Sensory representation of surface reflectances: assessments with hyperspectral images.

Specifying surface reflectances in a simple and perceptually informative way would be beneficial for many areas of research and application. We assessed whether a 3×3 matrix may be used to approximate how a surface reflectance modulates the sensory color signal across illuminants. We tested whether observers could discriminate between the model's approximate and accurate spectral renderings of hyperspectral images under narrowband and naturalistic, broadband illuminants for eight hue directions. Discriminating the approximate from the spectral rendering was possible with narrowband, but almost never with broadband illuminants. These results suggest that our model specifies the sensory information of reflectances across naturalistic illuminants with high fidelity, and with lower computational cost than spectral rendering.

Color Naming and Categorization Depend on Distinct Functional Brain Networks.

Naming a color can be understood as an act of categorization, that is, identifying it as a member of a category of colors that are referred to by the same name. But are naming and categorization equivalent cognitive processes and consequently rely on same neural substrates? Here, we used task and resting-state functional magnetic resonance imaging as well as behavioral measures to identify functional brain networks that modulated naming and categorization of colors. We first identified three bilateral color-sensitive regions in the ventro-occipital cortex. We then showed that, across participants, color naming and categorization response times (RTs) were correlated with different resting state connectivity networks seeded from the color-sensitive regions. Color naming RTs correlated with the connectivity between the left posterior color region, the left middle temporal gyrus, and the left angular gyrus. In contrast, color categorization RTs correlated with the connectivity between the bilateral posterior color regions, and left frontal, right temporal and bilateral parietal areas. The networks supporting naming and categorization had a minimal overlap, indicating that the 2 processes rely on different neural mechanisms.

When colours split from objects: The disconnection of colour perception from colour language and colour knowledge.

We investigated object-colour knowledge in RDS, a patient with impaired colour naming after a left occipito-temporal stroke. RDS's colour perception, object naming and verbal colour-knowledge (the ability to verbally say the typical colour of an object) were relatively spared. RDS was also able to state if an object was appropriately coloured or not. However, he could neither match colour names to coloured objects, nor match colour patches to grey-scale objects. Thus, RDS's colour-naming deficit was associated with an impaired ability to conceptually relate visually presented object shapes and colours. These results suggest that objects in their typical colour are processed holistically in the visual modality, and that abilities important for colour naming may also be involved in abstracting colours from visual objects. We discuss these findings in the context of developmental psychology and linguistic anthropology, and propose a model of neuro-functional organization of object-colour knowledge.

How to make a #theDress.

If we completely understand how a phenomenon works, we should be able to produce it ourselves. However, the individual differences in color appearance observed with #theDress seem to be a peculiarity of that photo, and it remains unclear how the proposed mechanisms underlying #theDress can be generalized to other images. Here, we developed a simple algorithm that transforms any image with bicolored objects into an image with the properties of #theDress. We measured the colors perceived in such images and compared them to those perceived in #theDress. Color adjustments confirmed that observers strongly differ in how they perceive the colors of the new images in a similar way as for #theDress. Most importantly, these differences were not unsystematic, but correlated with how observers perceive #theDress. These results imply that the color distribution is sufficient to produce the striking individual differences in color perception originally observed with #theDress-at least as long as the image appears realistic and hence compels the viewer to make assumptions about illuminations and surfaces. The algorithm can be used for stimulus production beyond this study.

Blue hues don't bring the blues: questioning conventional notions of color-emotion associations.

It is commonly held that yellow is happy and blue is sad, but the reason remains unclear. Part of the problem is that researchers tend to focus on understanding why yellow is happy and blue is sad, but this may be a misleading characterization of color-emotion associations. In this study, we disentangle the contribution of lightness, chroma, and hue in color-happy/sad associations by controlling for lightness and chroma either statistically or colorimetrically. We found that after controlling for lightness and chroma, colors with blue hue were no sadder than colors with yellow hue, and in some cases, colors with blue hue were actually happier. These results can help guide future efforts to understand the nature of color-emotion associations.

Similarities in Color Preferences Between Women and Men: The Case of Hadza, the Hunter-Gatherers From Tanzania.

