Figure 12.1 presents the mean discrimination ratio for six pigeons during the acquisition of a two-event task. The pigeons learned a two- event discrimination in which AB was the positive sequence type and AA, BB and BA were negative sequence types. The discrimination ratio shown in Figure 12.1 was calculated separately for each negative sequence during each session by dividing test-stimulus key pecks per sec for the positive sequence by test-stimulus key pecks per sec for both the negative and positive sequence. This ratio approaches 0.50 when, as in early sessions of training, the negative sequences are poorly differentiated from AB, and approaches 1.00 when, as in latter sessions, the differentiation of negative sequences is further advanced. These results, and those of our prior published experiments ( Weismanet al., 1980; Weisman & DiFranco, 1981) leave little doubt that pigeons can represent and remember the positive sequence, AB, and discriminate it from the negative sequences, AA, BA, and BB. Negative sequences that include the same final event as the reinforced sequence (i.e., B) are more slowly discriminated from AB; in most instances, BA is discriminated more slowly than AA.