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 dot would result in a global percept of a clockwise (CW) spiral while grouping with the other dot, a counterclockwise (CCW) spiral. All dots in each pattern were modulated in the same color direction but different contrasts. Four types of patterns were involved, namely modulating in +/-(L-M), and +/-S cardinal directions. The observers were to determine whether the spiral in each trial was CW or CCW. The probability of a context dot being grouped with the anchoring dot increased along with its color contrast to certain level before the probability started to drop. Our result cannot be explained by the existing models for perceptual grouping but a divisive inhibition model. The isoluminance contrast result observed is similar to the inverted U-shaped function for luminance contrast result previously reported (by us); except that color contrast model comprises a weaker self-inhibition component.