Chromatic induction: border contrast or adaptation to surrounding light?

Matching visual induction effects on screens of different size

In the film industry, the same movie is expected to be watched on displays of vastly different sizes, from cinema screens to mobile phones. But visual induction, the perceptual phenomenon by which the appearance of a scene region is affected by its surroundings, will be different for the same image shown on two displays of different dimensions. This phenomenon presents a practical challenge for the preservation of the artistic intentions of filmmakers, because it can lead to shifts in image appearance between viewing destinations. In this work, we show that a neural field model based on the efficient representation principle is able to predict induction effects and how, by regularizing its associated energy functional, the model is still able to represent induction but is now invertible. From this finding, we propose a method to preprocess an image in a screen-size dependent way so that its perception, in terms of visual induction, may remain constant across displays of different size. The potential of the method is demonstrated through psychophysical experiments on synthetic images and qualitative examples on natural images.

Specular highlights improve color constancy when other cues are weakened

Previous studies suggest that to achieve color constancy, the human visual system makes use of multiple cues, including a priori assumptions about the illumination ("daylight priors"). Specular highlights have been proposed to aid constancy, but the evidence for their usefulness is mixed. Here, we used a novel cue-combination approach to test whether the presence of specular highlights or the validity of a daylight prior improves illumination chromaticity estimates, inferred from achromatic settings, to determine whether and under which conditions either cue contributes to color constancy. Observers made achromatic settings within three-dimensional rendered scenes containing matte or glossy shapes, illuminated by either daylight or nondaylight illuminations. We assessed both the variability of these settings and their accuracy, in terms of the standard color constancy index (CCI). When a spectrally uniform background was present, neither CCIs nor variability improved with specular highlights or daylight illuminants (Experiment 1). When a Mondrian background was introduced, CCIs decreased overall but were higher for scenes containing glossy, as opposed to matte, shapes (Experiments 2 and 3). There was no overall reduction in variability of settings and no benefit for scenes illuminated by daylights. Taken together, these results suggest that the human visual system indeed uses specular highlights to improve color constancy but only when other cues, such as from the local surround, are weakened.

A Compound Computational Model for Filling-In Processes Triggered by Edges: Watercolor Illusions

The goal of our research was to develop a compound computational model with the ability to predict different variations of the “watercolor effects” and additional filling-in effects that are triggered by edges. The model is based on a filling-in mechanism solved by a Poisson equation, which considers the different gradients as “heat sources” after the gradients modification. The biased (modified) contours (edges) are ranked and determined according to their dominancy across the different chromatic and achromatic channels. The color and intensity of the perceived surface are calculated through a diffusive filling-in process of color triggered by the enhanced and biased edges of stimulus formed as a result of oriented double-opponent receptive fields. The model can successfully predict both the assimilative and non-assimilative watercolor effects, as well as a number of “conflicting” visual effects. Furthermore, the model can also predict the classic Craik–O'Brien–Cornsweet (COC) effect. In summary, our proposed computational model is able to predict most of the “conflicting” filling-in effects that derive from edges that have been recently described in the literature, and thus supports the theory that a shared visual mechanism is responsible for the vast variety of the “conflicting” filling-in effects that derive from edges.

The Gold Standard Paradox in Digital Image Analysis: Manual Versus Automated Scoring as Ground Truth

CONTEXT

- Novel therapeutics often target complex cellular mechanisms. Increasingly, quantitative methods like digital tissue image analysis (tIA) are required to evaluate correspondingly complex biomarkers to elucidate subtle phenotypes that can inform treatment decisions with these targeted therapies. These tIA systems need a gold standard, or reference method, to establish analytical validity. Conventional, subjective histopathologic scores assigned by an experienced pathologist are the gold standard in anatomic pathology and are an attractive reference method. The pathologist's score can establish the ground truth to assess a tIA solution's analytical performance. The paradox of this validation strategy, however, is that tIA is often used to assist pathologists to score complex biomarkers because it is more objective and reproducible than manual evaluation alone by overcoming known biases in a human's visual evaluation of tissue, and because it can generate endpoints that cannot be generated by a human observer.

OBJECTIVE

- To discuss common visual and cognitive traps known in traditional pathology-based scoring paradigms that may impact characterization of tIA-assisted scoring accuracy, sensitivity, and specificity.

DATA SOURCES

- This manuscript reviews the current literature from the past decades available for traditional subjective pathology scoring paradigms and known cognitive and visual traps relevant to these scoring paradigms.

CONCLUSIONS

- Awareness of the gold standard paradox is necessary when using traditional pathologist scores to analytically validate a tIA tool because image analysis is used specifically to overcome known sources of bias in visual assessment of tissue sections.