Evidence for cross-cultural patterns of sexual differences in color preferences raised the question of whether these preferences are determined by universal principles. To address this question, we investigated most- and least-favorite color choices in a nonindustrialized community, the Hadza that has an egalitarian hunter-gatherer culture, fundamentally different from those previously investigated. We also compared color preference patterns in the Hadza with published data from Poland and Papua. Our results show that Hadza have very different color preferences than Polish and Papuan Yali respondents. Unlike many industrialized and nonindustrialized cultures, Hadza color preferences are practically the same for women and men. These observations question the idea of universal differences of color preferences between sexes and raise important questions about the determinants of color preferences.

Red, yellow, green, and blue are not particularly colorful.

Colorfulness and saturation have been neglected in research on color appearance and color naming. Perceptual particularities, such as cross-cultural stability, "focality," "uniqueness," "salience," and "prominence" have been observed for red, yellow, green, and blue when those colors were more saturated than other colors in the stimulus samples. The present study tests whether high saturation is a characteristic property of red, yellow, green, and blue, which would explain the above observations. First, we carefully determined the category prototypes and unique hues for red, yellow, green, and blue. Using different approaches in two experiments, we assessed discriminable saturation as the number of just noticeable differences away from the adaptation point (i.e., neutral gray). Results show that some hues can reach much higher levels of maximal saturation than others. However, typical and unique red, yellow, green, and blue are not particularly colorful. Many other intermediate colors have a larger range of discriminable saturation than these colors. These findings suggest that prior claims of perceptual salience of category prototypes and unique hues actually reflect biases in stimulus sets rather than perceptual properties. Additional analyses show that consistent prototype choices across fundamentally different languages are strongly related to the variation of discriminable saturation in the stimulus sets. Our findings also undermine the idea that every color can be produced by a mixture of unique hues. Finally, the measurements in this study provide a large amount of data on saturation across hues, which allows for reevaluating existing estimates of saturation in future studies.

The most reasonable explanation of "the dress": Implicit assumptions about illumination.

Millions of Internet users around the world challenged science by asking why a certain photo of a dress led different observers to have surprisingly different judgments about the color of the dress. The reason this particular photo produces so diverse a variety of judgments presumably is that the photo allows a variety of interpretations about the illumination of the dress. The most obvious explanation from color science should be that observers have different implicit assumptions about the illumination in the photo. We show that the perceived color of the dress is negatively correlated with the assumed illumination along the daylight locus. Moreover, by manipulating the observers' assumptions prior to seeing the photo, we can steer how observers will see the colors of the dress. These findings confirm the idea that the perceived colors of the dress depend on the assumptions about the illumination. The phenomenon illustrates the power of unconscious inferences and implicit assumptions in perception.

Uncertainty of sensory signal explains variation of color constancy.

Color constancy is the ability to recognize the color of an object (or more generally of a surface) under different illuminations. Without color constancy, surface color as a perceptual attribute would not be meaningful in the visual environment, where illumination changes all the time. Nevertheless, it is not obvious how color constancy is possible in the light of metamer mismatching. Surfaces that produce exactly the same sensory color signal under one illumination (metamerism) may produce utterly different sensory signals under another illumination (metamer mismatching). Here we show that this phenomenon explains to a large extent the variation of color constancy across different colors. For this purpose, color constancy was measured for different colors in an asymmetric matching task with photorealistic images. Color constancy performance was strongly correlated to the size of metamer mismatch volumes, which describe the uncertainty of the sensory signal due to metamer mismatching for a given color. The higher the uncertainty of the sensory signal, the lower the observers' color constancy. At the same time, sensory singularities, color categories, and cone ratios did not affect color constancy. The present findings do not only provide considerable insight into the determinants of color constancy, they also show that metamer mismatch volumes must be taken into account when investigating color as a perceptual property of objects and surfaces.

Memory colours affect colour appearance.

Memory colour effects show that colour perception is affected by memory and prior knowledge and hence by cognition. None of Firestone & Scholl's (F&S's) potential pitfalls apply to our work on memory colours. We present a Bayesian model of colour appearance to illustrate that an interaction between perception and memory is plausible from the perspective of vision science.

Categorical facilitation with equally discriminable colors.

