Sensitivity of human color mechanisms to gratings and flicker.
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Color Perception↗
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Proximity factor in color-difference evaluations.
The effect of dividing-strip width (sample proximity) upon sensitivity to suprathreshold and threshold color differences is explored by three methods that involve ratio comparisons, liminal determinations, and repeated color matchings. The results suggest, in general, that sample separation impairs lightness discrimination more than chromaticness discrimination and that it may be necessary to introduce a proximity factor into color-difference formulas when evaluating threshold or small-size color differences.
Adult↗
Differences between red and green color-mechanism characteristics: interaction or artifact.
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Color Perception↗
Modulation sensitivity of human color mechanisms.
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Color Perception↗
Chromatic two-pulse resolution with and without luminance transients.
Two-pulse discrimination thresholds determined for homochromatic pulses presented as luminance increments or decrements are invariant with wavelength. Wavelength does affect two-pulse temporal resolution if chromatic pulses are presented in hue substitution, matched in luminance (with flicker photometry) to an achromatic background field. For relatively long pulses, the wavelength effect resembles trichromatic saturation discrimination, with poorest temporal resolution at 570 nm. For shorter pulses, temporal resolution for hue substitution stimuli appears related to saturation discrimination under conditions of artificial tritanopia. Under conditions of hue substitution, short-wavelength pulses yield good temporal resolution.
Color↗
Pi-4: adaptation of more than one class of cone.
The Pi-4 color mechanism was isolated in three observers with a 500-nm, 200-msec, l-deg foveal test flash. Stiles's field displacement law was tested with increment thresholds upon monochromatic adapting fields of several wave-lengths. The data of all three observers reject the displacement law. Data that use a 10-msec-duration test flash likewise reject the displacement law. We conclude that the Pi-4 branch represents light adaptation controlled by more than one class of cone. A model of the Pi-4 detection pathway is proposed that quantitatively describes the increment-threshold data.
Adaptation, Physiological↗
Temporal aspects of color discrimination.
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Adult↗
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↗
Chromatic induction as a function of wavelength of inducing stimulus.
Induced chromatic effects were determined for monochromatic, equal-luminance inducing stimuli from 460 to 680 nm by using a hue-cancellation procedure. The observed red-green-and yellow-blue-induced chromatic-response functions, which were different from the prediction based on the opponent-color hypothesis, could accurately explain the characteristics of the simultaneous color contrast effect. Good linear fits were obtained for the red-green function with a linear combination of R and G cones and for the yellow-blue function with a linear combination of R and B cones. These findings suggest that the opponent mechanisms for color contrast may be different from those for homogeneous color.
Adult↗
Functional relationship between chromatic induction and luminance of the inducing stimulus.
We determined the functional relationship between chromatic induction and luminance of the inducing stimulus for different spatial conditions and assessed whether the effects of luminance and spatial variables could be explained in terms of the total effective energy in the inducing field. The result showed that the relationship between chromatic induction and luminance of the inducing stimulus could be mathematically expressed by an exponential function of the luminance ratio between the test and inducing stimuli and that the coefficient of the exponent was independent of spatial variables, i.e., area and separation. This led to the conclusion that a luminance ratio between two fields, rather than a quantum energy of the inducing field, was a relevant determinant of the effect of luminance of the inducing stimulus on chromatic induction.
Adult↗
Relationship between chromatic induction and spatial variables: an integrated explanation in terms of element-contribution function.
Chromatic induction as a function of separation and as a function of area was determined by a hue-cancellation procedure. Both functions obtained were expressed by exponential functions with similar exponential coefficients. This led to the derivation of an element-contribution function, based on a linear summation model, that could explain both the relationship between chromatic induction and separation and that between chromatic induction and area. The effects of separation and area on chromatic induction could readily be determined in terms of an element-contribution function. In addition, the induction area that is due to a blue inducing stimulus was larger than those that are due to the other inducing stimuli, suggesting that the summation area of the blue response was larger than those of the other chromatic responses.
Adult↗
Evidence for an independent luminance channel.
There is a discrepancy between several studies that have shown the human luminous-efficiency function to vary with surround color and a recent study that failed to find this dependence. Data are presented that show that this discrepancy can be explained by differences in the matching techniques. Luminous efficiency measured by direct heterochromatic brightness matching does depend on surround color, whereas luminous efficiency measured by the flicker method does not. The independence of luminous efficiency as measured by flicker is evidence for an independent luminance channel.
Color Perception↗
Spatial and temporal discrimination ellipsoids in color space.
Three-dimensional discrimination ellipsoids are presented for a number of representative points in color space. These ellipsoids have been obtained not with the conventional split field but with flickering grating patterns. Thus our study extends the well-known results of Brown and MacAdam [J. Opt. Soc. Am. 39, 808-813 (1949)] to cases in which the image is structured in space and time. As expected, we find that the discrimination ellipsoids depend on the spatiotemporal structure of the stimulus. This has potential consequences for color-difference formulas as used in industry and commerce: no single formula will do when it is important to treat patterns with different structure. We present analytical descriptions, based on the Vos-Walraven [Vision Res. 12, 1327-1365 (1972)] line element augmented with spatiotemporal frequency-dependent coefficients that fit our results reasonably well. For coarse gratings (approximately 1 cycle per degree) or slowly modulated fields (approximately 1 Hz) our results prove to be compatible with the results of Brown and MacAdam obtained with a bipartite 2 degree field.
