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PubMed2005

The binocular combination of chromatic contrast.

How is chromatic contrast combined binocularly? One index of binocularity is the binocular contrast summation ratio (BCSR), which is the improvement in contrast sensitivity with binocular rather than monocular presentation. Simmons and Kingdom (1998, Vision Research 38 1063-1071) noted that BCSRs with some red-green isoluminant stimuli were suggestive of full linear summation. This suggestion was investigated further in four subjects by measuring binocular and monocular contrast thresholds for the detection of 0.5 cycle deg(1) isoluminant (red-green) and isochromatic (yellow-black) Gabor patches. These Gabor patches had either vertically or horizontally oriented carrier gratings and were either dichoptically in phase (same coloured bars in binocular correspondence) or in dichoptic anti-phase (opposite coloured bars in binocular correspondence). Full linear summation would be indicated by BCSRs of 2 for the in-phase and close to 0 for the anti-phase conditions. Mean BCSRs at isoluminance were 1.93 and 0.90, respectively, for the in-phase and anti-phase stimuli with horizontal carriers, the former being consistent with full linear summation, but the latter not. Despite these results, BCSRs obtained with isoluminant and isochromatic stimuli under similar conditions were not statistically distinguishable from each other, although there was a tendency for summation at isoluminance with in-phase stimuli to be higher and anti-phase stimuli to be lower. These data fall short of demonstrating full linear summation of chromatic contrast between the eyes under all presentation conditions, but they do indicate that there are strong binocular interactions at red-green isoluminance, which are similar to, and possibly even stronger than, those obtained with luminance stimuli.

Color Perception↗
PubMed1960

Electron microscopy of retinal photoreceptors. The use of chromation following formaldehyde fixation as a complementary technique to osmium tetroxide fixation.

The fine structure of the cone and rod outer segments of the toad was studied under the electron microscope after fixation in osmium tetroxide and fixation in formaldehyde followed by chromation. In the OsO(4)-fixed specimens, the rod outer segment appears to be built of a stack of lobulated flattened sacs, each of which is made of two membranes of about 40 A separated by an innerspace of about 30 A. The distance between the rod sacs is about 50 A. The sacs in the cone outer segment are originated by the folding of a continuous membrane. The thickness of the membranes and width of the spaces between the cone sacs is the same as in rod, but the sac innerspace is slightly narrower in the cone ( approximately 20 A). After fixation in formaldehyde and chromation, two different dense lines (l(1) and l(2)) separated by spaces of less density appear. One of the lines, l(1), has a thickness of 70 A and is less dense than the other, l(2), which is 30 A thick. The correlation of the patterns obtained with both fixatives is considered and two possible interpretations are given. The possibility that l(2) is related to a soluble phospholipid component is discussed. It is suggested that the outer segments have a paracrystallin organization similar to that found in myelin.

Animals↗
PubMed2001

Electrostatic correction of the chromatic and of the spherical aberration of charged-particle lenses (part I).

A feasible electrostatic corrector (ECO) is outlined and the principle of the electrostatic correction is elucidated by means of a light-optical analogue. The ECO compensates for the chromatic and the spherical aberration of charged-particle lenses and reduces the resolution limit of a special LVSEM (low-voltage scanning electron microscope) from 6 nm to 1.4 nm. The geometry of the electrodes of the corrector is optimized with respect to the chromatic correction, the maximum strength of the electric field, and the residual higher-order aberrations which limit the resolution. In addition the stability criteria of the electric power supplies are discussed in detail.

Journal Article↗
PubMed2003

Electrostatic mirror objective with eliminated spherical and axial chromatic aberrations.

Computational formulae for the coefficients of the third-order spherical aberration and the second-order axial chromatic aberration are presented for an axially symmetric electrostatic electron mirror. A technique for eliminating the high-order derivatives of the potential axial distribution in mirror systems from the integrands is described. Conditions for elimination of spherical and axial chromatic aberrations, either separately or simultaneously, are found for a three-electrode axially symmetric mirror composed of coaxial cylinders of the same diameter. A principal scheme of the transmission electron microscope, where an electrostatic electron mirror serves as its objective, is presented.

