Color discrimination repetition distorts color representations.

Perceptual learning is the improvement of perceptual performance after repeated practice on a perceptual task. Studies on perceptual learning in color vision are limited. In this study, we measured the impact of color discrimination repetitions at a specific base color on color perception for entire hues. Participants performed five sessions of color discrimination training (200 or 300 trials per session) over five days, at colors on either the negative or positive direction of the L-M color axis, based on group assignment. We administered three color perception assessments (unique hues, color category boundaries, and color appearance) before and after the sessions to evaluate perceptual changes after training. The results showed declines in color discrimination thresholds after training, as expected. Additionally, the training influenced outcomes across all three assessment types. After the training, the perceived color appearance changed near the trained color along the stimulus hue, and some of the unique hues and the color category boundaries moved significantly toward the trained color. These findings indicate that short-term repetitions of color discrimination training can alter color representations in the visual system, distorting color perception around the trained color.

Effects of specular roughness on the perception of color and opacity.

Previous studies have shown that information concerning object shape is important for the perception of translucency. This study aims to explore how the perception of semi-opaque objects is influenced by surface gloss. We varied specular roughness, specular amplitude, and the simulated direction of a light source used to illuminate a globally convex bumpy object. We found that perceived lightness and roughness increased as specular roughness was increased. Declines in perceived saturation were observed but were far smaller in magnitude with these increases in specular roughness. There were inverse correlations found between perceived gloss and perceived lightness, perceived transmittance and perceived saturation, and between perceived roughness and perceived gloss. Positive correlations were found between perceived transmittance and glossiness, and between perceived roughness and perceived lightness. These findings suggest that specular reflections influence the perception of transmittance and color attributes, and not just perceived gloss. We also performed follow-up modeling of image data to find that perceived saturation and lightness could be explained by the reliance on different image regions with greater chroma and lower lightness, respectively. We also found systematic effects of lighting direction on perceived transmittance that indicate there are complex perceptual interactions that require further consideration.

Texture statistics involved in specular highlight exclusion for object lightness perception.

The human visual system estimates the physical properties of objects, such as their lightness. Previous studies on the lightness perception of glossy three-dimensional objects have suggested that specular highlights are detected and excluded in lightness perception. However, only a few studies have attempted to elucidate the mechanisms underlying this exclusion. This study aimed to elucidate the image features that contribute to the highlight exclusion of lightness perception. We used Portilla-Simoncelli texture statistics (PS statistics), an image feature set similar to the representation in the early visual cortex, to explore their relationships with highlight exclusion for lightness perception. In experiment 1, computer graphics images of bumpy plastic plates with various physical parameters were used as stimuli, and the lightness perception on them was measured using a lightness matching task. We then calculated the highlight exclusion index, which represented the degree of highlight exclusion. Finally, we evaluated the correlation between the highlight exclusion index and the four PS statistic subsets. In experiment 2, an image synthesis algorithm was used to create images in which either the PS statistic subset was manipulated. The highlight exclusion indexes of the synthesized images were then measured. The results revealed that the PS statistic subset consisting of lowest-order image features, such as moment statistics of luminance, acts as a necessary condition for highlight exclusion, whereas the other three subsets consisting of higher order features are not crucial. These results suggest that the low-order image features are the most important among the features in PS statistics for highlight exclusion, even though image features higher order than those in PS statistics must be directly involved.

The dominating impacts of Helmholtz-Kohlrausch effect on color-induced glossiness enhancement.

