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PubMed1997

The development of chromatic and achromatic contrast sensitivity in infancy as tested with the sweep VEP.

Swept-contrast visual evoked potential (VEP) techniques were used to measure the development of contrast sensitivity functions (CSFs) for achromatic and red/green isoluminant chromatic gratings. Subjects were infants of 8, 14, 20 and 32 weeks of age, and adults. Stimuli were 20 deg, 0.3-4 cyc/deg sinusoidal gratings, counterphased at 6 Hz and modulated through white. Achromatic and chromatic CSFs for all ages could be fit simultaneously with a double exponential equation of a common, lowpass shape. Both achromatic and chromatic CSFs exhibited developmental shifts in both sensitivity and spatial scale. From 8 weeks to adulthood, sensitivity increased by 0.64 log units for achromatic gratings and by 0.91 log units for chromatic gratings, yielding an 0.27 log unit larger sensitivity change for chromatic than for achromatic stimuli. Spatial scale shifts were closely similar across achromatic and chromatic CSFs, and were consistent with the factor of about four predicted on the basis of changes in foveal receptor packing density and eye size. The question of uniform vs differential loss of sensitivity for chromatic vs achromatic stimuli at fixed spatial frequencies is discussed.

Adult↗
PubMedDate not supplied

Pattern-reversal electroretinogram in response to chromatic stimuli: II. Monkey.

We have recorded steady-state PERGs from five macaque monkeys in response to red-green plaid patterns reversed sinusoidally in contrast. The patterns had either a pure luminance contrast (red-black, green-black, yellow-black), pure red-green color contrast, or a variable amount of luminance and color contrast. By varying the relative luminance of the red-to-total luminance (color ratio) of red-green patterns, a value could be obtained at which the PERG amplitude was either minimum or locally maximum, and the phase was most lagged. This value was very similar to that producing equiluminance in human observers, and was considered to be equiluminance for the monkey. The phase of the PERG to chromatic stimulus was systematically lagged compared with that of luminance stimuli, by an amount corresponding to about 10-20 ms under our experimental conditions. The variation of phase with temporal frequency suggested an apparent latency of about 80 ms for color contrast compared with 63 ms for luminance. These estimates were confirmed with separate measurements of transient PERGs to abrupt contrast reversal. As a function of temporal frequency, the chromatic PERG function was clearly low-pass with a cutoff around 15 Hz, whereas that to luminance was double-peaked and extended to higher temporal frequencies, around 30 Hz. For both luminance and chromatic stimuli, the amplitude of PERGs increases with increasing stimulus contrast. By summing vectorially the luminance and chromatic responses of appropriate contrasts, we were able to predict with accuracy the response as a function of color ratio. In two monkeys, the optic chiasm was sectioned sagittally causing total degeneration of ganglion cells in the nasal retina, without affecting the temporal retina (verified by histology). In these animals, there was a strong response to both luminance and chromatic patterns in the temporal retinae, but none to either type of pattern in the nasal retinae, suggesting that the PERG to both luminance and chromatic stimuli arises from the inner-retinal layers. Electrophysiological studies suggest that the PERG to chromatic stimuli is probably associated with the activity of P-cells. P-cells may also make a major contribution to the PERG of luminance stimuli, although M-cells may also participate. The above findings on normal monkeys all agree with those reported in the accompanying paper for humans (Morrone et al., 1994), so similar conclusions can probably be extended to human PERG.

Animals↗
PubMedDate not supplied

Three-dimensional shape perception from chromatic orientation flows.

The role of chromatic information in 3-D shape perception is controversial. We resolve this controversy by showing that chromatic orientation flows are sufficient for accurate perception of 3-D shape. Chromatic flows required less cone contrast to convey shape than did achromatic flows, thus ruling out luminance artifacts as a problem. Luminance artifacts were also ruled out by a protanope's inability to see 3-D shape from chromatic flows. Since chromatic orientation flows can only be extracted from retinal images by neurons that are responsive to color modulations and selective for orientation, the psychophysical results also resolve the controversy over the existence of such neurons. In addition, we show that identification of 3-D shapes from chromatic flows can be masked by luminance modulations, indicating that it is subserved by orientation-tuned neurons sensitive to both chromatic and luminance modulations.

Color↗
PubMedDate not supplied

Temporal-chromatic interactions in LGN P-cells.

