Role of perceptual organization in chromatic induction

Illumination discrimination in the absence of a fixed surface-reflectance layout

Previous studies have shown that humans can discriminate spectral changes in illumination and that this sensitivity depends both on the chromatic direction of the illumination change and on the ensemble of surfaces in the scene. These studies, however, always used stimulus scenes with a fixed surface-reflectance layout. Here we compared illumination discrimination for scenes in which the surface reflectance layout remains fixed (fixed-surfaces condition) to those in which surface reflectances were shuffled randomly across scenes, but with the mean scene reflectance held approximately constant (shuffled-surfaces condition). Illumination discrimination thresholds in the fixed-surfaces condition were commensurate with previous reports. Thresholds in the shuffled-surfaces condition, however, were considerably elevated. Nonetheless, performance in the shuffled-surfaces condition exceeded that attainable through random guessing. Analysis of eye fixations revealed that in the fixed-surfaces condition, low illumination discrimination thresholds (across observers) were predicted by low overall fixation spread and high consistency of fixation location and fixated surface reflectances across trial intervals. Performance in the shuffled-surfaces condition was not systematically related to any of the eye-fixation characteristics we examined for that condition, but was correlated with performance in the fixed-surfaces condition.

Low levels of specularity support operational color constancy, particularly when surface and illumination geometry can be inferred

We tested whether surface specularity alone supports operational color constancy-the ability to discriminate changes in illumination or reflectance. Observers viewed short animations of illuminant or reflectance changes in rendered scenes containing a single spherical surface and were asked to classify the change. Performance improved with increasing specularity, as predicted from regularities in chromatic statistics. Peak performance was impaired by spatial rearrangements of image pixels that disrupted the perception of illuminated surfaces but was maintained with increased surface complexity. The characteristic chromatic transformations that are available with nonzero specularity are useful for operational color constancy, particularly if accompanied by appropriate perceptual organization.

The separation of monocular and binocular contrast

The contrast asynchrony is a stimulus configuration that illustrates the visual system's separable responses to luminance and luminance contrast information (Shapiro, 2008; Shapiro et al., 2004). When two disks, whose luminances modulate in phase with each other, are each surrounded by a disk, one light and one dark, observers can see both the in-phase brightness signals and the antiphase contrast signals and can separate the two. Here we present the results of experiments in which observers viewed a similar stimulus dichoptically. We report that no asynchrony is perceived when one eye is presented with modulating disks and the other eye is presented with the black and white surround rings, nor is an asynchrony perceived in gradient versions of the contrast asynchrony. We also explore the "window shade illusion" (Shapiro, Charles, & Shear-Heyman, 2005) dichoptically and find that when a modulating disk is presented to one eye and a horizontally split black/white annulus is presented to the other, observers perceive a "shading" motion up and down the disk. This shading can be seen in either direction in the binocular condition, but it is almost always seen as moving towards low contrast in the monocular condition. These findings indicate the presence of separable retinal and cortical networks for contrast processing at different temporal and spatial scales.

Scotopic hue percepts in natural scenes

Traditional trichromatic theories of color vision conclude that color perception is not possible under scotopic illumination in which only one type of photoreceptor, rods, is active. The current study demonstrates the existence of scotopic color perception and indicates that perceived hue is influenced by spatial context and top-down processes of color perception. Experiment 1 required observers to report the perceived hue in various natural scene images under purely rod-mediated vision. The results showed that when the test patch had low variation in the luminance distribution and was a decrement in luminance compared to the surrounding area, reddish or orangish percepts were more likely to be reported compared to all other percepts. In contrast, when the test patch had a high variation and was an increment in luminance, the probability of perceiving blue, green, or yellow hues increased. In addition, when observers had a strong, but singular, daylight hue association for the test patch, color percepts were reported more often and hues appeared more saturated compared to patches with no daylight hue association. This suggests that experience in daylight conditions modulates the bottom-up processing for rod-mediated color perception. In Experiment 2, observers reported changes in hue percepts for a test ring surrounded by inducing rings that varied in spatial context. In sum, the results challenge the classic view that rod vision is achromatic and suggest that scotopic hue perception is mediated by cortical mechanisms.