Dichoptic perception of brown

Two experiments assessed mechanisms underlying brown induction by presenting a foveal target disk and concentric annular surround stimuli that varied in contrast relative to larger backgrounds. Stimuli were presented under monocular, binocular, and dichoptic viewing conditions. Observers adjusted the luminance of the target disk to a criterion brown level. We found evidence for at least two separate mechanisms for brown induction: one mechanism that is dependent on physically contiguous contrast and operates in monocular pathways and another mechanism that responds to high luminance contrast anywhere in the visual field and can operate after convergence of signals from the two eyes.

No effects of surround complexity on brown induction

A yellow stimulus turns brown when it is made sufficiently darker than its surroundings. Most previous studies have used simple contiguous surround stimuli to induce brown, so we know little about how brown induction may be controlled by more distant and more complex surround features. We begin to address this issue by varying the complexity of two configurations of achromatic surround stimuli. It was shown that the area most immediately contiguous to the test stimulus has strong effects on brown induction. More importantly, we found that neither the number of surround features nor the distribution of light in the surround region had an effect on brown induction, as long as the overall size of the surround region remained constant. Instead, we found that brown induction depended on the total amount of light in the constant-size surround region, regardless of how that light was distributed. This potentially distinguishes the mechanisms of brown induction from those of brightness induction.

Luminance-dependent long-term chromatic adaptation

There is theoretical and empirical support for long-term adaptation of human vision to chromatic regularities in the environment. The current study investigates whether relationships of luminance and chromaticity in the natural environment could drive chromatic adaptation independently and differently for bright and dark colors. This is motivated by psychophysical evidence of systematic difference shifts in red-green chromatic sensitivities between contextually bright- versus dark-colored stimuli. For some broad classes of scene content, consistent shifts in chromaticity are found between high and low light levels within images. Especially in those images in which sky and terrain are juxtaposed, this shift has direction and magnitude consistent with the observed psychophysical shifts in the red-green balance between bright and dark colors. Taken together, these findings suggest that relative weighting of M- and L-cone signals could be adapted, in a luminance-dependent fashion, to regularities in the natural environment.

Colour contrasting between tissues predicts the resection in 5-aminolevulinic acid-guided surgery of malignant gliomas

Due to the various intensities of 5-aminolevulinic acid (5-ALA) fluorescence, neurosurgeons tend to be uncertain about which tissues to resect. This study aimed to reveal the shortcomings of the human visual perception of fluorescence, particularly the factors guiding the tissue removal and the correlation of fluorescence with contrast enhancement (CE) on magnetic resonance imaging (MRI). Various colour features [CIE L*a*b* colour space, colour difference described by ΔE and contrast ratio (CR)] of total 206 noticed fluorescent areas and their surroundings were measured from the video recordings of 21 primary high grade glioma (HGG) surgeries. The position of a fluorescent region was related to the corecorded navigational image. Following early postoperative MRI, 17 additional regions of corresponding to CE remnants were identified, their colour features were compared to the resected CEs. The targeted video post-processing method was designed, based on the results. There were no complications attributed to 5-ALA use and the median survival was <10 months. 82.5 % of recognised fluorescent areas were removed. Colour spaces of the resected regions and their backgrounds did not overlap. Opposite to the separate colour components (p > 0.05), the distant background colour (p < 0.05) and higher CR and ΔE (p < 0.01) determined the resection of a fluorescent region. Noneloquent location and CR both independently increased the resection rate in logistic regression. However, greater area under the receiver operating characteristic curve (AUC) in case of CR (AUC = 0.78; 95 % CI 0.71-0.83) determined its dominant role in neurosurgeon's fluorescence perception. CE regions presented with a significantly more saturated shade of violet (consistently higher a* and b*) than other tumour parts (p < 0.05). Regions corresponding to tumour remnants had a significantly lower a* component value (p = 0.02) as well as a lower ΔE than the matched background (AUC = 0.73; 95 % CI 0.65-0.80). In order to increase the resection rate, ΔE > 60 was needed. These results directed essential improvements in the 5-ALA fluorescence visualisation toward enhanced resection rate. The conventional filtering, unadjusted to the 5-ALA colour space converted some background shades to colours resembling relevant fluorescence. This is one of the first studies to demonstrate that perceived colours, their contrasting and CR are of significance in the decision-making during HGG 5-ALA fluorescence-guided surgery. Irrespective of the shortcomings of conventional video filtering, further development of a tailored post-processed contrast stretching will allow to achieve safe and radical tumour resection.