Glossiness can be increased by adding chromatic information to the object images. However, the mechanisms that create color-induced glossiness enhancement are unclear. In this study, we psychophysically measured the glossiness of object images to which various hue chromaticities were added to elucidate the perceptual and image factors that explain the color-induced glossiness enhancement effect. Two types of coloring conditions were tested: the both-colored (BC) condition, in which both specular and diffuse components were colored with the same chromaticity, and the diffuse-colored (DC) condition, in which only diffuse components were colored while specular components remained achromatic. The results showed that glossiness enhancement was more prominent in the BC than in the DC condition, and the dependency of glossiness enhancement on the stimulus color direction was similar to that of the Helmholtz-Kohlrausch (H-K) effect. Furthermore, we performed a regression analysis with a linear mixed model based on image features and an additional experiment in which an H-K effect-based increase in perceived brightness was imitated on achromatic stimuli by manipulating luminance. The results demonstrated that the H-K effect-based brightness enhancement in the highlight regions explains the glossiness enhancement effect well. These results suggest that the H-K effect, especially around the highlight region, is a dominant factor that creates the color-induced glossiness enhancement, although other color-related factors could also be partly involved.

The perception of translucency from surface gloss.

Translucent objects (like fruit and wax) reflect and transmit incident light to generate complex retinal image structure. Understanding how we visually perceive translucency from these images is challenging, but previous studies have demonstrated that perceived shape and shading is important for perceiving translucency. We considered the possibility that perceived translucency might also depend on 3D shape inferred from surface gloss (i.e., shape from specular highlights). Here, we performed experiments to test whether interactions between specular and non-specular image properties generated by different 3D shape information influences perceived translucency. Results revealed that perceived translucency could be explained by incongruence in 3D shape used to generate specular and non-specular image components. We proposed a new computational model based on measurable image features informative of shading relative to specular highlights that accounted for 59% of the variability in judgments of perceived translucency from the result of 10-fold cross validation. This model was found to outperform other models based on explicit subjective measures of perceived surface shape, suggesting it implicitly taps much of the relevant geometric information necessary for predicting observer judgments of translucency for glossy materials. These results provide new insight into how the visual system might infer translucency from the structure of specular and non-specular shading generated by glossy semi-opaque materials.

Luminance dependency of perceived color shift after color contrast adaptation caused by higher-order color channels.

Color adaptation is a phenomenon in which, after prolonged exposure to a specific color (i.e. adaptation color), the perceived color shifts to approximately the opposite color direction of the adaptation color. Color adaptation is strongly related to sensitivity changes in photoreceptors, such as von Kries adaptation and cone-opponent mechanisms. On the other hand, the perceptual contrast of colors (e.g. perceptual saturation of the red-green direction) decreases after adaptation to a stimulus with spatial and/or temporal color modulation along the color direction. This phenomenon is referred to as color contrast adaptation. Color contrast adaptation has been used to investigate the representation of colors in the visual system. In the present study, we measured color perception after color contrast adaptation to stimuli with temporal color modulations along complicated color loci in a luminance-chromaticity plane. We found that, after the observers adapted to color modulations with different chromaticities at higher, medium, and lower luminance (e.g. temporal alternations among red, green, and red, each at a different luminance level), the chromaticity corresponding to perceptual achromaticity (the achromatic point) shifted to the same color direction as the adaptation chromaticity in each test stimulus luminance. In contrast, this luminance dependence of the achromatic point shift was not observed after adaptation to color modulations with more complex luminance-chromaticity correspondences (e.g. alternating red, green, red, green, and red, at five luminance levels, respectively). In addition, the occurrence or nonoccurrence of the luminance-dependent achromatic point shift was qualitatively predicted using a noncardinal model composed of channels preferring intermediate color directions between the cardinal chromaticity and luminance axes. These results suggest that the noncardinal channels are involved in the luminance-dependent perceived color shift after adaptation.

Impact of local luminance contrast on facial skin lightness perception: A psychophysical reverse correlation study using sub-band contrast modulation noise.

