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PubMed1993

Dissociation of achromatic and chromatic processing of spatial form and temporal modulation by the titration method.

Using a titration method to attempt to silence the achromatic contrast subsystem, we tested the assumption that an equiluminant chromatic pattern does not stimulate the physiological achromatic contrast subsystem. We also examined the question whether contrast threshold is entirely or even partially determined by the chromatic contrast subsystem at equiluminance. We report a method for demonstrating when contrast threshold is entirely determined by the chromatic contrast subsystem.

Adult↗
PubMed1993

Motion at isoluminance: discrimination/detection ratios and the summation of luminance and chromatic signals.

Under some conditions, direction-of-motion thresholds are elevated with respect to detection thresholds for isoluminant chromatic stimuli. In the present study we investigated the effect of small luminance mismatches on ratios of direction-of-motion thresholds to detection thresholds (M/D ratios). The stimuli were 2 deg x 2 deg patches of a moving 1-cycle/deg, 2.75-deg/s sinusoidal grating, modulated spatially in luminance, chromaticity, or both. M/D ratios were close to 1 except when luminance modulation was < 1%, defined with respect to the individual subject's isoluminance point; in this range M/D ratios varied from 1.5 to 4.4. Sets of thresholds for both detection and motion tasks conformed approximately to ellipses. Thus, although motion processing shows a differential loss of sensitivity at isoluminance, similar summation rules appear to apply to the combination of luminance and chromatic signals for both tasks.

Color Perception↗
PubMed1995

Color appearance under chromatic adaptation varied along theoretically significant axes in color space.

Changes in color appearance caused by chromatic adaptation were measured with a wide range of adapting fields. Observers viewed a 39'-55' annular test field composed of an admixture of lights from the red phosphor and the green phosphor of a CRT. The annular mixture field was centered and superimposed upon a 4.7 degrees steady, circular background field. After the observer was completely adapted to the background, the luminance of the red phosphor in the test was held fixed while the observer adjusted the luminance of the green phosphor until the test appeared neither reddish nor greenish. Twenty-two equiluminant backgrounds (4.5 cd/m2, approximately 50 Td) were systematically selected along two axes in Judd chromaticity space. One axis was along tritanopic confusion lines, with middle-wavelength-sensitive- (M-) and long-wavelength-sensitive- (L-) cone stimulation held constant. The other axis maintained constant short-wavelength-sensitive- (S-) cone stimulation. The results show that adapting backgrounds that were varied along tritanopic confusion lines do not have a differential effect on color appearance at high test levels (well above the adapting level). At lower test levels there is a systematic change in color appearance of the test light, which is quantitatively described by additive redness. Along constant S-cone-stimulation lines, adapting backgrounds differentially affect color appearance in a systematic way, reflecting changes in receptoral gain and the additive contribution. The measurements taken with adapting fields throughout color space are described by the two-process model of chromatic adaptation.

Adaptation, Ocular↗
PubMed2000

Chromatic contrast sensitivity: the role of absolute threshold and gain constant in differences between the fovea and the periphery.

A model of foveal achromatic and chromatic sensitivity [Vision Res. 36, 1597 (1996)] was extended to the peripheral visual field. Threshold-versus-illuminance functions were analyzed to determine effects of eccentricity on absolute thresholds and gain constants of chromatic and luminance mechanisms. The resulting peripheral model successfully predicted peripheral contrast sensitivity as a function of wavelength, for both white and 500-nm backgrounds. We conclude that the short-wavelength-sensitive cone opponent mechanism may mediate thresholds in Sloan's notch in the normal periphery and that interpretation of reduced chromatic sensitivity in the periphery requires an explicit model of how eccentricity affects both the gain constant and the absolute threshold.

Color Perception↗
PubMed2000

Role of perceptual organization in chromatic induction.