The effect of background and illumination on color identification of real, 3D objects

For the surface reflectance of an object to be a useful cue to object identity, judgments of its color should remain stable across changes in the object's environment. In 2D scenes, there is general consensus that color judgments are much more stable across illumination changes than background changes. Here we investigate whether these findings generalize to real 3D objects. Observers made color matches to cubes as we independently varied both the illumination impinging on the cube and the 3D background of the cube. As in 2D scenes, we found relatively high but imperfect stability of color judgments under an illuminant shift. In contrast to 2D scenes, we found that background had little effect on average color judgments. In addition, variability of color judgments was increased by an illuminant shift and decreased by embedding the cube within a background. Taken together, these results suggest that in real 3D scenes with ample cues to object segregation, the addition of a background may improve stability of color identification.

Perceived segmentation of center from surround by only illusory contours causes chromatic lateral inhibition

When a light and also its surrounding context slowly oscillate in chromaticity over time, the color appearance of the light depends on the relative phase of center and surround. The influence of the surround is generally accounted for by retinotopic center-surround organization, with the surround inhibiting signals from the center. The traditional neural account, however, cannot rule out lateral inhibition due to cortical mechanisms sensitive to object segmentation cues. Experiments here reveal that illusory contours are sufficient to separate a center from its surround. Observers adjusted the Michelson contrast of a matching disk to equal the perceived modulation depth of a central area within a surround. Both the central test and matching disk were maintained at constant luminance and modulated in-phase at 2Hz along one chromatic axis (L/(L+M) or S/(L+M)). The center was perceptually segmented from the surround by either a physical (retinotopic separation) or illusory (cortically represented) triangle contour. Segmentation of center from surround by the illusory contour strongly attenuated the perceived modulation depth for both chromatic axes. Further, the strength of attenuation was consistently greater with the illusory than the physically segmenting triangle. This cannot be accounted for by retinal center-surround antagonism; instead it points to a cortical neural representation of contours, with lateral inhibition following neural mechanisms sensitive to object segmentation cues.

Almost complete colour constancy achieved with full-field adaptation

A successive asymmetric colour-matching task was used to study the changes in colour appearance of simulated Munsell samples. Colour shifts were induced with two Planckian illuminants, standard illuminant A (u'=0.256, v'=0.524) and illuminant S (u'=0.174, v'=0.392). Measurements were conducted with a 20 degrees field and a 120 degrees field. Adaptation period varied from 1 to 30s with the smaller field and from 1 to 60s with the larger field. Colour shifts were specified in terms of a modified Brunswik ratio (BR). Higher values of BR were associated with longer adaptation periods but only when the larger background was used. Supplementary experiments showed that the changes in colour appearance were related to a slight shift in the perceived colour of the background. The timing of the colour shifts are modelled in terms of cone opponent responses. High values of BR correspond to almost complete von Kries adaptation in all three cone types.

Induced steady color shifts from temporally varying surrounds

The color appearance of a physically steady central region can appear to vary over time if a surrounding chromatic light varies in time. The induced temporal variation, however, is strongly attenuated at surround temporal frequencies above approximately 3 Hz. At these higher temporal frequencies, the central region appears steady (De Valois et al., 1986). The posited explanation is a cortical low-pass temporal filter. Here, we investigate whether higher temporal-frequency surrounds induce color shifts in the steady appearance of the central test. Surrounds modulated in time along the l or s chromatic direction of MacLeod-Boynton color space were symmetric around equal-energy white (EEW). The temporal frequency of the surround was varied. If observers perceived the central test to be temporally modulating between two points in time, they set two separate matches to the extreme points of this modulation. If the central test appeared steady in time, then color matches were made to this steady appearance. Corroborating previous reports, measurements showed that surround temporal frequencies below approximately 3 Hz induced temporal modulation. At higher temporal frequencies, however, the surround induced steady color shifts, compared to a steady surround at its time average (EEW). The measurements imply that a nonlinear neural process affects chromatic induction from time-varying context.