This study investigates the impact of language on color perception. By categorical facilitation, we refer to an aspect of categorical perception, in which the linguistic distinction between categories affects color discrimination beyond the low-level, sensory sensitivity to color differences. According to this idea, discrimination performance for colors that cross a category border should be better than for colors that belong to the same category when controlling for low-level sensitivity. We controlled for sensitivity by using colors that were equally discriminable according to empirically measured discrimination thresholds. To test for categorical facilitation, we measured response times and error rates in a speeded discrimination task for suprathreshold stimuli. Robust categorical facilitation occurred for five out of six categories with a group of inexperienced observers, namely for pink, orange, yellow, green, and purple. Categorical facilitation was robust against individual variations of categories or the laterality of target presentation. However, contradictory effects occurred in the blue category, most probably reflecting the difficulty to control effects of sensory mechanisms at the green-blue boundary. Moreover, a group of observers who were highly familiar with the discrimination task did not show consistent categorical facilitation in the other five categories. This trained group had much faster response times than the inexperienced group without any speed-accuracy trade-off. Additional analyses suggest that categorical facilitation occurs when observers pay attention to the categorical distinction but not when they respond automatically based on sensory feed-forward information.

What determines the relationship between color naming, unique hues, and sensory singularities: Illuminations, surfaces, or photoreceptors?

The relationship between the sensory signal of the photoreceptors on one hand and color appearance and language on the other hand is completely unclear. A recent finding established a surprisingly accurate correlation between focal colors, unique hues, and so-called singularities in the laws governing how sensory signals for different surfaces change across illuminations. This article examines how this correlation with singularities depends on reflectances, illuminants, and cone sensitivities. Results show that this correlation holds for a large range of illuminants and for a large range of sensors, including sensors that are fundamentally different from human photoreceptors. In contrast, the spectral characteristics of the reflectance spectra turned out to be the key factor that determines the correlation between focal colors, unique hues, and sensory singularities. These findings suggest that the origins of color appearance and color language may be found in particular characteristics of the reflectance spectra that correspond to focal colors and unique hues.

Do focal colors look particularly "colorful"?

If the most typical red, yellow, green, and blue were particularly colorful (i.e., saturated), they would "jump out to the eye." This would explain why even fundamentally different languages have distinct color terms for these focal colors, and why unique hues play a prominent role in subjective color appearance. In this study, the subjective saturation of 10 colors around each of these focal colors was measured through a pairwise matching task. Results show that subjective saturation changes systematically across hues in a way that is strongly correlated to the visual gamut, and exponentially related to sensitivity but not to focal colors.

Getting the gist of multiple hues: metric and categorical effects on ensemble perception of hue.

This study investigated the perception of colorful ensembles and the effect of categories and perceptual similarity on their representation. We briefly presented ensembles of two hues and tested hue recognition with a range of seen and unseen hues. The average hue was familiar, even though it never appeared in the ensembles. Increasing the perceptual difference of ensemble hues inhibited this mean bias, and the categorical relationship of hues also affected the distribution of familiarity. The findings suggest there is an ensemble perception of hue, but this is affected by the categorical and metric relationships of the elements in the ensemble.

Color categories only affect post-perceptual processes when same- and different-category colors are equally discriminable.

Prior claims that color categories affect color perception are confounded by inequalities in the color space used to equate same- and different-category colors. Here, we equate same- and different-category colors in the number of just-noticeable differences, and measure event-related potentials (ERPs) to these colors on a visual oddball task to establish if color categories affect perceptual or post-perceptual stages of processing. Category effects were found from 200 ms after color presentation, only in ERP components that reflect post-perceptual processes (e.g., N2, P3). The findings suggest that color categories affect post-perceptual processing, but do not affect the perceptual representation of color.

Colour expectations across illumination changes.

This study investigates human expectations towards naturalistic colour changes under varying illuminations. Understanding colour expectations is key to both scientific research on colour constancy and applications of colour and lighting in art and industry. We reanalysed data from asymmetric colour matches of a previous study and found that colour adjustments tended to align with illuminant-induced colour shifts predicted by naturalistic, rather than artificial, illuminants and reflectances. We conducted three experiments using hyperspectral images of naturalistic scenes to test if participants judged colour changes based on naturalistic illuminant and reflectance spectra as more plausible than artificial ones, which contradicted their expectations. When we consistently manipulated the illuminant (Experiment 1) and reflectance (Experiment 2) spectra across the whole scene, observers chose the naturalistic renderings significantly above the chance level (>25 %) but barely more often than any of the three artificial ones, collectively (>50 %). However, when we manipulated only one object/area's reflectance (Experiment 3), observers more reliably identified the version in which the object had a naturalistic reflectance like the rest of the scene. Results from Experiments 2-3 and additional analyses suggested that relational colour constancy strongly contributed to observer expectations, and stable cone-excitation ratios are not limited to naturalistic illuminants and reflectances but also occur for our artificial renderings. Our findings indicate that relational colour constancy and prior knowledge about surface colour shifts help to disambiguate surface colour identity under illumination changes, enabling human observers to recognise surface colours reliably in naturalistic conditions. Additionally, relational colour constancy may even be effective in many artificial conditions.