Color matches between two small patches were made in a display containing ten larger regions of different chromaticities. The spatial organization of the ten regions was varied while keeping constant the immediate surround of each patch as well as the space-average chromaticity of the entire stimulus. Different spatial arrangements were designed to alter the perceptual organization inferred by the observer without changing the ensemble of chromaticities actually in view. For example, one arrangement of the ten regions was consistent with five surfaces under two distinct illuminations, with one edge within the display (an "apparent illumination edge") dividing the stimulus into two areas, one under illuminant A and the other under illuminant C. Another spatial arrangement had the ten regions configured to induce an observer to infer ten surfaces under a single illumination. When the ten regions were arranged with an apparent illumination edge, the patch within the area of illuminant C was perceived as bluer than when the same patch and immediate surround were presented without an apparent illumination edge. The results are accounted for by positing that observers group together regions sharing the same inferred illumination, with a consequent effect on color perception: A fixed patch-within-surround shifts in hue and saturation toward the perceived illumination. We suggest that the change in color perception in a complex scene that results from a difference in real illumination may be caused by the inferred illumination at the perceptual level, not directly by the physical change in the light absorbed by photoreceptors.

Adult↗
PubMed2002

Psychophysical model of chromatic perceptual transparency based on substractive color mixture.

Variants of Metelli's episcotister model, which are based on additive color mixture, have been found to describe the luminance conditions for perceptual transparency very accurately. However, the findings in the chromatic domain are not that clear-cut, since there exist chromatic stimuli that conform to the additive model but do not appear transparent. We present evidence that such failures are of a systematic nature, and we propose an alternative psychophysical model based on subtractive color mixture. Results of a computer simulation revealed that this model approximately describes color changes that occur when a surface is covered by a filter. We present the results of two psychophysical experiments with chromatic stimuli, in which we directly compared the predictions of the additive model and the predictions of the new model. These results show that the color relations leading to the perception of a homogeneous transparent layer conform very closely to the predictions of the new model and deviate systematically from the predictions of the additive model.

Color↗
PubMed2002

Estimating the scene illumination chromaticity by using a neural network.

A neural network can learn color constancy, defined here as the ability to estimate the chromaticity of a scene's overall illumination. We describe a multilayer neural network that is able to recover the illumination chromaticity given only an image of the scene. The network is previously trained by being presented with a set of images of scenes and the chromaticities of the corresponding scene illuminants. Experiments with real images show that the network performs better than previous color constancy methods. In particular, the performance is better for images with a relatively small number of distinct colors. The method has application to machine vision problems such as object recognition, where illumination-independent color descriptors are required, and in digital photography, where uncontrolled scene illumination can create an unwanted color cast in a photograph.

Journal Article↗
PubMed2005

Senescence of spatial chromatic contrast sensitivity. II. Matching under natural viewing conditions.

Age-related changes in the spatial chromatic contrast sensitivity function of detection, measured along S and L - M cone axes, were demonstrated in a companion paper [Hardy et al., J. Opt. Soc. Am. A 22, 49 (2005)]. Here senescent changes in chromatic contrast appearance were assessed by contrast-matching functions (CMFs). Luminance and chromatic CMFs (S and L - M axes) were compared for younger (age 18-31 yr) and older (age 65-75 yr) trichromatic subjects by using stimuli that were perceptually anchored to the same physical standard contrasts. Subjects matched the contrast of test gratings of various spatial frequencies (0.5-8 cycles per degree) to the standard stimuli under natural viewing conditions. Because of changes in the visual system with age, the standard stimuli were closer to threshold for older subjects; however, in general, the shapes of the CMFs were similar for both groups. The results suggest that the perception of relative contrasts across spatial frequencies is stable with age.

Adolescent↗
PubMed2005

Regulation of chromatic induction by neighboring images.

We deal with the regulation of chromatic contrast when the induction of a second stimulus (one of five neighboring surrounds) opposes the induction from a first stimulus (one of two remote vivid peripheral fields). Using a hue cancellation judgment, we show that, although every neighboring surround that we used has the same average chromatic content, the resulting color appearance of the target differs between surrounds, and this may be ascribed to the spatiochromatic organization of the surround. So, rather than the chromatic contrast amplitude or the frequential structure of the surround, it is the structure of proximity that matters.

Adaptation, Ocular↗
PubMed2006

Binocular interactions in random chromatic changes at isoluminance.

To examine the type of chromatic interactions at isoluminance in the phenomenon of binocular vision, I have determined simple visual reaction times (VRT) under three observational conditions (monocular left, monocular right, and binocular) for different chromatic stimuli along random color axes at isoluminance (simultaneous L-, M-, and S-cone variations). Upper and lower boundaries of probability summation as well as the binocular capacity coefficient were estimated with observed distributions of reaction times. The results were not consistent with the notion of independent chromatic channels between eyes, suggesting the existence of excitatory and inhibitory binocular interactions at suprathreshold isoluminance conditions.

