Computerized simulation of color appearance for anomalous trichromats using the multispectral image.

Most color simulators for color deficiencies are based on the tristimulus values and are intended to simulate the appearance of an image for dichromats. Statistics show that there are more anomalous trichromats than dichromats. Furthermore, the spectral sensitivities of anomalous cones are different from those of normal cones. Clinically, the types of color defects are characterized through Rayleigh color matching, where the observer matches a spectral yellow to a mixture of spectral red and green. The midpoints of the red/green ratios deviate from a normal trichromat. This means that any simulation based on the tristimulus values defined by a normal trichromat cannot predict the color appearance of anomalous Rayleigh matches. We propose a computerized simulation of the color appearance for anomalous trichromats using multispectral images. First, we assume that anomalous trichromats possess a protanomalous (green shifted) or deuteranomalous (red shifted) pigment instead of a normal (L or M) one. Second, we assume that the luminance will be given by L+M, and red/green and yellow/blue opponent color stimulus values are defined through L-M and (L+M)-S, respectively. Third, equal-energy white will look white for all observers. The spectral sensitivities of the luminance and the two opponent color channels are multiplied by the spectral radiance of each pixel of a multispectral image to give the luminance and opponent color stimulus values of the entire image. In the next stage of color reproduction for normal observers, the luminance and two opponent color channels are transformed into XYZ tristimulus values and then transformed into sRGB to reproduce a final image for anomalous trichromats. The proposed simulation can be used to predict the Rayleigh color matches for anomalous trichromats. We also conducted experiments to evaluate the appearance of simulated images by color deficient observers and verified the reliability of the simulation.

Color Constancy in Two-Dimensional and Three-Dimensional Scenes: Effects of Viewing Methods and Surface Texture.

There has been debate about how and why color constancy may be better in three-dimensional (3-D) scenes than in two-dimensional (2-D) scenes. Although some studies have shown better color constancy for 3-D conditions, the role of specific cues remains unclear. In this study, we compared color constancy for a 3-D miniature room (a real scene consisting of actual objects) and 2-D still images of that room presented on a monitor using three viewing methods: binocular viewing, monocular viewing, and head movement. We found that color constancy was better for the 3-D room; however, color constancy for the 2-D image improved when the viewing method caused the scene to be perceived more like a 3-D scene. Separate measurements of the perceptual 3-D effect of each viewing method also supported these results. An additional experiment comparing a miniature room and its image with and without texture suggested that surface texture of scene objects contributes to color constancy.

Attentional facilitation of detection of flicker on moving objects.

We investigated the influence of attention and motion on the sensitivity of flicker detection for a target among distractors. Experiment 1 showed that when the target and distractors were moving, detection performance plummeted compared to when they were not moving, suggesting that the most sensitive detectors were local, temporal frequency-tuned receptive fields. With the stimuli in motion, a qualitatively different strategy was required and this led to much reduced performance. Cueing, which specified the target location with 100% validity, had no effect for targets that had little or no motion, suggesting that the flicker was sufficiently salient in this case to attract attention to the target without requiring any search. For targets with medium to high speeds, however, cueing provided a strong increase in sensitivity over uncued performance. This suggests a significant advantage for localizing and tracking the target and so sampling the luminance changes from only one trajectory. Experiment 2 showed that effect of attention was to increase the efficiency and duration of signal integration for the moving target. Overall, the results show that flicker sensitivity for a moving target relies on a much less efficient process than detection of static flicker, and that this less efficient process is facilitated when attention can select the relevant trajectory and ignore the others.

Color constancy influenced by unnatural spatial structure.

The recognition of spatial structures is important for color constancy because we cannot identify an object's color under different illuminations without knowing which space it is in and how that space is illuminated. To show the importance of the natural structure of environments on color constancy, we investigated the way in which color appearance was affected by unnatural viewing conditions in which a spatial structure was distorted. Observers judged the color of a test patch placed in the center of a small room illuminated by white or reddish lights, as well as two rooms illuminated by white and reddish light, respectively. In the natural viewing condition, an observer saw the room(s) through a viewing window, whereas in an unnatural viewing condition, the scene structure was scrambled by a kaleidoscope-type viewing box. Results of single room condition with one illuminant color showed little difference in color constancy between the two viewing conditions. However, it decreased in the two-rooms condition with a more complex arrangement of space and illumination. The patch's appearance under the unnatural viewing condition was more influenced by simultaneous contrast than its appearance under the natural viewing condition. It also appears that color appearance under white illumination is more stable compared to that under reddish illumination. These findings suggest that natural spatial structure plays an important role for color constancy in a complex environment.

