Chromatic Contour Research Articles

Contour adaptation reduces the spreading of edge induced colors

Brief exposure to flickering achromatic outlines of an area causes a reduction in the brightness contrast of the surface inside the area. This contour adaptation to achromatic contours does not reduce surface contrast when the surface is chromatic (the saturation or colorimetric purity of the surface is maintained). In addition to reducing the brightness of physical luminance contrast, contour adaptation also reduces (or even reverses) the illusory brightness contrast seen in the Craik–O’Brien–Cornsweet illusion, in which two physically identical grey areas appear different brightness because of a sharp luminance edge separating them. Chromatic color spreading illusions also occur with chromatic inducing edges, and an unanswered question is whether contour adaptation can reduce the perceived contrast of illusory color spreading from edges, even though it cannot reduce the perceived contrast of physical surface color. The current studies use a color spreading illusion known as the watercolor effect in order to test whether illusory color spreading is affected by contour adaptation. The general findings of physical achromatic contrast being reduced and chromatic contrast being robust to contour adaptation were replicated. However, both illusory brightness and color were reduced by contour adaptation, even when the illusion edges only differed in chromatic contrast with each other and the background. Additional studies adapting to chromatic contours showed opposite effects on illusory color contrast than achromatic adaptation.

Quantifying the watercolor effect: from stimulus properties to neural models.

Visual illusions are perceptions that violate our expectations with respect to what we understand about the physical stimulus, for example, surfaces of identical spectral composition that appear to be of different colors. Such phenomena are thought to reveal mechanisms, biases, priors or strategies that the brain uses to interpret the visual environment. In natural viewing, we perceive all surfaces within a context of surrounding light and nearby objects, and this context affects their appearance. Change in color appearance due to the surrounding light is called color induction and can be of type contrast, when appearance of a test region shifts in chromaticity away from that of the surround and assimilation when the shift is toward that of the surround. Pinna et al. (Pinna, 1987; Pinna et al., 2001) demonstrated a long-range, color assimilation phenomenon called the Watercolor Effect (WCE) that provides an interesting example for studying such processes. The WCE occurs when a wavy, dark, chromatic contour delineating a figure is flanked on the inside by a lighter chromatic contour on a bright background. The lighter color spreads into the entire enclosed area so that the interior surface is perceived as filled in with a uniform color. Previous studies reported also a weak coloration effect exterior to the contour (Cao et al., 2011; Devinck et al., 2005). The WCE is distinguished from other assimilation illusions due by to its spatial extent; the phenomenon has been reported to be observed over distances of up to 45 deg (Pinna et al., 2001). Here, we review and discuss stimulus configurations, based on different procedures, that induce the WCE and their implications for a neural model of this phenomenon.

The watercolor effect: Spacing constraints

The watercolor effect (WCE) is a long-range color assimilation effect occurring within an area enclosed by a light chromatic contour, which in turn is surrounded by a dark chromatic contour. Here, we studied the effects of chromatic modulation of the WCE for different kinds of spacing between and within the inducing contours, using a hue-cancellation method. When an empty zone or interspace was inserted between the inducing contours (radial spacing), the hue shift required to null the induced coloration rapidly decreased with increasing spacing between the two contours. Similarly, when the continuous contours were replaced by dotted contours (lateral spacing), the shift in chromaticity quickly decreased with increasing distance between the dots. In this case, the decrease was similar for chains of paired dots (“in-phase”) and chains of unpaired dots (“out-of-phase”). Results demonstrate that the WCE is strongest when the two inducing contours are spatially contiguous and continuous. The neural implications of these findings are discussed.

Illusory spreading of watercolor.