Chromatic induction from surrounding stimuli under perceptual suppression

The appearance of colors can be affected by their spatiotemporal context. The shift in color appearance according to the surrounding colors is called color induction or chromatic induction; in particular, the shift in opponent color of the surround is called chromatic contrast. To investigate whether chromatic induction occurs even when the chromatic surround is imperceptible, we measured chromatic induction during interocular suppression. A multicolor or uniform color field was presented as the surround stimulus, and a colored continuous flash suppression (CFS) stimulus was presented to the dominant eye of each subject. The subjects were asked to report the appearance of the test field only when the stationary surround stimulus is invisible by interocular suppression with CFS. The resulting shifts in color appearance due to chromatic induction were significant even under the conditions of interocular suppression for all surround stimuli. The magnitude of chromatic induction differed with the surround conditions, and this difference was preserved regardless of the viewing conditions. The chromatic induction effect was reduced by CFS, in proportion to the magnitude of chromatic induction under natural (i.e., no-CFS) viewing conditions. According to an analysis with linear model fitting, we revealed the presence of at least two kinds of subprocesses for chromatic induction that reside at higher and lower levels than the site of interocular suppression. One mechanism yields different degrees of chromatic induction based on the complexity of the surround, which is unaffected by interocular suppression, while the other mechanism changes its output with interocular suppression acting as a gain control. Our results imply that the total chromatic induction effect is achieved via a linear summation of outputs from mechanisms that reside at different levels of visual processing.

Short-term memory affects color perception in context

Color-based object selection - for instance, looking for ripe tomatoes in the market - places demands on both perceptual and memory processes: it is necessary to form a stable perceptual estimate of surface color from a variable visual signal, as well as to retain multiple perceptual estimates in memory while comparing objects. Nevertheless, perceptual and memory processes in the color domain are generally studied in separate research programs with the assumption that they are independent. Here, we demonstrate a strong failure of independence between color perception and memory: the effect of context on color appearance is substantially weakened by a short retention interval between a reference and test stimulus. This somewhat counterintuitive result is consistent with Bayesian estimation: as the precision of the representation of the reference surface and its context decays in memory, prior information gains more weight, causing the retained percepts to be drawn toward prior information about surface and context color. This interaction implies that to fully understand information processing in real-world color tasks, perception and memory need to be considered jointly.

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.

Very-long-term and short-term chromatic adaptation: are their influences cumulative?

Very-long-term (VLT) chromatic adaptation results from exposure to an altered chromatic environment for days or weeks. Color shifts from VLT adaptation are observed hours or days after leaving the altered environment. Short-term chromatic adaptation, on the other hand, results from exposure for a few minutes or less, with color shifts measured within seconds or a few minutes after the adapting light is extinguished; recovery to the pre-adapted state is complete in less than an hour. Here, both types of adaptation were combined. All adaptation was to reddish-appearing long-wavelength light. Shifts in unique yellow were measured following adaptation. Previous studies demonstrate shifts in unique yellow due to VLT chromatic adaptation, but shifts from short-term chromatic adaptation to comparable adapting light can be far greater than from VLT adaptation. The question considered here is whether the color shifts from VLT adaptation are cumulative with large shifts from short-term adaptation or, alternatively, does simultaneous short-term adaptation eliminate color shifts caused by VLT adaptation. The results show the color shifts from VLT and short-term adaptation together are cumulative, which indicates that both short-term and very-long-term chromatic adaptation affect color perception during natural viewing.

Edges can eliminate the appearance of the contrast asynchrony

Recent work in the Shapiro laboratory has suggested that the visual response to changes in chromaticity/luminance can be separated from the visual response to changes in spatial contrast. Here, we examine how spatial edges affect the relative perceptual weighting of these two types of responses. In the experiments, we separate color from color contrast with a 'contrast asynchrony' stimulus in which the luminance of two identical rectangles varies sinusoidally over time. We use two different stimulus configurations: in one configuration, one rectangle is placed on a black background, and the other is placed on a white background; in the other configuration, the two rectangles are placed on a striped background (similar to Munker-White's background), with one rectangle set against a white stripe and the other against a black stripe. Experiment 1 documents that the rectangle placed on the solid white background appears to modulate out of phase with the rectangle placed on the solid black background, and that the two rectangles placed on the striped background appear to modulate in phase with each other. Experiment 2 measured the length the background stripes must be to shift from the perception of in-phase modulation to antiphase modulation (and vice versa). In the solid background configuration, the perceived shift from in-phase to antiphase occurred when edges above and below the rectangles were about 0.5°; and in the striped background configuration, the perceived shift from antiphase to in-phase occurred when the edges were < 10 min of arc. Experiment 3 showed that edges that could engender the perception of the contrast asynchrony in the striped background configuration had no effect on the perceived brightness of the bars. The results indicate that edges placed on opposite sides of the modulating field can inhibit the contrast response but do not necessarily affect the perceived brightness.

Color constancy

A quarter of a century ago, the first systematic behavioral experiments were performed to clarify the nature of color constancy-the effect whereby the perceived color of a surface remains constant despite changes in the spectrum of the illumination. At about the same time, new models of color constancy appeared, along with physiological data on cortical mechanisms and photographic colorimetric measurements of natural scenes. Since then, as this review shows, there have been many advances. The theoretical requirements for constancy have been better delineated and the range of experimental techniques has been greatly expanded; novel invariant properties of images and a variety of neural mechanisms have been identified; and increasing recognition has been given to the relevance of natural surfaces and scenes as laboratory stimuli. Even so, there remain many theoretical and experimental challenges, not least to develop an account of color constancy that goes beyond deterministic and relatively simple laboratory stimuli and instead deals with the intrinsically variable nature of surfaces and illuminations present in the natural world.

