The Craik-O'Brien-Cornsweet illusion in colour: quantitative characterisation and comparison with luminance

How anisotropy of CIELAB color space affects the separation effect: an experimental study

Almost all modern color difference equations have been developed based on partitioning the value of color difference into the difference of three perceptual color attributes, i.e., hue, chroma, and lightness. Separation of samples, as a parametric effect, has an undeniable impact on the perceived color difference. According to previous studies, there is no meaningful relationship between chromaticity and the separation effect. The present study has been carried out with the aim of investigating the possible relationships between the separation effect and the directions of color difference in CIELAB color space. For this purpose, five color centers were selected. A total number of 44 samples (10 samples around four chromatic color centers and four samples around the gray color center) were prepared employing acrylic water-based painted paperboards. Perceived color difference between each pair against three different backgrounds was assessed through the visual experiments using gray scale method. The results imply that the amount of separation effect depends significantly on the relative position of samples. It is deduced that the separation effect for tritan pairs (two samples in which the difference between them is only due to the response of S cone signals) is lower than deutan pairs (pairs in which the ratio of L cone signals to M cone signals is different for the two samples), which could be attributed to the negligible contribution of S signal difference in producing a naturally occurring border between two abutting samples. Moreover, it is revealed from the outcomes that dark background amplifies the separation effect regardless of color coordinates of the samples.

Does Colour Filling-In Account for Colour Perception in Natural Images?

It is popular to attribute the appearance of extended colour fields to a process of filling-in from the differential colour signals at colour edges, where one colour transitions to another. We ask whether such a process can account for the appearance of extended colour fields in natural images. Some form of colour filling-in must underlie the equiluminant colour Craik-O'Brien-Cornsweet effect and the Watercolour Effect, but these effects are too weak to account for the appearance of extended colour fields in natural images. Moreover, the graded colour disappearance effect reported as evidence for colour filling-in does not work under natural viewing conditions. We demonstrate that natural images do not look very colourful when their colour is restricted to edge transitions. Moreover, purely chromatic images with maximally graded (edgeless) transitions look fully colourful. Consequently, we conclude that colour filling-in makes no more than a minor contribution to the appearance of extended colour regions in natural images.

Representation of Color Surfaces in V1: Edge Enhancement and Unfilled Holes

The neuronal mechanism underlying the representation of color surfaces in primary visual cortex (V1) is not well understood. We tested on color surfaces the previously proposed hypothesis that visual perception of uniform surfaces is mediated by an isomorphic, filled-in representation in V1. We used voltage-sensitive-dye imaging in fixating macaque monkeys to measure V1 population responses to spatially uniform chromatic (red, green, or blue) and achromatic (black or white) squares of different sizes (0.5°-8°) presented for 300 ms. Responses to both color and luminance squares early after stimulus onset were similarly edge-enhanced: for squares 1° and larger, regions corresponding to edges were activated much more than those corresponding to the center. At later times after stimulus onset, responses to achromatic squares' centers increased, partially "filling-in" the V1 representation of the center. The rising phase of the center response was slower for larger squares. Surprisingly, the responses to color squares behaved differently. For color squares of all sizes, responses remained edge-enhanced throughout the stimulus. There was no filling-in of the center. Our results imply that uniform filled-in representations of surfaces in V1 are not required for the perception of uniform surfaces and that chromatic and achromatic squares are represented differently in V1.

SIGNIFICANCE STATEMENT

We used voltage-sensitive dye imaging from V1 of behaving monkeys to test the hypothesis that visual perception of uniform surfaces is mediated by an isomorphic, filled-in representation. We found that the early population responses to chromatic and achromatic surfaces are edge enhanced, emphasizing the importance of edges in surface processing. Next, we show for color surfaces that responses remained edge-enhanced throughout the stimulus presentation whereas response to luminance surfaces showed a slow neuronal 'filling-in' of the center. Our results suggest that isomorphic representation is not a general code for uniform surfaces in V1.

