Chromatic assimilation: spread light or neural mechanism?

AbstractChromatic assimilation is the shift in color appearance of a test field toward the appearance of nearby light. Possible explanations of chromatic assimilation include wavelength independent spread light, wavelength-dependent chromatic aberration and neural summation. This study evaluated these explanations by measuring chromatic assimilation from a concentric-ring pattern into an equal-energy-white background, as a function of the inducing rings’ width, separation, chromaticity and luminance. The measurements showed, in the s direction, that assimilation was observed with different inducing-ring widths and separations when the inducing luminance was lower or higher than the test luminance. In general, the thinner the inducing rings and the smaller their separation, the stronger the assimilation in s. In the l direction, either assimilation or contrast was observed, depending on the ring width, separation and luminance. Overall, the measured assimilation could not be accounted for by the joint contributions from wavelength-independent spread light and wavelength-dependent chromatic aberration. Spatial averaging of neural signals explained the assimilation in s reasonably well, but there were clear deviations from neural spatial averaging for the l direction

Stereo disparity improves color constancy

AbstractBinocular disparity is an aspect of natural viewing. This research investigates whether disparity affects surface color perception. Achromatic settings were obtained and compared for two stereograms of a scene with specular reflections, one stereogram with binocular disparity and one without it (cyclopean view). Binocular disparity was found to improve color constancy. Next, the geometry of specular highlights, which is distorted without binocular disparity, was specifically examined. Measurements compared color constancy with specular reflections that were either normal (with stereo disparity) or distorted (cyclopean view of the specularities). No significant change in constancy was found due to the geometrical distortion of specular highlights that occurs without stereo disparity, suggesting that constancy depends on other features of the percept affected by disparity. The results are discussed in terms of illuminant estimation in surface color perception

The Visual Photopigments of Simple Deuteranomalous Trichromats Inferred from Color Matching

AbstractDeuteranomalous trichromacy is the most common form of inherited color-vision deficiency. A modern description of its cause is a single abnormality: the normal middle-wave cone photopigment (M) is replaced by a shifted middle-wave pigment (M′) that is shared by all deuteranomalous trichromats. This explanation, however, fails to account for the individual differences in color vision observed even within the sub-group of deuteranomals with good chromatic discrimination. An ensemble of color matches is used here to test whether these individual differences reflect differences in the wavelength of peak sensitivity (λmax) of individual deuteranomals' cone photopigments. The results show variation in both the λmax and the effective optical density of their cone pigments. The individual differences found in λmax are in accord with recent molecular biological research that shows individual differences in the genes thought to encode deuteranomalous photopigments. © 1997 Elsevier Science Ltd. All rights reserved

Saturation in human cones

Increment threshold intensities were measured for a foveally presented 20msec test flash which fell on a flashed 4.1[deg] white background field. The test field was varied in color (red or green) and size (1[deg] or 0.57[deg]). In one condition (green 0.57[deg] test) the onset asynchrony of the background and test fields was also varied. The increment threshold measurements directly demonstrate saturation of the [pi]4 and [pi]5 color mechanisms and, based on the cone signal equation given by Alpern, Rushton and Torii (1970c), provide estimates of the semi-saturation constant [sigma]. For a given color mechanism, a single value of [sigma] was found to be adequate for all test flash sizes and asynchrony intervals tested; however, these values of [sigma] are about 100 times smaller than those found by Alpern et al. using the contrast-flash technique.The Alpern et al. cone signal equation is essentially a simple gain control model. It is shown that when light intensities are correctly expressed in terms of quantum catch, the gain control model can account explicitly for both the observed cone mechanism saturation in the absence of pigment bleaching and the absence of saturation in the presence of bleaching by steady backgrounds.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23058/1/0000630.pd

The dual role of chromatic backgrounds in color perception

This investigation explores the color appearance changes resulting from a continuously presented adapting field. In every experiment, an incremental mixture of red and green monochromatic lights was superimposed on top of a steady red background field. On each experimental trial the intensities of the red background and the red increment were fixed: the subject adjusted the intensity of the green light so that the incremental mixture appeared a "pure" (neither slightly reddish nor greenish) yellow. In one experiment, the increment was a steadily viewed thin annulus seen on a larger background: in another experiment the increment was identical to the background in size and retinal location but was presented as a brief ( 150 msec) flash: in the final experiment the increment was a briefly flashed thin annulus seen on a larger background.For any fixed, relatively dim background level the intensities of the red and green increments were approximately in constant ratio over a nearly 2 log unit range of test intensities. However, with more intense adapting fields the green light to red light incremental intensity ratio decreased as the test intensity was increased, with the ratio asymptoting at high test levels to an adaptation-intensity dependent value.The empirical observations reject both von Kries' Coefficient Law and the notion that only spatial (and or temporal) transients contribute to color signals. The results are consistent with a "two-process" theory where the adapting field is assumed both to contribute directly to the chromatic signal and simultaneously to alter the amplitudes (but not shapes) of the spectral sensitivity functions associated with the three receptor-types of color vision.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22782/1/0000337.pd

Chromatic Information and Feature Detection in Fast Visual Analysis

The visual system is able to recognize a scene based on a sketch made of very simple features. This ability is likely crucial for survival, when fast image recognition is necessary, and it is believed that a primal sketch is extracted very early in the visual processing. Such highly simplified representations can be sufficient for accurate object discrimination, but an open question is the role played by color in this process. Rich color information is available in natural scenes, yet artist's sketches are usually monochromatic; and, black-and-white movies provide compelling representations of real world scenes. Also, the contrast sensitivity of color is low at fine spatial scales. We approach the question from the perspective of optimal information processing by a system endowed with limited computational resources. We show that when such limitations are taken into account, the intrinsic statistical properties of natural scenes imply that the most effective strategy is to ignore fine-scale color features and devote most of the bandwidth to gray-scale information. We find confirmation of these information-based predictions from psychophysics measurements of fast-viewing discrimination of natural scenes. We conclude that the lack of colored features in our visual representation, and our overall low sensitivity to high-frequency color components, are a consequence of an adaptation process, optimizing the size and power consumption of our brain for the visual world we live in

Chromatic Information and Feature Detection in Fast Visual Analysis

The visual system is able to recognize a scene based on a sketch made of very simple features. This ability is likely crucial for survival, when fast image recognition is necessary, and it is believed that a primal sketch is extracted very early in the visual processing. Such highly simplified representations can be sufficient for accurate object discrimination, but an open question is the role played by color in this process. Rich color information is available in natural scenes, yet artist's sketches are usually monochromatic; and, black-and-white movies provide compelling representations of real world scenes. Also, the contrast sensitivity of color is low at fine spatial scales. We approach the question from the perspective of optimal information processing by a system endowed with limited computational resources. We show that when such limitations are taken into account, the intrinsic statistical properties of natural scenes imply that the most effective strategy is to ignore fine-scale color features and devote most of the bandwidth to gray-scale information. We find confirmation of these information-based predictions from psychophysics measurements of fast-viewing discrimination of natural scenes. We conclude that the lack of colored features in our visual representation, and our overall low sensitivity to high-frequency color components, are a consequence of an adaptation process, optimizing the size and power consumption of our brain for the visual world we live in