Chromatic discrimination: differential contributions from two adapting fields

Chromatic discriminations along two cardinal axes share a common attentional resource

Attentional modulation is specific to either luminance or chromatic contrast discrimination, implying separate attentional resources for processing luminance and chromatic information processing [e.g., Curr. Biol.12, 1134 (2002)CUBLE20960-982210.1016/S0960-9822(02)00921-1]. However, there are two distinct visual pathways that process chromatic information: the parvocellular (PC) and koniocellular (KC) pathways. It is unclear whether there are separate attentional resources modulating the chromatic processes in these pathways. Here, we examined the attentional modulation effects on chromatic contrast discrimination with chromaticities along the $l$l or $s$s cardinal axis on a cone chromaticity space for preferentially stimulating either the inferred PC or KC pathway, respectively. A dual-task interference paradigm was used, and chromatic contrast discrimination sensitivities under dual-task conditions were compared with that under a single-task condition. The results revealed that compared with the single-task condition, attending to a competing central task in the dual-task condition decreased the peripheral discrimination sensitivity in both chromatic cardinal axes, and sensitivity reduced regardless of whether the dual tasks were along the same or different chromatic cardinal axes. These findings indicate that attentional effects on chromatic processes are not specific to the cardinal axis, suggesting that the PC and KC pathways may share a common attention resource in modulating chromatic processing.

Evidence for an impact of melanopsin activation on unique white perception

Current models of human color vision only consider cone inputs at photopic light levels, yet it is unclear whether the recently discovered melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) contribute to color perception. Using a lab-made five-primary photostimulator that can independently control the stimulations of rods, cones, and ipRGCs in human retina, we determined the observer's unique white perception, an equilibrium point for signals arising from the opponent mechanisms of color vision, under different levels of melanopsin activation. We found changing melanopsin activation levels shifts the equilibrium point in the chromatic pathways. Our results suggest potential evidence for an impact of melanopsin activation on unique white perception and the existing color vision model for the periphery may need to be revised by incorporating melanopsin signaling.

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.

S-cone discrimination in the presence of two adapting fields: data and model

This study investigated S-cone discrimination using a test annulus surrounded by an inner and outer adapting field with systematic manipulation of the adapting l=L/(L+M) or s=S/(L+M) chromaticities. The results showed that different adapting l chromaticities altered S-cone discrimination for a high adapting s chromaticity due to parvocellular input to the koniocellular pathway. In addition, S-cone discrimination was determined by the combined spectral signals arising from both adapting fields. The "white" adapting field or an adapting field with a different l chromaticity from the other fields was more likely to have a stronger influence on discrimination thresholds. These results indicated that the two cardinal axes are not independent in S-cone discrimination, and the two adapting fields jointly contribute to S-cone discrimination through a cortical summation mechanism.

Contrast magnitude and polarity effects on color filling-in along cardinal color axes

Color filling-in is the phenomenon in which the color of a visual area is perceived as the color that is only presented in an adjacent area. In a stimulus with multiple edges, color filling-in can occur along any edge and in both centripetal and centrifugal directions when maintaining steady fixation. The current study aimed to investigate the role of chromatic contrast magnitude and polarity along the two chromaticity cardinal axes and the interaction of the axes in the color filling-in process. In Experiment 1, the color filling-in process was examined using stimuli with three different regions and two edges. The three regions had chromaticities that varied only in one of the chromaticity axes. In Experiment 2, the regions along both edges differed in chromaticity along both axes. The results showed that the contrast magnitudes and polarity relationship of the two edges worked together to determine the filled-in direction and time course of the filled-in percepts. Further, the results pointed to a common mechanism mediating the color filling-in process along the two cardinal axes, and the two axes did not act independently in this process.