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At least 973 records · Page 54Linked to original sources
PubMed1999

Effects of induced transverse chromatic aberration from an afocal prismatic lens on spatio-temporal sensitivity.

The goal of this study was to evaluate the effect of ophthalmic lens-induced transverse chromatic aberrations (TCA) in natural viewing conditions on spatio-temporal thresholds. Three psychophysical experiments were performed; the first isolating the spatial component, the second isolating the temporal component and the third assessing spatio-temporal combinations. Taken together, the results show that TCA has profound effects on medium to high spatial frequencies (above 1 cpd) consistent with previous studies, and that the ratio of deficit TCA/blur can be higher than 2:1 for equivalent dioptric strengths. Furthermore, the results of Experiment 1 and 3 taken together show that the relative sensitivity loss caused by TCA can differ significantly between individuals and that a possible factor for this differential loss is the overall baseline spatio-temporal sensitivity profile of the individual.

Adult↗
PubMed1981

Is accommodation colorblind? Focusing chromatic contours.

Two adjacent regions define an edge if they differ in either color or luminance. If the difference is purely chromatic, the edge is said to be isoluminant. Isoluminant contours are often perceptually unstable. Perhaps some of this instability could be explained if isoluminant contours were difficult to bring into focus. To test this hypothesis, a vernier optometer was used to measure the accuracy of steady-state accommodation for the vertical boundary of a red-green bipartite field. This edge was presented at optical distances of 0, 1.5, 3.0, and 4.5 diopters, with brightness contrasts between the two hemifields of 0% (isoluminant), 15%, 58%, and 100%. Accommodation was essentially unresponsiveness to the isoluminant edge and exhibited increasing focusing accuracy with increased brightness contrast. Control experiments replicated this finding for red-orange, green-blue, and white-white fields. These results imply that luminance contrast is a necessary stimulus for monocular accommodation. Inappropriate accommodation may be a factor contributing to the perceptual instability of isoluminant patterns.

Accommodation, Ocular↗
PubMed1979

Receptoral and postreceptoral visual processes in recovery from chromatic adaptation.

The time course of recovery from chromatic adaptation in human vision was tracked by determining the wavelength of light that appears uniquely yellow (neither red nor green) both before and after exposure to yellowish green and yellowish red adapting lights. Recovery is complete within 5 min after steady light exposure. After exposure to the alternating repeated sequence 10-sec light/10-sec dark, the initial magnitude of the aftereffect is reduced but recovery is retarded. The results are interpreted in terms of two processes located at different levels in the hierarchical organization of the visual system. One is a change in the balance of cone receptor sensitivities; the second is a shift in the equilibrium baseline between opposite-signed responses of the red/green channel at the opponent-process neural level. The baseline-shift mechanism is effective in the condition in which repeated input signals originating at the receptors are of sufficient strength to activate the system effectively. Hence, this process is revealed in the alternating adaptation condition when the receptors undergo partial recovery after each light exposure, but receptor adaptation during continued steady light exposure effectively protects the subsequent neural systems from continued strong activation.

Color↗
PubMed1995

Feedback from luminosity horizontal cells mediates depolarizing responses of chromaticity horizontal cells in the Xenopus retina.

It has been proposed that the depolarizing responses of chromaticity horizontal cells (C-HCs) to red light depend on a feedback signal from luminosity horizontal cells (L-HCs) to short-wavelength-sensitive cones in the retinas of lower vertebrates. In this regard we studied the C-HCs of the Xenopus retina. C-HCs and L-HCs were identified by physiological criteria and then injected with neurobiotin. The retina then was incubated with peanut agglutinin, which stains red-but not blue-sensitive cones. Electron microscopic examination revealed that L-HCs contact all cone classes, whereas C-HCs contact only blue-sensitive cones. Simultaneous recordings from C-HC/L-HC pairs established that when the L-HC was saturated by a steady bright red light, C-HCs alone responded to a superimposed blue stimulus. In response to red test flashes, the C-HC response was delayed by approximately 30 msec with respect to the L-HC response. Isolated HCs of both subtypes were examined by whole-cell patch clamp. Both responded to kainate with sustained inward currents and to quisqualate or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) with desensitizing currents from a negative holding potential; i.e., both have AMPA-type glutamate receptors. gamma-Aminobutyric acid or glycine opened a chloride channel in the L-HC, whereas the C-HC was unresponsive to either inhibitory amino acid. Since glycine has been shown to abolish selectively the depolarizing response of the C-HC, this finding and other pharmacological data strongly implicate the L-HC in the underlying circuit. Moreover, because the C-HC does not respond to gamma-aminobutyric acid, the neurotransmitter of the L-HC, by elimination, a feedback synapse from L-HC to blue cone is the most plausible mechanism for the creation of depolarizing responses in C-HCs.

