The role of color contrast gain control in global form perception

A Glass pattern consists of randomly distributed dot pairs, or dipoles, whose orientation is determined by a geometric transform, which defines the global percept perceived by an observer. The perception of Glass patterns involves a local process that associates dot pairs into dipoles and a global process that groups the dipoles into a global structure. In the present study, we used a variant of Glass patterns, which was composed of randomly distributed tripoles instead of dipoles, to estimate the influence of color contrast on perceptual grouping. Each tripole contained an anchor dot and two context dots. Grouping the anchor dot with one of the context dots resulted in a global percept of a clockwise spiral, while grouping with the other dot, a counter-clockwise spiral. All dots in each pattern were modulated in the same color direction but different contrasts. Four colors were involved, namely, red, green, blue, and yellow. The observers were to determine whether the spiral in each trial was clockwise or counter-clockwise. The probability of a context dot being grouped with the anchoring dot increased with its color contrast to a certain level, then decreased when the contrast continued to increase. Such probability decreased as the contrast of the other context dot increased. Our result cannot be explained by existing models in the literature, but with a divisive inhibition model. The equiluminance contrast result observed here is similar to the inverted U-shaped function for luminance contrast result previously reported by us, except that the color contrast model comprises a weaker self-inhibition component.