Color↗
PubMed1987

Magnitude of lateral chromatic aberration across the retina of the human eye.

Lateral chromatic aberration was measured in the right eyes of four adult observers as the physical misalignment between perceptually aligned short- and long-wavelength targets. The magnitude of aberration generally increased with retinal eccentricity but remained less than 10 arcmin within 40 deg of the fovea. At 60 deg, lateral chromatic aberration increased to approximately 30 arcmin but was reducible by refractive correction in the two observers retested. The results are consistent with previous reports of a sizable region of reasonably good optical quality extending into the retinal midperiphery. Within this region, lateral chromatic aberration approximates the average spacing between adjacent retinal cones, indicating that it does not substantially limit peripheral color vision.

Adult↗
PubMed1987

Calculation of the influence of lateral chromatic aberration on image quality across the visual field.

The magnitude of lateral chromatic aberration and its effect on image contrast were computed for a modified, reduced-eye model of the human eye, using geometrical optics. The results indicate that lateral chromatic aberration is a major factor affecting image quality for obliquely incident rays of polychromatic light. Modulation transfer functions for white sinusoidal gratings decline monotonically with spatial frequency, with eccentricity of the stimulus in the peripheral visual field, with grating orientation relative to the visual meridian, and with decentering of the pupil. Image contrast is largely independent of the color temperature of white light over the range 2800 to 12,000 K, but it improves significantly for the polychromatic green light of the P-31 oscilloscope phosphor. Selective filtering by macular pigment increases image contrast by an amount that grows with spatial frequency to about a factor of 1.5 at the foveal resolution limit. Reduced contrast caused by lateral chromatic aberration accounts for most of the threefold loss of acuity that occurs for foveal viewing through a decentered pupil. The aberration probably has negligible effect on peripheral acuity but may act to limit aliasing of peripheral patterns.

Color Perception↗
PubMed1988

Contrast dependence and mechanisms of masking interactions among chromatic and luminance gratings.

The contrast dependence of simultaneous masking has been measured using isochromatic yellow-black luminance sinusoids and isoluminant red-green chrominance gratings. Masking functions for all four combinations of chromatic and luminance masks and tests are reported. In the two same-on-same conditions (luminance mask/luminance test and chromatic mask/chromatic test) these functions (increment threshold contrast versus mask contrast) have the typical dipper shape and are almost identical when test and mask contrasts are normalized to the unmasked contrast thresholds. The contrast dependence of the luminance mask/color test and color mask/luminance test functions are quite different. The luminance mask/color test shows facilitation over a broad range of both subthreshold and suprathreshold contrasts of the luminance mask. In the color mask/luminance test condition facilitation is never observed, but at suprathreshold contrasts a 2-cycle/degree (c/deg) chromatic grating masks a 2-c/deg luminance grating as strongly as does a luminance mask. The luminance mask/chromatic test results are invariant over the 0.25-2-c/deg spatial-frequency range, whereas the robust masking of luminance by color at 2 c/deg diminishes at lower spatial frequencies. The spatial-frequency selectivity of the luminance-facilitates-color interaction is much broader than facilitatory interactions in either the color-color or luminance-luminance conditions. Possible mechanisms of color-luminance interactions are considered. The lack of facilitation in the color mask/luminance test condition precludes a simple pedestal interpretation of this masking interaction. The data are, however, consistent with models that invoke inhibitory or more elaborate excitatory masking interactions.

Color↗
PubMed1988

Effects of chromatic adaptation on phase-dependent sensitivity to heterochromatic flicker.

Temporal modulation sensitivity was measured as a function of the relative phase of two equiluminous chromatic sources (564 and 625 nm) for temporal frequencies from 6 to 20 Hz. The difference between 180 degrees and the phase of least sensitivity was computed as the measured phase shift. A 2 degree test field was superimposed upon 8 degrees chromatic adapting fields with luminances from 100 to 3000 Td and chromaticities of 500, 600, and 650 nm. For each adapting field, the 564-nm source was set to 175 Td, and the 625-nm source was matched to it with heterochromatic flicker photometry (giving an effective mean luminance of 350 Td). The 650-nm adapting fields produced large changes in photometric setting but only small changes in the measured phase shift. The 600- and 500-nm adapting fields produced smaller changes in photometric setting but larger changes in the measured phase shift. In general, increased adapting luminance resulted in an increase in the measured phase shift for 600-nm adaptation and a decrease in the measured phase shift for 500-nm adaptation.