We studied the interaction between the chromatic and temporal properties of parvocellular (P) neurons in the lateral geniculate nucleus (LGN) of macaque monkeys. We measured the amplitudes and phases of responses to stimulation by spatially uniform fields modulated sinusoidally about a white point in a three-dimensional color space, at a range of temporal frequencies between 1 and 25 Hz. Below about 4 Hz, temporal frequency had relatively little effect on chromatic tuning. At higher frequencies chromatic opponency was weakened in almost all cells. The complex interactions between temporal and chromatic properties are represented by a linear filter model that describes response amplitude and phase as a function of temporal frequency and direction in color space along which stimuli are modulated. The model stipulates the cone inputs to center and surround, their temporal properties, and the linear combination of center and surround signals. It predicts the amplitudes and phases of responses of P-cells, and the change of chromatic properties with temporal frequency. We used the model to investigate whether or not the chromatic signature of the surround in a red-green cell could be estimated from the change in the cell's chromatic properties with temporal frequency. Our findings could be equally well described by mixed cone surrounds as by pure cone surrounds, and we conclude that, with regard to temporal properties, there is no benefit to be gained by segregating cone classes in center and surround.

Animals↗
PubMed1995

Automated achromatic contrast and chromatic discrimination sensitivity testing in dysthyroid optic neuropathy.

Our experience of patients with dysthyroid eye disease shows that normal chromatic discrimination sensitivity precludes the diagnosis of optic nerve compression (31 patients), and that clinically confirmed optic nerve compression is invariably associated with decreased chromatic discrimination sensitivity thresholds (8 patients). Dysthyroid patients enrolled in this study underwent automated achromatic contrast and chromatic discrimination sensitivity testing on presentation, with repeat assessment of those patients suspected of developing optic nerve compression. If chromatic discrimination sensitivity was decreased, patients were followed up more frequently. If abnormal chromatic discrimination sensitivity was accompanied by a relative afferent pupillary defect (RAPD) or decreased Snellen visual activity (VA), then optic nerve decompression was performed. The automated chromatic discrimination sensitivity test described represents a quick, reproducible and cheap clinical test which we feel is of value in assessing patients with dysthyroid eye disease. We suggest that sequential chromatic discrimination sensitivity assessment is a sensitive and effective way of monitoring patients at risk of dysthyroid optic neuropathy.

Adult↗
PubMed1991

Effect of ocular chromatic aberration on monocular visual performance.

This brief review outlines the theory of ocular chromatic aberration and describes the three primary forms in which the aberration appears: chromatic difference of focus, chromatic difference of magnification, and chromatic difference of position. Our central theme is that all three aspects of chromatic aberration have as their common basis the chromatic dispersion of light. The magnitude of each form of the aberration is related to the others by simple linear formulas in which a key parameter is the location of the pupil relative to the nodal point of the eye. The way in which retinal image quality is affected by chromatic aberration is described and we assess the impact of the aberration on visual performance.

Color Perception↗
PubMed1992

Chromatic aberration and optical power of a diffractive bifocal contact lens.

Although diffractive contact lenses have been well documented in theory, no definitive experimental data have been reported which confirm that the near image is in fact created by diffraction rather than by refraction. We have tested the diffraction hypothesis for one type of diffractive contact lens (the Hydron Echelon bifocal) experimentally by measuring its longitudinal chromatic aberration in isolation and when worn on the eye. The basis of this test is that, according to theory, diffractive lenses should have chromatic aberration which is opposite in sign to that measured for the eye. Objective measurements of chromatic aberration were made with a focimeter when the lens was in a wet cell. Subjective measurements were made with a Badal optometer when the lens was worn on the eye. Four control experiments were conducted to provide baseline measurements of the eye's chromatic aberration, against which we compared the results obtained for the diffractive contact lens. The data were also compared with conventional measurements of refractive error obtained by standard subjective techniques and by an automated infrared refractor. Our results showed that the longitudinal chromatic aberration of the diffraction image of the Echelon bifocal lens was about one-half that obtained under the four control conditions: for the naked eye, for the nondiffraction image of the Echelon lens, or for either image of a refractive bifocal contact lens (CIBA Bisoft). These results are consistent with the theoretical prediction that the negative chromatic aberration of a diffractive contact lens should partially cancel the positive chromatic aberration of the human eye.(ABSTRACT TRUNCATED AT 250 WORDS)

Color Perception↗
PubMed1985

The contrast sensitivity of human colour vision to red-green and blue-yellow chromatic gratings.