Animals↗
PubMedDate not supplied

'Cyanidin volumetric index' and 'chromaticity coordinates ratio' to characterize red raspberry (Rubus idaeus).

The object of this work is presented in a larger research project concerning 'New indexes to evidence the nutritional quality of small fruits' in progress at the Analytical Food Research Laboratories, University of Milan. The present paper contains data that contribute to the analytical characterization of 12 varieties of red raspberry through the high-performance liquid chromatography determination of the aglycon 'cyanidin' derived from chemical hydrolysis of berries. Even more interesting results are the proposal of the 'cyanidin volumetric index', by which it is possible to compare different red raspberry varieties with higher meaningfulness. A new possible correlation between the ratio of chromaticity coordinates 'a/b' and the cyanidin content of red raspberries has been identified.

Anthocyanins↗
PubMed2003

Workplace airborne hexavalent chromium concentrations for the Painesville, Ohio, chromate production plant (1943-1971).

Hexavalent chromium [Cr(VI)] is recognized as an inhalation carcinogen, based primarily on the increased incidence of lung cancer among occupationally exposed workers. To assess the carcinogenic potency of Cr(VI), both the U.S. Environmental Protection Agency and the Occupational Safety and Health Administration have relied on data from a 1930s cohort of workers from the Painesville, Ohio, chromate production plant. However, the exposure information for this cohort has several shortcomings. In an effort to provide better exposure information, we present here recently identified historical exposure data for the Painesville workers. More than 800 measurements of airborne Cr(VI) from 23 newly identified surveys conducted from 1943 to 1971 are presented. The results indicate that the highest Cr(VI) concentrations recorded at the plant occurred in shipping (e.g., bagging of dichromate), lime and ash, and filtering operations, with maximum yearly average Cr(VI) concentrations of 8.9, 2.7, and 2.3 mg/m(3), respectively. The locker rooms, laboratory, maintenance shop, and outdoor raw liquor storage areas had the lowest average Cr(VI) air concentrations over time, with yearly average concentrations that rarely exceeded the historical and current Threshold Limit Value TLV(R) of 0.05 mgCr(VI)/m(3) (0.1 mgCrO(3)/m(3)). Concentrations generally decreased in the plant over time. The average airborne concentration of Cr(VI) in the indoor operating areas of the plant in the 1940s was 0.72 mg/m(3), that from 1957 through 1964 was 0.27 mg/m(3), and that from 1965 through 1972 was 0.039 mg/m(3). Although in some ways limited, these data are of sufficient quality to allow for exposure reconstruction for workers employed at this plant from 1940 to 1972, and to provide the basis for an improved cancer risk assessment.

Air Pollutants, Occupational↗
PubMed2004

Estimating historical occupational exposure to airborne hexavalent chromium in a chromate production plant: 1940--1972.

