Ichiro Kuriki

The color appearance of #TheDress image varies across individuals. The color of pixels in the image distributes mostly in blue-achromatic-yellow color direction, and so are the perceived color variations. One of the potential causes is differences in the degree of perceiving light-blue pixels as a part of white clothing under a skylight, referred to as “blue bias.” A deep neural network (DNN) application was used to simulate individual differences in blue bias, by varying the percentage of such scenes in the training-image set. A style-transfer DNN was used to simulate a “color naming” procedure by learning pairs of natural images and their color-name labels as pixel-by-pixel maps. The models trained with different ratio of blue-bias scenes were tested using the #TheDress image. The averaged results across trials showed a progressive change from blue/black to white (gray)/gold, indicating that exposure or attention to blue-bias scenes could have caused the individual differences in the color perception of #TheDress image. In an additional experiment, we manipulated the relative number of artificially blue- or yellow-tinted images, instead of varying the ratio of blue-bias scenes, to train the DNN. If the blue-bias scenes are equivalent with blue-tinted images of scenes taken under daylight, this manipulation should yield similar result. However, the resulting outputs did not produce a white/gold image at all. This suggests that exposure to skylight scenes alone is insufficient; the scenes must contain unequivocally white objects (such as snow, white clothing, or white road signs) in order to establish a “blue bias” in human observers.

We investigated how early human visual cortex processes color by analyzing individual variability in steady-state visual evoked potentials (SSVEPs). Sixteen participants viewed a flickering checkerboard that swept around the isoluminant hue circle at three chromatic contrasts. The current study analyzed the individual variability in the SSVEP data from the study to elucidate the hue-selective mechanisms in the early visual areas using a factor-analytic approach. The initial analyses of the correlations revealed that the responses to the nearby hues correlated highly, which is consistent with multiple overlapping color channels. Also, the correlational pattern showed consistent peaks and troughs at specific hue angles: 0° (+L-M), 30°, 120°, 180° (-L+M), 240°, and 300°. We further performed nonmetric multidimensional scaling, identifying four significant hue dimensions. Peaks and troughs of the dimension components were consistent with the hue angles revealed in the correlational pattern. Additional four hues also appeared in the last dimension: 90° (+S), 150°, 270° (-S), and 330°. The 10 (six plus four) hues suggested in these analyses may subserve the basis of early cortical color processing, including classical cone opponency and the mechanisms tuned to the intermediate hues.

Color perception has multiple aspects, such as categorical perception and the continuous perception of colors. Past studies on color information processing in the visual cortex have suggested that categorical color perception is processed mainly at higher-order visual areas, such as hV4 and VO, while more continuous color perception (color appearance) is considered to be processed in the early visual cortex, such as V1–3. The aim of this study is to report changes in cortical representation in the regions of interest in visual areas through the explicit manipulation of participants’ tasks using identical color stimuli. Our results showed that cortical activity patterns changed from the diverted attention condition when participants performed a color task. The difference was smaller in the early visual cortex during color appearance judgments than during the color category task but was significant in higher-order visual areas (V3, hV4, and VO1/2). The differences in cortical activity patterns were evaluated by direct comparisons between brain activities, and the differences between color appearance and categorization tasks were more pronounced in the anterior part of the ventral cortex. In summary, color representation at the earlier level of the visual cortex under diverted attention is more similar to that observed during the color appearance judgment, while color representation in higher-order visual areas changes more significantly from the diverted attention condition when conducting the color evaluation tasks.

Humans use environmental context for facilitating object searches. The benefit of context for visual search requires learning. Modeling the learning process of context for efficient processing is vital to understanding visual function in everyday environments. We proposed a model that accounts for the contextual cueing effect, which refers to the learning effect of scene context to identify the location of a target item. The model extracted the global feature of a scene and gradually strengthened the relationship between the global feature and its target location with repeated observations. We compared the model and human performance with two visual search experiments (letter arrangements on a gray background or a natural scene). The proposed model successfully simulated the faster reduction of the number of saccades required before target detection for the natural scene background compared with the uniform gray background. We further tested whether the model replicated the known characteristics of the contextual cueing effect in terms of local learning around the target, the effect of the ratio of repeated and novel stimuli, and the superiority of natural scenes.

