Different hue coding underlying figure segregation and region detection tasks.

Figure segregation from its background is one of the important functions of color vision for our visual system because it is a preliminary to shape recognition. However, little is known about the chromatic mechanisms underlying figure segregation as opposed to those underlying mere color discrimination and detection. We investigated whether there are differences in color difference thresholds between a shape discrimination task (involving figure segregation) and a simple region detection task. In the shape discrimination task the observer discriminated the shapes of two figures, which could be segregated from their background on the basis of a color direction (hue) difference. In the region detection task the observer simply detected a square region against its background. Thresholds of color direction differences from a range of background color directions were measured for each task. In addition, we added saturation variation in one condition to investigate the involvement of the cone-opponent channels in those tasks. First, the results showed that the saturation variation increased the thresholds evenly for all background color directions. This suggests that higher-order color mechanisms rather than the early cone-opponent mechanisms are involved in both of the two tasks. Second, the shapes of the background color direction-threshold functions were different between the two tasks and these shape differences were consistent across all observers. This finding suggests that hue information may be encoded differently for shape discrimination and region detection. Furthermore, differences in spatial frequency components and in the requirement for orientation extraction rarely affected the shapes of the threshold functions in additional experiments, suggesting the possibility that hue encoding for shape discrimination differs from encoding for region detection at a late stage of form processing where local orientation signals are globally integrated.