Low-level motion analysis of color and luminance for perception of 2D and 3D motion.

We investigated the low-level motion mechanisms for color and luminance and their integration process using 2D and 3D motion aftereffects (MAEs). The 2D and 3D MAEs obtained in equiluminant color gratings showed that the visual system has the low-level motion mechanism for color motion as well as for luminance motion. The 3D MAE is an MAE for motion in depth after monocular motion adaptation. Apparent 3D motion can be perceived after prolonged exposure of one eye to lateral motion because the difference in motion signal between the adapted and unadapted eyes generates interocular velocity differences (IOVDs). Since IOVDs cannot be analyzed by the high-level motion mechanism of feature tracking, we conclude that a low-level motion mechanism is responsible for the 3D MAE. Since we found different temporal frequency characteristics between the color and luminance stimuli, MAEs in the equiluminant color stimuli cannot be attributed to a residual luminance component in the color stimulus. Although a similar MAE was found with a luminance and a color test both for 2D and 3D motion judgments after adapting to either color or luminance motion, temporal frequency characteristics were different between the color and luminance adaptation. The visual system must have a low-level motion mechanism for color signals as for luminance ones. We also found that color and luminance motion signals are integrated monocularly before IOVD analysis, showing a cross adaptation effect between color and luminance stimuli. This was supported by an experiment with dichoptic presentations of color and luminance tests. In the experiment, color and luminance tests were presented in the different eyes dichoptically with four different combinations of test and adaptation: color or luminance test in the adapted eye after color or luminance adaptation. Findings of little or no influence of the adaptation/test combinations indicate the integration of color and luminance motion signals prior to the binocular IOVD process.

Effect of spatial structure on colorfulness adaptation for natural images.

We examined whether the perception of the colorfulness of an image is influenced by the adaptation of the visual system to natural and shuffled images with different degrees of saturation. In the experiment, observers first became adapted to several images with different levels of saturation and then their colorfulness perception of a test image was measured. The results show that their perception of colorfulness was influenced by their adaptation to the saturation of images. The effect was stronger following adaptation to natural images than to images consisting of a shuffled collage of randomized color blocks, which suggests that the naturalness of the spatial structure of an image affects the strength of the effect.

Nonlinear two-stage model for color discrimination.

We modified a two-stage model for color discrimination proposed in a previous study [Color Res. Appl.25, 105 (2000)]; in order to extend the model to wider conditions, we considered the conditions with luminance modulations in addition to color modulations. Using the modified model, we successfully predicted color discrimination data with test color changes along both the chromatic and luminance axes under a variety of background colors. Both qualitative and quantitative assessments in modeling showed that nonlinearity is required in both the cone and the cone-opponent stages to interpret adaptation effects of both color and luminance on color discrimination. This fact suggests that the nonlinear properties at each stage have different roles in color perception.

Measuring attention using flash-lag effect.

We investigated the effect of attention on the flash-lag effect (FLE) in order to determine whether the FLE can be used to estimate the effect of visual attention. The FLE is the effect that a flash aligned with a moving object is perceived to lag the moving object, and several studies have shown that attention reduces its magnitude. We measured the FLE as a function of the number or speed of moving objects. The results showed that the effect of cueing, which we attributed the effect of attention, on the FLE increased monotonically with the number or the speed of the objects. This suggests that the amount of attention can be estimated by measuring the FLE, assuming that more amount of attention is required for a larger number or faster speed of objects to attend. On the basis of this presumption, we attempted to measure the spatial spread of visual attention by FLE measurements. The estimated spatial spreads were similar to those estimated by other experimental methods.

Integration of monocular motion signals and the analysis of interocular velocity differences for the perception of motion-in-depth.

We investigated how the mechanism for perceiving motion-in-depth based on interocular velocity differences (IOVDs) integrates signals from the motion spatial frequency (SF) channels. We focused on the question whether this integration is implemented before or after the comparison of the velocity signals from the two eyes. We measured spatial frequency selectivity of the MAE of motion in depth (3D MAE). The 3D MAE showed little spatial frequency selectivity, whereas the 2D lateral MAE showed clear spatial frequency selectivity in the same condition. This indicates that the outputs of the monocular motion SF channels are combined before analyzing the IOVD. The presumption was confirmed by the disappearance of the 3D MAE after exposure to superimposed gratings with different spatial frequencies moving in opposite directions. The direction of the 2D MAE depended on the test spatial frequency in the same condition. These results suggest that the IOVD is calculated at a relatively later stage of the motion analysis, and that some monocular information is preserved even after the integration of the motion SF channel outputs.