Head-mounted displays (HMDs) are becoming more and more popular as a device for displaying a virtual reality space, but how real are they? The present study attempted to quantitatively evaluate the degree of reality achieved with HMDs by using a perceptual phenomenon as a measure. Lightness constancy is an ability that is present in human visual perception, in which the perceived reflectance (i.e., the lightness) of objects appears to stay constant across illuminant changes. Studies on color/lightness constancy in humans have shown that the degree of constancy is high, in general, when real objects are used as stimuli. We asked participants to make lightness matches between two virtual environments with different illuminant intensities, as presented in an HMD. The participants’ matches showed a high degree of lightness constancy in the HMD; our results marked no less than 74.2% (84.8% at the maximum) in terms of the constancy index, whereas the average score on the computer screen was around 65%. The effect of head-tracking ability was confirmed by disabling that function, and the result showed a significant drop in the constancy index but that it was equally effective when the virtual environment was generated by replay motions. HMDs yield a realistic environment, with the extension of the visual scene being accompanied by head motions.

We perceive the colors of moving objects. The color and motion signals are initially processed separately in different parts of the brain and later combined to yield a unified percept, but its details are not fully understood. To elucidate the underlying mechanisms, aftereffects of direction-selective adaptation (DSA) were measured with functional MRI (fMRI) technique using isoluminant color-defined motion (CM) and luminance-defined motion (LM) stimuli. In contrast to psychophysical studies, prolonged adaptation (>9 s) to CM yielded a higher response to LM test (opposite from typical motion aftereffects), while LM test after adaptation to LM did not evoke a significant DSA aftereffect. Additional experiments by using low-contrast or slow speed LM adapting stimulus suggested that residual luminance in CM could cause the DSA, but they did not explain its polarity or differences among visual areas. An experiment with a short adaptation (3 s) showed a typical DSA aftereffect in V3AB, a part of motion processing stream, when both adapting and test stimuli were LM. Overall, our results indicate that luminance channel mainly contributes to the DSAs, but the results of prolonged (>9 s) fMRI adaptation suggest interactions between color and luminance motion mechanisms that were not identified in the previous studies, suggesting the need for further investigation.

Previous claims of the number of color categories and corresponding basic color terms in modern Mandarin Chinese remain irreconcilable, mainly due to the shortage in objectively evaluating the basicness of color terms with statistical significance. Therefore the present study applied k-means cluster analysis to investigate native Mandarin Chinese speakers’ color naming data of 330 color chips similar to those used in World Color Survey. Results confirmed that there are 11 basic color categories among modern Mandarin speakers in Taiwan, one corresponding to each basic color term. Results also showed that observers overwhelmingly agreed in their use of Mandarin color terms, including those that had yielded ambiguous results in previous studies (gray, brown, pink, and orange). There is significant cross-language similarity when comparing the distribution of color categories in the World Color Survey chart with American English and Japanese data. The motif analysis and group mutual information analysis suggest that Mandarin color terms used in Taiwan describe very similar categories and are, hence, similarly precise in communicating color information as those in Japanese and American English. These results show that three languages of fundamentally different cultures and histories have very similar basic color terms.

Anomalous trichromacy is a common form of congenital color-deficiency resulting from a genetic alteration in the photopigments of the eye’s light receptors. The changes reduce sensitivity to reddish and greenish hues, yet previous work suggests that these observers may experience the world to be more colorful than their altered receptor sensitivities would predict, potentially indicating an amplification of post-receptoral signals. However, past evidence suggesting such a gain adjustment rests on subjective measures of color appearance or salience. We directly tested for neural amplification by using fMRI to measure cortical responses in color-anomalous and normal control observers. Color contrast response functions were measured in two experiments with different tasks to control for attentional factors. Both experiments showed a predictable reduction in chromatic responses for anomalous trichromats in primary visual cortex. However, in later areas V2v and V3v, chromatic responses in the two groups were indistinguishable. Our results provide direct evidence for neural plasticity that compensates for the deficiency in the initial receptor color signals and suggest that the site of this compensation is in early visual cortex.

Colors are represented in the cone-opponent signals, L-M versus S cones, at least up to the level of inputs to the primary visual cortex. We explored the hue selective responses in early cortical visual areas through recordings of steady-state visual evoked potentials (SSVEPs), elicited by a flickering checkerboard whose color smoothly swept around the hue circle defined in a cone-opponent color space. If cone opponency dominates hue representation in the source of SSVEP signals, SSVEP amplitudes as a function of hue should form a profile that is line-symmetric along the cardinal axes of the cone-opponent color space. Observed SSVEP responses were clearly chromatic ones with increased SSVEP amplitudes and reduced response latencies for higher contrast conditions. The overall elliptic amplitude profile was significantly tilted away from the cardinal axes to have the highest amplitudes in the “lime-magenta” direction, indicating that the hue representation in question is not dominated by cone-opponency. The observed SSVEP amplitude hue profile was better described as a summation of a perceptual response and cone-opponent responses with a larger weight to the former. These results indicate that hue representations in the early visual cortex, measured by the SSVEP technique, are possibly related to perceptual color contrast.

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.