The Reality of a Head-Mounted Display (HMD) Environment Tested via Lightness Perception.

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.

Different Mechanisms for Visual Attention at the Hand-movement Goal and Endogenous Visual Attention.

Visual perception is closely related to body movements and action, and it is known that processing visual stimuli is facilitated at the hand or at the hand-movement goal. Such facilitation suggests that there may be an attentional process associated with the hands or hand movements. To investigate the underlying mechanisms of visual attention at a hand-movement goal, we conducted two experiments to examine whether attention at the hand-movement goal is a process independent from endogenous attention. Endogenous attention is attention that is intentionally focused on a location, feature, or object. We controlled the hand-movement goal and endogenous attention separately to investigate the spatial profiles of the two types of attention. A visual target was presented either at the goal of hand movement (same condition) or at its opposite side (opposite condition) while steady-state visual-evoked potential (SSVEP) was used to estimate the spatial distributions of the facilitation effect from the 2 types of attention around the hand-movement goal and around the visual target through EEG. We estimated the spatial profile of attentional modulation for the hand-movement goal by taking the difference in SSVEP amplitude between conditions with and without hand movement, thereby obtaining the effect of visual endogenous attention alone. The results showed a peak at the hand-movement goal, independent of the location of the visual target where participants intentionally focused their attention (endogenous attention). We also found differences in the spatial extent of attentional modulation. Spatial tuning was narrow around the hand-movement goal (i.e., attentional facilitation only at the goal location) but was broadly tuned around the focus of endogenous attention (i.e., attentional facilitation spreading over adjacent stimulus locations), which was obtained from the condition without hand movement. These results suggest the existence of two separate mechanisms, one underlying the attention at the hand-movement goal and another underlying endogenous attention.

Representation of Cone-Opponent Color Space in Macaque Early Visual Cortices.

In primate vision, the encoding of color perception arises from three types of retinal cone cells (L, M, and S cones). The inputs from these cones are linearly integrated into two cone-opponent channels (cardinal axes) before the lateral geniculate nucleus. In subsequent visual cortical stages, color-preferring neurons cluster into functional domains within "blobs" in V1, "thin/color stripes" in V2, and "color bands" in V4. Here, we hypothesize that, with increasing cortical hierarchy, the functional organization of hue representation becomes more balanced and less dependent on cone opponency. To address this question, we used intrinsic signal optical imaging in macaque V1, V2, and V4 cortices to examine the domain-based representation of specific hues (here referred to as "hue domains") in cone-opponent color space (4 cardinal and 4 intermediate hues). Interestingly, we found that in V1, the relative size of S-cone hue preference domain was significantly smaller than that for other hues. This notable difference was less prominent in V2, and, in V4 was virtually absent, resulting in a more balanced representation of hues. In V2, hue clusters contained sequences of shifting preference, while in V4 the organization of hue clusters was more complex. Pattern classification analysis of these hue maps showed that accuracy of hue classification improved from V1 to V2 to V4. These results suggest that hue representation by domains in the early cortical hierarchy reflects a transformation away from cone-opponency and toward a full-coverage representation of hue.

Color Compensation in Anomalous Trichromats Assessed with fMRI.

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.

Basic color categories in Mandarin Chinese revealed by cluster analysis.

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.

Displacement detection is suppressed by the post-saccadic stimulus.

To establish a perceptually stable world despite the large retinal shifts caused by saccadic eye movements, the visual system reduces its sensitivity to the displacement of visual stimuli during saccades (e.g. saccadic suppression of displacement, SSD). Previous studies have demonstrated that inserting a temporal blank right after a saccade improves displacement detection performance. This 'blanking effect' suggests that visual information right after the saccade may play an important role in SSD. To understand the mechanisms underlying SSD, we here compare the effect of pre- and post-saccadic stimulus contrast on displacement detection during a saccade with and without inserting a blank. Our results show that observers' sensitivity to detect visual displacement was reduced by increasing post-saccadic stimulus contrast, but a blank relieves the impairment. We successfully explain the results with a model proposing that parvo-pathway signals suppress the magno-pathway processes responsible for detecting displacements across saccades. Our results suggest that the suppression of the magno-pathway by parvo-pathway signals immediately after a saccade causes SSD, which helps to achieve the perceptual stability of the visual world across saccades.

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.

A motion-in-depth model based on inter-ocular velocity to estimate direction in depth.