Psychophysical evidence for a purely binocular color system.

Two adaptation experiments were conducted to examine a hypothesis for a purely binocular color system that responds only to simultaneous inputs from the two eyes and that inhibits the activities of a pair of monocular color systems with each receiving input from their respective eye. In the first experiment, after a red or green stimulus was presented to both eyes to adapt the hypothesized binocular system, its compensatory color was presented alternately to each eye to nullify the adaptation effect of the hypothesized monocular systems. Results showed that after adaptation, the color appearance of a test stimulus shifted more to that of the compensatory color in binocular viewing than in monocular viewing. In the second experiment, a red or green stimulus was presented either to both eyes or to the left eye, and then its compensatory color was presented only to the left eye. Comparison was made to the adaptation effect between the binocular presentation of the color stimulus and its monocular presentation. Results showed that the color appearance viewed with the left eye shifted toward the compensatory color for the binocular adaptation and was constant for the monocular adaptation. These results are consistent with the idea of a "purely" binocular color system inhibiting the activity of a pair of monocular systems.

Differences in temporal frequency tuning between the two binocular mechanisms for seeing motion in depth.

There are two types of binocular cues available for perception of motion in depth. One is the binocular disparity change in time and the other is the velocity difference between the left and the right retinal images (inter-ocular velocity differences). We measured the luminance contrast threshold for seeing motion in depth while isolating either of the cues at various temporal modulations of velocity in the stimulus. To isolate disparity cues, dynamic random-dot stereograms were used (the disparity condition) while binocularly uncorrelated random-dot kinematograms were used to isolate velocity cues (the velocity condition). Results showed that sensitivity peaked at a temporal frequency (approximately 1 cps) in the velocity condition while the peak in the disparity condition was at the lowest frequency (0.35 cps) or at least at a frequency lower than that in the velocity condition. This suggests that the visual system has different temporal frequency properties for the velocity and disparity cues for motion in depth.

Stereo channels with different temporal frequency tunings.

To investigate the spatial and temporal frequency tunings for stereopsis, we measured the contrast sensitivity for depth discrimination with variable spatiotemporal frequencies and disparities using drifting sinusoidal gratings. The results showed that the contrast sensitivity changed with the stimulus disparity and the disparity tuning function varied with the spatial frequency. The disparity in the peak sensitivity decreased proportionally with the spatial frequency (size-disparity correlation). Although the temporal frequency exhibited a limited influence on the peak disparity, the temporal frequency tuning varied with the spatial frequency. The shape of the temporal frequency tuning function was lowpass for higher spatial frequencies, whereas it was bandpass for low spatial frequencies. These results suggest that more than one channel with different temporal as well as spatial frequency tunings contribute to stereopsis.

Detection of relative and uniform motion.

We measured the lowest velocity (velocity threshold) for discriminating motion direction in relative and uniform motion stimuli, varying the contrast and the spatial frequency of the stimulus gratings. The results showed significant differences in the effects of contrast and spatial frequency on the threshold, as well as on the absolute threshold level between the two motion conditions, except when the contrast was 1% or lower. Little effect of spatial frequency was found for uniform motion, whereas a bandpass property with a peak at approximately 5 cycles per degree was found for relative motion. It was also found that contrast had little effect on uniform motion, whereas the threshold decreased with increases in contrast up to 85% for relative motion. These differences cannot be attributed to possible differences in eye movements between the relative and the uniform motion conditions, because the spatial-frequency characteristics differed in the two conditions even when the presentation duration was short enough to prevent eye movements. The differences also cannot be attributed to detecting positional changes, because the velocity threshold was not determined by the total distance of the stimulus movements. These results suggest that there are two different motion pathways: one that specializes in relative motion and one that specializes in uniform or global motion. A simulation showed that the difference in the response functions of the two possible pathways accounts for the differences in the spatial-frequency and contrast dependency of the velocity threshold.

Smooth shifts of visual attention.

To examine whether visual attention shifts continuously across the visual field, we measured sensitivity to a small flash presented at various locations while the observer was tracking a moving target in an ambiguous apparent motion display. The sensitivity peaked near the target and the peak shifted smoothly along the apparent motion path. Since the peak-shift speed varied with the speed of the tracked target, we conclude that the attention mechanism selects the location to facilitate processing by tracking the target disk continuously. Attention does not simply select a location for enhanced processing, but rather predicts the future location of the object of interest based on its velocity.