How are lateral chromatic interactions computed from cone signals?

A small gray test field superimposed on a large colored background appears tinted in a color complementary to that of the surround. We tested the hypothesis whether photoreceptor sensitivity in the test field is altered in the presence of a colored surround. We investigated this effect using dichoptic viewing conditions. With the left eye, subjects viewed a small gray target superimposed on a large colored background. The right eye saw a gray target superimposed on a large gray background. When the two images were fused, observers perceived one homogeneous background and two targets. Observers matched the color of the target seen by the right eye to that seen by the left eye. A modified two-stage model for chromatic induction assuming that both receptor and postreceptor mechanisms contribute to the shift in color was fitted to the matched settings. We find that the dichoptic viewing effects presented here are well explained by an approximately equal contribution of receptor and postreceptor processes to the perceived shift in color.

Simultaneous Color Contrast in 4-Month-Old Infants

The present paper addresses the question of simultaneous color contrast in 4-month-old human infants. A temporal modulation paradigm was employed for infant testing. In this paradigm, infants viewed two test disks presented side-by-side: one of unchanging chromaticity (static) and another of the chromaticity varied in time (temporally modulated). The test stimuli were embedded in a surround that was either static or temporally modulated in phase with the modulated test stimulus. The temporally modulated test stimuli were chosen in such a way as to appear static to adults when viewed in the temporally modulated surround. On the basis of the observation that infants prefer to look more at flickering stimuli, the prediction is that, if infants have adult-like simultaneous color contrast, their preference for the temporally modulated stimulus should decrease and their preference for the static stimulus should increase when the surround is also temporally modulated as described. In concordance with this prediction, a significant increase in preference for the temporally static stimuli was observed with the introduction of temporal modulation in the surround. The data are consistent with the conclusion that infants as young as 4 months of age have simultaneous color contrast.

Perceiving colour at a glimpse: The relevance of where one fixates

We used classification images to examine whether certain parts of a surface are particularly important when judging its colour, such as its centre, its edges, or where one is looking. The scene consisted of a regular pattern of square tiles with random colours from along a short line in colour space. Targets defined by a square array of brighter tiles were presented for 200ms. The colours of the tiles within the target were biased by an amount that led to about 70% of the responses being correct. Subjects fixated a point that fell within the target's lower left quadrant and reported each target's colour. They tended to report the colour of the tiles near the fixation point. The influence of the tiles' colour reversed at the target's border and was weaker outside the target. The colour at the border itself was not particularly important. When coloured tiles were also presented before (and after) target presentation they had an opposite (but weaker) effect, indicating that the change in colour is important. Comparing the influence of tiles outside the target with that of tiles at the position at which the target would soon appear suggests that when judging surface colours during the short "glimpses" between saccades, temporal comparisons can be at least as important as spatial ones. We conclude that eye movements are important for colour vision, both because they determine which part of the surface of interest will be given most weight and because the perceived colour of such a surface also depends on what one looked at last.

Effects of surround articulation on lightness depend on the spatial arrangement of the articulated region

We investigated the effect of surround articulation on the perceived lightness of a target disk. Surround articulation was manipulated by varying either the number of wedges in a surround consisting of wedges of alternating luminance or the number of checks in a surround consisting of a radial checkerboard pattern. In most conditions, increased articulation caused incremental targets to appear lighter and decremental targets to appear darker. But increasing the surround articulation in a way that did not increase the number of target-coaligned edges in the display did not affect the target lightness. We propose that the effects of surround articulation depend on the relationship between the orientations and contrast polarities of the target edges and those of edges present within the surround.

Spatial frequency difference between textures interferes with brightness perception

Abrupt changes in luminance trigger and restrict brightness filling-in. If brightness was actively filled-in and mediated by cells signaling both luminance borders and surface brightness, then brightness spreading could also get disrupted by changes in texture. We measured psychophysically the brightness of a uniform luminance disk, which was segmented into two parts by different textures. The brightness of the central part of the disk was substantially reduced, and the reduction depended on spatial frequency, but not on the orientation difference between the textures. The results show that texture borders are able to block brightness filling-in. The bandwidth of brightness spreading was estimated to be approximately 1.5 octaves. This suggests that brightness information spreads only between neurons of similar spatial frequency characteristics.

Combining local and global contributions to perceived colour: An analysis of the variability in symmetric and asymmetric colour matching

Are surfaces' colours judged from weighted averages of the light that they reflect to the eyes and the colour contrast at their borders? To find out we asked subjects to set the colour and luminance of test disks to match reference disks, on various backgrounds, and analysed the variability in their settings. Most of the variability between repeated settings was in luminance. The standard deviations in the set colour were smallest when the disk and background were the same colour, irrespective of the colour itself. Matches were equally precise for greenish or reddish disks on a grey background, as for grey disks on a greenish or reddish background. The precision was less dependent on the colour contrast at the disks' borders when the backgrounds were more complex and when there was a large luminance contrast at the disks' borders. Subjects were less precise when different colours surrounded the two disks. These findings are consistent with the perceived colour at any position being a weighted average of the local cone excitation ratio and the change in the cone excitation ratio at the borders of the surface in question. However, the involved weights must be variable and depend systematically on parameters such as the luminance contrast at the surface's borders and other chromatic contrasts within the scene.

