Figure 1.

Each image quad appeared once in each daily testing session. Because one of the four images in each image quad was the sample more frequently than the others, there is opportunity for habits to select these high-frequency images to develop through repeated reinforcement. A total of 72 quads were used. Only one image quad is illustrated in the figure. The illustration shows a high-frequency image appearing as the sample on every single trial, as in Experiment 3. In Experiment 2, the three images in each image quad that were not the high-frequency image each appeared as the sample with a probability of 0.1.