persist, probably reflecting abnormal function of the neural visual pathway in this population. In addition, colour vision abnormalities are reportedly common in subjects with Down syndrome. We recorded transient visual evoked potentials in response to black-white and chromatic stimuli, in seven subjects with Down syndrome and 33 controls, to investigate function of the visual pathways underpinning spatial and chromatic visual function in Down syndrome. Our findings indicate, in agreement with previous studies, that retino-striate achromatic and chromatic processing in Down syndrome are abnormal. We find, however, that abnormal retino-striate processing of chromatic signals in this group may not give rise to colour vision deficits detected by the Colour Vision Test Made Easy or the City University test.

Adult↗
PubMed1994

Chromate-allergic patients challenged orally with potassium dichromate.

30 patients who had positive patch tests to potassium dichromate participated in a placebo-controlled oral challenge with 2.5 mg chromium given as potassium dichromate. 17 reacted to chromate but not to the placebo, 2 reacted to both chromate and the placebo, and 4 reacted to the placebo but not chromate. 7 patients had no reaction. A specific reaction to chromate was most common among patients with dermatitis of the hands and/or feet.

Administration, Oral↗
PubMed1969

Wave-length discrimination at the foveal chromatic threshold.

1. Wave-length discrimination has been measured at the chromatic threshold to test the assumption that when a monochromatic stimulus is reduced to its chromatic threshold, the activity of a single cone mechanism may be isolated.2. It has been shown that, at the chromatic threshold, all wave-lengths between 559 and at least 665 mmu are significantly confused. Similar ranges of confusion extend from 457 to 570 mmu and from 485 to at least 419 mmu.3. These results have been shown to be consistent with previous measures of the spectral sensitivities of the cone pigments. They support the view that the hue of a monochromatic stimulus at its chromatic threshold may be dependent on light absorbed by only one cone pigment.

Color Perception↗
PubMed1978

Influences of cones upon chromatic- and luminosity-type horizontal cells in pikeperch retinas.

1. The spectral sensitivity and spatial organization of cones and horizontal cells have been analysed by intracellular recording in pikeperch retinas. 2. The vast majority of cone recordings were obtained from orange-sensitive cones. They have an action spectrum which peaks at about 605 nm. Recordings from several green-sensitive cones have also been obtained. 3. The results of action spectrum measurements and spectral screening tests indicate that the vast majority of luminosity-type horizontal cells receive predominant input from the orange-sensitive cones. 4. Chromatic-type horizontal cells were recorded at more proximal levels of the retina than luminosity-type cells and were the classic red-depolarizing, green hyperpolarizing (R/G) type. 5. The action spectra of the depolarizing and hyperpolarizing responses of chromatic horizontal cells peak at about 650 and 530 nm, respectively. When the depolarizing mechanism is selectively depressed by a red background field, the action spectrum of the hyperpolarizing mechanism shows an enhanced sensitivity, peaks at 530--540 nm, and may approximate the action spectrum of the green-sensitive cones. 6. Small red fields evoke depolarizing responses from chromatic-type horizontal cells but do not seem to significantly activate the depolarizing surround mechanism of cones. 7. These and other results suggest that the colour-opponent properties of the chromatic-type horizontal cells are not fundamentally dependent upon feed-back to cones but rather originate from antagonistic interactions generated in post-receptor networks.

Action Potentials↗
PubMed1987

Thresholds to chromatic spots of cells in the macaque geniculate nucleus as compared to detection sensitivity in man.