Filling in, filling out, or filtering out: processes stabilizing color appearance near the center of gaze

Spectral sensitivity varies markedly across the center of gaze, in part because of the rapid decline in the density of macular pigment outside the fovea. Yet despite these retinal inhomogeneities, the color appearance of large uniform fields remains very uniform. We explored some of the processes contributing to these stable color percepts by measuring the effects of field size and eccentricity on saturated purples, whose spectra should show the largest biases with macular pigment screening. Small purple fields at 0° and 8° eccentricities differ in appearance but by much less than predicted by the macular screening or by compensation for the average effects of this screening at the two loci. This shows that the compensation is already nearly complete because of local adjustments that filter out the sensitivity variation and confirms that this filtering includes adjustments beyond average gain changes in the cones. In large fields, the appearance is dominated by the local peripheral color. This bias persists when the field edge is fixated or when abrupt edges are removed in Gaussian spots, suggesting that the spreading is not strongly dependent on luminance edges.

Blood oxygen level-dependent activation of the primary visual cortex predicts size adaptation illusion

In natural scenes, objects rarely occur in isolation but appear within a spatiotemporal context. Here, we show that the perceived size of a stimulus is significantly affected by the context of the scene: brief previous presentation of larger or smaller adapting stimuli at the same region of space changes the perceived size of a test stimulus, with larger adapting stimuli causing the test to appear smaller than veridical and vice versa. In a human fMRI study, we measured the blood oxygen level-dependent activation (BOLD) responses of the primary visual cortex (V1) to the contours of large-diameter stimuli and found that activation closely matched the perceptual rather than the retinal stimulus size: the activated area of V1 increased or decreased, depending on the size of the preceding stimulus. A model based on local inhibitory V1 mechanisms simulated the inward or outward shifts of the stimulus contours and hence the perceptual effects. Our findings suggest that area V1 is actively involved in reshaping our perception to match the short-term statistics of the visual scene.

Awareness of Central Luminance Edge is Crucial for the Craik-O'Brien-Cornsweet Effect

The Craik-O'Brien-Cornsweet (COC) effect demonstrates that perceived lightness depends not only on the retinal input at corresponding visual areas but also on distal retinal inputs. In the COC effect, the central edge of an opposing pair of luminance gradients (COC edge) makes adjoining regions with identical luminance appear to be different. To investigate the underlying mechanisms of the effect, we examined whether the subjective awareness of the COC edge is necessary for the generation of the effect. We manipulated the visibility of the COC edge using visual backward masking and continuous flash suppression while monitoring subjective reports regarding online percepts and aftereffects of adaptation. Psychophysical results showed that the online percept of the COC effect nearly vanishes in conditions where the COC edge is rendered invisible. On the other hand, the results of adaptation experiments showed that the COC edge is still processed at the early stage even under the perceptual suppression. These results suggest that processing of the COC edge at the early stage is not sufficient for generating the COC effect, and that subjective awareness of the COC edge is necessary.

The comparison of spatially separated colours

We have measured chromatic discrimination as a function of the spatial separation of the stimuli within the visual field. Pairs of stimuli were presented on an imaginary circle of 5 degrees radius and the distance between their centres was varied up to 10 degrees. Stimulus duration was 100 ms. Constructing an analogue of the MacLeod-Boynton diagram for an extra-foveal observer, we made separate series of measurements for the L/(L+M) and S/(L+M) axes of colour space. For both these axes, discrimination was optimal when there was a small spatial interval between the boundaries of the stimuli; thereafter thresholds rose moderately with increasing separation. Nevertheless, even at a separation of 10 degrees , subjects exhibited impressive discrimination, achieving thresholds in the range 0.4-2% on the L/(L+M) axis and in the range 3-6% on the S/(L+M) axis. Even when the two stimuli fell in different hemifields and transmission of information across the corpus callosum was required, accuracy did not differ significantly from that obtained when both stimuli fell within one hemifield. The human ability to compare remote stimuli requires an explanation. We argue that the discrimination is unlikely to depend on hard-wired neural comparators and may depend on neural representations that can be transmitted on a cerebral bus independently of the particular neurons carrying the code. Contrary to earlier reports, chromatic discrimination was not systematically better in the left visual field than in the right. And only one subject showed a significant advantage of the lower hemifield over the upper hemifield.