In conventional psychophysical reverse correlation methods using white or pink noise, the luminance noise is added to every pixel. Thus, the image features correlated with perception are often biased toward local mean luminance. Furthermore, spatial frequencies and orientations are represented in the primary visual cortex, which forms the basis of various visual perception. In this study, we proposed a new reverse correlation method using noise that modulated the spatial frequency sub-band contrast and examined its properties in psychophysical experiments on facial skin lightness perception. In the experiment, we asked the observers to compare the perceived skin lightness in a paired comparison manner on face stimuli with increased or decreased spatial frequency sub-band contrasts at random spatial locations. The results showed that the contrasts in the eyes or irises were strongly and positively correlated with the perceived skin lightness in most sub-bands, demonstrating that the proposed method reiterated the findings of previous studies that the sparkle of the irises makes the skin appear lighter. Contrarily, the conventional reverse correlation method using pink noise images was applied to the skin lightness perception. The results indicated that only the local mean luminance in some skin regions, such as the forehead, was correlated with skin lightness perception. In summary, with the proposed method, we found some image features in the facial parts other than the skin mean luminance relevant to skin lightness perception, which are difficult to detect using the conventional method. They are considered complementary given that the proposed method and the conventional method extracted considerably different image features. It depends on the psychophysical tasks and stimuli which one is more appropriate.

Spatial Frequency Effective for Increasing Perceived Glossiness by Contrast Enhancement.

It has been suggested that luminance edges in retinal images are potential cues for glossiness perception, particularly when the perception relies on low-luminance specular regions. However, a previous study has shown only statistical correlations between luminance edges and perceived glossiness, not their causal relations. Additionally, although specular components should be embedded at various spatial frequencies depending on the micro-roughness on the object surface, it is not well understood what spatial frequencies are essential for glossiness perception on objects with different micro-roughness. To address these issues, we examined the impact of a sub-band contrast enhancement on the perceived glossiness in the two conditions of stimuli: the Full condition where the stimulus had natural specular components and the Dark condition where it had specular components only in dark regions. Object images with various degrees of surface roughness were generated as stimuli, and their contrast was increased in various spatial-frequency sub-bands. The results indicate that the enhancement of the sub-band contrast can significantly increase perceived glossiness as expected. Furthermore, the effectiveness of each spatial frequency band depends on the surface roughness in the Full condition. However, effective spatial frequencies are constant at a middle spatial frequency regardless of the stimulus surface roughness in the Dark condition. These results suggest that, for glossiness perception, our visual system depends on specular-related information embedded in high spatial frequency components but may change the dependency on spatial frequency based on the surface luminance to be judged.

Hue selectivity of collinear facilitation.

Collinear facilitation (CF) is the improvement of the detection sensitivity of the target when two high-contrast flanking stimuli (flankers) have the same visual properties. While it is known that CF does not occur between achromatic flanking stimuli and chromatic targets, or vice versa, it remains unclear whether CF occurs when the hue of the target and flankers are different. We measured CF for Gabor stimuli defined in an isoluminant plane using stimuli defined by isoluminant colors along isolated cone-opponent axes and in two diagonal directions. The measured CF varied with the difference in hue between the target and flankers. Moreover, increased thresholds were also observed. These results suggest that CF exhibits hue selectivity and involves a suppression as well as a facilitation component. The hue selectivity profile of these factors infer that the CF cannot be simply explained by the assumption of two independent cone opponent mechanisms.

Luminance edge is a cue for glossiness perception based on low-luminance specular components.

The visual system is considered to employ various image cues from an object image to perceive its glossiness. It has been reported that, surprisingly, even for object images without specular highlights we can perceive glossiness by relying on low-luminance specular components (Kim, Marlow, & Anderson, 2012). This type of perceptual glossiness is referred to as dark gloss. However, it is still unclear whether dark gloss is observed commonly across various objects, and what image features are cues for dark gloss. To address these issues, we performed several psychophysical experiments. First, we measured perceived glossiness for a number of computer-graphics object images with natural specular reflection components (Full condition) and for those without high-luminance components of specular reflections (Dark condition). The results showed that dark gloss (glossiness perception in the Dark condition) was generally observed on almost all object images, while its intensity was rather different across the images. Then we psychologically or computationally measured several image features for the stimulus images, such as luminance edge number, recognizability of reflection images, and some highlight-related features, to examine their relations to perceived glossiness with a multiple regression analysis. The results demonstrated that luminance edge number was most strongly related to glossiness scores among the measured features only for object images with potent dark gloss. These results suggest that luminance edges are an effective cue for dark gloss under certain stimulus conditions.