A method of producing red-green and blue-yellow sinusoidal chromatic gratings is used which permits the correction of all chromatic aberrations. A quantitative criterion is adopted to choose the intensity match of the two colours in the stimulus: this is the intensity ratio at which contrast sensitivity for the chromatic grating differs most from the contrast sensitivity for a monochromatic luminance grating. Results show that this intensity match varies with spatial frequency and does not necessarily correspond to a luminance match between the colours. Contrast sensitivities to the chromatic gratings at the criterion intensity match are measured as a function of spatial frequency, using field sizes ranging from 2 to 23 deg. Both blue-yellow and red-green contrast sensitivity functions have similar low-pass characteristics, with no low-frequency attenuation even at low frequencies below 0.1 cycles/deg. These functions indicate that the limiting acuities based on red-green and blue-yellow colour discriminations are similar at 11 or 12 cycles/deg. Comparisons between contrast sensitivity functions for the chromatic and monochromatic gratings are made at the same mean luminances. Results show that, at low spatial frequencies below 0.5 cycles/deg, contrast sensitivity is greater to the chromatic gratings, consisting of two monochromatic gratings added in antiphase, than to either monochromatic grating alone. Above 0.5 cycles/deg, contrast sensitivity is greater to monochromatic than to chromatic gratings.

Color Perception↗
PubMed2003

Vibrio harveyi nitroreductase is also a chromate reductase.

The chromate reductase purified from Pseudomonas ambigua was found to be homologous with several nitroreductases. Escherichia coli DH5alpha and Vibrio harveyi KCTC 2720 nitroreductases were chosen for the present study, and their chromate-reducing activities were determined. A fusion between glutathione S-transferase (GST) and E. coli DH5alpha NfsA (GST-EcNfsA), a fusion between GST and E. coli DH5alpha NfsB (GST-EcNfsB), and a fusion between GST and V. harveyi KCTC 2720 NfsA (GST-VhNfsA) were prepared for their overproduction and easy purification. GST-EcNfsA, GST-EcNFsB, and GST-VhNFsA efficiently reduced nitrofurazone and 2,4,6-trinitrotoluene (TNT) as their nitro substrates. The K(m) values for GST-EcNfsA, GST-EcNfsB, and GST-VhNfsA for chromate reduction were 11.8, 23.5, and 5.4 micro M, respectively. The V(max) values for GST-EcNfsA, GST-EcNfsB, and GST-VhNfsA were 3.8, 3.9, and 10.7 nmol/min/mg of protein, respectively. GST-VhNfsA was the most effective of the three chromate reductases, as determined by each V(max)/K(m) value. The optimal temperatures of GST-EcNfsA, GST-EcNfsB, and GST-VhNfsA for chromate reduction were 55, 30, and 30 degrees C, respectively. Thus, it is confirmed that nitroreductase can also act as a chromate reductase. Nitroreductases may be used in chromate remediation. GST-EcNfsA, GST-EcNfsB, and GST-VhNfsA have a molecular mass of 50 kDa and exist as a monomer in solution. Thin-layer chromatography showed that GST-EcNfsA, GST-EcNfsB, and GST-VhNfsA contain FMN as a cofactor. GST-VhNfsA reduced Cr(VI) to Cr(III). Cr(III) was much less toxic to E. coli than Cr(VI).

Cloning, Molecular↗
PubMed2003

Topography of the chromatic pattern-onset VEP.

The chromatic pattern-onset VEP has been used successfully as a sensitive and objective technique to determine congenital and acquired color vision deficiency. It also has been applied to characterize development, maturation and aging of the chromatic visual pathways. Here we determine the topographic components of the full-field VEP using the multifocal technique. Recordings were made with the VERIS system that extracts topographic VEPs using a pseudorandom stimulus sequence. Chromatic pattern stimuli were presented in an onset-offset temporal sequence, with colors modulated along different axes in the MBDKL color space. Additional experiments were conducted to verify the S-cone axis for each observer and that our chromatic stimuli were close to isoluminant at different field locations. Our data show reliable and robust chromatic onset VEP responses for multiple retinal areas that conform to pattern-onset full-field VEP waveform characteristics. For stimuli with chromatic contributions, pattern-onsets produced reliable and consistent waveforms whereas for stimuli with large luminance contributions pattern-reversal stimuli were superior. Our method for recording chromatic multifocal pattern-onset VEPs holds promise for clinical application to detect and monitor early retinal and optic nerve changes related to aging and disease.

Adult↗
PubMed2004

Positional adaptation reveals multiple chromatic mechanisms in human vision.