This article presents a retrospective exposure assessment for 493 workers who were occupationally exposed to airborne hexavalent chromium, Cr(VI), at a Painesville, Ohio, chromate production plant from 1940-1972. Exposure estimates were reconstructed using a job-exposure matrix approach that related job titles with area monitoring data from 21 industrial hygiene surveys conducted from 1943 to 1971. No personal monitoring data were collected. Specifically, airborne Cr(VI) concentration profiles for 22 areas of the plant, termed job-exposure group (JEG) areas, were constructed for three distinct time periods (1940-1949, 1950-1964, and 1965-1972), with cut points based on known major plant and process changes. Average airborne Cr(VI) concentrations were the highest for the bridge crane operators (5.5 mg/m3) prior to 1965, although only four cohort members held this job title. Airborne concentrations for the rest of the production areas of the plant ranged from 1.9 mg/m3 for packers in the 1940s to 0.012 mg/m3 for ore mill operators after 1964. For nearly all JEG areas, exposures decreased over time, particularly after 1964. For example, average airborne concentrations in production areas of the plant decreased from 0.72 mg/m3 in the 1940s to 0.27 mg/m3 from 1950 to 1964, and the average was 0.039 mg/m3 after 1964. Former workers were interviewed to determine activity patterns in the plant by job title. This information was combined with Cr(VI) monitoring data to calculate cumulative occupational exposure for each worker. Cumulative exposures ranged from 0.003 to 23 (mg/m3) x years. The highest monthly 8-hour average exposure concentration for each worker ranged from 0.003 to 4.1 mg/m3. These exposure estimates have been combined with mortality data for this cohort to assess the lung cancer risk associated with inhaled Cr(VI), and a positive dose-response relationship was observed for increases in lung cancer mortality with measures of cumulative exposure and highest monthly exposure.

Air Pollution, Indoor↗
PubMed1994

White noise analysis of a chromatic type horizontal cell in the Xenopus retina.

The dynamics of color-coded signal transmission in the light-adapted Xenopus retina were studied by a combination of white noise (Wiener) analysis and simultaneous recordings from two types of horizontal cells: chromatic-type horizontal cells (C-HCs) are hyperpolarized by blue light and depolarized by red light, whereas luminosity-type horizontal cells (L-HCs) are hyperpolarized by all wave-lengths. The retina was stimulated by two superimposed fields of red and blue light modulated by two independent white noise signals, and the resulting intracellular responses were decomposed into red and blue components (first-order kernels). The first-order kernels predict the intracellular responses with a small degree of error (3.5-9.5% in terms of mean square error) under conditions where modulated responses exceeded 30 mV in amplitude peak-to-peak, thus demonstrating that both red and blue modulation responses are linear. Moreover, there is little or no interaction between the red- and blue-evoked responses; i.e., nearly identical first-order kernels were obtained for one color whether the other color was modulated or not. In C-HCs (but not L-HCs), there were consistent differences in the dynamics of the red and blue responses. In the C-HC, the cutoff frequency of the red response was higher than for the blue (approximately 12 vs 5 Hz), and the red kernel was more bandpass than the blue. In the L-HC, kernel waveform and cutoff frequencies were similar for both colors (approximately 12 Hz or greater), and the time-to-peak of the L-HC kernel was always shorter than either the red or blue C-HC kernel. These results have implications for the mechanisms underlying color coding in the distal retina, and they further suggest that nonlinear phenomena, such as voltage-dependent conductances in HCs, do not contribute to the generation of modulation responses under the experimental conditions used here.

Animals↗
PubMed1975

Action spectra and chromatic mechanisms of cells in the median ocelli of dragonflies.

Spectral sensitivities were recorded intracellulary in median ocelli of Anax junius, Aeschnatuberculifera, and Libellulapulcella. All cells had peak sensitivities at 360 and 500 nm while UV-blue+green cells found only in Anax had a third peak sensitivity at 440 nm. Ratios of UV-to-green sensitivities varied from cell to cell in each ocellus, but no UV-only or green-only cells were recorded. Half of the cells tested had a reverse Purkinje shift: They were more sensitive in the green at low illuminations but more sensitive in the UV at high illuminations; their intensity-response curves at 370 and 520 nm crossed but became parallel for large responses. Wave-lengths 420 nm and shorter elicited a family of low intensity-response curves with one slope; wavelengths 440 nm and longer elicities a family of curves with another slope. Orange-adapting lights selectively adapted sensitivity in the green, but UV-adapting lights had little selective effect. Amounts of log-selective adaptation were proportional to log orange-adapting intensity. It is concluded that two spectral mechanisms can be recorded from each cell, possibly by coupling of UV and green cells or possibly because each cell contains two visual pigments. Selective chromatic adaptations may provide the ocellus with a kind of "authomatic color control," while the reverse Purkinje shift could extend the ocellus' sensitivity to prevailing skylight.