Journal Article↗
PubMed2001

Chromatic variation of the abundance of PSII complexes observed with the red alga Prophyridium cruentum.

Chromatic regulation of photosystem stoichiometry in cyanophytes, green algae and probably vascular plants is achieved by regulation of the abundance of PSI in response to thylakoid electron transport state at least under our experimental conditions [cf. Fujita (1997) Photosyn. Res. 53: 83]. However, variation of not only PSI but also PSII, in reverse of each other, is characteristic of the stoichiometry regulation in red algae and some of marine cyanophytes. Our previous study with the red alga Porphyridium cruentum has revealed that PSII is inactivated by 50% upon a light shift from the light absorbed by Chl a, PSI light, to that mainly absorbed by phycobilisomes (PBS), PSII light [Fujita (1999) Plant Cell Physiol. 40: 924]. To evaluate the contribution of the photoinactivation to the chromatic variation of PSII, variation of the abundance of PSI, PSII and PBS, together with the fluorescence parameter and the activity of PSII, was followed after a light shift from PSI light to PSII light. Upon a light shift to PSII light, PSII, determined as Cyt b(559) per PBS, decreased rapidly, following the photoinactivation, down to the level a half of that before the light shift, and remained constant. Since the increase in PBS was not significant during this period, a rapid decrease of PSII/PBS led us to tentatively conclude that the degradation of PSII is a main cause for variation of the abundance of PSII. Photoinactivation of PSII, and also decrease in Cyt b(559), was accelerated, but only slightly, by the addition of chloramphenicol (CAP) at a moderate concentration while CAP at the same concentration significantly suppressed the increment of PSI determined as P700. A selective effect of CAP supports the above conclusion.

Chloramphenicol↗
PubMed2002

Luminance and chromatic contrast sensitivity in dyslexia: the magnocellular deficit hypothesis revisited.

The hypothesis of a magnocellular channel deficit in dyslexia was tested. Subjects were 10-year-old dyslexics and normal readers. Psychophysical thresholds for luminance and chromatic contrasts were estimated using black and white and red and green sinusoidal gratings of various spatial frequencies, presented in static and dynamic conditions (drift and reversal). Significant group differences were found for luminance contrast, with a higher sensitivity in dyslexics. No group differences were obtained for chromatic contrast. High luminance contrast sensitivity correlated with low reading and writing skills. The typical finding of an increase contrast sensitivity to low spatial frequency gratings, due to their dynamic presentations, was absent in dyslexics. The results provide support for the magnocellular deficit hypothesis. The pattern of this deficit, however, is much more complex than that emerging from previous research.

Child↗
PubMed1976

The state of accommodation during the measurement of axial chromatic aberration of the eye.

The axial chromatic aberration of the eye was determined as in previous studies by finding the lens required to eliminate or minimize blur of the fixation target when it is illuminated by a series of monochromatic sources. Objective measurement of the refraction of the eyes during this procedure showed that accommodation remained constant throughout. Hence, the conclusion that the accommodative state does not in this case respond automatically to changes in the vergence of light falling in the retinal plane. It is suggested that some other stimulus, perhaps perceived distance, predominates. Further, the least amount of plus lens required to blur a near target was about the same for white and red but much less for blue. This substantiates an earlier finding that the eye accommodates for near only sufficiently to place the blue end of the chromatic interval on the retina.

Accommodation, Ocular↗
PubMed1982

Chromatic and luminosity processing in retinal disease.