Sensory, computational and cognitive components of human colour constancy

When the illumination on a scene changes, so do the visual signals elicited by that scene. In spite of these changes, the objects within a scene tend to remain constant in their apparent colour. We start this review by discussing the psychophysical procedures that have been used to quantify colour constancy. The transformation imposed on the visual signals by a change in illumination dictates what the visual system must 'undo' to achieve constancy. The problem is mathematically underdetermined, and can be solved only by exploiting regularities of the visual world. The last decade has seen a substantial increase in our knowledge of such regularities as technical advances have made it possible to make empirical measurements of large numbers of environmental scenes and illuminants. This review provides a taxonomy of models of human colour constancy based first on the assumptions they make about how the inverse transformation might be simplified, and second, on how the parameters of the inverse transformation might be set by elements of a complex scene. Candidate algorithms for human colour constancy are represented graphically and pictorially, and the availability and utility of an accurate estimate of the illuminant is discussed. Throughout this review, we consider both the information that is, in principle, available and empirical assessments of what information the visual system actually uses. In the final section we discuss where in our visual systems these computations might be implemented.

Chromatic shadow compatibility and cone-excitation ratios

Logvinenko [Perception 31, 201 (2002)] asserts that Adelson's wall-of-blocks illusion [Science 262, 2042 (1993)], where identical gray-cube surface tops appear to differ in brightness, arises when the surfaces surrounding the cube tops are shadow compatible, creating a concomitant illusion of transparency. We replicated Logvinenko's main findings in the chromatic domain across three experiments in which observers match cube tops in hue, saturation, and brightness. A second set of stimuli adjusted cone-excitation ratios across the apparent transparency border [Proc. R. Soc. London 257, 115 (1994)], which enhanced lightness and brightness constancy but only when the stimuli varied in both chromaticity and intensity.

The spatial tuning of chromatic adaptation

A key question in colour research is how the colour and spatial analysis of an image interact. Traditionally, colour and form analysis have been regarded as parallel and separate processes, and documented effects of image complexity on chromatic adaptation have been attributed to a temporal integration process during eye movements. Evidence is presented here for a spatial mechanism, which tunes chromatic adaptation to the luminance structure (spatial frequency and orientation) of an image. This in turn suggests a close cooperation between colour and form analysis during chromatic adaptation. The results are discussed in relation to the "segregated pathway hypothesis" and the role of spatial aspects for the computation of colour constancy and adaptation to natural scenes.

Stereo disparity improves color constancy

Binocular 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.

Second-order statistics of colour codes modulate transformations that effectuate varying degrees of scene invariance and illumination invariance

We argue, from an ethology-inspired perspective, that the internal concepts 'surface colours' and 'illumination colours' are part of the data format of two different representational primitives. Thus, the internal concept of 'colour' is not a unitary one but rather refers to two different types of 'data structure', each with its own proprietary types of parameters and relations. The relation of these representational structures is modulated by a class of parameterised transformations whose effects are mirrored in the idealised computational achievements of illumination invariance of colour codes, on the one hand, and scene invariance, on the other hand. Because the same characteristics of a light array reaching the eye can be physically produced in many different ways, the visual system, then, has to make an 'inference' whether a chromatic deviation of the space-averaged colour codes from the neutral point is due to a 'non-normal', ie chromatic, illumination or due to an imbalanced spectral reflectance composition. We provide evidence that the visual system uses second-order statistics of chromatic codes of a single view of a scene in order to modulate corresponding transformations. In our experiments we used centre surround configurations with inhomogeneous surrounds given by a random structure of overlapping circles, referred to as Seurat configurations. Each family of surrounds has a fixed space-average of colour codes, but differs with respect to the covariance matrix of colour codes of pixels that defines the chromatic variance along some chromatic axis and the covariance between luminance and chromatic channels. We found that dominant wavelengths of red-green equilibrium settings of the infield exhibited a stable and strong dependence on the chromatic variance of the surround. High variances resulted in a tendency towards 'scene invariance', low variances in a tendency towards 'illumination invariance' of the infield.

Lightness constancy: a direct test of the illumination-estimation hypothesis

Many models of color constancy assume that the visual system estimates the scene illuminant and uses this estimate to determine an object's color appearance. A version of this illumination-estimation hypothesis, in which the illuminant estimate is associated with the explicitly perceived illuminant, was tested. Observers made appearance matches between two experimental chambers. Observers adjusted the illumination in one chamber to match that in the other and then adjusted a test patch in one chamber to match the surface lightness of a patch in the other. The illumination-estimation hypothesis, as formulated here, predicted that after both matches the luminances of the light reflected from the test patches would be identical. The data contradict this prediction. A second experiment showed that manipulating the immediate surround of a test patch can affect perceived lightness without affecting perceived illumination. This finding also falsifies the illumination-estimation hypothesis.