Figure 2. Acquisition functions for individual subjects in each of the four groups of Experiment A. The groups are labeled according to their fixed-ratio sample requirement, for example, pigeons in the FR20 group were required to peck the sample 20 times before the comparison stimuli were presented. Pigeons in the FR0 group were not required to peck the sample; all three cartoons of the display were presented at the same time. Within each group, pigeons trained on the upper set of Figure 1 have their data shown with unfilled points and those trained on the lower set of Figure 1 have their data shown with filled points.