Color vision loss can be an early sign of eye disease; in many retinal disorders the loss precedes any change in visual acuity. Noninvasive psychophysical methods allow factoring out of preretinal, receptoral, and postreceptoral (neural) components of the color vision change. A loss of chromatic but not achromatic sensitivity occurs for diabetics; the loss is selective for pathways subserving blue-sensitive photoreceptors. Both chromatic and achromatic pathways are altered in glaucoma and senile macular degeneration; the most marked change in central serous choroidopathy is loss of sensitivity somewhere in the blue-sensitive cone pathway. There is evidence that the pathways subserved by blue-sensitive cones have anatomically and physiologically different properties from those served by other receptor types, and they appear particularly vulnerable to disturbances of retinal integrity.

Adult↗
PubMed1983

Chromatic adaptation and the blue-green side of the color triangle.

This is an attempt to show with chromatic adaptation data that the line tangent to the spectrum locus at about 460 nm and passing through the tritanopic confusion point represents the blue-green leg of the triangle of fundamental colors. The inability to use chromatic adaptation to produce colors in the green corner of the triangle has been explained in terms of a zone theory of color vision.

Adaptation, Ocular↗
PubMed1985

Toward the design of an optimal filter for enhancement of dichromat monocular chromatic discrimination.

Based on Farnsworth's theories of color confusion and discrimination, "rose-colored" or "pink" filters such as the X-Chrom material of 0.16 mm in thickness or the Kodak Wratten no. 30 filter act as monocular chromatic enhancers for red/green color deficients by virtue of colorimetrically lengthening color space in a direction perpendicular to the dichromatic axis of color confusion. Using derived formulas for dichromat colorimetry, a set of representative isoluminant-isochroma spectral hues, and interactive computer graphic colorimetric tools, it is possible to determine an optimum spectrophotometric filter curve. Such a filter has minimal middle wavelength transmission and is optimal in terms of providing maximal monocular chromatic clues for discriminative tasks involving confused colors.

Color Perception↗
PubMed1985

Influence of age on chromatic aberration of the human eye.

We examined the difference in refraction when using portions of the visible spectrum from 437 to 671 nm, and report on the influence of age on the chromatic aberration in one subject 20 years after previous measurements. Changes in refraction were measured with a Rodenstock refractometer which has an exit pupil of 3 mm, and nine narrow band interference filters. Eight subjects were examined under cycloplegia and with dilated pupils. Results confirm previous reports that the magnitude of chromatic aberration decreases with age.

Adult↗
PubMed1989

Appearance of color matchings under various chromatic adaptation conditions.

Three chromatic reference stimuli (red, green, and blue) seen under five adaptation conditions (red, green, blue, achromatic, and darkness) were appearance-matched to subsequent color stimuli seen under achromatic conditions. The experimental results have been used to verify the opponent-color color-vision model and also to check the validity of some of the published transformation equations for predicting the tristimulus values of colors matched to a reference stimulus seen under different chromatic-adaptation conditions. It is concluded that the extant transformation laws are not sufficiently general to be applied to all adaptation conditions.

Adaptation, Ocular↗
PubMed1990

Visual neural performance for chromatic displays.

Various subjective procedures have been used in the past to examine the visual disorders and ocular symptoms often associated with prolonged usage of Video Display Units (VDU's). We examined visual neural performance for VDU stimuli which differed in size (14, 21, or 28 min arc), chromaticity (white, red, green, or blue), and retinal clarity, by transient pattern visually evoked potentials (t-p VEP's). Such information could prove useful in the design of electro-optical display systems that optimize visual neural performance and minimize ocular fatigue. Stimuli consisted of "monochromatic" (W, R, G, or Blue on black) and "multichromatic" (Blue/R, Blue/G, and R/G) checkerboards with brightness-matched chromatic elements displayed on a high resolution RGB monitor at 40 and 80 cm. The ambient lighting level was 54 lux. Group-averaged amplitudes and implicit times of t-p VEP's from 20 visually normal subjects indicated a differential neural response across target colors and three experimentally induced levels of blur. For monochromatic stimuli, the Blue/Black targets elicited t-p VEP's with the lowest amplitude, longest implicit time, and greatest sensitivity to optical defocus. Increasing the target element size reduced the VEP sensitivity to defocus across all colors except red. For multichromatic targets, Red/Green targets elicited the most vulnerable t-p VEP's. Several optical and neural explanations are given to explain these results. Implications for VDU designs are presented.