Adaptation, Physiological↗
PubMed1988

Optical and photoreceptor immaturities limit the spatial and chromatic vision of human neonates.

We examine the contributions of preneural mechanisms, i.e., the optics of the eye and the aperture, spacing, and efficiency of foveal cones, to poor spatial and chromatic vision in human neonates. We do so by comparing the performances of ideal observers incorporating the characteristics of the optics and the foveal cones of adults and neonates. Our analyses show that many, but not all, of the differences between neonatal and adult contrast sensitivities and grating acuities can be explained by age-related changes in these factors. The analyses also predict differing growth curves for vernier and grating acuities. Finally, we demonstrate that preneural mechanisms constrain chromatic discrimination in human neonates and that discrimination failures may reflect poor visual efficiency rather than immature chromatic mechanisms per se.

Aging↗
PubMed1988

Does the chromatic aberration of the eye vary with age?

The longitudinal chromatic aberration of the eye has been reported to decline with age. Using three different methods, we have measured the aberration in a group of young subjects (27-33 years old) and a group of older subjects (48-72 years old). In two of the methods we used a Badal optometer, either with or without an achromatizing lens incorporated, to examine the effect of wavelength on refractive error. In the third method we used a vernier-alignment apparatus to assess chromatic dispersion directly. None of the results of the experiments performed revealed any difference in aberration between the groups. Furthermore, a linear regression of aberration against age showed no relationship between these variables. We conclude that, for human adults, the magnitude of chromatic aberration is independent of age.

Adult↗
PubMed1988

Spectral sensitivity and chromatic discriminations in 3- and 7-week-old human infants.

The chromatic discrimination capabilities of 3- and 7-week-old infants were tested using 8 degrees, 417-, 448-, 486-, 540-, and 645-nm test fields embedded in a 547-nm surround and 486-nm test fields in a broadband red surround. In corroboration of earlier studies, few 3-week-old infants demonstrated chromatic discriminations, although their performance was somewhat better when one of the lights was long wavelength. Most 7-week-old infants could make chromatic discriminations, but they still demonstrated performance minima. The radiances of the test lights at the infants' performance minima were used to generate a spectral luminous efficiency curve. This curve agreed with both the adult heterochromatic brightness matches measured at 30 degrees of visual eccentricity in situ and the standard adult scotopic sensitivity curve V(lambda) over the short- and mid-wavelength range but deviated from both adult curves for the 645-nm test stimulus on a 547-nm surround. The results suggest that rod-initiated signals play a major role in infants' visual performance under the conditions tested.

Aging↗
PubMed1990

Orientation and spatial-frequency discrimination for luminance and chromatic gratings.

We have examined the accuracy of orientation and spatial-frequency discrimination for sine-wave gratings that vary in either luminance or color. The equiluminant chromatic gratings were modulated along either a tritanopic confusion axis (so that they were detectable on the basis of activity in only the short-wavelength-sensitive cones) or an axis of constant short-wavelength-sensitive cone excitation (so that they could be detected on the basis of opposing activity in only the long- and medium-wavelength-sensitive cones). Grating contrasts ranged from the detection threshold to the highest levels that we could produce; the contrasts of the luminance and color patterns were equated for equal multiples of their respective detection thresholds. Discrimination thresholds for all patterns showed a similar dependence on stimulus contrast, rising sharply at low contrasts and becoming nearly asymptotic at moderate contrasts. However, even at threshold contrasts, observers could still reliably discriminate sufficiently large differences in the orientation or spatial frequency of all patterns, and they could also reliably identify the type of variation (luminance or which color) defining the grafting. For most conditions the discrimination thresholds did not differ from the two types of color grafting and reached values as low as 1 deg (orientation) or 4% (spatial frequency). Thus observers were able to make accurate spatial judgments on the basis of either type of chromatic information. However, these thresholds were slightly but consistently higher than the thresholds for comparable luminance graftings. This difference in the color and luminance discrimination thresholds may reflect somewhat coarser orientation and spatial-frequency selectivity in the mechanisms encoding the chromatic patterns.

Color Perception↗