Color appearance can be classified into two modes, the surface-color mode and the illuminant-color mode. These modes of color appearance are determined depending on luminance of the stimulus. It has been reported that transition luminance between the two modes varies according to the chromaticity of the stimulus. We hypothesize that the visual system somehow knows the luminance of the optimal color and uses it to determine the transition luminance. The rationale for this hypothesis is that a stimulus with luminance higher than that of the optimal color is physically luminous. To confirm this hypothesis we compared the transition luminance with the optimal color luminance for various color surfaces under D65 illuminant. We found that the transition luminances between the two modes were not perfectly coincident with the luminances of the optimal color. However the transition of luminances varied in the manner of luminance of the optimal color varied. This suggests that the visual system might determine the transition luminance between the two modes using the luminance of the optimal color.

Human visual system can perceive 3D structure from optic flow or velocity field. We confirmed the 3D structure perception from the steady and continuous velocity field produced by cyclic presentation of several correlated random dot images. In addition, we investigated the velocity field perception for the factors of the number of phases (n), the temporal duration (△T) and the positional deviation (△d). Then, we found that there is suitable range of△T and △d for given n in which the velocity field could be perceived clearly.