Animals↗
PubMed1994

Optic neuritis. Differential losses of luminance and chromatic function near a scotoma.

Visual sensitivity to achromatic and chromatic stimulus flashes was determined at sites just inside, on the boundary and just outside scotomata in 11 patients with recovered optic neuritis. The colour of the flashes and the size of the steady background on which they appeared were such that detection was more likely to be mediated by either the large-diameter, magnocellular fibres or the small-diameter, parvocellular fibres of the anterior visual pathway. The spacing of the test sites ranged from 0.5 degrees to 4 degrees visual angle, depending on the shape and location of the scotomata. The greatest differences in sensitivity were between sites just inside and just outside the scotoma and in response to achromatic stimuli more likely to involve the magnocellular fibres. This effect may be due to the size of magnocellular fibres or to their relatively smaller numbers.

Adult↗
PubMed2002

Dual roles of photosynthetic electron transport in photosystem I biogenesis: light induction of mRNAs and chromatic regulation at post-mRNA level.

Light regulation of photosystem I (PSI) biogenesis was studied in a unicellular green alga, Chlamydomonas reinhardtii. When Chlamydomonas cells were transferred from darkness to the light, mRNAs for both nuclear- and chloroplast-encoded PSI subunits were induced in concert. This light induction was inhibited by photosynthetic electron transport (PET) inhibitors, 3-(3,4 dichlorophenyl)-1,1-dimethylurea and 2,5-dibromo-3-methyl-6 isopropyl-p-benzoquinone, but not by an uncoupler, carbonyl cyanide m-chlorophenylhydrazone. This indicated that PET plays a pivotal role in the light induction of PSI subunit mRNAs, but that photophosphorylation is not necessary. When we irradiated the Chlamydomonas cells with PSI-light (695 nm) or PSII-light (644 nm), which makes the plastoquinone pool oxidative and reductive, respectively, PSII-light caused the accumulation of PSI proteins more abundantly than did PSI-light. However, there was no difference for the PSI subunit mRNA levels between these light sources. From these results, we conclude that PET plays dual roles in the regulation of PSI biogenesis in Chlamydomonas: when cells are illuminated, PET first induces the PSI subunit mRNAs irrespective of the redox state of the intersystem electron carriers, and then their redox state fine-tunes PSI biogenesis at translational and/or post-translational steps to fulfil the chromatic adaptation.

Animals↗
PubMed1996

VEP in neglect patients have longer latencies for luminance but not for chromatic patterns.

In patients with unilateral neglect, visual evoked potentials (VEP) to stimuli displayed in the left visual field are delayed compared with responses to right visual field stimuli. In the present study, 10 patients with right brain damage and neglect were tested with contrast-reversed sinusoidal gratings, modulated either in luminance or in chromaticity. For gratings of luminance contrast modulated over relatively high temporal frequencies (4-10.5 Hz), latencies of VEP were about 30 ms longer for stimuli presented to the contralesional (left) visual field than to the field ipsilateral to the lesion. For equiluminant stimuli modulated at relatively low temporal frequencies (1-4 Hz), however, latency was the same for both hemifields. As this condition activates predominately the parvocellular pathway the results are consistent with our previous suggestion that the delay observed with luminance stimuli in neglect patients results from selective disruption of the faster response of the magnocellular pathway.

Cognition Disorders↗
PubMed1999

Differential inhibition of chromatic and achromatic perception by transcranial magnetic stimulation of the human visual cortex.