Color constancy in natural scenes explained by global image statistics

To what extent do observers' judgments of surface color with natural scenes depend on global image statistics? To address this question, a psychophysical experiment was performed in which images of natural scenes under two successive daylights were presented on a computer-controlled high-resolution color monitor. Observers reported whether there was a change in reflectance of a test surface in the scene. The scenes were obtained with a hyperspectral imaging system and included variously trees, shrubs, grasses, ferns, flowers, rocks, and buildings. Discrimination performance, quantified on a scale of 0 to 1 with a color-constancy index, varied from 0.69 to 0.97 over 21 scenes and two illuminant changes, from a correlated color temperature of 25,000 K to 6700 K and from 4000 K to 6700 K. The best account of these effects was provided by receptor-based rather than colorimetric properties of the images. Thus, in a linear regression, 43% of the variance in constancy index was explained by the log of the mean relative deviation in spatial cone-excitation ratios evaluated globally across the two images of a scene. A further 20% was explained by including the mean chroma of the first image and its difference from that of the second image and a further 7% by the mean difference in hue. Together, all four global color properties accounted for 70% of the variance and provided a good fit to the effects of scene and of illuminant change on color constancy, and, additionally, of changing test-surface position. By contrast, a spatial-frequency analysis of the images showed that the gradient of the luminance amplitude spectrum accounted for only 5% of the variance.

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.

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.

Minimalist surface-colour matching

Some theories of surface-colour perception assume that observers estimate the illuminant on a scene so that its effects can be discounted. A critical test of this interpretation of colour constancy is whether surface-colour matching is worse when the number of surfaces in a scene is so small that any illuminant estimate is unreliable. In the experiment reported here, observers made asymmetric colour matches between pairs of simultaneously presented Mondrian-like patterns under different daylights. The patterns had either 49 surfaces or a minimal 2 surfaces. No significant effect of number was found, suggesting that illuminant estimates are unnecessary for surface-colour matching.

Computational adaptation model and its predictions for color induction of first and second orders

The appearance of a patch of color or its contrast depends not only on the stimulus itself but also on the surrounding stimuli (induction effects-simultaneous contrast). A comprehensive computational physiological model is presented to describe chromatic adaptation of the first (retinal) and second (cortical) orders, and to predict the different chromatic induction effects. We propose that the chromatic induction of the first order that yields perceived complementary colors can be predicted by retinal adaptation mechanisms, contrary to previous suggestions. The second order of the proposed adaptation mechanism succeeds to predict the automatic perceived inhibition or facilitation of the central contrast of a texture stimulus, depending on the surrounding contrast. Furthermore, contrary to other models, this model is able to also predict the effect of variegated surrounding on the central perceived color.

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.

Does colour constancy exist?

For a stable visual world, the colours of objects should appear the same under different lights. This property of colour constancy has been assumed to be fundamental to vision, and many experimental attempts have been made to quantify it. I contend here, however, that the usual methods of measurement are either too coarse or concentrate not on colour constancy itself, but on other, complementary aspects of scene perception. Whether colour constancy exists other than in nominal terms remains unclear.

Colour constancy under simultaneous changes in surface position and illuminant

Two kinds of constancy underlie the everyday perception of surface colour: constancy under changes in illuminant and constancy under changes in surface position. Classically, these two constancies seem to place conflicting demands on the visual system: to both take into account the region surrounding a surface and also discount it. It is shown here, however, that the ability of observers to make surface-colour matches across simultaneous changes in test-surface position and illuminant in computer-generated 'Mondrian' patterns is almost as good as across changes in illuminant alone. Performance was no poorer when the surfaces surrounding the test surface were permuted, or when information from a potential comparison surface, the one with the highest luminance, was suppressed. Computer simulations of cone-photoreceptor activity showed that a reliable cue for making surface-colour matches in all experimental conditions was provided by the ratios of cone excitations between the test surfaces and a spatial average over the whole pattern.

Chromatic assimilation: spread light or neural mechanism?

Chromatic 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.

Chromatic assimilation unaffected by perceived depth of inducing light

Chromatic assimilation is a shift toward the color of nearby light. Several studies conclude that a neural process contributes to assimilation but the neural locus remains in question. Some studies posit a peripheral process, such as retinal receptive-field organization, while others claim the neural mechanism follows depth perception, figure/ground segregation, or perceptual grouping. The experiments here tested whether assimilation depends on a neural process that follows stereoscopic depth perception. By introducing binocular disparity, the test field judged in color was made to appear in a different depth plane than the light that induced assimilation. The chromaticity and spatial frequency of the inducing light, and the chromaticity of the test light, were varied. Chromatic assimilation was found with all inducing-light sizes and chromaticities, but the magnitude of assimilation did not depend on the perceived relative depth planes of the test and inducing fields. We found no evidence to support the view that chromatic assimilation depends on a neural process that follows binocular combination of the two eyes' signals.