Accommodation, Ocular↗
PubMed1991

Effect of chromatic aberration on isoluminance stereothreshold.

The results of a previous experiment have shown that stereothreshold varies as a function of the luminance difference between a target and its background. When the luminance of the target is the same as that of the background (isoluminance) the stereothreshold was elevated by a factor of 3 as compared with a situation where there was maximum luminance difference between the target and the background. Generally, if the target and the background have the same color, stereothreshold at a particular luminance difference level was lower (stereoacuity better) than if the target and the background have different colors. This indicates a possible effect of chromatic aberration on stereothreshold which could be responsible for the 3-fold increase in threshold at isoluminance. This possibility has been investigated in this experiment. The results show that even though stereothresholds are sensitive to chromatic aberration this factor cannot explain the elevated stereothreshold at isoluminance.

Color Perception↗
PubMed2002

Enhancement of contrast sensitivity and losses of chromatic discrimination with tinted lenses.

PURPOSE: Tinted lenses for everyday use should not impair visual acuity and contrast sensitivity or cause radical changes in color perception. The main aim of this study was to compare the performance in contrast detection and color discrimination tasks of a set of tinted lenses with that of gray filters of equal luminance under D65. METHODS: The contrast sensitivity functions of 10 observers were measured using sinusoidal gratings of mean luminance of 13 cd/m2 by the adjustment method. Color discrimination thresholds from white (x = 0.313, y = 0.330), green (x = 0.346, y = 0.407), and blue (x = 0.280, y = 0.253) were measured along 12 directions in the CIE-1931 xy diagram with and without lenses. RESULTS: Green, brown, and blue filters did not cause significant changes in contrast sensitivity compared with a gray filter of equal luminance, although chromatic discrimination was disturbed. Yellow and orange filters improved achromatic contrast at certain spatial frequencies, but impaired chromatic discrimination. CONCLUSIONS: Compared with gray filters of the same luminance, yellow filters may be useful when enhancement of low achromatic contrasts is desirable, although overall brightness decrements may occur. Nevertheless, these lenses cause tritan-like defects with discrimination losses increasing with the cutoff wavelength.

Accommodation, Ocular↗
PubMed1996

Effect of the nitrogen source on phycobiliprotein synthesis and cell reserves in a chromatically adapting filamentous cyanobacterium.

Cyanobacteria can utilize nitrate or ammonium as a source of fixed nitrogen for cell growth. In the filamentous Calothrix sp. strain PCC 7601, these two sources of nitrogen differently influenced the phycobiliprotein composition of the phycobilisomes, the major light-harvesting antennae. When compared to nitrate, growth in the presence of ammonium resulted in intracellular steady-state levels 35% lower for phycoerythrin and 46% higher for phycocyanin. Besides these differences in cell pigmentation, a rapid but transient accumulation of cyanophycin granule polypeptide occurred in ammonium-grown cells, while these macromolecules were not detected in cells grown with nitrate. In contrast, glycogen reserves displayed a dynamic pattern of accumulation and disappearance during cell growth which varied only slightly with the nitrogen source. The observed changes in cell pigmentation are reminiscent of the phenomenon of complementary chromatic adaptation, in which green and red wavelengths promote the syntheses of phycoerythrin and phycocyanin-2, respectively. As in complementary chromatic adaptation, the regulation of synthesis of phycoerythrin and phycocyanin-2 by the nitrogen source occurred mainly at the mRNA level. Moreover, the transcriptional start sites for the expression of the cpeBA and the cpc2 operons, which respectively encode the two subunits of phycoerythrin and phycocyanin-2, were the same in cells grown in nitrate or ammonium, and identical to those in green- and red-light-grown cells. The results of this study suggest that acclimation to the spectral light quality and to the nitrogen source share some common regulatory elements.