Color Perception↗
Spectral sensitivity and wavelength discrimination of the human peripheral visual field.
Spectral sensitivity and wavelength discrimination are determined along the nasal horizontal meridian of the human peripheral retina. The target size as a function of eccentricity is varied according to a particular cortical magnification factor. Spectral sensitivity is measured by flicker photometry parameterized for the flicker frequency (10-20 Hz) and is found to be independent of the eccentricity (0-80 degrees) for 20-Hz flicker photometry after correction of the foveal spectral sensitivity for macular pigment absorption. This 20-Hz function is chosen as being representative for the peripheral luminous-efficiency function and is used in the wavelength-discrimination experiments. The peripheral retina can perform wavelength discrimination up to an eccentricity of 80 degrees. If field-size scaling according to the eccentricity-dependent cone density, the cortical magnification factor, or the reciprocal of the interganglion cell distance is applied, then wavelength-discrimination performance from 8 degrees to 80 degrees eccentricity is roughly the same. Foveal wavelength discrimination is considerably better than peripheral wavelength discrimination.
Adult↗
Temporal sensitivities related to color theory.
Sensitivities of color-normal observers to temporal variations in stimulus luminance and chromaticity were measured for sine-wave stimuli between 1.5 and 20 Hz. Clear differences were found in observers' sensitivities to isochromatic luminance variations and to isoluminous chromaticity variations for wavelength pairs selected to test temporal discriminability along the red-green and yellow-blue dimensions, respectively. Despite interobserver differences in individual red-green functions, a given observer's sensitivity could be described by a single curve shape specific to that observer. Overall sensitivity for yellow-blue was less than that for red-green for all observers. Differences in curve shape between red-green and yellow-blue functions are found for individual observers, but group averages reveal that the differences are not systematic. Red-green temporal sensitivity is largely unaffected by adapting backgrounds in red-green equilibrium but is attenuated at low frequencies by nonequilibrium backgrounds of the same luminance. Isochromatic luminance sensitivity is largely independent of our adapting backgrounds, but heterochromatic luminance modulation functions undergo expected changes in form.
Color↗
Pigment tests evaluated by a model of chromatic discrimination.
Clinical color-vision tests are evaluated within the framework of a model of chromatic discrimination in terms of cone excitation. The motivation for this study was to derive a method for evaluation of test design, test sensitivity, and observer performance. The discrimination model is based on the assumption that chromatic discrimination is mediated in two independent channels, one for short-wavelength cones and one for long- and middle-wavelength cones. Luminance-dependent templates are derived for each channel, and they describe chromatic-discrimination behavior of the young color-normal observer. The templates incorporate receptor- and opponent-level gain controls. We show how the chromaticities of clinical tests can be calculated in cone-excitation units and how discrimination behavior on the tests can be plotted on the templates. The tests include the Farnsworth-Munsell 100-hue, the Farnsworth Panel D-15, the Farnsworth Panel D-15 desaturated, the American Optical Hardy-Rand-Rittler, the Farnsworth F2 plate, the Standard Pseudoisochromatic Plates, Part II, the Ishihara, and the Minimalist tests. Clinical-test data collected on young color-normal observers at different illumination levels show the validity of the techniques.
Color Perception↗
Color measurement and discrimination.
Theories of color-difference measurement provide a quantitative means for predicting whether two lights will be discriminable to an average observer. Consider the following color-measurement hypothesis. Suppose that two lights evoke responses from the color channels that we write as vectors, U and U'. The vector difference dU = U - U' is itself a set of channel responses that will result from the presentation of some light. I test the hypothesis that U and U' will be discriminable only if the light that gives rise to their differential, dU, is detectable. In the absence of a luminance component in the difference stimulus, dU, the vector-difference hypothesis holds well. In the presence of a luminance component, the theory is clearly false. When a luminance component is present, discrimination judgements depend largely on whether the lights U and U' are in separate, categorical regions of color space.
Biofeedback, Psychology↗
Psychophysical estimates of the number of spectral-reflectance basis functions needed to reproduce natural scenes.
Theoretical analyses of spectral reflectances of natural surfaces suggest that their perceived colors can be well reproduced by approximations comprising combinations of three or four spectral basis functions. The aim of the present work was to assess psychophysically the number of basis functions necessary to reproduce entire natural outdoor scenes. Hyperspectral images of 20 such scenes were each subjected to a principal component analysis and then reproduced with a variable number of basis functions. The quality of the color approximation under daylight illumination was quantified theoretically in CIELAB space and psychophysically by spatial and temporal two-alternative forced-choice measurements in which the original and the approximated images were compared on a calibrated color monitor. Although five basis functions produced on average unit error in CIELAB space, original images were visually indistinguishable from their approximations only if there were at least eight basis functions. The combination of the spectral diversity of the natural world and the observed levels of color discrimination suggest that estimates of the minimum number of basis functions necessary to reproduce natural scenes may need to be revised upward.
Algorithms↗