Relative contributions of low- and high-luminance components to material perception.

Besides specular highlights, image pixels that represent clues to recognizing the object material, such as shading between threads of fabrics, often yield relatively lower luminance in the image. Here, we psychophysically examined how lower and higher luminance components contribute to material perception. We created two types of luminance-modulated images-low- and high-luminance-preserved (LLP and HLP) images-and instructed observers to choose which modified image resulted in a material impression closer to the original. LLP images were created by compressing the luminance contrast of the higher half of the histogram in each original photograph and vice versa. The stimuli were photographs of various samples of stone, wood, leather, and fabric. Although the LLP and HLP images were equally chosen, the choice ratios of the HLP images largely differed across the samples and categories and moderately correlated with the luminance statistics of higher-spatial-frequency sub-bands. These results suggest that either the lower- or higher-luminance components play an important role in material perception, depending on the material category. However, the correlation with sub-band image statistics for stone/wood samples was much weaker than for leather/fabric samples, suggesting that more intricate image characteristics may be involved in evaluating the material impressions of the stone/wood samples.

Dissociation of equilibrium points for color-discrimination and color-appearance mechanisms in incomplete chromatic adaptation.

We compared the color-discrimination thresholds and supra-threshold color differences (STCDs) obtained in complete chromatic adaptation (gray) and incomplete chromatic adaptation (red). The color-difference profiles were examined by evaluating the perceptual distances between various color pairs using maximum likelihood difference scaling. In the gray condition, the chromaticities corresponding with the smallest threshold and the largest color difference were almost identical. In contrast, in the red condition, they were dissociated. The peaks of the sensitivity functions derived from the color-discrimination thresholds and STCDs along the L-M axis were systematically different between the adaptation conditions. These results suggest that the color signals involved in color discrimination and STCD tasks are controlled by separate mechanisms with different characteristic properties.

Contrast adaptation to luminance and brightness modulations.

Perceptual brightness and color contrast decrease after seeing a light temporally modulating along a certain direction in a color space, a phenomenon known as contrast adaptation. We investigated whether contrast adaptation along the luminance direction arises from modulation of luminance signals or apparent brightness signals. The stimulus consisted of two circles on a gray background presented on a CRT monitor. In the adaptation phase, the luminance and chromaticity of one circle were temporally modulated, while the other circle was kept at a constant luminance and color metameric with an equal-energy white. We employed two types of temporal modulations, namely, in luminance and brightness. Chromaticity was sinusoidally modulated along the L-M axis, leading to dissociation between luminance and brightness (the Helmholtz-Kohlrausch effect). In addition, luminance modulation was minimized in the brightness modulation, while brightness modulation was minimized in the luminance modulation. In the test phase, an asymmetric matching method was used to measure the magnitude of contrast adaptation for both modulations. Our results showed that, although contrast adaptation along the luminance direction occurred for both modulations, contrast adaptation for luminance modulation was significantly stronger than that for the brightness modulation regardless of the temporal frequency of the adaptation modulation. These results suggest that luminance modulation is more influential in contrast adaptation than brightness modulation.

Temporal properties of material categorization and material rating: visual vs non-visual material features.