Bacterial Proteins↗
PubMed1980

Effects of Chromatic Adaptation on the Photochemical Apparatus of Photosynthesis in Porphyridium cruentum.

Cells of Porphyridium cruentum were grown in different colors of light which would be absorbed primarily by chlorophyll (Chl) (red and blue light) or by the phycobilisomes (green or two intensities of cool-white fluorescent light), and samples of these cells were frozen to -196 C for measurements of absorption and fluorescence emission spectra. Cells grown in the high intensity white light had least of all of the photosynthetic pigments, a higher ratio of carotenoid/Chl, but essentially the same ratio of phycobilin to Chl as cells grown in the low intensity white light. The ratio of photosystem II (PSII) to photosystem I (PSI) pigments was affected by light quality; the ratios of phycobilin to Chl and of short wavelength (PSII) Chl to long wavelength (PSI) Chl were both greater in the cells grown in red or blue light.Light quality also exerted a strong influence on the structural and functional organization of the photochemical apparatus. Data on the relative optical cross-sections of PSI and PSII as a function of excitation wavelength indicate that cells grown in light absorbed primarily by the phycobilisomes package a large fraction of their Chl into PSI (PSI Chl/PSII Chl approximately 20), whereas cells grown in light absorbed by Chl distribute their Chl much more equitably (PSI Chl/PSII Chl approximately 1.5). In both types of cells the phycobilisomes transfer their excitation energy predominantly to PSII Chl with little or no direct energy transfer to PSI, but the yield of energy transfer from PSII to PSI is approximately twice as large for cells grown in the phycobilin wavelengths of light. These differences in functional organization and energy distribution account for the physiological expressions of chromatic adaptation. The effects of chromatic adaptation on O(2) evolution can be predicted from our calculations of energy distribution between PSI and PSII for cells grown in the different colors of light.

Journal Article↗
PubMed1982

Growth and Chromatic Adaptation of Nostoc sp. Strain MAC and the Pigment Mutant R-MAC.

A spontaneous, stable, pigmentation mutant of Nostoc sp. strain MAC was isolated. Under various growth conditions, this mutant, R-MAC, had similar phycoerythrin contents (relative to allophycocyanin) but significantly lower phycocyanin contents (relative to allophycocyanin) than the parent strain. In saturating white light, the mutant grew more slowly than the parent strain. In nonsaturating red light, MAC grew with a shorter generation time than the mutant; however, R-MAC grew more quickly in nonsaturating green light.When the parental and mutant strains were grown in green light, the phycoerythrin contents, relative to allophycocyanin, were significantly higher than the phycoerythrin contents of cells grown in red light. For both strains, the relative phycocyanin contents were only slightly higher for cells grown in red light than for cells grown in green light. These changes characterize both MAC and R-MAC as belonging to chromatic adaptation group II: phycoerythrin synthesis alone photocontrolled.A comparative analysis of the phycobilisomes, isolated from cultures of MAC and R-MAC grown in both red and green light, was performed by polyacrylamide gel electrophoresis in the presence of 8.0 molar urea or sodium dodecyl sulfate. Consistent with the assignment of MAC and R-MAC to chromatic adaptation group II, no evidence for the synthesis of red light-inducible phycocyanin subunits was found in either strain. Phycobilisomes isolated from MAC and R-MAC contained linker polypeptides with relative molecular masses of 95, 34.5, 34, 32, and 29 kilodaltons. When grown in red light, phycobilisomes of the mutant R-MAC appeared to contain a slightly higher amount of the 32-kilodalton linker polypeptide than did the phycobilisomes isolated from the parental strain under the same conditions. The 34.5-kilodalton linker polypeptide was totally absent from phycobilisomes isolated from cells of either MAC or R-MAC grown in green light.

Journal Article↗