The watercolor effect (WCE) is a phenomenon of long-range color assimilation occurring when a dark chromatic contour delineating a figure is flanked on the inside by a brighter chromatic contour; the brighter color spreads into the entire enclosed area. Here, we determined the optimal chromatic parameters and the cone signals supporting the WCE. To that end, we quantified the effect of color assimilation using hue cancellation as a function of hue, colorimetric purity, and cone modulation of inducing contours. When the inner and outer contours had chromaticities that were in opposite directions in color space, a stronger WCE was obtained as compared with other color directions. Additionally, equal colorimetric purity between the outer and inner contours was necessary to obtain a large effect compared with conditions in which the contours differed in colorimetric purity. However, there was no further increase in the magnitude of the effect when the colorimetric purity increased beyond a value corresponding to an equal vector length between the inner and outer contours. Finally, L-M-cone-modulated WCE was perceptually stronger than S-cone-modulated WCE for our conditions. This last result demonstrates that both L-M-cone and S-cone pathways are important for watercolor spreading. Our data suggest that the WCE depends critically upon the particular spatiochromatic arrangement in the display, with the relative chromatic contrast between the inducing contours being particularly important.

The watercolor effect: Quantitative evidence for luminance-dependent mechanisms of long-range color assimilation

When a dark chromatic contour delineating a figure is flanked on the inside by a brighter chromatic contour, the brighter color will spread into the entire enclosed area. This is known as the watercolor effect (WCE). Here we quantified the effect of color spreading using both color-matching and hue-cancellation tasks. Over a wide range of stimulus chromaticities, there was a reliable shift in color appearance that closely followed the direction of the inducing contour. When the contours were equated in luminance, the WCE was still present, but weak. The magnitude of the color spreading increased with increases in luminance contrast between the two contours. Additionally, as the luminance contrast between the contours increased, the chromaticity of the induced color more closely resembled that of the inside contour. The results support the hypothesis that the WCE is mediated by luminance-dependent mechanisms of long-range color assimilation.

The role of the Gestalt principle of similarity in the watercolor illusion

The watercolor illusion presents two main effects: a long-range assimilative color spreading (coloration effect), and properties imparting a strong figure status (figural effect) to a region delimited by a dark (e.g. purple) contour flanked by a lighter chromatic contour (e.g. orange). In four experiments, the strength of the watercolor illusion to determine figure-ground organization is directly compared (combined or pitted against) with the Gestalt principle of similarity both of color and line width. The results demonstrated that (i) the watercolor illusion and, particularly, its figural effect won over the classical Gestalt factors of similarity; (ii) the watercolor illusion cannot be due to the coloration effect as suggested by the similarity principle; (iii) coloration and figural effects may be independent in the watercolor illusion, and (iv) the watercolor illusion can be considered as a principle of figure-ground segregation on its own. Two parallel and independent processes as proposed within the FACADE model (Grossberg, 1994, 1997) are suggested to account for the two effects of coloration and figural enhancement in the watercolor illusion.

Chromatic edges, surfaces and constancies in cerebral achromatopsia

We tested achromatopsic observer, MS, on a number of tasks to establish the extent to which he can process chromatic contour. Stimuli, specified in terms of cone-contrast, were presented in a three-choice oddity paradigm. First we show that MS is able to discriminate the magnitude of chromatic and luminance contrast, but performance is inferior to that of normal observers. Moreover, MS can discriminate isoluminant borders of different chromatic composition. These abilities are not the result of unintended luminance differences and are abolished when chromatic borders are masked by sharp luminance change. In simple displays, local cone-contrast signals can make a significant contribution to surface colour appearance in normal observers. In more complex displays, the perception of a surface’s colour becomes largely independent of the local contrast to its background, via processes presumed to be similar to the edge integration and anchoring stages of Land’s Retinex algorithm. We show that in simple displays the percepts of both MS and normal observers are dominated by local chromatic-contrast. But, although the percepts of normal observers change in line with the predictions of retinex theory in more complex displays, those of MS do not, remaining dominated by local contrast signals. We conclude that MS has lost the ability to perform edge integration and that this loss is closely related to his absence of colour experience.

