Ichiro Kuriki

Human Visual System Integrates Color Signals along a Motion Trajectory

Whether fundamental visual attributes, such as color, motion, and shape, are analyzed separately in specialized pathways has been one of the central questions of visual neuroscience. Although recent studies have revealed various forms of cross-attribute interactions, including significant contributions of color signals to motion processing, it is still widely believed that color perception is relatively independent of motion processing. Here, we report a new color illusion, motion-induced color mixing, in which moving bars, the color of each of which alternates between two colors (e.g., red and green), are perceived as the mixed color (e.g., yellow) even though the two colors are never superimposed on the retina. The magnitude of color mixture is significantly stronger than that expected from direction-insensitive spatial integration of color signals. This illusion cannot be ascribed to optical image blurs, including those induced by chromatic aberration, or to involuntary eye movements of the observer. Our findings indicate that color signals are integrated not only at the same retinal location, but also along a motion trajectory. It is possible that this neural mechanism helps us to see veridical colors for moving objects by reducing motion blur, as in the case of luminance-based pattern perception.

Limitations of surface-color and apparent-color constancy

Color-constancy mechanisms have been studied and discussed in a number of investigations. However, there has been little attempt to reveal how color constancy deteriorates as the conditions for it become less than optimal. We carried out a series of asymmetric color-matching experiments, using two criteria: surface-color match and apparent-color match. With brief adaptation the degree of color constancy increased as chromatic cues were added in the surround. In the condition of black surround, the test stimuli appeared self-luminous, and chromaticities of the chosen matching stimuli were the same as the physical chromaticities of the test stimulus, indicating a total deficiency of color constancy. With 15 min of preadaptation to the illuminant, the surface-color matches showed almost perfect color constancy under illuminant change. In both adaptation conditions, the chromatic-shift of matches from what would be expected for perfect color constancy increased gradually between 1,700- and 30,000-K illuminant, as chromaticity of the illuminant departed from 6,500-K illuminant. Under 1,000-K illuminant the surface-color appearance became totally achromatic, and color constancy was completely lost. Our results show that, even with brief adaptation to the illuminant, the contribution of the surrounding stimulus is large enough to achieve a fair degree of color constancy, but complete adaptation to the illuminant helps to achieve almost perfect color constancy.

Adaptive shift of visual sensitivity balance under ambient illuminant change

We examined the relationship between the ambient illuminant chromaticity and changes in the sensitivity balance of the visual system, using illuminants of various chromaticities. The sensitivity of observers was measured in a room with a variable-chromaticity illuminant. The observers state of chromatic adaptation was measured with unique-white settings. Our results showed that the change in visual sensitivity has a nonlinear correlation with the change in illuminant chromaticity; chromatic adaptation was nearly complete for desaturated illuminants, but the degree of chromatic adaptation became worse as the illuminant became more saturated. We defined a new index, relative cone weights, which represents this relationship well. To measure the role of chromatic induction from the immediate-surround area of the matching stimulus, we performed additional experiments by presenting the test inside a colored or black immediate surround. The results showed that the unique-white settings were not disturbed by the change in immediate-surround color. Our results imply that the room illuminant chromaticity was the primary factor in changing the observers state of chromatic adaptation.

Brightness, not luminance, determines transition from the surface-color to the aperture-color mode for colored lights

Whether a color stimulus appears in the surface-color or in the aperture-color mode depends on the luminance relationship between the center color stimulus and its surround. We investigated how chromaticity of a color stimulus affected the luminance level at which the appearance of the stimulus changed from the surface-color to the aperture-color mode. Mode estimation points were obtained for 10-cd/m2 color stimuli with different chromaticities presented in the center of a white surround of variable luminance. The color stimuli tended to appear in the aperture-color mode as purity increased, similarly to the increase of the brightness-to-luminance ratio for equal-luminance colors. It was also found that the mode-transition sensitivity function was similar in shape to the brightness sensitivity function for 440–660-m monochromatic light. Our results indicate that brightness is a determining factor for mode transition between the surface-color and the aperture-color modes. We discuss a possible assumption for relationships between brightness and lightness limits of a surface color.

Time courses of attentional modulation in neural amplification and synchronization measured with steady-state visual-evoked potentials

Endogenous attention modulates the amplitude and phase coherence of steady-state visual-evoked potentials (SSVEPs). In efforts to decipher the neural mechanisms of attentional modulation, we compared the time course of attentional modulation of SSVEP amplitude (thought to reflect the magnitude of neural population activity) and phase coherence (thought to reflect neural response synchronization). We presented two stimuli flickering at different frequencies in the left and right visual hemifields and asked observers to shift their attention to either stimulus. Our results demonstrated that attention increased SSVEP phase coherence earlier than it increased SSVEP amplitude, with a positive correlation between the attentional modulations of SSVEP phase coherence and amplitude. Furthermore, the behavioral dynamics of attention shifts were more closely associated with changes in phase coherence than with changes in amplitude. These results are consistent with the possibility that attention increases neural response synchronization, which in turn leads to increased neural population activity.