Categorical sensitivity to color differences.

Categorical perception provides a potential link between color perception and the linguistic categories that correspond to the basic color terms. We examined whether the sensory information of the second-stage chromatic mechanisms is further processed so that sensitivity for color differences yields categorical perception. In this case, sensitivity for color differences should be higher across than within category boundaries. We measured discrimination thresholds (JNDs) and color categories around an isoluminant hue circle in Derrington-Krauskopf-Lennie (DKL) color space at three levels of lightness. At isoluminant lightness, the global pattern of JNDs coarsely followed an ellipse. Deviations from the ellipse coincided with the orange-pink and the blue-green category borders, but these minima were also aligned with the second-stage cone-opponent mechanisms. No evidence for categorical perception of color was found for any other category borders. At lower lightness, categories changed substantially, but JNDs did not change accordingly. Our results point to a loose relationship between color categorization and discrimination. However, the coincidence of some boundaries with JND minima is not a general property of color categorical boundaries. Hence, our basic ability to discriminate colors cannot fully explain why we use the particular set of categories to communicate about colors. Moreover, these findings seriously challenge the idea that color naming forms the basis for the categorical perception of colors. With respect to previous studies that concentrated on the green-blue boundary, our results highlight the importance of controlling perceptual distances and examining the full set of categories when investigating category effects on color perception.

Feature-based interaction between masks and target in continuous flash suppression.

Continuous flash suppression (CFS) has become one of the most popular tools in the study of visual processing in the absence of conscious awareness. Studies use different kinds of masks, like colorful Mondrians or random noise. Even though the use of CFS is widespread, little is known about some of the underlying neuronal mechanisms, such as the interactions between masks and stimuli. We designed a b-CFS experiment with feature-reduced targets and masks in order to investigate possible effects of feature-similarity or -orthogonality between masks and targets. Masks were pink noise patterns filtered with an orientation band pass to generate a strong directionality. Target stimuli were Gabors varying systematically in their orientational alignment with the masks. We found that stimuli whose orientational alignment was more similar to that of the masks are suppressed significantly longer. This feature-similarity (here: orientation) based enhancement of suppression duration can be overcome by feature orthogonality in another feature dimension (here: color). We conclude that mask-target interactions exist in continuous flash suppression, and the human visual system can use orthogonality within a feature dimension or across feature dimensions to facilitate the breaking of the CFS.

Why bananas look yellow: The dominant hue of object colours.

In this study, we propose a new approach to the perceptual representation of object colours. Three-dimensional objects have a polychromatic colour distribution. Yet, human observers abstract from the variation along the three perceptual colour dimensions when describing objects, such as when we say, "a banana is yellow". We propose that the perceived object colour is determined by the dominant hue. The dominant hue corresponds to the first principal component of an object's chromaticities. Across three experiments, we show for a sample of objects that the chromatic variation away from the dominant hue is almost completely neglected by human observers under non-laboratory viewing conditions. This is partly due to the low visibility of this variation, and partly to attentional change blindness. These findings reveal the potential role of dominant hue in the perception of object colours. Dominant hue may enable us to determine the most representative colours of objects because perceived object colours tend to be maximally bright and saturated. The present findings also imply that we can simplify the colour distributions of objects by projecting them onto their dominant hue. This may be useful for computational applications.

Is there a lateralized category effect for color?

According to the lateralized category effect for color, the influence of color category borders on color perception in fast reaction time tasks is significantly stronger in the right visual field than in the left. This finding has directly related behavioral category effects to the hemispheric lateralization of language. Multiple succeeding articles have built on these findings. We ran ten different versions of the two original experiments with overall 230 naive observers. We carefully controlled the rendering of the stimulus colors and determined the genuine color categories with an appropriate naming method. Congruent with the classical pattern of a category effect, reaction times in the visual search task were lower when the two colors to be discriminated belonged to different color categories than when they belonged to the same category. However, these effects were not lateralized: They appeared to the same extent in both visual fields.