Attentional capture by colour and motion in cerebral achromatopsia

Cerebral achromatopsia is a rare condition in which damage to the ventromedial occipital area of the cortex results in the loss of colour experience. Nevertheless, cortically colour-blind patients can still use wavelength variation to perceive form and motion. In a series of six experiments we examined whether colour could also direct exogenous attention in an achromatopsic observer. We employed the colour singleton paradigm, the phi motion effect, and the correspondence process to assess attentional modulation. Although colour singletons failed to capture attention, a motion signal, based solely on chromatic information, was able to direct attention in the patient. We then show that the effect is abolished when the chromatic contours of stimuli are masked with simultaneous luminance contrast. We argue that the motion effect is dependent on chromatic contrast mediated via intact colour-opponent mechanisms. The results are taken as further evidence for the processing of wavelength variation in achromatopsia despite the absence of colour experience.

The watercolor effect: a new principle of grouping and figure–ground organization

The watercolor effect is perceived when a dark (e.g., purple) contour is flanked by a lighter chromatic contour (e.g., orange). Under these conditions, the lighter color will assimilate over the entire enclosed area. This filling-in determines figure–ground organization when it is pitted against the classical Gestalt factors of proximity, good continuation, closure, symmetry, convexity, as well as amodal completion, and past experience. When it is combined with a given Gestalt factor, the resulting effect on figure–ground organization is stronger than for each factor alone. When the watercolor effect is induced by a dark red edge instead of an orange edge, its figural strength is reduced, but still stronger than without it. Finally, when a uniform surface is filled physically using the color of the orange fringe, figure–ground organization is not different from that for the purple contour only. These findings show that the watercolor effect induced by the edge could be an independent factor, different from the classical Gestalt factors of figure–ground organization.

Visual hallucinosis: the major clinical determinant of distorted chromatic contour perception in Parkinson's disease.

Recently distorted chromatic contour perception has been demonstrated in Parkinson's disease (PD). The aim of our study is to determine the clinical factors which influence chromatic contour perception in PD. Chromatic and achromatic contour perception, colour discrimination and clinical data were evaluated in 73 patients with PD. We used a computer-aided method to determine the chromatic fusion time (CFT) which indicates the acuity of monochromatic contour perception. Chromatic CFT was generally shortened in patients as compared to controls (p < 0.01), whereas achromatic CFT was not significantly different. Variance analysis revealed the ability of colour discrimination and the risk of visual hallucinations as statistically significant (p < 0.05) variables influencing contour perception of certain stimuli. In contrast, disease stage, disease duration and disease severity have no relevant effect on chromatic contour perception in Parkinson's disease. On the basis of those properties one may suggest that distorted chromatic contour perception is due to an impairment at a central stage of visual processing in PD and an imbalance of the serotonergic system. Whether CFT is a reliable method to predict the individual risk of hallucinosis in PD has to be evaluated.

Combined detection of intensity and chromatic contours in color images

In the literature on computer vision, very few contour detec- tion algorithms are designed to deal with color images. In this paper, we present the multispectral contour detection algorithm (MSCDA) which is designed to process multispectral digital images as well as monochro- matic ones. The MSCDA employs a bidimensional matrix of processing modules. The structure of a processing module is biologically plausible in that it consists of a bank of oriented filters. Each filter is a multispectral processing element (MSPE). A MSPE computes a contrast strength value locally from a receptive field characterized by specific orientation, shape, and size. The contrast strength value is a combination of an intensity contrast value with a chromatic contrast value, which are com- puted separately. Intensity contrast assesses the contrast due to local change in light energy, while chromatic contrast measures the contrast generated by local change in chromatic components. Even- and odd- symmetric MSPE pairs cooperate to extract a combined contrast strength value locally. Each processing module extracts one maximum combined contrast strength response from its bank of MSPEs. The maxi- mum values of the combined contrast strength, provided by the grid of processing modules, form the contrast image. The contour candidate and the contour pixels can be extracted from the contrast image accord- ing to a strategy which is developed through simulations on 1-D and 2-D data sets. The MSCDA is compared with existing contour detection al- gorithms theoretically, and experimental results are shown. The MSCDA accounts for several psychophysical effects which are related to the mammalian visual system and may provide new insights into the under- standing of the operational schemes employed by the visual cortex in combining energy, color and texture information for shape detection. © 1996 Society of Photo-Optical Instrumentation Engineers. Subject terms: contour detection; texture detection; chromatic and achromatic contrast; biological visual system; preattentive and attentive vision; Gabor filters; filter cooperation/competition.