Humans can visually recognize material categories of objects, such as glass, stone, and plastic, easily. However, little is known about the kinds of surface quality features that contribute to such material class recognition. In this paper, we examine the relationship between perceptual surface features and material category discrimination performance for pictures of materials, focusing on temporal aspects, including reaction time and effects of stimulus duration. The stimuli were pictures of objects with an identical shape but made of different materials that could be categorized into seven classes (glass, plastic, metal, stone, wood, leather, and fabric). In a pre-experiment, observers rated the pictures on nine surface features, including visual (e.g., glossiness and transparency) and non-visual features (e.g., heaviness and warmness), on a 7-point scale. In the main experiments, observers judged whether two simultaneously presented pictures were classified as the same or different material category. Reaction times and effects of stimulus duration were measured. The results showed that visual feature ratings were correlated with material discrimination performance for short reaction times or short stimulus durations, while non-visual feature ratings were correlated only with performance for long reaction times or long stimulus durations. These results suggest that the mechanisms underlying visual and non-visual feature processing may differ in terms of processing time, although the cause is unclear. Visual surface features may mainly contribute to material recognition in daily life, while non-visual features may contribute only weakly, if at all.

Experts and novices use the same factors--but differently--to evaluate pearl quality.

Well-trained experts in pearl grading have been thought to evaluate pearls according to their glossiness, interference color, and shape. However, the characteristics of their evaluations are not fully understood. Using pearl grading experiments, we investigate the consistency of novice (i.e., without knowledge of pearl grading) and expert participants' pearl grading skill and then compare the novices' grading with that of experts; furthermore, we discuss the relationship between grading, interference color, and glossiness. We found that novices' grading was significantly less concordant with experts average grading than was experts' grading; more than half of novices graded pearls the opposite of how experts graded those same pearls. However, while experts graded pearls more consistently than novices did, novices' consistency was relatively high. We also found differences between the groups in regression analyses that used interference color and glossiness as explanatory variables and were conducted for each trial. Although the regression coefficient was significant in 60% of novices' trials, there were fewer significant trials for the experts (20%). This indicates that novices can also make use of these two factors, but that their usage is simpler than that of the experts. These results suggest that experts and novices share some values about pearls but that the evaluation method is elaborated for experts.

Image regions contributing to perceptual translucency: A psychophysical reverse-correlation study.

The spatial luminance relationship between shading patterns and specular highlight is suggested to be a cue for perceptual translucency (Motoyoshi, 2010). Although local image features are also important for translucency perception (Fleming & Bulthoff, 2005), they have rarely been investigated. Here, we aimed to extract spatial regions related to translucency perception from computer graphics (CG) images of objects using a psychophysical reverse-correlation method. From many trials in which the observer compared the perceptual translucency of two CG images, we obtained translucency-related patterns showing which image regions were related to perceptual translucency judgments. An analysis of the luminance statistics calculated within these image regions showed that (1) the global rms contrast within an entire CG image was not related to perceptual translucency and (2) the local mean luminance of specific image regions within the CG images correlated well with perceptual translucency. However, the image regions contributing to perceptual translucency differed greatly between observers. These results suggest that perceptual translucency does not rely on global luminance statistics such as global rms contrast, but rather depends on local image features within specific image regions. There may be some "hot spots" effective for perceptual translucency, although which of many hot spots are used in judging translucency may be observer dependent.

Color signal integration for color discrimination along a long-range apparent motion trajectory.

In contrast to the classical view that fundamental visual attributes such as color and motion are independently processed in the visual system (e.g. Livingstone and Hubel, 1987; Marr, 1982), recent studies have revealed various forms of cross-attribute interactions, such as averaging of color appearance along the motion trajectory of an object (Nishida et al., 2007). In this study, we investigated whether such color signal integration along a motion trajectory can be induced only by motion mechanisms having large receptive fields, without simple integration within direction-selective neurons with small receptive fields, like those in V1. The stimulus consisted of discs with long-range apparent motion along a circular trajectory. The stimulus onset asynchrony (SOA) between disc presentations controlled the strength of the apparent motion perception. We measured observers' sensitivity in detecting color modulation on the discs. The results showed that the measured sensitivity was lowest at SOAs corresponding to the strongest motion perception. This can be interpreted as follows: color signals were integrated along an apparent motion path, and this integration reduced chromatic sensitivity by averaging color signals. Another experiment that controlled apparent motion perception in a different way also supported this idea. However, this integration effect seemed to be linked to responses of motion detectors for the apparent motion stimuli, not directly to perceptual motion representation in the visual system. These results suggest that the human visual system handles color information from retinal inputs regarding moving objects based not only on a retinotopic coordinate but also on object-based coordinates, even when the moving object yields only long-range apparent motion.