Precortical color vision is mediated by three independent opponent or cardinal mechanisms that linearly combine receptoral outputs to form L/M, S/(L+M), and L+M channels. However, data from a variety of psychophysical and physiological experiments indicate that chromatic processing undergoes a reorganization away from the basic opponent model. Frequently, this post-opponent reorganization is viewed in terms of the generation of multiple "higher order" chromatic mechanisms, tuned to a wide variety of axes in color space. Moreover, adaptation experiments have revealed that the synthesis of these mechanisms occurs at a level in the cortex following the binocular integration of the inputs from each eye. Here we report results from an experiment in which the influence of chromatic adaptation on the perceived visual location of a test stimulus was explored using a Vernier alignment task. The results indicate that not only is positional information processed independently within the L/M, S/(L+M), and L+M channels, but that when adapting and test stimuli are extended to non-cardinal axes, the existence of multiple chromatically tuned mechanisms is revealed. Most importantly, the effects of chromatic adaptation on this task exhibit little interocular transfer and have rapid decay rates, consistent with chromatic as opposed to contrast adaptation. These findings suggest that the reorganization of chromatic processing may take place earlier in the visual pathway than previously thought.

Adaptation, Ocular↗
PubMed2003

Simple reaction times in color space: the influence of chromaticity, contrast, and cone opponency.

PURPOSE: This study examined the influence of stimulus chromaticity on simple reaction times (RTs) to determine the stage of chromatic processing that is most influential in their generation. METHODS: Simple RTs were measured in response to the cosinusoidally ramped onset of small, equiluminant, colored Gaussian spots. The chromaticity of these stimuli was varied, to modulate along a series of vectors in color space that included red-green (L-M) and blue-yellow (S-[L+M]) opponent axes. RESULTS: RTs are highly sensitive to small departures from subjective equiluminance. They are also dependent on stimulus chromaticity. The longest RTs are generated in response to equiluminant stimuli that isolate S-cone activity, whereas the shortest are generated by stimuli that modulate the L-M opponent axis. However, temporal processing differences are highly dependent on how the chromatic stimuli are scaled in relation to one another. The differences are reduced when scaling is based on detection threshold. The relationship between chromatic contrast and RT can be described by the modified Piéron equation RT = RT(0) + k x C(-1). CONCLUSIONS: Simple RTs generated in this study conform to the idea that they are largely determined by cone-opponency mechanisms. The use of cone contrast as a metric for scaling chromatic stimuli exaggerates differences between the temporal responsiveness of L-M and S-(L+M) opponency mechanisms.

Adult↗
PubMed2002

Group delay and chromatic dispersion of thin-film-based, narrow bandpass filters used in dense wavelength-division-multiplexed systems.

Theoretical analysis is made for thin-film-based, 200- and 100-GHz narrow bandpass filters with respect to the intensity response as well as to the chromatic dispersion. The results indicate that the narrower the passband, the higher the chromatic dispersion. The maximum chromatic dispersion appears at the edges of the 0.5-dB passband, owing to the fast change of the group delay in the region. The deviation of chromatic dispersion induced by manufacturing error is simulated. Effective-medium approximation layers are added to simulate the contribution of surface roughness and the mixture interfaces to the passband ripple as well as the chromatic dispersion. The simulations are compared with the experimental results. The measured chromatic dispersion matches the general trend of the theoretical calculation. The imperfect surface and layer mismatch induce additional ripples across the 0.5-dB passband. The maximum chromatic dispersion within a 0.5-dB passband is 20.7 and 54.9 ps/nm for 200- and 100-GHZ narrow bandpass filters, respectively.

Journal Article↗
PubMed1980

Influence of the target size on the deterction threshold for luminance and chromaticity contrast.

The effect of the target size on the detecton of luminance and chromaticity flicker and gratings was studied. Discrimination ellipses in a luminance-chromaticity plane were determined with square test areas subtending from 1/4 degree to 1 degree. Pure luminance modulation thresholds and pure chromaticity modualtion thresholds were obtained from square targets subtending from 1/16 degree to 2 degree. Square-wave stimuli were presented on a color television monitor; the mean color of the screen was yellow, the average retinal illuminance was 350 td. The main effect of enlarging the field size is that the threshold for any luminance-chromaticity combination decreases monotonically except when spatial frequencies are high. The summation area for detecting a fine bar pattern is at least 8 x 8 periods; the integration area for detecting flicker is more than 1 degree x 1 degree. This holds for any luminance-chromaticity mixture. For a fixed spatial or a fixed temporal frequency the change in sensitivity sometimes depends strongly on the ratio of luminance modulation to chromaticity modulation. The main conclusion of this study is that if a target is predominantly yellow the target size has similar influence on the sensitivity to both luminance and chromaticity contrast.

Adult↗
PubMed1983

Simultaneous masking interactions between chromatic and luminance gratings.