Chromatic induction in humans: how are the cone signals combined to provide opponent processing?

We investigated the effect of 16 isoluminant chromatic surrounds on the perceived colour of an enclosed grey test-field at photopic (43 cd/m2) conditions. Stimuli were shown on a grey background identical to the test-field. Use of these stimuli implies that activations of receptoral (cS, cM, and cL) and postreceptoral (cM-cL, cS-(cM+cL)) mechanisms by surround colours are known quantitatively. This allows to predict shifts in colour of the test-field in terms of receptoral (adaptation) as well as postreceptoral (contrast) mechanisms assuming a standard two-stage model. Predictions are tested using matching and hue compensation procedures. Both procedures yield comparable results that are consistent with the assumption that postreceptoral mechanisms explain the observed shifts in perceived colour.

The roles of distractor noise and target certainty in search: a signal detection model

Observers searched arrays of briefly presented near-isoluminant colored disks for a single disk of known color (feature search) or unknown color (oddity search). Speed and accuracy were converted to a single, model-based measure of performance (Perf), in units of (d')2 per second of latency. Perf decreased with set size in feature search and increased in oddity. In both types of search, grouping the distractors, making them homogeneous in color, and reducing their saturation, all increased Perf. These commonalities suggested an SDT-based model in which distractors increase noise in the same way in both types of search. However, in oddity, though not in feature search, distractors must be attended and so adding distractors also boosts the effective target contrast, overcoming the added noise. A model with two free parameters for noise and one for attention accounted for every combination except for oddity searches among heterogeneous grouped distractors.

The peculiar nature of simultaneous colour contrast in uniform surrounds

We present evidence from asymmetric colour matching experiments which strongly suggests that uniform surrounds evoke induction effects of a very peculiar nature, not representative of colour induction effects in variegated surrounds. Given the widespread use of uniform surrounds in studies of colour vision, this finding is of interest in relation to a number of current research issues, such as contrast coding of colour, functionally equivalent surrounds and colour constancy. A framework that systematises the seemingly complex colour appearance changes induced by uniform surrounds is presented and its implications are discussed.

Investigation of color constancy with a neural network

The perceptual stability of an object's color under different illuminants is called color constancy. We created a neural network to investigate this phenomenon. The net consisted of one input channel for the background and one for the test object. Each channel had a set of three (L, M, and S) receptors that were transmitting to three opponent neurons. The signals from the opponent neurons were passed to hidden neurons, which were connected to the output neurons. The output signal was generated from the three components of a color vector. The neural net was trained to identify the color of Munsell samples under various illuminants using the back-propagation algorithm. Our study investigated the properties of a successfully trained neural network. Based on the cross-neuron weight analysis, we report that the successfully trained neural net calculates color differences between the test object and the background. By comparing the human visual system to the neural net, we conclude that to satisfy the color constancy phenomenon, the human visual system has to contain two separate components: one to approximate the background color and the other to estimate the color difference between the object and the background.

Color contrast: a contributory mechanism to color constancy

Color constancy--by which objects tend to appear the same color under changes in illumination--is most likely achieved by several mechanisms, operating at different levels in the visual system. One powerful contributory mechanism is simultaneous spatial color contrast. Under changes in natural illumination the spatial ratios of within-type cone excitations between natural surfaces tend to be preserved (Foster and Nascimento, 1994); therefore, the neural encoding of colors as spatial contrasts tends to achieve constancy. Several factors are known to influence the strength of chromatic contrast induction between surfaces, including their relative luminance, spatial scale, spatial configuration and context (Ware and Cowan, 1982; Zaidi et al., 1991). Here we test the hypothesis that color contrast is weakened by differences between surfaces which indicate that they may be under distinct illuminants. We summarize psychophysical measurements of the effects of relative motion, relative depth and texture differences on chromatic contrast induction. Of these factors, only texture differences between surfaces weaken chromatic contrast induction. We also consider neurophysiological and neuropsychological evidence and conclude that the mechanisms which mediate local chromatic contrast effects are sited at low levels in the visual system, in primary visual cortex (V1) or below, prior to image segmentation mechanisms which require computation of relative depth or motion. V1 and lower areas may therefore play a larger role in color constancy than previously thought.

Design and development of an indoor navigation and object identification system for the blind

In this paper we present a new system that assists blind users in orienting themselves in indoor environments. We developed a sensor module that can be handled like a flashlight by a blind user and can be used for searching tasks within the three-dimensional environment. By pressing keys, inquiries concerning object characteristics, position, orientation and navigation can be sent to a connected portable computer, or to a federation of data servers providing models of the environment. Finally these inquiries are acoustically answered over a text-to-speech engine.