Surface color from boundaries: a new 'watercolor' illusion

A colored line flanking a darker border will appear to assimilate its color onto the enclosed white area over distances of up to 45 deg (the Watercolor Effect). This coloration is uniform and complete within 100 ms. We found that thin (6 arcmin), winding inducing lines with different contrasts to the ground are generally more effective than thick, straight, and equiluminant lines. Blue and red lines induce the strongest effects, but watercolor spreading may also be seen with green and yellow. On a white background, color spreading is stronger than on chromatic, gray or black backgrounds. Little or no color is perceived when a narrow white zone (gap) is inserted in between the two inducing lines. However, chains of colored dots instead of continuous lines suffice to produce spreading. Edge-induced color is also observed when the two colored lines are presented dichoptically, suggesting a cortical origin. The Watercolor Effect described here may serve to enhance figure-ground segregation by imparting surface color onto the enclosed area, and to promote grouping between distant stimulus elements. As a grouping factor, watercolor coloration wins over proximity. Assimilative color spreading may arise in two steps: First, weakening of the contour by lateral inhibition between differentially activated edge cells (local diffusion); and second, unbarriered flow of color onto the enclosed area (global diffusion).

Evidence for the contribution of S cones to the detection of flicker brightness and red-green

We were interested in the question of how cones contribute to the detection of brightness, red-green, and blue-yellow. The linear combination of cone signals contributing to flicker detection was determined by fitting a plane to 64 points (colors) of equal heterochromatic flicker brightness. A small S-cone contribution to flicker brightness of similar amplitude in all five subjects was identified. The ratio of L- to M-cone contribution was found to vary considerably among subjects (1.7-4.1). Chromatic detection thresholds were determined for small patches in the isoluminant plane defined by flicker brightness. These stimuli were presented at an eccentricity of 40 arc min. By using color naming at the detection threshold, one can attribute different segments of the resulting detection ellipses to different chromatic mechanisms. Linear approximation of these segments provided an estimate for the contribution of the different cone types to the detection of red-green and blue-yellow. The results are consistent with the hypothesis that S cones contribute to the red-green mechanism with the same sign as that of the contribution from L cones. The blue-yellow mechanism very probably subtracts S-cone contrast from luminance contrast. The detection ellipse can be mapped into a circle in cone difference space. The base of this canonical transformation is a set of three cone fundamentals that differs from previously published estimates. Projecting the circle onto the three cone difference axes produces sinusoidal changes within the respective excitations. We propose that simultaneous sinusoidal changes of equal increment in the three cone difference excitations generate stimuli differing by equal saliency.

The missing-fundamental illusion at isoluminance

The missing-fundamental illusion describes how a square wave with its fundamental Fourier component removed appears as a square wave. This illusion is normally explained with reference to the bandpass nature of the luminance-contrast-sensitivity function, together with a 'default-to-square-wave' rule. Since the chromatic-contrast-sensitivity function is low-pass, we should not expect a missing-fundamental illusion at isoluminance. Using a simultaneous-detection-and-identification paradigm to eliminate contrast as a cue to discrimination, we nevertheless found that chromatic missing fundamentals and square waves could not be separately identified at detection threshold: just under twice the contrast required to detect the stimuli was needed to identify them. To test whether this was due to insufficiently narrow chromatic-channel bandwidths, we measured detection and identification thresholds for chromatic F and 3F sine-wave gratings. In this case identification was possible almost at detection threshold, suggesting that channel bandwidth limitations were not the critical factor. It is suggested that the weak missing-fundamental illusion observed at isoluminance probably reflects the operation of mechanisms similar to those that are responsible for the chromatic Craik-Cornsweet illusion.