Simultaneous masking using test and mask gratings composed of isochromatic luminance variations and isoluminant chromatic variations was studied. Masking of chromatic gratings by chromatic gratings shows less spatial-frequency specificity than does masking of luminance gratings by luminance gratings. Luminance gratings mask chromatic gratings of identical space-average luminance and chromaticity little and only when the spatial frequencies of the test and mask gratings are similar. Chromatic gratings, however, profoundly mask luminance gratings with a degree of spatial-frequency specificity akin to that of luminance-luminance masking. The insensitivity of the luminance-color masking results to the relative phase of the chromatic and luminance gratings indicates that the observed asymmetry is not due to local interactions.

Color Perception↗
PubMed1995

Time course of chromatic adaptation for color-appearance judgments.

Observer production of achromatic appearance has previously been used to measure the time course of chromatic adaptation for changes from daylight to incandescent illuminants at constant luminance, indicating an exponential decay of chromatic adaptation with a time constant of the order of 10 s. The work extends previous results in several ways. The psychophysical technique was significantly improved to provide more reliable estimates of color appearance as a function of adaptation duration, and the time course of chromatic adaptation was measured for six chromaticity changes. Three observers tracked achromatic appearance on a computer-controlled CRT display during transitions of 2-min duration between the various chromaticities. The results indicate that observer differences are statistically significant. However, differences in time course for different chromaticity changes are not statistically significant (within observer). Single or piecewise exponential decay functions cannot be fitted to the data. However, sum-of-two-exponentials functions provided accurate descriptions of the data. The results suggest two stages of adaptation: one extremely rapid (a few seconds) and the other somewhat slower (approximately 1 min). Chromatic adaptation at constant luminance was 90% complete after approximately 60 s.

Adaptation, Ocular↗
PubMed1989

Motion processing by chromatic and achromatic visual pathways.

We describe a family of stimuli consisting of colored bars of different orientations, which, when presented in rapid succession, may elicit unambiguous motion perception. These stimuli permitted the isolation of directional spatiotemporal information extracted from oriented luminance clues, from nonoriented chromatic-plus-luminance clues, or, when the stimuli were presented under equiluminant conditions, from pure chromatic clues. As a general rule, matching of orientation induces weaker motion-detection performances than does matching of color. When the orientation clues are in competition with the chromatic ones, motion perception based on the former is always overridden by motion perception based on the latter. We indirectly isolated an oriented chromatic mechanism that also contributes to motion perception. We finally showed that, under equiluminant conditions, matching of orientation across different colors is inefficient in eliciting motion perception, either because motion information is extracted poorly across different chromatic channels or because such channels show little orientational selectivity. Because motion strength determined by each of the manipulated attributes follows different functions with the displacement (or velocity) of the stimuli, we propose the existence of three underlying mechanisms, a luminance mechanism, a chromatic-plus-luminance mechanism, and a pure chromatic mechanism, each of which provides motion information.

Attention↗
PubMed1990

Visual interactions with luminance and chromatic stimuli.

The visibility of a 1 degree, 200-msec flash on a large yellow field was measured as a function of the intensity of a coincident pedestal flash (a flash that was the same in both temporal intervals of a two-alternative forced-choice trial). The various flashes were incremental (+Lum) or decremental (-Lum) yellow luminance flashes or green (+Chr) or red (-Chr) isoluminant chromatic flashes. With uncrossed conditions (Lum tests on Lum pedestals or Chr tests on Chr pedestals), we obtained the conventional dipper function, that is, the function of threshold test intensity was highly asymmetric about zero pedestal intensity, and strong pedestals induced strong masking. Crossed conditions produced neither effect: for example, with Chr tests on Lum pedestals, there was no dipper function: the function of threshold test intensity was symmetric about zero pedestal intensity, and strong pedestals produced no masking. Instead, the suprathreshold luminance pedestals facilitated chromatic detection by as much as 2-3X and also linearized the chromatic psychometric function, further enhancing sensitivity to weak chromatic stimuli. (Chromatic sensitivity on the suprathreshold luminance pedestal was approximately 25X higher than luminance sensitivity on the uniform field.) A pedestal consisting of a thin luminance ring that surrounded the chromatic test produced facilitation equal to that of the uniform-luminance pedestal: the pedestal may thus act to demarcate the test spatially and promote chromatic comparison with the surround. Removing the uniform yellow surround eliminated this crossed facilitation but did not eliminate the uncrossed facilitation (the dipper function), suggesting that different mechanisms mediate the crossed and uncrossed facilitations.

Color↗