Perception of motion in depth is one of the most important visual functions for living in the three-dimensional world. Two binocular cues have been investigated for motion in depth: inter-ocular velocity difference (IOVD) and changing disparity (CD). IOVD provides direction information directly by comparing velocity signals from the two retinas. In this study, we propose for the first time a motion-in-depth model of IOVD that predicts motion-in-depth direction. The model is based on a psychophysical assumption that there are four channels tuned to different directions in depth (Journal of Physiology 235 (1973) 17-29). We modeled these channels by combining outputs of low-level motion detectors that are sensitive to left and right retinal stimulation. Using these channels, we constructed a model of motion in depth that successfully predicted a variety of psychophysical results including direction discrimination, perceived direction, spatial frequency tuning, effect of speed on rotation in depth, effect of lateral motion direction, and effect of binocular and temporal correlations.

Steady-State Visual Evoked Potentials Elicited from Early Visual Cortex Reflect Both Perceptual Color Space and Cone-Opponent Mechanisms.

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.

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.

Extracting the orientation of rotating objects without object identification: Object orientation induction.

When a rotating object (inducer) is briefly replaced by a static face image (test stimulus), the orientation of the face appears to shift in the rotation direction of the inducer (object orientation induction, OOI). The OOI effect suggests that there is a process to continuously analyze and update the orientation of an object in motion. We investigated the perception of object orientation in motion, examining potential factors that contribute to OOI. Experiment 1 showed that the phenomenon is general to objects rather than specific to faces; OOI could be observed with non-face objects. Experiment 2 showed that OOI is a 3D effect, as the orientation shift for a bent-wire object depended on its configuration in the depth dimension. Experiment 3 showed that salient features are necessary to indicate the intrinsic orientation of the inducing object for producing OOI. Experiment 4 showed that change in the facing direction of the inducer object is a crucial factor for OOI, but neither the object shape nor its identity is important. A strong OOI effect was observed even when the inducer kept changing its shape and identity, as long as its direction change generated continuous rotation. Finally, Experiment 5 showed that OOI is a phenomenon in the pathway for fast visual processing. A single inducer presented shorter than 100ms before influenced the perceived orientation of the test stimulus. Together these results suggest that there is a predictive process that continuously analyzes and updates the orientation of rotating objects, independently of their identification.

Individual Variability in Simultaneous Contrast for Color and Brightness: Small Sample Factor Analyses Reveal Separate Induction Processes for Short and Long Flashes.

In classic simultaneous color contrast and simultaneous brightness contrast, the color or brightness of a stimulus appears to shift toward the complementary (opposite) color or brightness of its surrounding region. Kaneko and colleagues proposed that simultaneous contrast involves separate "fast" and "slow" mechanisms, with stronger induction effects for fast than slow. Support for the model came from a diverse series of experiments showing that induction by surrounds varying in luminance or color was stronger for brief than long presentation times (10-40 vs. 80-640 ms). Here, to further examine possible underlying processes, we reanalyzed 12 separate small data sets from these studies using correlational and factor analytic techniques. For each analysis, a principal component analysis of induction strength revealed two factors, with one Varimax-rotated factor accounting for brief and one for long durations. In simultaneous brightness experiments, separate factor pairs were obtained for luminance increments and decrements. Despite being based on small sample sizes, the two-factor consistency among 12 analyses would not be expected by chance. The results are consistent with separate fast and slow processes mediating simultaneous contrast for brief and long flashes.

A Novel Method of Color Appearance Simulation Using Achromatic Point Locus With Lightness Dependence.

The purpose of the present study is to propose a simple algorithm for color appearance simulation under a color illuminant. Achromatic point is a chromaticity of rays that appear neither red nor green, neither blue nor yellow under a given illuminant condition. Saturation and hue of surface colors are evaluated with respect to the achromatic point of the same lightness, while the achromatic point under a colored illuminant depends on the lightness tested. We previously found that this achromatic point locus can be simply approximated as a line with a parallel offset from the lightness axis of CIE LAB space normalized to daylight. We propose a model that applies shifts in the lightness direction after applying hue/saturation shifts using the cone-response (von Kries) transformation under an iso-lightness constraint, such that achromatic points would be aligned with the lightness axis in the CIE LAB space under daylight normalization. We tested this algorithm, which incorporates evaluation of color appearance in different lightness levels, using #theDress image. Resemblance between our simulation and subjective color-matching results implies that human color vision possibly processes shifts in color and lightness independently, as a previous study reported. Changes in the chromaticity distribution of the images were compared with conventional models, and the proposed model preserved relative color difference better, especially at the lower lightness levels. The better performance in lower lightness levels would be advantageous in displays with wider dynamic range in luminance. This implies that the proposed model is effective in simulating color appearance of images with nonnegligible lightness and color differences.

