Abstract
We found an illusion of which thin (0.031 deg) gray lines on a cyan background appeared red when the lines were surrounded by thin (0.031 deg) white gaps. This effect was known as simultaneous color contrast, however, the magnitude of the effect was greatly enhanced than the line without gaps. The effects were quantified by a color-matching experiment, and matched colors were significantly shifted toward a complementary hue than no gap conditions (p < 0.05, permutation test). Generally the appearance of gray lines on a color background reflects either simultaneous color assimilation or contrast depending on its luminance, whereas gray lines with white gaps always showed simultaneous color contrast regardless of gray luminance levels. Moreover, almost no simultaneous contrast nor assimilation was observed if the gap was black. This indicates any explanation based on spatial frequency or proximity would be difficult. The illusion occurred regardless of hues if the appropriate thickness of gray lines and gaps were used; preferred thickness was larger for blue and yellow (S cone axis) background than cyan and pink background (L-M cone axis). The illusion was prominent for thin lines and dots, however optical blur and chromatic aberration were not major factors. It might reflect the interaction between color and luminance in early vision stages which have small receptive fields. This phenomenon may be considered as an inference of color of small or thin objects which exceeds spatial resolution of color by remaining luminance pattern, and it help to recognize thin objects which have the specular reflection on the side edge, such as thin branches of a tree on the sky background.