Pigeons' discrimination of color proportion in computer-generated visual displays

Pigeons were trained to discriminate the proportion of red to green color i n paired stimulus displays. Initially, the stimuli were horizontal bars com posed of continuous blocks of color that varied from being all red versus a ll green to .5 proportions of these two colors. Discrimination accuracy dec reased as a function of the disparity in the proportions of the two colors. This relationship was maintained when the stimulus configurations were alt ered in various ways. Tests with horizontal bars indicated that the pigeons could utilize differences in the lengths (or areas) of one of the colors w hen choosing between stimuli. They did not rely only on this type of cue to assess proportion disparities but rather on multiple stimulus parameters. Also, the form of the discrimination function suggests that the pigeons dis tinguished ratio differences, so that Weber's law applies to this type of d iscrimination.