Characteristics of color memory for natural scenes

To study the characteristics of color memory for natural images, a memory-identification task was performed with differing color contrasts; three of the contrasts were defined by chromatic and luminance components of the image, and the others were defined with respect to the categorical colors. After observing a series of pictures successively, subjects identified the pictures using a confidence rating. Detection of increased contrasts tended to be harder than detection of decreased contrasts, suggesting that the chromaticness of pictures is enhanced in memory. Detecting changes within each color category was more difficult than across the categories. A multiple mechanism that processes color differences and categorical colors is briefly considered.

Eye-Head Coordination for Visual Cognitive Processing

We investigated coordinated movements between the eyes and head (“eye-head coordination”) in relation to vision for action. Several studies have measured eye and head movements during a single gaze shift, focusing on the mechanisms of motor control during eye-head coordination. However, in everyday life, gaze shifts occur sequentially and are accompanied by movements of the head and body. Under such conditions, visual cognitive processing influences eye movements and might also influence eye-head coordination because sequential gaze shifts include cycles of visual processing (fixation) and data acquisition (gaze shifts). In the present study, we examined how the eyes and head move in coordination during visual search in a large visual field. Subjects moved their eyes, head, and body without restriction inside a 360° visual display system. We found patterns of eye-head coordination that differed those observed in single gaze-shift studies. First, we frequently observed multiple saccades during one continuous head movement, and the contribution of head movement to gaze shifts increased as the number of saccades increased. This relationship between head movements and sequential gaze shifts suggests eye-head coordination over several saccade-fixation sequences; this could be related to cognitive processing because saccade-fixation cycles are the result of visual cognitive processing. Second, distribution bias of eye position during gaze fixation was highly correlated with head orientation. The distribution peak of eye position was biased in the same direction as head orientation. This influence of head orientation suggests that eye-head coordination is involved in gaze fixation, when the visual system processes retinal information. This further supports the role of eye-head coordination in visual cognitive processing.

Direction-specific adaptation of magnetic responses to motion onset

We investigated the direction-specificity of motion adaptation, by recording magnetic responses evoked by motion onsets under both adapted and control conditions. The inter-stimulus interval was equated between the conditions to precisely evaluate the effect of motion adaptation itself. The onset stimuli at 1.5, 3.0 or 6.0 deg/s moved in the same direction or in the opposite direction to an adaptation stimulus at 3.0 deg/s. The perceived velocity of each test stimulus was measured in separate sessions. The most prominent peak (M2) of evoked responses appeared around 200-300 ms after motion onsets, and the dipoles were mainly estimated in the temporo-occipital area. Adaptation largely affected both perceived velocities and the M2 amplitudes. The M2 amplitudes were decreased by adaptation for both directions of test stimuli, and the decreases were significantly larger for the test stimuli in the adapted direction (49-63% of control condition) than for the test stimuli in the opposite direction (17-27% of control condition). The present study, for the first time, found that magnetic responses evoked by motion onsets reflect the activities of neurons that have direction-specificity.

The loci of achromatic points in a real environment under various illuminant chromaticities

Under colored illumination, the achromatic point (the point in the chromaticity diagram seen as colorless) shifts toward the chromaticity of the illuminant. This investigation measured the loci of achromatic points for various intensities of a test field presented in a real rather than a simulated environment, lit by illuminants of various chromaticities. The achromatic point varied markedly with the intensity level of the test field: for dim test fields it was close to the surround chromaticity, but for high luminance test fields it was almost invariant with the surround chromaticity. The varying achromatic settings imply a variation in the relative effectiveness of the different cone types, but this variation originates in the postreceptoral system rather than at the photoreceptors themselves: flicker photometric sensitivity was almost independent of the illuminant in all cases. Nor does the variation take the simple form of a sensitivity-scaling coefficient; such a model can not predict the observed dependence of the achromatic setting on test intensity. The data could, however, be modeled with a scheme in which the log of the relative cone weight implicit in the achromatic setting depends almost linearly on (1) the log of the relative cone excitation by the illuminant and (2) the log of the test field intensity.

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

Significance There has been much debate on the Sapir–Wharf hypothesis regarding whether language affects our perceptual world. Despite much research on this topic, there remains no clear consensus on whether and how language affects categorical color perception. Here, we provide the first evidence, to our knowledge, that categorical color perception has a universal starting point prior to language acquisition. We measured the neural correlates of categorical color perception in prelinguistic infants. We found increased brain activities to colors in different categories, but not to colors in the same category. These results indicated that different color categories are differently represented in the visual cortex of prelinguistic infants, which implies that color categories may develop in the visual system before language acquisition. 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.