1. The relation between wavelength and psychophysical threshold for chromatic spots on a white background provides evidence for the existence of chromatic channels in the primate visual system. To find the physiological substrate of this task, we compared increment thresholds of different cell types in the macaque lateral geniculate nucleus with human psychophysical thresholds to the same stimuli, using two spot sizes, 4 and 0.4 deg. 2. At different wavelengths, different opponent cell classes in the parvocellular layers of the nucleus were most sensitive, so that at long wavelengths (greater than 600 nm) red on-centre cells were most sensitive, while at short wavelengths (less than 500 nm) S-cone, blue on-centre cells were most sensitive, from 500 to about 550 nm green on-centre cells being most sensitive. A rare cell type with inhibition from S-cones was most sensitive at about 570 nm, although its maximum contrast increment sensitivity was poor compared with that of other cell types. Variation in strength of cone opponency caused a considerable range in threshold in each of the opponent cell classes of the parvocellular layers. 3. On- and off-centre cells from the magnocellular layers were more sensitive than opponent cells to white and yellow spots (as is the case with achromatic gratings). 4. With different wavelengths and spot sizes, the most sensitive cells found approached (to within 0.1-0.3 log units) human psychophysical sensitivity, suggesting that the most sensitive cells available may underlie detection. 5. Measurements of psychophysical chromatic discrimination thresholds, both with nearly monochromatic spots and with spots of differing saturation (purity), support this hypothesis. When magnocellular cell sensitivity corresponded to psychophysical threshold, a suprathreshold stimulus, capable of activating opponent cells, was required for chromatic discrimination.

Animals↗
PubMed1992

Longitudinal chromatic aberration and emmetropization: results from the chicken eye.

1. Due to the chromatic dispersion of the ocular media, the focal length of the optics of the eye is about 3 diopters longer for red light than for blue light. Because emmetropization in the chicken (Gallus domesticus) does not require colour cues and operates properly in monochromatic light, one can, therefore, expect that chickens raised in red light become more myopic (with longer eyes) than chicks raised in short wavelength light. Prior to conducting this experiment, we matched the brightness of both light conditions by means of flicker electroretinograms such that equiluminance was obtained for the chickens. 2. Unexpectedly, refractive development was not different from controls in white light for either red or near-ultraviolet light. 3. We tested whether the visual mechanisms guiding refractive development were still sensitive to defocus under both illuminations by treating the chicks with spectacle lenses. 4. Similar to a previous experiment in white light, the growth of the eye in red light also changed such that it compensated for the imposed defocus. It failed to do so, however, in near-ultraviolet light. 5. A histological analysis of the sampling intervals for the ultraviolet receptor system revealed that its spatial resolving power was too low to detect the defocus imposed by the lenses, whereas the long wavelength receptors provided sufficiently good visual acuity. 6. The results show that, during emmetropization, the chicken eye elegantly bypasses the problem of multiple chromatic focal planes by having a low sensitivity to defocus in the blue end of the spectrum. Because the chromatic dispersion function is steep in the blue range but flat at the red end of the spectrum, the remaining chromatic defocus in the spectral range of high visual acuity is low and may match the depth of field of the eye.

Animals↗
PubMed2006

Neural adjustments to chromatic blur.

The perception of blur in images can be strongly affected by prior adaptation to blurry images or by spatial induction from blurred surrounds. These contextual effects may play a role in calibrating visual responses for the spatial structure of luminance variations in images. We asked whether similar adjustments might also calibrate the visual system for spatial variations in color. Observers adjusted the amplitude spectra of luminance or chromatic images until they appeared correctly focused, and repeated these measurements either before or after adaptation to blurred or sharpened images or in the presence of blurred or sharpened surrounds. Prior adaptation induced large and distinct changes in perceived focus for both luminance and chromatic patterns, suggesting that luminance and chromatic mechanisms are both able to adjust to changes in the level of blur. However, judgments of focus were more variable for color, and unlike luminance there was little effect of surrounding spatial context on perceived blur. In additional measurements we explored the effects of adaptation on threshold contrast sensitivity for luminance and color. Adaptation to filtered noise with a 1/f spectrum characteristic of natural images strongly and selectively elevated thresholds at low spatial frequencies for both luminance and color, thus transforming the chromatic contrast sensitivity function from lowpass to nearly bandpass. These threshold changes were found to reflect interactions between different spatial scales that bias sensitivity against the lowest spatial grain in the image, and may reflect adaptation to different stimulus attributes than the attributes underlying judgments of image focus. Our results suggest that spatial sensitivity for variations in color can be strongly shaped by adaptation to the spatial structure of the stimulus, but point to dissociations in these visual adjustments both between luminance and color and different measures of spatial sensitivity.

Adaptation, Ocular↗