Spatial representations of the viewer's surroundings.

Spatial representation surrounding a viewer including outside the visual field is crucial for moving around the three-dimensional world. To obtain such spatial representations, we predict that there is a learning process that integrates visual inputs from different viewpoints covering all the 360° visual angles. We report here the learning effect of the spatial layouts on six displays arranged to surround the viewer, showing shortening of visual search time on surrounding layouts that are repeatedly used (contextual cueing effect). The learning effect is found even in the time to reach the display with the target as well as the time to reach the target within the target display, which indicates that there is an implicit learning effect on spatial configurations of stimulus elements across displays. Since, furthermore, the learning effect is found between layouts and the target presented on displays located even 120° apart, this effect should be based on the representation that covers visual information far outside the visual field.

Brief presentation enhances various simultaneous contrast effects.

Kaneko and Murakami (2012) demonstrated that simultaneous contrast for brightness and color (chromatic saturation) were enhanced by flashing the stimulus very briefly (10 ms). Here we examined whether this effect of duration generalized to other visual features. Tilt illusion and simultaneous hue contrast were both shown to be much stronger with a stimulus duration of 10 ms compared with 500 ms. The similar temporal dynamics for simultaneous contrast across visual features suggest common underlying principles.

The modern Japanese color lexicon.

Despite numerous prior studies, important questions about the Japanese color lexicon persist, particularly about the number of Japanese basic color terms and their deployment across color space. Here, 57 native Japanese speakers provided monolexemic terms for 320 chromatic and 10 achromatic Munsell color samples. Through k-means cluster analysis we revealed 16 statistically distinct Japanese chromatic categories. These included eight chromatic basic color terms (aka/red, ki/yellow, midori/green, ao/blue, pink, orange, cha/brown, and murasaki/purple) plus eight additional terms: mizu ("water")/light blue, hada ("skin tone")/peach, kon ("indigo")/dark blue, matcha ("green tea")/yellow-green, enji/maroon, oudo ("sand or mud")/mustard, yamabuki ("globeflower")/gold, and cream. Of these additional terms, mizu was used by 98% of informants, and emerged as a strong candidate for a 12th Japanese basic color term. Japanese and American English color-naming systems were broadly similar, except for color categories in one language (mizu, kon, teal, lavender, magenta, lime) that had no equivalent in the other. Our analysis revealed two statistically distinct Japanese motifs (or color-naming systems), which differed mainly in the extension of mizu across our color palette. Comparison of the present data with an earlier study by Uchikawa & Boynton (1987) suggests that some changes in the Japanese color lexicon have occurred over the last 30 years.

Visual attention spreads broadly but selects information locally.

Visual attention spreads over a range around the focus as the spotlight metaphor describes. Spatial spread of attentional enhancement and local selection/inhibition are crucial factors determining the profile of the spatial attention. Enhancement and ignorance/suppression are opposite effects of attention, and appeared to be mutually exclusive. Yet, no unified view of the factors has been provided despite their necessity for understanding the functions of spatial attention. This report provides electroencephalographic and behavioral evidence for the attentional spread at an early stage and selection/inhibition at a later stage of visual processing. Steady state visual evoked potential showed broad spatial tuning whereas the P3 component of the event related potential showed local selection or inhibition of the adjacent areas. Based on these results, we propose a two-stage model of spatial attention with broad spread at an early stage and local selection at a later stage.

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.

Cortical response to categorical color perception in infants investigated by near-infrared spectroscopy.

Perceptual color space is continuous; however, we tend to divide it into only a small number of categories. It is unclear whether categorical color perception is obtained solely through the development of the visual system or whether it is affected by language acquisition. To address this issue, we recruited prelinguistic infants (5- to 7-mo-olds) to measure changes in brain activity in relation to categorical color differences by using near-infrared spectroscopy (NIRS). We presented two sets of geometric figures to infants: One set altered in color between green and blue, and the other set altered between two different shades of green. We found a significant increase in hemodynamic responses during the between-category alternations, but not during the within-category alternations. These differences in hemodynamic response based on categorical relationship were observed only in the bilateral occipitotemporal regions, and not in the occipital region. We confirmed that categorical color differences yield behavioral differences in infants. We also observed comparable hemodynamic responses to categorical color differences in adults. The present study provided the first evidence, to our knowledge, that colors of different categories are represented differently in the visual cortex of prelinguistic infants, which implies that color categories may develop independently before language acquisition.