Enhancement of glossiness perception by retinal-image motion: additional effect of head-yoked motion parallax.

It has been argued that when an observer moves, a contingent retinal-image motion of a stimulus would strengthen the perceived glossiness. This would be attributed to the veridical perception of three-dimensional structure by motion parallax. However, it has not been investigated whether the effect of motion parallax is more than that of retinal-image motion of the stimulus. Using a magnitude estimation method, we examine in this paper whether cross-modal coordination of the stimulus change and the observer's motion (i.e., motion parallax) is essential or the retinal-image motion alone is sufficient for enhancing the perceived glossiness. Our data show that a retinal-image motion simulating motion parallax without head motion strengthened the perceived glossiness but that its effect was weaker than that of motion parallax with head motion. These results suggest the existence of an additional effect of the cross-modal coordination between vision and proprioception on glossiness perception. That is, motion parallax enhances the perception of glossiness, in addition to retinal-image motions of specular surfaces.

Statistical significance testing with mahalanobis distance for thresholds estimated from constant stimuli method.

The t-test and the analysis of variance are commonly used as statistical significance testing methods. However, they cannot assess the significance of differences between thresholds within individual observers estimated from the constant stimuli method; these thresholds are not defined as averages of samples, but they are rather defined as functions of parameters of psychometric functions fitted to participants' responses. Moreover, the statistics necessary for these statistical testing methods cannot be derived. In this paper, we propose a new statistical testing method to assess the statistical significance of differences between thresholds estimated from the constant stimuli method. The new method can assess not only threshold differences but also main effects and interactions in multifactor experiments, exploiting the asymptotic normality of maximum likelihood estimators and the characteristics of multivariate normal distributions. This proposed method could be used in similar cases to the analysis of variance for thresholds estimated from the adjustment method and the staircase method. Finally, we present some data on simulations in which we tested assumptions, power and type I error of the proposed method.

Spatiotemporal averaging of perceived brightness along an apparent motion trajectory.

Objects are critical functional units for many aspects of visual perception and recognition. Many psychophysical experiments support the concept of an "object file" consisting of characteristics attributed to a single object on the basis of successive views of it, but there has been little evidence that object identity influences apparent brightness and color. In this study, we investigated whether the perceptual identification of successive flashed stimuli as views of a single moving object could affect brightness perception. Our target stimulus was composed of eight wedge-shaped sectors. The sectors were presented successively at different inter-flash intervals along an annular trajectory. At inter-flash intervals of around 100 ms, the impression was of a single moving object undergoing long-range apparent motion. By modulating the luminance between successive views, we measured the perception of luminance modulation along the trajectory of this long-range apparent motion. At the inter-flash intervals where the motion perception was strongest, the luminance difference was perceptually underestimated, and forced-choice luminance discrimination thresholds were elevated. Moreover, under such conditions, it became difficult for the observer to correctly associate or "bind" spatial positions and wedge luminances. These results indicate that the different luminances of wedges that were perceived as a single object were averaged along its apparent motion trajectory. The large spatial step size of our stimulus makes it unlikely that the results could be explained by averaging in a low-level mechanism that has a compact spatiotemporal receptive field (such as V1 and V2 neurons); higher level global motion or object mechanisms must be invoked to account for the averaging effect. The luminance averaging and the ambiguity of position-luminance "binding" suggest that the visual system may evade some of the costs of rapidly computing apparent brightness by adopting the assumption that the characteristics of an object are invariant over successive views.