Alterations in Chromatic Contour Perception in de novo Parkinsonian Patients

Recently abnormalities of chromatic visual perception were reported in treated patients with Parkinson's disease (PD). We studied 28 previously untreated Parkinsonian patients and 28 age- and sex-matched controls using a computer-aided method to determine the chromatic fusion time (CFT). CFT indicates the acuity of the perception of monochromatic contours. Patients with PD have a shortened CFT especially for the light-blue and dark-green stimuli. The chromatic perception disorder is correlated with the severity of PD. It can be concluded from our data that the dysfunction of colour perception is related to the pathophysiology of PD.

Depth capture and transparency of regions bounded by illusory and chromatic contours

Spillmann and Redies noted that when a transparent textured pattern is held above the Ehrenstein figure, the subjective surfaces appear to lie not in the plane of the figures but in the plane of the overlying texture. In Experiment 1, we tested this phenomenon with chromatic squares and found that the perceived depth of regions bounded by the chromatic contours was captured by overlying texture planes when the square was equiluminous with the background. We then tested this phenomenon with a variety of illusory contour stimuli and found that it only occurs with figures involving fine line terminators, and not, for example, with the solid Kanizsa triangle. These results suggest that chromatic contours and the illusory contours induced by line terminators provide only weak binocular disparity signals and that these signals are easily overwhelmed by the disparity signals from the overlying luminance texture.

The gap effect revisited: Slow changes in chromatic sensitivity as affected by luminance and chromatic borders

Chromatic discrimination was studied with two half-fields that were either precisely juxtaposed or were separated by a narrow gap. When present, the gap was in some conditions filled with light isoluminant to the test fields. When the fields were juxtaposed, chromatic sensitivity declined with viewing duration. For a discrimination based solely on S cone activity, separating steadily-viewed fields by either a luminance or a purely chromatic gap caused similar enhancements of sensitivity. Neither type of gap had much effect when the fields were flashed. The results may be interpreted as showing that either a luminance or chromatic contour can spatially delimit the two half-fields, thus preventing a slow spatial integration from reducing the discriminability of the two sides of the field.

Binocular rivalry with chromatic contours.

The contribution of colored contours to binocular rivalry was investigated. Orthogonal circular and radial figures were used as stimuli. Binocular rivalry was measured between a fixed achromatic target presented to the left eye and a two-color target presented to the right eye. Various color combinations were presented to the right eye to assess the rivalry strength of these color combinations. The main conclusion is that the contributions of color contours in binocular rivalry and minimally distinct border experiments are closely related; that is, signals from shortwavelength-sensitive cones do not make an appreciable contribution to rivalry.

Effects of flicker-induced depth on chromatic subjective contours.

Two experiments examined the dependence of illusory colors on boundary salience and depth stratification by using flicker-induced depth. The first used a subjective-contour stimulus that appeared as a translucent colored rectangle covering a set of inducing circles and a dark background. The circles were then flickered so as to be perceived as background, and the previously dark background moved forward and appeared as foreground. Simultaneously, the chromatic subjective contour was eliminated. In the second experiment, a subjective-contour (faces/vase-concentric squares) figure was tinted with the McCollough effect, which produced a strong subjective color edge. This edge was visible only with the faces/vase percept and not in the squares percept. Flickering the target locked it into the square configuration because in this case the flicker held the entire pattern in the same depth plane. This eliminated the subjective color edge. Depth stratification and subjective color blockage were maximal at a flicker rate of 6 Hz.

Stereopsis with chromatic contours

In this experiment stereopsis occurred for stimuli defined by chromatic contours. A chromatic contour was defined as a contour formed by a spatial chromatic change with the chromatic components heterochromatically equated using the criterion of a minimally distinct border. Discrimination differed for different combinations of target and background hue. Discrimination was better when the stereoscopic target was at a 30′ binocular disparity than at a 7′ binocular disparity. It was suggested that the discrimination level was related to the distinctness of border of the target against the background for a given magnitude of binocular disparity. Several theories of the cue to stereopsis were discussed in regard to the findings of this experiment.