Large shifts in color appearance from patterned chromatic backgrounds

The perceived color of a light varies with the background on which it is seen. In the present study, patterned backgrounds composed of two different chromaticities caused larger shifts in perceived color than did a uniform background at either chromaticity within the pattern. Cortical receptive-field organization, but not optical factors or known retinal neurons, can account for the color shifts from patterned backgrounds.

Chromatic induction and the layout of colours within a complex scene

A target's apparent colour is influenced by the colours in its surrounding. If the surrounding consists of a single coloured surface, the influence is a shift 'away' from the surface's colour. If the surface is more than 1 degrees from the target area the shift is very small. If there are many surfaces, then not only the average luminance and chromaticity of the surfaces matters, but also the chromatic variability. It is not yet clear whether it makes any difference where the chromatic variability is within the scene, so we constructed stimuli in which the chromatic variability was restricted to certain regions. We found that it made very little difference where the chromatic variability was located. The extent to which the average colour of nearby surfaces influences the apparent colour of the target seems to depend on the average chromatic variability of the whole scene.

Contextual influences on visual processing

The visual image formed on the retina represents an amalgam of visual scene properties, including the reflectances of surfaces, their relative positions, and the type of illumination. The challenge facing the visual system is to extract the "meaning" of the image by decomposing it into its environmental causes. For each local region of the image, that extraction of meaning is only possible if information from other regions is taken into account. Of particular importance is a set of image cues revealing surface occlusion and/or lighting conditions. These information-rich cues direct the perceptual interpretation of other more ambiguous image regions. This context-dependent transformation from image to perception has profound-but frequently under-appreciated-implications for neurophysiological studies of visual processing: To demonstrate that neuronal responses are correlated with perception of visual scene properties, rather than visual image features, neuronal sensitivity must be assessed in varied contexts that differentially influence perceptual interpretation. We review a number of recent studies that have used this context-based approach to explore the neuronal bases of visual scene perception.

Cone contributions to colour constancy

Colour constancy refers to the stable perception of object colour under changing illumination conditions. This problem has been reformulated as relational colour constancy, or the ability of the observer to discriminate between material changes and changes in illumination. It has been suggested that local cone excitation ratios play a prominent role in achieving such constancy. Here we show that perceptual colour constancy measured by achromatic adjustments is to a large part complete after 25 ms. This speaks against a prominent role for receptor adaptation, which takes significantly longer. We also found no difference in colour constancy between colour changes that were compatible with a change of illuminant, and between colour changes where local cone ratios were uncorrelated between the two illuminants. Our results show that constant cone ratios are not necessary for colour constancy.

The influence of chromatic and achromatic variability on chromatic induction and perceived colour

Judgments of the colour of a surface are influenced by the colour of the surrounding. To determine whether only the average colour of the surrounding matters, or also the chromatic variability, judgments in colourful scenes are often compared with ones in which a target is surrounded by a plain background that provides the same average physical illumination of the retina as the colourful scene. The variability sometimes makes a difference (eg Shevell and Wei, 1998 Vision Research 38 1561-1566), and sometimes it does not (eg Brenner and Cornelissen, 1998 Vision Research 38 1789-1793). Is this because of the nonlinearity in cone responses? We designed scenes that stimulated the cones in an equivalent manner, both on average and in terms of variability, and yet differed markedly in chromatic variability. The more colourful surroundings had considerably less influence on subjects' colour judgments. We conclude that early cone-specific regulation of sensitivity cannot be responsible for the change in perceived colour, and deduce that chromatic induction takes place after contrast gain control.

Simultaneous S-cone contrast

Chromatic induction is the change in appearance of one light caused by a second, nearby light. We measured chromatic induction in a central test viewed within an inducing field that was varied in only short-wavelength-sensitive (S) cone stimulation. The observer matched the appearance of the central test by adjusting the chromaticity of a haploscopically presented comparison field, seen by the other eye on a dark background. When the central test weakly stimulated S cones, the S-cone level in the surround caused little change in the color appearance of the test. When the central test substantially stimulated S cones, on the other hand, the appearance of the center showed S-cone contrast: raising the level of S in the surround reduced the level of S set to match the central test. Further, a surround that weakly stimulated S cones raised the matching S-cone level above that required without a surround (dark-adapted condition). These results cannot be explained by S-cone sensitivity loss or by a two-process model of adaptation. A cortical mechanism is proposed to mediate S-cone antagonism.