The magnocellular visual pathway is devoted to low-contrast achromatic and motion perception whereas the parvocellular pathway deals with chromatic and high resolution spatial vision. To specifically separate perception mediated by these pathways we have used low-contrast Gaussian filtered black-white or coloured visual stimuli. By use of transcranial magnetic stimulation (TMS) over the visual cortex inhibition of magnocellular stimuli was achieved distinctly earlier by about 40 ms compared with parvocellular information. A nonspecific inhibition of all stimuli could be seen peaking at 75-90 ms, significantly higher for magnocellular stimuli. The particular vulnerability of magnocellular stimuli to TMS is correlated with distinct physiological properties of this pathway such as faster conduction velocity and non-linear stimulus encoding.

Analysis of Variance↗
PubMed2002

Temporal summation of magnetic response to chromatic stimulus in the human visual cortex.

The temporal-summation characteristics of the human visual cortex were investigated by recording the magnetic responses to isoluminant red-green gratings. In one condition, exposure duration (ED) of a single-pulse stimulus was varied between 16.7 ms and 200 ms, and in the other, stimulus-onset-asynchrony (SOA) of a double-pulse (presented for 16.7 ms each) stimulus was varied between 16.7 ms and 200 ms. The magnetic responses showed an initial peak at a latency of around 100 ms, the origin of which was estimated to be in the vicinity of the striate cortex. The peak amplitude increased with increasing ED and decreased with increasing SOA, showing a clear sign of temporal summation. The critical ED and SOA estimated from the peak amplitude vs. ED/SOA functions were about 50 ms. These values indicate the upper limit of temporal summation for chromatic stimuli in the human early visual cortex.

Adult↗
PubMed1984

Effect of ophthalmic filter thickness on predicted monocular dichromatic luminance and chromaticity discrimination.

The majority of ophthalmic filters, whether they be in the form of spectacles or contact lenses, are absorbance type filters. Although color vision researchers routinely provide spectrophotometric transmission profiles of filters, filter thickness is rarely specified. In this paper, colorimetric tools and volume color theory are used to show that the color of a filter as well as its physical properties are altered dramatically by changes in thickness. The effect of changes in X-Chrom filter thickness on predicted monocular dichromatic luminance and chromaticity discrimination is presented.

Color Perception↗
PubMed1986

Chromostereopsis and chromatic dispersion.

Slits placed before the eyes measured the chromatic dispersion of the eyes as a function of the separation of the slits. Prisms in front of the eyes determined the separation of the visual and null axes. The product of these separate measurements predicted the apparent depth expected with full-pupil stereoscopy. Agreement was good suggesting that chromostereopsis depends primarily on dispersion. The pupils were important modulators; a term for the Stiles-Crawford effect was not needed. Theoretical calculations to predict the effects of dispersion by slits and prisms agreed with experiments.

Color Perception↗
PubMed1986

Chromatic and luminance difference contribution to stereopsis.

Stereoacuity has been measured at various luminance difference levels and also at the isoluminance point, where the target and the background have equal luminances. The results of the experiment show that acuity at isoluminance is three times worse than the value obtained when there is maximum contrast between target and background. possible reasons for these findings were investigated. Results of control experiments show that neither chromatic aberration nor defective accomodative response could be responsible for this finding.

Color↗
PubMed1992

Glenn A. Fry Award Lecture 1991: perceptual manifestations of imperfect optics in the human eye: attempts to correct for ocular chromatic aberration.

The profession of optometry has been very successful in providing optical corrections for spherocylindrical refractive errors. In this paper, I examine one attempt to improve retinal image quality beyond that afforded by a standard refractive correction. Ocular chromatic aberration is one of the factors that prevent retinal image quality from reaching the upper limit set by the wave nature of light. It can be subdivided into three primary aberrations (wavelength-dependent differences in imaging plane, image position, and image size). We have been able to measure all three of these using psychophysical techniques. Although attempts to provide an optical correction for wavelength-dependent refractive errors have been optically successful, they have failed to improve vision. Several possible explanations are given for this failure.

Awards and Prizes↗