Nonlocal interactions in color perception: nonlinear processing of chromatic signals from remote inducers

The perceived color of an object depends on the chromaticity of its immediate background. But color appearance is also influenced by remote chromaticities. To quantify these influences, the effects of remote color fields on the appearance of a fixated 2 degrees test field were measured using a forced-choice method. Changes in the appearance of the test field were induced by chromaticity changes of the background and of 2 degrees color fields not adjacent to the test field. The appearance changes induced by the color of the background corresponded to a fraction of between 0.5 and 0.95 of the cone contrast of the background change, depending on the observer. The magnitude of induction by the background color was modulated on average by 7.6% by chromaticity changes in the remote color fields. Chromaticity changes in the remote fields had virtually no inducing effect when they occurred without a change in background color. The spatial range of these chromatic interactions extended over at least 10 degrees from the fovea. They were established within the first few hundred milliseconds after the change of background color and depended only weakly on the number of inducing fields. These results may be interpreted as reflecting rapid chromatic interactions that support robustness of color vision under changing viewing conditions.

The spatial transformation of color in the primary visual cortex of the macaque monkey

Perceptually, color is used to discriminate objects by hue and to identify color boundaries. The primate retina and the lateral geniculate nucleus (LGN) have cell populations sensitive to color modulation, but the role of the primary visual cortex (V1) in color signal processing is uncertain. We re-evaluated color processing in V1 by studying single-neuron responses to luminance and to equiluminant color patterns equated for cone contrast. Many neurons respond robustly to both equiluminant color and luminance modulation (color-luminance cells). Also, there are neurons that prefer luminance (luminance cells), and a few neurons that prefer color (color cells). Surprisingly, most color-luminance cells are spatial-frequency tuned, with approximately equal selectivity for chromatic and achromatic patterns. Therefore, V1 retains the color sensitivity provided by the LGN, and adds spatial selectivity for color boundaries.

A central mechanism of chromatic contrast

The color appearance of a light can be altered by introducing a second, surrounding field. This phenomenon, called chromatic induction, is attenuated by chromatic variation within a remote region outside the surround [Shevell & Wei (1998). Vision Research, 38, 1561-1566]. We now consider the locus of the neural mechanism mediating the attenuation caused by the remote chromatic contrast. In the first experiment, the magnitude of chromatic variation within the remote region is changed either: (i) in the same eye that views the patch judged in color; or (ii) in only the opposite eye. The measurements are virtually the same in both cases, which implies attenuation of chromatic induction is mediated by a central, binocular mechanism. In the second experiment, the patch with its immediate inducing surround is changed in binocular disparity relative to the remote region with chromatic variation. The patch and surround, seen together in one depth plane, are perceived to be in front of, behind, or in the same plane as the remote region with chromatic variation. Attenuation of chromatic induction is strongest when the patch and surround are in the same depth plane as the remote region. This change of color appearance with disparity is consistent with a central binocular process. Overall, the color-appearance measurements are explained by monocular encoding of chromatic differences at edges, and a central binocular mechanism of chromatic-contrast gain control.

Time course of chromatic adaptation for color appearance and discrimination

Adaptation to a steady background has a profound effect on both color appearance and discrimination. We determined the temporal characteristics of chromatic adaptation for appearance and discrimination along different color directions. Subjects were adapted to a large uniform background made up of a CRT screen and a 45x64 degrees wall, illuminated by computer controlled lamps. After an instant change in background color along a red-green or blue-yellow color axis, we measured thresholds for the detection of increments along the same axes at fixed times between 25 ms and 121 s. Analogously, color appearance was determined using achromatic matching. Three components of adaptation could be identified by their temporal characteristics. A slow exponential time course of adaptation with a half-life of about 20 s was common to appearance and discrimination. A faster component with a half-life of 40-70 ms--probably due to photoreceptor adaptation--was also common to both. Exclusive for color appearance, there was a third, extremely rapid mechanism with a half-life faster than 10 ms. This instantaneous process explained more than 50% of total adaptation for color appearance and could be shown to act in a multiplicative manner. We conclude that this instantaneous adaptation mechanism for color appearance is situated at a later processing stage, after mechanisms common to appearance and discrimination, and is based on multiplicative spatial interactions rather than on local, temporal adaptational processes. Color appearance, and thus color constancy, seems to be determined in large part by cortical computations.

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.

Chromatic induction with remote chromatic contrast varied in magnitude, spatial frequency, and chromaticity

Chromatic induction from a surround is attenuated by chromatic contrast within a remote region outside of the surround (Shevell & Wei, 1998, Vision Research, 38, 1561-1566). The present study reports hue-cancellation measurements that show the attenuation depends on the magnitude, spatial frequency and chromaticity of remote chromatic contrast. Spatial-frequency tuning is shown by maximal attenuation of induction with remote contrast elements of the same size as the test. Experiments with various chromaticities of remote contrast show that S-cone stimulation within the remote region has a much weaker effect than L-/M-cone chromatic contrast, and does not depend on whether the S-cone stimulation is uniform or uneven across the region. Overall, the results show that remote L/M contrast affects classical chromatic induction, with its effect depending on the spatial frequency and magnitude of contrast. The influence of remote S-cone stimulation, on the other hand, is relatively weak and depends on only the S-cone spatial average, at least when S-cone stimulation by the test and its immediate surround is minimal (as in all experiments here).